oscillation help

[JoshBernstein]

So this is probably my 4th or 5th thread about my ac30 build, but its really start I to drive me nuts. Im convinced I've got some parasitic oscillation going on. Ive got some relatively unpleasant distortion going on, and a little synthesizer like ghost note that occurs at all volume levels. Ive tried several different preamp tubes in all positions, and 3 sets of power tubes. Chopsticking around has revealed nothing. Ive pulled the cathode caps off of v1 and v2, and have removed the normal channel because it was basically useless to me. Ive also tried large grid stoppers (100k) on the phase inverter and the power tubes. Ive got a 47 pf cap accross the plates of the pi, and a dr z-ish conjunctive filter across the ot primarie. I dont have a scope which is a huge pain in the ass because in basically shooting in the dark. In pretty stumped and dont know what else to try. Any suggestions are greatly appreciated.

[Phil_S]

How about if you post gut shots and a voltage chart? For the gut shots, we need "orientation photos" of the whole thing, topside and underneath plus details. If you followed a standard schematic post a link to it, or post it here. I don't know that much, but from what you write, I am suspecting a lead dress problem. Maybe someone will spot something.

[pdf64]

A scope is really needed to see what's going on.
If you've not got one, any pc with soundcard can be used, see http://www.thegearpage.net/board/index. ... s.1620645/

[Firestorm]

Yes, you should scope it. But don't cure every "problem" you see. Vox didn't use scopes. Just the Jr. engineer's attempt to draw a layout based on the Sr. Engineer's schematic. Or something like that. Vox amps were notorious for catching fire. Not necessarily likely for an amp operating properly. But an oscillating amp can get so hot the tubes melt their own glass.

Cure the obnoxious sound but don't kill the peculiarities.

[JoshBernstein]

Well, while taking voltages just now, ive got 94 volts on the grids of the phase inverter. This seems very wrong to me. Could this be my problem? A bad cap somewhere?

[JoshBernstein]

Also, ill most likely go for that pc based o scope, unless i can find something on ebay.

[pdf64]

Well, while taking voltages just now, ive got 94 volts on the grids of the phase inverter. This seems very wrong to me. Could this be my problem? A bad cap somewhere?

Grid voltage is with respect to its cathode; that is elevated way above 0V by the LTP tail resistor and cathode current.
If your meter had infinite input resistance, the LTP grids should measure about 1 to 2V below the cathode, ie pretty much equal to the voltage across the cathodes bias resistor, eg 1k2.
But the meter resistance will be ~10M, which will act to pull the grid voltage down below that, as the meter and 1M grid reference resistor will form a potential divider to 0V.

[martin manning]

Well, while taking voltages just now, ive got 94 volts on the grids of the phase inverter. This seems very wrong to me. Could this be my problem? A bad cap somewhere?

Voltage measured at the grid is not correct due to loading by the meter, but 94V sounds high. Check the voltage at the plates (pins 1 and 6, 230V on the schematic), and the cathodes (pins 3 and 8, 55V on the schematic). You can get something close to the grid voltage by measuring at the junction of the 1k2 bias resistor and the 1M grid resistors.

[Stevem]

Take a lenght of wire and solder it to a gain statges plate, then rap that around the stages grid wire 5 or 6 times, then try out the amp, if the issue is gone or greatly reduced than at least you have then pinned down the stage making for your issue!
Just be safe with the end of that live plate wire!

Most issues like this are due to parts lay out. ( long grid wires ) and or not enough power supply filtering for the level of gain the amp has.
The signal must go thru 3 gain statges that flip phase to make this issue , and in that case the stages to look at are 1 and 3 .

[pdf64]

..The signal must go thru 3 gain statges that flip phase to make this issue , and in that case the stages to look at are 1 and 3 .

2 voltage gain stages are all that's needed here, as the 2 polarity flips result in the required 360 degree phase shift between the input of one stage and the output of the next.

[JoshBernstein]

Will do all of those things when i get home. My camera completely crapped out so i cant get any new pics, but the last ones i took are on thus thread https://tubeamparchive.com/viewtopic.php?t=28717

[Firestorm]

..The signal must go thru 3 gain statges that flip phase to make this issue , and in that case the stages to look at are 1 and 3 .

2 voltage gain stages are all that's needed here, as the 2 polarity flips result in the required 360 degree phase shift between the input of one stage and the output of the next.

PI will do the third flip on one side (or sometimes the forth or fifth). Big signals there. Watch the lead dress.

[JoshBernstein]

So a couple of things have happened since my last post. I dropped my 5ar4 on th ground and it busted... didn't want to order another one so ive switched to a solid state rectifier and added more filtering. Thr plate wire around the grid trick revealed nothing. Im still puzzled with the high dc voltage of the grid. Triode 1 plate voltage is 278.5, triode 2 at 278.6v, cathode at 52v. Still dont have a scope. Chopsticking still has not revealed anything. Any more suggestions? Thanks

[Phil_S]

If you are in the market for a scope, this one might do for you at a reasonable price. I am not familiar with this particular item and I have no attachment to the vendor. Just trying to be helpful. The pricing on it for a relatively full featured, low end item, looks very good to my eye. http://www.tequipment.net/Siglent/SDS10 ... es/?v=7401