Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Voxbox on February 09, 2011, 04:53:07 pm

Title: Acceptable Cap Leakage in a vintage AC30?
Post by: Voxbox on February 09, 2011, 04:53:07 pm

I have a small dilemma...

I have a friends '63 AC30 in for repair as it has DC on the treble and bass pots which is making them scratchy. Easy enough to fix as the caps in that part of the circuit are leaky (150V caps in a 166V circuit...doh!)

Anyway, in checking around for other leaky caps, the two anode DC blocking caps on the PI are leaking so that there is a DC voltage of 0.8V on one and 0.4V the other on the output side feeding the power valves.

The two anode caps on the Bright /normal input valve are also leaking with 168mV and 0.4V on them. (or is that out of them?)

I'm reluctant to go changing anything in this amp as its near immaculate and original and also sounds really good.

So the question is, do I change the leaky caps with these amounts of DC leakage?

And if so, is there a good make of cap to use that sounds the same?  The caps on the PI are 0.15uF/630V WIMAs.

I also have to change the 16+16uF main smoother as the original is ruptured and dribbling, but I can use an F&T brand cap for that.
BTW, I havent run the amp at full power yet to see if I get ripple on the output.

Would the other electrolytics be worth changing as a matter of course? I would have thought the power supply caps would almost be essential,  but would others like the cathode caps be necessary to change?
 :dontknow:
help!
Thanks

Title: Re: Acceptable Cap Leakage in a vintage AC30?
Post by: FYL on February 09, 2011, 05:27:10 pm

Quote

So the question is, do I change the leaky caps with these amounts of DC leakage?

Leaky coupling caps offset the bias point, the amp will run even hotter... I'd change them and keep the originals - useful if the amp is to be resold and put back to original.

Quote

And if so, is there a good make of cap to use that sounds the same?  The caps on the PI are 0.15uF/630V WIMAs.

Today's Wima caps use a very different form factor, none come in tubular axial form. I'd use quality axial tubular MKT caps.

Quote

Would the other electrolytics be worth changing as a matter of course? I would have thought the power supply caps would almost be essential,  but would others like the cathode caps be necessary to change?

I'd do a full cap job and change all the 'lytics.

Title: Re: Acceptable Cap Leakage in a vintage AC30?
Post by: Voxbox on February 10, 2011, 02:49:52 am

Thanks FYL,

Good point about the bias.

Agree about the electrolytics, they are all past their sell-by date.

So I'll do one at a time and see if the sound changes.

However, the Engineer in me wants to make things as good as possible, but the Guitarist in me says if it aint broke dont fix it!

So the power supply caps are a deffo to change, and go more cautiously on the other caps.

Cheers, and thanks for your advice. :smiley:

Colin