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Old 16th February 2012, 05:30 AM   #1641
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Albert, from #1627: please make
R1 = 0.5
R2 = 1.5 + 25mH, series connected.

Choke in R1 position will reduce voltage. (If you have a 20V transformer, inductor here works well)
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Old 16th February 2012, 02:18 PM   #1642
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Quote:
Originally Posted by Rod Coleman View Post
Albert, from #1627: please make
R1 = 0.5
R2 = 1.5 + 25mH, series connected.

Choke in R1 position will reduce voltage. (If you have a 20V transformer, inductor here works well)
Rod,

Thanks for the advice

Albert
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Old 19th February 2012, 09:25 AM   #1643
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Has anybody taken into account Steve Bench's findings about the 26 in starved filament. Look at the figures below!
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File Type: png 26 starved filament.png (29.8 KB, 341 views)
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Old 19th February 2012, 09:46 AM   #1644
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by andyjevans View Post
Has anybody taken into account Steve Bench's findings about the 26 in starved filament. Look at the figures below!
Hi Andy,
I have and with great results. I tried 30, 30sp, 112,01a,10Y, 46, 4P1L, 6P21S in addition to 26. If you use the Rod Coleman filament regulators you can reduce the filament current precisely whilst measuring THD.

See some of my comments here:

4P1L DHT Line Stage

I found effective not only to reduce THD but also microphonics, as this was the suggestion to address that issue on the 4P1L preamp.

When using triode strapped DHT, is interesting to find that there is a point where starvations magnifies the H3 and H5 above H2.

Currently I'm playing a 71a preamp with filament bias and gyrator loaded. I actually like it very much:

My Blog

Distortion levels can be reduced from 0.04-0.06% to 0.01-0.02% depending the valve

I'm currently experimenting with the 6P21S:
My Blog

Cheers,
Ale
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Old 19th February 2012, 09:50 AM   #1645
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Hi Ale

So where are you running the 26 filaments then?

Andy
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Old 19th February 2012, 11:10 AM   #1646
regal is offline regal  United States
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Mogliaa did you find a decrease in transductance or increase in Ra with the reduced filament current on the 4p1l?
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Old 19th February 2012, 11:55 AM   #1647
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by andyjevans View Post
Hi Ale

So where are you running the 26 filaments then?

Andy
Hi Andy,
Can't find my notes and will need to measure it again but I remember it was around 60-70% (650-700 ma) of the data sheet value, so close to the values you posted before.

I spent quite a lot trying other valves so will need to come back to the 26 to refine it
cheers,
Ale
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Old 19th February 2012, 11:56 AM   #1648
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by regal View Post
Mogliaa did you find a decrease in transductance or increase in Ra with the reduced filament current on the 4p1l?
Hi Regal,
Haven't measure both, so can't say. I focused on minimising microphonics and THD, which did with excellent results...
cheers,
Ale
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Old 19th February 2012, 12:21 PM   #1649
regal is offline regal  United States
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Originally Posted by mogliaa View Post
Hi Regal,
Haven't measure both, so can't say. I focused on minimising microphonics and THD, which did with excellent results...
cheers,
Ale
Well I imagine if the RA went up much you would have seen a drop in the frequency response if you used a choke or output tranny, but I remembered you were using CCS OTL for the test setup.


I'd recommend others using an inductive load to check this by doing a freq response sweep if running lower currents. I ended up shelving the pair of #26's I had and bought a few 4P1L's, so as usual I won't be any help.
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Old 20th February 2012, 11:52 AM   #1650
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Mouser has 100 watt 10 ohm wirewound resistors for about 8 bucks. They also have thick film 100 watters for bolting to heatsinks for about the same price. Which would be a better choice for filament bias for the 26? Does the inductance of the wirewound have an impact on the circuit?
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