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Old 27th November 2009, 07:42 AM   #1
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Default Filament current bias

Does anyone remember this topology discussed here? I did a search and could find no info. I am interested in how this sounds compared to CCS on the cathode and floating filament supply. Is there a lot of noise from the current passing through the cathode resistor?

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Old 27th November 2009, 08:08 AM   #2
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The 12,5V must be coming from a CCS. I have shown the same topology (with CCS) in the "26 preamp thread" but not built it yet.
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Old 27th November 2009, 08:12 AM   #3
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Yes, great thread! Thank you Lars. No bypass cap needed is very good. This could also work well with IDH tubes, cathode tied to the top of the bias resistor on the heater.
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Last edited by greenvalve; 27th November 2009 at 08:19 AM.
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Old 27th November 2009, 03:13 PM   #4
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The first person I ever recall utilizing such a setup was Thomas Meyer about 15 years ago...but, if I remember correctly, there was no CCS, just an L-C-L-C-L filter.
Such a setup works well, but can generate a lot of heat! Probably most practical with DHTs that have low filament current draw, like the 112A.

Jim

Last edited by dowdylama; 27th November 2009 at 03:18 PM. Reason: typo
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Old 27th November 2009, 08:44 PM   #5
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Thank you Jim,

Ha ha, that WOULD be a lot of heat for something like a GM70 Even with 7.5 V filament at 1.5A a 15W part would get mighty hot. Do you remember what tubes Thomas was using? I am surprised this is not used much more often. I think it would be great for the input tube of a phono-stage for instance where the typical bypass capacitor on the cathode bias resistor is so audible. Or where the graininess of LED bias is evident.

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Old 27th November 2009, 08:58 PM   #6
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I believe Thomas was biasing WE 101Ds (or maybe it was 101Fs) in this manner.
My 'still-under-construction-after-about-a-year-and-a-half' phono project biases the 12BZ7 (V1) this way.

Jim
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Old 28th November 2009, 12:16 PM   #7
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Instead of a big heat dissipating resistor, why not use another tube ? For example two 6.3 V tubes in series, 12.6 VDC supply. One tube has 0 - 6.3 Vdc, average "bias" 3.15 V, the other has 6.3 - 12.6 Vdc across it, average bias 9.45 V.
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Old 29th November 2009, 10:58 AM   #8
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Robert, yes using the filament of tubes for bias is another good idea. Remember, on DHT, bias measurement is always taken from the more negative voltage cathode pin. On IDH tubes connect the cathode to the positive side of the heater. There is no 'average' bias. A 12.6v supply feeding two series tubes with 6.3v heaters, that have the same current draw, would have bias voltages of 6.3v and 12.6v respectively.
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Last edited by greenvalve; 29th November 2009 at 11:11 AM.
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Old 29th November 2009, 11:59 AM   #9
Sendy is offline Sendy  Czech Republic
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I think this topology was used in some WW2 German radio station. Single system tubes.
I saw it in old military RX too.
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Old 29th November 2009, 03:21 PM   #10
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Quote:
Originally Posted by Sendy View Post
I think this topology was used in some WW2 German radio station. Single system tubes.
I saw it in old military RX too.
Yah - I figured there was an earlier use of this approach - there always seems to be

If you have more info, I'd be curious to know what tubes this was used with back in the good ol' days...as I indicated earlier, heat becomes a major consideration with this approach.

Jim
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