Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dsteinschneider on December 18, 2024, 03:16:14 pm
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I play bass and do audio engineering (both recording and live sound) for our band. I have a singular experience in tube amp repair work - I got zapped as a middle schooler trying to get a behemoth early 50's receiver working. I've learned a lot from watching Uncle Doug videos on YouTube but have no hands-on experience working on tube amp circuitry. I'm fairly good at soldering from repairing computer motherboards (mostly replacing capacitors) and have a Hakko FX888 soldering station.
I have two CHB-20A units. One is working fine but the second one blew the fuse when I turned it on. The one that works is Series p79, the one that needs work is Series s41.
Here are photos of the one that blows the fuse - visual inspection looks ok to me, maybe someone will see an issue in these photos.
Bogen CHB-20A
https://links.flickr.com/u/click?_t=40ac9f0c38d540238d36950e58265982&_m=8d437314e00546de9ab572cd7affa8fb&_e=I0iERQspEJ6w7MpICdyJqjoOjcE0LcFVvpYGED26Qfzd1l7aJzTwcNo4wKnQ9a4fju__OQep-cKroZNqggmtuwU3txCluO5mkjRvYRnMqXwRzAPmgM23VZZsSbrc2JNhpnCLaslZZp4of2hcXZJILpJrAmlTvBZKsH8FTptkcF9khYQY58jtNUF9nkFE3xl2kAl65OOIq4s2RyvduO0qC8jmFRq53n-il1BHnMjo5OhQJAnCE5psjyyw4WNdtw9m_xpjmi3T_yRWMwmgRZiLrA%3D%3D (https://links.flickr.com/u/click?_t=40ac9f0c38d540238d36950e58265982&_m=8d437314e00546de9ab572cd7affa8fb&_e=I0iERQspEJ6w7MpICdyJqjoOjcE0LcFVvpYGED26Qfzd1l7aJzTwcNo4wKnQ9a4fju__OQep-cKroZNqggmtuwU3txCluO5mkjRvYRnMqXwRzAPmgM23VZZsSbrc2JNhpnCLaslZZp4of2hcXZJILpJrAmlTvBZKsH8FTptkcF9khYQY58jtNUF9nkFE3xl2kAl65OOIq4s2RyvduO0qC8jmFRq53n-il1BHnMjo5OhQJAnCE5psjyyw4WNdtw9m_xpjmi3T_yRWMwmgRZiLrA%3D%3D)
Questions
1. Can I simply run yellow in the speaker strip to tip and black to sleeve to add a 1/4 speaker output?
2. The G and C are jumpered and the impedence wire is attached to C - what ohms is that configuration? I couldn't hear a difference between that setting and unjumpered with the black lead going to 8 ohms
Thanks,
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Cool amps - Bogen is good quality in my experience.
Here is a good thread from Platefire - he will likely jump in: https://el34world.com/Forum/index.php?topic=19555.0 (https://el34world.com/Forum/index.php?topic=19555.0) and it includes a schematic. :icon_biggrin:
Visually finding a short is hard.
Your questions:
1 Yes - the yellow wire is the 8 ohm lead.
2. If I understand your question - that jumper is simply grounding the "common" wire. C=common G=ground.
BTW - same tube compliment as Sluckey's conversion of an AO-44. Worth a look.
And a second BTW - I would consider making those into a pair of mono-blocks for stereo audio usage, unless you already have a tube stereo amp. With a pair of efficient speakers, they would sound awesome.
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I have already played some music through it into a Tannoy PBM 6.5 - it did sound good. The stereo mono block(s) idea is a good one. I always rue the day I gave a friend my 60's Fisher receiver.
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The stock configuration is surprisingly good at being a guitar amp. The amp that works came with a TRS input next to the coax one. I used a Hosa adapter to split tip and ring and the tip input works with my guitar. I soldered some pins (from a kit I've had for years to make DB-25 connectors) plugged into 1 and 3 on the amps output connector to the tip and ring of a 1/4 jack to connect a Kustom Defender 1x12. Just realizing now that cabinet is 16 ohm. Will patch the back terminal strip for that and see how it sounds.
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Yup - that amp is not far away at all from being a good guitar amp. A few tweaks to that 6eu7 and you are there. The electrolytic camps could probably use replacing. The ceramic disc caps will likely last till the next ice age.
For the other amp with the short. Do you have/use a multimeter?
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For the other amp with the short. Do you have/use a multimeter?
I do have a multimeter.
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Well before applying power again you could go through the circuit with the meter and check for a short. I would especially look at any places where the connections are really crowded and/or places where previous work has been done. You could also compare with the good amp.
And you could pull all the tubes, put in the proper fuse and see if it blows without the tubes.
Assuming you do not have a variac, next step is probably to make a simple light bulb limiter and follow a standard procedure for finding a short. Rob Robinette site has a good instruction for this.
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Thank so much for these tips, its exactly what I need to get started. I need to order some 1.0 amp slo blow fuses - where do people buy them?
UPDATE 1 - found them :icon_biggrin:
https://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=FusesCords&ORDER_ID=357474397
UPDATE 2 - Didn't realize store is in UK....
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Thank so much for these tips, its exactly what I need to get started. I need to order some 1.0 amp slo blow fuses - where do people buy them?
UPDATE 1 - found them :icon_biggrin:
https://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=FusesCords&ORDER_ID=357474397 (https://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=FusesCords&ORDER_ID=357474397)
UPDATE 2 - Didn't realize store is in UK....
Doug's store is in the US.
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Doug's store is in the US.
Thanks - I clicked on the enter Store button and the shipping was normal US shipping prices.
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Rob Robinette site has a good instruction for this.
Robin's design is really clever - will build it before I proceed.
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Question:
In stock configuration this August 20, 1969 CHB-20A doesn't have much "clean range". I can turn the master volume up to 5 or 6 and then start around 2 with the input volume and it's somewhat clean but from there it quickly goes into gain overdrive. Is that what I should expect? It sounds good but I would like more volume while still clean before it goes off into overdrive.
I believe the amp is completely stock from 1969. The tubes say Bogen on them and wiring and caps and resistors all look original to me. I suspect the amp only saw moderate use for perhaps 1 or 2 hours a week and was probably retired sometime in the 80's when solid state PA's became widely available. So at most 15 years of 100 hours of moderate volume use.
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Who knows what to expect?
So this is the amp that hasn't blown the fuse yet?
If everything is original from 1969, likely the electrolytics need to be replaced, all resistors checked, pots and sockets cleaned. Then check voltages, and you will have a better foundation to tell what the amp might have sounded like, as well as feel safer testing it.
1500 hrs on the tubes at moderate use could be at the end of their useful lifespan, but you won't know unless you get the rest of the amp in working order. Don't create 2 identical amps with blown fuses.
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A schematic (https://el34world.com/charts/Schematics/files/Bogen/Bogen_chb_20a.pdf) would answer a lot of questions.
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Attached is the schematic.
Although the amp is close to a guitar amp, it is not a guitar amp. But of course something could be wrong, worn out (like the filter caps), out of adjustment (like the bias) etc.
Fortunately a few Forum members and members of other Forums have converted these amps to guitar with success.
There are searchable threads here, and web searches will bring up others.
The route you take is up to you in terms of your experience and the amount of time you want to put in.
My best advice - read, read, read - including safety procedures before you plug in the soldering iron. Its not a lot of work, but its easy to screw it up. Don't ask me how I know.
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I have read all the CHB-20A posts here and on the TDPRI forum (Platefire has modified his CHB-20A in two stages - first minor tweaks - then he replaced the pre-amp section with a 12AX7 Ampeg Gemini II inspired design).
I think what I need is a training course or book on how to troubleshoot and repair tube amps.
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If you're a video learner, Uncle Doug's videos are probably some of the best content for beginners learning about theory. He rebuilds a lot of old amps, but also has about a 10 part series where he goes in depth about the circuits which really helped me in understanding how they work.
Mr. Carlson's lab is also a great resource, again he rebuilds a lot of old radios, etc., and goes in depth about theory and parts in a way that's fairly simple to understand. Mr. Carlson would be more of an intermediate to advanced learning source, but he also explains thoroughly why they used the types parts they used, benefits, limitations, etc. I think he has a few good soldering videos as well.
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I think what I need is a training course or book on how to troubleshoot and repair tube amps.
That is a very wise step that too often is skipped. It keeps you safe and relatively sane.
If you prefer online sources, I think Rob Robinette's pages are very good.
And Doug and users have provided a ton of good information on these pages.
Sluckeyamps.com including his scrapbook is very good.
Book: https://www.tubesandmore.com/products/guitar-amplifier-electronics-basic-theory (https://www.tubesandmore.com/products/guitar-amplifier-electronics-basic-theory)
Videos - in addition to AlNewman's list. Also on YouTube, Fazio is good, Coleen has good technique and is patient. Psionic is also good, but you need to get past the ego.
If you check out Dr. Carlson's Lab, try not to be intimidated by all the gear at his workbench - most is for show.
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I've been watching Uncle Doug's videos for years even though I didn't own a tube amp. Always want to see what treats are in the box for Jack :laugh:.
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Have ordered tools I think will make this work safer and easier (bought some recommended by Roy Cox on YouTube) and am building the dim bulb tester. I've scoured YouTube for basic tube amp repair instruction videos for beginners and found about 10 that were helpful. I can post the links if anyone is interested.
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Just seen this. My CHB-20A project works pretty good and gets a good measure of clean headroom. It took me a coupleof attempts and some help from this forum to get it right but I think the schematic I made on it is a good recipe:>)
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No R24 in that schematic...Notes state R24 was changed to 22K, no 22K R either. :dontknow:
--Pete
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Thanks Dummyload, yelp mistake here. I went back to the original Bogen schematic and ID'ed the R24 & R25 locations. Revised schematic is attached