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Post subject: Super Reverb Tonestack Lift Noise
Posted: Mon Jan 03, 2011 11:31 pm
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Hi,
This is my first post I am still in the learning process not that it ever ends.

I own a 71-73 SF Super Reverb it looks like it started a AB568 but now when I got it all resistor/cap values were AB763 it still has the hum balance pot though.

I was after more gain and reverb out of the normal channel I removed the vibrato and the circuit is now as follows-

Input is standard V1a is 100k plate 1.2k bias 2.2uf bypass cap V2a is standard 820/25uf bias shared with V2b.
The tonestack is .1uf/.022uf/470pf with a 4k fixed mid resistor and .047 coupling cap connected to 330k resistor wich is connected in parallel to V5a [old vibrato tube] and another 330k resistor to ground.
V5a is 100k plate 10k bis resistor then the signal goes through a .022 coupling cap and joins the reverb circuit at Z the junction of 470k/3.3m/10pf the circuit is then as normal besides two switches first bypass' NF resistor with .047cap second removes NF completely.

After all this I had a slightly different voice on the normal channel and slightly more gain but not enough for the heavy rock sound I'm after so I tried one last thing lifting the ground on the 4k fixed mid resistor what do you know there is all the gain I ever wanted and I like the tone it is slightly dark compared to the vibrato channel but it sounds fine.

The problem is 1 it is with the eq ground lifted it is to loud and introduces hiss I tried a 12AY7 in V1and a 12AU7 in v2 this seems to balance it out a bit but don't know if the bias on V2 should now be changed for the12AU7?

Secondly when you turn the volume up past six the hiss seems to be canceled out by something I dont have a scope or anything so I'm worried this could be sqaure waving or something I can't see is the signal level now way to high for the following stage?

Thanks in advance any suggestions would be of help and yes I now I should leave this stuff to someone who knows but I have to start somewhere .
Dan.


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Post subject: Re: Super Reverb Tonestack Lift Noise
Posted: Tue Jan 04, 2011 3:04 am
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What you write ,,,a hum balance pot, is a BIAS balance pot, not hum balance


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Post subject: Re: Super Reverb Tonestack Lift Noise
Posted: Tue Jan 04, 2011 3:04 am
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Try going back to a 12AX7 or even a 12AT7 for your V2. I'm guessing that the relatively low-mu 12AU7 is being driven into premature cut-off (ie: the bias is in fact excessive).

Incidentally, the last non-MV Super Reverb circuit revision was the AA270. If you're basing your mods on the AB568 you're working from the wrong schematic.

HTH

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Super Reverb Tonestack Lift Noise
Posted: Tue Jan 04, 2011 3:04 am
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stratele52 wrote:
What you write ,,,a hum balance pot, is a BIAS balance pot, not hum balance


Correct.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Super Reverb Tonestack Lift Noise
Posted: Tue Jan 04, 2011 3:23 am
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I have Lifted the tonestack ground only on the normal channel witch has increased the signal into the second half of the first 12ax7.

Sounds great but high background noise until about 6 on the volume pot after that it like square waves or something it actually cancels the noise when you play but obviously is not normal.
Is there a max input signal level I am exeeding for the second triode of v1?

V2 is the first preamp tube for the vibrato channel.


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