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Post subject: Re: Noisy effects loop in Fender Supersonic Twin Combo amp
Posted: Thu Mar 24, 2016 8:40 pm
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Ok, so I don't know whether anyone is following this thread, as I know much has been written on the noisy Super Sonic effects loop, which seems to infect many copies of the 100 watt and 60 watt versions (but not the 22, because that effects loop is really a preamp out, power amp in loop - I know, I have both the 22 and the Twin). But I've been experimenting with my Super Sonic Twin and found what really is working for me is to replace the 12AX7s in the effects loop send and return (v7 and v8) with 12AT7s. The the amp remains a little noisier with the effects loop engaged than without, but rather than having a loud annoying hiss+hum, it is just a hum, no hiss, and a quiet hum at that. Not objectionable at all. Also I hear no loss of tone whatsoever from using the 12AT7s in place of the 12AX7s, just a quieter effects loop. Interestingly, swapping the 12AX7 for a 12AT7 in only one position-either v7 or v8- did NOT reduce the hiss one bit; it took a 12AT7 in BOTH positions to result in the dramatic difference, which I would call a "fix". I'm not a tech and I have no idea why it worked, but it certainly worked for me. So if anyone is still struggling with a noisy loop on a SS 60 or Twin/100 - give the 12AT7s a try, and good luck.

[Disclaimer - I'm not a tech - just a dangerous noob - and I've read the 12AX7 - 12AT7 swap is generally safe but I also understand there are significant differences in the tubes regarding plate current and voltage, as well as gain. The swap may have different consequences depending on the specific circuit and what it was designed for. So far the swap hasn't fried my amp but your results may differ!]


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Post subject: Re: Noisy effects loop in Fender Supersonic Twin Combo amp
Posted: Mon Mar 28, 2016 7:26 am
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Testing over the weekend confirmed that substituting 12AT7s for the 12AX7s in V7 and V8 dramatically reduced the noise (hiss/hum) introduced by engaging the effects loop in my Super Sonic Twin. The result was even better after I put shiny new matched 12AT7s from TAD in place of some old worn GT 12AT7s I'd been using.

From what I've gathered from the schematic and other posts, V7 and V8 have these functions within the circuit:

V7 - V7A is a buffer stage after the switching relay between clean and burn. V7B is the effects loop send buffer stage

V8 - V8A is the effects loop return buffer stage, V8B is the reverb mixer stage, which is the last gain stage before the phase inverter (V9 - 12AT7)

Does any one have thoughts on why running a 12AT7 in both V7 and V8 is reducing the hiss/hum of the effects loop? (Substituting the 12AT7 in either V7 or V8 did not reduce the noise BTW; I had to switch both 12AX7s for 12AT7s.) I would guess that the reduced noise results from reduced gain in the effects send and return buffer stages (V7B and V8A) but I wonder what other effects I might be getting from reduced gain in the buffer stage after the clean/burn switching relay (V7A) and the reverb mixer stage (V8B).

It appears that I have to turn the effects send and return level controls up higher to achieve unity gain than I had to with the 12AX7s, but I do not notice any other sonic or operational differences (other than the reduced effects loop noise).


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Post subject: Re: Noisy effects loop in Fender Supersonic Twin Combo amp
Posted: Sun May 01, 2016 8:00 am
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UPDATE
I'm still posting to this zombie thread in case my observations are useful for anyone with a noisy effects loop in a Super Sonic Twin/100 or Super Sonic 60. After reading up on why effects loops can be prone to noise, I pulled the 12AT7s from V7 and V8 from my Super Sonic Twin and replaced each with a TAD 7025 WA. Result = no more objectionable noise.

The TAD 7025 WA is equivalent of a 12AX7 and is is reviewed as having very low noise. My observations bear this out. All other 12AX7s I tried were too noisy for the effects loop send/return (new production Tung Sol, JJ, original stock Groove Tubes) but TAD 7025 WA is nearly silent.

So now the effects loop is working as it should -- I just needed to find good quiet tubes to drive it. I am glad I found 12AX7 equivalents that work well with the loop so I don't have to keep using the 12AT7s. The circuit was designed for 12AX7s and I didn't know what unintended consequences might result from using the non-spec 12AT7s.

I'm still running the volume pedal in the preamp out/power amp in loop, and find this further mitigates excess noise, without objectionable tone loss.

I hope this helps someone enjoy their Super Sonic Twin/100 or 60 as much as I'm enjoying mine.


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