AC30 on the bench : volume drop drives me crazy

[archaos]

Hi guys,

             My latest AC30 building which sounds awesome has a volume drop issue, yet. The amp powers up & play, but occasionally half volume pops for some times, then we're back to full volume.
             It tends to happen either with the Bright or Normal channel & sometimes there is a little static and crackling before a volume drop. My OT is a Hammond 1750V, & I've already tried a lot of things
             to find out the issue. Unfortunately it remains unsuccessful :

               - checking for bad/cold solder joints - OK
               - checking the cathode resistor + bypass cap - OK
               - checking the input grid of power tubes - OK
               - measuring the resistance of triode & power tubes cathodes - OK
               - the shielded wire is OK : it doesn't touch the core wire
               - continuity test is OK everywhere
               - chopsticking the amp while using a CD player as a generator gives nothing
               - both OT secondary & supply voltage are OK too
               - swapping the triodes doesn't change anything

Nonetheless, I've noticed something interesting when I've wanted to make another voltage chart (with the CD player plugged) : sometimes when touching some components leads (PS caps, plate/grid resistors...not the cathode bypass components yet) with the probe, full volume pops back but not always...

Any suggestions/thoughts ?
I've already spent more than 20 hours bent over the amp, & my back is broken.
I know that I should suspect my work first, but could it be due a bad component for instance ?

Thanks.
               

[kagliostro]

Have you tested the potentiometers ?

input female jack are ok ?

Kagliostro

[archaos]

Have you tested the potentiometers ?

input female jack are ok ?

Kagliostro

Thanks for the tips.

That's done too. Though, as far as the problem occurs either with Hi/Lo Brilliant or Normal channels, I think it doesn't concern the jacks/pots.

[Merlin]

I had a problem exactly like the one you describe, also in an EL84 amp. It was due to intermittent screen-grid failure. I replaced the output valves and upped the screen resistance to make me feel better, and all was well.

[archaos]

I had a problem exactly like the one you describe, also in an EL84 amp. It was due to intermittent screen-grid failure. I replaced the output valves and upped the screen resistance to make me feel better, and all was well.

Merlin, thanks a lot !
Firstly, I must say that I'm a great fan of your website : http://www.freewebs.com/valvewizard/ Excellent information !
Then, I'm both puzzled by your remark & very happy because I was very far away from thinking about a power tube issue. IMO, it could be due either to the phase inverter section, either the OT or the power section.
Anyway, I will swap the power tubes with my other AC30 so as to listen to what happens. My control grid resistors are 1K5 1/2W Allen Bradleys ; the screen grids are 100R 2W Allen Bradley CCs.

I'll report back...

[archaos]

Guys, I'm the happiest now thanks to Merlin.   :hello2: :hello2:    I have made a test using his suggestion :

  - last Sunday evening I have swapped the EL84s in my AC30s
  - I have used my amps +/- 2 hours a day since Sunday & all is well : no more volume drop or crackling

Yes, he's a wizard !

[tubeswell]

Yes tubes is the first thing you should swap in a tube amp when it starts making funny noises. Crackling/ hissing/ popping are well-known symptoms of output tubes on their way out, especially EL84s.  You should always carry around a spare set anyway.

[archaos]

You should always carry around a spare set anyway.

Yes, sure !

[FYL]

You should always carry around a spare set anyway.

Plus a spare set of amps jut in case. Voxen love to self-destruct on stage.

[Fresh_Start]

You should always carry around a spare set anyway.

Plus a spare set of amps jut in case. Voxen love to self-destruct on stage.

Do you offer a "second amp for $500" discount for working musicians?

Chip

[archaos]

Do you offer a "second amp for $500" discount for working musicians?

Chip

I would even accept a kit for that price ! 

[archaos]

I had a problem exactly like the one you describe, also in an EL84 amp. It was due to intermittent screen-grid failure. I replaced the output valves and upped the screen resistance to make me feel better, and all was well.

Hi friends, I'm rather curious, & thinking again about that (endemic ?) EL84 screen-grid failure concerning AC30s in general on the one hand, & since a buddy of mine has seen the 100Rs burning out in his AC30 too   (see below) on the other hand, I was wondering what was the best way(s) to reduce screen stress ?

Yep, I've already heard of 1kΩs, but what you guys would you recommend ?

Likewise, in which manner 1kΩs for instance would affect the sound ?

 

[archaos]

Anyone ??? Come on !

[octal]

Randall Aiken (who seems knowledgeable about tube theory) did some experiments replacing the screen resistors in EL84 amps with larger values and claims that there was no detectable difference in tone. I think he went up to 1.5 or 2K. It's a simple & inexpensive change- why not try it and see what you think?

[archaos]

Randall Aiken (who seems knowledgeable about tube theory) did some experiments replacing the screen resistors in EL84 amps with larger values and claims that there was no detectable difference in tone. I think he went up to 1.5 or 2K. It's a simple & inexpensive change- why not try it and see what you think?

Thanks for your input. Yep indeed I'd read that excellent tube comparison using different values for the Rs on aikenamps.com

Moreover, I've just had the luck to find a discussion about that subject, see attached.