A bit off topic for RAP, but I thought someone here might know right
off.
I was replacing burnt plate resistors in a Fender Blues Deville and
noticed a diode on each of the power tubes reverse biased between the
plate and cathode. I assume this would only conduct if the plate
voltage went negative (or exceeded PIV and the diode shorted out). What
does this do in terms of circuit protection (I'm assuming that's what
it's there for)? Under what condition would the plate voltage actually
go below 0?

apa wrote:

A bit off topic for RAP, but I thought someone here might know right
off.
I was replacing burnt plate resistors in a Fender Blues Deville and
noticed a diode on each of the power tubes reverse biased between the
plate and cathode. I assume this would only conduct if the plate
voltage went negative (or exceeded PIV and the diode shorted out). What
does this do in terms of circuit protection (I'm assuming that's what
it's there for)? Under what condition would the plate voltage actually
go below 0?

That's a novel one ! New to me and I've seen a few.

You're sure it was plate / anode - cathode ? What kind of diode btw ?

Graham

Chel van Gennip wrote:

On Fri, 17 Jun 2005 16:48:32 +0200, apa wrote:

A bit off topic for RAP, but I thought someone here might know right
off.
I was replacing burnt plate resistors in a Fender Blues Deville and
noticed a diode on each of the power tubes reverse biased between the
plate and cathode. I assume this would only conduct if the plate voltage
went negative (or exceeded PIV and the diode shorted out). What does
this do in terms of circuit protection (I'm assuming that's what it's
there for)? Under what condition would the plate voltage actually go
below 0?

The load of the power tubes is highly inductive: a transformer. This can
result in high voltage peaks. If one side of the transformer is going up,
the other side is goging down and will cross zero. A diode from anode to
cathode clamps that peak,

No damn good for an output stage !

It *relies* on the tube with -ve a-k volts being reverse biased at the grid.

That's how tube push-pull amps work !

Graham

Pooh Bear wrote:
apa wrote:

A bit off topic for RAP, but I thought someone here might know right
off.
I was replacing burnt plate resistors in a Fender Blues Deville and
noticed a diode on each of the power tubes reverse biased between the
plate and cathode. I assume this would only conduct if the plate
voltage went negative (or exceeded PIV and the diode shorted out). What
does this do in terms of circuit protection (I'm assuming that's what
it's there for)? Under what condition would the plate voltage actually
go below 0?

That's a novel one ! New to me and I've seen a few.

You're sure it was plate / anode - cathode ? What kind of diode btw ?

Graham

Well the cathode is grounded - so plate to ground. I'm 99% sure that's
what it was, but I'll look again to be sure.

apa wrote:

Well the cathode is grounded - so plate to ground. I'm 99% sure that's
what it was, but I'll look again to be sure.

Check to see that it's not the screen grid to ground! That would seem
like a reasonable thing to have.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On 17 Jun 2005 07:48:32 -0700, "apa" wrote:

A bit off topic for RAP, but I thought someone here might know right
off.
I was replacing burnt plate resistors in a Fender Blues Deville and
noticed a diode on each of the power tubes reverse biased between the
plate and cathode. I assume this would only conduct if the plate
voltage went negative (or exceeded PIV and the diode shorted out). What
does this do in terms of circuit protection (I'm assuming that's what
it's there for)? Under what condition would the plate voltage actually
go below 0?

This apparently protects from overvoltage/arcing in the output
transformer primary and connected components. This won't normally
happen even at full output with speakers connected, but tube amps can
be damaged if played with the load (speaker) disconnected. High signal
levels under a no-load situation, especially clipping, can cause
inductive 'kick' and high voltage spikes (much higher than the already
high operating voltage on the tubes), high enough to cause arcing
inside the transformer, in the tube, at the tube socket or elsewhere
in the circuitry, and generally causing component damage with
overvoltage.
Specifically, with a high-level input signal into a push-pull
output stage, if one tube conducts hard and then suddenly switches
off, the transformer (with no load, effectively an inductor) voltage
will go up to several times the quiescent plate voltage (and the other
plate will likewise go several times below that), but the diode on the
other side will limit that side's voltage to zero, and through
transformer action the side of the primary that just turned off will
be limited to twice the plate voltage.

Someone asked what type diodes these are, I'm guessing 1N4007 (a
few pennies each in quantity), 1A 1000 PIV, good for outputs powered
at 450V where they should see a peak reverse voltage of no more than
900V.

-----
http://www.mindspring.com/~benbradley

Chel van Gennip wrote:

On Fri, 17 Jun 2005 18:49:56 +0200, Pooh Bear wrote:

The load of the power tubes is highly inductive: a transformer. This
can result in high voltage peaks. If one side of the transformer is
going up, the other side is goging down and will cross zero. A diode
from anode to cathode clamps that peak,

No damn good for an output stage !

It *relies* on the tube with -ve a-k volts being reverse biased at the
grid.

You have not understand what I wrote.

That's how tube push-pull amps work !

When I was working with tubes they never showed any current with a
negative a-k potential.

They won't. It's impossible. Emission is going the wrong way.

You might get nasty flashovers between grids though.

That why I suggested the control grid should be driven negative.

Graham

Chel van Gennip wrote:

On Fri, 17 Jun 2005 18:49:56 +0200, Pooh Bear wrote:

The load of the power tubes is highly inductive: a transformer. This
can result in high voltage peaks. If one side of the transformer is
going up, the other side is goging down and will cross zero. A diode
from anode to cathode clamps that peak,

No damn good for an output stage !

It *relies* on the tube with -ve a-k volts being reverse biased at the
grid.

You have not understand what I wrote.

That's how tube push-pull amps work !

When I was working with tubes they never showed any current with a
negative a-k potential.

Sorry, it's unseasonably warm here and my brain went to sleep !

The push-pull stage shouldn't result in -ve a-k volts.

I expect the diodes are indeed a protective measure against inductive
flyback caused by no load or load mismatch.

Graham

Scott Dorsey wrote:
apa wrote:

Well the cathode is grounded - so plate to ground. I'm 99% sure that's
what it was, but I'll look again to be sure.

Check to see that it's not the screen grid to ground! That would seem
like a reasonable thing to have.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Definitely the plate. What good would a screen to ground do?

On Fri, 17 Jun 2005 17:43:34 GMT, Ben Bradley
wrote:

This apparently protects from overvoltage/arcing in the output
transformer primary and connected components.

excellent analysis snipped for bandwidth

In other words, this is how you get to "11".

Chris Hornbeck