Gibson T-8790 IT in EH-100,125,150, maybe others

[murrayatuptown]

I'm encouraged to post a new question since the other discussions in this forum category were more than 120 days old.

EH-150's with T-8790 interstage transformer and it seems all versions have the same power transformer...I think T-40069. They're usually identified (online) as Thordarson. Thordarson has a 1938 catalog online and that era DID has a T prefix, but a letter in the middle of the numbers. I slowly realized the T-####(#) format was probably Gibson's #, and same for L-6904 or whatever for ones with a filter choke in addition to the field coil choke.

I got tired of trying to find info on the T-8790 IT that doesn't seem to exist, other than Mercury Magnetics replica. Somewhere I thought I read a description of turns ratio, which is enough to get an idea of the signal levels.

I didn't need one, but MM had a 10% off code thru August so I ordered one. I may need one soon for a build and rationalized.

So I'll measure the turns ratio when it arrives.

Meanwhile, does anyone know if the Thordarson p/n and turns ratio are known somewhere I haven't looked hard enough for?

Thank you   

[Willabe]

Classic Tone transformers that closed, said they built most of the Gibson PT/OT/chokes for them.

They still had all the Gibson spec order sheets. Problem was Gibson used different numbers on the order sheets than what the amp model was.

So Classic Tone had no idea which PT or OT was for what model. 

[murrayatuptown]

Mark Sacketti at MCI/Classic Tone always answered my questions there, no matter what I asked...until that one day. I thought maybe he had had enough, but the letter about closing came out I think the next day.

I once asked him about a missing output transformer in a Gibson BR-9 (that I think was a GA-8 transitional circuit). I didn't know what to look for, but probably didn't want a used one if my amp was missing it because it failed.

They made the PT, but not the OT, and still had a folder for the latter from when Gibson sent them a request for quote, so he was able to tell me what its requirements were.

Thank you

I suspect the T-8790 isn't exotic or magical, since the EH-100 & EH-125 circuits are substantially different from EH-150's IT PI variant. I have a '150 with that transformer, but it works & I don't want to mess with it unnecessarily.

I ordered a 'replica' from Mercury, will try to convince myself I can extract enough info to describe it. I thought I saw turns or impedance ratios described somewhere, but it's hard to find random finds again sometimes...no luck yet.

When I think I have a set of 'terminal characteristics', I'll pull tubes from the '150, and repeat measurements in the chassis.

FWIW, MM's specification drawing, consisting of a schematic and wire colors, with primary and secondary reversed (or mislabeled) isn't very inspiring. I understand why they might be protective, but there isn't even size info. They must only want to sell them to people who are going to use it for EH-150, and nothing else.

The kind of information they give out freely, in abundance, is not specification.

But I think their products appear to be well-engineered and built. Until I read the marketing, which doesn't change any facts...just my impression.

If I need the same IT again, I can buy one of theirs again, but I think knowing some fundamental specs is important. I don't need to know how they make them, and they don't have to tell me. Other companies who sell to individuals answer reasonable questions.

I'm going to build something '150-inspired for a friend, and I'm not too obsessed about the PT, OT or choke (mine had choke + field coil as choke).

Thanks again