Bogen CHB-10A

[bluzmn]

Last week I received in the mail a Bogen CHB 10-A that I bought on Ebay. Someone had installed a 6L6 in place of the 7868, and there was a Post-it note that said "6L6 SE-working". So, I plugged it into the Variac and started bringing up the voltage slowly; at 60 volts the obnoxious humming started. I brought it up to 120 volts, and plugged in my harp; it sounded kinda like a SF Champ. Not a bad sound, but I'd like more crunch; plus, it started feeding back with the volume at the 12 o'clock position. I switched a 5751 for the 12AX7; I was able to turn to maybe 1 o'clock. Tried a 5814; could turn up almost all the way, but still no crunch. I guess I'm going to work on this amp first, since I'm at least getting some sound from this one, but I have a few questions. First of all, the filter caps are in a 20-20-10-5mfd can; I'm thinking of replacing it with a 40-20-20-20mfd, but the feedback I was experiencing was not high-pitched, but lower like a foghorn. Will replacing with some higher value caps exacerbate that problem? Also, I was getting all the sound from the mic input; the phono input was extremely weak. Is that because that input is only connected to one half of the 12AX7, or is there something wrong with that part of the circuit? I'm asking this because if that input is useless, I'd want to remove the phono volume control and put a 1/4" jack there. I'm attaching a schematic of the pre and power amp; unfortunately, the full schematic I have exceeds the forum's size limit.bogen-chb-10a.pdf

[JHow]

What are your design goals and parameters for this project? Are you trying to retain the characteristics of the orginal amp - change as little as possible and have a working amp? Or, are you in more of a I-have-a 12ax7-and-6L6-and-all-bets-are-off- kind of mood?

To address your question about filter caps, I don't think changing those will have any effect one way or the other on feedback, if that is what you are hearing. Feedback and oscilllation will likely stem from other causes and have other cures.

Regarding the phono input, It does only use 1/2 the 12AX7, so it should be lower vol. The phono volume doesn't appear to do anything more than an "extra" volume control on your guitar. I think you can freely dispense with it if it doesn't fit your goals. Ditto for the "tone" control.

[MWJB]

"Someone had installed a 6L6 in place of the 7868, and there was a Post-it note that said "6L6 SE-working". - So the socket has been rewired? What is the value of the cathode resistor feeding pin 8? What are the plate & cathode voltages? R5 at 1M is too large for a 6L6, try 220K to 470K.

"So, I plugged it into the Variac and started bringing up the voltage slowly; at 60 volts the obnoxious humming started. I brought it up to 120 volts, and plugged in my harp; it sounded kinda like a SF Champ. Not a bad sound, but I'd like more crunch;" There is very little negative feedback, what speaker are you using (ohms), I'd start by increasing R11 to 1.5K, try A feedback loop ratio between 10:1 & 30:1. What happened to the horrendous hum at 120v...stil there, gone away? Fix the horrendous hum first.

I'd start with at least 50uf for the filter right after the rectifier...I'd leave the others at stock values for now.

Yes, phono input is weak because there is only one preamp triode, I'd keep the phono vol fully up & usethe other channel to limit signal bleed.

[MWJB]

"I'd start with at least 50uf for the filter right after the rectifier...I'd leave the others at stock values for now." The full B+ rail is not available on your schem, so this is assuming that there is no brute force filter upstream of the OT supply.

[bluzmn]

"Someone had installed a 6L6 in place of the 7868, and there was a Post-it note that said "6L6 SE-working". - So the socket has been rewired? What is the value of the cathode resistor feeding pin 8? What are the plate & cathode voltages? R5 at 1M is too large for a 6L6, try 220K to 470K.

"So, I plugged it into the Variac and started bringing up the voltage slowly; at 60 volts the obnoxious humming started. I brought it up to 120 volts, and plugged in my harp; it sounded kinda like a SF Champ. Not a bad sound, but I'd like more crunch;" There is very little negative feedback, what speaker are you using (ohms), I'd start by increasing R11 to 1.5K, try A feedback loop ratio between 10:1 & 30:1. What happened to the horrendous hum at 120v...stil there, gone away? Fix the horrendous hum first.

I'd start with at least 50uf for the filter right after the rectifier...I'd leave the others at stock values for now.

Yes, phono input is weak because there is only one preamp triode, I'd keep the phono vol fully up & usethe other channel to limit signal bleed.

To answer JHow's question, it's not stock to begin with (6L6 in place of 7868), but I'm just going to replace the filter caps now and see how I like it. If I'm not satisfied, I'll make some adjustments.
To MWJB- Yeah, the socket's been rewired; I checked the pin-outs to make sure they were correct before I fired it up. There are two resistors in parallel on pin 8; the color codes aren't making any sense because I think they may be discolored with age and some of the paint is worn out. I'll have to take them out of the circuit to get an accurate reading on an ohmmeter, right? I'll have to get back to you on that. I notice on the schemo for the 5f2h (which your buddy Bruce designed) the value is 270 ohms. What do you think it should be? I was using an 8 ohm speaker wired to the 8 ohm taps on the amp. R5 is already 100K; do you mean decrease it to 1.5K or increase it to 1.5M? Actually, I was thinking of disconnecting the NFB completely; that would give me more crunch, wouldn't it? And you have to understand that I'm new to this; I don't understand what those ratios mean. Nor do I understand what "signal bleed" means. The hum was still there at 120v; that's why I've been talking about changing the filter caps. Why would you not want to beef up the filters on the preamp tube and the screen grid of the 6L6? I've found another complete schemo; hopefully it's not too big. Thanks for all your help.

[MWJB]

Try 330ohms for the 6L6 cathode resistor (or whatever gives around 65mA plate current).

R11 the 2nd stage cathode resistor was the one I was talking about increasing to 1.5K, also reducing the value of the 100K NFB loop resistor...I'd also remove the 150K resistor from the B+ rail that meets the hot side of R11 & the NFB resistor. Personally I wouldn't remove the NFB loop altogether, you won't increase crunch...try a larger 6L6 cathode resistor if you want a crunchier sound, anything down to 45mA on the 6L6 plate might work fine.

You have a Pi filter prior to the B+ supply (390ohm resistor) , if you remove this then you can try enlarging the first filter after the rectifier. 6L6 plate voltage will rise if you do this, so rebias accordingly (maybe 470ohms cathode resistor minimum, up to 680ohms?)

As you turn the phono volume down, you reduce resistance to ground from the preamp signal, either keep the phono volume up, or remove it.

[bluzmn]

It appears that the 2 resistors in series on the cathode of the 6L6 are 130 ohms and 300 ohms. Does that make 215 ohms or is there a more complex equation I have to use?

[MWJB]

If they are in series (end to end) then that is 430ohms, go with that for now.

If they were in parallel (side by side) then they would be 90ohms (130*300/430).

[bluzmn]

I know why we had that miscommunication about resistor values; you where looking at that partial schemo and I've been looking at the complete one the whole time. The resistors are numbered differently; now I understand what you've been saying. What does a pi filter do? Maybe I won't want to disassemble it.

[MWJB]

It's a power supply hum filter, instead of using a large value cap after the rectifier you can use 2 smaller value caps either side of a resistor or choke. The PI filter drops the B+ voltage supplied to the OT a little...as your amp might only be running in the early 300's voltage wise, you can afford to lose the 390ohm resistor & use 1 larger value main filter (50-100uf) along with a 470ohm+ cathode resistor (to restore a reasonable bias current).

I'd run it as it is at the moment, if you decide you want more cleans bear the other format in mind.

[Marc]

looking at the first two caps in the Bogen cut sheet, I think the power supply is a voltage doubler. the 20 mfd caps and the diodes are stacked wrt ground. This configuration is pretty common in Bogens.
I'd avoid going beyond 40 mfd if you replace them as the B+ will rise...
Marc

[J M Fahey]

Yes, it is a voltage doubler.
Stick to the original schematic as much as possible, that's why you bought a 50 years old amplifier instead of a new one.
To check where that hum comes from, short R12 (I'm always referring to the original manual scan).
If it keeps about the same, it's coming from a poorly filtered supply (dry caps?); in that case replace them but respecting original values; then it'll hum as much (or as little) as when it was a newborn baby. Do that anyway, simply because of the age.
If it all but disappears, pull that shorting wire and short R4 ends instead.
If you have hum only on the mic input, try to better your cables, but I guess you'll have to live with some of it, just as old Bluesmen did !!!!
As far as the feedback is concerned, if it's acoustic (it depends on your diustance to the speakers), well, it's not the amp's fault.
Good luck.

[wainemaine]

On my Bogen 10 there was a green wire from a resistor on the speaker terminal 16 ohm tap that was the feedback for the amp, cut that wire and your "crunch will come back, volume will increase a lot too, you will have to turn it down.