Bogen CHB-50 Rebuild

[physics]

Hi all!

As mentioned in the CHB-20 rebuild thread, my friend decided that 50W was a better fit for his needs so I am working on making a Bogen CHB-50 usable as a clean pedal platform. Schematic for the curious:
   

My primary mission right now is rebuilding the power supply with some new electrolytics and also a film cap to replace the paper-in-oil one from the bias supply. However, there are a few other caps (coupling caps) which I'm unsure on whether to replace or not. I think they are film types since they don't look like paper-in-oil caps that I have seen before, but I was hoping some of you would recognize them. I've heard chatter on other forums that old coupling caps between stages should be replaced too even if film types, but given the same forums also worship at the altar of all sorts of audiophoolery that doesn't seem like a reliable source of advice. Are there any guidelines for determining whether coupling caps are bad, assuming one doesn't have a fancy high voltage capacitor checker?

Pictures of the caps in question:

   
   
   

Thanks for any input!

[Sherlok Ohms]

Hello Physics

That is an interesting circuit which should be very pedal friendly as is. I would suggest that you simply get it working first and have your friend assess the tone. It should be very "high-fidelity" as it was intended to be a clean public address amplifier, and not anything approaching the kind of tone altering beast modern guitar players use.

The capacitive-coupled bias supply alarms me. I am in agreement with KOC that such bias supplies should be replaced with proper transformer-generated supplies, and a true independent bias-set network applied with cathode current-sense resistors for the output valves. At the moment there is a mix of fixed-biasing with cathode biasing. This is not bad unto itself, and if I recall, Mr. O'Connor did that in previous versions of his Studio power amplifier.

Regarding the capacitors: I think it is safest to replace the ones that say "PMK" on them as the "P" may mean paper? I do not know this as fact, but the age of the unit suggests it could be true. All paper dielectric capacitors have a limited life regardless of what oil is used or how well sealed the package may seem. In fact, both the oil and the paper are organic and cannot have the durability and stability that plastic has.

This is obviously a hand-built amplifier with large solder connections that themselves are quite durable over time. However, as experts have warned us, solder is eroded by electron flow, so you may want to check certain connections that seem dubious.

Also, for completeness, you may want to replace 20% tolerance resistors with 5% or 10% as the former are cracked carbon and the latter would be carbon film. All carbon resistors exhibit distortion unique to carbon, but the earlier construction forms were quite poor and lacked stability, and lacked even the accuracy of manufacture one expects today. These deviations introduce nuisance noises, snaps, and pops, that would simply be annoying especially after investing time and effort to restore an interesting amplifier.

Cheerio

[physics]

Hello Physics

That is an interesting circuit which should be very pedal friendly as is. I would suggest that you simply get it working first and have your friend assess the tone. It should be very "high-fidelity" as it was intended to be a clean public address amplifier, and not anything approaching the kind of tone altering beast modern guitar players use.

The capacitive-coupled bias supply alarms me. I am in agreement with KOC that such bias supplies should be replaced with proper transformer-generated supplies, and a true independent bias-set network applied with cathode current-sense resistors for the output valves. At the moment there is a mix of fixed-biasing with cathode biasing. This is not bad unto itself, and if I recall, Mr. O'Connor did that in previous versions of his Studio power amplifier.

Regarding the capacitors: I think it is safest to replace the ones that say "PMK" on them as the "P" may mean paper? I do not know this as fact, but the age of the unit suggests it could be true. All paper dielectric capacitors have a limited life regardless of what oil is used or how well sealed the package may seem. In fact, both the oil and the paper are organic and cannot have the durability and stability that plastic has.

This is obviously a hand-built amplifier with large solder connections that themselves are quite durable over time. However, as experts have warned us, solder is eroded by electron flow, so you may want to check certain connections that seem dubious.

Also, for completeness, you may want to replace 20% tolerance resistors with 5% or 10% as the former are cracked carbon and the latter would be carbon film. All carbon resistors exhibit distortion unique to carbon, but the earlier construction forms were quite poor and lacked stability, and lacked even the accuracy of manufacture one expects today. These deviations introduce nuisance noises, snaps, and pops, that would simply be annoying especially after investing time and effort to restore an interesting amplifier.

Cheerio

Thanks for the feedback! Would you suggest going into one of the mic inputs, or the auxiliary input? I had figured that the mic inputs would not be ideal for guitar given the grounded cathode (mu amp?) configuration and was figuring on either going into the aux inputs if I wanted to be quick or adding some cathode resistors to the mic input stages if I wanted to go through there.

Regarding the bias supply, my tentative plan is to add per-tube adjustment and current sensing, however I have not given much thought to supply impedance beyond planning to bypass the wipers of the high-value pots (250k each) to ground with capacitors to keep a low-impedance grid leak circuit at signal frequencies as mentioned in TUT2. Now that you mention it, I'll have to look more closely how well the supply will actually do at supplying current. I'm trying to avoid non-essential costs, but then a 115V to 6.3V chassis mount hammond transformer is only around $10 plus a few extra bucks for lower value pots, and I think I have the rest of the parts lying around. RBX + BMK is probably out of the budget for this (around 60USD) otherwise I'd likely just go with that instead of sourcing parts myself.

Yeah, definitely hand built. Some stuff is nicely done as well, such as local decoupling for the mic gain stages and (some) wired grounds. Other stuff less so, like multiple chassis ground points.

The age of the unit is definitely part of why I'm worried they may be paper, however I'm trying to find something definitive along those lines to avoid adding a possibly unnecessary cost. If I can't find anything though, then yeah definitely agree it's safer (and not that costly) to just replace them with some plastic caps.

I'll keep the resistors in mind for sure, thanks for mentioning that

Thanks!

[makinrose]

Nice amp! The I'd suggest keeping one of the channel as grid-leaked baised because it is interesting sound but the main channel should be converted to cathode-bias since the grid-leak biased inputs cannot tolerate really high input signals that some pedal players may use. The Aux inputs are meant as line ins so they may not be too useful.

These are not all that hi-fi. They have character. The overall build is rather tweed-like. For a more modern player you probably will need to roll off some low end to keep it from being woofy and there are places to add gain if you want to. You could also experiment with the negative feedback loop.

It looks like a fun project!

[physics]

Nice amp! The I'd suggest keeping one of the channel as grid-leaked baised because it is interesting sound but the main channel should be converted to cathode-bias since the grid-leak biased inputs cannot tolerate really high input signals that some pedal players may use.  The Aux inputs are meant as line ins so they may not be too useful.

These are not all that hi-fi.  They have character.  The overall build is rather tweed-like.  For a more modern player you probably will need to roll off some low end to keep it from being woofy and there are places to add gain if you want to.  You could also experiment with the negative feedback loop.

It looks like a fun project!

Thanks for the feedback! Especially regarding the grid-leaked bias stages. Was originally planning on just rewiring both of them, but if the sound is at all usable I'd say it's worth leaving at least one like that for my friend to try.

[physics]

The capacitive-coupled bias supply alarms me. I am in agreement with KOC that such bias supplies should be replaced with proper transformer-generated supplies, and a true independent bias-set network applied with cathode current-sense resistors for the output valves.

Could you expand on what alarms you? So far the issues I've found while digging around online/in TUT is that it may not be able to supply enough current in certain situations (like using BMK, or high grid drive) and that some amps have a reputation for blowing the coupling capacitor (Marshall something or other, with a plastic coupling cap). I agree that a transformer-based supply would be best, at the moment I'm trying to work out if it's necessary in my situation either for current supply capability or immediate danger (hence asking about why you're alarmed). Even if it does turn out that I don't need a higher current supply, is there any benefits towards building a higher current supply anyway, whether from a tone or robustness perspective?

Thanks for any info.