I purchased a small collection of audio items from an estate sale yesterday (I had to buy them all in one lot) and one of the items was a really nice Dynaco Mark III tube amp with a chrome chassis. Here is my question - when I opened it up, the output tubes were 2 Genelex KT88s with metal bases made in the UK. I don't know much about these tubes - are they equivalent to 6L6s (can 6L6s be used instead of the KT88s in this amp) and, to test these tubes, will the settings on my Hickok 752 tube tester for 6L6s (or 6550s) work? Thanks for any info.

By the way, among the other items was a Heathkit Audio Analyzer Model AA-1. Does anyone know anything about this and does it have any value? Thanks again...

On my tube tester the settings for the tubes are different,
I would guess "No"
You still are pretty lucky !
Peter

KT 88's are a lot closer to 6550's than 6L6's , KT66 are the equivelent of 6L6G's , also EL37 s.

If the KT88 are good they are worth a pretty penny . The B+ is to high for 6L6G's and GB's in the MKIII's ,you might be ok with 6L6GC if you sub a 5U4GB in place of the 5AR4 ans set the bias corectly .

Thanks for the information - I wonder how I can test the KT88s? My Hickcok 752 does not list any settings for these tubes.

Test as 6L6 but don'e be 100% sure of findings. Do your own resesarh on basings and ratings for both. They all did the same thing, amplify.

If your tester will test type 6550, it will test a KT88. The two types are interchangeable.

Larry

As mentioned, the KT88 is more like the 6550 than anything else. In fact, within the limits of the 6550 (voltage and plate dissipation- the KT88 is listed at 800v plate, 600v screens and 42w plate dissipation, while the 6550s are 600v/400v/35w) they are functionally identical.

As for other tubes in the Mk III- the output transformer on those is about a 4.2K primary. Given that, a 6L6GC won't really be "happy" there... it's too "heavy" of a load, for their characteristics. A 6L6 in ultralinear would be much happier with 6.6K to 8K primary impedance...

OTOH, EL34 tubes WILL work in a Mk III, just fine- as they are quite happy with 4.2K primary output transformers. You do need to, as suggested above, replace the rectifier with a 5U4 (direct plug in as well) to drop B+ a bit, and re-bias the output tubes (you will wind up very near one end of the bias pot, as it's hard to get many EL34s to draw enough bias current, as they need less bias negative voltage than 6550/KT88s). But, it will work. I have EL34s in my pair of Mk IIIs on my test bench now... some Russian EL34s that I wouldn't particularly miss, if something went wrong and one of them got damaged by an "oops" on the bench. They were convenient for me to use, when re-working the amps (re-cap and power supply upgrade). Once I finish everything, I'll put in some good (new?) KT88s or 6550s... or maybe some really good EL34s (such as Matsushitas, Mullards, or such)...

Regards,
Gordon.