Mark II simul-class switchable to 60watt mod?

[erickompositör72]

Does such a mod exist? Either something like a 3-way switch (simul 75w, 60w class a/b inner tubes, 15w class A outer tubes), a separate switch, or changing the normal switch (simul class/class A) to a simul/60w switch?

I would actually find this way more useful than the simul/class A switch.

Would this even be possible?

 

[Vinzenz]

Possible? - yes
Worth the effort? - no, afaik

 

[erickompositör72]

Possible? - yes
Worth the effort? - no, afaik

Please tell me what leads you to believe this would be universally "not worth the effort."

I find the difference between simul-class (2 tubes class A triode + 2 tubes class A/B pentode) and class A/B full pentode to be so distinct that it would absolutely be worth it for me (if it's something Mike B. would do).

 

[VisaL]

Take out the outer pair. Doing that though might make audible the fact that inner pair is biased so cold that cross-over distortion is present. I think usually that’s not audible because the outer pair is making up for that. That is particularly good option if you have not fixed the bias issue with the pseudo-class A of the outer pair. It’a not cathode biased class a, but class a/b biased to over 100% of plate dissipation and wired pentode. Very much no-no based on the spec sheets for el34/6l6 tubes. That ’simul-class’ just fries the tubes without any benefit.

I think it would be funny mod to wire a switch which makes you able to choose between Simulclass and properly biased class ab(inner pair) + properly biased class ab wired pentode. So basically one position would be simulclass and the same basic circuit but in compliance with tube spec sheets. That would be awesome to demonstrate what is the actual benefit of shortening the tubelife.

I think one kind of compromise would be to bias the outer pair properly and add triode/pentode switch. Running the amp in ”class a” mode and pentode would achieve what your after.

 

[Vinzenz]

Please tell me what leads you to believe this would be universally "not worth the effort."

I find the difference between simul-class (2 tubes class A triode + 2 tubes class A/B pentode) and class A/B full pentode to be so distinct that it would absolutely be worth it for me (if it's something Mike B. would do).

If I'm right, the 60W class a/b pentode is available anyway and much cheaper than simul class.

 

[Vinzenz]

Take out the outer pair. Doing that though might make audible the fact that inner pair is biased so cold that cross-over distortion is present. I think usually that’s not audible because the outer pair is making up for that. That is particularly good option if you have not fixed the bias issue with the pseudo-class A of the outer pair. It’a not cathode biased class a, but class a/b biased to over 100% of plate dissipation and wired pentode. Very much no-no based on the spec sheets for el34/6l6 tubes. That ’simul-class’ just fries the tubes without any benefit.

I think it would be funny mod to wire a switch which makes you able to choose between Simulclass and properly biased class ab(inner pair) + properly biased class ab wired pentode. So basically one position would be simulclass and the same basic circuit but in compliance with tube spec sheets. That would be awesome to demonstrate what is the actual benefit of shortening the tubelife.

I think one kind of compromise would be to bias the outer pair properly and add triode/pentode switch. Running the amp in ”class a” mode and pentode would achieve what your after.

 

[Vinzenz]

Afaik class A only means that a tube conducts 100% of its cycle no matter if self biased or fixed bias. Class A can be triode or pentode Mode

 

[Fastforded]

Very confusing to me.

 

[erickompositör72]

Take out the outer pair. Doing that though might make audible the fact that inner pair is biased so cold that cross-over distortion is present. I think usually that’s not audible because the outer pair is making up for that. That is particularly good option if you have not fixed the bias issue with the pseudo-class A of the outer pair. It’a not cathode biased class a, but class a/b biased to over 100% of plate dissipation and wired pentode. Very much no-no based on the spec sheets for el34/6l6 tubes. That ’simul-class’ just fries the tubes without any benefit.

I think it would be funny mod to wire a switch which makes you able to choose between Simulclass and properly biased class ab(inner pair) + properly biased class ab wired pentode. So basically one position would be simulclass and the same basic circuit but in compliance with tube spec sheets. That would be awesome to demonstrate what is the actual benefit of shortening the tubelife.

I think one kind of compromise would be to bias the outer pair properly and add triode/pentode switch. Running the amp in ”class a” mode and pentode would achieve what your after.

Thanks so much for this insight. I do know that Mike B does a pentode/triode switch for Mark II's.

And I have experienced the simul-class burning through tubes (SED winged C EL34's were the latest victim, which are supposedly a pretty robust NOS tube). However, I am a believer in the simul-class tone. I have a simul and a 100/60 watt, and the simul produces incredible overtones/ harmonics that I just don't get out of the 100/60, not to mention the added "bounce."

The 100/60, however—even in 60 watt mode—has amazing clarity, punch, and "spank." I would love to see how my simul behaves if I'm able to switch it to full class a/b pentode. I'll see if I can have a conversation with Mike B about it.

 

[erickompositör72]

If I'm right, the 60W class a/b pentode is available anyway and much cheaper than simul class.

Cheaper is relative. IIc+ 60W amps go for 4K these days, and that's the non-EQ model.