Re: Ampeg V4B instability

On Apr 20, 8:35 pm, mykeymyke...@xxxxxxxxx wrote:
On Apr 20, 8:50 am, Mike Schway <msch...@xxxxxxx> wrote:





In article <1177041580.418466.136...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,

mykeymyke...@xxxxxxxxx wrote:
On Apr 17, 3:22 pm, Jonathan Krogh <johnc...@xxxxxxxxx> wrote:
Oh BTW I did take out those 'flyback' diode strings off the power
plates, surely this wouldnt be the cause....? I mean.. no other amps
have them

you need to keep the diodes on the power plates,
this is what prevents the oscillation, and yes
a bunch of amps DO have them including Marshalls.
we have cured many a mesa boogie design screw up
using the same method.
prefer to use 1000V diodes (two) in series, it
works better than one; on each plate.
this will also cure ultra high frequency oscillation
problems in many amps.
reinstall the diodes and the problem will be cured.
is this a real V4B or a fake SLM (crate) V4B?
from what you describe, sounds like a real one.
used to run an ampeg repair center (real magnavox
ampeg) that's how i know about this problem.
and as far as SLM (fake) ampegs, they do make excellent
boat anchors; or in case you don't have a boat, Reynold's
pays 15 cents a pound and it's all CASH!

How could these diodes POSSIBLY prevent oscillation? In order for them
to conduct, the plate would have go lower than NEGATIVE 2-3 V (assuming
two 1000V diodes). That just ain't gonna happen unless you have
grid-to-plate short, and if you do, you'll have bigger problems than
oscillation!!!.

What they do accomplish is to act as a sort of sacrificial inverse-fuse
which will short plate-to-ground if the plate voltage ever exceeds the
PIV of the diode string. This CAN happen under flyback conditions (high
signal with no load on secondary. If your fuse is of the correct value,
the diode string will protect sockets and your trannies.

I do agree, however, about the difference between East Coast Ampeg and
Midwest Ampeg. Different beasts entirely, though I'm not sure SLM ever
made a V4.

To the OP: you may want to reflow all solder joints in the power amp
section. These things notoriously overheat and/or crack in a V4,
especially if there's been a previous tube failure.

--Mike

--------------------------------------------------------------------
Mike Schway | [Picture your favorite quote here]
msch...@xxxxxxx |
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it definitely does prevent oscillation, ultra high frequency,
low frequency and "motor boating". that's why the manufacturer
installed the diodes to start with.
if you want to modify the amp by removing them, you must
accept the consequence.
" I took out the diodes and now my amp oscillates"
just where is the mystery in this?
if you are brave enough to re install them you would soon
find out that I am 100% correct.
you can resolder all you want, check the capacitors all
you want, etc, etc, etc... the oscillation will not stop
until you install the diodes.
(finding a bad solder connection in a magnavox ampeg
would really be something, cause there aren't any, these amps
are not PVS!)
for many years i have shared knowledge and many laugh at it,
but sooner or later what i have been saying all this time
will be revealed as truth. i sincerely doubt that anyone reading
AGA has repaired more tube amps than i have, and you could
possibly learn something from a person with more experience
than yourself- if you are willing to accept new ideas.
many amps including Marshall contain the same circuit.
see old Marshall schematics, learn something new.
you will also find the same circuit in some versions of
(real) SVT's.
the diode is used as an audio feedback path to stabilize
the power supply. its not a DC path. the fact that two out of phase
audio signals are converging at the B+ is creating a "zero point"
reference balancing the plates to that reference.
its not my fault it works.- Hide quoted text -

- Show quoted text -

I have had to use some tricks on a lot of V4's to prevent actual
melting of solder joints on the screen-grid supply resistor, which can
result in that resistor losing an electrical connection. This
effectively removes the screen-grid voltage, and the output stage gain
drops to almost nothing. I've solved this problem by replacing the
resistor, along with the individual S-G feed resistors with new
wirewounds, keeping the lead long, bending them along the traces and
each other to create a larger area to dissipate heat, then soldering
the whole nest of leads. It has worked great in every case, and I
haven't seen a single one come back for that problem again. I mention
this because it is one example of solder joint related problems I've
found in many amps, and Ampegs are definitely *not* immune to this
problem.

I don't see how those diodes could possibly have any effect on low
frequency oscillation- not saying you're wrong, as I've seen weird
stuff that didn't make sense but just worked. I have removed these
diodes in a few V4's and never had any motorboating occur, and many
high power amps work just fine without these diodes, so...

I have worked on a few dozen Ampegs in my time, as well as some
Fenders, Marshalls, Traynors, Carvins, Mesas, Sovteks, Gibsons,
Epiphones, Peaveys (ugh), Crates, etc. etc. etc.

-dave M.

.

Relevant Pages

  • Re: Ampeg V4B instability
    ... a bunch of amps DO have them including Marshalls. ... works better than one; on each plate. ... this will also cure ultra high frequency oscillation ... reinstall the diodes and the problem will be cured. ...
    (alt.guitar.amps)
  • Re: Ampeg V4B instability
    ... a bunch of amps DO have them including Marshalls. ... works better than one; on each plate. ... this will also cure ultra high frequency oscillation ... reinstall the diodes and the problem will be cured. ...
    (alt.guitar.amps)
  • Re: Ampeg V4B instability
    ... a bunch of amps DO have them including Marshalls. ... works better than one; on each plate. ... this will also cure ultra high frequency oscillation ... reinstall the diodes and the problem will be cured. ...
    (alt.guitar.amps)
  • Re: Ampeg V4B instability
    ... you need to keep the diodes on the power plates, ... a bunch of amps DO have them including Marshalls. ... works better than one; on each plate. ... ampeg) that's how i know about this problem. ...
    (alt.guitar.amps)
  • Re: Another one for the Setties...
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