Tweaking an amp

[wicker]

During the weekend, I've made some changes, first, I've figured out that I like the amp with 47k NFB res, since changing anything by switch (smooth) occurred with a noticeable pup in the speaker (due to DC) I decided to get rid of this. Since I couldn't decide if I prefer amp with 10n cap across the bass pot, I've made it switchable using this smooth switch. Then, I wanted to get a bit less bass and make bass pot more usefull, setting it over 10 o'clock was too much. I've lowered cathode cap in V1A to 4.7u and changed cap between V1B anode and EQ to 10n, now it's better, I can go over 12 o'clock and switching cap make the difference more noticible, I'm thinking also about those 47n caps before power tubes.
I'll also struggle with presence, but all in all, it's heading the right way.

[Structo]

Like this?

If it were me, I would probably just copy the clean channel of Dumble #124 and tweak that to taste.

[wicker]

Ok, presence works, it's 5k with 1uF in series, I don't know why it didn't worked before, I think I like 1u better than 2.2u, I also reduced cathode cap on reverb driver from 47u (waaaay to much) to 10uF and now I like, I think that amp sounds great, I'll have to fight with some noticible hiss, but it might be problem with some caps, changing one of the caps made it better. When it'll be ok, I'll send some photos, and will try to record something in future.

[Structo]

What is your grid stopper on V1a?

You can try a larger value to see if it helps the hiss.

Also tubes can vary with hiss as well. So try a few different ones if you have them.

[wicker]

It's 47k on V1A, I'll try maybe 68k or higher.

[10thTx]

Structo,

Is that presence control schematic that you posted correct? It's showing the 4.7k to ground.

OR is it like this one?

With respect, 10thtx

[Structo]

You are right, that is wrong.

When I snipped it from the schematic I edited it wrong.

Thanks I will fix that.

[wicker]

I've got 47k NF res and 4.7k to ground, at first, I had 470ohm parallel to 5k pot, but thrown it and left only 5k pot.

[wicker]

Ok, there's too much bass indeed, I think that I'll try change caps in eq to 22n bass, and 47n mid (!), also, try different cap across bass pot (4.7n ?). What other mods do you suggest guys, not only talking about the bass, but the whole amp, PI, Reverb etc.

[bcook]

Before you go too far afield, try changing the 220k in the OD entrance to 150k.

[Structo]

Tweaking a non HRM amp is always a compromise because you tweak the tone stack to make the clean side sound great then you have to juggle things in the OD side to match up.

Having a tone stack on the OD side alleviates that.

[wicker]

Anyone have seen schematic on previous site ?? Here's actual, it's not dumble amp, clean only amp with reverb:
http://imageshack.us/photo/my-images/685/schemat1.png/

[Structo]

Is this the schematic for your amp?

How is the gain as you turn up?

Do you want it to stay real clean or do you want a little break up as you turn up?

The reason I ask is because you have a 150K plate resistor on V1b as well as a 2K2 cathode resistor.
That is going to increase the gain there.

If you want a cleaner sound change that to 100K/ 1K5.

Also the resistance on the local feedback loop on V1b is a bit lower than standard.
Try a 22M or 44M there.

Why the 47K plate load resistors on the phase inverter?
Try 100K/82K there.

Let us know how it sounds.

[wicker]

I thought that increasing V1b plate res to 150k will compensate a bit loss of gain that local NF provide, I rather like it to stay clean as long as possible, that's why there's 47k in PI plates - not to get too much gain before power tubes.

[Structo]

OK, I understand.

The tail resistor is a bit large on the GNFB.

Have you tried the Dumble combination with a 8K2 (or 6K2) FB resistor (50w amplifier on 4 ohm tap) and a 390R tail?