Superlead hiss at high volume

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Elad E
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Posts: 137
Joined: Sat Jun 29, 2013 9:19 am
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Superlead hiss at high volume

Post by Elad E » Wed Aug 05, 2015 6:13 pm

Hi,

I recently incorporated a few changes in my 1969 Superlead build and noticed RF-like hiss on channel 1 when volume 1 is at 8 or higher. volume 2 has much less noticeable hiss when on 8 or higher. plugging in a guitar makes it worse (including humbuckers).

I'm pretty sure the amp was dead quite when it was bone stock (as per the Metro kit instructions except for a Lar/Mar PPIMV and the split/share cathode switch) but since completing it I lowered the filtering in the PI to 50uf, the screens to 32/32 (with the 2 watt 56k balance resistors on) and the preamp cap to 32/32. mains filter caps remain 50/50. I also changed the V1b cathode from 2.7K/.68 to 820/.68.
switching between split and shared cathode merely changed the hiss so I guess it's probably not the split cathode or the switch. also have a V2a .68 bypass cap.

I also changed the PPIMV Alpha pot for a PEC pot which made a world of difference to the sound but the hiss seems to have gotten more noticeable at about the time I put the PEC in.

when the PPIMV is set anywhere below 90%, or the volume is set below 8 the hiss disappears or becomes negligible, which makes me think the problem stems from the preamp section.

for the record - I tried taking out the 250pf bright cap, tiding up the PPIMV, changing the way the presence cap is wired as to make sure it's not acting as an RF antenna, took out the old 440pf Lemco silver mica mixer cap and changed it for new production silver mica, tried moving around the leads going to V1-3 etc. with only a minimal change in the hiss.
the amp is in a head case with an aluminum plate and yet when I pulled my home built JTM 45 out of it's head box for a bias two days ago - it was dead quiet on 10 (I got a fluorescent and a Mac plugged in to the same power strip).

also, when the Superlead is out of the head case and taken out of Standby there's an audible electrical buzz emanating from the rectifier diode bridge/snubber cap area (which doesn't get amplified).

attached are two pics I took prior to the elimination process described above.
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Elad E
Senior Member
Posts: 137
Joined: Sat Jun 29, 2013 9:19 am
Just the numbers in order: 13492

Re: Superlead hiss at high volume

Post by Elad E » Thu Aug 06, 2015 5:25 pm

well, I found Ken Fisher's advice about the preamp tubes' grid leads - raised them from the chassis as far as possible and there is definitely less hiss at high volume now though still not as quiet as my JTM 45.

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