Question on Gain of "Comit 60" vs. AB165 Bassman?

[ChrisGS]

Hi,

I was looking at some of the Francesca drawings and also of the "Asteroid" and comparing them to Fender Bassman schematics for AB864 and AB165 , and I'm confused as to where all of the extra gain comes from ?

In the case of the "Asteroid" layout, some of the values seem as though there would be less gain...As in the case of the first and third gain stages cathode resistors. An other thing that I can see is that the resistor going to ground at the grid of the third stage is 220K on the Fender and 330k for the asteroid

I see that theAA864 has some oddball values for the volume bass and treble pots, but the AB165 has the same values for the pots as in the "Asteroid" drawing.

I'm sure that I'm missing somthing , but it seems like the preamp circuits going into the phase inverter of both the "Asteroid" and the Bassman should be similar in their gain...aside from the tone stack voicing.

So where does all of the gain/drive come from in the circuit ? I see that there is one resistor different in the phase inverter (22K vs. 10K) and the NFB resistor is 47K on the "Asteroid" vs. 820 ohms on the AA864 ( but, the AB165 has the same 47K NFB resistor as in the Comit)...I guess the other thing is the EL-34's vs. the 6L6 tubes.

The reason why this intrests me, is that I have a Bassman chassis that I want to do something with and I have a Replacement Super Reverb power xformer that I think should have similar voltage to the "Asteroid", so I want to try to build a Comit/Bassman...

I was originally thinking that if the amp won't be stabil, I can bring the pre amp back closer to Fender values, but there doesn't seem to be a very big difference between the Bassman and the "Asteroid" values.

Back to my question, where does all of the gain and drive come from in this circuit?...And why isn't everyone talking about screaming wailing Bassman heads.

It seems kind of funny that one of the most humble tube amps could have so much in common with such a High-end expensive amplifier.

Could it be , that the only big differences are a few resitor and cap swaps and an output transformer?

Chris.

[keithrick]

The AB165 has a global NFB loop which could account for less gain. I have mine adjustable on my AB165 Bassman and Trust me there is plenty of gain on tap if you crank it!

BF AND most SF Bassman's are badass amps!

[bnwitt]

I didn't think the Komet 60 was a high gain amp. All of the clips I've heard aren't high gain at all, so I'm not sure where that label comes from.

[ChrisGS]

I heared the Missisippi Queen clips on the Komit site and I thought that there was a lot of fat sustain. Although a never heared the Komet in person, I understand that it's supposed to be right there with an Express, and I did get to hear a couple Express clones and they were awesome.

I guess if high gain is the wrong term , I'd definitely say that these amps are very sustain-y

[dobbhill]

The local feedback (470K resistor) around the 3rd gain stage in the AB165 Bassman kills the gain.
HTH, D

[bnwitt]

The local feedback (470K resistor) around the 3rd gain stage in the AB165 Bassman kills the gain.
HTH, D

That makes sense. I suppose eliminating it, or increasing the value would bring the gain up. One could take a 1,2 or 3 meg linear pot and substitute it in place of the 470k resistor and dial in the gain I suppose.

[Buschman]

Don't the Express, Liverpool, & Comet have an extra triode as combared to the Bassman??

[bnwitt]

The comet has three triodes prior to the Phase inverter just like the AB165. The AB165 does have two channels though but no more gain stages. That's the same thing on the express and liverpool, three triodes pre PI.

[ChrisGS]

There are a few different Bassman head schematics with different wacky variations (differnt locations in the NFB loop , extra caps, resistors from the input to the output of the same tube)...The AB165 has three stages in each channel as in both Bass and Normal channel go through the third pre amp stage...The AA864 on the Bass channel, the signal goes through the third pre-amp stage, but the Normal channel only gets two stages.

The Bass channel on the AA864 looks the most similar to the "Asteroid" drawing,(it doesn't have those feedback resistors across the third stage or across the power tubes) but I was surprised that there isn't a bunch of resistors in series or bleeding signal to ground.

Oh by the way , thank you Bnwitt for the beautiful layout drawing of the Asteroid...

Maybe everyone should call these amps Asteroids and say that they are Hot-Rodded Blackface Bassman inspired amps and not refer to Comit at all.

I'm going to order some stuff to try and put this thing together eventually.

Thanks,
Chris