High B+ Low Heater Voltages on Deluxe Reverb

[BCorbett]

I recently bought Jeffry Falla's book; Hot Rod Your Fender... and was installing his Vox preamp mod using an EF86 pentode. I Fired it up and it sounded sweet 15 minutes then it stopped making sound. Bummer. I installed it in V1, V2 still works. I don't know what the voltages for the pentode are supposed to be but they were quite low. Like 56v on the plate and 14 on the control grid so I doubt I burned it up.

Checking the heater voltage, I got 2.4 volts and the plates for V2 and V4 were at 256.

A few years ago I put a tweed mod on V1 and all the voltages were correct so have not looked at it in a few years.

I am going to check the batteries in my multimeter because I can't believe the readings.

Any advice would be appreciated.

[johnnyreece]

You mention high B+. What is it, exactly?

[BCorbett]

The dc voltage going to the plates, pin 1 and 6, coming through the 100k 1w resistors.

[Stevem]

Explain thus mod more,is the ef86 a second channel, or just replacing V1?

[BCorbett]

Both, V1, or the normal channel, I originally converted to a tweed channel. A design taken from a different book. It has volume and tone, a .22uf coupling cap. A simple design set up as a high gain channel. It ran on a 12 AX7 and had an extra 8uf 450volt filter cap.

I kept the tone stack and the extra filter cap and just changed the wiring on the tube socket so it would work with the pentode. It has the same schematic as a Vox AC 15. I did keep the .22uf cap instead of switching to a .01uf coupling cap.

I did here a slight pop before the tube went silent and perhaps it was a defect. I am going to call the TubeDepot and see if they will send me another.

I replaced the battery in my multimeter and the results are the same except my Vox channel is getting way more DC voltage 300 on the plate and 160 on the grid, still no sound though.

[johnnyreece]

The dc voltage going to the plates, pin 1 and 6, coming through the 100k 1w resistors.

Gotcha...when I think "B+", I think the VDC on the first filter after the rectifier.

Your heater voltage sounds off...are you measuring AC or DC? Between the heaters, or from one leg to ground? You should be awfully close to 6.3 VAC between the heater connections.

[BCorbett]

I am measuring AC, the Positive testing the heater wire and The negative lead to ground.

[BCorbett]

Here are the readings I am getting off the filters: 436, 432, 409, 356.

436 is a little higher than the schematic, 420. I don't know if that will matter to the 6v6's.

Between 432 and 409 it has a 2.7k resistor and between 409 and 356 there is a 10k.

I am going to swap the 10k to the 2.7k spot and since I have an 18k on hand put that in the 10k's spot and see that lowers the voltage to V1, V2 and V4 to around 200.

[Firestorm]

Are you talking about the supply voltages or the actual plate voltages on pins 1 and 6 of the 12A_7s? As supply voltages, those don't look far off. If plate voltages are that high, you have a cathode ground or resistor problem.

[pops]

If this is an older black face then the voltage given on the schematic will be wrong as there was 110 volts on the main ac now it is between 120 and 125 depending how far you are from a transformer so the power transformer will be giving you much higher voltages than the schematic.

[BCorbett]

I changed those two resistors, now the supply voltages read 436, 432, 357, 279. Now the plate voltages range from 195 to 204 across the preamp tubes. They started out around 250.

To my ears it sounds a little mellower, less brittle. I can turn up the reverb a little more with out it killing my ears.

There is a decent pop when I turn on the standby now that was not there before I changed resistor values.

Any ideas why this is ?