Roost Session Master Presence Parasitic

[Ted]

I have a 50W Roost Session master in front of me.

Schematic here:

http://www.andy-duke.co.uk/roost/Non...2006_05_14.png

It goes into a ~40kHz oscillation in the last ten degrees or so of presence control rotation.

Things I have tried:

1. Checked that NFB sense is correct.
2. Checked all resistor values
3. Checked tube voltages against schematic
4. Cleaned and tightened all ground connection
5. Extensive playing with lead dress


About the only thing that does stop it is slugging the grids of the output valves a la SF Fender.

The owner has only recently acquired it and doesn't have any history on the amp

I guess my question is not really Roost-specific. What's the next move to identify the source of the problem?

[g1]

What is "slugging" ?
Perhaps it will provide some clue.

[The Dude]

That, and what do you mean by "Checked that NFB sense is correct."? Are you talking phase/polarity? My first suspicion would be that the OT is reverse wired and that you have positive feedback instead of negative............but if you've already checked that?........

[loudthud]

The amp has a stability issue. A possible cause is the EL34s don't like the 22K grid stoppers. Are the tubes original or replacements? I would first contact the amp manufacturer to see what they suggest. On a Marshall type amp I simply insert a small resistor (470 Ohm) in series with the Presence cap. On this design the resistor would go between C17 and the Presence pot. Perhaps 4.7K.

[Ted]

That, and what do you mean by "Checked that NFB sense is correct."? Are you talking phase/polarity? My first suspicion would be that the OT is reverse wired and that you have positive feedback instead of negative............but if you've already checked that?........

Exactly that.

[Ted]

Slugging - the use of capacitors to create a low pass filter.

[Ted]

Actually, I just lifted the NFB resistor and the 40Khz is still present - just very, very faint. So phase shift in the NFB loop isn't the root cause, it just happens to make it worse.

[Ted]

Surely the 22k grid stoppers should prevent high frequency oscillation?

[J M Fahey]

Help, but not enough.
There is excess treble gain:
* R21//C16 (// means in parallel)
* R24//C23
* C18//R32

Plus active presence control adding C17//R32.

I am AMAZED that the amp does not oscillate all the time, no matter what.

2 possibilities:
1) add an extra 270 ohms resistor in series with C18
2) if not enough, rise R27 to 470r and add 470r in series with C17 , that should do.

[Ted]

Ah. A bit of actual engineering analysis. Thanks you. Once you start looking for all those extra treble paths, it's obvious.
I actually added a 4K7 in series with C17 and that at least made it stable.
Still the faintest hint of 40Khz but it was needed for a gig. I'll get it back and finish the job next week.
Thanks Juan.

Help, but not enough.
There is excess treble gain:
* R21//C16 (// means in parallel)
* R24//C23
* C18//R32

Plus active presence control adding C17//R32.

I am AMAZED that the amp does not oscillate all the time, no matter what.

2 possibilities:
1) add an extra 270 ohms resistor in series with C18
2) if not enough, rise R27 to 470r and add 470r in series with C17 , that should do.

[Gingertube]

THis looks like a HiWatt knockoff.
See this schematic trace I did quite a few years back when I did a HiWatt restoration.

http://bmamps.com/Schematics/hiwatt/hiwatt_sa412.pdf

Note: That the PI tube is a 12AU7 in the HiWatt. The ROOST has a 12AX7.
There are other minor but maybe significant changes around the presence control.

Cheers,
Ian