DIY Hybrid amp - "disappearing distortion" help needed

[Cursor]

Hi all,

For my first tube projects I've been playing with the low-voltage side of things, with some great tubes forumite Brett kindly sent me and others.

I've tried putting this to the DIYAudio forum folks, but to no avail. For reference the original thread is here. Apologies for copying and pasting part of that here; if it's poor form to cross-post in this way please let me know.

This is my first tube project, and I'm building a hybrid practice amp using an ECH83 (low-B+ triode and heptode) as a preamp and a TDA2030A as the power amp. The speaker's a 12" Celestion, the PSU is from a gutted Park G-10 with a seperate 6.3V transformer for the heaters (rectified and smoothed).

Schematic (sorry for crapiness):


It already sounds really nice - punches the Celestion quite impressively.

However, I don't think I'm handling the gain or volume controls correctly. The gain control produces no sound below a certain level, full sound thereafter - making me think I should replace it with a 680K grid resistor and look elsewhere to add a gain control.

The volume control is working, but for the bottom 20% of it's travel a distorted tone is present underneath the otherwise cleanish tone. This distortion is very pleasant but it goes away suddenly as the volume reaches 20% or so. The closest analogy I can find sound-wise is that when it's in the sweet zone it sounds like the way a JFET stuggles when it's on the point of starvation, except much richer and warmer (bear in mind I'm coming from JFETs to tubes - I liked the sounds the runoffgroove.com guys were getting so much I wanted to check out the real thing). I'm aware that tube distortion generally comes from overdriving them not starving them!

I'd really welcome any advice on how I could get the nice distortion to stay present throughout the travel of the volume knob.

[Steben]

Cursor,

Gain control: You make a mistake with your gain pot. The way you wire it up, it is an input impedance control for V1B, yet it also acts as a parallel resistor to the 100k resistor on V1A, lowering the gain of the first tube. Wire the gain pot just like you wired the volume pot and you'll be on the right path! 

Volume control: put another resistor of 100k to ground between the .47 cap and input 1 of TDA2030.

6.3V: I'm not sure the 6.3V should be rectified and filtered. I guess it surely heatens the tubes more than it should. Not only the energy to the heaters is constant (DC), it has a bigger DC level than 6.3V (Filtering bumps the voltage a bit with the caps as charge pumps)!