OT impedance ratio WAY high - why?

[EFK]

I have two Supro 1690T amps, one a 57 and one a 61. They're almost identical although the OT are slightly different. Here's the thing: they use two 10" 8 ohm speakers in parallel for 4 ohms. All fine and dandy. When I measure the OT impedance (disconnected) I use a 9V wall power supply. Under load, it's putting out 7.58 VAC. I apply this voltage to the speaker side and get 590VAC on the tube side. This means a turns ratio of 77.8 to 1. So, the impedance ratio works out to about 6K to 1. So these two 6L6 tubes (cathode bias, about 390 plate voltage) are seeing 24K? What the heck is THAT about? Can someone expound upon why Valco would have done this, and what sonic effect this might be having as opposed to a more "typical" 4.5 to 6.6K? I thought the first one was a fluke but since I was converting to a 12" speaker and a much bigger OT anyway, I didn't care. But this second one I just picked up is the same as the first!

Am I doing the math wrong somewhere?

[loudthud]

Your math looks correct so there must be something bad with the measurements. Most of the errors I can think of would tend to cause the calculated primary impedance to be low. Do you have another wall wort you can try or maybe use the heater supply from a working amp?

[EFK]

I just tried this with two different wall supplies, same results. I also decided to use a JTM45 transformer I have here as a test case or baseline and I got about 7.9xxx K, which is what the transformer is marked (8K). If this was just one amp, I'd think either the OT was faulty or something was wrong but TWO identical amps? Unfortunately it's almost impossible to find info on these, but I'm going to have to assume it's part of the design. So again, what effect might this be having as opposed to a "normal" impedance? Cutting the volume way back? (They are not loud at all for a 2X6L6 amp.)

[loudthud]

Well, you sure couldn't expect much power out of these amps. What is the B+? Does anything on the amp specify 6L6GC like chassis marking, tube layout diagram or schematic? Could it be designed around the low power versions 6L6, 6L6G, 6L6GA, 6L6GB?

[EFK]

B+ is about 380-390 V, cathode bias. The earlier one is (was originally) 6L6 GB, the later is 6L6 GC. I have the earlier one really ripping with a 12" speaker and JTM45 OT 16 ohm into an 8 ohm load for 4K using JAN 7581A now, but I wanted to keep this one stock as it is pretty much untouched. This OT situation has thrown me for a loop though, as I originally thought the first one was just screwy when I got it, but now it seems the norm for this model. ???

[Chuck H]

I originally thought the first one was just screwy when I got it, but now it seems the norm for this model. ???

But a primary Z of 24k isn't normal. It has to be a mistake. Not on your part but perhaps a bad part spec and low QC or something like that. It would have to be really low QC for the 'problem?' to go unnoticed for at least four years though. When I search for info on the amp most of what I find is your previous threads. But that 24k primary Z is just so far out of any typical spec that I can't imagine it was intentional.

Chuck

[EFK]

I would certainly agree with you. I initially thought they may have been using (2) 2 ohm speakers in parallel, but I think I might have made a mistake with my measuring the first time around because with this amp, I pulled everything apart and measured the speakers individually and they are reading @ 7.2 ohms each. I suppose if I could find two 2 ohm speakers, this transformer would be well suited! I've got other transformers which would probably work much more effectively, so that's likely the easiest way out.