Fender Noise !

[Valvehead]

positive its not the tubes.

with 2 different pairs i can only get 20mv at that test point. The scem calls for 60mv

[Enzo]

OK if the tubes are good, pull one, then replace it and pull the other. I want to know what each socket individually does. Check the two screen resistors. They should be 470 ohms. If they have gone way up in value, the B+ at the screen would be OK< but the tube would not feel it and conduction would be limited.

Just for grins, I might disconnect the OT and clip in a different one. I have a old Bassman OT I keep onhand for this. Impedance wouldn't matter to test current at idle or hum problems.

[Valvehead]

OK if the tubes are good, pull one, then replace it and pull the other. I want to know what each socket individually does. Check the two screen resistors. They should be 470 ohms. If they have gone way up in value, the B+ at the screen would be OK< but the tube would not feel it and conduction would be limited.

Just for grins, I might disconnect the OT and clip in a different one. I have a old Bassman OT I keep onhand for this. Impedance wouldn't matter to test current at idle or hum problems.

so check it with just one tube in at a time?

[Valvehead]

ok----update time.

I clipped in a known good OT. The noise went away and all the voltages came back. 443v at the plates , bias is now 64mv .


However...........heres where it gets weird.

i started putting the pre tubes back in one at a time . with V2 and V3 in , amp seems ok.
When i put in V1 the hum comes back !! Now it varies with the gain and master vol. You can turn them down and the amp is silent . If i pull V1 the amp is silent. Yes , i swapped out all the pretubes for known good ones.

What the hell ????

Also--i ran the Preamp out from the fender into the fx return of my 5150--the hum was super loud out of the cab driven by the 5150

so now the hum seems to be in the preamp section ? wow...

[Enzo]

That hum was always there, you just couldn't hear it with the power amp in such a whacked condition. Once you get the power amp squared away, then work on the first stage hum you have.

At this pont it is moot, but yes, you can run one tube at a time for troubleshooting. it wound up the transformer, but it was possible one or the other of the tubes or their sockets was acting weirder than the other. Pulling one is a way to see what one tube does by itself. By the sub test, you have bypassed all that and found the tranny problem.

[Valvehead]

ok, so we have a bad OT.

possibly a bad cap somewhere in the pre amp section causing all that hum.


I wont be able to mess with the amp until maybe saturday , so ill keep you posted.

and thanks for all the help so far guys !!!!!

[Valvehead]

Well , just my luck. Put in a new OT today . same problem , same noise .

tried all different tubes.

screen resistors ok

all tubes removed, i get 466 B+ and 462 at the sockets

power tubes in , i get 360v

bias is low at 22mv, adjusted all the way up.

heater voltage is ok

pulling one power tube while leaving one in shows the same readings.

pin 8 on the sockets shows ground

[mykey]

Well , just my luck. Put in a new OT today . same problem , same noise .

tried all different tubes.

screen resistors ok

all tubes removed, i get 466 B+ and 462 at the sockets

power tubes in , i get 360v

bias is low at 22mv, adjusted all the way up.

heater voltage is ok

pulling one power tube while leaving one in shows the same readings.

pin 8 on the sockets shows ground

it's not worth guessing to repair amps but you are learning
the semi expensive way. why not take it to a shop?

[Valvehead]

it's not worth guessing to repair amps but you are learning
the semi expensive way. why not take it to a shop?

??? i clipped in a good Ot and the amp improved greatly. Did you not read that part? Thats why i ordered a new one.

Im also not paying the only shop in town $85 an hour.



Id rather come on here when im stumped, hopefully pick Enzo's brain , and learn a few things while im at it.

[Valvehead]

Update .

The noise is gone . It was one of the Filter caps .


So i get it all back together and now it wont switch channels.....

I am getting 54v at both sides of R79 (470 ohm/5w) , where it shows i should have 48v on one side and 16v on the other. R78 looks normal . Going to swap those 2 resistors out and see what i get.

[52 Bill]

So i get it all back together and now it wont switch channels.....

I am getting 54v at both sides of R79 (470 ohm/5w) , where it shows i should have 48v on one side and 16v on the other. R78 looks normal . Going to swap those 2 resistors out and see what i get.

Check the Zener diode CR14 and the traces around that area of the board. If the Zener open circuits, it will do what you say and not drop the voltage down to 16v.

[Valvehead]

it was a trace or one of those resistors. It switches fine now after i replaced r78/79 .


this thing is a nightmare...... NOW, the clean channel is slightly distorted ......plate voltages are all around 230-240v on all 3 12ax7's. I did try some different pre tubes--no go .

[Valvehead]

wow--its the new GT tubes causing the breaking up. Ive swapped to different 6L6's and the amp sounds great .