Bogen CH8 to 3-knob champ with 6L6 conversion filter capacitor questions

[Randall]

This amp ended up coming to me for help. It was running waaayyy hot and burning up 6L6s with a 470 ohm 5 watt cathode resistor. It would go over 100mA. I had to double the resistor to get it down to 17 watts. Someone check my methods, please? Plate voltage = 440. cathode voltage = 39.8v. Cathode resistance = 931R. So, 39.8v/931R = 42.7 mA. (440v - 39.8v) x .0427A = 17.1 watts, correct?

If this is correct, does 931 ohms seem like a high value for a cathode resistor?

[bmccowan]

No offense taken. I’m obviously not an expert on all of the Champs and Princetons. For some reason I thought there were single ended production Princeton Reverbs. Guess I was wrong, probably confused it with some custom builds as you said. That said, there’s always more than one way to Egypt. With a high B+ I would be tempted to use a 6L6 and a couple of big zeners. But as you say... The OT might be an issue.

"More than one way to Egypt." I've never heard that before - that's funny. I've never tried the zener route, but know of it. The CH8 uses a 6L6. I have dropped the voltage more than I need to to protect the tube, but I like that "brown" sound you get from lower voltage. And hey, if you are interested in checking out all the various Fender amps, check out the Fender Amp Field Guide; The Fender Amp Field Guide There are a few mistakes but its pretty interesting and comprehensive.

[olddawg]

And my point was simply to correct some of the Princeton info you provided, so that Chemtooler would not spend effort looking for what does not exist; that's all. I get stuff wrong sometimes and others correct it - no offense taken. I have that Bogen CH8 - the voltage is higher than desired for the circuit and Bogen scrubbed off voltage with a couple of big wattage resistors to ground. In fact according to their schematics, they did that for several of their smaller amps. Why? I can only guess that with the huge number of models Bogen produced, they tried to standardize as much as possible on power transformers. That left some models with transformers designed for larger amps. Following a fender power supply schematic, would not fix that, unless you also replaced the PT itself. In many cases, you are right; follow a well know power supply/filtering architecture and you will be ok - but not for this one.

No offense taken. I’m obviously not an expert on all of the Champs and Princetons. For some reason I thought there were single ended production Princeton Reverbs. Guess I was wrong, probably confused it with some custom builds as you said. That said, there’s always more than one way to Egypt. With a high B+ I would be tempted to use a 6L6 and a couple of big zeners. But as you say... The OT might be an issue.

[bmccowan]

I thought the issue was a "weird" power supply problem? My point simply was to compare some similar schematics and modify the power supply more conventionally if the old PA setup is causing the issue.

And my point was simply to correct some of the Princeton info you provided, so that Chemtooler would not spend effort looking for what does not exist; that's all. I get stuff wrong sometimes and others correct it - no offense taken. I have that Bogen CH8 - the voltage is higher than desired for the circuit and Bogen scrubbed off voltage with a couple of big wattage resistors to ground. In fact according to their schematics, they did that for several of their smaller amps. Why? I can only guess that with the huge number of models Bogen produced, they tried to standardize as much as possible on power transformers. That left some models with transformers designed for larger amps. Following a fender power supply schematic, would not fix that, unless you also replaced the PT itself. In many cases, you are right; follow a well know power supply/filtering architecture and you will be ok - but not for this one.

[olddawg]

Yes - there are single ended Princetons; but there were never single ended 6L6 Princetons as all single ended Princetons were 5 watt 6V6 amps.
Tweed Princetons - single ended - same as champ circuit, but with an added tone control. Same wattage (5 watts) as Champ, 1-6V6, but depending on vintage, a larger speaker.
After the tweed era - all Princetons were push pull 2-6V6.
Brownface Princetons - push pull, 2-6V6.
Blackface & Silverface Princetons - push pull, 2-6V6.
All Princeton Reverbs - push pull, 2-6V6.
Scratch built, kits, or converted PA amps that people sometimes call Princetons - Any of the above and more, including single ended 6L6 amps. Some people even change the power tube cathode resistor and use a 6L6 in SE Champs and Princetons, but the power transformer is working harder than it was designed. Dave Hunter's two-stroke started out as a Champ type circuit with parallel 6V6 and then moved to a single 6L6. Weber's Maggie is the same. Yes people like them, I have built them and like them.
The single ended Champ type circuit was used by just about everybody back in the day: Fender, Gibson, Valco, Webster, etc. It was not an original Fender circuit, they are all derived from a Western Electric circuit.
Princetons are Fenders, and they are all good amps as far as I am concerned. I have rebuilt several and scratch built a few.
Chemtooler referred to what DLabs had done with their Bogen CH8- gutted it and rewired it with a Champ type circuit (same as a Princeton type circuit.) Terry hooked up the preamp sockets for the use of either a 9 pin 12A_7, or an octal 6SL7.
What I did with my CH8 was to use a Matchless Clubman type preamp 6AT6 (7 pin triad) into a 6SJ7 (Octal Pentode) with the 6L6 power tube. Different - not better, not worse, just different.

I thought the issue was a "weird" power supply problem? My point simply was to compare some similar schematics and modify the power supply more conventionally if the old PA setup is causing the issue.

[bmccowan]

There are are versions of the Princeton that use a single ended architecture and have a higher output than a Champ. You may be correct that all of the Princeton Reverbs were push-pull. You might want to peruse the schematics online. There was a long thread on this forum a while back about 6L6 single ended Princetons also. People seem to like them.

Yes - there are single ended Princetons; but there were never single ended 6L6 Princetons as all single ended Princetons were 5 watt 6V6 amps.
Tweed Princetons - single ended - same as champ circuit, but with an added tone control. Same wattage (5 watts) as Champ, 1-6V6, but depending on vintage, a larger speaker.
After the tweed era - all Princetons were push pull 2-6V6.
Brownface Princetons - push pull, 2-6V6.
Blackface & Silverface Princetons - push pull, 2-6V6.
All Princeton Reverbs - push pull, 2-6V6.
Scratch built, kits, or converted PA amps that people sometimes call Princetons - Any of the above and more, including single ended 6L6 amps. Some people even change the power tube cathode resistor and use a 6L6 in SE Champs and Princetons, but the power transformer is working harder than it was designed. Dave Hunter's two-stroke started out as a Champ type circuit with parallel 6V6 and then moved to a single 6L6. Weber's Maggie is the same. Yes people like them, I have built them and like them.
The single ended Champ type circuit was used by just about everybody back in the day: Fender, Gibson, Valco, Webster, etc. It was not an original Fender circuit, they are all derived from a Western Electric circuit.
Princetons are Fenders, and they are all good amps as far as I am concerned. I have rebuilt several and scratch built a few.
Chemtooler referred to what DLabs had done with their Bogen CH8- gutted it and rewired it with a Champ type circuit (same as a Princeton type circuit.) Terry hooked up the preamp sockets for the use of either a 9 pin 12A_7, or an octal 6SL7.
What I did with my CH8 was to use a Matchless Clubman type preamp 6AT6 (7 pin triad) into a 6SJ7 (Octal Pentode) with the 6L6 power tube. Different - not better, not worse, just different.

[olddawg]

Hi - I should have been more specific. Even with a 5Y3, B+ voltage is very high (500+) without dumping some to ground. Your amp is a somewhat different version of the CH-8, so the power transformer spec may be different. Based on real Bogen schematics I've seen for some of the other PAs, I suspect the CH-8 has two 47K large wattage resisters in parallel to ground on the power supply. That would be 23.5K of resistance. This also allows the voltage to bleed off after the amp is turned off. But if that was the only purpose the resistance would be much higher; 220K is common. I used 20K and that brought the plate voltage down to 445.
I'd be happy to show you what I did, but you may be looking for something different, as there are a lot of options. Although Old Dawg mentioned a Princeton Reverb, that's push-pull and the CH8 is single ended. I was looking to stay single ended, as I have built many push-pull 6V6/6L6 amps. And I wanted to experiment with a pentode (6SJ7) in the second preamp position. Very few commercial amps did so, the Matchless Clubman being one of them. the early clubman had a 6AT6 > 6SH7 or 6SJ7 in the preamp, later they changed to 12AX7 > EF86.
I know that what D Labs did will work fine, as I have built that same configuration in the past - a Champ circuit that can use either a 12AX7 or a 6SL7, and its an easier build as there are accurate schematics out there. Angela instruments has/had a post for that option on the Super Single Ended amp schematic/instructions. I built a Valco circuit 6SL7 > 6V6 champ type amp a few years back, and its a favorite.

There are are versions of the Princeton that use a single ended architecture and have a higher output than a Champ. You may be correct that all of the Princeton Reverbs were push-pull. You might want to peruse the schematics online. There was a long thread on this forum a while back about 6L6 single ended Princetons also. People seem to like them.

[bmccowan]

No, I have not got it up and going yet. I had a bad output transformer and then ran into an issue with the burning out the 6L6 tube. I put it on the shelf, but I am looking to get back into it in a few weeks. I am interested in your comment about the power transformer being too strong. I need to look at that to see if that might have been my issue.

Hi - I should have been more specific. Even with a 5Y3, B+ voltage is very high (500+) without dumping some to ground. Your amp is a somewhat different version of the CH-8, so the power transformer spec may be different. Based on real Bogen schematics I've seen for some of the other PAs, I suspect the CH-8 has two 47K large wattage resisters in parallel to ground on the power supply. That would be 23.5K of resistance. This also allows the voltage to bleed off after the amp is turned off. But if that was the only purpose the resistance would be much higher; 220K is common. I used 20K and that brought the plate voltage down to 445.
I'd be happy to show you what I did, but you may be looking for something different, as there are a lot of options. Although Old Dawg mentioned a Princeton Reverb, that's push-pull and the CH8 is single ended. I was looking to stay single ended, as I have built many push-pull 6V6/6L6 amps. And I wanted to experiment with a pentode (6SJ7) in the second preamp position. Very few commercial amps did so, the Matchless Clubman being one of them. the early clubman had a 6AT6 > 6SH7 or 6SJ7 in the preamp, later they changed to 12AX7 > EF86.
I know that what D Labs did will work fine, as I have built that same configuration in the past - a Champ circuit that can use either a 12AX7 or a 6SL7, and its an easier build as there are accurate schematics out there. Angela instruments has/had a post for that option on the Super Single Ended amp schematic/instructions. I built a Valco circuit 6SL7 > 6V6 champ type amp a few years back, and its a favorite.

[loudthud]

Some of these old amps were built around specs for the metal 6L6{nothing}.

Design center values were:

Plate Voltage 360V
Screen Voltage 270V
Plate Dissipation 19W

The metal shell is connected to pin 1 so designs like this usually have pin 1 on the socket grounded. Some old Fender amps are wired like this. You see it on the layouts.

[olddawg]

No, I have not got it up and going yet. I had a bad output transformer and then ran into an issue with the burning out the 6L6 tube. I put it on the shelf, but I am looking to get back into it in a few weeks. I am interested in your comment about the power transformer being too strong. I need to look at that to see if that might have been my issue.

What exactly is meant by “too strong”? There is just voltage and current. It shouldn’t matter unless your B+ is too high or it can’t deliver enough current. If the PT is more “robust” it usually just means less sag. I put a PT meant for a 30(ish) watt amp in a 20(ish) watt amp the other day. It may be a little stiffer but it sounds fine and is reliable as hell.

[Chemtooler]

No, I have not got it up and going yet. I had a bad output transformer and then ran into an issue with the burning out the 6L6 tube. I put it on the shelf, but I am looking to get back into it in a few weeks. I am interested in your comment about the power transformer being too strong. I need to look at that to see if that might have been my issue.

[olddawg]

I am wondering if this amp got finished? I have one I converted - It is a 6AV6 > 6SJ7 > 6L6 with a 5y3 rectifier.
I did not want to go the D Labs route - as I have built many champs. I wanted to keep the tube compliment and have the 6AV6 as the first gain stage, feeding the 6SJ7. I learned that the early Matchless Clubman had a preamp like that. But I could not find an accurate schematic. It took me a lot of experimenting, but I got there. There is weirdness in the Bogen power supply. The power transformer is too strong, so they bled some power to ground through large resistors. And in some Bogen amps like the db15 they ran the 6L6 plates 100 volts higher than the screen grids - WTF? I think that CH8 revA schematic has mistakes.
So - did you get that amp done?

BMac

As was stated before.. look at some Fender Princeton schematics. I doubt the conversion is too complex to copy their power supply architecture for the output stage. I would probably gut it and just build a Princeton if the PT specs are close...

[Jazz P Bass]

"And in some Bogen amps like the db15 they ran the 6L6 plates 100 volts higher than the screen grids "

The reasoning behind the lower screen voltage is to limit the overall amplifier output.

After all, it is only a PA amp.

[bmccowan]

Have a CH8 that I converted

I am wondering if this amp got finished? I have one I converted - It is a 6AV6 > 6SJ7 > 6L6 with a 5y3 rectifier.
I did not want to go the D Labs route - as I have built many champs. I wanted to keep the tube compliment and have the 6AV6 as the first gain stage, feeding the 6SJ7. I learned that the early Matchless Clubman had a preamp like that. But I could not find an accurate schematic. It took me a lot of experimenting, but I got there. There is weirdness in the Bogen power supply. The power transformer is too strong, so they bled some power to ground through large resistors. And in some Bogen amps like the db15 they ran the 6L6 plates 100 volts higher than the screen grids - WTF? I think that CH8 revA schematic has mistakes.
So - did you get that amp done?

BMac

[olddawg]

Since this amp uses a 6L6 single ended... you might want to look at a few Princeton schematics. If the pilot lamp is run off the filament voltage, if you run a neon or LED lamp off of the power switch, you might even free up enough heater current for a Princeton Reverb build. If you use an eyelet or turret board it is easy to mount all of the power supply filter caps on one side. You can also move the OT to the other side if the chassis to give you more room.