plate and screen voltage

[screenplate]

Hey guys,

I just finished my masterpiece monoblocks. each consists of 1 12AX7 that feeds 2 6SN7`s. Each 6SN7 feeds a pair of 6550`s. they sound great and with 4 6550`s there is plenty of power. My question is I got 550v on the plate. What is a good voltage to hav on the screens? I currently have 525. Im being told that that is too much and my screen voltage should be about 400 with 550 on the plate.

 

[GMuffley]

With a separate screen supply, 300-325V would be ideal. I presume you are not running ultralinear.

 

[hemgjord]

It really depends on what primary impedance you are using.
Draw the loadline on the plate curves for a few different G2 voltages and try to estimate the G2 voltage required to hit the "knee".
Alternative if you have the possibility to test with various G2 voltages to find your preferred mix of max power and distortion profile.

/olof

 

[dcgillespie]

What hemgjord said. It is a product of the combination of the primary impedance that your OPT offers, ideally the value of the plate supply B+ under full power conditions, and the type of bias system being used. I assume that for a project of this size, it is using fixed bias, and a pretty healthy plate supply current wise. If that is the case, then it all really boils down to the plate-to-plate impedance your OPT provides. If you know that, you can work backwards through the curves to hit the knee, or pass the info along and we can give a good target value to shoot for. Since the quality of screen voltage (i.e regulation) plays such a big part in the performance of a pentode design, the screen supply will need to be very stable over the full dynamic range the output stage can produce at the target voltage.

Dave

 

[Ian444]

I have found in one amp that regulated supplies for the screens sounded worse than unregulated supplies. Also that increasing the screen grid resistor in 2 amps decreases distortion according to my ears. This is with pentode mode on PP amp. Let your ears be the judge. YMMV.

Ian

 

[Tubelab_com]

I have found in one amp that regulated supplies for the screens sounded worse than unregulated supplies. Also that increasing the screen grid resistor in 2 amps decreases distortion according to my ears. This is with pentode mode on PP amp. Let your ears be the judge. YMMV.

What output tubes?

Some tubes like the EL34 seem to like the right value of screen resistor and Mullard mentions a 1K resistor in the data sheet. On the other hand I find myself building a screen regulator for my hot rodded Simple P-P (EL84's).

 

[Ian444]

What output tubes?

6P1P and EL84. I tried LR8N regs (one reg per tube) on the screens of a 6P1P PP amp and didn't like it, after a week of listening pulled them out. In the Simple P-P (JJ EL84) I'm using 820 ohms on the screens in pentode mode with no global feedback. There is a very noticeable difference in sound using anything from 100 ohms to 1K5 on the screens, the higher values seem to have a similar effect to adding NFB. I am too short on theory to explain the reasons why.

Ian

 

[Tubelab_com]

There is a very noticeable difference in sound using anything from 100 ohms to 1K5 on the screens, the higher values seem to have a similar effect to adding NFB. I am too short on theory to explain the reasons why

A higher value of screen resistor is a form of local feedback. As the tube is pushed harder into conduction both plate and screen draw more current. The additional screen current results in a drop across the screen resistor lowering the screen voltage which reduces the tube gain.

As expected, I have a few Simple P-P's to play with. One of them has become my test board for some extreme experiments. It is using JJ EL84's. I tried several different screen resistors in the board when it was built as per the parts list but my speakers and my musical tastes tended to prefer the 100 ohm resistor and just a little (5 db) of GNFB. As you noted other users may have a different preference.

I have been exploring ways to crank up the Simple P-P without blowing it up. I have found that the JJ tubes and some old Sylvania 6BQ5's that I have will work just fine at plate voltages over 400 volts. The screen voltage however must be kept near 300 volts. Generating the 300 volts the typical way with a resistor and a cap off of the main B+ just sounds horrible since the voltage varies all over the place as the volume is turned up, hence the regulator.

 

[Ian444]

A higher value of screen resistor is a form of local feedback. As the tube is pushed harder into conduction both plate and screen draw more current. The additional screen current results in a drop across the screen resistor lowering the screen voltage which reduces the tube gain.

Thanks for explaining the local feedback. I went back through my notes and noticed I never tried GNFB with 100 ohm screen resistors, something that should have been done in the very first instance! Guilty!

It amazes me how many possibilities there are for fine tuning a tube amp using various voltages/operating points, bias schemes, regulators etc. I must say the sound of the Simple P-P well and truly surpassed all my expectations.

 

[airboss]

I must say the sound of the Simple P-P well and truly surpassed all my expectations.

You and me both Ian. Though I will admit to never having built a single ended triode power amp, I just don't see how the numbers add up to the kind of musical reproduction I prefer. Push pull cancels most harmonic distortion, especially if the amp is not pushed hard.

But, if you ain't tried it don't complain. That's my thinkin'. I read some (who could read all?) of the #26 line stage pre-amp thread the other night though, and I'm prepared to go to work on one after the other 126 projects I got on the bench right now. Aaaand, there are some dudes I have mucho respect for who dig that SET sound.

When my model 19s are done and the Harmon Kardon receiver (the very first, with the simulcast AM/FM receive set up. I've been looking for three years for a multiplex for it) is de-noised enough to listen to them, I'll go after something a bit more esoteric. I don't need much more than a watt. The H&K uses some esoteric hand picked 6V6. 7044?? I forget. 6V6 work fine.

Altecs need about 1-2mv noise floor to be listenable. Simple stuff and big metal film resistors eh?

Eh, and ditto on the regulated screens. I don't have much experience in the hi-fi world, but I do know that in the old Broadcast transmitters, it was the first mod to do if the modulation tubes were 813s or some other pentode. If you use a voltage divider off the plate supply the screens just flop around all over the place. Humpy Pumpy. Handful of glow O tubes and you had real music. The screen dropping resistor scheme was the product of accountants, guitar amp builders, and other foolishness. Read dcgillespie's post, and tubelabs a few times. It's really all you need to know about screens in and around and between the lines.

When am I going to be off probation. I wonder.