GM70 PP

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izzy wizzy
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Re: GM70 PP

Post by izzy wizzy » Sat Aug 10, 2019 4:58 pm

Nick wrote:
Fri Aug 09, 2019 10:48 pm
Once the grid goes positive, it becomes an anode.
Thanks Nick. Obvious really isn't it.

So that makes me wonder why I see some circuits have caps in that position (bias point on the IT to ground) and some don't. Maybe I should go and play in the sim and try and answer that.

In my head, I've kinda been thinking should I put some in to ensure the AC impedance of both halves are exactly the same independently of where the bias pots are set i.e. bypassing that resistance so the signal is developed only across the IT not the IT and the bias resistance. But then I'm adding caps in series with the signal and that doesn't feel right.

Cheers,
Stephen

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Nick
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Re: GM70 PP

Post by Nick » Sat Aug 10, 2019 6:11 pm

Sometimes a little more complexity allows for simpler and therefore better operation.
Or, sometimes more complex is better full stop. Without needing the mental gymnastics making black into while so it fits in with a dogmatic position.

"It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong. Richard P. Feynman"
Resistance isn't futile it's V / I.

izzy wizzy
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Re: GM70 PP

Post by izzy wizzy » Sat Aug 10, 2019 6:31 pm

Nick wrote:
Sat Aug 10, 2019 6:11 pm
Or, sometimes more complex is better full stop. Without needing the mental gymnastics making black into while so it fits in with a dogmatic position.
That's why I like it round here. So many other forums decend into a pissing contest of what's "right" ... whatever that is.

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IslandPink
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Re: GM70 PP

Post by IslandPink » Sat Aug 10, 2019 6:44 pm

Nick wrote:
Sat Aug 10, 2019 6:11 pm
.... If it doesn't agree with experiment, it's wrong. Richard P. Feynman"
Now then Nick - remember what we said over email, about citing Richard Feynman against people in an argument ?
"The bass is the king of the instruments - it has no known natural predator" (Wobble)

izzy wizzy
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Re: GM70 PP

Post by izzy wizzy » Sat Aug 10, 2019 6:53 pm

IslandPink wrote:
Sat Aug 10, 2019 6:44 pm
Nick wrote:
Sat Aug 10, 2019 6:11 pm
.... If it doesn't agree with experiment, it's wrong. Richard P. Feynman"
Now then Nick - remember what we said over email, about citing Richard Feynman against people in an argument ?
I wasn't game to tackle that one ;)

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Nick
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Re: GM70 PP

Post by Nick » Sat Aug 10, 2019 7:33 pm

IslandPink wrote:
Sat Aug 10, 2019 6:44 pm
Nick wrote:
Sat Aug 10, 2019 6:11 pm
.... If it doesn't agree with experiment, it's wrong. Richard P. Feynman"
Now then Nick - remember what we said over email, about citing Richard Feynman against people in an argument ?
Not sure I was citing it "against" anyone (other than myself), and I am not sure I was even arguing :-).
Resistance isn't futile it's V / I.

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Re: GM70 PP

Post by RhythMick » Mon Aug 12, 2019 8:36 am

Ok I overstated. Fair cop. I'm well chastised.

"Everything should be made as simple as possible, but not simpler" isn't right either - it references what's possible but not what is optimal. And of course everyone will have their own view of what is optimal.

I was simply trying to say that if it works and sounds fantastic, why introduce additional complexity? But then I don't live by that guidance myself, as my regular experiments to see where the envelope can be stretched demonstrate. That's how I learn and I've no doubt others in the group do similar.

In other words ignore me. I'll get me coat.

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Re: GM70 PP

Post by izzy wizzy » Tue Aug 13, 2019 7:15 pm

Hooking up the SMPS direct to the filaments was never going to be the final solution but it got me started. Put in 8mH common mode chokes between to great effect. Then you can hear the signature of the SMPS on the sound once a fair portion of the ripple has gone. Spec for the PSUs are 120mV of ripple and I'd guess that will be mostly HF stuff.

Looking at the spec sheet for the choke cites 100R impedance at 1k. With the filament at about 7R, that should equate to roughly 6mV of ripple, less at higher frequencies. That's a decent reduction and it sounds it. Plan is this will feed Lehane VCCS filament supplies. With a cap following the CM choke, I'm hoping the VCCS will get a relatively smooth ride. These chokes have good response out to 300kHz.

However having read this, there is a very real possibilty I have the theory wrong. I only went for the theory after listening to the reduction of the SMPS effect on the sound. Maybe more work to do on the theory.

Modelling the bias supply in PSUD shows 150mV of ripple. That seems a lot. I've often wondered why fixed bias supplies are so crude in general and then all this gets injected into the grid(s) of the final(s). A LC of 10H and 20uF drops this to about 1mV so that could be my next move.

Cheers,
Stephen

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