Deluxe Reverb problem

[Leo_Gnardo]

Yes, they're between center tap and ground. There is a separate blue 50v winding for the bias supply.

Is that an entirely separate winding OR a tap on the hi voltage winding? Most transformers intended for Deluxe have the tap. I say most because I haven't seen 'em all, maybe somebody makes a Deluxe power transformer with an entirely separate bias winding.

I hear what you're saying on the standby, but somehow the reverse is happening. Doesn't make sense because the standby wires are going to their proper lugs on the switch. I will have to try a new one.

If the "sense" of the standby function is inverted then a new switch won't help. Easy enough to apply an ohmmeter & find out which direction to point the bat so that the switch is closed; when the meter reads zero ohms, that's the "operate" position and should allow current to pass thru the power supply to run the amp. Yes it's a bit confusing: Fender and lots of other companies print "standby" on the chassis so that when the bat is pointed to the lettering, that's the operate position, and bat away from the lettering is amp on standby, iow not passing signal. Might be all you need to do is invert the switch position and that's that.

[pdf64]

I think Leo's hunch is right, the zeners may be screwing up the bias supply.
I suggest to take them out of circuit and review.
I think that a light bulb limiter is second only to a multimeter as being most important resource on a tech's bench.
I suggest to build one before proceeding further.

[g1]

Doesn't make sense because the standby wires are going to their proper lugs on the switch. I will have to try a new one.

Don't go by the looks of the lugs on the standby switch, measure resistance. Zero ohms is the closed position.

[tyler8611]

Is that an entirely separate winding OR a tap on the hi voltage winding? Most transformers intended for Deluxe have the tap. I say most because I haven't seen 'em all, maybe somebody makes a Deluxe power transformer with an entirely separate bias winding.



If the "sense" of the standby function is inverted then a new switch won't help. Easy enough to apply an ohmmeter & find out which direction to point the bat so that the switch is closed; when the meter reads zero ohms, that's the "operate" position and should allow current to pass thru the power supply to run the amp. Yes it's a bit confusing: Fender and lots of other companies print "standby" on the chassis so that when the bat is pointed to the lettering, that's the operate position, and bat away from the lettering is amp on standby, iow not passing signal. Might be all you need to do is invert the switch position and that's that.

It's the one here: Fender Power Transformer, Deluxe, Deluxe Reverb, Tweed Tremolux, 125P23B, 025130, 041316, 120V

And you were right about me stupidly forgetting the direction of the standby switch. Unbelievable. Haven't dealt with blackface amps in a long time. Totally slipped my mind!

Now, the problem is the bias is only 4 mA max. So the zeners may indeed be causing this to happen. Currently, the bias tail resistor is 10k. The range resistor, which I may try changing next, is the standard 470 ohm. I was thinking of upping it to 1.5k, which was the next highest metal oxide resistor I could find.

Then again, maybe I should go ahead and remove the zeners. I'll let you guys chime in.

[tyler8611]

I think Leo's hunch is right, the zeners may be screwing up the bias supply.
I suggest to take them out of circuit and review.
I think that a light bulb limiter is second only to a multimeter as being most important resource on a tech's bench.
I suggest to build one before proceeding further.

Thanks I will look into that.

[Leo_Gnardo]

Then again, maybe I should go ahead and remove the zeners. I'll let you guys chime in.

Ring-a-ding-ding, chiming we are! Sounds like progress, good. Zeners in the hi voltage center tap line are almost certainly forcing the bias voltage to be greater than it should be. More negative voltage that is. You could bridge them with a clip lead instead of eliminating them, until you're sure of what you want to do. With the zeners out of the way I'll bet you can set your bias to some more normal current draw and that will pull your main hi voltage down to what it should be. If the amp starts behaving as we expect then it's time to nix the zeners entirely.

[tyler8611]

Ring-a-ding-ding, chiming we are! Sounds like progress, good. Zeners in the hi voltage center tap line are almost certainly forcing the bias voltage to be greater than it should be. More negative voltage that is. You could bridge them with a clip lead instead of eliminating them, until you're sure of what you want to do. With the zeners out of the way I'll bet you can set your bias to some more normal current draw and that will pull your main hi voltage down to what it should be. If the amp starts behaving as we expect then it's time to nix the zeners entirely.

Ok cool so bridge or get rid of the zeners and not mess with the range resistor?

[tyler8611]

So I unsoldered and grounded the center tap and left the bias arrangement per schematic. Switched it on (standby open) and while I was at it took a measurement of the B+ coming off the rectifier. It eventually reached 497v, and a few minutes later, exactly 500.

I took it off standby and it immediately began motorboating and the tubes were glowing blue. They measured 17 mA. I turned the bias pot and there was no change in the reading. I figured that may be bad and have another to replace it.

Last thing I did was measure the B+ and it was 404v. I should have measured everything but I couldn't take the motorboating anymore. So next is change the bias pot and maybe the tail resistor, and if that doesn't work I'll try to see if it's the tubes.

[pdf64]

The bias pot operation should be able to be verified without the power tubes in place, or with them in but with the amp in standby mode, eg measure the Vdc on power tubes pin 5 whilst rotating the bias control over its range.
Motorboating should be able to be stopped by removing the tube in V6, the LTP phase splitter, with negligible effect on the power tube idle conditions.

[tyler8611]

The bias pot operation should be able to be verified without the power tubes in place, or with them in but with the amp in standby mode, eg measure the Vdc on power tubes pin 5 whilst rotating the bias control over its range.
Motorboating should be able to be stopped by removing the tube in V6, the LTP phase splitter, with negligible effect on the power tube idle conditions.

Thanks I'll try it and report back.

[tyler8611]

The bias pot operation should be able to be verified without the power tubes in place, or with them in but with the amp in standby mode, eg measure the Vdc on power tubes pin 5 whilst rotating the bias control over its range.
Motorboating should be able to be stopped by removing the tube in V6, the LTP phase splitter, with negligible effect on the power tube idle conditions.

Just now I took measurements with the tubes installed since they were already in there. Left it in standby and took pin 5 readings, which did change as I moved the bias pot. So it's working. I left it at -35v.

Now I'll remove the 6v6s and the phase splitter as you advised and then take the measurements with standby closed.

[tyler8611]

Just now I took measurements with the tubes installed since they were already in there. Left it in standby and took pin 5 readings, which did change as I moved the bias pot. So it's working. I left it at -35v.

Now I'll remove the 6v6s and the phase splitter as you advised and then take the measurements with standby closed.

I realized what you were saying about the phase splitter- take it out with the 6v6s in. Anyway, with all but 6v6s installed, I took voltage readings as follows:

B+: 480v
Screens: 477v
PI: 382v
Preamp: 317v

V1 pin 1: 211v
V1 pin 6: 221v

V2 pin 1: 199v
V2 pin 6: 192v

V3 pin 1: 469v
V3 pin 6: 469v

V4 pin 1: 202v
V4 pin 6: 191v

V5 pin 1: 393v
V5 pin 6: 471v

V6 pin 1: 215v
V6 pin 6: 252v

I checked the preamp cathodes and all except vibrato tube (no reading) and phase splitter (forgot to check) aligned with schematic voltages.

So next I will measure with the 6v6s installed and the phase splitter removed.

[tyler8611]

The bias pot operation should be able to be verified without the power tubes in place, or with them in but with the amp in standby mode, eg measure the Vdc on power tubes pin 5 whilst rotating the bias control over its range.
Motorboating should be able to be stopped by removing the tube in V6, the LTP phase splitter, with negligible effect on the power tube idle conditions.

Ok just checked it with phase splitter removed. No motorboating, thankfully. The max current I could get was 17mA. That is with standard 10k pot/10k tail resistor and 470 ohm range. At this setting the negative voltage is -23. So I suppose some resistor changes are in order. And hopefully the motorboating issue will also be resolved.

[tyler8611]

Also, the B+ was 433, screens 429, PI 365, preamp 304. Preamp plates still running pretty high but maybe they'll drop when the bias is set right.

[tyler8611]

Would it be preferable to increase the range resistor value first? I have a 1.5k I can substitute.