• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Any idea what the primary impedance of the X101-B OPT is?

Status
Not open for further replies.
Jump to Latest
jmillerdoc

jmillerdoc

Member
Joined 2009
#1
Hello, I just acquired an old, beat up, Fisher X101-B 7591 PP amp for parts. It has good iron but that is about it. I think I will ultimately striip the iron and build something else but I haven't been able to find the OPT ratings anywhere on the net. Does anybody here have any idea what the primary impedance ratings are for the OPT's in this unit are? It uses a PP 7591 output stage in fixed bias....that is all I have found.

Thanks,
Jeff
 
#3
Do you have signal generator and a DVM (two DVMs would be better)? Hook the generator up to the primary (brown and blue?) along with a DVM, connect the other DVM to the secondary ( C and 16 Ohms). Turn the generator up all of the way, note each DVM reading. Divide the primary by the secondary, that's your basic voltage ratio. To find the impedance ratio square the voltage ratio. Now multiply that by 16 and there's your primary impedance. If you want to see how close the 4 and 8 Ohm taps are repeat on each of those. The frequency of the generator really doesn't matter, but you can experiment with different ones. You can also use a Variac instead of the generator also.

Craig
 
#4
jmillerdoc said:
Does anybody here have any idea what the primary impedance ratings are for the OPT's in this unit are? It uses a PP 7591 output stage in fixed bias....Jeff
Click to expand...
Just from memory the 7591A typically like about 6600 ohms P-P. I think the Fishers (and Scotts) were very close to that.

Edit: The RCA tube manual confirms 6600 ohm P-P in fixed bias w/450 Vp
 
Last edited:
#6
According to my Sams it is 6.6k. PM me if you are open to trading. I happen to have an x-101-b with a non-original output transformer so would be willing to sacrifice something nice for them.

Interestingly, the Sams also lists that power transformer at 200mA capacity. The problem is, that amp draws slightly more than that at idle. Mine went up in smoke one evening. I wouldn't recommend using it for a very big amp.
 
#7
Doc,

A possible use for X101B O/P trafos is in combination with Class "AB2" triode wired 6L6GCs. The "iron" will be roughly 6.6 KOhms (end to end) on the primary side and 35 or so W. capable.

Heed Spread Spectrum's warning and acquire a better power trafo, regardless of what you use for signal topology.
 
Status
Not open for further replies.
Top Bottom