Modulator question

[kw7y]

I'm building a modulator using two EL34s.

I am testing it in to a matched resistive load. The output between 30% and 100% of max (50Watts) gets quite distorted. Driving with a sine wave the grids look good, but the output starts to get more triangular the harder you push it.  FYI the plate voltage is about 660V and the screen tapped at around half of that.

Any thoughts from you audio buffs?

- Paul   KW7Y

[KA2DZT]

What frequency are you using??  Low freqs below few hundred hertz may do that with a poor output xfmr.

Fred

[kw7y]

I'm testing it from 300Hz to 3KHz, and observe the effect over that range? I am using a Thordarson trany.

- Paul

[KA2DZT]

It would help to see a schematic of what you're doing.

[kw7y]

It's very simple and conventional. The plates of the EL34s are connected to the primary of the push pull mod trany. The secondary is connected to a 5K load. The center tap of the mod trany goes to about 660V. The screens go to a fixed 330V supply. The grids are driven from a push pull transformer. The signal on the grids is a nice clean sine wave. The signals an the load becomes triangular (pointed) above about 20Watts out and gets worse as you drive it to 50Watts. The grids are biased so that each tube draws around 25mA quiescent current.

[KA2DZT]

Make sure the screen voltage is correct for the tubes.  Check the bias voltage instead of the resting current, make sure the negative grid voltage is correct.  I guess you have the cathodes grounded.  Make sure the 3 supply voltages are stable and well filtered.  Could also be something with the scope or the way you're connecting it to the test points.  Class A operation those tubes probably should have more resting current, even with Class AB-1.  At full output make sure the supply sine wave is still clean.  Only other things left are the two xfmrs and the tubes.

Fred

[Opcom]

from the standard load you are using, go up 20% and down 20%  and see if you get more clean power. An easy way to check if you may have a mismatch of some kind. You can find the load for the max clean power by changing the load in the sirection of more power, and if you get the 50W then you know it is a mismatch.

[w4bfs]

you might try more screen voltage .... AB1 operation of the 6L6 family of tubes has the screen at nearly the same dc level as the plate .... 660V is high and I doubt you could reliably run the screen that high ... you might consider a sweepie such as a 6DQ5 which would be happier with a lower screen than plate voltage and makes a dandy modulator tube

[KA2DZT]

you might try more screen voltage .... AB1 operation of the 6L6 family of tubes has the screen at nearly the same dc level as the plate .... 660V is high and I doubt you could reliably run the screen that high ... you might consider a sweepie such as a 6DQ5 which would be happier with a lower screen than plate voltage and makes a dandy modulator tube

What tube charts are you looking at??  Where do you see screen voltage that high??

Fred

[kw7y]

You are right that the screen voltage is typically the same as the plate voltage for a pair AB1 EL34s. I did not do this because of my extra high plate voltage. I will experiment increasing the screen voltage (and adjusting the bias of course).

Thanks.

- Paul

[KA2DZT]

My mistake, 6CA7/EL34 the screen voltage does run high, my book shows 450 volts for both plate and screen in AB-1.  I was thinking more about 6L6 type tubes where the screens don't run that high.

The resting plate current should be 120 ma in AB-1.  Still think the bias on the tubes is not correct.

Fred

[W7TFO]

Any audiofool knows that if they are not NOS Mullard metal-based they will never work!

D

[flintstone mop]

You can copy the venerable Eico modulator
Using the nicer EL34's

http://www.ohio.edu/people/postr/bapix/eico730.htm

[kw7y]

It looks like they run 460V on the plate and 445 on the screen. The idle cathode current is 70mA, subtracting 10mA for screen current, that gives 28W plate dissipation.

I'm running 660V on the plate (330 on the screen) so for the spec value of 25W max dissipation I need to keep the cathode the idle current below 38mA (28+10). That's what I'll set it to and see how it looks. I'm going to try upping the screen voltage a little also.

- Paul

[kw7y]

Math correction:

I'm running 660V on the plate (330 on the screen) so for the spec value of 25W max dissipation I need to keep the cathode the idle current below 48mA (38 plate+10 screen)

[KA2DZT]

You may have the dissipation within limits, but it doesn't mean you're operating in the center of the plate curve.  Can you bring the plate voltage down to 450-500 volts??  Book shows a 1K resistor between the plate supply and screens.  Also shows a 232ohm cathode resistor if you don't use grid bias.  Plate load 6500 ohms  What Thordarson xfmr are you using??

Fred

[kw7y]

I've upped the screen voltage a little, and upped the idle plate power to 25W per tube and waveform looks much better now up to 50W out.

Thanks for everyones thoughts.

- Paul

[WD5JKO]

Glad your getting a handle on it. With a lower B+ you can run a higher anode current. That might work better. Or perhaps look at an alternate tube in the same family. Also I remember a brand of the E34L that had a Pd rating of 32 watts. Could also switch over to 6550 or KT88/90 as well. If the output transformer is a little low on primary inductance, then a lower drive impedance helps. Also as Pat KD5OEI suggested, try varying the load impedance +/- 20% or more. Could be your better off at 9K anode to anode instead of something lower like 4500 ohms anode to anode. If best quality is your concern with NO negative feedback, a higher plate to plate load might be better, but will sacrifice some wattage capability.

The following thread from Patrick Turner of Turner Audio has some very detailed text on the EL-34 (Dynaco Mark IV) and more:

http://turneraudio.com.au/Dynaco-mkIV-reformed.html

Jim

[Opcom]

all else I got to say on those tubes is:
Don't couple the screens to the plates unless you don't care for output power. No ultralinear phoolery.
Run the screens to 400V minimum if the load Z is low in order to increase peak current.
Use high plate volts and understand that these like higher plate to plate load and higher voltage.
Also I never run a tube near its maximum dissipation. 25W in the book although some might claim 35W.
If you are stuck with the voltages you got and the plate load is low like 5-6K then try a pair of 6550s or lower the screen to 150V and use eight-dollar 6CD6s that can deliver high currents at low voltages.