1980 fender twin Ultralinier help needed

[spinfresh]

hello Forum,

I have been building and repairing amps for the past 15 or so years off and on and I have finally met my match, maybe one of you kind folks can help me here.

I have a customers 1980 blackface fender UL twin reverb in my shop to fix, it has been in 3 times, and here is what I have done / replaced

he had new JJ tubes installed (it appeared to work fine until then) when I got it the first time the final dropping resistor was bad (open) the 10watt 220 ohm I believe it was. so I replaced it, and it immediately blew out one of the 470ohm screen resistors and the fuse.

I then tested all the tubes (they tested fine) and I replaced the resistor and put in the tubes and bam the resistor blew (so I stopped using those tubes and used a set of my own)

I got it to work on the bench - the customer took it home (without tubes) and I had him order some NOS RCA metal bottle tubes - I had him bring it back with the new tubes so I could bias the amp (it had been modded - the tube balance pot was converted to a bias pot - correctly I may add)


after it was biased I played it on the bench - and returned it to him -he got it home and he complained of popping noises - he brought it back once again -

this time when i touched a tube it would pop and I could hear arching so I replaced the tube sockets (the original ones were total trash and I have seen bad sockets in amps of that vintage before) again i had it playing on the bench - with the customers NOS RCA tubes installed.

he got it home and started playing it and he said it was popping and clicking and generally sounding bad. I had him put the original tubes in it (the ones that it came with - that he originally replaces with the JJ's) and he says it plays ok but then after 1/2 hour or so when it heats up it starts popping again.

any ideas? so far I have replaced

1. tube sockets
2. grid stoppers
3. cathode bypass resistors
4. filter caps (they were original now new f&t)
5. all the other electrolytic s (bias and cathode bypass on 12ax7 / at7's)

i have also biased it super conservatively

any ideas or help would be appreciated - the only thing I have not done is put it on the scope to check for oscillation. which will be my next step if he brings it back (and I cant blame him if he doesn't at this point)

thanks in advance for your help

[pdf64]

NOS RCA metal bottle tubes

What exactly do you mean by this, eg what tube type reference?

Most 6L6 type tubes with a metal case tend to have a very low limiting plate voltage.
The UL TR may run at a B+ of 500V, it requires a very robust power tube.

From the schematic http://www.webphix.com/schematic%20heav ... _schem.pdf
I think that the resistor referred to is the 2k7 10W B+ dropper to the LTP phase splitter node.
I can't see any reason why repairing that should cause a power tube to conduct fault current, unless a coupling cap was leaky.
Pete

[martin manning]

What about the plate load resistors in the preamp? If you remove preamp tubes one at a time does the popping stop at some point?

[Firestorm]

NOS RCA metal bottle tubes

What exactly do you mean by this, eg what tube type reference?

Most 6L6 type tubes with a metal case tend to have a very low limiting voltage.
The UL TR may run at a B+ of 500V, it requires a very robust power tube.
Pete

+1. Metal 6L6 tubes have a plate max of 360 and dissipation of 19W. That amp will kill them and they will make all sorts of noises as they are getting ready to die.

The circuit in that amp is odd. The 10W resistor is 2K7 and is the dropping part of a resistive voltage stabilizer used because there is no choke. The shunt resistor for that network is the 30K 20W which is located way over by the power side chassis wall. There are wires running willy nilly and you have to keep your wits about you to make sure they're all connected to the right places.