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Old 1st August 2011, 03:14 AM   #1
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Default Small tube amp conversion help 6x4, 6AU6, 6AQ5

Hi Everyone,

I have bought a small tube amp from Ebay. It is converted to a guitar amp from some other application. With a DOD 250 pedal and an 8 Ohm speaker it is nice, but without the pedal it is not loud for my guitar. Please look at the schematics and advise mods that would make it a bit louder.

The Electrolytic caps are 350V 30 uF and 20 uF, but don't know which one is which. I hope there is an easy fix.

Also, could I put a "tone block" between the tubes, like before the coupling capacitor (such as at in http://www.netads.com/~meo/Guitar/Amps/PA2Guitar/)?

Thanks!
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Old 1st August 2011, 04:52 AM   #2
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Voltage readings would help greatly. I think I have a couple of ideas, but really, some DC voltage readings would help.
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Old 1st August 2011, 03:11 PM   #3
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I can't see the schematic, but increasing plate resistor (and cathode resistor) on the 6AU6 will increase gain. Cathode resistors should be bypassed. Tone controls will require another gain stage.
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Old 1st August 2011, 04:25 PM   #4
DF96 is offline DF96  England
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They seem to be running the 6AU6 with a low screen voltage and unbypassed. I assume they had a good reason for this, but it is reducing gain.
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Old 3rd August 2011, 01:33 PM   #5
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Default Correction and some mods

Thank you all for your replies! I apologize for some incorrect readings. The 10M resistor WAS soldered to the Vol pot...
I took some advise and here is the current status of the amp. By removing the feedback and changing the 33 nF to a 20 mF the amp got somewhat louder, but still not 3-4W, I think. Also, now has a much nicer tone.

Can you suggest some additional mods? What would happen if I change the coupling capacitor to 0.1 mF? Someone did that in a similar circuit to get more gain.

Thanks again!Click the image to open in full size.
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Old 3rd August 2011, 03:45 PM   #6
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I would suggest remove the existing 150K and 27K screen resistors, and then supplying it through a 680K resistor. Then use a .05 mfd cap to bypass the screen to the cathode. That will raise the screen voltage somewhat, eliminate any remaining degeneration taking place, and therefore improve your gain as well.

Dave
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Old 4th August 2011, 05:41 AM   #7
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Replacing the coupling caps don´t give you more gain it just alters the frequencys.

If you have a small OPT i wouldn´t go to cracy on big caps.
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Old 4th August 2011, 06:08 AM   #8
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Quote:
Originally Posted by dcgillespie View Post
I would suggest remove the existing 150K and 27K screen resistors, and then supplying it through a 680K resistor.
Is there a typo?
In the hand-numbered schematic, the screen supply resistors (voltage divider) are 3k7 and 150k.
In the second schematic with the typed numbering, they are 27k and 150k.

And, what is the problem with supplying the screen from a voltage divider? Is it because the resistor to ground is in parallel with the screen-cathode in the tube? I'm a real beginner at this stuff.
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Old 4th August 2011, 11:12 AM   #9
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I saw the change in screen divider resistor values as well (from the original unrealistic values), and thought it to be an adjustment the OP made after they "took some advice" in post #5.

A pentode's performance is always maximized when the potential between the screen and cathode is stabilized as much as possible. While the divider resistor scheme for the screen (along with the bypassed cathode resistor) likely did a pretty good job of this, using the screen cap approach bypassed directly to the cathode simply assures the issue. Also, for small signal pentodes, using a single large value dropping resistor to achieve a target screen voltage (as opposed to the divider resistor approach) tends to help stabilize the performance of the circuit between different examples of the specified tube being used.

Dave
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Old 4th August 2011, 11:25 AM   #10
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Is it just me or does the 18V on the 6AU6 look like a big mistake. Surely this should be 180V ???

Shoog
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