RCA Tube amp tone stack

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TubeMonkey

TubeMonkey

Well-known member
Joined
Jan 24, 2014
Messages
164
Location
Nashville, TN
Hey All,

I've just brought an RCA MI-12223 PA amp back to life as a guitar amp. The last guy who had it stole the mic transformers off the front....  It actually sounds pretty great as a guitar amp but i'm unhappy with the tone stack....  I had the idea of implementing the simple bass/treble tone stack from a 5e6-a bassman but don't know quite how to implement it in this circuit. I thought I would throw the idea out there to see if you guys had any suggestions. I've changed the feedback resistor to 27k which has opened up the presence a lot but the bass and treble knobs are still pretty useless. Thanks for any ideas in advance!  Schematics to follow!
 

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That tone stack comes off the plate so I would copy a fender twin tone stack. Then just fiddle with the caps till you find something you like.
 
Check out Duncan's Amp Tools  application 'Tone Stack Calculator'  http://www.duncanamps.com/tsc/

It's free and quite useful for tone stack noodling  :)

I quite like some of the 2-knob  fender stacks for instrument amps  and the baxandal/james stack for hifi.
 
TubeMonkey said:
I've just brought an RCA MI-12223 PA amp back to life as a guitar amp. The last guy who had it stole the mic transformers off the front....  It actually sounds pretty great as a guitar amp but i'm unhappy with the tone stack....
Click to expand...
It is actually a cut-only tone control, resulting in a mid-accented response, contrary to a typical guitar amp response, which is scooped in the midrange.

  I had the idea of implementing the simple bass/treble tone stack from a 5e6-a bassman but don't know quite how to implement it in this circuit.
Click to expand...
It's quite easy; you just insert the Fender-style tone stack between the anode of the 3rd triode (at R27) and the grid of the 4th tube. You need to disconnect C16, 17 & 18, and R20, 23 & 29, as well as the pots.
S2 could be used as a midrange control, but one step at a time.


I've changed the feedback resistor to 27k which has opened up the presence a lot
Click to expand...
That is quite surprizing, since it should only reduce gain and not have significant effect on the frequency response.

but the bass and treble knobs are still pretty useless.
Click to expand...
That is not surprizing, since the amount of cut is only about 3dB at 80Hz, the lowest note on guitar.
 
abbey road d enfer said:
That is quite surprizing, since it should only reduce gain and not have significant effect on the frequency response.

Hi all, just an update.

I ended up sticking in the tone stack from a Fender twin 5c8 because it was a similar plate follower design. Sounds good! Comparing to my AB165 bassman which is also a 6L6 power section and its pretty low output.  I'm still relatively new to some of the push pull power section stuff...... 

Abbey Road what can you teach me about the feedback loop? Less resistance=more gain (until it blows up?)  Yes when i changed the feedback resistor it definitely changed the presence of the amp....
Click to expand...
 
TubeMonkey said:
Abbey Road what can you teach me about the feedback loop? Less resistance=more gain (until it blows up?)
Click to expand...
Assuming you're referring to R45, going from 82k to 27k should reduce gain by about 8dB.

  Yes when i changed the feedback resistor it definitely changed the presence of the amp....
Click to expand...
That is strange, since only teh gain should change, with only very minor effect on the frequency response.
 
abbey road d enfer said:
Assuming you're referring to R45, going from 82k to 27k should reduce gain by about 8dB.
That is strange, since only teh gain should change, with only very minor effect on the frequency response.
Click to expand...
I am NOT the tube guy here but if that global feedback changes the amp's source impedance it could affect interactions with speaker/cabinet loads.

Just my uninformed guess, unless you are not normalizing for level/gain.

JR
 
JohnRoberts said:
I am NOT the tube guy here but if that global feedback changes the amp's source impedance it could affect interactions with speaker/cabinet loads.
Click to expand...
Indeed, the actual output Z of the amp would go from maybe 1 ohm to 0.3 ohm, which could  attenuate the speaker's LF resonance by a fraction of dB, I don't think it makes a significant difference.

Just my uninformed guess, unless you are not normalizing for level/gain.
Click to expand...
That's really what I suspect. The poster's opinion should be substantiated with measurements.
 

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