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Old 24th September 2005, 02:01 AM   #1
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Default dht p-p filament question

hello. i have been working on building a dht p-p 10 (vt-25) amp, and ran into something that gave me pause. i am using a shared cathode resistor (bypassed), and wired the filaments backwards on each tube of a p-p pair. i did this, as i am running dc filaments, and had been told by jack elliano years ago to do that (on a single ended amp). have the + reversed on the pins between the two dht output tubes, and then just switch the tubes every once in a while (to help ensure a full life). anyhow, that is where i got the idea to do the same between each p-p pair. i recently read something, however that said wiring the filaments to different pins is bad (ie: pin 1 of one tube is wired to pin 4 of the other). is this the case? will i have to change them, or will they be fine as is? really annoying as i have spent a lot of time making the wiring look nice. won't be too bad to undo, but still i would rather just leave it. if there is a problem, what is the technical basis? thanks a lot!
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Old 24th September 2005, 09:28 AM   #2
billr is offline billr  New Zealand
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Hi, It does not make a blind bit of difference. Filaments are filaments, if you wire them in parallel as you have, it will work ok.

best wishes

bill
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Old 24th September 2005, 03:01 PM   #3
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great...thanks! i thought it was fine too, but as i mention, i had read somewhere that it is bad. guess you can't trust everything you read
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Old 24th September 2005, 03:08 PM   #4
jsn is offline jsn  United States
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I have seen it suggested to wire the filaments out of phase with each other to gain a bit of hum cancelling in the output transformer. To do this, you should wire pin1 to pin1, etc.

This should cause any filament related hum to cancel in the output transformer.

I'm not totally sure about that increased life thing, but if that is your goal, you could add a DPDT polarity switch in the filament supply circuit and flip it every hundred hours or so :)

jsn

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Old 24th September 2005, 03:11 PM   #5
kmtang is offline kmtang  Canada
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Of course, it does make sense for supply the filament with separate power supply.

Hammond 266M2 is a good candidate which provides 2 separate 2.5V 3A AC supply for the filaments.


Johnny
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Old 24th September 2005, 03:30 PM   #6
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hey jsn. thanks. i think that might have been what i read. my thinking was that it wouldn't matter so much since i was using dc on the filaments (ie: no hum problems).

hey johnny. based on my research, i think that shared cathode bias is the best way to go, as opposed to a separate bias component for each tube. and i'm using the dc because the voltage is so high on the vt-25 (7.5 volts).
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Old 25th September 2005, 03:58 PM   #7
kmtang is offline kmtang  Canada
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Oppsss, sorry. My mind was stuck with 2A3.

Johnny
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Old 25th September 2005, 04:31 PM   #8
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hehehe...that's ok. it's a good place to be stuck
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