Fender 1969 Twin Reverb bias pot caps?

[bnwitt]

Mick,
the reason I recommend the replacement of the 50uf/50V bias cap is twofold. One, age, two the 50 volt rating is below the actual voltage present in that part of the circuit. The commonly available cap today is a
47uf/100Volt. Don't worry about the choclate drops. I would rather see you change all of the white Mallory electrolytic caps first.

[darrellcheng]

I have seen a resistor in the same place where one of the caps is on other schematics such as the AA769

That is because it was a bias adjust circuit, instead of your bias balance. You can easily change to bias adjust if you want, to correctly bias your output valves.

[Dave Curtis, dB AudioTech]

Mick,
the reason I recommend the replacement of the 50uf/50V bias cap is twofold. One, age, two the 50 volt rating is below the actual voltage present in that part of the circuit. The commonly available cap today is a
47uf/100Volt. Don't worry about the choclate drops. I would rather see you change all of the white Mallory electrolytic caps first.

The AC568 TR used a 50uF/70V cap here. There is nothing wrong with reusing one of the Sprague Atom 100/100s; they're not very old, and are date-coded so you can check. It's the same cap I use for bias circuits in Fenders. It'll stiffen up the bias supply, which is a good thing.

[Dave Curtis, dB AudioTech]

That is because it was a bias adjust circuit, instead of your bias balance. You can easily change to bias adjust if you want, to correctly bias your output valves.

Or even better, add a pot and have bias balance and bias level. I did that on my '76 Pro Reverb. The AC568 has a wonky bias balance anyway; it only adjusts the bias on one pair of tubes. Overall bias is set by changing the 15K dropping resistor.

[MickG]

Caps

The AC568 TR used a 50uF/70V cap here. There is nothing wrong with reusing one of the Sprague Atom 100/100s; they're not very old, and are date-coded so you can check. It's the same cap I use for bias circuits in Fenders. It'll stiffen up the bias supply, which is a good thing.

Good I like reusing....it just feels right. I'll replace the 50/50 with one of the 100/100s, and as for the white Mallory 25/25s, how are the Sprague Atom blue 25/25 axials or are the Sprague premiums a lot better as replacements? Any opinions?---Mick

[bnwitt]

I would use 50 volt caps for the white mallories. The sprague's are fine so are Illinois caps

[MickG]

why 50 volt?

I would use 50 volt caps for the white mallories. The sprague's are fine so are Illinois caps

I am learning a lot and want to understand it all so....why? Why the 50 volt instead of stock 25?

[bnwitt]

Modern electrolytic capacitors are smaller than the ones from that time period and todays 50 volt units are a little smaller than those old Mallory 25 volters. With a 50 volt rating, you've got more headroom and insurance so to speak on those caps. It's pretty standard with many techs to go to the 50 volt units for that application. And they fit right in there.

[MickG]

Red Hot Tubes! This is how I learn!

Well I have done most of the mod but upon trying a test to hear what it is sounding like I got way too much power somehow into the tubes....they were glowing pretty strong so I turned her off and prayed....anyway....Here's what I have done so far to bring me to this point:
1. clipped out the cathode resistors and grounded all pin 8s..
2. removed the resistors on the tube grids (2 of them)
3. removed the 10k resistors on the bias pot
4. Removed 2 100uf caps off the bias pot
5. Changed the bias supply cap from the stock 50/70? to 100/100.
Now I have not replaced the 68k resistors with 220k yet and I have not yet replaced the middle resistor in the phase inverter to stock 470ohm....mine is still 270ohm.
I have not adjusted the bias at all....I have no bias rite or sockets to use wioth a multimeter....yet. Can I turn it down just to be safe by just using the bias pot?
I may have some cold joints on the new bias cap.....will check and/or redo that, also the ground on one of the power tubes pin 8 looks like a cold joint too. So I will check and report back.....all you comments really are helping a great deal so thanks again----Mick

[bnwitt]

If you put a 1 0hm resistor on each cathode to ground you can use them to bias the amp with your multimeter. I wouldn't put the power tubes in until you prove the range of negative voltage out of the bias circuit to pin 5 of the power tubes. I also wouldn't crank it up until I finish all of the component replacements.

[Dave Curtis, dB AudioTech]

Modern electrolytic capacitors are smaller than the ones from that time period and todays 50 volt units are a little smaller than those old Mallory 25 volters. With a 50 volt rating, you've got more headroom and insurance so to speak on those caps. It's pretty standard with many techs to go to the 50 volt units for that application. And they fit right in there.

Since the voltage on those caps in this amp are well under 5V, the 25v parts are fine, and are plenty overrated. (IMHO)

BTW, if you use the Spragues, don't tweak the leads very much, I've had one break off.

[bnwitt]

Since the voltage on those caps in this amp are well under 5V, the 25v parts are fine, and are plenty overrated. (IMHO)

BTW, if you use the Spragues, don't tweak the leads very much, I've had one break off.

You're probably right but The voltage on many 12AT7's in the fender amps is at 10 volts and it is a personal preference of mine to use the 50 volters through out the amp. Saves buying two values. I admit, it's overkill.