Twin Reverb Problems

[Gibsonman63]

Gents,

I am presently restoring a 1965 twin in pretty rough shape. With the power tubes removed, I am getting only -1.5V of bias voltage. Turning the bias pot varies this slightly, but it is still in the same low range. The amp powers up with the tubes and plays at very low volumes for a little less than a minute and blows the fuse.

I have replaced the filter caps, the cathode bypass caps and the bias cap and double checked orientation at least three times.

Four of the pots were frozen, so I replaced all of the pots (one at a time) and double checked the wiring. I will double check my wiring again.

The voltages from the power supply transformer are good as are the voltages downstream from the rectifier diodes.

The voltages on pins 1 and 6 on the phase inverter tube socket are running high at 340V each. The voltage on pin three of the rectifier tube is low at 67V.

Any help would be appreciated.

[JoeM]

Gents,

....The voltage on pin three of the rectifier tube is low at 67V.

???

What version of Twin do you have that has a rectifier tube (maybe you meant something else)? I'm confused, you mention rectifier diodes too.

[Gibsonman63]

It is the AB763 circuit.

http://www.schematicheaven.com/fende...b763_schem.pdf

[Enzo]

But AB763 doesn;t have a rectifier tube - not on the schematic you linked anyway.

If your bias measures only 1 or 2 volts, something is wrong. There is a tap off the power transformer for the bias, then that AC voltage feeds through a high value resistor to the rectifier. Is the resistor open? is the diode open?

[Gibsonman63]

Thanks for the reply, Enzo. You are correct, this amp has rectifier diodes rather than a rectifier tube. I presume that the 48VAC from the transformer is my bias supply and I read about 50VAC at the point shown on the layout drawing. I have replaced the 470 Ohm 1W resistor as well as the diode, but it would probably be a good idea to ring out the circuit to check my soldering. If you follow the lead up from the wiper of the bias pot, I have also replaced the 220K, 100K and 82K resistors as well as the two .1uF capacitors. (shotgun approach out of frustration) I noticed that the vibrato circuit also ties into the annode of the bias supply diode. The opto-coupler does not flash and I had planned on troubleshooting this circuit after I got the amp up and running, but now I am beginning to think that it may be bleeding down my bias supply voltage. I have replaced the cathode capacitors on all of the preamp tubes and the only other path to ground seems to be the pots.

[MWJB]

The optocoupler will only flash if the vibrato footswitch jack is shorted, either by a pedal or a shorted RCA plug/crocodile clip. If it still doesn't flash, replace the optocoupler & the 3 oscillator caps (0.01/0.01/0.02).

You mention -50vdc, you have this at pin 5 of all the powe tubes? What do you read as regards actual plate current (you can't set the bias purely by reading negative bias voltage)?

"The voltages from the power supply transformer are good as are the voltages downstream from the rectifier diodes." please specify by what you mean as "good", supply plate, screen & grid voltages for power tubes, plate & cathode voltages for preamp tubes, AC heater voltages.

"The voltages on pins 1 and 6 on the phase inverter tube socket are running high at 340V each." Confirm that all other tubes are installed & that you have a 12AT7 in V6, not a 12AX7.

[Gibsonman63]

MWJB

"You mention -50vdc, you have this at pin 5 of all the powe tubes? What do you read as regards actual plate current (you can't set the bias purely by reading negative bias voltage)?"

Actually it is 50VAC from the power transformer to the rectifier board. I only have about -1.5VDC to pin 5 of the power tubes, so something is happening between the transformer and the power tubes. I can't put the power tubes in without blowing the fuse, unless I use my handy-dandy light-bulb current limiter. I have a bias meter, but I didn't think I could set power tube bias when the amp is powered through the current limiter. In the past, I set the bias pot to provide -52VDC without power tubes to get it in the ballpark and then readjusted with my bias meter and the tubes in.

"The voltages from the power supply transformer are good as are the voltages downstream from the rectifier diodes." please specify by what you mean as "good", supply plate, screen & grid voltages for power tubes, plate & cathode voltages for preamp tubes, AC heater voltages."

By good, I mean they are all close the the schematic. Actually all the voltages are slightly higher (5-10%), but I assume it is from slightly higher wall voltage now than typically supplied in 1965. My B+ is around 470V and my heater voltage is 6.8VAC. I don't remember the other readings off-hand and my schematic is at the house.

"Confirm that all other tubes are installed & that you have a 12AT7 in V6, not a 12AX7. "

The power tubes have been pulled. I tried pulling the rest of the pre-amp tubes except for the 1rst channel pre-amp tube and the phase inverter, and it moved the voltages for the phase inverter tube in the right direction, but it is still way too high and still no bias voltage. I have a 12AX7 in the phase inverter position, but I didn't realize it would create problems. For my purposes, the twin is too clean, too loud. I thought I could add extra gain by going to a 12AX7, but if it causes grief for troubleshooting, I can go back to the 12AT7 until the amp is working and experiment with tubes later.

Would the readings be more meaningful with all of the tubes installed and the amplifier on current limited power?

[MWJB]

OK trace back from the power tube pin 5, then to the 220K bias feed resistors, then back to the bias adjust pot...see where your -50vdc from the diode disappears. Look for a short/break in the circuit. DO NOT put the power tubes back in until you have a constant -50 to -60vdc at the grids, set the bias pot for most negative voltage before installing & fine tuning as you have in the past. The fuse is blowing because the current is going sky high without significant neative voltage at the grids.

Regarding the PI tube, running a 12AX7 won't cause problems (as long as you like the sound of it) but a 12AX7 draws less current than a 12AT7, therefore plate voltages will be higher and cathode voltage will drop, so you can't really compare voltages with the schem unless you know roughly what percentage voltage rise you get from switching to 12AX7...

"Would the readings be more meaningful with all of the tubes installed and the amplifier on current limited power?" Only when everything runs OK with all tubes installed, via the current limiter, should you try plugging straight into the wall, then check/fine tune voltages & plate current.

[Gibsonman63]

Thanks for pointing me in the right direction. I will do a little more signal tracing when I get home. I am getting my 48VAC bias supply voltage from the transformer, but very minimal DC voltage downstream of the half-wave rectifier circuit. I measured ~20 VAC on the downstream of the 470 ohm resistor, but downstream of the diode DC voltage is only ~-1.5V. Tonight, I will pull the white wire feeding the Vibrato circuit from the -52VDC supply and see if it makes a difference.

[MWJB]

Replace the bias diode (1N4007, stripe to PT tap) & cap (remember "+" to ground, use a 100uf/100v cap).

[Gibsonman63]

I have replaced all of the diodes with 1N4007s and checked the orientation. I replaced the bias capacitor with a 50/50, but have since read that this cap needs a higher voltage rating. I don't have and 100/100 caps. I know that adding another 50/50 in parallel with increase my capacitance to 100uF, but the voltage rating will stay the same, but it may be good enough for testing purposes until I can order one.

[MWJB]

Can you post a pic of the bias supply & connections to yop of the circuit board?

[Gibsonman63]

This picture was taken after I replaced all of the electrolytic capacitors, but before I changed the diodes and resistor. I used 1N4007s for the diodes and a 2W 470K ohm metal film resistor. I won't have access to the amp until this evening.

I have done quite a bit of cleaning since this photo, but you can see how rough this one was when I got it... but I got a screaming deal on it and I am learning how to work on amps.

[Gibsonman63]

Update

I pulled the white wire from the vibrato section, no change. Pulled the wire from the wiper of the bias pot, still no bias voltage. Pulled the wire from the bias supply to the bias pot and the voltage slowly climbs. I got to 15VDC and killed the power because I am not sure that I have enough load on the transformer secondary and don't want to risk damaging a vintage transformer. The 15 volts slowly bleeds down after killing power. The screws holding the diode board are rusted in and I have them soaking now. Based on the amount of rust on this thing it probably worth have a look underneath and cleaning things up a bit.

Suggestions?

Here's some pics of the project so far. I seem to be doing much better with the cosmetics than the electronics for now.

Login to a private Photobucket.com album

The password is fender

Band practice tomorrow and Anniversary Thursday, so Friday is my next chance to work on the amp.

[MWJB]

That is a pretty rusty amp, must have been outside/underwater for some time. The trouble with exposing amps to moisture is that virtually every part suffers - resistors & board material absorb moisture & play up, xformer laminations rust, cloth wire suffers, chassis corrosion can play havoc with grounds especially behind the brass plate (remove this & remove any rust behind it).

I hope you got this amp for peanuts, literally. You could end up having to replace the whole board & wire...however if you got the amp cheap enough and it functions when you have finished you might be OK. I worked on a Twin that was even rustier than yours, it needed both xformers replacing but after that it functioned OK.

Replace the bias board & the bias pot.