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Old 12th February 2009, 04:18 AM   #31
FRE is offline FRE  United States
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The last tube amplifier I had was a Dynaco stereo 70 which I assembled in about 1961. The schematic shows the EL34 screens directly connected to the screen taps on the output transformers, but I've seen some amplifier schematics with resistors connecting the screens to the output transformer taps. I had assumed that the resistors were intended to drop the voltage and was unaware that they were to damp parasitic oscillations. I don't know exactly when zeners became available, but probably it wasn't 'till after about 1960.

Of course zeners would not damp oscillations, so I can see that if oscillations are a problem, resistors should be used. I guess that that shows that making changes to circuits could have unexpected results.

After I move into my new house, I'll try to find a schematic for an amplifier using 6V6s in push-pull mode.
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Old 12th February 2009, 09:47 AM   #32
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The usual series resistors of 1K in EL34 screens are advised by Mullard. In graphs with and without those resistors, distortion is lower with the resistors, especially at higer outputs. I have taken this as a peculiarity of EL34s (manufacturers are not often at fault on their own products!).

I have never used zeners as screen droppers, but can see no reason why they would not work, especially in UL where one is limited to the same h.t. while it is sometimes advantageous to have lower Vg2. I would bypass them with say 470nF, and for the EL34 one can have the 1K extra. (Zeners are shorts to signals.) one must just remember that zeners have a temperature co-efficient; at 100V it can be quite 10V, which can have an effect on maximum power output. (Amplified zener packages can be built - but that is another story.)
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Old 12th February 2009, 10:07 AM   #33
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...applying negative polarity to screen grids on peaks?
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Old 12th February 2009, 11:49 PM   #34
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Hi Anatoliy,

If that comment was addressed to me, I do not quite understand; perhaps time of night! Negative polarity with respect to?
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