Fender Twin 135 Watt

[bsco]

Hey guys....I have a question...I posted about this before but thought I would start a new thread.....I have this amp which draws excessive current when you plug a guitar in and try to play it....the volume is very very low and with a current limiter in place of the fuse will glow and increase in brightness......and this is a 300W blub....all electrolytic caps have been replaced....new screen resistors and different tubes....all grounds repaired...amp has been completely cleaned up....tube voltages seem to measure ok but when trying in put a signal through the amp, all the voltages pretty much drop to the point where if a fuse was installed, it would blow out.....I had orginally thought there was a problem with the power transformer but that is not the case.....any suggestions??? It is 5 AM here now and I am going to call it a night.....

[Jazz P Bass]

Disconnect the output transformer & see if the voltages hold up & the current draw diminishes.

I hope that is not the problem 'cause replacements are not cheap.

[Tom Phillips]

...I have this amp which draws excessive current when you plug a guitar in and try to play it....the volume is very very low and with a current limiter in place of the fuse will glow and increase in brightness...tube voltages seem to measure ok but when trying in put a signal through the amp, all the voltages pretty much drop...

The amp may be going into parasitic osculation when you introduce a signal.
Did it exhibit they symptoms you describe before you serviced it?
What other test equipment do you have available? (Scope, dummy speaker load etc?
Can you post detail gut shot photos?

Edit: Can you provide more info about how the current draw increases when you try to play the amp?
Does it return to normal if you 1) Unplug the guitar after current draw begins? 2)Turn all the front panel controls down?
Do you have the same exact problem in both channels?

[bsco]

Disconnect the output transformer & see if the voltages hold up & the current draw diminishes.

I hope that is not the problem 'cause replacements are not cheap.

Thanks Jazz....I'll check that out later today and let you know......

[bsco]

The amp may be going into parasitic osculation when you introduce a signal.
Did it exhibit they symptoms you describe before you serviced it?
What other test equipment do you have available? (Scope, dummy speaker load etc?
Can you post detail gut shot photos?

Edit: Can you provide more info about how the current draw increases when you try to play the amp?
Does it return to normal if you 1) Unplug the guitar after current draw begins? 2)Turn all the front panel controls down?
Do you have the same exact problem in both channels?

Actually it has been in storage for a long time.....and the caps are very old.....so I decided to strip it down and clean it up and replace caps, etc before I even applied power to the unit....when you plug in a guitar and set the controls to get some sound, this current limiter starts to light up and then keeps increasing in brightness...the sound is very,very low and distorted.....both channels are the same....so it looks like I have a power supply problem or an output ckt problem or both.....when the amp is idling, it appears to be fine.....no excessive current draw...current limit blub is not lit....I am going to check out a few things today and post them here for you......
Cheers,
Bernie

[bsco]

Disconnect the output transformer & see if the voltages hold up & the current draw diminishes.

I hope that is not the problem 'cause replacements are not cheap.

Quick question....I had installed two 1.2 ohm resistors on the cathodes....one for each pair.....I wonder if I have the right tubes paired up??? orginally the cathodes were tied to ground.....
Cheers,
Bernie

[Jazz P Bass]

Not too sure what you are saying here but you can check each tube, one at a time, for idle current draw (across the cathode resistor).
Then you can match up 'pairs'.

For that matter you can run one tube at a time to see if the excessive current draw is being caused by one of the output tubes (or all of them.).

[bsco]

Not too sure what you are saying here but you can check each tube, one at a time, for idle current draw (across the cathode resistor).
Then you can match up 'pairs'.

For that matter you can run one tube at a time to see if the excessive current draw is being caused by one of the output tubes (or all of them.).

I did some voltage checks and here is what I found.....the bias voltage across the cathode resistor for one pair of tubes measured about 38mV..the other pair measured about 29.5 mV....so I adjusted the output matching control to balance them out at around 32mV per side......my current limit blub glows very dim when the amp is just idling with no input signal........it should not light at all....

as long as the master volume control is turned down,and the other controls are set somewhere to produce a signal, all voltages remain stable.....if I input a signal....any type of signal for that matter, and turn up the master volume control, the plate voltages will decrease......the more I increase the level of the signal, the greater the voltage drop......When I check the bias voltage, I find that it will increase....If the plate voltage drops to say 370V, the bias voltage increases to almost 90mV across the cathode resistor....1.2 ohm.....the larger signal you put through the amp, the brighter the blub, the lower the plate voltage and the higher the bias voltage drop across the cathode resistors....these changes in measurements will be very quick....a couple of seconds.....so I don't keep the level very high for very long for fear of damaging something.....

I am going to go downstairs again and do some more tests but I wanted to post this first to get some opinions.....and i realize this is hard to do over emails.....so bear with me........
Cheers.....

[g1]

You definitely have oscillation. The increase in cathode current and drop in plate voltage proves it. If you can't hear it then it is above our hearing frequency.
Try disconnecting the NFB resistor in case the OT is out of phase.

As far as the pairing for the cathode resistors, each half of the OT primary goes to a pair of power tubes, each of those pairs should have one of your cathode resistors.

[bsco]

You definitely have oscillation. The increase in cathode current and drop in plate voltage proves it. If you can't hear it then it is above our hearing frequency.
Try disconnecting the NFB resistor in case the OT is out of phase.

As far as the pairing for the cathode resistors, each half of the OT primary goes to a pair of power tubes, each of those pairs should have one of your cathode resistors.

Ok. G-one....I'll check that out....you can't hear any oscillation....well.....I can't hear it....yes...I have one resistor for a pair of tubes.......I'll check that feedback resistor....but the output wiring hasn't been tampered with.....
Cheers,
Bernie

[g1]

If lifting the NFB makes no difference, then you will no the OT wiring is good. After that you can try removing V1 and V2. I think that will stop it as the volume controls were affecting the idle current.

[52 Bill]

Do you have access to an oscilloscope? That would help in finding ultrasonics.

You mentioned repairing all of the grounds, please explain.

[g1]

Missed that about the grounds, very good possibility.

[Tom Phillips]

...my current limit blub glows very dim when the amp is just idling with no input signal........it should not light at all...

That’s incorrect. At idle the 135 Twin is drawing significant line current. With that current is flowing through the bulb, the bulb it is going to glow a little

...What other test equipment do you have available? (Scope, dummy speaker load etc?
Can you post detail gut shot photos?

It would help us to help you if we knew the answers.


Also...are you using a dummy load or are the speakers hooked up.?
If you have the speakers connected and it turns out that the amp is going into oscillation that you cannot hear the voice coils will still be dissipating all the output power. That could damage the speakers.

[bsco]

Do you have access to an oscilloscope? That would help in finding ultrasonics.

You mentioned repairing all of the grounds, please explain.

They were corroded and/or had old flux which had dried up and looked like crap....There were two grounds connected to the power transformer mounting bolts...and they were black with corrosion....so I cleaned everything up....I cleaned where the pots are secured to the chassis...the input jacks as they were corroded as well.....lots of rusty nuts and washers............I do have a scope which I will connect up to it today to check to see what the output is like....
I used both a test speaker and a dummy load for testing.......I'll check the V1 andV2 as well.....they have their shields in place but I had to clean these up as well......i wonder.....is it possible to be the turret board itself??? I am going to put the heat gun on it to see if it might be holding some sort of moisture....thanks for all the suggestions guys....I'll post some pics of the inside as well......
Cheers