Fender 65 Twin Reverb

[The Dude]

Disclaimer: I can't open the schematics in post one, so I'm not 100% sure which amp we're dealing with here. It could be on my end. We've been having storms here and my internet connection has been crap as of late.

My assumption is that we have a quad of 6L6's and a typical Fender output circuit. Some have a hum balance control and some a bias control. I'm assuming from the discussion that we have a bias control. If I read you right, we have approximately 59mA idle current for each tube. IMO, this is way high. 35mA is a good number and many of the old Fenders were biased on the cold side. I've seen many in the 25mA or less area. I believe this is the problem. At idle, your bias is high enough that any little bit of noise is more noticeable. This could be due to a problem in the bias circuit, the set of tubes that are in the amp, or just a bias V circuit that can't get where you need it to get. With the bias as cold as you can get it, what is the bias voltage?

[Enzo]

I don't care what else may or may not be wrong with it, FIX the bias supply. If the PI is involved that would be strange, unless a coupling cap is WAY leaky.


Sawtooth on the bias supply says failed filter cap. Excess tube current says bias voltage is too low, which is one result of loss of filtration.


Tack a couple 100uf 100v caps on the bias supply and see what happens.

[Mick Bailey]

With these kind of jobs the cost of replacing a cap or two to eliminate a possibility can save time. If it makes no difference, well, it takes just a minute to do and pennies for parts.

[Enzo]

Parts are way cheaper than labor, when it comes to figuring the bill.




It is false economy to spend half an hour to decide of some 30 cent part is bad or not.

[bsco]

Dude...the bias reading I gave to you is the lowest that it will go...It also has a hum control...Enzo, I connected a 47uf,63V cap across the one that is connected to the bias wiper arm and the noise increased slightly.....I will tack a cap across the other one later today and let you know what I find...Thanks for the reply....
Cheers,
Bernie

[g1]

Your schematic attachment is not working so I've re-attached.

Do you have your -65V at TP25? What is your min and max -DC at the pot wiper? You should be able to set the bias for the tubes to be idling very low, so as Enzo stated, fix the bias first, then maybe it will solve the hum.
Also, have you set the hum balance according to the procedure in the schem. notes?

[Leo_Gnardo]

Dude...the bias reading I gave to you is the lowest that it will go...It also has a hum control...Enzo, I connected a 47uf,63V cap across the one that is connected to the bias wiper arm and the noise increased slightly.....I will tack a cap across the other one later today and let you know what I find...

It IS bizarre that you have more hum at the second filter cap than at the first. A real head-scratcher. Just for fun do you measure any AC at the "ground" end of the second cap? I'm thinkin' if it's not at a real solid ground you'd pick up noise and that would be passed thru the cap*. That noise increased when you paralleled another cap increases my suspicion. Then again I could be wrong. Wouldn't be the first time. Put "scratchin' head" smiley thing here.

*for instance if the ground end was at the same spot the main power supply filter is grounded, and if that's not a very secure ground noise could distribute from there up into the bias supply. Had a similar Fender in where none of the bolts holding the PT in were tight, therefore none of the grounds made at those bolts was good. Sometimes corrosion finds its way between chassis & ground lugs too, gotta disassemble, scrub corrosion off chassis, put on a fresh lug & reconnect.

[52 Bill]

I'm with g-one here, test the bias voltage levels. It seems we are all assuming a bias supply cap going open, but if one is shorting, bridging in another cap across it will not show an improvement of the symptom.

[bsco]

Hi....Had the flu for a few days....Just checked out the amp again....There was a break where the pad meets the circuit trace at the positive terminal of C35...I was getting roughly negative 50 volts at TP25.....and the break was very hard to see visually......had to find it with a meter.....soldered a small jumper between the two solder points of the trace on the underside of the board, re-installed the board, hooked up all the wiring, installed the tubes and it worked....the bias can now be adjusted.....will have to set the bias according to the plate voltage but all this noise and hum is now gone.....I am so happy.......Thanks for all the help and suggestions.......
Cheers....
P.S. I wish I knew one tenth of what you guys have forgotten...you're all geniuses....thanks once again....

[bsco]

I set the bias for 30mA per tube...that's 60mA per side....plate voltage of 441volts....amp sounds great...so all I have to do now is install the chassis in the cabinet and re-test everything....