How to change from EL34 to KT88

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tjohn1968
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How to change from EL34 to KT88

Post by tjohn1968 » Sun Jun 19, 2022 9:40 pm

Need help changing from EL34 tubes to KT88 tubes. I would like to do this with a 1969 Super Bass build and a 1968 1986 build. What would I need to do to make this switch? I also have the Lamar type master volume in these amps. Any recommend bias settings for the KT88’s?
Has anyone done this? Any help would be appreciated.

Thanks!

American Viking
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Re: How to change from EL34 to KT88

Post by American Viking » Mon Jun 05, 2023 10:46 am

The main part is the bias feed resistors coming off the phase inverter. They will be 220K for EL34 based amps and you should use 150K for KT88's or 6550's.

The bias range resistor will likely also need to be adjusted but I would put the tubes in first and worry about that part later if they won't fall into range. I think the stock circuits usually have something like a 47K or 56K resistor in the bias circuit. Sometimes that has to be lowered a little bit. If it's too high, piggybacking a resistor will work to get to the desired total value. The KT88's may fall in range without altering this component which is why I say leave it for now.

You would bias the KT88's as if they were EL34's. Basically continue to use 25W max plate dissipation in your equation to set the bias. The reason for this is that the transformers are not designed to use a 35W tube so if you bias them that way you'll be putting unnecessary stress on the power transformer. There's no real benefit to running them that hot either. I always use 60% to 70% of max idle current. I used to use 70% all the time but I've been more conservative in recent years with the cost and availability of tubes going through the roof.

Hope this helps

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BehindBlueEyes
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Re: How to change from EL34 to KT88

Post by BehindBlueEyes » Tue Jun 06, 2023 11:49 pm

I asked this question a decade ago on the plexi palace and was told the only necessary part was to the bias range. I used a 50k trim pot and adjusted the bias resistor (forget to what) so I could go back to EL34's, which I haven't. Still working fine on my self built 1987 using the metro instructions

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