KT88 heaters

croccodillo · 29th December 2005, 03:24 PM

Hi all,

I'm trying to build a PSE amp using two paralleled KT88 for each channel, driven by a 12AX7 with the help of a PowerDrive circuit (Thanks to TubeLab).
I have a lot of surplus transformers, but no one with a suitable voltage for the KT88s heaters (6.3V).
I have instead some 12V transformers, so my thought was: can I place the two KT88 heaters of each channel in series (in a string)? This should give me a 12V string, and I could use some resistor, in parallel with each heater, to equalize the two voltages across the two heaters themselves.
Is this feasible?

Thanks,
Giovanni

Giaime · 29th December 2005, 03:42 PM

Obviously you can! Better would be to rectify the heater supply and regulate it to 12.6V precisely. Or better yet, with a CCS as I've seen. But this may be overkill... I think that simply putting them in series without any resistors and tricks will do good.

Btw, Giovanni, are you a member of some italian forums? I don't remember to have seen your nick anywhere else.

Edit: could you provide a schematic of your amp? Are you driving the KT88 in A2 with the powerdrive?

Shoog · 29th December 2005, 05:55 PM

Series should be fine- no resistors necissary.. Running them slightly lean wont be a problem. Don't bother going DC, there is absolutely no advantage with KT88's.
Just make certain that your transformer is beefy enough and wont sag under the strain.

Shoog

dhaen · 29th December 2005, 06:38 PM

I would add the caveat that both valves be of the same manufacturer and year.

croccodillo · 4th January 2006, 02:02 PM

Quote:

I would add the caveat that both valves be of the same manufacturer and year.

Yes, I have a matched quartet from JJ Tesla, coming from the same lot of production (at least this is what they told me...).

Giaime:
I have no schematic of my amp yet, I'm still trying different configurations and building some parts before to put all togheter.

However my amp will be a PSE (two KT88 for each channel), with para-feed output topology feed by a SS CCS, a PowerDrive to drive the two KT88s (modified in manner it can equalize continuously the current into the two paralleled tubes) and a not-yet-decided input stage (my favorite input stage at the moment is a long tail with a CCS).
The KT88s cathodes will be connected to ground, while the PowerDrive will provide the needed negative bias voltage.
Some sort of local feedback will be added, from the output stage to the input one (no feedback from the OPT, due to phase shift).

The B+ will be about 600V, regulated by an high voltage mosfet.
The PowerDrive will be powered by two voltage (Bias+ and Bias-), of about 24V and -120V (both of them regulated with mosfets).
This should permit me to drive the KT88's grid from about +20V to -80V, going into A2 zone.

I have built the four CCS (two with a current of about 5mA -input stage-, the others two with a current of about 150mA -output stage-).
At now I'm building the power supply, next will be the two modified PowerDrive boards and, at the end, the amp itself.

Ciao,
Giovanni

richwalters · 4th January 2006, 02:14 PM

What cheeses me off is stupid images been put on thread pages with an imbedded virus been put on before virus system updates.

Moderators wake up and stop this stupid time wasting practice.

richj

Giaime · 4th January 2006, 03:33 PM

Quote:

Originally posted by richwalters
What cheeses me off is stupid images been put on thread pages with an imbedded virus been put on before virus system updates.

Moderators wake up and stop this stupid time wasting practice.

richj

What are you referring to Rich?

Giovanni: I never heard of running KT88 to A2. Can you point me at some links of someone who did this? How much power are you expecting to get from this PSE? Maybe 50 or more watts...

dhaen · 4th January 2006, 05:42 PM

Quote:

Originally posted by richwalters
What cheeses me off is stupid images been put on thread pages with an imbedded virus been put on before virus system updates.

Moderators wake up and stop this stupid time wasting practice.

richj

Rich if you know of any on diyAudio.com please let us know their whereabouts and how you detected them.

richwalters · 4th January 2006, 08:32 PM

Although I'm implying Gaime got this on his system...(I don't want a fight with a nice fella) but the attachment http://www.danasoft.com/sig/Croccodillo.jpg contained a bug that upset my computer clock. If I'm wrong Ill take it all back but the forum mail was the only one I had in the "in box". I had opened other diyaudio threads, perhaps the villan is elsewhere.

richj

Giaime · 4th January 2006, 09:55 PM

I still can't understand.

29th December 2005, 03:24 PM	#1
croccodillo diyAudio Member Join Date: Mar 2005 Location: Bergamo	KT88 heaters Hi all, I'm trying to build a PSE amp using two paralleled KT88 for each channel, driven by a 12AX7 with the help of a PowerDrive circuit (Thanks to TubeLab). I have a lot of surplus transformers, but no one with a suitable voltage for the KT88s heaters (6.3V). I have instead some 12V transformers, so my thought was: can I place the two KT88 heaters of each channel in series (in a string)? This should give me a 12V string, and I could use some resistor, in parallel with each heater, to equalize the two voltages across the two heaters themselves. Is this feasible? Thanks, Giovanni __________________ In Nomine Libertatis Vincula Edificamus, In Nomine Veritatis Mendacia Efferimus.

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29th December 2005, 03:42 PM	#2
Giaime diyAudio Member Join Date: Oct 2004	Obviously you can! Better would be to rectify the heater supply and regulate it to 12.6V precisely. Or better yet, with a CCS as I've seen. But this may be overkill... I think that simply putting them in series without any resistors and tricks will do good. Btw, Giovanni, are you a member of some italian forums? I don't remember to have seen your nick anywhere else. Edit: could you provide a schematic of your amp? Are you driving the KT88 in A2 with the powerdrive?

29th December 2005, 05:55 PM	#3
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	Series should be fine- no resistors necissary.. Running them slightly lean wont be a problem. Don't bother going DC, there is absolutely no advantage with KT88's. Just make certain that your transformer is beefy enough and wont sag under the strain. Shoog

29th December 2005, 06:38 PM	#4
dhaen diyAudio Moderator Emeritus Join Date: Jun 2002 Location: U.K.	I would add the caveat that both valves be of the same manufacturer and year.

4th January 2006, 02:14 PM	#6
richwalters diyAudio Member Join Date: Sep 2003 Location: Alps:Tube amp designs over 150W, SMPS guru.	What cheeses me off is stupid images been put on thread pages with an imbedded virus been put on before virus system updates. Moderators wake up and stop this stupid time wasting practice. richj

4th January 2006, 08:32 PM	#9
richwalters diyAudio Member Join Date: Sep 2003 Location: Alps:Tube amp designs over 150W, SMPS guru.	Although I'm implying Gaime got this on his system...(I don't want a fight with a nice fella) but the attachment http://www.danasoft.com/sig/Croccodillo.jpg contained a bug that upset my computer clock. If I'm wrong Ill take it all back but the forum mail was the only one I had in the "in box". I had opened other diyaudio threads, perhaps the villan is elsewhere. richj

4th January 2006, 09:55 PM	#10
Giaime diyAudio Member Join Date: Oct 2004	I still can't understand.

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