Twin Reverb troublesooting

[martin manning]

I'm wondering about the PT. You are right voltages should be high rather than low. Does it seem to run hot? What happens if you just leave it on with no tubes installed?

[Firestorm]

Some the late 60s Fenders ran lower voltages. My AA769 schematic shows 405VDC on the plates. With AB763 values in the bias supply, the bias voltage may always be too negative.

[Eric_Allard]

Some the late 60s Fenders ran lower voltages. My AA769 schematic shows 405VDC on the plates. With AB763 values in the bias supply, the bias voltage may always be too negative.

Good catch! I never thought to check; I just assumed they changed the circuit without changing the power transformer. Comparing the values to the AA769 Schematic, mine are slightly higher as expected due to the higher line voltage.

So I should revert to the 3.3K and 15k resistors in the bias supply and on the pot, respectively? Wouldn't the higher resistance in the bias supply cause more of a voltage drop?

[Eric_Allard]

I'm wondering about the PT. You are right voltages should be high rather than low. Does it seem to run hot? What happens if you just leave it on with no tubes installed?

I don't really have a reference for what is "hot" for a PT. When I was playing around with it a lower volume, I think the PT was slightly warm to the touch, but that may have been partially due to the heat from the tubes.

What should I be looking for when I run it without tubes? I ran it like that briefly when I was checking for shorts with my DIY lightbulb current limiter. I think there was a faint hum coming from the PT. I assumed that it was normal.

[Firestorm]

I'd just adjust the shunt resistor unless the raw bias is over 50V, since you only have 50/50 as a bias cap.

[martin manning]

If a transformer has shorted turns it will overheat even with no load connected. Too hot is too hot to touch for more than 5-10 seconds; a faint hum and warm to the touch is normal.

[Eric_Allard]

I'd just adjust the shunt resistor unless the raw bias is over 50V, since you only have 50/50 as a bias cap.

I don't have all of the lingo down yet. The shunt resistor is the one on the back of the bias adjustment pot, correct?

By "raw bias" you mean the AC voltage coming from the PT? Mine was 52.6 VAC.

That same circuit appears to feed the vibrato circuit. Could there be some negative side effects from providing an incorrect voltage to this portion of the circuit? Perhaps the odd oscillation I'm encountering (here's to hoping one change fixes everything).

I can replace the capacitor if required. I think that 50mfd/100v and 100mfd/100v Sprague Atoms are available at my local music store. Is there any benefit to increasing the capacitance at this location?

Thanks for the help.

[Eric_Allard]

If a transformer has shorted turns it will overheat even with no load connected. Too hot is too hot to touch for more than 5-10 seconds; a faint hum and warm to the touch is normal.

I'll check and report back.

[martin manning]

That same circuit appears to feed the vibrato circuit. Could there be some negative side effects from providing an incorrect voltage to this portion of the circuit? Perhaps the odd oscillation I'm encountering (here's to hoping one change fixes everything).

Not likely. The connection to the bias voltage is only used to help stop/start the trem oscillator, and the voltage isn't all that critical.

[Firestorm]

I'd just adjust the shunt resistor unless the raw bias is over 50V, since you only have 50/50 as a bias cap.

I don't have all of the lingo down yet. The shunt resistor is the one on the back of the bias adjustment pot, correct?

By "raw bias" you mean the AC voltage coming from the PT? Mine was 52.6 VAC.

That same circuit appears to feed the vibrato circuit. Could there be some negative side effects from providing an incorrect voltage to this portion of the circuit? Perhaps the odd oscillation I'm encountering (here's to hoping one change fixes everything).

I can replace the capacitor if required. I think that 50mfd/100v and 100mfd/100v Sprague Atoms are available at my local music store. Is there any benefit to increasing the capacitance at this location?

Thanks for the help.

Raw bias is DC you have at the negative end of the bias cap. You don't want it to exceed your cap's voltage rating.

[Eric_Allard]

Something just occurred to me:

When I started this, the bypass cap connected to pin 3 of the vibrato tube was not on the board per the layout; it was wired point to point from pin 3 to the vibrato pedal plug.

When I replaced the cap, I put it on the board per the layout, but now that I think about it, I suspect Fender moved it to the alternate location for a reason.

I think the lead dress in that area is likely the source of my oscillation.

I'll try improving it and see if that helps.

[Eric_Allard]

Raw bias is DC you have at the negative end of the bias cap. You don't want it to exceed your cap's voltage rating.

I just checked it and with the 470R on the bias board, I'm getting -66V through that cap. I'm definitely going to need to change something! That's over 30% over its rated capacity.

[Firestorm]

Since you likely have too much negative bias anyway, try a 1K to 3K3 dropping resistor and see if comes down to a safe range.

[Eric_Allard]

Since you likely have too much negative bias anyway, try a 1K to 3K3 dropping resistor and see if comes down to a safe range.

I don't have any new resistors of the appropriate value so I dug out the one I took out of there in the first place. It's supposed to be a 3.3k, but had drifted to about 3.5k. I put it in and left the 27k on the back of the pot. My raw bias voltage is now -48V and I was able to bias the amp to about 20W static plate dissipation. With the lower plate voltage, it took around 50 MA if current to get it there. Does that seem like a reasonable bias setting? Are these things ever loud! After all this effort, I think this may be too much amp for me .

I'll keep working on the lead dress to address the oscillation issue.

Thanks for all the help.

[Eric_Allard]

I was a little perplexed that despite having the same part number, the power transformer in AA769 Twin Reverbs do put out significantly less voltage than those in the AB763. As a result, short of swapping out the power transformer, it's not going to sound like a AB763.

With that in mind, would this amp sound better closer to its stock configuration (with the exception of the bias balance circuit)? As it is now, the circuit is the same as an AB763 with the exception of the bias supply resistor, which I had to change to a 3k5 to get adequate bias voltage.

In order to return it close to the original configuration, I would need to change two resistors under the cap pan, two in the phase inverter, and four plate resistors for the power tubes and phase inverter tube. I wouldn't bother reinstating the suppression caps. One of the bypass caps in the tremolo circuit is also different (I'm guessing to filter some low end from the tremolo circuit).

Does anyone have any experience with the AA769 Twin Reverbs in their stock form and how they compare with a "blackfaced" AA769 with its low plate voltage? The amp sounds pretty good as-is, but of course, I want it to sound the best it possibly can.

Thanks