Effect of lower G2, EL84 PP - diyAudio
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Old 26th February 2013, 03:11 AM   #1
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Default Effect of lower G2, EL84 PP

Hi All,

I have been lurking here for a while and learnt plenty reading the topics here.
I have a couple of EL84 push-pull amps that I bought off fleabay a while ago, they are supposed to be "Suppo" but their internal layout and schematic seems a lot different to ones I've seen posted here.

I've managed to work out why they've chosen most of the values on the board, but I can't work out why they have 10K screen resistors, so when it's running normally with 300V B+, the screens are only seeing ~250V.

Would there be a technical reason for this? Does this suppress the valve/tube from providing its full output? Finally, would there benefits in raising the G2 voltage closer to what the EL84 spec sheet says for class AB PP operation? (which is 300V B+ & 300V G2). Should mention that it's currently running the std Chinese 6P15s.

cheers!

Jacob
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Old 26th February 2013, 06:30 AM   #2
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Is it a shared 10K screen resistor or one per valve. Shared reduces distortion without the need for a screen regulator.

Shoog
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Old 26th February 2013, 06:48 AM   #3
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I made a quick LT Spice simulation with and without 10 k screen resistor, which was common for both tubes. Power was decreased and distortion increased when the 10 k resistor was added.
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Old 26th February 2013, 06:59 AM   #4
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It may not be universally applicable, but I know of at least two designers (Gary Pimm and Peter Millet) who have measured lower distortion with shared screen resistors.
Quote:
Another oddity is that the driver stage - a diff pair of 7W7 pentodes - share a common screen grid dropping resistor, and a very small (0.001uF) bypass capacitor. Again, experimentally, this worked considerably better than the usual choices, including a VR tube to regulate the screen supply, and separate screen dropping resistors, and using large bypass capacitor(s). Interestingly, using a big bypass cap increased THD significantly across the board. Adding a small cap lowered THD at high frequencies (above ~10kHz) without increasing distortion at low frequency.
307A Push-Pull Amp

I wonder if SPICE is accurate enough to resolve this?

Shoog
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Old 26th February 2013, 07:14 AM   #5
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Thanks for the speedy replies

Yes a shared resistor for both output valves. A bit remiss of me to not include a schematic, but as I the OP was when I was at work, it was a bit tricky

The first image is the schematic as provided by the seller. When I popped the lid off I noticed there was some significant differences so the second image is the original schematic modified. That got a bit messy I thought so I sketched out my own schematic, which are the last files.

Are there any gotchas or pitfalls I should look out for if I increase the G2 voltage by lowering the resistor value?

Cheers, Jacob
Attached Images
File Type: png viewer.png (53.7 KB, 252 views)
File Type: jpg Suppo_EL84_AmpB_noMods.jpg (355.6 KB, 255 views)
File Type: jpg suppo_no_mods_jschem.JPG (79.7 KB, 248 views)
Attached Files
File Type: pdf suppo_no_mods_jschem.pdf (34.0 KB, 36 views)

Last edited by Vulcanbb18; 26th February 2013 at 07:17 AM.
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Old 26th February 2013, 08:24 AM   #6
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The only thing I am a bit dubious about is the 1nf cap on the OT. It looks like a bit of a sticking plaster to solve instability issues, but its only on one side so its introducing high frequency imbalalance which equates to higher distortion. In the original it looks like it should be across the whole primary of the OT. It looks like a poorly implemented zobel without the series resistance.
I would take it completely out to see what effect it has.
Yes you could restore the original 1K5 screen resistor, which should increase power output a little.

Shoog
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Old 26th February 2013, 08:37 AM   #7
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Hey Shoog,

argh, my bad I just noticed on review I posted an older image where I had the 1nf in the wrong place It is actually across the primary of the OT.

I've done quite a few mods to one of the amps I have and the other is stock as a comparison. They sound pretty good for how cheap they were!

Ok thanks I might try the 1k5 and see how it goes.

I've made resistor changes to the preamp stage to accomodate a 12AY7, and the phase splitter (inc switchable heater wiring) for a 6DJ8 or 12AY7. I've currently got adjustable GNFB feedback and I'm also experimenting with LED bias for the preamp. I like valve amps! (always had SS until 8 months ago).

There's lots of other changes I've made along the way as well, including some safety ones, for example who makes and sells and amp with the earth pin of the IEC socket connected to nothing??

cheers,

Jacob
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Old 26th February 2013, 10:54 AM   #8
DF96 is offline DF96  England
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Unbypassed shared cathode and screen resistors in a push-pull output may reduce even-order distortion by helping to force balance. They may increase odd-order distortion, via a re-entrant process. The even-order (mainly second) still happens and can be seen at the cathode/screen even though it cancels in the OPT. This even stuff then reacts again in the valve with the incoming signal to make more odd-order. Tghis extra odd-order may be inphase or antiphase to the odd-order already created by the valves, so it can add or subtract (but adding is more likely!). I guess it depends what you are trying to achieve, and how much global feedback there is. I'm not saying you should worry about this, but be aware of it.

Increasing g2 voltage will run the valves hotter, which will probably increase output and reduce valve lifetime, and may or may not change the distortion.
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Old 26th February 2013, 12:20 PM   #9
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6P15 has a lower screen rating, I believe, than the 6P14 (EL84) - probably the reason for the higher screen resistor. If the screen resistor is lowered, the cathode resistor may have to go up to keep a reasonable operating point.
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Old 26th February 2013, 12:23 PM   #10
DF96 is offline DF96  England
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6P15 is not an EL84. It is more like an EL83?
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