Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jim on February 08, 2021, 07:16:29 pm
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I modified a young friends 1974 Silverface Twin with a preamp out. He plugs that into a McIntosh 250 solid state power amp. We removed the power tubes and PI. Had a recent cap job --all 500V caps. He is going for a particular clean tone. The normal channel sounds OK but there is a tiny bit of distortion on the other channel. The degree of distortion is constant throughout the rotation of the volume control--it is not natural gain and is not a pretty sound. Lower gain preamp tubes do nothing. I have replaced all the components in the vibrato channel and the distortion has remained the same. Question: could distortion arise from lead dress? or maybe a bad pot? or maybe one of the new filter caps is bad and ripple is modulating the tone? What do you think?
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It looks like the preamp has been modded too, everyone knows that BF sounds better, right?
Well, maybe not so much; try removing the cathode bypass from the reverb driver V3 and report back. With it in place, the earliest clipping point is the V3 g1-k diode, when signal level is sufficient to forward bias it.
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That helped quite a bit. I did not know that the reverb stage adds gain whether the tank is attached or not. I re-soldered the hot lead back to the trem intensity pot and that helped more. But it still does not sound absolutely clean. I replaced the 12AX7 with a 6072 and it helped a tiny bit more but volume dropped a little. I had replaced the .1/.47 bass and mid tone caps with .022 caps but I think I need to replace the pots with 1meg/ 25K. Is there anything else I can do to make this preamp spanky clean up to 11? Jim
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In regard of this issue, the gain (ie between grid and anode) of V3 is immaterial. It’s purely due to diode clipping.
Does removing the valve in V3 help any further?
Is the V4 cathode bypass cap definitely good?
Are all the resistor values good, not drifted up too far?
Have you checked for the dreaded conductive board issue, ie that every eyelet which should measure 0VDC, does? eg tone stack caps, V4 pin2.
Have you done a voltage survey, comparing your actual measured voltages to those noted on the schematic?
May be this one
https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_100_schem.pdf
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Yep...35mv between open eyelet and ground. Look at this board...like it was slimed by an alien. I will try to lift all the components and leads off the board and listen again before I waste any more time on this.Thanks for the tip....I did not anticipate this. Jim
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Also, check and replace the filter caps (including the bias supply caps) if they are old. 'Mushy' fizzy distortion can be a bad pre-amp filter cap (check the idle voltages across all B+ nodes - no two nodes should be the same. If two of them are the same, suspect a bad filter cap)