dht p-p filament question

enochRoot · 24th September 2005, 02:01 AM

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!

billr · 24th September 2005, 09:28 AM

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

enochRoot · 24th September 2005, 03:01 PM

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

jsn · 24th September 2005, 03:08 PM

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

http://boozhoundlabs.com

kmtang · 24th September 2005, 03:11 PM

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

enochRoot · 24th September 2005, 03:30 PM

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).

kmtang · 25th September 2005, 03:58 PM

Oppsss, sorry. My mind was stuck with 2A3.

Johnny

enochRoot · 25th September 2005, 04:31 PM

hehehe...that's ok. it's a good place to be stuck

24th September 2005, 02:01 AM	#1
enochRoot diyAudio Member Join Date: Mar 2004 Location: ohio	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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24th September 2005, 09:28 AM	#2
billr diyAudio Member Join Date: Dec 2003 Location: new zealand	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

24th September 2005, 03:01 PM	#3
enochRoot diyAudio Member Join Date: Mar 2004 Location: ohio	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

24th September 2005, 03:08 PM	#4
jsn diyAudio Member Join Date: Sep 2004 Location: The Atomic City	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 http://boozhoundlabs.com

24th September 2005, 03:11 PM	#5
kmtang diyAudio Member Join Date: Nov 2004 Location: Vancouver	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

24th September 2005, 03:30 PM	#6
enochRoot diyAudio Member Join Date: Mar 2004 Location: ohio	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).

25th September 2005, 03:58 PM	#7
kmtang diyAudio Member Join Date: Nov 2004 Location: Vancouver	Oppsss, sorry. My mind was stuck with 2A3. Johnny

25th September 2005, 04:31 PM	#8
enochRoot diyAudio Member Join Date: Mar 2004 Location: ohio	hehehe...that's ok. it's a good place to be stuck