transconductance effect on peak power?

[speakerfritz]

OK, we are talking about peak power not continuos.

Assume bias is no problem, filament current OK, and any other electronic circuit requirement has been satisfied.

situation A. amp using 4 8417's claim power output of 100 watts continuos. We put in kt-88's and rebias, and we say, now our amp has become a 50 watt amp since the 8417's were 23,000 transconductance and the kt-88 only 11000. unless you can do something to increase the drive levels such as add a pre driver.

situation B. amp using 4 kt-88's claim power of 100 watts continuous. We put in 8417's and rebias. is this amp now a 200 watt peak power and 100 watts continuos power amp. The power supply probally can not support a 200 watt continuos rating. the pre driver circut could obviously drive the 8417s to max power peaks.

so, the question, what effect does transconductance have on peak power output as you cross over from high to lower transconductance tubes and vice versa.

 

[Tubelab_com]

These are my thoughts based on the way I design amplifiers:

A modern tube HiFi amplifier should have ample gain and voltage swing everywhere in the signal chain such that the plate circuit of the output tubes is always the place where clipping occurs when the amp is overdriven.

You want the driver and earlier stages to have at least twice the drive voltage and gain that the output tubes require. You also need them to be fast enough to avoid slew rate limiting when producing a major transient like a drum stick whack on the rim of a snare drum at full power without clipping or ringing. Gain and drive voltage capability doesn't cost too much there.

The output stage will usually run out of voltage swing (hard clipping) or in some cases be unable to source enough current into the speaker if driven hard at a frequency coincident with a low speaker impedance. If this is the case transconductance will have little effect on the total power output capability of an amplifier. The peak power output capability has to do with the power supply voltage, the load impedance, and the peak cathode current capability of the output tubes.

For a lightly loaded amp that is not drive voltage limited the power output is mostly determined by the power supply voltage and the load impedance. As the load impedance is reduced to increase power, the output tubes will have to work harder. I have observed 1/2 amp peak currents when cranking KT88's into a 3300 ohm load on 550 volts. You loose about 100 volts across the tube under these conditions. Swap in a big fat sweep tube that can pull its plate allthe way down to 30 volts,

 

[speakerfritz]

ok, that makes sense, we don't know how much extra drive ability exists in any given circut and we don't know how much extra voltage swing exists.

so when someone says putting kt-88's in an 8417 quick silver, bogen mo100 or dynco mark VI amp will cut your power in half, we need to first establish what if any extra driver drive capability exists and what if any extra voltag swing exists.

I have a peak and hold meter so I think i will do some measurements of peak power at and beyond the continous power rating using a few amps that are 8417 based and sapping to other tubes as well as amps that are other tubes based and swapping to 8417's.

thanks for the reply.

 

[speakerfritz]

Update. In reviewing how to set this up, ran into a twist that will require some planning. The 8417 internally ties pin 1/8 together where as el-34's, kt88's leave that as a circut option (for example, manlet amps tie 1/8 together on the board). So i need to make sure any amp I want to try 8417's in, are not effected by the internal tie of 1/8

 

[Sch3mat1c]

Peak current capacity determines maximum power output (assuming optimal load impedance). This is generally controlled by screen voltage and perveance. (It's also controlled by peak grid voltage, but I'm assuming peak grid voltage is 0V at clipping, as usual for class 1 operation.)

Lower transconductance means more grid voltage swing is required, which may cause the driver to clip, which would hide the true peak power level.

Tim

 

[speakerfritz]

OK, i have been looking for some real world examples. lets look at specific examples. A) a manley 240/250 series map. model 250 is 10 6l6 tubes, 1800uf of power supply capacitance, 575 volts b+, 240 watts output. Model 500 is 10 KT90 tubes, 1800uf power supply capacitance, 575 b+, 500 watts output. b) cary cad 50 amp, 2 el34's, 750uf power supply capacitance, 525 b+, 50 watts output. Cary cad 100, 2 KT-88's, 750uf power supply capacitance, 525 b+, 100 watts output. both amps use the same PCB for both models indicated, same power supply tranny, same power supply capacitors. minor bias circutry changes. just the utbes are different.

 

[Tubelab_com]

same power supply capacitors. minor bias circutry changes. just the utbes are different.

The OPT has to be different. You are not going to double the power output out of any given amplifier just by swapping tubes. You might increase the peak current capability through the tubes by swapping them for bigger tubes, but that reserve current capability will not be realized until you lower the load impedance.

For a real world example spend some time looking through the Red Board thread:

http://www.diyaudio.com/forums/tubes-valves/151206-posted-new-p-p-power-amp-design.html

Look for my posts. Pete designed a little P-P amplifier board and started selling them. His conservative design was rated at 17 WPC using 6JN6 sweep tubes and 340 volts of B+ with a load impedance of 8K ohms (I think). He admitted that it was a conservative set up designed for reliability and a high probability of first time success for novice builders.

I built one, but used a 6600 ohm OPT since that is what I had. I got more power output (about 30 WPC) since I was using up more of the tubes current capability with the lower impedance load. Not satisfied, I turned up the B+ voltage. I found the limit for the 6JN6's. With 520 volts of B+ and a 6600 ohm load I got 70 WPC from the board. Any attempt to lower the load or increase the B+ further was met with higher distortion, red glow from the tubes, or both. I had reached the dissipation AND peak current carrying capability of the tubes. All tubes with a 17 watt dissipation rating seemed limited to about 70 WPC.

I swapped the output tubes for 6HF5's which have a higher dissipation and peak current carrying capability. Resuming the testing with the SAME conditions, I got the same 70 WPC. I could increase the B+ voltage to get more power but I was already pushing the limit of the capacitors on the board, so I reduced the load impedance. At 3300 ohms I could get 125 WPC. So using bigger output tubes allows me to use a lower load impedance to get more power. Attempts to use a 2500 ohm load were met with glow and distortion. I tried several tubes in the 24 to 28 watt dissipation class and found 110 to 125 watts to be the limit.

Again, I reached for bigger tubes. This time I used 6KD6's and I could get to 2500 ohms, still at 520 volts and got 150 WPC. I believe that none of the well used 6KD6's were in good condition limiting the power somewhat. I tried other tubes in the 30 to 35 watt range and got 150 to 175 WPC.

I modified the board to handle more voltage and installed some NOS 35LR6's. With 650 volts and a 2500 ohm load I could get 250 WPC. These were 35 watt tubes, but operating over spec. This was from the same board that Pete designed for 17 WPC.

I currently have E130L output tubes in the amp (27 watt dissipation rating). These have a transconductance rating of 27500. THe higher transconductance does not allow for more output power, it does however bring the input sensitivity down where I can hit clipping (120 watts) with 1/2 a volt of drive.

I have stuffed at least 20 different output tubes into this board and I am not done yet. What have I learned?

Given a good driver design the output tube determines the maximum output power CAPABILITY of a given amp. The operating conditions applied to those output tubes may use all or part of that capability. This is a trade off between reliability, power output, distortion, and transient handling.

Within reason the transconductance of the output tubes only affects the amps input sensitivity. If negative feedback is used around the output tubes, the transconductance will affect the feedback level. Swapping to a tube with a different transconductance will require tweaking the feedback components.

 

[speakerfritz]

""Given a good driver design the output tube determines the maximum output power CAPABILITY of a given amp. The operating conditions applied to those output tubes may use all or part of that capability. This is a trade off between reliability, power output, distortion, and transient handling.

Within reason the transconductance of the output tubes only affects the amps input sensitivity. If negative feedback is used around the output tubes, the transconductance will affect the feedback level. Swapping to a tube with a different transconductance will require tweaking the feedback components. ""


Well put.