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diy VOX AC4 rip-off, HELP!
diy VOX AC4 rip-off, HELP!
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Old 12th January 2018, 12:00 PM   #21
DrFireBottle is offline DrFireBottle  Montenegro
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Okay so here's the schematic, hope it's readable.
Shema SE.jpg
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Old 12th January 2018, 04:39 PM   #22
GeorgK is offline GeorgK  Austria
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Your triodes share one plate resistor? No surprise that the amp is lacking gain. What was your intention for this? Apart from that, its value is pretty low.
The HV for the preamp is probably too high and screen voltage maybe too low. You could add a separate filtering stage for the pre, this should go into the right direction.
I also would add a 10uF cap to B+.
Can you add voltages to the schematic?

Last edited by GeorgK; 12th January 2018 at 04:49 PM.
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Old 12th January 2018, 04:46 PM   #23
Thomasha is offline Thomasha
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that's odd. I guess it's missing a plate resistor, otherwise there is a local feedback loop from the plate of the second triode to the first and then to the grid.

If it's really built like that removing that connection and adding a plate resistor from the second triode's plate to the PRE point could be the solution.

Look for the schematic of a valve junior.
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Old 12th January 2018, 09:37 PM   #24
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by GeorgK View Post
Your triodes share one plate resistor? No surprise that the amp is lacking gain.
GeorgK is absolutely right, that is not a workable circuit (shared anode resistor for two triodes driving out-of-phase signals!)

Honestly, I'm amazed it works at all, if this is really how it's wired. But it's an easy fix, just add a second anode resistor for the second triode.

-Gnobuddy
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Old 13th January 2018, 01:16 AM   #25
PRR is offline PRR  United States
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Quote:
Originally Posted by DrFireBottle View Post
...hope it's readable....
Light helps.

As said, stage 2 needs its own plate load. And I would suspect you have the GR and PRE power nodes swapped. G2 is significant current and does not need to be real clean. PRE is less current and should be real clean.
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Old 13th January 2018, 08:03 AM   #26
DrFireBottle is offline DrFireBottle  Montenegro
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Okay Thank You, IT WORKS!!

My intention on this was using only one plate resistor to supply the both tubes,
Now I get why was my amp so quiet! I was so stupid to put it that way because I taught one resistor two plates supplied, less resistors used, but now you helped me point out that tings, also replaced GR's(grid) 6.8kOhm to 2kOhm, added a filtering stage and amp was on. It manages to pull off some funky sounds for an EL84, and I am also glad that the amp has warm cleans like 6L6, probably because ECC88's that have low gain factor.

If i was careful enough, I would notice it no-time.
Thank you again, I will take a closer look at a schematic next time before posted!
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Old 13th January 2018, 10:45 AM   #27
GeorgK is offline GeorgK  Austria
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Congratulations!
In fact, in an anode follower stages like this, the resistor is not just a supply for the tubes, its varying voltage drop is what actually makes the tube an amplifier. The way it was wired, the triodes were working against each other instead of one after the other, as their plates carry opposite signal phases. Or, like Thomasha said, you incorporated a massive local negative feedback.
Still I would suggest a separate HV filtering for the preamp. This also prevents the power stage from feeding into the pre, which may cause oscillation and other nuisances.
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