Does anyone care to ask an amplifier designer a technical question? My door is open.

@enobenetto  I own your RM-10mkii, which is a mysteriously addictive amp. How did you approach low-frequency response? I ask because the bass the amp produces is punchier than any 100-watt solid state I've owned.

I have zero knowledge of electronics, so my other question is a simple one of your opinion on newer Russian tubes and their sonics/quality. I might be the only person in audiophile land who actually likes the JJ brand. They seem the most linear and balanced, albeit with a dry top end.

I am pleased how many people love the bass of that amp. I think I just lucked out. I designed and prototyped all the transformers myself. The application is unique. Very high voltage, perhaps its that. I gave a talk about it at Burning Amp2018 that outlines the application and compared it to the typical applications 

 http://berkeleyhifischool.com/having-fun-at-burning-amp-2018/

I stock several brands and vintages of EL84s. I do not stock JJ so I can't say much. Im glad you like them.

@geek101  @ramtubes with Class D switching freq over 450/500 KHz these days do you still think output filtering is an issue?. How do I educate more about this ?. If the issue of filtering not a function of switching freq please correct me. Thanks again.

One would want to see the measurements John Atkinson style. There is no other way to know but measure it. If these things interest you Stereophile is your best bet and its a good deal at $14 or so.

The filtering is easier the higher the frequency. That does not mean its done right. JA recently reviewed a D amp that had what I assume is pre EQ to make up for the drop. I think it was NAD.

Of course there are flaws in that idea because it EQ's for a resistive load. At least one can play with it and choose by ear.

@terry9 

Thanks for the nod ... I guess the takeaway is, "Don't be so lazy. Match HFE at operating temperature and current." Thanks!

Have you measured the idle current of each transistor at operating temp in the amplifier? You of course know just to measure the Emitter drop divided by R. How well is it distributed? Then you could put a scope on them at full power and see how equal those are. 

Dont spread the emitter resistors too much because they might not track and share over full swing. I would make all the emitter resistors the same and do good transistor matching. You might make them a little higher like 0.22 ohm. You won't loose much output.

The concept of varied emitter resistors is a thought I have not had. :)

@terry9

I would use a dual trace scope, make sure the two channels have identical gain, use one emitter resistor on channel 1, call that the reference. Then use channel 2 to see how the others (one at a time) match up at various power levels. I would find the results interesting.

Have you also looked at the output transistor current driving the speaker at high levels, high frequencies? Trumpet music, thats a killer? Have you measured the impedance of the primary side over frequency? I have found some stepups have so much capacitance that more of the energy goes into the transformer than the panels. FYI, the 57 and 63 transformers, though different, are very cleverly wound. A torroid typically has lots of shunt capacitance that you have to drive. The QUAD transformer does not.

BTW, I am not impressed at all with the Plitron audio transformers. Van Der Veen’s paper seemes flawed to me. He is a great speaker promoting his idea of automatic bias. He believes you can use a gapless tube output transformer to make a practical amplifier. I take great issue with that. I think there are some unasked thus unanswered questions. I would ask him, "what happens when there is a DC offest in the output stage. It will happen when the amp is pushed?"

Similarly if you have any DC across your primary the core flux will be off center. I cant say how many mV is ok, but i think at 50 it could be noticed and at 100 mV it might be a real problem.

Here’s how to set up your test:

Scope ground goes to hot output terminal of course. Im assuming the output stage is emitter follower.

Dont forget to float the AC power on your scope. Othewise you will be shorting your output.

On my bench I only have one piece grounded. Everything else floats. Even my soldering iron floats because sometimes I solder amplifiers while they are on. There are parts of the circuit that dont much mind. Dont do that with SS amps, the are unforgiving.