KT88 in triode or pentode?

[yaffstone]

And there you have it. Has anyone compared KT88 beam power tubes

wired in triode versus pentode configuration? I have to replace

the octal sockets in my heavily modified ST-70 and I need to know

whether to go triode on the output section. Beam power tubes were

designed to solve the sonic issues with pentode so did they?

Inquiring minds need to know....

Thanks,

David

[Dylanl]

I didn't know the Dynaco ST 70 used Kt88s? I thought only the MKII AN IIIs did.

[yaffstone]

The Dynaco ST-70 doesn't use KT-88, but mine does. I guess it's

not a Dynaco any more, but it sure sounds great. I replaced the

driver board with a VAT board capped with Auricaps and Orange Drops and

have some ECC-82 (12au7) on order for it. The quad cap was

replaced with an under chassis unit while keeping the original quad cap

can for asthetic purposes. I've replaced all the sockets with

ceramic and am using JJ Tesla KT-88 for output tubes. Currently

running bias current of 40 mA and in ultra-linear mode. It's

hooked up to the teak horns you may remember which have had their

crossovers re-capped with polypropylene. The whole thing just

sounds awsome. I'm now considering design and construction of a

surround sound (seven channel) tube amp based on what I've

learned from the Dynaco. I'm thinking dimond plate surrounded by

cherry and point to point wiring under.

But back to the question: Are KT-88 better in triode mode than

ultra-linear or did they do a good job designing the beam power tube in

the first place????

[NOSValves]

Ultra linear is half way to Triode as it is. To run your pentode/Beam in Triode you connect the plate to the screen with a 100 ohm resistor. Ultra linear is connecting the plate through a 40% tap of the winding in the transformer effectively connecting the plate to the screen. It give you almost the power of the pentode with better linearity then if strapped in triode. Triode will cut your over all power in half. If you really want to have the best of both worlds why don't you install a switch in each channel and make it so you can change back and forth and see what you like. I find Triode sound great with Jazz and acoustic music and Ultra linear sounds great with it all.

two switched like this will do the job and fit right between the two output tubes. If you need wiring instructions just give me a call.

http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=388644&e_categoryid=84&e_pcodeid=01008

Craig

[Dylanl]

The Dynaco ST-70 doesn't use KT-88, but mine does. I guess it's not a Dynaco any more, but it sure sounds great. I replaced the driver board with a VAT board capped with Auricaps and Orange Drops and have some ECC-82 (12au7) on order for it. The quad cap was replaced with an under chassis unit while keeping the original quad cap can for asthetic purposes. I've replaced all the sockets with ceramic and am using JJ Tesla KT-88 for output tubes. Currently running bias current of 40 mA and in ultra-linear mode. It's hooked up to the teak horns you may remember which have had their crossovers re-capped with polypropylene. The whole thing just sounds awsome. I'm now considering design and construction of a surround sound (seven channel) tube amp based on what I've learned from the Dynaco. I'm thinking dimond plate surrounded by cherry and point to point wiring under.

But back to the question: Are KT-88 better in triode mode than ultra-linear or did they do a good job designing the beam power tube in the first place????

Can you send me a few pictures of your current set up and amps? I would love to see them.

Thanks, Brett

[yaffstone]

Pics are perhaps attached.

[yaffstone]

And one from afar. The EQ is no longer in use since I went to the

tube amp! Sorry about the pic quality (lack of) The only digital

camera I have is the one on my cell phone. No flash, and not

enough light in the room without the corner lamp on. Anyhow, you

get the idea.

[Dylanl]

Wow an ST70 with KT88 Tubes

[NOSValves]

Brett,

Its a very simple thing to do just adjust the a few values in the Bias supply for proper biasing of the KT88's. Takes about 5 minutes. But there is a price to pay the power transformer will not like it for ever. Also the KT88's are running very much on the class B side of the AB push pull spec.

Craig

[yaffstone]

Class A/B or B/A, this was an improvement over the EL-34. I

suspect the difference is in the efficiency of the beam design.

Far less internal scatter of electrons with the commensurate loss of

power and potential noise. So far the X-former is happy. I

did lose a JJ GZ34 though. I don't think this is due to the KT88s

since there is less B+ draw than before as the bias current is reduced

from 100 mA / tube to 50. I think I just got a bad GZ34.