late 70s AC30 master volume mod

[Chuck H]

The link doesn't recognize the attachment. At least it doesn't for me.

[Enzo]

Link took me to EL34world forum, with a thread about MV, and several schematics relevant. At first I saw a register box, but scrolled down to find the material.

[Chuck H]

Link took me to EL34world forum, with a thread about MV, and several schematics relevant. At first I saw a register box, but scrolled down to find the material.

Yep. When I clicked the photo it said "invalid link"

[g1]

The exact image does not appear in that thread. This seems to be the relevant snippet of it, with same yellow highlighted area.

[Helmholtz]

This seems to be the relevant snippet of it, with same yellow highlighted area.

I don't understand the choice of resistance values and I'm not sure if 2.2M is low enough to prevent thermal runaway with a bad wiper contact.

The EL84 datasheet specifies a max grid leak of 1M with cathode bias, makes 500k for 2 tubes in parallel.
So I would use a dual 1M or 500k pot and 470k safety resistors with an AC30.

[Chuck H]

This is what I was talking about before. The difference between the fixed loads and pots can certainly be tempered. This just pulled from files I have, but is what I had in mind. Actual PI values not relevant. It's the master dual pot values and grid leaks I'm considering.

[boroman]

Okay.

I decided to go simple route and try replacing 0,047uF cap on the CUT with a wire.
Amp looks like it has been recapped at some point (some elements)

I don't see anything attached to the (500k) cut potentiometer... Also the brown and black wire goes to 1M resistor (blue squared). Can't raise that plate more without resoldering every point from the top and bottom. So, where's exaclty the point I need to jump?

[boroman]

To be clear - these are those point where CUT potentiometer goes to:

[g1]

I think what you are calling "1M resistor (blue squared)" is probably the .047 cut cap.
But you say both wires from pot are connecting to it? That is not what schematic shows.

[boroman]

You were right about it being a cap... Unsoldered it and yes. 1TD mark but it's a 47nf cap. I cut it and placed a wire instead. Yes, it works, but kidn of limited range like on the scale of 10, on 1,5 turn I have full throttle...

Now, questions is, this is A500k pot there, should I got to 1M, or to the other side, 50k for example?

[Mick Bailey]

I would replace the pot with a pot+resistor. Measure the resistance at the point where you have maximum volume 1.5/10 by your reckoning and select the nearest value pot of that resistance. Subtract the resistance reading from 500K and insert a fixed resistor of that value in series with the pot (to replace the original cap). Now when the pot is at 10, it will give the same overall resistance as your original setup at 1.5.

You may be able to add a parallel resistor to your existing pot (from the capacitor-side lug to the wiper) to change the value, though this also alters the taper. You still need the series resistor, though.

[boroman]

I would replace the pot with a pot+resistor. Measure the resistance at the point where you have maximum volume 1.5/10 by your reckoning and select the nearest value pot of that resistance. Subtract the resistance reading from 500K and insert a fixed resistor of that value in series with the pot (to replace the original cap). Now when the pot is at 10, it will give the same overall resistance as your original setup at 1.5.

You may be able to add a parallel resistor to your existing pot (from the capacitor-side lug to the wiper) to change the value, though this also alters the taper. You still need the series resistor, though.

Thanks for the tip!

But isn't it the same/close as using 100k ok 50k pot, for example?
I've tried 1M now and it's even worse...

[Helmholtz]

The pot is wired as a variable resistance and without the cap it acts as a variable load to the PI.
So the lower its resistance, the less output.

If it's a log pot (schematic specifies 250k log), it makes a difference to which end the wiper is connected.
So it might be a solution to connect the wiper to the other end.

Using a 250k pot should be no problem. Lower values might reduce full output power.

[Mick Bailey]

But isn't it the same/close as using 100k ok 50k pot, for example?

It isn't the same and effectively re-scales the pot to act through its entire range. Think about how your control currently works; you describe that at a rotation of 1.5/10 you have 'full throttle' with a 500k pot. So your whole effective range is between 0 and 1.5 (whatever that resistance is). Keeping the overall resistance close to the original value means that you won't lose volume. Try a low value pot (maybe start at 25k or 50k) in series with a resistor to make up the total resistance to 250k or 500k.