BOGEN CHB 20A

[Colas LeGrippa]

Hola amigos !

A friend of mine just bought a Bogen amplifier, model CHB20A for 80 bucks.  He wants to use it as a guitar amp. What mod could  I perform for a decent tone ? It's got one 6EU7 and two ECL86.

Muchas gracias !

Cola LeGrippaaaaaaaaaaaHHHH

[sluckey]

Maybe something like geezer's Little Wing (mini bassman). Look in the archives. I believe he actually used a 6BM8 but it is very similar to the 6GW8.

But... First just plug into the mic jack. It may already be all you want it to be.

[jjasilli]

Yes, lots of old posts on this.  Hope they're not lost due to age. 


One point is the tonestack:  James/Baxandall.  Way too dark for me for el guitar.  But some people prefer it. 


BTW:  it does make a great, reasonably light-weight tube PA amp for voice & acoustic guitar amplification in bars and small venues.  I use mine that way as a power amp with a SS mixer in front of it.

[sluckey]

One point is the tonestack:  James/Baxandall.  Way too dark for me for el guitar.  But some people prefer it.

You can try a simple mod that should voice the TS more for a guitar. Just change the components around the tone pots to match this Ampeg TS.

[Colas LeGrippa]

Hola hombres ,

I have removed the phono and mic inputs and volume, put a 1/4 jack in the corresponding hole as a guitar input, drilled a1/4'' hole in the back for the speaker out,  keeping operative the black impedance lead .

The  guitar now is connected to where the mic was connected before, right onto the 6EU7 that I kept as is, as well as the ECL 86. I did not have to perform any circuit mod, the tone of that thing was incredibly loud and tasty ! My friend LOVED IT .

One thing : the price of the tubes is twice the price of one 12ax7 and EL84, though we would need one more 12ax7 to drive the EL84's. The ECL86's got the triode and the pentode into the same tube.....so it's worth it.

Good rock amp, without lots of changes. I tried it in a 8in,  20w  Alnico speaker. Much better than an Eminence PATRIOT Red Coat.

Thanks buddies !
Colas LeGrippa

[Platefire]

I got one I did very few changes and loved the way it sounded almost stock. Attached is a marked up schematic of what I did:

1-As you did installed 1/4 jack and build a fender like input to V1. Removed wiring for Aux 1 and Aux 2 inputs.
Think I also removed C3 and R3.

2-Rewired the existing tone stack to be immediately after C2 and just before R4, the first vol.

3-Rewired the Volume R5 to be a master volume after V1b.

4-I also added a standby switch.

5-Also installed a speaker jack to the 8 Ohm tap of OT.

6-I may have changed v1a plate resistor R2 to 220k to get the plate voltage up.

To my ears, the tone stack works good for guitar as re-wired. I guess all this info is To LATE! Platefire

[Colas LeGrippa]

Thanks Platefire, no it's never too late I may have another one to mod one of these days. Now my friend wants an effect loop in it and I got to do it right. I may perform oher mods, e.g. line out. He wrote me last week to tell me his amp was FANTASTIC. And this is a comment all the techs are hoping for , for sure.
 I find the mix of 6EU7 and ECL86 is AWESOME for guitar, and this f....little Bogen amp's got guts like  very few amps designed for guitar have got. I may design and build one with these tubes, gee it turns me on !!!! I would hook up  a pair of Jensen P8R ( Alnico ) to it , and the tone would be incredible. These tubes are expensive like hell,though, over 100$ for the three.

Colas LeGrippa

[Platefire]

Well if you customer is happy, it might be better to leave well enough alone---he might not like it if you did more? 

[TIMBO]

Hey Colas, This is a local built amp from the 70's http://el34world.com/Forum/index.php?topic=19430.0
And many other builders used the 6GW8 with great success.

[Colas LeGrippa]

Exactly what I thought Platefire, I said to myself: «Jack, do the minimum and try the thing,» And that was it. Less is more, often.

Timbo, thanks for the link !

Colas