65' Pro Reverb Questions

[Hobart]

Hey,

Thanks for taking the time. As the subject suggests, I have a few questions about my 65' Pro Reverb (aa165). When I bought the amp, it came with a much appreciated bag of replaced parts, particularly some capacitors. I don't have the ability to ask the former owner why these parts were swapped out, and was looking for some input on what effect these capacitor values have on the overall tone of the amp, or any other reason why these new values are commonly used.

Here are the capacitors I found in the bag and what they were replaced by:

Stage Filter Caps x2
80µF/450V to 220µF/300V

Cathode Bypass Cap
25µF/25V to 4.7µF/50V

Bias Cap
47µF/160V to 100µF/100V

So again, any info on the purpose behind these swaps would be appreciated.

[xtian]

These are all electrolytic capacitors, And these need to be replaced as a matter of course Every 20 years or so.

[rfgordon]

The smaller cathode bypass cap can help tame bass response if the amp is too woofy.

The bigger bias cap value can yield a quieter bias supply.

[Hobart]

Oh ok, so I forgot to mention that all of the capacitors look mechanically perfect. I have yet to test them, but it makes sense that he wouldn't have sent them to me if they were faulty parts.

As for the large valued bypass cap, I read that something as high as 100µF when the stock value is 25µF will produce a sterile sound. Any thoughts? Perhaps I will try the 47µF back in to see what the feel is. If I do put the 47µ back in, will there be a need to rebias?

Also, any thoughts on the two 220µF filter caps?

[jelle]

Yeah, it is like changing tires on a car, at some point it is unwise not to change them.

[jelle]

Wait, does it still have a rectifier tube? If so, 2x 100uF is the max in that amp for the mains filters. This results in max 50uF combined or effective capacitive load on the rectifier tube. More would damage the rectifier tube. Jelle

[Hobart]

The Spragues that were removed look great, and the other three in the power section are all Spragues, at 20µF I believe. My assumption is that the 220µ's were put in to provide some sort of advantage. Does this value seem too high? I couldn't find the specs on a schematic or layout, but a lot of people have been using caps in the 70-80µ range.

[Hobart]

Ya, it's tube rectified, and I've actually had to replace the rectifier already! I'll swap those two out as soon as I can.

If you have a chance to see my other post, I'm also experiencing an intermittent crackle/pop. Would that symptom have any relation to the overly large capacitor values?

[xtian]

Oh ok, so I forgot to mention that all of the capacitors look mechanically perfect. I have yet to test them, but it makes sense that he wouldn't have sent them to me if they were faulty parts.

Not true. The repair person should always return any parts he/she removed. Don't put these old e-caps back in. In this case, "vintage" is worse than new.

Now, the values of these caps is a different issue...

As for the large valued bypass cap, I read that something as high as 100µF when the stock value is 25µF will produce a sterile sound. Any thoughts?

Right. Higher reservoir cap can help tame hum, but also gives "tighter" bass response. Metal amps want tight bass, Fender Tweed are usually "looser". In amps with Tube rectifiers, there is an upper limit for this reservoir cap. Your Pro had only 35uF stock. 47uF is a good modern replacement. The original GZ34 data sheet calls for a maximum capacitance of 60uF at the input filter. Your tech put a pair of 220uF (110uF in effect) there?!? That's way out of spec.

Ah--I see you just wrote that you had to replace your rectifier already. QED.

Perhaps I will try the 47µF back in to see what the feel is. If I do put the 47µ back in, will there be a need to rebias?

No rebias needed. Again, don't put in the old caps.

Also, any thoughts on the two 220µF filter caps?

Yes. That sounds bad.

http://www.thevintagesound.com/ffg/sche ... _schem.gif

[jelle]

Maybe your recifier tube is arcing internally during transients. Please perform the two repairs and report back.

[Hobart]

Great, thanks for the info. I'm not sure who serviced the amp last, but I aim to get it back within spec, and hopefully fix that awful crackling sound. I'll put in an order for a couple of new 70µF caps.