While biasing a Fender Tremolux (AA763) the biasing is current is increasing?

[Dandrix]

These 2 caps equate to C27 & C28 on the super reverb schem...in case you didn't hear me...THESE 2 CAPS EQUATE TO C27 & C28 ON THE SUPER REVERB SCHEM.
first of all i have not seen a schem that shows a designator of C27 & C28
the first and 2nd caps from the left would of helped this confusion. did the voltages still sound off???

[Dandrix]

don't be sorry, it does have 2-1.5k res across pins 1&5 going to both ends of the .1 uf coupling caps, now known as (c27&c28) if they are the same designator as the fender Tremolux serial #TA3964 (production model #33) which label on side states ab763. big oops on my part. sorry to MWJB and you. would replacing all blue molded caps & brown turd caps be best for fulfilling my maintenance on the head? and conclude this problem once and for all?

[MWJB]

We need to see a comparison of the voltages when things are OK & when the tube is redplating, so at idle everything OK we have -44v at pin 5 & 445v at pin 3...what do you have at redplating? We are looking for an imbalance between the 2 pin 5's, does the negative voltage at the redplating socket drop in comparison to the idle setting & with respect to the non-redplating tube. I would just go & sub the 2 coupling caps & see if the bias imbalance & redplating continues.

Go to Fender.com, they have all schems for current Fenders, the "65 Deluxe/Twin Reverb/Super Reverb" are pretty much the same as the Tremolux, just the Tremolux lacks the reverb circuit. This is what Pdf64 was referring to.

[MWJB]

Personally I would change all the 25/25 caps on the board, they are the same age as the filter caps & bias electrolytic, just as prone to failure...you don't need to be going back in there every 5 mins because, for example your tremolo has stopped working because the bypass cap has failed.

I wouldn't out of hand change all the blue molded caps, these rarely need changing, it appears that you have an issue that needs correcting...once we have identified what the issue is for sure (the voltages in the amp are definitely drifting, not just the bias resistor value if it's getting hot), then it may require the changing of one or both of C27 & C28...if you just go and change ALL the coupling caps & tone caps, you will just have more work for yourself & your amp will just look like it's had a hachet job to any prospective buyers. It's not unusual to se one or two new coupling caps in a vintage Fender, but when I see ALL the coupling & tone caps changed, my first thought is, "this amp had a problem that a tech couldn't identify, so they shotgunned it & I can't be sure that it is even cured now"

[Dandrix]

i have one 6l6 tube giving me an orange glow after playing 15 mins (vol is @4-5) if i move the tube that would glow orange to the other side(other socket), it would do it again. i have a bias current of 35ma. with the standby switch off the voltage on pin6 reads 440v if i play the amp ( like a E run shuffle) the 6l6 starts to glow orange,the voltage starts to drop to 370v. if i stop playing it, goes back to 440v.changed all 25/25v caps, c27&c28. and filter supply caps (sprauge 16uf 475) its a fender tremolux. ab763. any insight would be very helpful. dan

[52 Bill]

If the same tube glows orange in either tube socket, then there is something wrong with the tube.

[Dandrix]

i replaced the TAD 6l6 with the Tung-sol 6l6's got the same result. was thinking it was something other then that?

[52 Bill]

Are you saying that both sets of tubes have one tube that red plates and that when you switch the tubes from socket to socket the same tube always red plates?

[Dandrix]

yes, sir
did you read the top part of the thread? i have replaced some caps tried different tubes. needed info as what else could effect the tubes in this manner?

[MWJB]

"if i play the amp ( like a E run shuffle) the 6l6 starts to glow orange,the voltage starts to drop to 370v. if i stop playing it, goes back to 440v.changed all 25/25v caps, c27&c28. and filter supply caps (sprauge 16uf 475) its a fender tremolux. ab763. any insight would be very helpful. dan"

When the current rises, a tube (any tube in either socket) redplates and the plate voltage drops, what is happening to the negative voltage at pin 5 of the 6L6. What value caps to you have at the main & screen filter nodes? 475v rating is not enough for these positions.

How hard are you driving the amp? Any boost pedals? What happens if you rebias to 20mA per tube (not that I think you want 20mA per tube, just want to see what happens).

[Dandrix]

mwjb,
you asked "What value caps to you have at the main & screen filter nodes?"the filter caps underneath the chassis? they are (sprauge 16uf 475v)
i pulled out five ASTRON minimite 16uf 450v caps, Sprauge did not have a 450v so, i got the 475v.
i even went a little better then what they had, also factoring in that we run at 120v ac these days not 110v.

How hard are you driving the amp? i have one 6l6 tube giving me an orange glow after playing 15 mins (vol is @4-5)
no, i would not use any pedals during a repair.
i'll rebias the tubes but , not to sure that 20ma is a good idea? that means my new volume would be like 6-8? right?

[MWJB]

You should have at least 50vdc headroom cap rating over the B+, in fact I'd rewire the main caps like a Super Reverb AB763, using 2x100uf 350v caps, each cap bypassed by a 220K 2W resistor. I'd want a 500v cap at the screens too.

You haven't answered my question about the negative voltage at pin 5?

20mA is not a bad idea in terms of the fact that it's not going to break anything, i just want to see what happens when the idle current is reduced...the tone may not be great, but before we worry about that, we need to establish everything is running OK. Don't talk about volume numbers on the dial, they are meaningless, not measurements of anything, pot tapers vary so you can't use them as a datum. An amp may exceed it's clean W RMS rating at "3" on one volume control...it may not get into its stride until "6/7" on another.

Best to measure AC voltage at the speaker to establish how hard you are driving things. Determine an average voltage, square it, then divide by speaker load in ohms.

[Dandrix]

just fired her up, get more info for you soon.

[Dandrix]

got your bias voltage down to -20ma
pin 5 : -053
pin 6: 456v

after 15 mins of warm up time.
played it for 3mins, the usally 6l6 started redplate (orange glow). bias v dropped to -13ma, pin 6:380v
if i would stop playing, orange glow stops, you hear a little pop, current and volt back up
-20 ma is alot cleaner of a sound just to add.

[Dandrix]

rebias tubes to 31ma
-46 off of pin 5
440v off of pin 6

took reading off of filter supply nodes
two caps going to standby switch are at 440v
node A to the left of 10K res: 437v (pin 6 of tube socket)
node C brown wire to 100k&82k :378v
node B right of 10k goes to two 100k res: 350v

HAS ANYBODY HEAR OF THIS:
If the transformer is in fact open, it would cause the tube with the open primary to draw very little (screen only) current through the cathode. Then, if the cathodes are biased by one common cathode resistor, there would not be enough current drawn through that resistor to develop the correct amount of bias for the one tube that is still functional, which would cause it to red plate

i even tried this tonight, replacing output trany. i had one brand new, installed, after 15 mins of warm up, two mins with the standby off, tube red plated again.

whats left to replace???