Marltone PA amp parasitic oscillation

[Steelwitch]

Hey y’all! I have an old marltone on the bench that is having parastic oscillation issues. There are 470 ohm resistors on the grids of the 6v6’s. Should I consider replacing them with 1.5k?

[Steelwitch]

From pin 4 to pin 6

[52 Bill]

From pin 4 to pin 6

Pin 6 is unused by the tube, so that socket pin is just a tie point. Pin 4 is the screen grid and should be fed by the 470 ohm resistor. The 1500 ohm value is most often used as the input grid resistor.

[Steelwitch]

So how would that be wired on a 6V6

[Steelwitch]

Any other sources of parasitic oscillation I should investigate?

[Steelwitch]

If it helps to pin point, I can see it as early soon as the volume pot

[g1]

Like 52Bill said, those are screen grid 470's. Standard grid stoppers would go in series with pin 5, wired at the socket.

Do you have a schematic for the amp? Otherwise we can't even guess where the volume pot is actually located in the circuit.

[J M Fahey]

Can you get the schematic somewhere and post it here?
Not exactly a popular amplifier and I doubt many know it, if at all.

At the very least, post a couple gut pictures and maybe a scope screen capture showing the oscillations.

Are they constant or appear when driving speaker with some signal?

Just a suggestion: try to think what you want to ask and do it in a single post, then wait for answers.
Adding a new question every 15 minutes just to bump the thread up does not help.

[Steelwitch]

Thanks, JM! You’re right! Here’s an in depth run down of what I know is going on. I’ve added some gut shots and a close up of the oscillation on a scope.

The oscillation is not audible through a speaker, and contributes to loss in tone, volume. When the treble knob and volume knob are at Max it is most present, turning the treble pot to 0 gets rid of the oscillation.. so— I’m trying to track down the source of this. I had a suspicion that it may have been oscillation between the output tubes, but the preamp tube (6SN7) is hyper sensitive. Even putting your hand close to the tube will affect the frequency. Tapping on the tube has a ringing quality, and sounds like it could be microphonic, but I also have my doubts about that. The mounting on the socket is somewhat loose, and is not mounted with screws, just a metal ring to hold tension against the chassis (pictured). It could be lead dress related, but I’m waiting on a new 6SN7 to see it maybe there is oscillation between the two triodes. Which could be likely as well. Just wondering if anyone has any suggestions for places to look for potential sources of oscillation!

https://ibb.co/Bjkt3Bf
https://ibb.co/k3vFZQ0
https://ibb.co/QKb2SCZ

[Steelwitch]

Unfortunately a schematic does not exist, and the builder has passed away. I may draw something up when I’m back at the bench if the search for oscillation gets more involved

[J M Fahey]

I’ve added some gut shots and a close up of the oscillation on a scope.

The oscillation is not audible through a speaker, and contributes to loss in tone, volume. When the treble knob and volume knob are at Max it is most present, turning the treble pot to 0 gets rid of the oscillation.. so— I’m trying to track down the source of this. I had a suspicion that it may have been oscillation between the output tubes, but the preamp tube (6SN7) is hyper sensitive. Even putting your hand close to the tube will affect the frequency.

Then itīs not acoustic/microphonic but high frequency.
may come from poor grounding, poor shielding, poor decoupling or simply too sensitive parts tooclose to high level ones.

Tapping on the tube has a ringing quality, and sounds like it could be microphonic, but I also have my doubts about that. The mounting on the socket is somewhat loose, and is not mounted with screws, just a metal ring to hold tension against the chassis (pictured).

The tube may also be microphonic, but that oscillation would be acoustic, closer to a howling type.
Not the one thatīs worrying you today.

It could be lead dress related, but I’m waiting on a new 6SN7 to see it maybe there is oscillation between the two triodes.

Quite possible, and not inside the tube but also from interaction between it and others.







Nice amp, but very compact.
Obviously rebuilt, no way those are original components, also the modern jack mounted on an aluminum plate, fresh wiring, new pots, etc.

No clue on the scope capture since we do not know frequency or amplitude.

Is that the oscillation itself or you show a regular audio sinewave (say 1 kHz) and the oscillation is the tiny wave riding on top?

If the first, post frequency (even approximate) and amplitude (Vpp).

In any case, I think itīs quite solvable, basically layout/shielding problem and not a fault of the tube itself.

EDIT: and doubt any original Marltone schematic can help you, I very much suspect some Fender Tweed amp there or same era Gibson or Ampeg.

EWDIT 2: I bet you can draw that schematic in under 2 hours, late at night, nice background music, some coffee, not much else needed.

[Steelwitch]

Admittedly I’m not the best at reading frequency on my scope AND it does need a calibration... but I will post something approximate here soon! Waiting on a new 6SN7 to see if the oscillation is originating from the preamp tube

[TimmyP1955]

I'd say measure the HF response of each stage, and anything that goes way beyond what's needed gets a small cap from plate to ground.

[J M Fahey]

nd meanwhile start drawing the circuits.

The beauty of Tube Technology is that itīs at least 60 years old and most people copy each other to death .
Couple that to original designers using basically datasheet recommendations and you "already know" whatīs in there even before opening chassis cover.

Unlike transistors which can be very complex and direct coupled end to end in 1000 different configurations, Tube amps are LEGO type constructions, made out of very few basic component blocks ... which being AC coupled can be troubleshooted on their own.

So when you draw each gain stage, from input jack to speaker out, rather than trying to find something unknown pulled out of the blue, you are basically "checking" which one of the LEGO blocks you have there.

As in: 6SN7 triode stage?
I very much doubt you find anything much different than one of these combinations:



the beauty of it being that even if your amp is physically missing the resistors, or they are hunks of carbon or whatever, you use one of those in the table (matching available voltage) and you KNOW they will work properly.

You may find some variation in tone controls but those are usually quite crude, meaning few parts easy to identify.

I bet in a couple hours, soft music in the background (no wifey/kids/pets around) plus a coffe thermos will result in a working schematic