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"El Cheapo" with EL86-triodes?
"El Cheapo" with EL86-triodes?
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Old 30th June 2020, 09:41 AM   #11
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by rongon View Post
The reason I'm going here is that I have some Russian 6P43P-E I'd like to use, and I have a pair of 25W-rated 6.6k OPTs as well.
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Doesn't the 6CW5 family of output tubes require plate to plate loads in the 3k to 3k5 range, rather than 6k6, when operating as pentodes? Hence, tied as triodes the load impedance should be expected to be even lower?
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Old 30th June 2020, 10:51 AM   #12
rongon is offline rongon  United States
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"El Cheapo" with EL86-triodes?
Well, with triodes in the output stage, using a higher load impedance reduces THD but also reduces output power. Most designs use 2x the triode's rp to get the most power out possible, but that will be at higher THD for a given circuit.

I once built a PP 300B amp using 6.6k output transformers (UTC LS63, which I should have kept!). You couldn't really tell the power output was less, but you sure could hear how clean it was and the nice damping in the bass.

Given the higher-than-usual load impedance, I was hoping to get away with no global NFB loop.

Another possibility might be to use a 12AX7 LTP into ZVN0545 MOSFET source followers, into the EL86s, with 6dB gNFB. What do you think? Would that be a solution? It seems to work on paper...
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Last edited by rongon; 30th June 2020 at 10:55 AM.
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Old 30th June 2020, 11:02 AM   #13
Kay Pirinha is offline Kay Pirinha  Germany
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Do you intend class AB2?
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Old 30th June 2020, 11:05 AM   #14
rongon is offline rongon  United States
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"El Cheapo" with EL86-triodes?
No, class A would be fine.
If I'm not mistaken, higher load impedance keeps the outputs in class A longer (less power).
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Old 30th June 2020, 12:58 PM   #15
rongon is offline rongon  United States
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"El Cheapo" with EL86-triodes?
The reason for source followers after a 12AX7 LTP would be to avoid slew limiting into the Miller capacitance of the output triodes' grids. The 12AX7's Ip of 1mA or slightly less would not be enough. I figure a ZVN0545 sourcing 3 to 4mA would do it. Maybe even better would be 5mA+ from FQPF2N60C or something like that (with Crss of 5pF or less).

Any class AB capability would be gravy, but not the goal here.
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Old 1st July 2020, 05:23 PM   #16
artosalo is offline artosalo  Finland
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As a reference, this is a version I have tested.

As triode connected:
-Pout = 11.3 W, +Ub = 300 V, OPT = 5k
-Pout = 7.8 W, +Ub = 200 V, OPT = 5k

As UL:
- Pout = 23.5 W, +Ub = 300 V, OPT = 5k
- Pout = 16.8 W, +Ub = 250 V, OPT = 5k
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