Re: HiWatt DR504 phase splitter problem

On Thu, 3 Jul 2008 20:53:24 -0700 (PDT), vikingamps@xxxxxxxxxxx wrote:

On Jul 3, 10:48 pm, RS <R...@xxxxxxxxxx> wrote:
On Thu, 03 Jul 2008 23:18:14 GMT, "Gareth Magennis"

<gareth.magen...@xxxxxxxxxxxx> wrote:

Well this amp came to me in a sorry state, and now after lots of work, it is
nearly there. I am going to sell it, hopefully to someone who is going to
be able to use, love and cherish it for many years.
Sure, I could put in a Band-Aid ECC82, but that kind of makes me one of
those rip-off merchants pedalling expensive years of dissapointment, and I
don't want to be one of those people. If it was mine to keep though, that
is probably exactly what I would do.

Cheers,

Gareth.

Keep in mind that the gain from that stage will be very low with
ECC82/12au7. Probably only 8 or so, compared to theoretical 25 with
ECC81/12at7. I bet that is why you're not seeing the same
glitch--you've attenuated the signal to the point that it's not
pushing the output tube grids into conduction.

A 12AT7 would have about the same gain as the 12AX7 in that circuit
that "fixes" the problem.

Yeah, actually a 12at7 can have slightly -more- gain than a 12ax7 in a
split tail. I believe that would be the case here, given the component
values. Not much difference of course, so I'd attribute problems to
rp, etc., presuming that they really are not turning up in both tubes.

I would make sure there isn't something
causing oscillation, I've seen that symptom before and that was the
cause. Lead dress is important as well as making sure the OT polarity
is correct. The Hiwatt PI and output section are quite a bit different
from a Fender, including the feedback. I wouldn't rule out a screen
resistor either, I've seen those cause oscillation in the output
section.

It will be interesting to see what this actually is. It didn't sound
like he was describing HF oscillations on the scope trace, but that
doesn't explain the problems with the sound. It did seem like he was
seeing the 'spike' only at extreme drive levels, but I got the
impression that he didn't think it sounded good at any volume.
.

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