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Is changing the power tubes in an amp incredibly complicated like changing things on the circuit or is it just rebiasing it for that type of power tube?

depends on the amp

some need quite a bit of modification amp; some don't

so, what amp are we discussing?

Amps in general really, I've always wondered what some Fender amps would sound like with EL34s, and some Marshalls with 6L6s, and other out-of-the ordinary tube/amp combinations.

I believe that it is a faily involved concept far beyond just tossing in a different tube type into your favorite amp. Different power ratings for example, will require serious mods to your amp so that it is not a dangerous hazard. Other factors are involved as well.

Amps wired exclusively for 6L6 types such as Fenders, can not run EL34's without modifications. However, amps wired for EL34's, such as most Marshalls, can run 6L6 and 6550/KT88 types with the correct bias voltage. This is because of the diffrences in how the internal componants within the tubes are constructed and connected to the pins.

Generally most Marshall's can run 6550/KT88, 6L6/5881, KT66, or EL34 with minor modifications to the resistors that determine the bias voltage range. In some cases the grid resistors on amps designed to run only 6L6 will need to be changed to preserve tube life, and prevent ossculations, when running KT66, 6550 or EL34.

Usually the primary impedance of the output transformer is matched to the specific tube type. For example, KT66 tubes ran slightly higher impedance than EL34, or KT88 that are close to the same. Early JTM45/100 amps originally equiped with KT66 used higher primary side impedance transformers than post early 67 plexis opt's that ran EL34's. Impedance mismatches with the transformer primary from tube type to tube type, are usually not worried about too much when swapping tube types. The original JTM45 output transformer had a mismatch with KT66's, but that's part of the mojo.

I put a toggle switch on the underside of my Edana 45 chassis that switches between 2 resistors, one for the EL34 bias range, the other for the KT66 bias range. Even that though is based readings I take off of two sets of power tubes of each style. I then average out the results.

It could be that down the road, an end user puts in an EL34 and the resistor I put in doesn't cover the proper range. But, it's an incredible easy spot to get to and swap out. Even to amp noobs, although I don't want anyone not qualified opening up one of my amps.

I have a Peavey Classic 60/60 with 6L6s that I'm thinking of switching to either KT88s, KT66s, or 5881s. How do I know if the PT can handle other types of tubes? I'm mainly interested in KT88s.


Originally Posted by n00bI have a Peavey Classic 60/60 with 6L6s that I'm thinking of switching to either KT88s, KT66s, or 5881s. How do I know if the PT can handle other types of tubes? I'm mainly interested in KT88s.Email Peavey and ask them. That's ultimately the safest route. Going with 5881's or KT66HP's is probably very easy to do. KT88... I have no clue about.

If you want less clean headroom, give the TAD 6L6WGB's a try. Nice tubes.


Originally Posted by n00bI have a Peavey Classic 60/60 with 6L6s that I'm thinking of switching to either KT88s, KT66s, or 5881s. How do I know if the PT can handle other types of tubes? I'm mainly interested in KT88s.

There's virtually no significant difference between 5881's and 6L6's, so only a regular bias adjustment should do. KT66's are the electrical eqivilants to 6L6's too. However, the KT66 sometimes like a larger grid resistor. 6550's have the same pin configuration as the 6L6, but also like larger grid resistors.