Re: Ampeg V4B instability

On Apr 20, 9:48 pm, Jonathan Krogh <johnc...@xxxxxxxxx> wrote:
On Apr 20, 9:35 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 -

Well I did reinstall them, so we shall soon find out, apparently
theyre originally a pair of 1400piv in each string, so ive put in
three IN4007 per string, I will leave out the coupling cap replacement
and get it going. I was at this time installing some spring retainers
and a set of KT88s, but their matching is way off, so back to the
6L6GEs ive had in.- Hide quoted text -

- Show quoted text -


Ok, back to the diode strings installed, no effect on the low/sub
sonic oscillation/motorboating, im also seeing that it can be
triggered by noise on the vol pot of one of the channels, im not
getting any DC on that pot either, so I need to take that control
board out and start shotgunning caps, I cant work on that board in
situ. I did shotgun out caps on the main board, didnt help any. GE
6L6's reinstalled all biased up around 35ma per tube.

.

Relevant Pages

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  • Re: Ampeg V4B instability
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  • Re: Ampeg V4B instability
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