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Old 1st August 2014, 09:23 AM   #1
Ceglar is offline Ceglar  Australia
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Default DHT PSE 2A3

I'm stuck with a common filament xfmr for both OP tubes, and the OPT UBT-1(?) is rated according to Mr Atwood, as marginal for stacked supplies.. and with a surplus 1:1 IT, figured to try this.

With previous near melt down experience (3 stage DC), I'm liking the idea of some fixed grid voltage.. liking the clipping characteristics of cathode bias, happy to add some there up an until about RL = 3x Rp. No bypass..

Realising that -ve supply is regulated and B supply floats, not sure if some partial Rk is enough to mitigate the majority of the effect (?).

Cap labelled xuF is something I've not really seen before in SE cct.. usually from C supply to ground.. not C supply to filament CT, but it does seem to close the AC loop nice and tightly, however I'm unsure as how to arrive at a suitable value (keep Xc at LF < 22k? _ that'd be good).

I can use a relay coil in series with the 20mA CCS and have it switch either 30VDC (don't really want it in the cathode cct of the OP stage), or 230Vac, in which case it'd be the primary of the filament transformer, B supply primary is off-board. In the latter case, B OP supply could be set up with a timer, so various angles to work with there.

Bit out of my depth, but its on the bread board, any comment much appreciated.

pls. no comment on parallel vs single SE or PP etc etc…


Shane
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Old 1st August 2014, 10:30 AM   #2
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xuF decouples the fixed bias, so ~1uF will suffice (cp 22K || 82K = 17.3K) at LF.

A warmup relais can short the 82K (to say, 10K) to protect the output stage filament during warmup. Grids get -100V or more while 82K shorted, to give full protection of cold filaments - this is much better than delayed B+, where an abruptly rising 250V may send sharp current pulses through the 2A3s.
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Old 1st August 2014, 11:03 AM   #3
Ceglar is offline Ceglar  Australia
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Thanks for that, would you change anything in the OP stage filament cct:?
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Old 1st August 2014, 11:05 AM   #4
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So long as the driver has plenty more headroom than the endstage, the overload will be independent of the RC there.

The signal current in the cap is very small, so the dielectric will not have much effect. For this reason, choosing a 1uF with small physical size, for low pickup, and low mechanical effects, is best.
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Old 1st August 2014, 11:09 AM   #5
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Quote:
Originally Posted by Ceglar View Post
Thanks for that, would you change anything in the OP stage filament cct:?
ac may give acceptable hum levels, but the sound is far behind a purpose-designed current driven filament regulator.

The 10 ohm signal-balancing resistors are a good idea though.
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Old 1st August 2014, 11:10 AM   #6
Ceglar is offline Ceglar  Australia
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As in the end stage runs out of steam first, yes.

I was thinking TCC PIO 1uF/400 (on hand..) not exactly small but radial stumps would see lead length kept to a minimum..
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Old 1st August 2014, 11:13 AM   #7
Ceglar is offline Ceglar  Australia
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Quote:
Originally Posted by Rod Coleman View Post
ac may give acceptable hum levels, but the sound is far behind a purpose-designed current driven filament regulator.

The 10 ohm signal-balancing resistors are a good idea though.
Yeah, I'd noticed before that it goes nearly unmentioned about the Lsec of filament transformers and how impedance varies with frequency. Maybe not an issue in practice, but no harm in being empirical. (Hi, MI.)

Last edited by Ceglar; 1st August 2014 at 11:19 AM.
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Old 1st August 2014, 11:13 AM   #8
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Physical size of the cap may make a difference.
Why not try your PIO, and compare to a modern MKP stacked type (Panasonic, EPCOS etc).
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Old 1st August 2014, 11:15 AM   #9
Ceglar is offline Ceglar  Australia
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Can do.. easiest might be 2.2uF M supremes on hand.

WRT DC regs, I'm eating cat food ATM and limited to what I have.. 2.5/5A Ac.
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