... I ... changed several caps and got it working. It sounds great until I get over the halfway point on the volume control the the volume drops and it starts breaking up badly. ...
All filter caps are new? If not, the amp probably needs them. Even if it doesn't now, it will in the next year or 2 as they start failing.
Let's
assume you didn't goof during any of the repair process and create a poor solder joint or intermittent ground. Try lifting one leg of the bypass cap you added to the 2nd gain stage.
This thing is very much like a tweed Princeton (I used to own a ~54 tweed Princeton that used a 6SL7 like this Valco; I guess that makes it the
5B2). It got plenty of distortion pretty quick on the dial without a bypass cap on its 2nd stage (though it does use a grid-leak biased input stage).
Gain will drop when you remove that bypass cap. Your problem may be resolved, or may not. Tell us what happens.
With or without the bypass cap, the output tube
may need a grid stopper and/or screen resistor. These help prevent oscillation in the output tube. I'm thinking you're running into too much gain and and oscillation that's popping up (again, *if* there was no simple wiring/solder error during the repair).
If the grid stopper on the output tube doesn't help, and you need the extra gain of the bypass on the 2nd gain stage, you may have to look hard at the way the preamp tube wiring is routed. Any way the output of the 2nd stage gets anywhere near the grid wiring of the input stage? Any way the wiring of the output tube (especially plate/OT wiring) gets anywhere near any of the preamp wiring or resistors?
Last possibility (and maybe one to look into regardless) is grid blocking. If you slam the output with a massive signal (from the added gain of the 2nd bypass), you can drive the grid positive which charges the 0.05uF coupling cap, and when the signal swings negative the cap's charge keeps the tube cut off longer than it would be with just the control of the input signal. A nasty form of distortion usually results.
Quick solution/test for grid blocking might be to reduce the coupling cap to 0.01uF. Yes, it will make the amp much brighter, but the smaller cap discharges faster and tends to recover from grid blocking easier. You'd just try this as a test.
If you had an o'scope, you could look at the signal on the output tube grid when then effect starts to happen and see how big the driving signal is. If the peak voltage is much more than the d.c. voltage across the 330Ω cathode resistor, you're probably running into grid blocking. You might even see the input signal's waveform change when blocking starts occurring.
If you verify blocking is what's happening, there's a couple things you can try:
- Remember the grid stopper on the output tube to combat oscillation? It can also help reduce the effect of blocking. You could increase that from a starting value of 1.5kΩ to as much as you'd like (but don't go over about 100-220kΩ, and hopefully not even close to that much). The downside is you'll eventually hear it roll off highs.
- If the bass response doesn't suffer too much according to your taste, you can keep reducing that coupling cap value.
- You can reduce gain in the preamp.
- You can keep gain in the preamp, but add some kind of volume control or voltage divider just before the output tube to knock down the size of the signal.
Topic: Valco 510-11 issues (Read 5116 times)