63 fender pro cap specs. subs?

[dmartn149]

I got this sad looking and broken 63 pro amp on craigs list. I got it working now, but I don't want to use it with the old caps. So, in doing a little research, I find that the schem. shows 600v filter caps, my amp has 525v filter caps, and there is one on ebay now with a picture showing it's 500v filter caps. So can I use 500v caps? They're much cheaper than the 600v, so I would really like to use them if I can, but I am new to this whole field and don't want to screw it up.
Thanks
PS I know about draining them, lethal voltages, etc.

[txstrat]

I've had a look at the hammond replacement transformers for the Fender Pro and they claim a secondary of 330vac. This would end up in 465vdc using a solid state rectifier at no load (no tubes plugged in). The voltage will drop slightly when the tubes draw current, due to ohms law.
I'd say you're perfectly fine with 500v caps. Just to be sure you could measure the secondary voltage. If it's around 330-340vac I'd use 500v caps. Use quality caps, though.

[dmartn149]

Thanks Txstrat. I'll get the 500s

[Enzo]

Why wonder. You have the amp working now, right? With the old caps? SO measure the B+ voltage. What voltage is in there? If it is less than 500v, then 500v caps will work all day.

[MWJB]

"I've had a look at the hammond replacement transformers for the Fender Pro and they claim a secondary of 330vac. This would end up in 465vdc using a solid state rectifier at no load (no tubes plugged in). The voltage will drop slightly when the tubes draw current, due to ohms law.
I'd say you're perfectly fine with 500v caps. Just to be sure you could measure the secondary voltage. If it's around 330-340vac I'd use 500v caps. Use quality caps, though." Whoa there!

A '63 Pro should be putting out 495v (+/- a few volts), under a reasonable load. 370-0-370VAC-ish is more typical.

Some tweed & brown amps came with 525v filter caps as stock. 600v Spragues won't fit in the doghouse, scrap that idea.

Rewire the first 2 filter caps like a Super Reverb, use 2x100uf or 2x220uf @ 350-450v, each bypassed with 220K 2W resistors. For the screen supply try a 20uf 550v F&T cap, or 2x47uf/450v in series, bypassed with 2x220K like the main filters, it's easy enough to find 2 small caps that will fit in series in the space vacated by the old 20uf 525v cap.

Remember to remove one of the jumpers between the first 2 filter caps, I remove the one at the positive ends of the old caps, where the HT wires meet the board.

20uf 500v caps will be fine for the 4 other positions.

[guitician]

The old caps are fine. leave them in.

If you replace them....105 Centigrade are better than 85 for tube amps.

[SoK66]

Ditto on the power supply caps in series recommendation. SS rectified Brown amps were designed to run at between 110 and 115vac line voltage, which rendered a B+ in the 440 - 450vdc range. Today's 120vac+ line voltage can push the B+ over 500vdc, which not only stresses the tubes and components but makes the amps sound awful (JMHO, of course). Standby B+ can run nearly 550vdc, making it imperative you have enough voltage capacity in the power supply.

I've tried a number of fixes for this, including use of a Weber Copper Cap rectifier in place of the SS. Unfortunately, while this will drop the B+, it does nothing to correct the high heater voltage. So, the only real fix for this is to either replace the power transformer with a modern unit designed to operate at 120vac (Mojo/Heyboer, Mercury, etc. all make replacements), or better yet, just feed the amp the correct line voltage using a buck transformer, a device called the Amp Preserver (my favorite) or the Weber Browner. You can also use a variac, but they heavy and cumbersome to use.

Using an Amp Preserver set to -12v drop (about 110vac) my '61 Bandmaster's B+ dropped from 510vdc to the spec 445vdc. Heater voltage is now spot on at 6.3vac. Heater voltage is the key factor in determining what line voltage your amp should have.

[MWJB]

Sok66 wrote:

"Unfortunately, while this will drop the B+, it does nothing to correct the high heater voltage"

"Heater voltage is the key factor in determining what line voltage your amp should have."

Heater voltage on these amps is typically <7VAC, therefore only 0.1VAC over spec...it's not a big problem. If you like your heaters at 6.3VAC then I'm not going to try and persuade you otherwise, but exactly 6.3VAC on a 6.3VAC winding is usually a sign of borderline overload, healthy Fender usually run around 6.5VAC - heater windings usually have some current headroom & are therefore over voltage.

It's worth noting that most guys who have heard 50's & 60's Fenders being played have heard them at contemporary wall AC (e.g. Fender RI, Clark's, Victorias & Kendricks etc. all run a 5F6As at the 470vdc that you find today, not 432v as per the schem), I have heard a Concert with a rewound PT that gave 444v on the plates...I really didn't care for it.

Though with either the copper cap, or the bucking xformer approach you can switch between specs (rebiasing accordingly) and make up your own mind...but don't think that the amp is about to spontaneously combust, or is guaranteed to sound bad at the uncorrected voltages.

[SoK66]

Sok66 wrote:

"Unfortunately, while this will drop the B+, it does nothing to correct the high heater voltage"

"Heater voltage is the key factor in determining what line voltage your amp should have."

Heater voltage on these amps is typically <7VAC, therefore only 0.1VAC over spec...it's not a big problem. If you like your heaters at 6.3VAC then I'm not going to try and persuade you otherwise, but exactly 6.3VAC on a 6.3VAC winding is usually a sign of borderline overload, healthy Fender usually run around 6.5VAC - heater windings usually have some current headroom & are therefore over voltage.

It's worth noting that most guys who have heard 50's & 60's Fenders being played have heard them at contemporary wall AC (e.g. Fender RI, Clark's, Victorias & Kendricks etc. all run a 5F6As at the 470vdc that you find today, not 432v as per the schem), I have heard a Concert with a rewound PT that gave 444v on the plates...I really didn't care for it.

Though with either the copper cap, or the bucking xformer approach you can switch between specs (rebiasing accordingly) and make up your own mind...but don't think that the amp is about to spontaneously combust, or is guaranteed to sound bad at the uncorrected voltages.

To each his own, but the spec is there for a reason and vintage amps sound "correct" at those voltages. Dialing in the optimum line voltage on a vintage amp is simply a matter of getting the heaters close to spec., not necessarily spot on. Important to note that reissue Fenders, with the BMRI as an exception, have line AC corrected PTs and at 120+vac will run at the old spec B+, plate and heater voltages. However, running a BMRI at 117vac will drop the B+ and all other voltages right into spec ranges.

Aside from causing spiky tone, excessive heater and plate voltages are a primary cause of premature tube failure, particularly with new production units. It's only prudent to mitigate high line voltages when setting up a vintage amp, both from a tone and component life perspective.

[dmartn149]

Hey guys thanks for all the interest and input, but don't forget I'm a neophyte. This discussion went over my head with the suggestion that I measure B+ voltage, then it just got deeper from there.
I am sure I can solder in some new caps, but I am just as sure that I am not qualified to power this thing up with it open and measure anything. All I really want to do is be able to operate the amp with out too much risk of a capacitor going and causing more damage.

So....... 500v caps?

[txstrat]

That's a wise decision not to fire up the open amp when you're not used to it. The problem is, (like everybody arguing here want's to tell you) if the voltage is higher than 500V your NEW 500V caps probably blow up.
Depends on what the power transformer puts out as secondary voltage for the B+

[dmartn149]

Are the 525v caps obsolete?

[MWJB]

"To each his own, but the spec is there for a reason and vintage amps sound "correct" at those voltages." I don't agree with the use of the word "correct", it will indeed sound "as the designer intended, at the time that the amp was first built" (assuming that the figures on the schem were ever correct), but players have been listening to & enjoying the tone of these amps running 10-15% higher voltages for many years and it's this sound that is the datum line.

"Aside from causing spiky tone, excessive heater and plate voltages are a primary cause of premature tube failure, particularly with new production units." New production units don't usually have excessive heater voltage, it's usually vintage amps that have heaters nearer 7VAC, or dc heaters running 19-20v...some vintage amps exceed modern amps in regard to plate voltage and they still don't eat tubes. Excessive plate current will kill a tube quicker than either of the aforementioned perameters. Within reason (6.2-7VAC) heater voltage is less of a tonal consideration than plate voltage & current, it is certainly not a concern regarding reliability. I have run up to 7.5VAC, still with great tone, because I wanted to compare higher & lower plate voltages -of course, in the end I bought a new PT that gave the higher B+ voltage at reasonable heater voltages, but take it from me, high heater voltage alone will not destroy your tone (low heater voltage will).