Request for input and feedback on a circuit idea

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Nick
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#46 Re: Request for input and feedback on a circuit idea

Post by Nick »

Instability of some sort, maybe better power supply isolation between stages? They are all low gm valves but maybe try some grid stoppers.
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andrew Ivimey
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#47 Re: Request for input and feedback on a circuit idea

Post by andrew Ivimey »

I wonder if there is a virtue in the parralel heaters...Ive only done this on output pairs pp'd. 26s are ime very difficult to get right and a.c. heating which I did with dropper resistors and Hammond transformers ( centre tapped) still impossible! I stopped at the lead battery solution for a rainy day.
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Nick
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#48 Re: Request for input and feedback on a circuit idea

Post by Nick »

I wonder if there is a virtue in the parralel heaters.
Maybe, why do you think there could be?
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IslandPink
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#49 Re: Request for input and feedback on a circuit idea

Post by IslandPink »

What does the power supply look like ?
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andrew Ivimey
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#50 Re: Request for input and feedback on a circuit idea

Post by andrew Ivimey »

Nothing sensible Nick... just pondering. I found a.c. intolerable in 26 the hum just getting worse through the stages. 71as were fine. 2a3 and 300b, a doddle.
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RhythMick
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#51 Re: Request for input and feedback on a circuit idea

Post by RhythMick »

IslandPink wrote: Sun Mar 18, 2018 12:13 pm What does the power supply look like ?
I'll put up a circuit later but... 2 LCLC supplies, the 26 and 71A share 1st LC then branch.

45 is

400-0-400 tx > 2 x 1n4004 FWR > LCLC
LL1673-20H / Mundorf TubeCaps 200uF
Repeat

26 and 71A
350-0-350 TX > 2X1N4004 FWR
LL1673-20H / Mundorf TubeCaps 200uF

26 LC is LL1667-07mA, 100uF
71A is LL1667-15mA (parallel), 100uF

Bleeds are at the first cap.
Grounds all from the bottom of the LTP from each pair.
B+ to the centre of the pp tx primaries. 10r sense resistors to the anodes.
Cathodes joined and use a combination of resistors and high inductance chokes for the LTP.
Bias is tapped off the cathode resistors, for now they are pots to allow some control. Bias goes to centre tap of pp secondaries and from there to the grids.

The input is across ll1540. I have the 22k and 1nF cap across the outputs as per the 1540 datasheet.

The pp interstage have 1uF caps across input to output, again per datasheet.
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#52 Re: Request for input and feedback on a circuit idea

Post by RhythMick »

Nick wrote: Sun Mar 18, 2018 10:42 am Instability of some sort, maybe better power supply isolation between stages? They are all low gm valves but maybe try some grid stoppers.
Thanks Nick. I think the PSU isolation is pretty good but I'll add grid stops to my list of things to try.
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