Bogan CHB35 - Circuit question.

[LooseChange]

Curious about V2A and V2B.
It looks like they added high voltage to the cathodes through a 220k resistor. Is this correct?
See schematic.

[jjasilli]

they added high voltage to the cathodes through a 220k resistor. Is this correct?  Yes, but the cathode voltage is still under 1VDC, per the voltage chart.  This circuit is similar in the Bogen 20, 50 and 100 amps also.  This must be due to the voltage divider formed by the 220K series resistor (which you mention), with the small value cathode resistor.  The cathode resistor serves double-duty as the shunt resistor for the "injection" circuit.  However, my simple theory may be complicated by the inner workings of the tube.  Anyway, the net result is that the cathode voltage is "in the ballpark".  My guess is that the injection of +VDC allows the use of a smaller cathode resistor.  But why???

I suspect that the low value cathode resistor (in conjunction with the plate resistor) has a major effect on tone, and IMHO not a good one.  Others may disagree, but to me this particular baxandall circuit does not sound good for guitar -- way too mid-heavy and muddy.  YMMV.

[LooseChange]

Maybe it creates more headroom at the grid.  We don't do that with guitar amps. Probably could be removed.

[sluckey]

It's just fixed bias. Could have done the same thing by putting an equal but negative voltage on the grid.

[LooseChange]

But why do that?? It would work without that just fine.
I do see that the preamp tubes are "Grid biased" (I think that's what it's called).
Thanks!

[PRR]

That was a Bogen hallmark. Note that the 220K bleeder current is as-large as the tube current, often more. As Sluckey says, nearly "fixed" bias. It allows a small cathode impedance and high gain without a cathode cap. It may also reject B+ ripple, similar to John B's "Akido" thinking.

The "grid-leak" inputs will handle up to 0.1V inputs well. Mikes in Bogen systems would rarely have more. If they do, the input stage overloads semi-gracefully: the cap charges from positive peaks, forces the grid negative, reduces tube current and thus gain. It's not a great sound but may be less rude than outright clipping.

[jjasilli]

It's just fixed bias. Could have done the same thing by putting an equal but negative voltage on the grid.

Well, altogether it's not just fixed bias.  But yes, Bogen could have done the same thing with a (-)bias on the gird.  Altogether, it's mixed bias -- part cathode bias + part fixed bias.  For DC operation: with 240V B+ > 220K plate resistor > 12ax7 plate, we could to guesstimate maybe a 3K cathode  resistor to get the cathode (+) enough for the grid.  But Bogen's cathode resistor values seem well under 800R (hard to read).  That's hot bias.  For colder bias with small K_r, either :  a) inject a negative fixed bias voltage on the grid (as sluckey says); or B) inject a (+) voltage on the cathode.  So, now Bogen hits it's DC operating point.  

AC (signal) Operation:  Frequency response of the circuit is affected by this choice.  The value of the cathode resistor acts in conjunction with the (plate impedance║plate resistor, etc.) to affect frequency response.  So this circuit will sound different than the pure cathode bias we usually see, because the different value cathode resistor changes the AC response of the circuit.  

Try it and see if you like the stock tone circuit.  If not, maybe a James or baxendall as cooked-up by Geezer & Tubenit is worth a try.

[67polara]

I have a CHB-50 it sounds terrible as a guitar amp.  No base no high's great for voice sucks for guitar, but at least it's more parts for another project LOL.

Tony

[jjasilli]

It's possible I'm being overly critical of the tonestack in these amps.  V1's plates are fed by huge 470K plate resistors with only about 45 plate volts.  (PRR pointed that out some time ago.)  Not exactly gutar friendly; and could darken and muddy tone befoe it gets to the tonestack. 

[SoundmasterG]

That was a Bogen hallmark. Note that the 220K bleeder current is as-large as the tube current, often more. As Sluckey says, nearly "fixed" bias. It allows a small cathode impedance and high gain without a cathode cap. It may also reject B+ ripple, similar to John B's "Akido" thinking.

These amps were pretty quiet as far as hum is concerned in a stock configuration. My modded one so far is hummy is certain preamp setups.

I have a CHB-50 it sounds terrible as a guitar amp.  No base no high's great for voice sucks for guitar, but at least it's more parts for another project LOL.

The CHB100 was the same...pretty lousy and uninteresting for a guitar amp stock. But for $30, I wans't complaining, and it gives me a good mod platform to try stuff out on. So far it is coming together and sounding quite nice, if a little hummy. One thing at a time though.

Greg

[sluckey]

Well, altogether it's not just fixed bias.

I didn't mean to imply that the tube was operating with 'just' fixed bias. I was specifically talking about the high voltage applied thru the 220K to the cathode. That is just fixed bias.

Any current that flows thru the tube will also cause a 'self' bias (AKA cathode bias) voltage to be developed across the cathode resistor. The net result is that the tube 'operates' with a combination of fixed and self bias.

[PRR]

> not just fixed bias ... mixed

Do math. All the clues are there.



Say 2/3rd fixed.

We can also look-up the tube parameters and estimate that at 119V 0.64mA a 12AX7 cathode is about 770 ohms, so the 470 ohms gives only small degeneration.

[LooseChange]

Thanks.
But why go thrugh all that extra trouble? Is it gain they are trying to achieve or headroom?

[PRR]

What extra trouble? It's cheaper than a cap.

[jjasilli]

Do the math. Got me again!