EL34 to KT88

[pegwill]

Hi Guys

I see that some amps can use bith EL34 and KT88 valves. They seem to be able to do this at the flick of a switch. Is it only the bias arrangements they are switching or is there something else involved?

Manh thanks

Regards

Bill

[Richard]

Hi Bill,

Not really, just as you say, KT88s draw a bit more current. If it's cathode bias it's usually 470R for EL34 and 560R resistor for KT88 so 560R will allow either to be used even without a switch. G3 needs tying to cathode for EL34 which will the suit either as it's already ic in KT88. I did have a nice pair of monoblocs configured that way but tbh they sounded better with EL34 and I suspect that was because they used UL and were optimised as 5-20. I've had KT88 running nicely as tetrode but think either valve really needs its own optimisation and nether are really interchangeable as you'll find one is better than the other depending on UL/tetrode and primary load etc.

[bikerhifinut]

Originally Posted by Richard

Hi Bill,

Not really, just as you say, KT88s draw a bit more current. If it's cathode bias it's usually 470R for EL34 and 560R resistor for KT88 so 560R will allow either to be used even without a switch. G3 needs tying to cathode for EL34 which will the suit either as it's already ic in KT88. I did have a nice pair of monoblocs configured that way but tbh they sounded better with EL34 and I suspect that was because they used UL and were optimised as 5-20. I've had KT88 running nicely as tetrode but think either valve really needs its own optimisation and nether are really interchangeable as you'll find one is better than the other depending on UL/tetrode and primary load etc.

But you can run KT88's as UL too, I agree though that they both have their own individual character. Plus 88's generally run a higher HT I thought?

I did a PCB for KEGGER on the AK site based on the Dynaco St70 format with a different driver tubes and feedback scheme. Adjustable fixed bias for the outputs. For tube rolling, I tried the Russian 6P3S 6L6, Russian 5881 Wafer bases, EL34, Svet 6550 and Kt88's. Output iron was 6.6K and 4.3K Hammonds and cheep 8K-8 25watt iron with a B+ from 360 to 480 volts. The 6P3S 6L6 did really well with the 6.6K iron, ran out of steam with the 4.3K at 360V. 5881's were more authorative on the 6.6k, but ran out of steam with the 4.3K, the EL34's ran best on the 6.6K and required heavier feedback with the 4.3K, 6550 and KT88's ran well with both 6.6k and 4.3 iron.
The lowest cost was 6P3S and cheep 8K-8 25watt iron into 8ohm speakers.
My favorite combo was Svetlana 6550=C= at 480V with the 6.6K lower GNFB, for more volume, heavier feedback 4.3K iron.
I also preferred Triode connected over UL for the speakers that I'm driving 4ohm that dip down to 3ohm... ACI Orions.
Tubes were drop in replacements, just had to tweak the bias voltage, KT88's needing the lowest voltage bias to throttle them to 45ma.