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Recommended course of action - 1st tube project

T

Tommy_B

Well-Known Member
I'm a toob noob, so looking for some advice on a potential 1st tube project. I have an opportunity to buy a couple of 100W Bogen Challenger PA amp chassis (no tubes) for $25 ea to use as a base for building a couple of monoblocks. I've seen some positive comments from others that have done such a conversion, so it sounds promising. I'm just wondering if I might I be better off starting with a different approach as a first tube project, like building something from scratch, or starting with something like a stereo Magnavox chassis.

The Bogen chassis is attractive because it seems to have fairly beefy output transformers and could provide decent output power (maybe 40W at reasonable fidelity?), but each uses four 7868 power tubes, so a little pricey. If I went the Bogen route, I'd probably just bypass the tone and input sections for stage 1 of the project, and maybe later work on the preamp section (I would eventually like to have a good phono input).

I should probably mention that my power needs are low-moderate, as I would be using the amps in a small room. The Bogens might be more power than I need, but I figured, why not? (aside from the higher cost).

Anyway, any advice would be appreciated.

P.S. Guess I should also mention that I'm an electrical engineer, but my career has been spent in the digital domain. Looking to do something a bit more fun now! :-)
 
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For the $25 it would be a good start for a chassis and power transformer at least. I will have to look for more info before I could say anything on the OPT's, Dave G did some MO200's with excellent results.
 
No real input for you. Just interested to see what you come up with. I picked up one of these with tubes for $50 last fall. Eventually I want to pick up a pair of appropriate OPTs and convert it to a stereo amp.
 
I had a Bogen CHB-100 in my early teens. I heavily modified the preamp section,then used it throughout high school as my bass amp with an old Traynor cabinet that had 8 ten inch speakers. My recollection is pretty clean up to ''really loud'', and overdriving it was the closest I've ever come to sounding like John Entwistle on Live At Leeds.

I would imagine this output transformer could be successfully used in high fidelity applications,especially under less strenuous conditions:)

From a technical standpoint,a pair of 7591's usually have a plate to plate load of about 6K6. Two pairs would bring that down to about 3K3,which gets you close to the requirements for a pair of EL34 or 6550. Lower tube costs,and much more variety available.

https://www.bogen.com/support/discontinued/pdfs/CHB100m.PDF
 
You'd have to do some testing, but Bogen did seem to use fairly good transformers. With a little massaging of the circuit, it might come out OK. I expect the stock preamp stuff is not favorable though.
 
arts said:
I had a Bogen CHB-100 in my early teens. I heavily modified the preamp section,then used it throughout high school as my bass amp with an old Traynor cabinet that had 8 ten inch speakers. My recollection is pretty clean up to ''really loud'', and overdriving it was the closest I've ever come to sounding like John Entwistle on Live At Leeds.

I would imagine this output transformer could be successfully used in high fidelity applications,especially under less strenuous conditions:)

From a technical standpoint,a pair of 7591's usually have a plate to plate load of about 6K6. Two pairs would bring that down to about 3K3,which gets you close to the requirements for a pair of EL34 or 6550. Lower tube costs,and much more variety available.

https://www.bogen.com/support/discontinued/pdfs/CHB100m.PDF
Click to expand...

Thanks for the schematic link. I read somewhere that there were two variations of the CHB100 schematic, so I will look for the other one as well.
The two Bogen amps I'm considering are of different vintage, so might be some differences between them.
I was curious about the possibility of using different output tubes as a way to get my feet wet, but I don't know enough about tubes yet to consider the options.
I'll start looking into the EL34s as a possibility though.

For posterity, here's a pic of the Bogen PT and the OPT on one of the units. If they're in good condition, I think they should do :-)

Pic of PT and OPT.jpg
 
Didn't see the actual schematic at the link previously posted, but this may be it:
(from http://www.makearadio.com/schematics/images/bogen-chb100b-6.jpg)

bogen-chb100b-6.jpg

I think you could make this work without too much trouble, at least enough to try it out to see if you like it as a Hi-Fi amp. Here's what I'd probably do for starters, as a least invasive approach:
  • Disconnect all wires from the grid of V2B and tape them off and set them aside.
  • Add somewhere between a 100K and 470K resistor from the grid of V2B to ground.
  • Add an input RCA jack with center connected to the grid of V2B.
Then drive it with an outboard active preamp. Of course you'd need to do the "normal" maintenance on an old amp if it's not already been done. If you like the way it sounds you could make things more permanent.

The next set of things to do I would call mandatory if you are going to keep this unit in long term service as a hi-fi amp:
  • Address is the biasing of V2B 12AX7. It looks to me that it is under biased with only a 470 ohm cathode resistor. I'd like to see the frontend of the power amp section revisited for proper biasing for a standalone amp. Since the 12AX7 is direct coupled to the 6C4, any changes made in biasing of the 12AX7 stage will cascade downstream to require the 6C4 to also be rebiased. The goal here is to make the frontend so that it can cleanly drive the output stage to full power. There will be slight changes to the forward voltage gain of the amp if you start tweaking the bias of the frontend, so to do a proper job of this you should then at that point revisit the feedback and HF/LF stability to make sure you haven't upset something.
  • Parallel both sections of the 12AX7 together (since this is a mono amplifier), adjusting the plate and cathode resistors to account for double the current through the first stage. Alternatively you could replace the 12AX7 and the 6C4 in the power amp section with a single 12DW7.
All of that was not meant to put up a big roadblock or sound difficult. It is rather straight forward, and people here on the forum can help out.

The rest of this is perhaps more optional or cosmetic:
  • To give a little more flexibility, add adjustable biasing so that you don't need closely matched output tubes.
  • Remove the 6EU7 and tone controls all together.
Not sure how the output transformers will perform for Hi-Fi, but I bet they will be okay. I think the idea of swapping the output tubes out for a single pair of EL34s or 6550s is interesting, but you'd have to reexamine the drive and bias requirements, possibly making adjustments to the input stage and/or the inverter, as well as certainly making adjustments to the negative bias supply to accommodate. You don't see this method of obtaining negative bias voltage very often. Also if you change output tube types, you'd need to revisit the feedback circuit and HF/LF stability.

Good luck. This sounds like a really interesting project.
 
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Thanks guys. I'm going to see about picking up the two Bogen chassis this week. I'll probably have to put this project on the backburner while I learn about tube amps (and tubes for that matter), but will try to document and post my progress once I do get started. I'm sure I'll be back asking more tube questions in the meantime. :-) I just love AK!
 
The actual power amp section of this doesn't look very different from the stock MO-100A design. Its got that same wanky fixed bias supply and extremely minimal balance setup at any rate. The MO-100A used a quad of 8417's, which are basically like a supersized set of 7868 tubes. Very similar drive requirements, but they will handle a lot more current and voltage.


The MO-100A used a 12DW7, which is basically half of the one 12ax7 and the 6C4 in one bottle. Really the only thing different here is that there is a preamp section.

The end solution on the MO-100A was to replace the quad of 8417 tubes with a pair of 6550's and change the driver tube to a 6AN8 to get enough voltage gain to drive them to full output. It works pretty darn well too.
 
And as I recall, the final solution on the MO-100A did not use the 70V winding for the feedback tap as was done on the stock unit.

Tommy_B: here is a little "light reading" for you on a similar unit ;)
http://audiokarma.org/forums/index.php?threads/the-mighty-bogen-mo-200a.556575/

The major differences are that unit used something like 700V B+ to the center tap of the output transformers to support 100 watts/channel, whereas your units deliver a more sane 465V, at least it appears so from the schematic, which supports about 1/2 that wattage at (apparently) 50 watts/channel.
 
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yeah, thats quite a long thread, but the tech info will probably be useful. I'm pretty pleased with the results on my amplifiers. I never heard them in stock form, but by all accounts its nothing special as-built.
 
The CHB-100 transformer measures quite well at low levels, but isn't big enough to go down to 20 Hz at full power. Should do quite well at lower power. Biggest problem is the low impedance - 2350 Ohms. Push-pull parallel EL34 or 6L6GC? Since it has 2,4,8,16 secondary taps, you could use it as 4.7K to 4,8,16, 32 - but you'll lose an octave at the low end, and I'm not sure how the high end will turn out.
 
I'm also thinking their ratings are a bit fanciful. 20-20k within 2db at 100w from a quad of 7868 ? Not bloody likely.
 
Tom Bavis said:
The CHB-100 transformer measures quite well at low levels, but isn't big enough to go down to 20 Hz at full power. Should do quite well at lower power. Biggest problem is the low impedance - 2350 Ohms. Push-pull parallel EL34 or 6L6GC? Since it has 2,4,8,16 secondary taps, you could use it as 4.7K to 4,8,16, 32 - but you'll lose an octave at the low end, and I'm not sure how the high end will turn out.
Click to expand...

I guess it might be helpful to list my goals for this project, in order of prioritiy:

1. Moderate Cost
2. Good Fidelity
3. Reliability / Low Maintenance
4. Cosmetically Attractive
5. Power Output (am willing to accept lower power output to achieve items 1 - 3 above; 15-20W should be plenty. If I happen to get more than that, great!).

LOL ... I just realized that items 1 thru 4 would also apply if one was looking for a spouse instead of an amp! :-)

My undestanding from what I've only recently learned about tubes/amps is that the 7868 power tubes sound quite good and produce good power, and since the Bogen chassis is already setup for them, seems like that would be a fine way to go. However, I wouldn't be opposed to using a different set of output tubes and appropriately modifying the circuit if that helps achieve items 1 - 3 above.

Anyway, thanks for the advice so far ... looks like I've got a lot to learn. I'm going to pick up the two Bogen chassis tomorrow.
 
Yeah, I'd keep the 7868's, at least for now, as that makes the problem of getting these amps working much simpler. It's a taller order to rethink the drive/frontend to support a different output tube type. I just finished building a scratch amp using the EH 7868's, and while they are a bit expensive, I think they sound fantastic.
 
The grid drive requirements are very similar between EL34's and 7591's. The 6C4 phase splitter is more than up to the task. As economy was mentioned,I thought it was a worthwhile suggestion to sub in a pair of EL34's per amp. For the sake of changing a couple of tube sockets,4x EL34 versus 8x 7868 seemed to make more sense from the $ standpoint.
 
To get started with checking out each amp, could I just use two instead of the four 7868's or would that just complicate things? Just wondering how susceptible a new set of tubes might be to damage from an unknown, and potentially faulty, chassis. Or maybe I should try to find a cheap/used set of tubes to do the checkout?

It appears the Bogen OPT impedance is ~2500ohm (when using an 8 ohm speaker load). I assume that is the impedance between the center tap and either plate tap of the OPT, and not the total impedance across the two plate taps ... is that correct? I need to start looking at some tube specs to see how tube output impedance is specified and how that gets matched up with the OPT/load.
 
It would make the power supply loading odd, but at the end of the day its possible that it will work out better with only 2 tubes per amp anyway. With half the tubes, I'd expect voltages to be higher since there is less load.

One thing I would do is install a real bias supply. Not real complex, just a simple 6.3vac filament transformer wired backwards. Feed it 6v off the heater string, get 120vac out and rectify that. The cap pilfering it off the HV supply is just way too chintzy for my liking, though I guess you could make the argument that it tracks B+ and in theory makes some attempt to keep the tube element voltages track relative to one another. Still just seems like an "oops, we forgot to add this" type of "fix".
 
OK, so I just read a short Wiki about push-pull tube amp architecture and I think I get it now. So, if I were to reduce the power tubes from 4 to 2, that would increase (by 2) the power stage output impedance (i.e., instead of two sets of push-pull tubes in parallel, only one set). So one set of 7868 tubes in a push-pull configuration would present a total series resistance of about 6000 ohms, whereas two sets in parallel would exhibit a total series resistance of about 3000 ohms. So that now leads me to believe that the 2500 ohm Bogen OPT impedance is in fact across the entire winding (plate tap to plate tap). That would indicate the Bogen amp is somewhat optimized (from a power transfer standpoint) for an 8 ohm speaker load.

I'm beginning to appreciate the inherent tradeoffs involved in tube selection, OPT selection, and speaker selection, when designing a tube amp, but still much more to learn. :-)
 
Actually,while rare in hi-fi applications,this manner of deriving bias voltage is actually quite common. Some of the most respected manufacturers have used it,and it is well proven and reliable. In my mind an elegant and uncomplicated engineering solution to what is,in essence,a very simple requirement.And in a lifetime of service work,I have never seen it fail. Different or unfamiliar does not equal wrong.

Tommy-where did you get OPT impedance? If it's plate-to-plate it seems a little low,and if it's center-tap to plate,a little high.
 
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