Pro Tube Twin amp recap - trust the schematic?

[Stratoblaster]

Hi everybody,

Backstory: A few weeks ago I picked up a minty 2004ish “pro tube” “Twin Amp”. ( It’s the model with the single input jack, dirty channel, 1/4 power switch, fx loop, bias test points and adjust pots, etc. ) It had been plastic film wrapped and placed in storage for over a decade. I got it home, found a mains cord for it, and fired it up. No real issues per se, however I noticed when I played through it with the 1/4 power switch engaged the PT got quite hot after 1/2hr of play time. 150F plus I’d guess? After 2 hours of play on full power, the PT was barely warm above ambient.

I decided I’d like to keep the amp, so I ordered an FCK-17 recap kit from Amp repair parts. This came with (4) 47uF 500V and (3) 22uF 500V caps, which appears to match the schematics I’ve found.

So here’s my dilemma: When I went to install, I pulled (4) 47uf 500V, (1) 22uF/500v, and (3) 47uF/350v caps from the board.

So I’m wondering if I’m good to trust the schematic and install what I’ve got on hand, or order up some more 47uF and put it back like the factory shipped it? Was there a reason Fender went to a larger value or maybe it was a goof-up at the factory? Appreciate any thoughts here.

And yeah… I’m not sure I’ll ever be buying another PCB Fender amp like this if I can help it. I recapped my 63 VVRI PCB, but this beast is a real pain to get the board out in comparison.

Thanks,
Chris

[glebert]

I guess I would ask why you would recap if it is working OK? It's not old enough that I would replace caps "just because."

[Stratoblaster]

Yes, I get your point. I’m a fan of leave well enough alone in general, but I had understood the IC caps to be from the electrolyte plague era, and figured it would be worth $70 and a couple hours time to recap for an amp I was going to hang onto. (I was emboldened by recent recap of my VVRI, which was a hassle, but nothing terrible.) Also was curious if I might see a change in the PT temp at 1/4 power. Figured that would get me to a good baseline and maybe avert some future failures, since this amp is more complex than what I’ve worked on in the past.

[nosaj]

Well you've come to the right place. There is a lot of hogwash out there and a lot of egos on the Internet. What you mostly find here are the techs in the trenches who offer up there knowledge to those who are daring enough to fix there amp. These guys have helped me immensely over the years an still continue.

Welcome to the place.
nosaj

[glebert]

Looking at the schematic I can't see why the PT would get hot in 1/4 power mode but not full power. I would see what current the power tubes were drawing in both modes and see if there is something odd going on. I'd probably check the node voltages and see how they compare to the schematic too.

[Stratoblaster]

I’m stalled out with most of the new F&T’s installed with the exception of C203,206,207. I need to know if I can put the .22/500v like the schematic shows or need to get .47’s like came out of it from the factory. I’m not sure why the disparity exists.

[Helmholtz]

I’m stalled out with most of the new F&T’s installed with the exception of C203,206,207. I need to know if I can put the .22/500v like the schematic shows or need to get .47’s like came out of it from the factory. I’m not sure why the disparity exists.

I guess you mean 22µ and 47µ, not 0.22µ or 0.47µ.

Both 22µ or 47µ should work. Higher value means a little less ripple.

[g1]

The amp had 8 caps but the kit (and schematic) has 7 ?
Were 2 of the 47's in series, making a ~23uf @ 700V ?
Maybe there is another version of schematic?

[Stratoblaster]

Sorry, for my sloppiness with the values and count. For clarity, (2) 47/500, (4) 47/350, (1) 22/500 are what I pulled off the board.

The kit and schematic both show 22uf/500 for C203, 206,207. The amp originally had a 22/500 in C203, and 47/350’s in both C206 and C207.

I’m not keen to remove this circuit board from the amp again in the future if I can help it, so the disparity between what the schematic showed and how the board was actually built distressed me a bit. I did find a pic of someone else’s board which shows 47’s in position C206,C207 (photo of the other guys board attached). I supposed Fender could have done this for multiple reasons including cost, component availability. If the little extra ripple will be basically harmless, I’ll just move forward with what I’ve got.

Thanks guys!

[eschertron]

What did you replace C201/202 and C204/205 with? the schem shows 350vdc caps, you have 500v caps there?
if the factory installed 47/350 caps in c206 and c207, it may be because that's what they had that day. 22u caps may make the amp a bit looser, but that's what the schem calls for. Plus a bit more overvoltage protection.

[g1]

The amp originally had a 22/500 in C203, and 47/350’s in both C206 and C207.

The photo shows those 47's as 500V.

I supposed Fender could have done this for multiple reasons including cost, component availability.

The only schematic that seems available online for these amps is from 2001 (rev.B).
The amps were made til 2010 I think, and you said this particular one is around 2004? So the bigger uF caps in the newer amps would be more expensive. I would guess they upped the value to deal with some particular issue, or customer feedback. It is possible they ran out of 22uF for some part of the production run, as eschertron mentioned, but then it seems odd they still had 22uF's for C203.

[Stratoblaster]

Thanks guys. So I’ve installed 47/500V F&T caps everywhere but C203, 206, 207 at this point. I’ve got a new 22/500V installed in C203, since that matches the schematic and matched what came out of the amp. I guess I’ll play it safe and order up a pair of 47/500V and just put it back like the factory shipped it to play it safe. I’ve got a few other amps I can likely use the 22/500’s in. Appreciate all your thoughts and suggestions. If by any unlikely chance this recap results in a cooler running PT on 1/4 power I’ll report back

Thanks!
Chris