Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AmberB on June 25, 2021, 02:10:11 am
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Hello! It's been a while since I've had much to write about here.
I recently came across a few Bogan amplifiers, a couple of CHB-20s and a couple more CHB-35s. The CHB-20 caught my attention, so it got me enthusiastic about working on another amp. One really interesting thing about this particular amp is that it's a rack mount amplifier. This is the first Bogan amp I've seen that is a rack mount.
Anyway, the first thing that I did was to remove the preamp socket with most of the parts still attached and put in a new 9 pin socket. Removing parts from a Bogan chassis is a pain in the butt...
I wired the new socket for a 12AX7. Deciding what kind of preamp to build was interesting, my research took me through an old thread here on the forum. After looking at the possibilities, I went with the Ampeg Gemini preamp, which included changing pretty mush all the components in the tone control section to the Ampeg values. I'm at the point right now where I just need to put in a speaker output jack and I'll be able to test it. I could hardwire a speaker cable for a first test, I suppose, to see if the thing is functional.
I'll post some pictures of the build once I get them off my camera.
One thing that I was thinking about with this amp build is that there's plenty of room in the chassis to build a reverb section for this amp. I'm not sure if I'd rather have a reverb transformer, or transformerless system. The Ampeg reverb circuit is interesting... I don't know how much extra heater current I have available for another tube.
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Here's a picture of the face of the amp. There's no top cover, just the faceplate and frame.
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This is the rebuilt tone section. I ended up taking the big blue capacitor out after this picture because I didn't want the plate voltage on the wire all the way to that cap from the preamp tube. The blue cap is the cap from the plate to the next stage, in this case, the tone circuit. I'm not sure why Bogan designed it that way...
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This is the chassis after finishing the assembly of the preamp circuit. The Ampeg schematic calls for a .1uf bypass cap for the cathode on the second stage. I didn't have anything like that except for my 600 volt .1 tone caps, so I used a 1uf radial 'lytic cap that I happened to have in my parts stash. I might decide to replace it with an axial cap, but that's probably not necessary.
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I pulled all the speaker connections out of the back of the chassis and put in a tag strip for the speaker transformer wires. Now I just have to put a plate over the holes and mount an impedance switch and a speaker output jack. I just use a spdt switch for the impedance switch, I have it set up to switch between the 4 ohm lead and the 8 ohm lead. There doesn't seem to be much use for the 16 ohm connection these days. I take the feedback loop off the 16 ohm lead.
I also installed a power cord jack in the hole where the aux power plug was. You have to file the hole a bit wider, but it works pretty well in that position.
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I'm guessing that there's enough extra transformer power to put in an effects loop, just a 12AU7 tube and associated parts, but I don't know if I could put in a reverb circuit. I suppose if I use a Fender type transformer reverb, that wouldn't be too much extra draw.
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https://el34world.com/Forum/index.php?action=search2
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Well, the original Bogan 6GW8s both test good, so that's a good start. The printing on the tubes says Gt Britan, so they are probably pretty decent tubes.
I got the impedance switch and a speaker jack mounted on the back of the chassis, and I brought the power supply up to voltage ofer the course of several hours with a variac without any problems showing up, so I think it's ready to be tested.
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I have a voltage discrepancy on one of the 6GW8 tubes, V2 on the schematic. According to the schematic, I'm supposed to get 171 volts on the plate of the triode, pin 9, I'm getting about 90 volts or so. The other 6GW8, V3, is reading more or less normal voltage on pin 9. I swapped the tubes to see if the problem would follow the tube, but it did not. So now I have to check the components associated with that tube to see if I have a defective part. All the caps in that area are original ceramics and the resistors all seem to be original also.
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220K plate resistor is most likely suspect.
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The problem turned out to be the way Bogan wired the amp to feed the 70 volt output line. They run the B+ for the plates of the preamp tubes through an 82K resistor and connect it to the cathode of the triode of V2 where the feedback loop from the output transformer is also connected. That feedback loop also serves as the 70 volt output through a separate winding in the output transformer. The cathode of the triode goes to ground through a 470 ohm resistor. It's an odd thing that took me a while to figure out.
When I rewired the amp, I disconnected that B+ connection and removed the 82K resistor, R14 on the schematic. I never thought to replace the 470 ohm cathode resistor, R15, so that triode was biased way different from the triode in V3.
It took me a lot of staring at the schematic to realize that the bias on V2's triode was way different from V3. I pulled the 470 ohm resistor and replaced it with a 2.2K resistor, the same as on the cathode of V3, R27. Now both triodes have roughly the same voltage, within about 20 volts.
It seems to work pretty well now, except that it motorboats when I turn the master volume all the way down. The Scott Amps Bogan CHB-20 conversion schematic that I'm using for some of my refrence, (as well as an original Bogan schematic for comparison) shows the Ampeg Gemini 2 preamp, including the 270K isolation resistor between the output of the treble pot and the master volume control. I left that out because I didn't think I would need it for a single channel amp. I guess I'm going to put that resistor in and see if it solves the motorboating problem. I'm guessing that with the MV all the way down, it creates an oscillator in the driver circuit...I could be wrong...
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I think your analysis is incorrect, and rather: B+ flows "up" through R14 (the 82K R), back up and around through the feedback loop; then through the OT secondary to ground. Along the way it provides 1.7 VDC to the cathode of V2A. It does not supply any plate voltage.
V2A & 3A are the PI and part of the power amp (not the preamp). Their plates are supplied with 260VDC (dropped to 150 @ their plates) from the junction (dot) immediately below R14.
The preamp tube plates are supplied with 260VDC (dropped to 145 & 65 @ the plates) from the junction (dot) immediately below that.
The 70V line from the OT secondary is a feedback loop which carries only (inverted) AC signal; no DC (except for the unusual cathode bias voltage mentioned above).
Bogen's PI & power tube bias schemes are idiosyncratic and complex, as you discovered: with different plate R values for the PI plates; fixed bias voltage added to the cathode bias of V2A; and fixed bias voltage added to the minimal cathode bias of the power tube sections. (It's also possible that the weird feedback arrangement is necessary to keep the amp stable and free of issues such as motorboating.)
IN SHORT: Leave the power amp section - from PI to Power Tube sections - stock; or a complete re-design is needed, say, like Fender or Marshall circuits.
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I agree with your analysis of the amp, I didn't explain it very well, I guess. The B+ line supplies the preamp tube plates as well as going through the 82K resistor.
I was looking at the schematics for some of the old Ampeg amps that have a similar PI setup using a separate tube for the PI. They seem quite similar in most respects, other than Bogan's 70 volt line circuit.
I think this is the cathodyne PI setup, but I could be wrong...
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The PI is a paraphase type, not cathodyne. A cathodyne will always have one output on the plate and the other on the cathode.
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The PI is a paraphase type, not cathodyne. A cathodyne will always have one output on the plate and the other on the cathode.
Yeah, I figured I had that one wrong. Thanks!
I put a 270K resistor from the MV pot to the input of the PI circuit, pin 1 on V2-a instead of putting it between the treble pot and the MV pot. I figured it wouldn't do anything to help the motorboating if it was between the treble pot and the MV, because the motorboating was happening when the MV pot was all the way down, basically grounding the grid of V2-a. It seemed logical that putting the 270K resistor at the top of the MV pot wouldn't change anything. Testing the amp showed that the resistor solved the motorboating problem. I had to deal with some buzz by moving a couple of ground points in the circuit, putting all of the preamp and tone control grounds in the same place. The factory layout has a terminal strip next to the preamp tube with the attachment lug soldered to the chassis, so I used that for everything. That seems to have solved about 95% of the noise. I had my Squire strat plugged into the amp and couldn't figure out why I had so much hum until I switched to the mini humbucker that I put in the guitar a couple of years ago. The hum went away...DUH!
There's still a little hum at full volume, but that's not unexpected. I think this amp rebuild is more or less done.
I made a couple of changes to the way the amp is wired in this schematic. I'll try to get an updated schematic posted in a couple of days, I have to change it by hand and then scan it.
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I still haven't gotten the modified schematic scanned into my computer...
I did it by hand, so it's a bit of a mess. I don't have a program for drawing schematics, so if I were to redraw the entire schematic, I would have to do it by hand.
I do have a note about the Scott Amps schematic for the CHB-20A, there's a mistake on the pin numbering on V1a, the plate and cathode are numbered backwards.
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Lots of Forum members download the free app: Express Schematics. Pretty small learning curve.
You can download schematics and more component symbols on this Forum. Then you can block, copy & save circuit segments, like: gain stage, tonestack, PI, etc., to paste into your schematics.
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And look at this 1 too;
https://el34world.com/Forum/index.php?board=26.0