How do Peavey stop thermal run-a-way in their Transtube amplifier

[Dusk]

Hi All
can anybody tell me how Peavey stop thermal run-a-way in their Transtube Amplifier power stage,do they use an vbias transistor circuit??

Dusk

[J M Fahey]

They use a very classic 4 diode string, with one of them in parallel with a 33 ohm resistor, to lower bias voltage *a bit*.
They *could* use a Vbe multiplier, of course.

[Roly]

As an aside; while repairing a Gibson amp a while back I happened to notice that it was thermally unstable, in fact the idle current was actually oscillating regularly up and down over a matter of minutes.  This was because the thermal compensation diodes were some distance from the actual heatsink, and thermally formed a phase shift network that turned the amp into an ultra low frequency oscillator.

I replaced two of the three thermal compensation diodes with two BD139 transistor B-E junctions, and mounted these transistors on the same bolts as the ones holding the output transistors to the heatsink so they were in intimate thermal coupling.  By removing the excessive delay between heating and response this made the amp totally thermally stable.

Since the idle current determines the amount of crossover distortion, and crossover distortion sounds pretty awful, this problem could well result in an amp that sounds fine one minute and literally a minute later sounds pretty terrible.

[J M Fahey]

Nice to know  :tu:

[Dusk]

Thanks J M Fahey
One more question,are these diode mounted on the driver heatsink ?

Oh thanks you also Roly,for the input too.

Dusk

[Roly]

The thermal compensation diodes (or transistor(s)) should be in intimate coupling with the thing they are trying to stabilise, but it has become quite common practice, particularly in home stereo amps, to mount the compensation components some distance from the output stage and heat sink, and I can't help but wonder how many of these are also unstable and thermally oscillating.   :loco

The reason I used BD139's (or 140's) is mainly because they have a hole through the middle which makes them a whole lot easier to mount on the heatsink than say a 1N4007-class diode.

[J M Fahey]

Lately I have been using BC160, epoxied to the heat sink, and mounted so the flat side of the TO92 case touches it, for better contact.
Will post a picture when I find my camera.
And yes, if space is available, a medium power transistor bolted to theHS is best.
You need some free space though.
Fender and Peavey make a hole in the heatsink and place the diode body inside, coupled with a dab of thermal grease.

Home Hi Fi stuff gets by because nobody abuses them.
Many work for years ... in the Living Room ... , and die the first Saturday night you have a backyard party.

[Enzo]

WHICH transtube amp do you have?  The schematic from the factory - which they will gladly send you if you contact customer service (include your serial number) - includes a parts layout.

In general Peavey mounts their compensation diodes on the heat sinks. Maybe in a hole through the sink or perhaps clamped to it.  Depe4nds on which model.

For the most part, Peavey solid state amps do not have adjustable bias, they are stable by design.   A little crossover distortion in a guitar amp is inaudible.

[Dusk]

Hi Enzo

I have the Bandit 2 with the red stripe and block Peavey" bage" (made in USA),and it's a great amplifier imho,full of nice tones.

Dusk :dbtu: