Fender 6G6-B Voltage way too high

[Bassist001]

So i just finished a convertion to a '63 6G6-B bassman in an old AB165 Bassman. My problem is my voltage right off the rectifier is hitting around 480v+ when its only suppose to be around 430v. My plates on the pre-amp are way too high! around 450v any ideas what could be going wrong?

[olddawg]

That's only about a 10% difference. The line voltage has changed since 63. You can always use some zeners to bring down the B+.

[Enzo]

How about this, did you take all these readings with the tubes removed? The only voltage readings that will mean anything are taken with all the tubes installed and operating.

[Intergalactic Tourister]

The line voltage has changed since 63.

Check the schematic to see if the xformer primary its rated 117v or 115v.
Also check if your filaments are upwards of around 6.8v. If so, olddawg has a very convincing point.

[Bassist001]

Ya, i did take the readings with the tubes out. But, 450v seems way to high for a Preamp tube plate and i didn't want to throw one in and blow it up. Especially considering that the schematic says the pre plates should be at 230v. I may also have something wrong in my filter section Im guessing. But haven't found an issue yet. How could i go about lowering it with some zeners? I've never had to lower the B+ before haha

[mac dillard]

With the tubes removed, no current flow, no current flow no voltage drop.

[Bassist001]

So, if I drop the power tubes in and get them some what biased. Should that help lower the overall voltage? Sorry this is my first major build. So I guess I'm a little extra cautious haha

[mac dillard]

Yes installing the power tubes as you said will drop the voltage..If this is your first build I would recommend that you go to
Paul Ruby Amplifiers and read through his initial start up procedure. Also would recommend that you
build and use a "light bulb limiter" for powering up. Search for light bulb limiter on Google to see how to build one. It is worth the trouble. Basically a light bulb in series with your wall power cord to limit the current draw.

[Bruce / Mission Amps]

Put all the tubes in, not just the power tubes... yes the power tubes will load the B+ rail down a bit but the drop across the B+ rail, with respect to the preamp section, only occurs because of the load the actual preamp tubes put on the B+ rails dropping resistors... as previously mentioned above... ha ha
You won't need to drop any B+ with zeners in this amp.
Set the bias supply -VDC to the power tubes grids (lugs 5) so the power tubes idle around 30-40ma each and you'll be golden.
If you don't know how to do that bias setting part... well you have some homework to do...

[Bassist001]

ok so i threw in the rest of the tube and its giving me pretty close reading of where i want to be. Thanks for the help! Now i know haha but now i just have anothere problem its looking like. So about every other time i flip the amp out of standby i get this horrible squeek through the speaker. All the volumes are turned down. and i did pull the phase inverter and it stopped. So its gotta be in the pre amp section somewhere. Any of you see this before?

[Bassist001]

and the squeek is just a quick little sound. Lasting little 250ms at the most

[ThermionicScott]

Could be a momentary oscillation in the output tubes. The 6G6B doesn't show grid-stoppers on the output tubes, so I'd try them.

[Bassist001]

O that's a good idea! I'll try it out tonight and see if it works! Thanks!

[Bassist001]

So, i tried some 1.5ks on the grids of the output tubes and it didnt change anything. And once again pulled the inverter and it stopped. There was no tube in the stage before it. so im guessing its in the inverter section. Could this be caused by a bad component? Like something i heat up too much in the process of building it maybe?

[ThermionicScott]

Well, it was worth a shot, and good practice to have anyway. I'll just throw out a couple more thoughts:

1) Bad tube? Try other 12AX7's or 12AT7's in the PI slot.
2) Double-check that all the socket pins are tight.
3) Negative feedback wired backward? See if the oscillation still happens if you disconnect one end of the 56k feedback resistor.
4) Just for kicks, try disconnecting the 0.001uF anti-oscillation cap across the PI plates.
5) Check for leakage on the coupling caps on either side of the PI.
6) Consider putting a high-value wirewound resistor across the standby switch so that the voltage spike isn't as extreme.

- Scott