Silvertone 1431 6AU6 Screen Grid Trimmer

[Zephyr]

I have a Silvertone 1431 that has a 6AU6 preamp configuration I haven't seen before.
If I'm looking at this right, it has a trimmer on the screen grid connected to B+.
Have done quite a bit of searching on the net and have not found any info on how this
works or how to adjust it.

Anyone seen this before?

[52 Bill]

The schematic shows a trimmer capacitor connected there. It's probably there to kill unwanted high frequency oscillations. To adjust it, scope the output and set it to stop any ringing in the signal.

[trobbins]

It is a vintage method for hum neutralistion - somewhat common back then. That simple amp has a few means for hum to enter - one would be power supply ripple in pre and output stage. I'd be interested in how well it can null hum.

[J M Fahey]

Probably so, but what I don't understand is why would this be necessary, since the grid is grounded at RF frequencies by a .05uF cap.
But it's unusual that the supply node feeding the screen does *not* have its own electrolytic capacitor to ground.

Even if a trimmer is cheaper than an electrolytic, replacing a "solder and forget" element with one that needs adjustment sounds like a poor design decision to me.

Why don't you just add the missing cap and give it a try?

At least you'll have less hum.

[trobbins]

Reference: Radiotron Designers Handbook 3 -chapter 24, hum.

[drewl]

Remember, these were cheap Fender knock offs, so I'm not surprised.

Recently worked on an '80's Hiwatt combo that had a trimmer pot and ceramic disc cap INSIDE the reverb tank to kill noise/oscillations.

[52 Bill]

Forgive my earlier well meaning but ignorant post. I just saw the trimmer cap and posted an idea.

[trobbins]

It was pure timely luck for me, as I had just been searching around and located that type of hum related material for an article - so it was right at my fingertips, so to speak! Otherwise I would have scratched my head too.

[Mick Bailey]

Recently worked on an '80's Hiwatt combo that had a trimmer pot and ceramic disc cap INSIDE the reverb tank to kill noise/oscillations.

Some Cascade tanks also have a cap soldered on the back of the output socket. The caps are to tune the resonant frequency of the reverb and improve the output, rather than to kill oscillation. Think of the cap being in parallel to the coil inductance to form a resonant circuit.

This method is a really good way to fine-tune the reverb. I built a standalone unit with a 12-position rotary switch and 11 caps (couldn't get a trimmer with a wide enough range for what I wanted) and you can get pretty extreme reverb effects as the emphasis shifts from low to high.

[J M Fahey]

Agree, peaking at some high frequency to get a twangy, shimmery reverb can work very well.

As of the use of trimmers to kill high frequency noise or instability is a tried and true classic trick in Radio Amateur transmitters
Neutralizing Amplifier
they post a crude but eye opening drawing showing the "unseen" parasitic capacitance which messes things up everywhere:


Some high gain amps use a couple twisted but insulated wires as very small value trimmers, I think they call them pinch trimmers or something, while Mesa are known to solder small pieces of wire going nowhere, as tiny compensating "antennas".

Delights of high gain, high impedance, cramped designs.

[Mick Bailey]

I've always known the twisted wire caps as 'gimmick caps'.

[J M Fahey]

Yes, those, didn't remember the exact word
Haven't been involved with Amateur Radio for decades

[Zephyr]

Thanks All.

That helped a lot.

Read that RDH3 section and was able to follow the description enough to get a general idea of how it works.
Will probably leave it as is for now.
Adjusted the trimmer but didn't hear any change. But at my age that's not surprising.
The hum is less than I would have expected without a filter cap at that node
I wondered if if was something to stop oscillations. Never would have thought of hum reduction.
Found a discussion that described it or something like it as perfect neutralization.
When I get some other things caught up, I'm going to dig into this a bit.