1968 vs 1969 Super Lead sound

[shakti]

Yep, something is wrong with the '68. Could be a signal cap going bad. You could check the de-coupled side of the caps for the preamp and tonestack for the presence of dc voltage using a multi-meter. How's the lead dress on the '68?

I would think the lead dress is OK, I have done things fairly similar in all my amps which have generally been very quiet and stable. Grid wires are generally kept well away from heater wires or plate wires. I was always able to crank this amp with all controls on 10 without any oscillation. I can try to check for DC on the caps.

[shakti]

Forgot to mention one thing; between video 1 and 2, I had the amps on standby while I took a break to recharge the phone. When I flipped standby again I got a terrible howl from one of the amps...but I was so surprised I didn't notice which one, just flipped them back off, then turned down the volumes and tried again. The noise was gone then, but neither of these amps have *ever* done anything like that, they've always worked without any extraneous noise. Now assuming it was the 68 that did this, perhaps it's another sign that something is going bad? The noise did continue even after I flipped the standby switch, FWIW, so as long as there was B+ present and the filter caps were discharging the noise continued, but that's probably not telling us much.
In any case, I've never noticed the 68 sounding as bad as it did in those two last clips, so something must be going on.

I've had a quiet evening at work (knock wood), so had some time on Youtube Learning more about the principles of tube amps. I am too much of a hack, just painting by numbers and pushing my luck. I need to develop a deeper understanding of how these things work, but I am getting there slowly...

[Carbia]

I don't know how you recorded the clips, but both sounds bad to me

Really? The 69 sounds OK to me...remember it has 6550s and a 12AT7 PI tube. The recording leaves something to be desired though, it's just an iPhone into a cheap Fostex interface, so that puts its own tonal imprint on things. If you see any of my other videos you will probably get an idea of what the recording itself does to the sound.

ok, being recorded with the phone could be the point.

I hear some bright harsh in both amps, but could be the recording.

Check the caps, even without DC leakage it could sound terribly bad.

Some months ago I received a '69 SB to repair.
I fixed it but when I tried the amp, the sound wasn't great at all.
The RS mica cap on the tone stack was bad. There was no leakage, but I was changing all the caps in the amp, one by one, for new ones until I reached the RS cap. When I changed it for a cheap ceramic one, the amp started to sound GREAT. Then put all the old good ones in place and job done.

Another thing that you can do before, that is easy and helps a lot is plugging the '69 preamp into '68 power amp, and then the 68 preamp into 69 power amp.
Doing that you can know if the problems comes from the preamp or the power amp.

[Tazin]

Forgot to ask this earlier....Do both channels on the '68 have that distortion?

[shakti]

That "new" distortion that you hear particularly on the Strat clip may not be easy to hear in the normal channel since it is so rounded and muffled sounding, but the early breakup is there.

[shakti]

The OT seems to check out fine, I get 17 ohms on one side and 18 on the other.

I get no DC on the output side of either of the first stage coupling caps (did not desolder). I desoldered the phase inverter output coupling caps from the output side, and get no DC on those. I also tried to desolder the wire to the treble pot from the 560pF silver mica in the tone stack, and clipped in a new production silver mica, no difference there.

I do get DC on either side of the 0.1uF feeding into the phase inverter, but looking at the schematic that seems to be OK? I had 40V on one side (by the 1M resistor) and 20V on the other side (by the 10k resistor).
Can I check the other caps in the tone stack without desoldering?

[shakti]

Sorry, misread the schematic...should I be getting DC on either side of that 0.1uF cap? It's the one on the board, not the one across the presence pot.

[shakti]

Well, I've gone over the voltages now, and in the B+ line they are pretty much the same for both amps. Same DC voltage readings on either side of that 0.1uF cap in both amps. I have DC on the HV side of the caps in the tone stack, but none on the opposite side. So those would also appear to be OK.

Bad bias cap??

[Carbia]

Remember, even if there's no DC leaks... the caps can sound BAD.

I have a couple of mustards here that test ok but sounds horrible...

[shakti]

I suppose so, but I hate shotgun type of amp fixing...surely, there must be some sort of technical way to determine if the cap is good? I have that cheap cap tester in the mail that you mentioned earlier in the thread.

Two things that I haven't tried; I've never tried to replace the PI filter cap which is a F&T 50+50. IIRC I did try to parallell in another 50+50 just to see what that did and it didn't do much. And I haven't looked into the bias circuit. It biases up fine, but I suppose ripple on the bias feed could cause some distortion?

[Tazin]

As others have suggested, I think jumpering in the '68 preamp to the '69 power section; and vice versa, in order to isolate where the early breakup is coming from should be your next step.

[RACKSYSTEMS]

Not sure if this was brought up but are there resistors on the series screen caps? What value and are they on both amps?

[Carbia]

I suppose so, but I hate shotgun type of amp fixing...surely, there must be some sort of technical way to determine if the cap is good? I have that cheap cap tester in the mail that you mentioned earlier in the thread.

Two things that I haven't tried; I've never tried to replace the PI filter cap which is a F&T 50+50. IIRC I did try to parallell in another 50+50 just to see what that did and it didn't do much. And I haven't looked into the bias circuit. It biases up fine, but I suppose ripple on the bias feed could cause some distortion?

First, do what I said earlier. Plug the 68 preamp into 69 power amp and see what happens. And the opposite.
In that way you will know where is the problem, in the preamp or the poweramp.

[Tazin]

Not sure if this was brought up but are there resistors on the series screen caps? What value and are they on both amps?

Curious as to what roll the balancing resistors on the screens filter caps might play?

[Carbia]

Lower value resistors increase capacitor leakage and sag. And, can contribute to ghosting.