retox

I need some help deciding what to do with two Fisher 400s. I got both of them from the same person and he was the original owner. One has smooth plate Telefunken tubes and the other has The Fisher branded tubes from Great Britain. I am going to sell one of the receivers and wanted opinions if i should keep the Telefunkens of Fisher 12AX7s from GB. What are your thoughts on this. I could also keep all the 12AX7s and put new JJs in. I would have to eat the cost for the JJs.

Any thoughts on the subject are welcome...

Hi Mark,

It could be possible one side of the 12AT7/ECC81 has a weaker side compared to the other. A tube tester would be one way of checking each section to see the difference.

Or power up the amplifier with either a dummy load or speaker load, and check over the voltages for each side of the 12AT7/ECC81. Plate/grid/cathode voltages...
The voltages should be pretty close, but won't be exact.

I suppose if there is a big difference in voltage between each section of the 12AT7, then there could be a problem with the tube having unbalanced sections with regard to emission. One side could be pulling more current than the other side.

I usually test the 12AT7/ECC81(or any driver tube in question)with a tube tester.

Basically look and inspect the circuit over real well...grounds...etc...
You never know what you might find.

Mike.