AB763 new build V5 voltage question

[Planobilly]

Hi,

I just finished building a AB763 with Mid pot and bright switch.

Transformers are all Hammond. JJ GZ 34 rectifier tube. B+ is 437V.  All voltages appear more or less normal. Turns on with no noise. All controls work in a normal way except volume. Biased around 17mV. Reverb working.

Here are the issues.

1 low output volume
2 vibrato circuit not working

I don't clearly understand the vibrato circuit.

The voltage at V5 pin 1 is 415 V and pin 6 is 435 V. I ASSUME this is correct because there is 435 V at the filter cap (B on the layout)  that is directly connected to pin 6 and a 470K resistor to drop the voltage to pin 1. The normal plate voltage  for a Fender with a "roach" type circuit is around 210 V.

I am currently exploring what I have not connected correctly. I measured every resistor with a DMM as I installed them to prevent mis reading the code. I am 99.999% sure they are all correct. Having said that, something is obviously not hooked up correctly.

The Bright switch and Mid Pot layout indicates a 4.7K and a 1K resistor in the filter circuit. The schematic indicates 10K resistors. I installed the 4.7K and 1K first and then changed them to 10K as I thought the plate voltages were to high. V1a for example was 246V and now is 180 V which is normal for a Deluxe Reverb.

Do you think the V5 plate voltage is correct at 435 V ? Should I use the 10K resistors?

Thanks,

Billy

Do you guys need me to post the schematics?

[sluckey]

4.7K and a 1K resistor in the filter circuit. The schematic indicates 10K resistors.

10Ks are for Deluxe reverb. 4.7K and 1K are for 6L6 amps.

As for the tremolo circuit, we need to know if this is a real AB763 with roach, or if it is a Hoffman hybrid AB763. If it's something else we need to see your schematic and layout. Let us know which.

[Planobilly]

Hi Sluckey,

It is a Hoffman hybrid AB763. No roach. I used the Hoffman  layout diagram called Hoffman Amplifiers AB763-2 Channel "Bright switch and Mid Pot layout".I also have the Hoffman Deluxe Reverb layout and the Hoffman Hoffman AB763-2 Channel schematic. Other than the added Mid Pot and having both high and low inputs, the two layouts are the same except for the 10K resistors in the filter caps.

The only thing I used differently was 25uf cathode caps instead of the 22uf that were in the Hoffman schematic. That was the original Fender value and what I had on hand. I can change them if need be but I assume using either value will not change things much.

Also the Hoffman schematic is almost identical to the original Fender schematic except for the vibrato circuit as best I can tell. My output transformer has  4,8,16 ohm outputs and not just 8 ohms. NFB is coming off the 8 ohm tap. That is not related to any of the issues as far as I can tell.
I am still checking my work(have have not found an mistakes as of yet) and as soon as I get feedback from you guys about the V5 voltage question I will start injecting a 1000Hz 50mA signal and do some tracing.

Thanks,

Billy

[sluckey]

The voltage at V5 pin 1 is 415 V

This high voltage indicates the tremolo oscillator is not working and that half of the tube is not conducting. This could be a bad tube or the filaments not lit on V5 or a wiring error. The most likely wiring error would be that the cathode has no path to ground. Look at the attached pic. See the 5 turrets I've circled in red? Are they all strapped together and connected to ground?

[Planobilly]

I found part of the issue. The 470K resistor is in the wrong place. I will redo things and report back.

Silly mistakes...lol

Thanks,

Billy

[Planobilly]

Sounds like a Deluxe Reverb now.

Everything is working normally. Thanks for the support!!

After you mention the cathode to ground issue I started tracing the circuit to ground. No jumper wire from the board to ground. I don't know why I could not see that!! I guess that what scopes and DMM's are for...lol

Now I get to start tuning this beast up and play some guitar.

This amp build was a little over my head, several things I don't clearly understand such as how the vibrato circuit actually works....most likely not a good choice for a first time scratch build. At least I learn a lot in the process.

Thanks to everyone for all the advice.

Billy