5f1 slight audible oscillation

[martinman]

It sounds like a really fast trem -- but very subtle. It's easier to hear when playing in the upper registers. Any ideas? The amp sounds great otherwise - no issues to report.

I suspect something with high voltage going to the power tubes. It's a weber build w/ heyboer transformers. I have 2 100kohm resisters across pin 7/8 and 2/8 of the power tubes.

What do you think?

[Jazz P Bass]

I have 2 100kohm resisters across pin 7/8 and 2/8 of the power tubes.

Why the resistors?
I would monitor what is going on in the high voltage circuit.
It 'sounds' like it is motorboating.

[martinman]

Why the resistors?

No center-tapped filament supply on the Heyboer PT. More info here:
http://music-electronics-forum.com/t27891/#post244262

It's such a subtle problem... Hard to hear when playing, and only somewhat obvious when a single note is sustained right after it has been plucked.

[Jazz P Bass]

Thanks for the info on the resistors.
I must ask the obvious: Have you tried a different speaker?

[soundguruman]

It's very possible that tubes are old, or that even a new tube acts up.
Tube tester does not check the tube for noises, etc...and that's one noise that can be in a tube.
it always helps to have a set of new ones, and swap them one at a time.
Another cause of the subtle distorted decay is that the bias is not hot enough.
And, I think it's one of those 2 things.
but also just crappy plate and cathode resistors in the stages of the preamp.

Whats the bias voltage read at this time?

[mac dillard]

That should be 100 ohm, not 100K ohm.

[Chuck H]

Good catch.

And FWIW a trem like oscillation could be motorboating (as mentioned above) and that is usually related to the power supply. If this is a new build check and recheck the voltages and wiring for the power rail and filters. If it's an old build check components in these circuits.

Also, on the off chance that it's NOT motoboating and is actually a wierd oscillation artifact that your hearing, try reversing the OT primary leads. Swapping the phase in the output will likely reverse the phase of whatever crosstalk is causing oscillation and change it from positive feedback to negative feedback. Which shouldn't cause the problem.

[martinman]

Sorry for the delay everyone. I _finally_ took some voltage readings.

V1
Pin1 200
Pin2 0
Pin 3 1.4
Pin 4/5 3.5ac
Pin6 190
Pin7 0
Pin8 1.5
Pin9 3.3ac

V2
Pin1 --
Pin2 3.3ac
Pin3 415
Pin4 343
Pin5 0
Pin6 0
Pin7 3.3ac
Pin8 20dcv

V3 (5Y3GT)
Pin1 0
Pin2 430
Pin3 0
Pin4 370ac
Pin5
Pin6 370
Pin7 430

Mac - I did verify - they are 100Ohm, not 100KOhm. I mis-typed above.

Oh, and my pilot light burned out. Hrm. bad day.

[mac dillard]

If you are using a 470 ohm cathode resistor you are really biased hot. near about 17 watts. That is not the problem but is something that should be looked at.

[martinman]

I currently have a 1.5k ohm resistor for cathode bias.

I implemented the standard weber layout:
http://taweber.powweb.com/store/5f1_layout.jpg

And added the 100ohm resistors for center-tapped FS

[mac dillard]

The 1.5k would put you at about 5.2 watts which is pretty cold..I generally try to bias the 6V6 in a SE amp at 12 watts. Have you tried disconnecting the NFB?

[martinman]

Are you thinking something along the lines of 1k?

NFB?

[Chuck H]

The 22k resistor between the output jack and the last gain stage is the NFB line. Lifting one end of that resistor disables the negative feedback circuit. Have you tried reversing the primary or secondary OT leads?

[mac dillard]

Try something around 650 ohm..Your plate (anode) voltage will change so you will have to remeasure it. (also your cathode voltage). Something else to think about(not the existing problem) is that your rectified voltage (voltage at the first filter cap) is a lot higher than what a tweed likes to run at..That voltage should be between about 340 vdc and 380 vdc..This is the fun part of building amps..as Chuck suggests, swap the output transformer primary leads, get your bias in the neighborhood and see where you are..

[martinman]

Can you guys briefly take me through:

Removing the NFB

and

Swapping the transformer primaries?

When this is done, what are you hoping to see or hear?