Bandito Amps: The Online Mag for Tube Amp Lovers
Volume No. 10-3 March 2010
Volume No. 10-3 March 2010
Bandito's Garage: Where the rubber meets the road
Here is the place to find lots of pictures of all the great names in tube amplification, famous dudes and dudets of modern music, and maybe even a surprise or two for you adventurous bolt benders in the banditoamps crowd.
For January, February .... etc
Bandy has gone on the road in his '65 Impala to find some new tube amp stuff. If he ever comes home, he promises to make "everything right" with all his loyal readers and post some hot tube amp articles here. Maybe?
Here is the place to find lots of pictures of all the great names in tube amplification, famous dudes and dudets of modern music, and maybe even a surprise or two for you adventurous bolt benders in the banditoamps crowd.
For January, February .... etc
Bandy has gone on the road in his '65 Impala to find some new tube amp stuff. If he ever comes home, he promises to make "everything right" with all his loyal readers and post some hot tube amp articles here. Maybe?
This part is the original tear down, layout and first build that Mark Moser did for Banditoamps in November.
Here is the Mark Moser description of what makes a 'Proluxe' amp:
The Proluxe circuit I believe was developed by Ted Weber as a cross between the Tweed Deluxe and a Tweed Pro circuits.
The Tweed Deluxe uses 2 input channels (usually with a pair of input jacks per channel, one high on low input). One volume control for the Normal input side and one volume control for the Brite input side. It uses one tone control for both. The volume controls and the tone control are all interactive- meaning no matter which channel you are plugged into, the other channel's volume control will effect the signal. This is a cool feature of the Tweed Deluxe and is a big factor in its' creamy tones. It uses 2- 6V6 power tubes, two preamp tubes and a 5Y3 rectifier tube for about 12-15 watts of power. The amp is Cathode biased.
The Tweed Pro and its very close siblings, the Super and the Bandmaster, all use 6L6 power tubes and a Leo Fender fixed bias feature to get more clean headroom and power from the amp. These amps produce about 30 watts or more. The Proluxe takes the preamp front end of a Tweed Deluxe and marries it to the power section of a Tweed Pro using the bigger power tubes and fixed tube biasing and, walah!, the Proluxe was born. I believe that Ted Weber came up with a real winner here and that's the idea behind this Bogan Challenger retro rebuild.
Ok amigos, let's kick this new segment off with a couple pics of the Bogen Challenger CHA-20. It started life as a cathode biased 6V6 power tube public address amplifier in the mid-sixties. David Bogen built a lot of high quality tube amps in the 60's and quite a few of them have found their way back onto the stage. I will be adding pictures and notes as Mark from Frugalamps does the modifications. All of Mark's remarks will be in red and have 'MM' in them.
MM Front panel shot of Bogen amp. Two inputs, 3 control pot openings and a spot for a double action power/standby switch. Good setup for a tweed era control layout for a Proluxe. You can see the amp has an old modification to one of the input jacks to convert it from a microphone input to a 1/4" guitar jack input. Note the large washer used to close the oversized hole when the mic jack was removed.
Notice in this picture of the Bogen Challenger before any mods are done, that it uses the very popular 6V6 power tubes and 12AX7 preamp tubes. Also note that it has a mono Phono jack in the right hand corner for hooking up an early mono record player. There are also lots of windings on the output transformer offering 4,8 and 16 ohms which is always a plus when you know that you are going to be hooking up to different speaker cabinets.
Here is the Bogen CHA-20 with her innards exposed. Note that both the power transformer and the output transformer have nice clear markings so Mark can easily check voltages and plan the layout of the new circuits. It's a big plus having lots of open space inside the chasis to make the tear out and rebuild as trouble free as possible.
MM Whenever you consider a conversion of an old amp you always should- if possible- fire it up and take some readings from the power transfomer to test for function and voltages. Check the output transformer for shorts, etc. You need to know if the voltages are going to be appropriate for the circuit you are building. Old transformers were made to operate under lower AC voltage than today's wall voltage provides. This will make them run higher voltages to the circuit. This unit is within the range I need to push the 2 6L6 power tubes my Proluxe circuit calls for. Your rectifier choice will affect the final voltage to the circuit as well and there are ways to drop some voltage via zeners. I like to be close to start with and just modify the power circuit resistors to get the sweet spot I am looking for.
MM Here a shot of the interior almost cleaned out. I do not like to reuse any of the old componenets from an amp this age. Resistors drift, electrolytics leak, old pots also drift or loose their values (sounds like Harry and Nancy P down there in Dumbass DC). No sense going to all the trouble to build a new circuit with less than quality new parts. I like to build my new layouts on tag boards but I keep the wiring runs as short as possible. In this amp the power tubes run at right angles to the preamp tubes, so I will divide up the preamp circuit and the power circuit to fit the space and keep wiring runs to the preamp and tone controls short. In this amp I like the twisted heater runs they bused to the preamp tubes, so I will keep them in place. Just make sure the wiring is still sound and retouch the solder joints to be safe.
MM After I look over the existing tube sockets and carefully remove the old components that were wired to them using the point to point method Bogen used on these amps, I then remove all the solder and wiring tags from each socket. If the socket is damaged or I view it as too "used" I will drill out the old fastening rivets and replace the socket with a new one. Bogen used Cinch brand sockets here and they are a good quality part, so I try to reuse them.
MM The front panel after all pots and switches are removed and the old mic hole has been resized and riveted in place. Time for a coat of paint. (ok Mark, anything but pink on that paint job, eh?!)
MM This is the pair of terminals I will use to wire up the power filter chain. It's separate from the preamp board but there is a perfect spot in the chassis near the power and standby switches to install. I have two of the dropping resistors in place.
MM Here is a shot of the tag board I use for the new Proluxe circuits. All dressed out with the circuit for the preamp and bias requirements. The round black pot is for adjusting the bias via a small screwdriver.
MM Here is the first preamp tube wired with the input jacks going straight through the resistors and into the corresponding socket pins. This is called point to point style. I like to keep the distance from input jack to preamp tube as short as possible. It can't get much shorter than this. (Amen, brother Mark. And it can't get much nicer looking either, eh banditos?)
Ok, now we're talking output business here. Mark shows us the final wiring for the output jack and the retro wiring to the Bogan's ohm selector strip.
The outside view of the Ohm Selector swich posts. This is the "old style" way of wiring the ohm selector so the user can choose which type of speaker cabinet they are using and match it to the correct output ohms.
MM Output transformer connection done old school via the wire I installed and the original screw tab board from the Bogen. Simple but effective. This shows the output jack in the back of the chassis with a wire going from the tip junction out the back of the amp to connect with one of the output transformer taps. I do it this way to keep costs down but you can get fancy and get a good selector switch and wire it that way as well. This works fine and its simple. (Simple is good for guys who are interested in getting the equipment set up and throwing tonal grenades at the crowd, not playing around on a dark stage.
Here is the new Proluxe preamp circuit board ready for install. Note that Mark is going to mount it on a pair of adjustable machine bolts to make getting the height above the tube sockets and chasis wiring just right.
Ok, here is our Proluxe amp built up with all the major items connected and placed in the proper spot. Very clean and nice looking build from our tube amp hero, Mark from Frugal Amps.
At this point, after Mark had the amp all together and running, he found that the voltages coming off the Power Transformer were too hot for the circuits he had designed. Mark offers that this is a common problem when you are using a vintage tranny. Let's see what he has to say about this issue for future reference.
MM Yes, I have found the Bogen amps of this vintage to be the most fussy to get the voltages inline for a guitar circuit. Some of the other amps I have refitted like the Bell or Dukanes are much closer to where I want them for the guitar amp circuit- however Bogens are probably one of the most available of this ilk and therefore, would be a common amp for guys to consider for a project like this. The later 60's Challengers (Bogans) are even tougher but I do not find the power transformers to be as robust as the earlier vintage Bogans like this one we're building.
One of the vintage tranny gurus from Mercury Magnetics claims that many 50's and early 60's amp transformers were using the old supply of high grade steel once stockpiled for the WW2 war effort. When that supply of steel started to wane, the makers started using cheaper steel grades in the power and output transformers and quality went down...Now there is a interesting theory which, all else considered, certainly could be true.
We are going to take another time-out at this portion of the Retro-Rebuild article while Mark is waiting for some Zener diodes to arrive that he will use to bring the voltages into spec for the Proluxe circuit. Check back soon and catch all the action here in the Bandito's Garage. Last updated on November 25th Gobble Gobble vatos!
Tired of waiting already kids? Ok, uncle Bandy's gonna take care of that. Here's a little time waster we call Lucinda.
"Bandy, can I get my underpants please? I'm kinda cold sitting here waiting for you to come warm my bum. And, if you don't quit tubanating with that electro thingy, I'm gonna stand up and show my yum-yums to the boys out there." Lucinda get's her way again fellas.
Here is the Mark Moser description of what makes a 'Proluxe' amp:
The Proluxe circuit I believe was developed by Ted Weber as a cross between the Tweed Deluxe and a Tweed Pro circuits.
The Tweed Deluxe uses 2 input channels (usually with a pair of input jacks per channel, one high on low input). One volume control for the Normal input side and one volume control for the Brite input side. It uses one tone control for both. The volume controls and the tone control are all interactive- meaning no matter which channel you are plugged into, the other channel's volume control will effect the signal. This is a cool feature of the Tweed Deluxe and is a big factor in its' creamy tones. It uses 2- 6V6 power tubes, two preamp tubes and a 5Y3 rectifier tube for about 12-15 watts of power. The amp is Cathode biased.
The Tweed Pro and its very close siblings, the Super and the Bandmaster, all use 6L6 power tubes and a Leo Fender fixed bias feature to get more clean headroom and power from the amp. These amps produce about 30 watts or more. The Proluxe takes the preamp front end of a Tweed Deluxe and marries it to the power section of a Tweed Pro using the bigger power tubes and fixed tube biasing and, walah!, the Proluxe was born. I believe that Ted Weber came up with a real winner here and that's the idea behind this Bogan Challenger retro rebuild.
Ok amigos, let's kick this new segment off with a couple pics of the Bogen Challenger CHA-20. It started life as a cathode biased 6V6 power tube public address amplifier in the mid-sixties. David Bogen built a lot of high quality tube amps in the 60's and quite a few of them have found their way back onto the stage. I will be adding pictures and notes as Mark from Frugalamps does the modifications. All of Mark's remarks will be in red and have 'MM' in them.
MM Front panel shot of Bogen amp. Two inputs, 3 control pot openings and a spot for a double action power/standby switch. Good setup for a tweed era control layout for a Proluxe. You can see the amp has an old modification to one of the input jacks to convert it from a microphone input to a 1/4" guitar jack input. Note the large washer used to close the oversized hole when the mic jack was removed.
Notice in this picture of the Bogen Challenger before any mods are done, that it uses the very popular 6V6 power tubes and 12AX7 preamp tubes. Also note that it has a mono Phono jack in the right hand corner for hooking up an early mono record player. There are also lots of windings on the output transformer offering 4,8 and 16 ohms which is always a plus when you know that you are going to be hooking up to different speaker cabinets.
Here is the Bogen CHA-20 with her innards exposed. Note that both the power transformer and the output transformer have nice clear markings so Mark can easily check voltages and plan the layout of the new circuits. It's a big plus having lots of open space inside the chasis to make the tear out and rebuild as trouble free as possible.
MM Whenever you consider a conversion of an old amp you always should- if possible- fire it up and take some readings from the power transfomer to test for function and voltages. Check the output transformer for shorts, etc. You need to know if the voltages are going to be appropriate for the circuit you are building. Old transformers were made to operate under lower AC voltage than today's wall voltage provides. This will make them run higher voltages to the circuit. This unit is within the range I need to push the 2 6L6 power tubes my Proluxe circuit calls for. Your rectifier choice will affect the final voltage to the circuit as well and there are ways to drop some voltage via zeners. I like to be close to start with and just modify the power circuit resistors to get the sweet spot I am looking for.
MM Here a shot of the interior almost cleaned out. I do not like to reuse any of the old componenets from an amp this age. Resistors drift, electrolytics leak, old pots also drift or loose their values (sounds like Harry and Nancy P down there in Dumbass DC). No sense going to all the trouble to build a new circuit with less than quality new parts. I like to build my new layouts on tag boards but I keep the wiring runs as short as possible. In this amp the power tubes run at right angles to the preamp tubes, so I will divide up the preamp circuit and the power circuit to fit the space and keep wiring runs to the preamp and tone controls short. In this amp I like the twisted heater runs they bused to the preamp tubes, so I will keep them in place. Just make sure the wiring is still sound and retouch the solder joints to be safe.
MM After I look over the existing tube sockets and carefully remove the old components that were wired to them using the point to point method Bogen used on these amps, I then remove all the solder and wiring tags from each socket. If the socket is damaged or I view it as too "used" I will drill out the old fastening rivets and replace the socket with a new one. Bogen used Cinch brand sockets here and they are a good quality part, so I try to reuse them.
MM The front panel after all pots and switches are removed and the old mic hole has been resized and riveted in place. Time for a coat of paint. (ok Mark, anything but pink on that paint job, eh?!)
MM This is the pair of terminals I will use to wire up the power filter chain. It's separate from the preamp board but there is a perfect spot in the chassis near the power and standby switches to install. I have two of the dropping resistors in place.
MM Here is a shot of the tag board I use for the new Proluxe circuits. All dressed out with the circuit for the preamp and bias requirements. The round black pot is for adjusting the bias via a small screwdriver.
MM Here is the first preamp tube wired with the input jacks going straight through the resistors and into the corresponding socket pins. This is called point to point style. I like to keep the distance from input jack to preamp tube as short as possible. It can't get much shorter than this. (Amen, brother Mark. And it can't get much nicer looking either, eh banditos?)
Ok, now we're talking output business here. Mark shows us the final wiring for the output jack and the retro wiring to the Bogan's ohm selector strip.
The outside view of the Ohm Selector swich posts. This is the "old style" way of wiring the ohm selector so the user can choose which type of speaker cabinet they are using and match it to the correct output ohms.
MM Output transformer connection done old school via the wire I installed and the original screw tab board from the Bogen. Simple but effective. This shows the output jack in the back of the chassis with a wire going from the tip junction out the back of the amp to connect with one of the output transformer taps. I do it this way to keep costs down but you can get fancy and get a good selector switch and wire it that way as well. This works fine and its simple. (Simple is good for guys who are interested in getting the equipment set up and throwing tonal grenades at the crowd, not playing around on a dark stage.
Here is the new Proluxe preamp circuit board ready for install. Note that Mark is going to mount it on a pair of adjustable machine bolts to make getting the height above the tube sockets and chasis wiring just right.
Ok, here is our Proluxe amp built up with all the major items connected and placed in the proper spot. Very clean and nice looking build from our tube amp hero, Mark from Frugal Amps.
At this point, after Mark had the amp all together and running, he found that the voltages coming off the Power Transformer were too hot for the circuits he had designed. Mark offers that this is a common problem when you are using a vintage tranny. Let's see what he has to say about this issue for future reference.
MM Yes, I have found the Bogen amps of this vintage to be the most fussy to get the voltages inline for a guitar circuit. Some of the other amps I have refitted like the Bell or Dukanes are much closer to where I want them for the guitar amp circuit- however Bogens are probably one of the most available of this ilk and therefore, would be a common amp for guys to consider for a project like this. The later 60's Challengers (Bogans) are even tougher but I do not find the power transformers to be as robust as the earlier vintage Bogans like this one we're building.
One of the vintage tranny gurus from Mercury Magnetics claims that many 50's and early 60's amp transformers were using the old supply of high grade steel once stockpiled for the WW2 war effort. When that supply of steel started to wane, the makers started using cheaper steel grades in the power and output transformers and quality went down...Now there is a interesting theory which, all else considered, certainly could be true.
We are going to take another time-out at this portion of the Retro-Rebuild article while Mark is waiting for some Zener diodes to arrive that he will use to bring the voltages into spec for the Proluxe circuit. Check back soon and catch all the action here in the Bandito's Garage. Last updated on November 25th Gobble Gobble vatos!
Tired of waiting already kids? Ok, uncle Bandy's gonna take care of that. Here's a little time waster we call Lucinda.
"Bandy, can I get my underpants please? I'm kinda cold sitting here waiting for you to come warm my bum. And, if you don't quit tubanating with that electro thingy, I'm gonna stand up and show my yum-yums to the boys out there." Lucinda get's her way again fellas.
Back in the saddle again. Yeah, that's right Roy Acuff and Doc Watson, Mark Moser over at Frugal Amps is back and has the babeolicious retro rebuild Proluxe in the can and ready for publication on America's hottest tube amp magazine......and, everyone says, "BANDITOAMPS"
So, let's stop dilly dallying around and get right to it. We pick up the retro rebuild with Mark explaining some problematic voltage issues he found during startup and burn in.
MM After all the initial wiring and testing the circuit with a full compliment of tubes, I was getting
voltages a bit too high for my liking in this circuit. I sent off the the Weber folks and ordered a "power dump" zener to place on the power transformers high voltage ground tap. The unit also comes with a high voltage capacitor that solders on from the zener's tip to ground to eliminate any noise a big zener can sometimes introduce into an amp. In addition, it comes with a white thermal grease that helps insure good temperature transfer from the zener to the ground plate. Its the same thermal grease you put on a computer processor when you mount a heat sink to it- if you ever have made your own computer that is?
MM After final tweaking and playing the amp, making sure the voltages are good and the bias on the power tubes are in spec, its time to button this amp up. Here I have applied the labels to the jacks, knobs and switches. In addition, I have placed labels on the topside to indicate the tubes where they have changed from the origianl labeling. This amp has also gained a 5 volt filament transformer seen in the left hand corner which I added to power the 5 volt 5y3 rectifier tube as the original amp used a 6 volt 6X4 rectifier. I wanted there to be more options down the road in any future tweaks or circuit changes the amp may have and a 5 volt rectifier option is the way to go to accomplish this versatility.
MM Another word on 5Y3 rectifiers. NOS 5Y3 rectifiers are preferred by me in most cases when building a tweed circuit amp. They "drop" a lot more voltage when compared to a new production rectifier tube (at least all the Russian made types I have used) and since my voltages were high on this amp to begin with, I wanted that feature from the tube. The other 5 volt rectifiers like the 5U4 or the 5AR4 (GZ34) drop less current than a 5Y3 and are sometimes referred to as a "stiffer" rectifier. A solid state diode based rectifier is the stiffest of all.
MM Ok, here we have the Boganitious Proluxe dressed out in chicken head knobs as befits a Tweed based amp. I use a "hammertone" finish paint most of the time on my amps. It gives a nice old fashioned look and hides a lot of old labeling from the original faceplate. Here you can also see the two old 12ax7 tubes that power the preamp section. Its a well known trick to try different members of the 12??7 family especially in the first preamp spot. Using a 12AU7 or a 12AY7 tube will lower the overall gain of the amp and also increase your clean headroom a bit. But you can also try swapping out the second preamp tube for a different effect as well. You will not hurt the amp trying these swaps.
MM The backside of the amp gets a simple method for changing the output tap as mentioned earlier. When we switched this amp from 6V6 power tubes to 6L6 tubes we must pay attention in the shift downward of the output transformer taps. Our original 4,8 and 16 ohm taps all take a step down to 2, 4 and 8 ohms. This matches the load now seen by the output transformer with the different power tubes. On a side note, it would be possible to swap in a variety of "big bottle" power tubes in this amp. The voltages are high enough to use EL34, KT66, 5881 or 6L6 tubes. Just be forewarned to pay attention to bias and make sure you ground pin 1 and pin 8 for the EL34 family of tubes- they need pin 1 grounded to the chassis unlike the 6L6 which just wants pin 8 going to ground.
editor's note: Bandy asked Mark to further explain the downward "shift" in the output taps. Here is Mark's description. Thanks again Mark. YOU THE MAN!
MM The reason for changing output transformer taps on the amp goes like this: The original 6V6 power tubes liked to see around an 8,000 ohm primary resistance (the primary is the part of the output transformer that connects to the power tubes and power supply circuit of the amp). This was how the amp was built when new. A pair of 6L6 power tubes in the amp now want to see around a 4,000 ohm primary resistance. Since I can't change the actual output transformer, I need to vary the load on the secondary of the output transformer (the taps where you connect your speaker). Moving the taps "down" a step accomplishes this electrically and brings the load in line.
MM Finally, our Proluxe all buttoned up and with its cover on, you can see the power light behind the nameplate. How cool is that! This amp took a little extra time to get the voltages down to make the circuit happy (I'm glad Mark took the time to 'get it right' since I'm not partial to Mushroom clouds pluming from my rig while I'm basting the chicas' booties with my version of Innagoddadavida Baby). As with many older electronics from the 50's and 60's, the wall voltages in the US have risen steadily over the years. Where I live in Vermont, the power coming out of the wall in my workshop is 127 volts. The power expected when these amps were made are 112 to 115. Doesn't sound like a lot, but I find its a magnifying effect when you measure the actual high voltage taps coming out of the power transformer and compare it back to the original schematic and the expected voltages back in the day. Newly built transformers can take this into account but using vintage gear of any kind, you have to be aware of the changes. Bogen amps are very available and I find they really run high voltages during conversions, so if you try one of these just be ready to deal with it.
This amp is sounding really nice- like a high powered tweed deluxe (Ok, kinda like a Proluxe right?! Right on Mark). Some future tweaks that could be tried? Well, how about a little switch to tie the two inputs together internally for higher gain. Or add a VVR (variable voltage resistor) style master volume. This would take a bit of moving things around to fit it into the chassis as it needs another pot/knob up front, but you can use them to control the high voltage issue of the amp instead of a zener and you get to really crank the preamp if you need to get overdrive but keep the volume down on the home front. Lastly, I would consider an output transformer upgrade to some bigger iron for this amp. It would get you some piano like bass. The current transformer is a little small and while it sounds fine, with more iron comes more bass. This is a simple change and easy to do.
I find these amps react well with pedals and other effects. You really can set the volume where you want it for great overdrive then use your guitars volume to clean it up or let it rip.
So, let's stop dilly dallying around and get right to it. We pick up the retro rebuild with Mark explaining some problematic voltage issues he found during startup and burn in.
MM After all the initial wiring and testing the circuit with a full compliment of tubes, I was getting
voltages a bit too high for my liking in this circuit. I sent off the the Weber folks and ordered a "power dump" zener to place on the power transformers high voltage ground tap. The unit also comes with a high voltage capacitor that solders on from the zener's tip to ground to eliminate any noise a big zener can sometimes introduce into an amp. In addition, it comes with a white thermal grease that helps insure good temperature transfer from the zener to the ground plate. Its the same thermal grease you put on a computer processor when you mount a heat sink to it- if you ever have made your own computer that is?
MM After final tweaking and playing the amp, making sure the voltages are good and the bias on the power tubes are in spec, its time to button this amp up. Here I have applied the labels to the jacks, knobs and switches. In addition, I have placed labels on the topside to indicate the tubes where they have changed from the origianl labeling. This amp has also gained a 5 volt filament transformer seen in the left hand corner which I added to power the 5 volt 5y3 rectifier tube as the original amp used a 6 volt 6X4 rectifier. I wanted there to be more options down the road in any future tweaks or circuit changes the amp may have and a 5 volt rectifier option is the way to go to accomplish this versatility.
MM Another word on 5Y3 rectifiers. NOS 5Y3 rectifiers are preferred by me in most cases when building a tweed circuit amp. They "drop" a lot more voltage when compared to a new production rectifier tube (at least all the Russian made types I have used) and since my voltages were high on this amp to begin with, I wanted that feature from the tube. The other 5 volt rectifiers like the 5U4 or the 5AR4 (GZ34) drop less current than a 5Y3 and are sometimes referred to as a "stiffer" rectifier. A solid state diode based rectifier is the stiffest of all.
MM Ok, here we have the Boganitious Proluxe dressed out in chicken head knobs as befits a Tweed based amp. I use a "hammertone" finish paint most of the time on my amps. It gives a nice old fashioned look and hides a lot of old labeling from the original faceplate. Here you can also see the two old 12ax7 tubes that power the preamp section. Its a well known trick to try different members of the 12??7 family especially in the first preamp spot. Using a 12AU7 or a 12AY7 tube will lower the overall gain of the amp and also increase your clean headroom a bit. But you can also try swapping out the second preamp tube for a different effect as well. You will not hurt the amp trying these swaps.
MM The backside of the amp gets a simple method for changing the output tap as mentioned earlier. When we switched this amp from 6V6 power tubes to 6L6 tubes we must pay attention in the shift downward of the output transformer taps. Our original 4,8 and 16 ohm taps all take a step down to 2, 4 and 8 ohms. This matches the load now seen by the output transformer with the different power tubes. On a side note, it would be possible to swap in a variety of "big bottle" power tubes in this amp. The voltages are high enough to use EL34, KT66, 5881 or 6L6 tubes. Just be forewarned to pay attention to bias and make sure you ground pin 1 and pin 8 for the EL34 family of tubes- they need pin 1 grounded to the chassis unlike the 6L6 which just wants pin 8 going to ground.
editor's note: Bandy asked Mark to further explain the downward "shift" in the output taps. Here is Mark's description. Thanks again Mark. YOU THE MAN!
MM The reason for changing output transformer taps on the amp goes like this: The original 6V6 power tubes liked to see around an 8,000 ohm primary resistance (the primary is the part of the output transformer that connects to the power tubes and power supply circuit of the amp). This was how the amp was built when new. A pair of 6L6 power tubes in the amp now want to see around a 4,000 ohm primary resistance. Since I can't change the actual output transformer, I need to vary the load on the secondary of the output transformer (the taps where you connect your speaker). Moving the taps "down" a step accomplishes this electrically and brings the load in line.
MM Finally, our Proluxe all buttoned up and with its cover on, you can see the power light behind the nameplate. How cool is that! This amp took a little extra time to get the voltages down to make the circuit happy (I'm glad Mark took the time to 'get it right' since I'm not partial to Mushroom clouds pluming from my rig while I'm basting the chicas' booties with my version of Innagoddadavida Baby). As with many older electronics from the 50's and 60's, the wall voltages in the US have risen steadily over the years. Where I live in Vermont, the power coming out of the wall in my workshop is 127 volts. The power expected when these amps were made are 112 to 115. Doesn't sound like a lot, but I find its a magnifying effect when you measure the actual high voltage taps coming out of the power transformer and compare it back to the original schematic and the expected voltages back in the day. Newly built transformers can take this into account but using vintage gear of any kind, you have to be aware of the changes. Bogen amps are very available and I find they really run high voltages during conversions, so if you try one of these just be ready to deal with it.
This amp is sounding really nice- like a high powered tweed deluxe (Ok, kinda like a Proluxe right?! Right on Mark). Some future tweaks that could be tried? Well, how about a little switch to tie the two inputs together internally for higher gain. Or add a VVR (variable voltage resistor) style master volume. This would take a bit of moving things around to fit it into the chassis as it needs another pot/knob up front, but you can use them to control the high voltage issue of the amp instead of a zener and you get to really crank the preamp if you need to get overdrive but keep the volume down on the home front. Lastly, I would consider an output transformer upgrade to some bigger iron for this amp. It would get you some piano like bass. The current transformer is a little small and while it sounds fine, with more iron comes more bass. This is a simple change and easy to do.
I find these amps react well with pedals and other effects. You really can set the volume where you want it for great overdrive then use your guitars volume to clean it up or let it rip.
