Bogen CHB-35 - guitar tube amp conversion (another)

[fenug]

Hi all - (I think this is my first post despite being a lurker for years)

I got a nice all-tube Bogen CHB-35 PA amplifier and like many others
before me, want to re-work it. However I'm a total "toob-noob" ,
my experience until now being 100% solid state.

Here are some quick specs on the amp (I also attached a copy of
the schemmy) :
1x 6EU7 input stage (electrically equivalent to 12AX7, I'm told)
1x 12AX7 wired as the tone stack
1x 6C4 phase splitter
2x 7868 output pentodes.

I already did a number of mods on my bogen (silvertone 1484
tonestack, re-wired pre-amp with the two 6EU7 triodes cascaded,
standby switch & some other inconsequent stuff...)
They mostly didn't work out too well so I'm coming here for some
help !

A few questions : (many, actually)

1- when run at higher power, the power tubes (2x7868) seemed
to produce a mechanical rattling sound when striking notes. One
of the tubes actually glowed brighter blue than the other... Are
they dying on me ? Is my output stage badly biased ? (I noticed
some PP output stages on "real" amps have a bias pot.)

2- In contrast with the orig. schematic, my amp did not have 1k
resistors between the 7868 grids and the rest of the biasing
network (if that's the correct term). I marked them in red in
the attached schematic. What is their function ?

4- Stock, the CHB35 runs the 6EU7 with a fairly low plate voltage,
around 50-60V. All guitar amps I have data on, or have simulated
in SPICE, run at ~125V for the first stage. Apart from being
easier on the tubes, is there any reason to run the tube on such
a low Va ?

5- The most common pre-amp stage seems to be with a resistor
from cathode to ground. The voltage at this point is "usually"
between 1 and 2 V. Am I right in thinking this will limit the
input voltage range before clipping hard (when the grid goes more
negative than the cathode) ? My first tentative mod of the
input stage had a Vk of ~ 0.5 V. It sounded terrible with min
attenuation on my guitar. I measured up to 2Vp-p peaks from
the pick-ups when picking hard !

6- I haven't seen any guitar amp with a similar phase-splitting
setup as the 6C4 topology in the Bogen. However, I don't have
enough tubes to reproduce the common 3x12AX7 architecture
(1 on input, 1 for the tonestack, 1 to split phases) . Is the 6C4
"bad" for guitar applications ? Can I just feed just about anything
to the 6C4 input ?

7- What about feedback ! This CHB35 takes one of the secondary
windings, feeds it back to the last 12AX7 cathode in the tonestack
before the 6C4. If I modify the tonestack configuration (probably
will), can I just route & adjust the feedback path to the last triode
in the chain?

Well, congratulations on reading all of that ! Any help is appreciated,
I could sure use some more tube knowledge p-)

Thanks & happy holidays,
~C

 

[Tom Bavis]

The 1K resistors are referred to as "grid stoppers" - they help prevent oscillations, which are likely with high gain tubes like the 7868. You DO want to add them. I suggest removing the feedback in a guitar amp, to slow the transition into clipping. This will increase the gain quite a bit. Just remove that resistor and cap.

The split-load phase inverter works great in the linear region but has very asymmetrical distortion when overdriven - this igets much worse with feedback applied. You COULD use a 6J6 dual triode, by rewiring the 7-pin socket. Of course this will give even MORE gain...

I guess you've already discovered the "grid leak" bias (no cathode resistor) doesn't work well with guitar - I think they used it to save parts. Low noise for mike use, but can't take much input. Some like it for harp though...

 

[jjman]

That was my 1st “guitar” amp. My uncle gave it to me when I was 15 or 16 around 30 years ago. It sounded pretty good, at least to me, with the ’65 Epiphone Casino I had at the time but crapped out one day. I suspected it had tubes but did not respect the positive significance of that. I may have assumed the tubes made it antiquated and thus difficult to diagnose and repair. I did a bad thing. I tossed it. 😱 I had completely forgotten about this amp until someone posted one on another site recently.

I’m sure you can get it set up to sounds good for guitar. I’m not familiar with that PI and output tubes. What’s up with the cathodes on the 12ax7 being attached to the power supply thru 220k resistors? Is that for real? Certainly not a guitar amp approach.

 

[fenug]

Hi & Merry Christmas all,

I suggest removing the feedback in a guitar amp, to slow the transition into clipping. This will increase the gain quite a bit. Just remove that resistor and cap.

The split-load phase inverter works great in the linear region but has very asymmetrical distortion when overdriven - this igets much worse with feedback applied.

Do you mean when the 6C4 is overdriven or when overall the tube stages are overdriven?
That part of the circuit is a bit shady to me. I see the 6C4 grid biased -15V under the
cathode, which is well into the cutoff zone (based on a plate graph I may be reading
wrong). So one phase of the input signal would do nothing (already in cutoff) and the
other half would need to reach at least +15V peaks to reach Vgrid=Vk...
Ah, but then Vk must vary as the input signal changes. I'll try simulating that...

What’s up with the cathodes on the 12ax7 being attached to the power supply thru 220k resistors? Is that for real? Certainly not a guitar amp approach.

jjman : I think it's just to set up a DC bias on the cathodes ?
But you're right, the original CHB35A circuit is totally not a guitar amp.


Another discrepancy I noticed: my Bogen is lacking the 1n2@1kV cap in series with a
3k3@2W resistor between B+ on the OPT and the plate of the top 7868...
Something else to guard against HF oscillation ?


I'm still learning tons of stuff, in good part thanks to www.tubecad.com with its journal / blog. Great articles !

I'll try to have a functionnal amp this year...

Anyone got any more light for me ?

Thanks,
C

 

[Slatocaster]

1x 6EU7 input stage (electrically equivalent to 12AX7, I'm told)

Probably too late for you Fenug, but:

Not directly interchangeable without rewiring the socket. The pinouts are not the same and plugging a 12ax7 into the 6EU7 could damage the tube, the amp or both. Just getting that out there before someone reading this from now on gets a big surprise.

Also to jjman:

What’s up with the cathodes on the 12ax7 being attached to the power supply thru 220k resistors? Is that for real? Certainly not a guitar amp approach.

When I first looked at my CHB-50 schematic I saw the same thing, and thought, "How is it possible to connect the cathode to 320Vdc, even through a 220K resistor?" I knew this was not feasible and started looking for the voltage divider. Found it: on the CHB-50 the 220Kohm from the B+ is in series with a 470ohm resistor. This produces a voltage divider in which nearly all the voltage is dropped across the 220K and the cathode connected at the divider only gets about 1Vdc. The two resistors are drawn on different lines and at a right angle making the series circuit less obvious during a quick scan.

My first time here. I recently picked up a Bogen CHB-50 through an ad on craigslist. This will be my first conversion. I am in the study and planning stage right now, and am checking forums for knowledge and guidance.

What I've learned about the process is that information tends to come in piecemeal, with no one forum giving a view of the whole. I too have the 6C4 inverter and can't do the 12ax7 mod without changing sockets. NOt out of the question, but not something I want to take on now. Thanks to Tom Bavis for the 6J6 mod. I think I have one of those in an old RCA o-scope i've had for the last 30 years.

Any, just saying hello and I will be looking around, and may offer some posts of my own as I learn more about conversions.

 

[Enzo]

Hello.

let me restate your discovery another way. When I train technicians I like to point out that it is all about the circuit, not the parts. So as you found out, just seeing a resistor to B+ is not the whole circuit.


Similarly I often see discussions of "the NFB resistor", and they wonder why two amps might have very different values for a feedback resistor. What they didn;t do was look to see that in both cases, that resistor was part of a voltage divider, and that all in all, the rations provided a very similar amount of feedback.