AB763 reservoir caps: why not using a single 500V instead of two 350V in series?

[Lambertini]

Hello!

I want to increase the reservoir cap capacitance to reduce 100hz buzz in a Twin Reverb.
I suppose in the older time 500V caps were expansive and rare, so Fender decided to put 2 x 350V 75µF reservoir caps in series in the AB763 circuit, for a total of 37.5µF capacitance.
I know 700V > 500V but my PT is delivering 430VDC, maybe 440 VDC at peak.
What would be the risks/drawbacks to replace these 2 caps in series by a single 500V cap 75µF, or even 100µF?
Would that also mean to replace the two 220K resistors in parallel of each cap, by a single 440K in parallel of the single cap?

Thanks!

[Dave]

Not a problem. The two 220k serve two purposes, first they equalized the voltages seen by each of the series capacitors and second they provide a slow drain power the power rail when the amp is off. If you just use one cap, you don't need the resistor/s for equalizing the voltages, but its still a good idea for safety.


Dave

[sluckey]

Measure the AC volts on the PT high voltage winding. Now multiply that number by 1.414. The result is the voltage that will be on that first filter cap whenever the standby switch is OPEN.

[Lambertini]

Not a problem. The two 220k serve two purposes, first they equalized the voltages seen by each of the series capacitors and second they provide a slow drain power the power rail when the amp is off. If you just use one cap, you don't need the resistor/s for equalizing the voltages, but its still a good idea for safety.

All right, thank you.

Measure the AC volts on the PT high voltage winding. Now multiply that number by 1.414. The result is the voltage that will be on that first filter cap whenever the standby switch is OPEN.

So, roughly, 330 x 1.414 = 466 V ~ 340 x 1.414 = 480 V. Looks good to go. Thank you.

[AmberB]

Some Fender amps, such as the silver face amps running 6L6s, can go higher than 500 volts when you flip the power switch before the filter caps charge up.  The closer the amp runs to 500volts when it's all warmed up, the more likely the initial voltage surge will pass 500 volts when you turn the amp on.

[DummyLoad]

Typically 500V rated caps have a surge rating of 525 to 550V depending on brand/part. I have seen initial surges of up to 510V on my service bench with a TwinReverb with just over 120V line. In that some areas can see north of 125V line, that may be too marginal for comfort. MIEC does make 600V axial part, however, I personally have no experience with or knowledge of the realm of reliability of those. 

I like to keep things looking stock, so I use 220uF/300V for the plate supply and 47uF/500V for the screen/trem osc/reverb drvr tap, 22uF/500V for the rest of the ckts. I use F&T axial parts whenever I service caps on 100W or greater Fender tube amps, they fit under the doghouse fine. The added filter 70UF of plate and 27uF of screen capacitance do make a noticeable improvement in the PS noise over stock.   

--Pete 

[Lambertini]

I like to keep things looking stock, so I use 220uF/300V for the plate supply and 47uF/500V for the screen/trem osc/reverb drvr tap, 22uF/500V for the rest of the ckts. I use F&T axial parts whenever I service caps on 100W or greater Fender tube amps, they fit under the doghouse fine. The added filter 70UF of plate and 27uF of screen capacitance do make a noticeable improvement in the PS noise over stock.   

Thank you Pete, I think this is the wisest advise regarding the plate caps. I'll follow it.
However, I've put 3 x 30 µF (JJ) for the others. Do you think it really worth the move to replace them by 47 / 22 / 22 µF noise-wise and tone-wise?

[DummyLoad]

I like to keep things looking stock, so I use 220uF/300V for the plate supply and 47uF/500V for the screen/trem osc/reverb drvr tap, 22uF/500V for the rest of the ckts. I use F&T axial parts whenever I service caps on 100W or greater Fender tube amps, they fit under the doghouse fine. The added filter 70UF of plate and 27uF of screen capacitance do make a noticeable improvement in the PS noise over stock.   

Thank you Pete, I think this is the wisest advise regarding the plate caps. I'll follow it.
However, I've put 3 x 30 µF (JJ) for the others. Do you think it really worth the move to replace them by 47 / 22 / 22 µF noise-wise and tone-wise?

No, what you have done is fine. With the exception less PS noise, the added capacitance to preamp circuits should be sonically transparent.

--Pete