Tuning up a vintage SVT, any advice?

[nateflanigan]

It's been a bit but I thought it'd be nice to post an update. It took me a while to order some replacement pre-amp tubes, get my scope back from a friend, order a dummy load etc etc. After taking apart the chassis and taking some time to get familiar with it (the amp's a bit overwhelming when you first open it up) I found some problems. The pre-amp was exhibiting really weird, poor frequency response. I could amplify a 1k test tone cleanly but as I lowered the frequency the power went out the window and the wave form would break down with hardly any amplification. This was occurring on both channels, so I figured the 6c4 cathode follower which is after the point where the two channels mix might be suspect. Replacing the tube did nothing, but then I found a suspicious resistor in that part of the circuit (R45). It had obviously been replaced at some point, it should be a 47k resistor, color codes can be hard to read but this really really looked like a 470 ohm resistor. I pulled it out, it tested at 62k, weird, but I replaced it with a new 47k anyhow. Seems all is well now, I can amplify from 1k down to 40hz at ~30v across the dummy load and get a clean sine wave on my scope.

[vibratoking]

You fixed it! Congrats.

[nateflanigan]

Thanks for the hi-5 my friend.

[dartanion]

Excellent! I have worked on a bunch of SVTs this year, and when they are right, they are awesome. When something is amiss, they are nothing special. Biggest problem is keeping good output tubes in place. I did burn ins with all of them and bias drifted a lot over the first 2-3 hours on all of them. It's an expensive affair to retube these, but having good outputs makes a huge difference in performance. I have had good luck with SED and surprisingly, the new Tung Sol 6550s. I actually liked the Tung Sols a bit better as the low end was tighter and cleaner than with the SEDs.

[nateflanigan]

I've heard good things about the new tung sols, I'll have to try them out some time