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Old 25th July 2006, 10:10 PM   #1
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Default Bogen CHB-10A Conversion, need help and advice

Well, I finally decided that my M60A was too powerful to be my only guitar amp so I picked up a 1971 CHB-10A off eBay. 10 watts of single-ended and cathode-biased fun. Loudest 10 watts I've ever heard too. Without any changes to the circuit it sounds pretty good, it's a bit bass heavy playing clean but it screams when cranked. Right now it has a Mullard 12AX7 and a Sylvania 7868.

First off, I seem to be having a bit of trouble with the grid (screen grid I believe, I can see it through the holes) overheating when the amp is pushed really hard. It's a cool effect to watch for a while as the tube glows brighter or dimmer along with your playing, but I can't imagine this being good for the tube. I've heard this is common with US made 7868s, but I don't want to buy new production. From what I've seen, EH makes decent ones now but I'd like to stick with NOS. Any way around this?

Stock schematic, preamp and power amp sections:
http://home.rochester.rr.com/groundh...nd%20Power.gif

Power supply:
http://home.rochester.rr.com/groundh...r%20Supply.gif

Also I've drawn up a quick schematic of how I intend to change the amp to make it more suitable for guitar. I relabeled all the components on this schematic so it may be a bit confusing, sorry. I searched all over the internet for what other people have done with these and tried to take the best out of all of them for a unique amp. The first preamp stage is copied from a Fender 5D2 Princeton, I like a grid-leak bias first preamp stage and I wanted to copy a good one. I made the RCA jack connected in parallel with the 1/4" input just incase I needed to plug in a CD player or something. The phono input circuit is getting removed and I'm using whe hole where the pot was to mount the input jack. I kept the tone control the same for now, I may have to change the cap value but I like its simplicity and the fact it's pretty much bypassed with the knob all whe way up. I copied the second preamp stage including the NFB/bypass cap switch from another CHB conversion schematic I found. Should I keep the NFB wire hooked to the 16 ohm tap if I do this, or should I move it to the 8 ohm tap? I kept the output stage fairly the same although I changed C9 (on my schematic) to a .022 and the cathode bypass cap to a 33/160. I'll probably use a lower voltage rating in the end but that's what someone on another forum said he used with good results. Some of the cap values may change a bit depending on what's available at a good price.

So what do you guys think? Will these mods make this amp more suitable for guitar? Thanks in advance.

-Darren
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Old 25th July 2006, 11:41 PM   #2
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The grid-leak bias on the first stage can't take much signal - biasing it like the second stage would be better if you will have a pedal with gain ahead of it. Bogen did it that way to save two parts.

Lowering the 7868 grid resitor by half would be good. Will lose a little low end you don't need with a guitar. If you want to add a little treble, wire a "bright" switch across the volume control, in series with maybe 220-470 pF.

You could try an additional 1K/2W resistor in series with the screen... may help. Will cause some loss ofr power and second harmonic distortion, which might sound GOOD... this is part of a musical instrument, not a hi-fi amp! Which is why the feedback must go...
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Old 26th July 2006, 01:47 AM   #3
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I'm more of a stright into the amp guy, which is why I needed a smaller amp because I usually rely on the amp's natural clipping for my tone. I have an overdrive pedal I use when I need a specific sound, but that's it. I already know I like the sound of a grid-leak bias preamp stage, it's just a matter of finding a particular one that sounds good.

So you're talking about lowering R9 to 500k? I can try that. Thanks.
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Old 29th July 2006, 04:06 PM   #4
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Alright, this should be the last revision of the schematic. I'll probably order the parts tomorrow sometime.
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Old 29th July 2006, 05:00 PM   #5
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A quick observation without further design review:

You will hear a loud THWAACK when you switch S2 from NFB to the 22uF cap, if that cap has bled down to zero volts or if it was never charged since turn-on. When the cap is first switched in, the 12AX7 will momentarily have a grounded cathode, causing the plate voltage to sharply step down, until the cap charges. And then youíll hear a smaller thwack when you switch back to NFB, since the NFB resistor will take some DC current through the secondary. Better to connect a large resistor (~100K) between the top of the cap and the left end of the 27K NFB resistor. Remove the switch as wired and permanently connect the junction of the 100K and 27K to the cathode. Then place the switch (now a SPST) across the 100K resistor. Switch it to short out the 100K to get rid of the NFB, or open it to let the NFB work. As long as the cap is not leaky and itís had time to charge to cathode potential through the 100K, youíll have no pops. The 100K will be in parallel with the 1.5K cathode resistor at all audio frequencies, but it only drops the parallel value to 1.48K, which is not criminal.
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Old 29th July 2006, 10:10 PM   #6
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You mean something like this? Thanks for the help.
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Old 29th July 2006, 11:42 PM   #7
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Yep, that should do it. The 100K value is not critical; I suppose you could go lower in value if you wanted, 47K perhaps.
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Old 2nd August 2006, 11:39 PM   #8
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I suppose I should have checked voltages before I started complaining about my tube overheating .

The screen grid is running at about 305VDC while there is only 301VDC getting to the plate. The screen grid should never have a higher voltage than the plate, correct?

This is odd because there is 313VDC going into the output transformer, but I guess it loses just enough to create a problem. An extra 1k resistor on the screen grid should hopefully take care of this. I'm just waiting for my parts to get here and then I'll start tearing into it .
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Old 3rd August 2006, 12:31 AM   #9
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Darren,

The Octal based 7591A, the Novar based 7868, and the Noval based 6GM5 are electrically equivalent. AFAIK, older production of the 3 types was strictly a North American affair. NOS of all 3 types is SCARCE and COSTLY. 6GM5s seem to be the most available and lowest priced of the 3, but replacing a Novar socket with a Noval (9 pin miniature) socket could be problematic.

After the Sylvania 7868 dies, I think changing to an Octal socket and an EH 7591A is the way to go. The only potential "show stopper" I can envisage is the EH 7591A's bottle being too big.
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Old 3rd August 2006, 12:42 AM   #10
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EH now makes the 7868 as well so I think there won't be any modification needed. If I swapped in an octal for the 7591A it would let me use the JJ version too and give me more options. I'll just deal with it when I get there I guess. My other amp uses 8417s, so that's the amp I'm really concerned about .
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