[starvinmarvin]

hello everyone,
I confess I don't know much about tubes except I like the way they sound.
i have a couple of modded dynaco mkIII's I could use help with here.
I have had them for about 15 years now and love the way they sound.
my question, (if you can tell from the pictures that is)
 what did they do and can I substitute the kt90's for kt88's or 6550's?
 (one of the kt 90's finally gave out)

[ElliottStudio]

Looks like external polypropylene caps were added for power supply bypassing. They also removed the tube rectifier and replaced with solid state (under the chassis), added more PSU caps (under the chassis) and who knows what else?

You should be able to use KT88's or 6550's in place of the KT90's, but not knowing the circuit and the mods done to it I can't say for sure. Depends on where the self-bias ends up with the alternate tubes. I would stick with the '90's unless you have a tech test it out for you with the other types.

[FullRangeMan]

The current crop of 6550 are more suited to Guitar combos than hi-fi.
In the regular Dynaco the EL34 was used also.

[ElliottStudio]

StarvinMarvin, I probably needn't tell you of the dangerous high voltages present on those external caps. The HT is on the bottom and the ground is on the top by the looks of the wiring but do be careful.

I don't think that if the amp was modified for KT90's that the EL34's would work anymore- unless the bias was adjustable but it appears that this amp is auto bias. But nobody can tell just by looking at those pictures. OP- Did the person who did the work provide any drawings?

[JohnR]

The Mk 3 is fixed bias. It looks like there's a second pot in the top of the chassis so my guess is it's been modified for independent bias of both output tubes. If so there should be a second wire for the bias measurement takeoff point running to the octal socket on the front, but can't see from these photos.

[starvinmarvin]

I had these professionally checked out when I first got them to make sure they were in fact safe to run, as the seller had no info on the mods that were done.
 they assured me the voltage on the caps was not lethal,
 but keep them away from children and curious fingers anyway,
(which I do)
I know they look kinda of a mess, but I like the sound and better bass I get than my stock st 70
another question I have is what are the different characteristics between the tubes,
 and is it safe to swap different tubes.

[ElliottStudio]

If it is indeed fixed bias (I couldn't make out any bias pots in the photos) then should be able to try the KT88's and 6550's. Just be careful not to bias them too hot.

As for different characteristics, it's all dependent on brand and the circuit you use them in. The only way to know is to try them - see if you can borrow sets of the ones you wish to try.

[JohnR]

If you look at the first photo, you can see them this side of the output tubes. The one in the middle of the "X" is the original one, the one to the right is the added one.

@starvinmarvin, might not be a bad idea to get the person that checked them over before to show you how to adjust the bias for the output tubes and the voltage/bias current ratio.

[Edit: I'm assuming that they're actually connected, hard to know from the pics]

[ElliottStudio]

^^^Good advice there..