Fender Silverface Twin Reverb Repair - 1973

[74-335]

Hi,

I'm chiming in late, and my very casual observation suggests to me to mention you might wish to replace a couple of things (if you haven't already)

from your Pic it looks like you haven't replaced the Bias Cap (silver can mallory 50µF 70V) or the 3.3KΩ resistor from the pot to the bias supply board.
There's also a 68K from the bias pot to the coupling cap -  I'd suggest that for stability and longevity, that you replace these with some vishay pro1/2 series resistors (https://www.vishay.com/docs/28729/pr010203.pdf) Not that there aren't other brands out there, but I have found these to be the most reliable metal films with a good tight tolerance.

Everyone has a preference - I am not saying my preference is better, only a suggestion to use a quality resistor in this application. (i.e. through all the areas in the amp where component drift is more than 5%( despite fender's 10% tolerance from back in the day, we have more advanced manufacturing processes these days, so take advantage of this).


Edit:  For sh* and giggles, I've got a 73 quad reverb on my bench right now, and apart from the filter caps and valves, it's alarmingly unmolested.
Don't feel like you're trying to summit Everest - these things are easily tamed. Like eating an Elephant.... you can do it - One bite at a time.

I'll attach a couple pics of it for you to reaffirm your progress on your twin... Best of luck - You've been given excellent advice already by others.

[stw307]

With PT heater leads disconnected from LED, Rs, and heater wires. And with no tubes and no load.

One PT heater lead has 17.5 vac and the other is 23.5 vac. Should I be concerned they are different?

Like someone said, this is not the way to measure heater voltage. But this is an excellent way to confirm that you have elevated heaters. Note the 6V difference? I'd say there is (roughly) a nice 6VAC floating on 20.5 volts of DC. Very many amps have elevated heater voltages due to tube specs, arc avoiding etc.

- Mike, another noob

Edit: Schemas show only an artificial ground. So, are these really elevated, because "right" AC readings to ground should be -3.15 and +3.15? 

[Joneg44]

74-335 - thanks for chiming in and the affirming the progress.

from your Pic it looks like you haven't replaced the Bias Cap (silver can mallory 50µF 70V) or the 3.3KΩ resistor from the pot to the bias supply board.

Good eyes - that pic is a bit old at this point. Have replaced both.

There's also a 68K from the bias pot to the coupling cap -  I'd suggest that for stability and longevity, that you replace these with some vishay pro1/2 series resistors (https://www.vishay.com/docs/28729/pr010203.pdf) Not that there aren't other brands out there, but I have found these to be the most reliable metal films with a good tight tolerance.

Have not yet replaced this - seems worth doing while it is on the bench.

Thanks again.

[Joneg44]

stw307 - thanks for your post. Yeah I got sorted on the heater measurement and ended up at 6.9vdc at the sockets. Us n00bs gotta stick together so thanks again!

[Joneg44]

Update:

Following AlNewman's suggestion tightened up everything. A couple PT bolts needed snugging and a couple of pre-amp tube sockets were looser that the other sockets.

I changed out 7025 to 12AY7 in the first socket and cleaned that socket.) Swapped 12AT7 to another 12AT7 on sixth socket and cleaned that socket.

Fired it up again and here are the differences day to day:

Clean channel:
1. The tube changes and tightening things up definitely cleaned up a good amount of distortion on low end.
2. When compared to reverb channel - the clean channel has half or less of the volume of the reverb channel.

Reverb channel
1. Has the expected twin reverb loudness.
2. Reverb and tremolo work as advertised
3. Seems like lows are less distorted

Feels like I am getting closer. But always welcome input.

My next steps are:

1. chopstick the thang to see if there are any loose connections
2. clean remaining sockets and pots (did have some noise on one or two when turned.)
3. only have one preamp tube at this point that was not replaced - will likely see if swapping it with one of the new tubes tells me anything.
4. See if replacing any of the preamp metal tube covers affects sound.

[TitaniumValhalla]

Have you checked the board for conductivity yet? The super waxy mid 70s Silverface boards seem to be the most susceptible to becoming conductive. Check for DC on the fiber board leaking around the eyelets where there is B+.

[ac427v]

Testing the Normal Channel with a 12AY7 may not give accurate results because the "Y" version makes much less gain than the "X" version or a 7025.

You really should put in a 12AX7 to verify that the distortion on the Normal Channel has been fixed.

[Joneg44]

ac427v - will put back the 12ax7

TitaniumValhalla - will test the board for conductivity.

Thanks you both.

[Joneg44]

Plodding along - preamp tube sockets and pots cleaned, tested board and there was no leakage on board around B+. Replaced the 12AY7 with the 7025. verified blue blob caps are not leaking dcv.

Did some more listening afterward.

• Reverb channel remains louder than Normal channel
• Reverb channel highs are nice and sparkely
• Normal channel highs are muddy
• Normal and Reverb channels have distortion on E and A string which goes away-ish when bass at 1. The distortion is more pronounced on the Normal channel.
  

Will continue tomorrow. Will drag another cabinet over to rule out speakers. Will study the schematic and chop stick the connections and think of some logical approach to problem isolation. 

[AlNewman]

Carbon comp resistors have a tendency to drift.  Old caps have a tendency to drift.  Almost always they drift to higher values, meaning an out of spec resistor or cap will often tend towards a muddier tone.
Resistors can usually be tested in circuit, caps not so much.

For the rattle, have you tried new power tubes, or just pre amp tubes?

[Joneg44]

Thanks again AlNewman

For the rattle, have you tried new power tubes, or just pre amp tubes?

Yeah new power tubes in, Willabe suggested that and put in new matched set. But since I did have an arc at the tube socket while they were in, maybe I'll pull a pair at a time to test.

Almost always they drift to higher values, meaning an out of spec resistor or cap will often tend towards a muddier tone.
Resistors can usually be tested in circuit, caps not so much.

Have previously tested and replaced out of spec Rs - will double check.

On capacitors - understood. How far out of spec warrants replacement or is it just replace if not at spec?

[Joneg44]

Going through layout and schematic. I do not understand what this section is and what it is doing. Symbols are R and solder bridge.

Also it appears to be "flashing" under its covering - normal or not? (Not sure either a resistor or a solder bridge should flash)

[TitaniumValhalla]

That is your tremolo “roach” - a light bulb on one side and a light dependent resistor on the other, shrink wrapped together. The light should strobe with the tremolo and the resistance on the other side should rise and fall with it.

[Willabe]

Also it appears to be "flashing" under its covering - normal or not?

Normal.

That is your tremolo “roach” - a light bulb on one side and a light dependent resistor on the other, shrink wrapped together. The light should strobe with the tremolo and the resistance on the other side should rise and fall with it.

[Joneg44]

Finally back at it pulling and checking caps. Quick question.

Have a .047 cap measuring .0514 which is close to 10% tolerance.

Do I only change the cap when clearly outside of tolerance or for ones that are close like this should I swap it them out?

[tubeswell]

.047 cap measuring .0514 which is close to 10% tolerance.

Same difference in a guitar amp.

Taken any set of caps from a factory run and test them and they'll be all over the place 'within tolerance'. So situation normal

Also, these size caps typically function in electrical circuits as filters of one sort or another that work in conjunction with a range of resistances  (i.e. in R/C or C/R filters) and so the end aim - which is usually some frequency roll-off point or other, is usually achievable with no audible difference detectable to the human ear. Guitar amp technology is a 'ballpark' game. You'd have to change the caps values by a factor of 10 or more to notice changes