Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tdvt on April 07, 2024, 06:18:00 pm
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I am working on a no-trem Princeton Reverb project using a modded Hoffman board (Sluckey's LTPI-version) & the friend I am building it for wants to use EL84s as he is partial to them.
While looking for examples of EL84 amps to get an idea of desirable node voltages (particularly plate-to-screen), I noticed on a DR Z MAZ schematic I have, that they have the power tube screens on a side-node of the B+1 or "A"/plate supply.
The rest of the power rail is more typical with a larger dropping resistor between "A" & "B", etc.
Advantages or disadvantages?
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Mullard advise that a screen grid resistor be common, shared by both (push pull) valves, see p3 https://frank.pocnet.net/sheets/129/e/EL84.pdf
My understanding is that doing so can help to improve linearity, compared to a resistor per screen grid.
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Mullard advise that a screen grid resistor be common, shared by both (push pull) valves, see p3 https://frank.pocnet.net/sheets/129/e/EL84.pdf (https://frank.pocnet.net/sheets/129/e/EL84.pdf)
My understanding is that doing so can help to improve linearity, compared to a resistor per screen grid.
That is interesting, thank you for that link.
Most other examples I have looked at use B+B voltage with additional screen resistor(s) to the screens, 100R-ish. Seems DR Z was maybe more "correct" in the implementation.
I am adding a choke & using a single side-node dropping/screen resistor will actually simplify the layout.
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Can you post the entire DR Z schematic?
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I can...
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Mullard advise that a screen grid resistor be common, shared by both (push pull) valves, see p3 https://frank.pocnet.net/sheets/129/e/EL84.pdf (https://frank.pocnet.net/sheets/129/e/EL84.pdf)
My understanding is that doing so can help to improve linearity, compared to a resistor per screen grid.
That is interesting, thank you for that link.
Most other examples I have looked at use B+B voltage with additional screen resistor(s) to the screens, 100R-ish. Seems DR Z was maybe more "correct" in the implementation.
I am adding a choke & using a single side-node dropping/screen resistor will actually simplify the layout.
That's probably an improvement. It's beneficial for the screen grid HT supply to be well filtered. Because the screen grid is a grid, ripple there will be amplified and modulate anode current.
That's mitigated at idle and the class A operating area by it being a common mode interference in a differential (balanced) system.
But high signal levels move operation to class B, and while a push pull side is in cut off, the common mode cancellation effect is lost.
The Mullard 17W conditions will probably have been developed using a good spec 300V DC supply, eg a CLC pi filter.
The Maz 18 mitigates for the screen grid ripple by its unbypassed shared cathode resistor. That will act to limit current peaks (hence reduce HT ripple voltage) and introduce cathode degeneration NFB in the potentially problematic class B area.
I can...
Thanks!
Dr Z certainly likes hot switching therapy for valve rectifiers :)
Perhaps based on the well established medical principle of 'what doesn't kill 'em makes 'em stronger'?
And a linear pots for Fender black panel tonestack; an improvement for the mid, tolerable for the treble, a somewhat annoying hair trigger for the bass.
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I can...
Looks like a virtual earth mixer on the other half of that 12AT7. You can play around with the 2M2 LFB value to untap some gain. YMMV