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6L6 Triode AB Push/Pull Pout
6L6 Triode AB Push/Pull Pout
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Old 9th March 2015, 10:36 PM   #1
famousmockingbird is offline famousmockingbird  United States
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Default 6L6 Triode AB Push/Pull Pout

I have 355V plate voltage and output iron is 8k:8R no global feedback.

Screens tied to plates through 100 ohm resistors.

Cathode bias, shared 360R common resistor, fully bypassed w/ 1000uF cap.

I have plenty of drive but I see clipping at 7V into 8 ohms. I thought I would get more closer to 10-12 watts not 6

I have tried different tubes so they are not the problem.

Thoughts?

-bird
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Old 9th March 2015, 10:50 PM   #2
nigelwright7557 is offline nigelwright7557  United Kingdom
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In theory you should be getting 355*355/8000 watts peak.
This gives 15 watts peak.

Is the signal symmetrical all through the amplifier ?
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Old 10th March 2015, 01:10 AM   #3
famousmockingbird is offline famousmockingbird  United States
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Hi Nigel, Everything looks symmetrical enough. Coming from the long tailed pair I am seeing 20.5Vac at G1 of one 6L6 and 21.3Vac at the other 6L6's G1, these are the readings when clipping is observed from speaker out, clipping is symmetrical at speaker output.
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Old 10th March 2015, 01:13 AM   #4
nigelwright7557 is offline nigelwright7557  United Kingdom
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What is the voltage driving the output transformer ?
Perhaps the output valves aren't been driven hard enough ?
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Old 10th March 2015, 01:17 AM   #5
nigelwright7557 is offline nigelwright7557  United Kingdom
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It could be because you have screen resistor to anode.
If I remember correctly you don't get as much power in triode mode.

I had a look at one of my old designs and I have put the screen through 1K to the B+.
This is pentode mode.

Another mode is ultra linear where the screen goes through ha resistor to a 40% tap on the output transformer.
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Last edited by nigelwright7557; 10th March 2015 at 01:25 AM.
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Old 10th March 2015, 04:38 AM   #6
LPMark is offline LPMark  United States
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It seems to me that load impedance is on the high side.
Have you tried different load line by subbing different dummy load values of RL?
Try 4 ohm load and see if that increases your output.
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Old 10th March 2015, 04:57 AM   #7
SpreadSpectrum is offline SpreadSpectrum  United States
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I would say that what you are getting seems about right. In order to get more power, you would have to drive the output tubes in AB2. Do you have a driver that is capable of doing that?
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Old 10th March 2015, 05:47 AM   #8
Miles Prower is offline Miles Prower  United States
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Quote:
Originally Posted by famousmockingbird View Post
I have 355V plate voltage and output iron is 8k:8R no global feedback.

Screens tied to plates through 100 ohm resistors.

Cathode bias, shared 360R common resistor, fully bypassed w/ 1000uF cap.

I have plenty of drive but I see clipping at 7V into 8 ohms. I thought I would get more closer to 10-12 watts not 6

I have tried different tubes so they are not the problem.

Thoughts?

-bird
Loadline (attached)

Given what you've told us, 10-12W is unrealistic here. The value I get for output is 9.49W. Given the I2R and core losses in the OPT, and the use of cathode bias (always costs some output watts) I'd say 6W of output isn't too far out of line here.

If you want more output, you'll have to go with pentode operation.
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Old 10th March 2015, 05:54 AM   #9
famousmockingbird is offline famousmockingbird  United States
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I do have a driver capable of AB2, it's Pete Millets board: A2 Buffer

I need a negative supply for this to work. Is there any problems with paralleling rectifiers? What I am asking is it okay to take say a 1N4007 and wire it's cathode to pin 6 of the rectifier tube (5Y3GT) and it's anode to a voltage divider or zener string to get my negative rail? The amps normal power supply caps will get charged through the tube rectifier on the positive cycle of that HT tap, the negative cycle will charge the cap at the voltage divider through the 1N4007 for the negative rail?

I will say with 103db sensitivity speakers the power is sufficient as is, I just thought I calculated something closer to 10 watts. I will try and do all the math out again to be sure, if it's correct then I might live with it. BUT I am tempted to try Pete's board and drive them into AB2.
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Old 10th March 2015, 06:03 AM   #10
famousmockingbird is offline famousmockingbird  United States
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Quote:
Originally Posted by Miles Prower View Post
Given what you've told us, 10-12W is unrealistic here. The value I get for output is 9.49W. Given the I2R and core losses in the OPT, and the use of cathode bias (always costs some output watts) I'd say 6W of output isn't too far out of line here.
Hi Miles, I think you are correct, now that I am thinking about it when I was starting this project I planned on using sand rectification, essentially giving me a much higher B+, I think it was around 420Vdc, that would have given me closer to the 10 watts.
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