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Hammond PT-125E

11/6/2000 12:22 AM
Ted Pimm
Hammond PT-125E
A universal output transformer must make some compromizes in power handling or frequency response at some of its ratios. Correct?  
Having said that, anyone know where this little OT performs best? i.e. anyone have any idea at what primary/secondary impedence ratio this transformer would operate best in push-pull? I am thinking mostly of power handling rather than frequency response.  
Just curios! I have some flexibility in design at this point because I have low plate voltage (325V). I have a few different sets of old tubes to consider, including 6F6, 6K6, 6V6, 6L6, EL34.  
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11/6/2000 12:23 AM
Ted Pimm
Correction Hammond OT-125E(end)
Thats the OT, 125E (not PT).
11/6/2000 3:28 AM
Winnie Thomas

The 125E is rated for 15 Watts, but that's at class A and conservative at best. I am using one to drive two 6V6's in one of my amps with 348 Volts on the plates, and it works just fine.  
11/6/2000 5:56 PM
Ted Pimm
6F6G,6V6GT,6K6GT and 6L6GC with same Rk and Zout?
Ok this might be a goofy question BUT...  
If I have around 325V plate voltage, what load impdence and cathode resistor values would compromize for use of either 6V6, 6k6, 6F6 and 6l6 tubes. Is a compromise possible? Looks like a load impedence of around 7-8k and a cathode resistor of around 300ohms might do it. Thinking just about cathode bias right now.  
Any suggestions? It might be a lot to ask but it would be cool to switch tube types around in the same amp.  
11/8/2000 8:52 PM
Carl Z

If you want to be able to swap several different power tubes in and out I think your best bet would be to use fixed bias. Cathode bias will be a big hassle. Even though it's cathode biased you will have to rebias after every change in tube type.  
With a fixed bias arrangement, once you get the bias circuit calibrated you can wire up a switch to change the bias resistor with every tube change. For some ideas on how to do this check out the schematics of some amps that have EL34/6L6 selector switches.  
Carl Z
11/8/2000 9:05 PM
Ted Pimm

Thanks Carl,  
I guess you're right about fixed bias for multiple tube types. I think I'll start with a 6L6 circuit cathode biased and work towards a fixed bias switching scheme. My PT has an extra secondary winding at 120VAC so I have a dedicated bias supply to work with. I just wanted to start with cathode bias because it was going to be easier.  
Q. What Z-out is favoured for 2x6L6 in PP (around 4500ohm?) with 325V plate. What about 6V6? Anyone have suggestions for the 6F6 and 6K6's -> I have seen the typical data suggesting 10,000ohm plate to plate for these at around 300V plate and screen.  
I guess I should consider a switching system for the 125E ratios to get the proper Zout for each of these tube types (or try to compromise with two impedences). I could do this fairly easily with a DPDT (if it could handle the power).  
Oh, lots of questions...but getting closer.  

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