Tube amp fluctuating volume on moderate gain

[jule]

Schematic is simillar to this:

http://diyguitarfreak.files.wordpress.com/2010/09/miniamp2-schematic1.jpg

My changes:

I have master volume with stereo pot after Phase Inverter.

I also changed the tone stack to Marshall one.

I changed first 2 stages bias to resistor 1k instead 1k5.

I have a switch to bypass third gain stage.

Power tube is biased 23mA each half.

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I have 2 problems:

1.) when I use only 2 gain stages the volume fluctuates and it sounds crappy.

2) when I turn on the third stage the distortion becomes very harsh. See the video.

any suggestions? Tubes are new.

tube amp works like **** - YouTube

 

[Osvaldo de Banfield]

Is the power supply fluctuating in some amount rhythmically with sound?

 

[razorrick1293]

How have you configured the switch? there may be some DC feedback from the PI. It also looks as the 3rd stagegives some NFB.

It probably doesnt help having the tone stack after the first stage, especially if you want good distortion.

 

[chris661]

I can see why you're not keen on the sound.

A quick something-to-try: drop the mid and treble, boost the bass a little. Use the bridge pickup. Use the first gain setting.

What's the speaker?

 

[jule]

Sorry for late response.

I have the lead switch configured like this( I had to open the amp since I forgot how I wired it)

http://img13.imageshack.us/img13/8656/miniampprede.jpg

I suppose this negative feedback is ruining it on "clean" channel. I will try to disable it.

Speaker is a copy of celestion Tube10. But everyone is just saying how good this speaker is. It shouldnt be that bad.

I am going to put tone stack after second gain stage?

 

[razorrick1293]

I would suggest putting the tone stack after the second stage, and just a 1M pot where the tone stack is, for a gain control. then i would get rid of the switch, while you bypass the third stage the nfb is doing nothing, you could use the switch to turn the feedback on/off.

Though looking at the schematic, this may not be very good as you have quite a low impedance input for stage 3. maybe put the tone stack after stage 3?
I would also change R6 to 68k and C3 to 2.2u

 

[jule]

I put 500k pot instead of gain(+ coupling cap 22n) and moved tonestack before phase inverter.

I also wired DPDT switch instead SPDT and used the other half to switch off negative feedback while using only 2 gain stages. Now volume no longer fluctuates!

But still is kind of too trebly on full 3 stage gain. It is much better now, but I still need to roll the Treble knob all the way down to get normal bright sound while this setting is a bit muddy when using only 2 gain stages.

I am thinking of switching that 2n2 coupling cap after second stage to 22n, it seems unusually small?

 

[razorrick1293]

2n2 may be a bit small, try 10n first 22n may be a little much if theres any mud, also you can put a 10-47pF cap accross the plate and grid of V2a, that should knock off some treble, or put a 1n cap in parallel with R20