KT-88's in Marshall amps

[Mark]

I've put a quad of KT-88's into a SLP Marshall. I have modified the bias so it can supply up to -60VDC. I've put a other set of KT-88's into the amp and they are idling at 32mA's with minimum bias voltage (-55VDC), the amp is putting out 100 watts, but I'm wondering if there is any merit biasing the tubes harder?

Please let me know of your experiences?

[xtian]

Wow, Mark--that's a lot of tube. Is it killing all the small animals in the area? Do you need to turn up the air conditioner when you play?

[Reeltarded]

How hot is the PT right now? This might be a Star Trek engine room call.

[cbass]

It can double as toaster oven.Strudles anyone?

[Mark]

Thanks for the replies.

The rail voltage is 470VDC and with signal it drops to 440VDC. I was initially worried as the power transformers can run quite hot on Marshall's but this one is okay. Come to think off it EL-34's would be biased hotter than this according to this calculator, it has EL-34's requiring 37mA's of bias while KT-88's require 52mA's of bias.

http://www.webervst.com/tubes1/calcbias.htm

I wouldn't bias the KT-88's at 52mA's as this is throwing caution to the wind, but it does seem to me these tubes are biased quite cold.

Thoughts and experiences?

[Reeltarded]

I dunno where the PT gives up. That is a tough call. You might want to search Metro Amp's forum for experiences.

I know if you bias them at 52mA you are basically opening a foundry in your amp where there was a PT.



Be gentle!

[Mark]

Thanks for the reply Miles. The owner wants more headroom in the amp, I've put the CRO on the amp and it distorts at the power tubes first, the amp may be able to put out 120W RMS, the extra 20W won't be audible noticed but will there be a better tonal response, with it be worth the extra hassle, this is the question?

I just noticed this from Eurotubes:

We re-tube almost as many Marshall amps as we do Fender amps. We see quite a few old ones but mainly we see 800's, 900’s and 2000’s. We do still see a few of the old 60’s and early 70’s Marshall's along with the mid 70’s Mark ll Master Volume amps. We will usually load them with the E34L's to get a better low end but we have also been using a lot of the JJ KT77's which sound great or the JJ Big Bottle 6CA7's which are quickly becoming a favorite. For the front end we almost always go with high gain ECC83S's and balanced phase inverter tubes but another real nice option is using the ECC803S in V1. If you're looking for a real fat sound from a 50 or 100 watt Mark ll the JJ KT88’s will do it! We’ve tubed a lot of them with KT88’s biased up to about 60 to 65mA and the tone is very thick and fat.

http://www.eurotubes.com/eurotubes-Mars ... modern.htm

Did I say WOW!

[Reeltarded]

Nope. My take. When Marshall started using 6550s they kept them cold for a reason. This reason.

What do you think about those tubes in there? It makes them sound like Texas to me.

[katopan]

That's cold and as you say would only give you a bit on top of EL34s. Set up with even higher voltages and a pair (rather than a quad) of KT88s can give you 100W, as per the datasheet.

[Mark]

Agreed the data sheet does mention this. However I only have a Marshall Super Bass to work with.

[Mark]

Agreed the data sheet does mention this. However I only have a Marshall Super Bass to work with.

http://www.marshallforum.com/workbench/ ... ormer.html

http://www.marshallforum.com/workbench/ ... ation.html

[Mark]

Here is a bit of feedback on the KT-88's. I biased them as high as 52mA's and it didn't appear to be that benefial, I decided to bias the amp at 44mA's and run it at full undistorted output. The amp ran for 45 minutes before the power transformer was getting too hot to touch. In my books that s a problem, I'll listen to the amp tomorrow at various bias settings to see if I'm missing something.

[JazzGuitarGimp]

I would think KT66's would be a better replacement for EL34's, or as mentioned, use just a pair of KT88's rather than a quad.