OT Primary impedance

[trem]

Looking at a pair of 6K6 power tubes.
Am I reading the data sheet correctly.? I believe it is page 3.
I would need an OT with a (approximately) 12k primary.
Does that seem right to you guys.?
Thank You

http://www.tubebooks.org/tubedata/HB...t_2/6K6-GT.PDF

[pdf64]

Yes, 12k seems to be it, but scroll down to page 6 and it shows that a wide band of load impedances give pretty much similar performance. So anything in the range 8k - 16k would probably be fine.
Compared to data sheets, guitar amps typically operate at higher plate / screen voltages and use load impedances below the suggested circuit values.
However, when power supply sag and the wide variation in speaker impedance with frequency is taken into consideration, it probably evens out.
Pete.

[Bruce / Mission Amps]

For lower power output, I've used TONS and TONS of 6K6GTs (at a little lower voltage with 5Y3GT rectifiers and zener diodes) in amps like a 5E3, Champs, tweed Vibrolux, etc.) ... I just use the same OT and they work great.

[TransLucid]

The proper primary impedance depends on the supply voltage. The higher the supply, the higher the primary impedance to obtain the same power output. Remember that each tube sees 1/4 of the total primary impedance. If you use composite characteristic curves (two sets of curves, one upside down, joined on the x-axis at the supply voltage) and draw a load line that just touches the maximum power dissipation curves, the slope of the load line will be the desired primary impedance for each tube. Multiply by four and that's the optimal total primary impedance.

[soundmasterg]

The proper primary impedance depends on the supply voltage. The higher the supply, the higher the primary impedance to obtain the same power output. Remember that each tube sees 1/4 of the total primary impedance. If you use composite characteristic curves (two sets of curves, one upside down, joined on the x-axis at the supply voltage) and draw a load line that just touches the maximum power dissipation curves, the slope of the load line will be the desired primary impedance for each tube. Multiply by four and that's the optimal total primary impedance.

Thats if you are using four power tubes right?

Greg

[TransLucid]

that's for any number of tubes. the tubes are just "valves" that control the current though the primary. You just need to make sure they're operating within their limits. If two can't handle the job you can use four, or use a higher-power tube.

[trem]

I think he asked "4 tubes", because you said each tube sees 1/4 of the primary impedance.

[trem]

For lower power output, I've used TONS and TONS of 6K6GTs (at a little lower voltage with 5Y3GT rectifiers and zener diodes) in amps like a 5E3, Champs, tweed Vibrolux, etc.) ... I just use the same OT and they work great.

Thanks for the info.
I am thinking of converting an old Signal Generator. So it already has the PT running 2 x 6K6, 2 octal preamp tubes, and a 6 volt rectifier (cannot remember the exact number at the moment).
So I was thinking to get an OT that has the "right" primary impedance for the pair of 6K6. Not sure what the HV of the PT is. Guess I should measure that. It might be needed info for future questions I will be asking you guys when I get going on this thing.
Perhaps an OT in the 10K Ohm range.....
Thank You

[soundmasterg]

I think he asked "4 tubes", because you said each tube sees 1/4 of the primary impedance.

Yes that was why I asked that particular question...thanks for the clarification.

Greg

[Gingertube]

I would try a Hammond 1609 (10K Raa).
You need to be carefull with the analysis, when we say that each tube of the push pull will see 1/4 of the Raa Impedance that is only true up to the point where one tube cuts off (i.e. the Class A to Class AB transition level). Above that level the tube sees 1/2 of the Raa value. That reflects to the output as double the oputput impedance and less speaker damping. Good players with "magic" ears say they can pick this transition and get into arguements about how "hot" to bias the tubes, the "hotter" the bias the higher the power level at which the Class A to AB transition happens. That can lead to all sorts of missinformation like the old claim that VOX AC30 is a class A amp. (Actually it runs hot bias so is Class A up to about 10 Watts out).
Also that 12K is the perferct match to a 4 Ohm resistor on the 4 Ohm tap. A typical 4 Ohm speaker varies impedance with frequency, from say 3 Ohms up to say 30 Ohms. Impedance matching is less important than often represented. The "recommended" load impedance is a good "centre value" to go with, but don't obsess over plus or minus 30 or 40%.

Cheers,
Ian

[jpfamps]

I would try a Hammond 1609 (10K Raa).
You need to be carefull with the analysis, when we say that each tube of the push pull will see 1/4 of the Raa Impedance that is only true up to the point where one tube cuts off (i.e. the Class A to Class AB transition level). Above that level the tube sees 1/2 of the Raa value. That reflects to the output as double the oputput impedance and less speaker damping. Good players with "magic" ears say they can pick this transition and get into arguements about how "hot" to bias the tubes, the "hotter" the bias the higher the power level at which the Class A to AB transition happens. That can lead to all sorts of missinformation like the old claim that VOX AC30 is a class A amp. (Actually it runs hot bias so is Class A up to about 10 Watts out).

Ian

Actually that's the wrong way around, in class A loading each valve (assuming 2 valves )sees 1/2 the Raa value. When driven into class B loading the conducting valve sees 1/4 the raa loading.

This revelation answered a question that confused me for a long time, namely "in a class AB amp, when one side is driven into cut off surely you can only deliver half the power?". Of course once I realized that the loading also halfs, then it's obvious how one valve can deliver twice the power.

[kg]

Actually that's the wrong way around, in class A loading each valve (assuming 2 valves )sees 1/2 the Raa value. When driven into class B loading the conducting valve sees 1/4 the raa loading.

This revelation answered a question that confused me for a long time, namely "in a class AB amp, when one side is driven into cut off surely you can only deliver half the power?". Of course once I realized that the loading also halfs, then it's obvious how one valve can deliver twice the power.

for more info: http://ken-gilbert.com/images/tubes/ppamp.jpg

[Gingertube]

jpfamps + kg,
oops - is my face red? - You are quite correct, typed it up in a hurry without fully engaging the brain.
Thanks for correcting my mistake.
Cheers,
Ian