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Restoring a Bogen CHB100 - advice appreciate for a novice
Restoring a Bogen CHB100 - advice appreciate for a novice
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Old 11th January 2021, 08:05 PM   #1
Verbstank is offline Verbstank
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Default Restoring a Bogen CHB100 - advice appreciate for a novice

Hello there,


You guys helped me tremendously when I put together a F2B preamp clone so I thought I'd shoot some more questions here, I'm still fairly new to tube amps but I learned a lot with the preamp build.


I've been using this Bogen CHB100 as a bass amp for about a year now. The only things I've done to it was replace the old-style microphone jack to a 1/4 inch jack for ease of use, and swap the 12ax7 tube for a newer one I had laying around. I really dig the sound I've been getting from this... it's clean but gives a nice distortion if I set the gain at 7-8 and keep the volume low. I understand people do more modding to these for guitar amps to get a more colored tone, but the clean-ness works well for my bass application.

However, recently I've been getting more and more distortion even with gain set low and regardless of volume position.

It seems the amp has the original 7868 tubes in it, as well as the original filter caps, and from what I've read one of these two could be the problem. I've wanted to replace both for a while now just to keep this amp healthy and avoid any spontaneous failures, so I feel like now is the time to do it, and if it solves my distortion problem, that's grand, and if not I'll address it afterwards.

The tube swap will be easy, I'm getting 7868 to 7591 pin adapters and going to buy a matched quad of 7591s to use, and all that makes sense to me. Replacing the filter caps is what I'd like advice on. Attached are pictures of the guts, and a schematic of the CHB100. On the schematic I have circled the four caps that correspond to the large brown caps in the pictures. Are these the only ones that I should be replacing or should the blue caps in the photos also be replaced?


Will these caps from Stewmac be the appropriate replacement? Sprague Atom Electrolytic Caps | stewmac.com (See 100uf, 350v choice for brown caps)

I have a heavy-duty resistor/alligator clamp setup that I made for draining the caps and have done that before, so know that I am being safe about this. I would just like opinions from those more knowledgeable about what caps need to be replaced and what exactly I should be looking for as replacements.

Any info appreciated, thanks so much.
Attached Images
File Type: jpg 20210111_123503.jpg (537.0 KB, 91 views)
File Type: jpg 20210111_123517.jpg (484.8 KB, 91 views)
File Type: jpg 20210111_123548.jpg (438.6 KB, 88 views)
File Type: jpg bogen-chb100b-6-edited.jpg (838.2 KB, 87 views)
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Old 11th January 2021, 08:59 PM   #2
mctavish is offline mctavish  Austria
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I would replace EVERY SINGLE ELECTROLYTIC in that amp.

dont wait.

Use a good cap from a reputable manufacturer.

You can do better that the Sprague.

Look to Nichicon or Chemicon or such. I have had poor results with the Sprague atom. They have a name that no longer deserves admiration.

Have a look at the coupling caps as well.

Ping Eli duttman.
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Old 11th January 2021, 09:09 PM   #3
mctavish is offline mctavish  Austria
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plus the prices on that link are insane!
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Old 11th January 2021, 09:36 PM   #4
Demonkleaner is offline Demonkleaner  United States
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I'm sure Mouser has what you need.
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Old 11th January 2021, 09:39 PM   #5
Eli Duttman is offline Eli Duttman  United States
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Let's rotate the schematic, for ease of viewing.

I agree with mctavish; eschew Sprague "Atoms".

FWIW, Panasonic and Nichicon are my most favored 'lytic brands, but other brands can be good too. Definitely select 105o C. rated parts. It may be necessary to employ either snap in or radial lead parts, instead of the OEM axial lead parts. Taking advantage of Sheldon Stokes' idea of mounting a substrate on the power trafo chassis screws frequently eases the burden of switching to the modern 'lytics.

Mouser, DigiKey, etc. will charge you less than an instrument specialty house. Speaking of instruments, if bass meant bass guitar, Instruments and Amps is the correct "board" for this thread.

716P series "Orange Drop" caps. will be highly suitable in the signal path.

The triodes in the 6EU7 are the same as those in a 12AX7. What's special about the 6EU7 is a spiral wound, hum bucking, heater and its pin out. You can get the hum bucking at lower cost by switching to a 7025 equivalent, like the Sovtek 12AX7LPS. Of course, the socket needs rewiring.
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Old 11th January 2021, 09:54 PM   #6
Verbstank is offline Verbstank
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Ok thank you all.


So to replace all electrolytic caps I'm pretty much just looking for all the cylindrical, larger caps regardless of their place in the circuitry, and leaving any ceramic caps in place? And there should be no problem either using a radial (the ones that stand up) vs axial (the ones with lead on either side) besides space considerations as long as ratings and mf values are the same?


Your suggestion for using "Orange Drop" caps is to replace some of the ceramics? or just where i have small mf value electrolytics I could instead use an orange drop there?



Will look for 105degree parts per your suggestion, and will most likely buy from Mouser, I just had the stewmac link up at the time.


Thanks all
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Old 12th January 2021, 01:50 AM   #7
audiowize is offline audiowize  United States
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Any caps that attach to the plate of one tube and feed the grid of another ought to be pulled out and replaced, and the Orange Drops are a nice choice. Caps in this position are referred to as coupling caps.

It won't hurt anything and it won't cost much to replace the rectifier diodes with some UF4007s.

The first two power supply caps after the rectifier diodes that form the voltage doubler could be much more than 100uF without causing problems, and larger caps may fit a bit better in the space you have. These would be totally fine for replacing the 100uF/300V caps in each amp:
https://www.mouser.com/ProductDetail...DbhPL69A%3D%3D

The 0.047uF cap that's part of the bias supply should be replaced as well.

I'm wishing I had saved photos of the inside of the pair I redid.
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Old 12th January 2021, 06:01 AM   #8
Eli Duttman is offline Eli Duttman  United States
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Increasing the value of the PSU 'lytics is a very reasonable idea. However, take out insurance against the power trafo making a clanging noise, at power turn on time. A CL-140 inrush current limiting thermistor inserted in the line that connects the power trafo to the junction of the doubler stack caps. provides the requisite protection.

2X of this part in the doubler stack and 2X of this part after the 750 Ω resistor should work out nicely. The suggested parts are rated for 105o C. and have long service lives.

@audiowize - Is there room to "shoehorn" in something like this inside the enclosure? FWIW, I intensely dislike deriving the negative bias rail in the manner Bogen chose.
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Old 13th January 2021, 12:20 AM   #9
Verbstank is offline Verbstank
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Eli, are you talking about right after CR1 on the schematic as the place to insert the thermistor, then from that to the 750Ω resistor and 100uf cap?



And Audiowize thank you for the suggestions I'll replace the diodes as well as coupling caps.


In terms of higher value uf caps for the filter caps (they are filter caps right? or different term? you guys seem to call them power supply caps?) would the higher value have an effect on the tone of the amplifier? Or are they just to have more "storage space" to not bog down on transients? Or am I way off on that?
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Old 13th January 2021, 02:18 AM   #10
Eli Duttman is offline Eli Duttman  United States
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Quote:
Originally Posted by Verbstank View Post
Eli, are you talking about right after CR1 on the schematic as the place to insert the thermistor, then from that to the 750Ω resistor and 100uf cap?



And Audiowize thank you for the suggestions I'll replace the diodes as well as coupling caps.


In terms of higher value uf caps for the filter caps (they are filter caps right? or different term? you guys seem to call them power supply caps?) would the higher value have an effect on the tone of the amplifier? Or are they just to have more "storage space" to not bog down on transients? Or am I way off on that?
Filter caps. or PSU caps., it's 6 of 1 and a 1/2 dozen of the other. The increased capacitance reduces residual hum and may "tighten" bass. Yes, energy storage is increased.

The CL-140 location is shown in blue. Sorry about my twitchy hand on the mouse working with MS Paint.
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