Fender Twin Reverb Blackface problems

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tardishead

Well-known member
Joined
Aug 11, 2004
Messages
623
After recapping my Fender Twin Reverb - had a few minutes play and then the output just died absolutely. Checked speakers - fine. Checked voltages from power supply - fine. How do I tell if there is a problem with my output transformer?? The primary reads 80ohms or so and then secondary reads 0.3 ohms.
I changed output tubes aswell - no change.
What else would totally stop all output to the speakers?????
 
Check the voltages on all of your plates (pin 1 and 6 on the preamp and PI tubes) There are a ton of schematics on the net to compare them to.

Also, maybe something silly like the speaker wire coming loose.

You could check the points in the signal path to make sure they are not being grounded. You may just have to end up tracing the signal path bit by bit, but if you don't get sound out of either channel (Normal or Reverb) then the problem is most likely somewhere between the phase inverter and the speakers.
 
There is no second fuse only one. Like I said voltages read good at the caps.
It's weird it was working beautifully then suddenly nothing.
There is no output from the output tubes.
 
What is the best way to check the output transformer. I read someone on the net say take the transformer out - connect a 240///6v transformer to the secondaries of the output transformer and you should get 100v or so AC on the primaries. Is this a good idea??
For the life of me I cant work out what is going on. Like I said the amp was working like a dream.Plate volteges read right without the tubes. 470vDC is getting to the tubes.
one strange reading which is bugging me is that I get 100VAC between ground and both sides of the heaters. WHY WHY WHY
 
A rough way to check the primary side of the output transformer, remove the 4 output tubes, then measure from each side of the transformer to the centre tap, the DC resistance from the two readings should be close to being identical, ie, it could be about 40 ohms and 40 ohms to the centre tap. If there is a big discrepancy, maybe it has had a melt down.
Michael
 
First did the twin work before you recapped it?

If it did then you introduced the issue. First thing to do it check the connections that were soldered during the recapping. Next check for solder blobs shorts. Check anything you changed in the amp.

What brand caps?

What schematic? that will possibly help with the heater issue

Go to geofex.com and look for the tube amp troubleshooting and also look for the transformer tester.
 
Look for horizontally mounted 100 ohm resistor in phase splitter. The board is notorious for flexing at this point and cracking the solder contact there. It will shut down the whole amp.

Other than that, just make sure you have good plate voltage drop across each plate resistor. It's kinda hard to fry an OT like that without serious abuse.
 
Which caps did you replace? All of the electrolytics? Did you get the polarity right on the bias supply cap (the one at the right front over near the jewel lamp)? I like to use a higher voltage rating on that one as it can operate near 50V in a normal amp.

A P
 
Amp worked fine for 10-20 minutes after replacing caps.

Took some readings. Is it possible for the choke to become capacitive??? When I measure between choke windings and chassis I get a rising or falling value like a capacitor charging or discharging. When I unbolt the choke from the chassis I get a larger reading but it still seems to rise and fall when alternating to different sides of the choke winding. Just like a cap. Leaky choke possibly???
 
Look for horizontally mounted 100 ohm resistor in phase splitter. The board is notorious for flexing at this point and cracking the solder contact there. It will shut down the whole amp.

I think this is a good call. I had a similar issue with the reverb completely going out in a similar manner. Check continuity in the signal path.
 
You're right about the caps either side of the choke,
How do I test for a leaky choke????

Checked the 100R resistor thru the phase splitter seems fine.

I am still getting 120VAC when measuring between the chassis and one or other of the filament wires. That does not seem right to me.
 
One measurement that is throwing me aswell-------

the two 220k grid bias resistors that split the bias voltage going towards the push pull sides. Measure one side and it is 220k. Measure the other side (should be exactly the same) starts high and gets lower as if measuring a capacitance. Whats that. Take it out of circuit and it works right. This is with no tubes in at all. Its as if I am measuring a capacitance between the bias supply and one of the push pull sides?????????????
 
[quote author="Gus"]First did the twin work before you recapped it?

If it did then you introduced the issue. First thing to do it check the connections that were soldered during the recapping. Next check for solder blobs shorts. Check anything you changed in the amp.

What brand caps?

What schematic? that will possibly help with the heater issue

Go to geofex.com and look for the tube amp troubleshooting and also look for the transformer tester.[/quote]

Did the amp work before recapping?
 
Thanks for any help on this.

Yes the amp did work prior to recapping and for about a good half hour to hour after recapping. Sprague atoms - yes the correct polarity I have fixed and recapped a lot of units but not much in the way of tube stuff thats why I am a bit timid and unconfidant with this circuit. I am not a complete electronics idiot but I am no wizzard either.

The output started to stutter and stop after I put the amp back in the wooden cabinet. Which makes me wonder if something happened that physically changed position of wiring etc.

I checked all the components over and over again and everything seemed to measure exactly right. Also ran through the checklist on geofex about tube amps. Output transformer primaries measure the right DC resistance. All voltages seem to correspond with Fender schematic.

After a few hours of readings and pondering I found that on the 2 output jacks(somebody modded it to have two speaker outs ie two 8 ohm cabs in parallel) I discovered that a wire was suspiciously close to shorting onto another terminal whether the jack was in or out. If this was shorting even if the speaker was connected at the same time - would this be enough to damage the output transformer???

After the signal did stop and I took the chassis out again to have a look - there was a smell like burning rubber - it was hard to tell from where exactly it was coming from.

Also have another confession to make. I was running the amp with 2 output tubes into an 8 ohm load. Instead of normal use which is 4 output tubes into a 4 ohm load. I had read this was an acceptable load for 2 tubes.

So the scenario is - there might have been a short across the output transformer with speaker connected. What would that do???????? And would driving a 8 ohm load with 2 tubes affect this or make the impact worse????
 
ouch nasty!!!!!

found something major. THe grid bias feed that goes to 2 of the tubes ie one side of the push pull had come loose under the board and was intermittently touching the HT choke side. Hence that 220k resistor that was reading like a cap - it was connected to the 20uf cap after the choke as a result.

probably blew a few power tubes while it was happening hey????
So on that side of the push pull the grids were getting something in the region of 467 DC volts. Would that kill power tubes???

Have not had time to finish and try it out becuse its late and I should do it in the morning.
 

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