Yet another output power thread

[35L6]

Don't know if anything is "wrong" but I'm not getting the output power I expected . Push pull 50L6's .HT1 is 160V . Screens 120V .Cathode bias 470 Ohm 14V .Drew up a load line , looked like 4k plate to plate would be best , load line through the knee . After I built it I got 1.5 Watt at onset of clipping . I was expecting about 4 Watt . Switched to the 8k tap and got 2 Watt . Perplexed , I started taking measurements . With sine wave input , 500 Hz , at onset of clipping the screen voltage dropped to 95V . Is this normal ? Spec sheet suggests 5ma increase in screen current time two tubes through 22k dropper would explain this . Need the dropper to get below max screen voltage . Is there any way around this ? 2 Watts would be OK , I've been asked to turn down my 1 Watt amp more than once , but I would like a better understanding of power amp design .

[Helmholtz]

Please post schematic and tube datasheet.

[35L6]

I will try , I'm not good with computers . Vacuum tubes are logical , computers don't make any sense .

[35L6]

[35L6]

Look at that , it came up ! Hope you can read it . I'll try the data sheets .

[nosaj]

What is the voltage of the sine wave input?
looks to be an incomplete schematic. What is driving the 50l6's?
https://frank.pocnet.net/sheets/127/5/50L6GT.pdf datasheet
nosaj

[nosaj]

Another pushpull amp and excellent forum right up there with this one.
nosaj

[35L6]

50L6 with 4k plate to plate load .

[35L6]

50L6 with 8k load

[Helmholtz]

What made you choose a 470R cathode resistor?
That obviously results in very low plate current (15mA per tube).
Aim for a cathode voltage around 8V.

Start with 150R and lower if necessary.

The design examples are for SE operation.
With PP class A operation, plate load should be doubled.

[35L6]

The amp is a 2204 tribute . Low voltage , low power . Preamp is taken direct from 2204 but with 20EZ7 instead of ECC83 . Cathode resistors scaled to give a proportional operating point . Power supply and outputs are different . PT is N-68X . 50 VA 1:1 , just one secondary . Bridge rectifier . 194 V unloaded . 160V with load .OT primary from HT1 . Splits in 3 to supply series heaters ( 50 , 50 , 20 ,20 20 ) , preamp supply through 1k , and screen supply through 22k shared and 1k individual . Max screen voltage is 110V or 125 depending on whose spec sheet you use . I think whats happening is when the signal goes up screen current goes up and causes a drop across the 22k . that drop compresses the characteristics so the load line hits the 0V line above the knee . I think at 20 something ma . The 8k load line gets you closer to the knee so more power but not as much as you would get if the screen voltage held . problem is you need the 22k to get the voltage down . I think the drive voltage at the power tube grids at onset of clipping was in the high teens maybe 20 . I could check again . I had wondered if I had enough drive but it seemed to clip right around the bias voltage and I still had more to go . I measured 2.5 watts dimed , if you believe my meter reading a square wave .

[35L6]

I chose 470R to get a deep AB to mimic the 2204 . Fixed bias seemed a little fussy . 8V is spec sheet value for single ended . Very little info on 50L6 push pull . Don't know how different it would sound class A . Probably louder . I guess this shows why they use that dual bridge dual rail supplies with those high voltage amps . Hotter bias may be the way to go . I don't think I want to get into regulated screen supplies on this project .

[Helmholtz]

Class AB requires a cathode bypass cap. Otherwise you get considerable cathode degeneration.
And - as said plate load needs to be about doubled for PP, so no wonder 8k worked better..

[35L6]

Nosaj , thanks for the link . He use 90R on the cathodes . He also used AC heaters and half wave rectifier .I put the heaters on the dc side and a bridge . The bridge gave me 160V , perfect for 2x 50L6 and 3 x 20EZ7 . The 20EZ7 is a 12AX7 with a 20V heater . THe 20EZ7 has 100ma heaters so I had to parallel the with 1k2 5Watt to soak up the other 50ma .

[35L6]

Forgot to mention 22u cathode bypass . I'll draw a complete schematic when I stop making changes . I've just been marking up a 2204 schematic . By the way does anyone have a real one and is treble emphasis really that strong on the Hi channel ?