Bogen CHB-20A Rebuild

[physics]

Hi Guys

Arc voltage is the limit for plate voltage in a tube and this is rarely listed. Many tubes have other notes in their spec sheets that suggest the true capability. For the EL-84 / 6BQ5A, note that the pins to either side of A are left empty to provide isolation for the voltages that may be present on the anode.

Get rid of the dropping resistors and go fixed-biased as the original schematic shows and you will have 20W.

EL-84s are heater current hogs for sure, pulling 760mA each compared to a 6V6 at 450mA and 6L6 at 900mA., and a pair of them is already near the stock tube complement's full heater load (1850mA; 2x700 + 300 +150)). A cathode-biased amp tends to be garbled at and near clip, where fixed-biased amps tend to have some definition left.

You could save current by switching to 6V6s, or just change one tube to 6V6. The mix of the '84 brightness and the V's cream is quite nice. This mix would require separate Rk+Ck or fixed-bias with individual controls.

Every tech has his own style and deals with customers differently, accepting or rejecting certain customers and/or their projects. As I mentioned elsewhere, I would ask a player what the goal was tonally and/or functionally, and then tell him the cost of different options. It is up to me to interpret his goal into tech. I would choose components and circuits based on what I know to be reliable and that suit my beliefs and experience. Once the player starts demanding the use of yellow caps and green resistors and red transformers we are done. There are other techs willing to cater to those compromises.

Your friend is obviously reading forums or blogs and assuming what he reads is accurate. He may not even know what his own goal actually is? In electronics there are many ways to achieve the same goal. The 50W amp is a better choice if he wants something that will stay clean to use pedals with. The 20W may be closer to the loudness he wants with inefficient cabinets and a cranked sound. He is getting a bargain having you do the work for him. You are doing it for the experience and to help your friend and he is just paying for parts. You will end up going around in circles although in the end if he is not happy with the result it is only because he dictated what you should do rather than properly defining the goal and let you loose. It will be "your fault" either way.

I suppose the original tubes are history? You could easily make a nice amp using those with the output stage and PSU left in their stock form, either rewire the splitter and PA front-end or leave as is, and revoice the preamp.This applies to either amp. The stock power tubes are a similar construction to the '84 and will sound about the same when driven hard.  The key PA point is to not have a feedback loop to the secondary - which most old PAs did not have anyway.

Did the PA work at the beginning? Did your friend assess the sound? Open the NFB and the tone would likely be how he wants. 6EU7 is like a 12AX7 with a different pinout, so it should get the same tones. The 6GW8 triode is identical to 12AX7 and the pentode is remarkably similar to the '84.

Yes, the 2A slow-blow mains fuse is not protecting the PT at all. TUT3 explains how to select fuse values.

You see Schmitt splitters in most of the TUT3,5 projects simply because that is what the iconic circuit had and what hobbyists expect. I haven't used them since 1985, and then only for a hifi amp. Concertina is king for versatility, stability and tone.

I see, ok. So it's mostly an empirical process guided by educated guesses to find the max voltage, assuming it's not published?

That's a good point about the original schematic, I had not consulted with that since seeing that a previous owner had (mostly) obviated it.

I like the tube swapping idea, installing all 6V6's was one of the things I proposed as an option initially both because of the heater current and due to the plate voltage question being up in the air at the time.

I may have given the wrong idea about my friend, pretty much the only constraint I have is to not blow the budget. When I mentioned convincing him to go with fixed bias, I meant purely from the standpoint of convincing him that it's worth the extra cost in parts vs running the tubes at full tilt in cathode bias and trying to resolve the issue of the missing 5W. With just about everything so far he's pretty much said he'll leave it up to my discretion, with cost being the only limiting factor. I agree that the 50W would be a better choice, and after further discussion yesterday and some rough ballpark figures in what it will cost to make either the 20W or 50W useful now that I have a better idea of what's needed, he decided to go with the 50W instead. So the 20W is now my personal playground.

Yes, the original tubes are gone as far as I know. The same guy I bought the amps from though also gave me a pile of vintage tubes he didn't feel like selling, and I think I saw a full complement of the original tubes among them. Not original (some are labeled hammond), but the same types and mostly USA or Britain in origin. The 50W does have original tubes though, at least the twin 6L6 tubes are Bogen-labeled, and it is pretty much stock (down to the two-prong power cord ) including the 6C4 concertina inverter, so I'm glad to hear the ringing endorsement of that particular topology.

The 20W PA was already gutted and rebuilt as marshall clone when I got it, the 50W PA appears to still be a PA, I've yet to turn it on since he and I agreed it would be worth appraising the 20W first since it looked really close to being useful.

I'll have to re-read TUT3 then, thanks for the pointer to it.

Regarding schmitts and TUTs, I suppose that makes sense. I'll have to do a deep dive into the behaviors of both at some point, and do some comparisons. While this 20W will probably go on the shelf for a bit while I work on the 50W and my Peavey VK100, I think it would be interesting to change the 20W to a concertina and do some before/after recordings and measurements for a more quantifiable description of the differences. When I have time I guess.

Thanks for the informative reply!