Ace Peppers P-Matic - slight amount of hiss

[txstrat]

I've mentioned it before, but this is the most quiet tube amp I've ever heard, BUT only in the drive channel.

Since it is at a very low level I wouldn't bother with the hiss if it was in both channels (that way I would have thought it's the circuit) but my build does hiss in the clean channel only. As long as I know it could be quiet... well, you know what I mean.

As you can see in this schematic (http://www.sonicdeli.com/ThunderTwea...o/P-Matic2.pdf ), the drive channel adds one more gain stage and I would have expected the drive channel to develop a hiss rather than the clean channel.
Instead the drive channel is absolutely quiet.

I've used chielded cable from the input to the first grid and from the clean volume pots wiper to the 10K grid stopper of the cathode follower stage.

I've got the idea the hiss might be from the "drive channel stage" when it's grid connection is grounded and the rest of this stage amplifying a hiss that strays into the signal. Does this make sense?

@Ace Pepper: does your amp do the same?

Any ideas?

Thanks

Matt

[tubeswell]

Hi Matt

Have you tried using metal film plate resistors? (or even just swapping tubes around first?)

[txstrat]

Yes, both. Thanks.
What makes me wondering is the fact that the hiss goes away completely, when I switch in the second stage. I assume it has to do with the second stage then?

[tubeswell]

It could be the way that 2nd stage is set up. Theoretically, with the relay on the "NC" setting, the plate AC signal should be grounded, so any hiss ought not to be coming from that. But if the relay was left open instead of being grounded, then the plate of that 'o/d' stage would be free to randomly hiss and splutter I guess. How is the relay really set up?

[txstrat]

I don't use a relay, just a DPDT switch. The plate signal is grounded in between the 1M and the 220k resistors, like in the schematic. When I throw the switch the ground connection flips over to the clean volume pots wiper (see pic).
The 100K resistor between the treble wiper and the o/d channel master pots input is not in the schem but in the layout drawing. I assume it's for attenuation reasons.

[txstrat]

I still can't get rid of the hiss in the clean channel mode.
Have changed the switchable ground to the grid of V1B. Still hiss.
Disconnected the plate of V1B. Still hiss.
Changed the cable from the switch to the cap before the PI with a shielded one. Still hiss.
Changed all tubes again (not the power tubes). Still hiss.
And still if I switch to the dirty mode, the hiss is completely gone.

[tubeswell]

Just wondering if the problem is with the DPDT switch you are using to ground the OD channel. If might be just a 'bad', unshielded part of the signal chain. If you bypass the switch, does the hiss go away?

[txstrat]

If you bypass the switch, does the hiss go away?

It's worth a try.
What I also noticed when I threw the std by switch into play mode was some sputtering which went away after a couple seconds. Don't know if it was one of the other tubes.

[txstrat]

Got rid of the sputtering . Was the 100K resistor across V2 (cathode follower). It was way out of spec (measured 200+K). Changed it and the sputtering was gone. I haven't changed the switch yet, cause the hiss is also less than before. Think I leave it as it is, as long as I don't feel my oats.