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

[Dandrix]

Hello,

While biasing a Fender Tremolux (AA763) the biasing is current is increasing? is this an indicater that there is a compent is going bad? i got new tubes TAD's. all new sprauge filter caps. any ideas??? thanks dan

[pdf64]

Increasing from X mA to Y mA over time T? Both 6L6?
Whats happening to the bias supply voltage?
How about the bias voltage at the tube socket/s?
Did you replace the bias supply cap?
Have you got any functional 6L6 to swap in (even new tubes can have problems)?
Pete.

[Enzo]

And are we waiting 15 minutes for the tubes to get fully warm and stable?

[überfuzz]

Did you mean 10% or 10 mA..?

[MWJB]

How are you reading the bias, if you are using probes with 1ohm resistors AND you are biasing very hot, you might find the 1ohm resistors are heating up & drifting out of value.

Also make sure that the tremolo is switched off, not just turned down on the speed & intensity.

[Dandrix]

thanks everybody for the help,
Enzo,
i have used this amp for about two weeks, the first time i bias the tubes i followed the Dan Torres book (inside tubes amps). a week later i rechecked it noticed it has increased from -35ma to -37. hooked my bias checker back up and brought it back to -35ma(both tubes within 4ma of each other). that was after an hour of use. the tubes are matched Tung-sol 6l6gc-str tubes. about three days ago i used the amp, took a peek around the back and noticed one of the tubes had an orange glow in the center of it, the other did not. turn off the amp, let them cool down for about 20mins or so. checked with the bias checker again one tube was at -18ma the other was at -100ma (-100ma was glowing) tried the TAD tubes the next day. again bias at -35ma, noticed the current was increasing very very slow up 3 tenths then down 1-2 tenths back and forth for about 10mins till it got -35.55ma. had enough of that , wrote a thread to this forum for help.
i'm using a hoffman bias current checker to bias the tubes
pete,
Whats happening to the bias supply voltage? where can i get that reding for you?

Did you replace the bias supply cap? you mean filter supply caps? yes, new sprauge filter caps

Have you got any functional 6L6 to swap in (even new tubes can have problems)? yes, tried the TAD's 6l6
hope i answer everybody question, thanks dan

[pdf64]

Picasa Web Albums - Jonathan Lacey
See the bias supply cap on the board in the bottom right hand corner of the chassis. Have you got the positive end to ground?
Anyway, the bias supply voltage is across that cap. But if only one tube is red plating then the supply itself probably isn't bad.

Monitor the voltage on pin 5 of the tube socket that red plated. Be ready to switch the amp to standby if it red plates again.
I'm assuming that you are competent to work on live, high voltage, open chassis; if you are in any doubt, then don't. You may be best advised to take it to a tech, or you could replace the all components that might be causing the problem. Pete.

[Dandrix]

B+ supply / filter supply / bias supply caps all the same? if the positive end was not going to ground would that cause the bias supply to go bad? not work at all???the chassis is on the bench, i'll have know problem monitor the voltage for you but, i changed over to the TAD's tubes as stated in the last thread. i moved the Tung-sol tubes into the super reverb amp that i have. i bias them at -37ma and played it for an hour rechecked bias current after the hour of usage, it was fine. tung-sol tubes were within 4ma of each other.

or you could replace the all components that might be causing the problem. HOW MANY COMPONENTS ARE TALKING ABOUT?

dan

[pdf64]

'B+ supply / filter supply / bias supply caps all the same?'
No, the B+ supply is positive, typically 400 to 500V, and has several stages of filtering, see TP 1 to TP4 on
http://support.fender.com/schematics..._schematic.pdf
not a tremolux, but similar enough for our purposes.

The bias supply, also known as C-, is negative, typically about -50V and only has one stage of filtering (occasionally 2), see TP5.
Didn't the photo on my previous post make any sense, ie did you replace the blue cap on the small board by the pilot lamp?
If the cap was the wrong way around it may work for a minute or five, but would overheat; what happened then would depend on the actual cap, some burst, even explosively.
Re components that may be faulty, with reference to the super reverb schematic, C27, C28, R56, R57, R60, R61, R69, D1, C36.
May also be a broken connection, eg broken solid core wire, dry joint or poor contact between tube pin and tube socket (see Billm Audio » Re-tension, resolder your tube sockets). Pete.

[MWJB]

Take the bias checker out of the equation...measure the plate voltage & the negative grid voltage at idle cool & again at idle when the power tube is redplating.

How hard/loud are you playing the amp? A tube's plate dissipation isn't totally governed by idle current, the work that the tube has to do under signal also affects where it will redplate.

It only takes a few minutes to change the PI coupling caps, so you could check thesefor leakage, or just sub them out.

Confirm that you are setting idle current with the vibrato switched off, by unplugging/turning off the footswitch.

[Dandrix]

pdf64,
what was i looking for on that picture? kinda far away? neg voltage is -44
yes, i replaced a yellow cap on the small board (50uf/150v) with a 100uf/100v sprauge cap. yes, caps polarity is correct.
super reverb schematic, C27, C28, R56, R57, R60, R61, R69, D1, C36. ARE ANY OF THE CAPS COUPLING CAPS THAT YOU MENTION?
thanks for the re-tension, resolder your tube sockets. you never know how much maintenance you have to get into with these older amp? i'm just starting to learn.

MWJB,
the plate voltage pin3: 445v
idle voltage how do i get that reading for you? i think what your talking about is 488v, with standby on back to 445v
the amp volume is on 4.
i was really waiting for someone to say coupling caps. i have the Mallory caps, 2 in 1 cap (25uf/ 25v) 3 total. brown caps.
the rest are blue molded caps, hate to lose them....

[MWJB]

On the Super Reverb, schem the caps I am talking about are C27 & C28.

Check the dc voltage at pin 5 (-44vdc, which sounds low-ish to be honest, what do you get at 30mA per tube?) & pin 3 (445v) of each 6L6 at cold idle (standby in play mode, after the amp has warmed up, but before you have played & driven the amp into redplating), then check those pins again when the amp has been played for a while & is actually redplating.

If the caps in question are leaking they might be throwing out your bias, IF they are the cause of your problem, then you need to get rid of them, I don't care what colour they are, or what sentimental attachement you have to them...do you want the amp to work or not.

The Mallory 25/25 caps are "cathode bypass" not "coupling" caps, these should probably be changed too for the amp to sound its best, but they are not related to your bias issue.

[Dandrix]

Check the dc voltage at pin 5: -44v bias current 33ma/35ma
(my super reverb pin 5 reads -48 the tremolux sounds like it's the ball park???)
pin3: reads 0 w/standby on, standby off 445v
On the Super Reverb, schem the caps I am talking about are C27 & C28 DON'T KNOW WHICH ONE C27&C28 WOULD BE? SAME ON THE TREMOLUX?

[MWJB]

Locate pin 1 on each 6L6 socket (this is linked to pin 5 via 1.5K resistor - brn/grn/red), follow the wire from pin 1 back to the cicuit board, it terminates at one of the blu caps at the top of the board, each power tube has its own blue cap, each one is soldered to it's own 220K resistor (Red/Red/Yel) at the top, at the bottom on the right is an 82K (grey/red/or) on the bottom of the left cp is a 100K (brn/blk/yel). 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.

[pdf64]

Yes, same on your tremolux.
Sorry, just checked the http://www.ampwares.com/schematics/tremolux_aa763.pdf and it doesn't have R60 and R61, grid stoppers on the power tubes, that the SR has. So that's 2 less things it could be (though an amp without grid stoppers could have similar symptoms as yours!). Pete.