Fender 5E8-A reissue hum at idle

[fester gomez]

I have a late model 5E8-A Twin Amp reissue that was modded with torres stuff...i.e. bias supply voltage increase kit and driver stage kit w/ precision matched components! Also the entire preamp/driver stages have 12AY7's installed. Anyway the problem is it has a rather nasty 60cyl hum at stand still....still has the non-adj 100ohm hum balance resistors. The bias is set at 14W @ idle, if you increase the bias the hum increases as well.....any thoughts....thanks in advance

[Jazz P Bass]

You need to isolate where the hum is coming from.

Do any of the controls affect it?

Does pulling preamp tubes one at at time affect it?

Does pulling one power tube at a time affect it?

There are a number of Vdc test points that you should check.
The plate voltages of the preamp tubes should be checked for abnormal Vac riding them. (bad decoupling cap)



Link: http://support.fender.com/schematics..._schematic.pdf

[Enzo]

What he said^^^^


\Only one small thought. Increasing the bias current, making the power tubes run hotter, will often increase the signal output some, so that might be the reason goosing the bias raises the hum. But just MAYBE the bias supply filter cap is weak or otherwise compromised, and so ripple on that supply is involved.

Just a thought.

[Gingertube]

I would hazard a small bet that it is simply a badly matched pair of output tubes. That is those "precission matched" components does not include the output tubes.
Why these bozos keep re-issuing stuff without individual bias adjustments or at least a bias balance adjustment is beyond me.

Ah!!! - I feel much better now with that rant off my chest.

Cheers,
Ian

[fester gomez]

hi JPB, the controls have no effect, neither does removing pre-amp tubes,( except for PI), removing one power tubes makes it a bit louder.....I tried another matched set of power tubes and swapped out all preamp tubes. Probing pin 1 & 6 on preamp tubes i'm getting .004acv.....also any suggestions on a decent meter thanks for help

[fester gomez]

I want to thank you all for the response

[Jazz P Bass]

I would check the plate resistors on the PI tube.
One may have taken a hit when the misaligned power tube was inserted.

[52 Bill]

I would check the plate resistors on the PI tube.
One may have taken a hit when the misaligned power tube was inserted.

Huh? Was there a power tube incident that we don't know about?

On any newer Fender I'll ask, have the main filter caps been replaced or are they original? If they are original, have you checked them?

Oh, and did you do the Torres mods? Did the amp ever work right after they were installed?

[Jazz P Bass]

"Huh? Was there a power tube incident that we don't know about?"
Sorry, I got this confused with a different post.

[fester gomez]

no problem JPB, I thought you may have gotten confused with another post, as far as I know there was not any issues with the power tubes.....52 Bill, the filter caps appear to be original, I did not install the Torres mods and the owner says it was quiet when installed which was about 4 yr.s ago. As far a checking the filter caps what procedure would you recommend

[pdf64]

'As far a checking the filter caps what procedure would you recommend'
I temporarily fit a good, new cap (eg 10-22uF) across each one in turn, whilst monitoring for reduced ripple (or other problem being resolved).
Pete

[loudthud]

I would check the bias supply filter, C21 on the schematic. Did the Torres mod install screen stopper resistors?

[Dave H]

hi JPB, the controls have no effect, neither does removing pre-amp tubes,( except for PI), removing one power tubes makes it a bit louder.....I tried another matched set of power tubes and swapped out all preamp tubes. Probing pin 1 & 6 on preamp tubes i'm getting .004acv.....also any suggestions on a decent meter thanks for help

Is there more or less hum with the PI tube removed?

If the hum really is 60Hz as stated in the OP then I would suspect the voltage double bias supply. Hum from the B+ supply would be 120Hz. Why did Torres build a voltage doubler for more voltage when he could just change R47 and D1 and connect it to one end of the B+ winding?

[Dave H]

...That is those "precission matched" components does not include the output tubes.

Cheers,
Ian

And even with "precision matched" components it won't be matched unless the bottom 6L6 grid is connected to the other end of R36 as you can see from the VAC readings on the schematic.