1962 Fender Twin Caps

[keithb7]

Hi folks. I am planning a cap job for a 1962 Fender Twin. Looking at F&T Caps right now. 6G8-A schematic shows 20uf at 600V. Originals in amp read 20 uf at 525V. Closest new F&T I can find is 22uF at 500V. These should work fine? Sprague & Atom offer 20uF at 600V but you'd think they had gold in them. About 6x the price of F&T. Thanks.

[mort]

I sub in F&T 22uF when it calls for 16uF all the time. 20 to 22 is even closer. No problemas, amigo.

[Jazz P Bass]

22uf is fine.

I would not go lower on the voltage rating.

Fender spec'd a 600V & installed a 525V.
Go figure.

Three 22uF at 600V 105C Axial Leaded Electrolytic Capacitors 600 Volt Ecaps | eBay

[Tom Phillips]

Hi folks. I am planning a cap job for a 1962 Fender Twin. Looking at F&T Caps right now. 6G8-A schematic shows 20uf at 600V. Originals in amp read 20 uf at 525V. Closest new F&T I can find is 22uF at 500V. These should work fine? ...

I agree. If I were doing the re-cap I would use the F&T caps. However, I would upgrade the first stage filter section from the 6G8 circuit , which used two 20uF caps in parallel, to the series cap configuration using two 100uF / 350V caps in series with balancing resistors. Reference the Fender Twin Reverb AB763 schematic for an example.

The reason Fender made the change is because the first filter stage voltage is very high when the amp is in standby mode. Additionally, today, the 600V caps are expensive and more difficult to source. Furthermore, the 600V caps are less trustworthy because the manufacturers are pushing limits that require them to reduce other specifications such as the life rating when they push the voltage rating to 600V.

Let us know if you have questions about this.

[Tom Phillips]

The caps that Jazz P Bass linked look like good parts. The seller even provides the manufacturer's detailed data if you click through the provided links. It was a little confusing that the price was for Qty 1 but the "1" is a batch of three.

One interesting test that you could do is to measure the actual voltage on the caps in your amp. Especially the first stage cap with the amp in standby mode. That would give you additional data. Could be interesting depending on your local line voltage.

[keithb7]

Thanks guys.

With Twin amp on, standby on, I am getting:

Cap 1 477V
Cap 2 477V
Cap 3 347V
Cap 4 377V
Cap 5 474V
Cap 6 443V
Cap 7 329V


This is idea new for me, re-wiring first stage filter caps. It sounds like a good idea, I just have to be sure I understand how to do it, where to put the resistors. I have a 1971 Super Rev and the first two caps are in series I think. That's where one is installed beside the other, opposite polarity. Correct?

[pdf64]

What's the voltage Vdc after the rectifier, with standby in the 'non-operational' mode?

[keithb7]

pdf64 the Voltage after the SS rectifier, with the standby switch off is 531V DC.
Thinking the 22uF at 600V caps shown above may be the simple answer.

[Enzo]

And yet those 525v rated caps have worked in this amp for the last 53 years.

remember, caps have a working voltage rating, and at 500 or 525, your 477v B+ will be happy as a clam. Caps also have a surge rating, just to cover things like power on elevated voltages.

[Tom Phillips]

pdf64 the Voltage after the SS rectifier, with the standby switch off is 531V DC.
Thinking the 22uF at 600V caps shown above may be the simple answer.

I agree that simply buying and installing the 600V Caps would be the best approach given your experience level. Why complicate maters and taking the chance of taking a wrong turn?

The issue with the circuit in your amp is that the voltage on the first filter cap (actually two caps in parallel) is that the voltage is highest when the amp is in standby mode because there is no load on the high voltage supply. Good design practice is to specify the components to handle the higher voltage. It's a matter of service life not necessarily that the part will fail as soon as the voltage goes over the rating.

As to the "Standby On" / "Standby Off" / "Yes we have no bananas" issue I think that one clear way to state the situation is to say either the amp is in "Operate mode" or it is in "Standby mode." "On standby" seems to be perpetually confusing.

Cheers,
Tom

[loudthud]

I've used that seller on ebay before and have no complaints. Checkout his store, he has a wide selection of caps for tube amps.

Edit: The seller has 16uF and 20uF at 600V available.

[Chuck H]

I'm actually on board with the series main filter. More reliable and it incorporates a voltage bleed circuit into the design. On a side note it would probably be closer to the period correct ESR than parallel modern caps for that amp. Small taters either way. Enzo made a very practical post regarding surge ratings. And parallel caps would represent the stock circuit so it could be wired EXACTLY as it is now with less possibility of error.

[Enzo]

people have reported cutting open Sprague Atom caps of current stock and finding a smaller ordinary cap within the larger package. ne then has to wonder if they are REALLY making 600v caps, or if they are just derating 500v caps to a shorter life span and calling them 600v. 525v caps do not explode at 526v any more than 1A fuses blow at 1.01A.

[keithb7]

I think I can figure this out. So here are the 1962 Twin stage 1 caps:


Here are the 1965 Twin Reverb stage 1 caps.


I looked at my 71 Super Reverb to compare layout and where to place the 220k resistors.
The two little resistors seen tucked down beside the stage 1 caps seen here in my Super R?


Thanks for your help. Much appreciated.

[g1]

Yes, that is correct. One 220K resistor is in parallel with each of the caps, as shown in your 65Twin drawing.
Note that for the 2 drawings you posted, the 62Twin stage one works out to 40uF@600V, the 65Twin stage one works out to 35uF@700V.
The series arrangement is sometimes referred to as "totem pole".