guitarmike2107 wrote:The frustrating thing about the zener fix is its just that .. its a fix rather than understanding what’s causing it.
VTAMPSUSA wrote:Paul's document never really honed in on what exactly could be the root cause...
I don't think I can completely agree with those comments. Paul Ruby did a thorough analysis of that one particular problem and accurately pin-pointed the actual cause. He then proposed an effective remedy for that problem which goes directly to the source of the problem but has no impact on the amp's gain or overall voicing - all the R and C values stay exactly the same. I think that's pretty clever.
The only thing that Paul was unable to figure out was why in some amps you didn't hear the buzz although it could be seen on the 'scope trace. That's the remaining mystery.
JdJ wrote:In my case I think I have 2 things going on. The ruby buzz and the fizz which is represented in the big overshoot on my previously posted oscilloscope traces.
Yes, there's this separate issue of a general tendency to a bit of fizziness in some 18W amps, which Paul didn't address. I have seen some reports of this issue being affected by choice of OT as well as power tubes. With regard to the OTs I wonder if primary winding resistance is an important factor. AFAIK early RS 18W OTs had a higher primary winding resistance than later ones (650 ohms vs 480 ohms), and I suspect that perhaps the higher resistance in some OTs may help snub overshoot spikes that contribute to a fizziness in the tone.
JdJ wrote:I'd imagine there has to be a way to keep the PTs in or close to class a while maintaining a healthy (but not too healthy...) signal to the pt grids. Aiken on his site, and zaphod_phil here have mentioned decreasing ground reference resistors and/or increasing grid blocking to help keep the PT's from premature cutoff.
Reducing the reference resistors simply reduces the effective gain of the PI and hence drive to the power tube grids. Increasing the size of the grid blocking resistors limits grid current and also rolls of more upper frequencies. Also FYI the EL84 design guide says to use a minimum value of 10k for grid block resistors. So I've often wondered how come the Marshall 18W (or Watkins Dominator) uses 8.2k and Vox/Matchless amps use 1.5k.
JdJ wrote:I'm beginning to wonder if I have some PP imbalance that is resulting in the overshoot. I'm going to try replacing the coupling caps and all of the grid blockers/refs for the power section - there seems to be a small difference on my ohm meter between both sides...
I would really doubt that. And in any case I always maintain that some imbalance is desirable to help all the nice even harmonics not get heavily attenuated.
guitarmike2107 wrote:Makes you wonder if Paul actually knows more than he is telling about the cause of the buzz, the rocket builds will be slamming the power valves too.
IIRC TW Rocket amps use the same 220k grid reference value as Vox/Matchless amps. So they aren't really slamming the power tubes .
stevesuk wrote:According to a Tonequest article interviewing Dr. Z, they fit a conjuctive Filter in every 15/18W EL84 amp as standard.
The Watkins Dominator, which the Marshall 18W derives from, also has a conjunctive filter. But I thought they were only meant to be used to correct the frequency response of cheap OTs.