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Re: Can't escape crap distortion

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5/29/2000 4:24 AM
John Fisher	Re: Can't escape crap distortion Just a remote posibility: I had the same problem once to find out that one of the caps in the signal path was bad. John Fisher

5/29/2000 3:57 PM

Thanks for all your input, I've been away from the computer for a few days so I've only just read the posts. I'll try some of the suggestions you mention and get back to you all.

The B+ on the PA valve is around 350V, but it probably sags a lot under load. I've just recieved some EL34's so I might change to one of those and much larger PT. I couldn't use it with the 6V6 because it gives me a 550V HT, which is a bit hot for 6V6's. As soon as my new OT's arrive I'll be going over to PP opperation, but surface mail from the other side of the pond will take a while, and I can't wait that long to start experimenting. Is it possible that my OT is saturating because it doesn't have an air gap? How would this affect tone?

Parasitic oscilation is a distinct possibility as I have had large problems with it and I may have not got rid of it completely.

Thanks again

Chris

5/29/2000 11:00 PM

quote:
"Is it possible that my OT is saturating because it doesn't have an air gap?"

It's practically guaranteed... the oT for a single ended amp MUST MUST MUST have an air gap. The saturation will make the effective inductance of the OT too small, so the amp will be unable to reproduce loud bass notes without distorting horribly (and probably overloading your output tube)

quote:
"How would this affect tone?"

It would very probably produce exactly the kind of 'crap distortion' you're complaining of. You'ld probably be better off running PP with a mains transformer that had a 120 volt tap (or two 120 volt windings)

Steve C.

5/30/2000 12:59 PM
Chris Harden	Me thinks that OT may be the main source of the problem, DOH! I was too lazy to build a PI when I started so I went for SE, I knew that it wouldn't be good, but I didn't realise quite how bad it would be. Time to make radical changes to the output topology, I need those OT's! I got the invoice for them today, but I don't expect them to be with me for another few weeks. Chris

5/30/2000 4:21 PM

Not so crap distortion!

I've just spent the afternoon completely rebuilding the output stage, and it now consists of 2x6V6 in PP configuration, using the same OT. It incorporates some of the features I've discussed on this site. The bias is a mix of cathode and fixed. Each tube has a 330R resistor in the cathode, and there is a negative bias voltage of around 10V. The bias is set to 29mA and the B+ is 350V.

The PI is a design I made up as I went along. It is basicly a 12AT7 see-saw design, with a constant current circuit from the cathodes tied together to ground.

The preamp is almost the same, except for the tone stack being moved to between stages 1 and 2, and some experimentation with swapping the 12AX7 and 12AT7 over, as I find that the 12AT7 can drive the tone stack better than an 12AX7.

First thoughts, It's louder! Due to the increase in efficiency and the extra preamp valve. The distortion is smoother, but the bottom end is still a bit fuzzy although better than before. Next problem, it's too powerful for my speakers now, so I now have to wait for my new Celestion Silver Series to arrive. I hate waiting for parts!

All your suggestions have been very usefull in getting me back on track, and heading towards useful sounds. Cheers all.

Chris

5/30/2000 7:10 PM
Brian	Re: Can't escape crap distortion Is there any way to put in an air gap in a transformer? I have a hammond 125E universal that I am using in my AC4 clone, but it sounds really farty when it distorts no matter what I do in the preamp. BT

5/31/2000 12:10 AM
MBSetzer	The 125E is already an air-gapped unit. I am not familiar with the AC4, what tubes, voltages, and transformer connections are you making, and how many ohms is your speaker load? There are so many possibilities with the 125E, that odds are in favor of disoptimal connection . . .

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