Build Journal: Bogen CHB35a to switchable Bass/2204

[thetragichero]

there are smarter folks that are much better at setting up psud for modeling the B+ rail, i believe using constant current sources. all we're concerned with is the maximum voltage at T0. i used the 190VAC i measured previously on the chb35 power transformer on my bench. i assume it's about what you have

 

[_Steve]

OK thanks

there are smarter folks that are much better at setting up psud for modeling the B+ rail, i believe using constant current sources. all we're concerned with is the maximum voltage at T0. i used the 190VAC i measured previously on the chb35 power transformer on my bench. i assume it's about what you have

Thanks. Im gonna check my assumptions again I was of the opinion that before the tubes start conducting and drawing current (ie at startup) the whole string of nodes will be at the full potential of B+1 (which ive measured at 503VDC).

Being able to use the cheap 450V radial caps I already have on-hand would definitely be better

 

[_Steve]

I made a schematic of the 1st stages and the switching between JTM45/Bass circuit and 2204.

I'll be using a 500K DPDT switching pot for the preamp gain as there is a 1/2 inch gap between the chassis and the shell/faceplate so i need a pot with a long shaft. I couldn't find a A1M with DPDT and a long shaft anywhere online, and I already had a 500K one on-hand.

 

[Pete Farrington]

before the tubes start conducting and drawing current (ie at startup) the whole string of nodes will be at the full potential of B+1 (which ive measured at 503VDC).

I think at least that high is a safe bet.
It’s one of the downsides of voltage doubler power supplies.
I agree that R4 at 5k seems a bit extreme.

 

[_Steve]

I updated the schematic to move the 270k/568pF network before the volume pot:



I also tweaked the resistor value to offset the change in pot value from 1M to 500K

 

[gldtp99]

Yes, Bogen CHB35A's !!!!

I used to buy up old Bogen PA heads and re-purpose them for guitar use and sell them to local bands/players

Bogen CHB35A and CHB50 were the most common but I also re-did Knight, Stromberg-Carlson, Bell, and many other brands

Bogen CHB35A and CHB50 iron was usually made by Schumacher (606 EIA code) or Better Coil & Transformer (831 EIA code) ---- the same companies supplied transformers for many Blackface era Fender amps --- so they are quality units in my experience

I often would gut the pre-amps and build a Plexi type preamp (usually single channel/single guitar input) into CHB35A and CHB50 amps

Most of these amps were used for years without problems by many local bands

Here's one re-built CHB35A I re-did (w/Plexi based preamp) that was used by many different bands for years ----- It was bought by a local studio, after a while, and used for countless hours for rehearsal/recording/shows by many different bands ---- one owner had the original metal case powder coated in white before the studio bought it

It's the rectangular white amp sitting on top of two speaker cabs behind the guitar player in this live-in-studio vid:

(2) The Strange Transmission - The Hallways - Lies - YouTube

People told me that bands would usually turn everything up to "10" and play it without pedals --- I believe that's how it was used in the video

I finally bought back this particular amp just so local bands would stop using it in public shows

I was building much better amps at the time and I thought this old Bogen wasn't up to the standards of the amps I was building so it was "bad advertising" for the amps I was building

This white CHB35A is now sitting in my storage area --- unused for several years

Best of luck on your CHB35A re-build !!!!

 

[_Steve]

Yes, Bogen CHB35A's !!!!

I used to buy up old Bogen PA heads and re-purpose them for guitar use and sell them to local bands/players

Bogen CHB35A and CHB50 were the most common but I also re-did Knight, Stromberg-Carlson, Bell, and many other brands

Bogen CHB35A and CHB50 iron was usually made by Schumacher (606 EIA code) or Better Coil & Transformer (831 EIA code) ---- the same companies supplied transformers for many Blackface era Fender amps --- so they are quality units in my experience

I often would gut the pre-amps and build a Plexi type preamp (usually single channel/single guitar input) into CHB35A and CHB50 amps

Most of these amps were used for years without problems by many local bands

Here's one re-built CHB35A I re-did (w/Plexi based preamp) that was used by many different bands for years ----- It was bought by a local studio, after a while, and used for countless hours for rehearsal/recording/shows by many different bands ---- one owner had the original metal case powder coated in white before the studio bought it

It's the rectangular white amp sitting on top of two speaker cabs behind the guitar player in this live-in-studio vid:

(2) The Strange Transmission - The Hallways - Lies - YouTube

People told me that bands would usually turn everything up to "10" and play it without pedals --- I believe that's how it was used in the video

I finally bought back this particular amp just so local bands would stop using it in public shows

I was building much better amps at the time and I thought this old Bogen wasn't up to the standards of the amps I was building so it was "bad advertising" for the amps I was building

This white CHB35A is now sitting in my storage area --- unused for several years

Best of luck on your CHB35A re-build !!!!

Thanks for posting this - I enjoyed reading that!

Interested what you opinions are on 7868s? Did you leave them in or swap out the power section? Ive been thinking that if the 7868s are not great maybe some KT66s might work with the TXs..

Finally do you have any idea what the big resistor and cap hanging of just one of the 7868 plates are for? Did you keep it like that?

 

[gldtp99]

Most CHB35A's I used 7868's ---- In some amps I replaced the output tube sockets and used 6L6GC ---- In one I used cathode biased EL34's

They all worked well and it didn't make a huge difference what exact type of output tube was used

The 7868's might have a slightly earlier breakup but they are a good tube

Just rare and more expensive than other perfectly suitable types

The current production EH 7868's sound good and are well made --- but they are more expensive

I've used many Sylvania 7868's ---- again they are good tubes but expensive

Most CHB35A's I've seen didn't have that 3.3k/2 Watt resistor and small cap ----- those that had them i just removed them and the amps ran for years trouble free

 

[_Steve]

Most CHB35A's I used 7868's ---- In some amps I replaced the output tube sockets and used 6L6GC ---- In one I used cathode biased EL34's

They all worked well and it didn't make a huge difference what exact type of output tube was used

The 7868's might have a slightly earlier breakup but they are a good tube

Just rare and more expensive than other perfectly suitable types

The current production EH 7868's sound good and are well made --- but they are more expensive

I've used many Sylvania 7868's ---- again they are good tubes but expensive

Most CHB35A's I've seen didn't have that 3.3k/2 Watt resistor and small cap ----- those that had them i just removed them and the amps ran for years trouble free

Awesome thanks for the valuable intel!

 

[_Steve]

Here's a pic showing the gap between the chassis and the shell. Even 3/4" bushing length pots are borderline and limiting which knobs I can use.

 

[gldtp99]

I used to buy Alpha long shaft 24mm pots from AES (tubesandmore.com) that had very long solid 1/4 inch shafts

I'd cut the shafts to the length I needed for the CHB series Bogens

They don't appear to be available at AES anymore

Maybe you could google "long shaft pots" and see if anyone still sells them

 

[_Steve]

I used to buy Alpha long shaft 24mm pots from AES (tubesandmore.com) that had very long solid 1/4 inch shafts

I'd cut the shafts to the length I needed for the CHB series Bogens

They don't appear to be available at AES anymore

Maybe you could google "long shaft pots" and see if anyone still sells them

Problem is a couple of the pots need to be switched and there doesn't appear to be any long shaft versions available anywhere. That's what's causing the headache. I have a plan... Im not proud of it but it should work.

 

[Pete Farrington]

Problem is a couple of the pots need to be switched and there doesn't appear to be any long shaft versions available anywhere.

Buy the various pots, each with one or more of the necessary attributes. Bend back the tabs that hold them together and assemble your own custom Frankensteins.

 

[gldtp99]

These switched pots seem to have the long solid shaft

You'd just need to make them into the value(s) you need:

Potentiometer - Alpha, Audio, SPST Switch | Antique Electronic Supply (tubesandmore.com)

 

[paul-e-mann]

This morning I fired it up with just the PI and power tubes. I wanted to see if the 7868s I received with the unit work or not because I've realized none of the US retailers have them in stock - so I'd need to either buy Russian/Ukrainian equivalents off Ebay or switch out the sockets to octals to run 7951 tubes which are the same tube with a different base.

Thankfully the 7868s seem to work as I was getting output signal. However my LTP PI is not working correctly - passing very little signal. It feels like ive spent the last 3-4 months wrestling with LTPs so I just couldnt be arsed and put it aside to investigate later.

I wired up V2 ready for the tonestack and V1 tomorrow. With a bit of luck I might finish by Friday.

Spot the oopsie:
View attachment 91287

Wow that sure looks like a mess inside

 

[_Steve]

These switched pots seem to have the long solid shaft

You'd just need to make them into the value(s) you need:

Potentiometer - Alpha, Audio, SPST Switch | Antique Electronic Supply (tubesandmore.com)

Thanks - I actually had one of these on hand which im using for the Bass pot & Tonestack lift. Unfortunately the other (Plexi/2204) switch needs to be DPDT.. I have a solution im working through though.

The 500V caps came yesterday so I should be able to get this finished very soon.

 

[_Steve]

Wow that sure looks like a mess inside

Sorry to disappoint you

 

[Pete Farrington]

Does it matter in the above schematic that V1b's grid is left floating when in 'plexi' mode?

I don't see that it is floating ever? V1b grid is connected to the 2204 gain pot wiper and so referenced to 0V common.

 

[_Steve]

I don't see that it is floating ever? V1b grid is connected to the 2204 gain pot wiper and so referenced to 0V common.

Thanks I see how that works now!

 

[_Steve]

Arrrgh I'm having exactly the same issue with the LTP that I had with my JTM45 here: JTM45 vs Idiot | MarshallForum.com

The input-side triode is not conducting. I thought I had fixed this on the JTM45 by reflowing the solder on the heaters, but clearly there must be a different root cause - other than me being a total idiot!