KT 88 project request

[catalin gramada]

Hello. Looking for a tested power amp schematic using 4 kt88 .hammond 278cx and 1650t into ultralinear mode. Could this stage be full driven from a 12at7 ltp please? For a kt88 a max 100k leak bias resistor is requested.How convenient is
from a 12at7 perspective. to use a 47k for a pair? I.m not familiar with this type of ul circuit.it will be used for bass gt aplication. Can someone to help me please? Thanks. Catalin

[Enzo]

http://bmamps.com/Schematics/Sunn/Sunn_Sentura_2.pdf

Try that.

[catalin gramada]

Do you know if is enough room inside 278cx over the isolation to do some turns for an other 6.3v secondary for preamp heaters.please ? I think the 6A sec. will not drop so much under 4 kt88 filaments...maybe a litle more heat but think still far safe for a hammond PT... Also I intend to use another small tranny for bias circuit conected directly to the main plug with no switch in between and 'shuntable' limiter current resistors in primary side of 278 like soft start circuit. What do you think...? Thanks. Catalin

[catalin gramada]

Hallo .what to expect from a pair of kt88-90 driving a hammond 1650R . 5k Raa at 550v in UL mode please? Can it deliver 100w into a 8 ohm load .please? I am very restricted by hammond choices. Thanks. Catalin

[Jazz P Bass]

I would think yes.

With a very hefty grid signal.

With the disclaimer of about 2.5% total harmonic distortion.

[catalin gramada]

Thank you. Thinking to drive it from 12au7 at 400v or so. Looks pretty classic
Transformers was ordered. Hammond 278cx 400-0-400v. 1650R 5K Raa. 159v choke 1.5H/0.5A and 4pc. 500u/500v

[catalin gramada]

As I have to wound the tranny for bias circuit have to ask to experts if it worth the effort to wound one more sec. for a fixed bias supply for drivers instead cathode bias.to take as benefit full driving capacity from 12au7 .please?

[big_teee]

Thank you. Thinking to drive it from 12au7 at 400v or so. Looks pretty classic
Transformers was ordered. Hammond 278cx 400-0-400v. 1650R 5K Raa. 159v choke 1.5H/0.5A and 4pc. 500u/500v

Won't the 400-0-400, run thru a full wave rectifier, generate 560-600v, depending on the AC primary?
I would think your 500/500u 500v Caps would have a short life?
Unless you are wiring them series? I may have missed that part?
T

[catalin gramada]

I intend to use it in series bateries two by two half ac ripple voltage on each don.t think so much stress on it

[pdf64]

what to expect from a pair of kt88-90 driving a hammond 1650R . 5k Raa at 550v in UL mode please? Can it deliver 100w into a 8 ohm load .please?

I got about 80 watts at the onset of overdrive, using JJ.
Yes it's a little disappointing not to get the magic 100, but it's a monstrous sound and I don't think anyone will feel short changed.

[catalin gramada]

Thanks. I want to add a litlle bit complexity using a fixed bias circuit vs cathode bias instead for a pair of 12au7 drivers. Balance trim pot of course to take all benefit. I.m not familiar with fixed bias mode for triode. Do you see any incovenience.please? Thank you.Catalin

[catalin gramada]

Allright. This is the plan...

[catalin gramada]

and a update: separate negative bias power supply.

[catalin gramada]

Hello. How should I run the input wires for an UL transformer.please? Anode wires twisted in a pair then ul taps twisted in another and b+ separate it is good enough.please,?thanks.

[tedmich]

Twisted pairs on the OT can help shield and stop oscillation in the primary and secondary, especially if they are longer. I run the B+ close to a ground plane (chassis) and of course keep the primary/secondary far apart.
Any noise experienced when using a chop stick to adjust leads during initial testing might respond to winding (or unwinding I suppose) the OT leads, so keep an open mind!
PS Don't forget the bleeding/balancing resistors on your series caps, 220k being an oft used value.