Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: goldstache on October 28, 2015, 07:38:59 pm

Title: Problem with 6sl7 as V1
Post by: goldstache on October 28, 2015, 07:38:59 pm

I built a real charmer of an amp.  Only thing is that when preamp (gain control) and treble are at max my preamp cuts out completely. 
My topology is:
Input V1 > Gain > V2 > fenderish TS > LTP 6sl7 PI > Master Volume > 5881's cathode biased UL screens.

Is my phase inverter being bombarded or lead dress????

Tweaks that may give you all some insight:
I changed the gain of V1 as there was some oscillating happening at same settings described above.  I have 220k on the plates and I had an 820E cathode resistor.  Bumped cathode up to 1k5 and it shut it up and focused it a bit.  But with gain and treble maxed it completely shuts off (silent).  If you back off either gain or treble (10 to 9) it disappears and is back to being sweet.  Master Volume which is PPIMV, does not effect the phenomenon so I'm sure it's in the pre.
Any ideas?
Thanks

Title: Re: Problem with 6sl7 as V1
Post by: Paul1453 on October 28, 2015, 07:48:19 pm

Could you post a schematic?  Might help if we knew the specific details of your circuit.

Title: Re: Problem with 6sl7 as V1
Post by: Willabe on October 28, 2015, 07:57:24 pm

Does it have NFB? If so temporarily disconnect it and see if the problem goes away. 

Title: Re: Problem with 6sl7 as V1
Post by: shooter on October 28, 2015, 08:36:19 pm

Quote

If you back off either gain or treble

I've had more than 1 pot be *dead* at the extremes, ohm it and see if it goes open.

Title: Re: Problem with 6sl7 as V1
Post by: sluckey on October 28, 2015, 08:57:50 pm

Try a 4.7K on the cathode of V1. Any better?

Title: Re: Problem with 6sl7 as V1
Post by: PRR on October 29, 2015, 11:29:55 pm

> gain and treble maxed it completely shuts off

Tack 220pFd from plate to cathode on some stage, then another.

Keep your OT plate leads FAR from the small tubes, especially the input and the post-tone/volume tube.

There is a total gain of 10,000 from jack to OT leads. That means even 0.01% sneakage back to the input WILL oscillate. High (even supersonic) frequencies sneak best through thin air. The oscillation quickly builds to high level, swamping signal and/or bias.

Title: Re: Problem with 6sl7 as V1
Post by: goldstache on October 30, 2015, 06:29:36 am

Thanks everyone.  I suspect it's parasitic like PRR suggests.  Tried lowering gain, and checked all pots.  Won't be moving OT (tight combo chassis).  All signal wires over 2" are shielded and physically moving them doesn't change the oscillation.  I will try the cap P to K and report back!  Thanks!

Title: Re: Problem with 6sl7 as V1
Post by: sluckey on October 30, 2015, 07:15:15 am

Have you tried 4.7K cathode resistor yet?

Here's the schematic for an amp I have that uses 6SL7s. It was converted from a Rockola juke box amp. I used 4.7Ks because that's what was in the original design. I don't have the problems you are having.

Title: Re: Problem with 6sl7 as V1
Post by: Paul1453 on October 30, 2015, 04:01:30 pm

Quote from: goldstache on October 30, 2015, 06:29:36 am

Thanks everyone.  I suspect it's parasitic like PRR suggests.  Tried lowering gain, and checked all pots.  Won't be moving OT (tight combo chassis).  All signal wires over 2" are shielded and physically moving them doesn't change the oscillation.  I will try the cap P to K and report back!  Thanks!

I was looking at some designs with the 6SL7 and then I check my tube inventory and I didn't have any.  So then I went looking for subs for the SL.  It appears to be a very high gain tube along the lines of the 12AX7, but as a PI some of the things I read were that a 6SN7 is a much lower gain sub that would work in the PI spot where the max gain is not necessarily needed.  I could be mistaken, and I'm sure sluckey would have a much better understanding of the issues here.  But I'm thinking that maybe you could just swap in a 6SN7 and things would be better.   :dontknow:

Now that I checked sluckey's rocky schematic, I see that it has the 6SN7 pulling PI duties.  If you have a 6SN7 tube and put in the PI spot, you would have a spare 6SL7 to switch in the V1 spot.  This might help determine if it is an issue with one of the SL tubes or something else.

Title: Re: Problem with 6sl7 as V1
Post by: goldstache on October 31, 2015, 07:21:09 am

Paul, thanks.  Great ideas.  I swapped many tubes and also lower gain octals and even adapters to use Noval.  (5751 being the closest match to 6sl7). Problem persists in V1 I'm pretty sure.  I will tinker a bit today and see if I get anywhere with it.  Thanks all!

Title: Re: Problem with 6sl7 as V1
Post by: goldstache on October 31, 2015, 09:25:17 am

Alas the problem is gone.  PRR had it nailed 250pf was all I had on hand.  But it no longer swamps next stage!  I applied it to V1A P and K.
I will be more attentive on lead dress next build.  Amp has floated heaters, but OT's layout kind of straddled the preamp so, it probably started there as it's pretty high gain for it's topology!
Back to rocking!!!!