I just finished puting together a "Ratrod Deluxe"
from a bunch of parts I got off $ebay-I am a novice technician and do it to learn and have fun. I have very little gear just a iron and a multimeter. I am playing the amp and it sounds like it should for the most part.
There seems to be a problem with the more-drive mode causing occilations when it is engaged, it does this also in drive mode. When the drive pot is backed off, but its okay when on full. The clean mode is unaffected.
The strange thing(or perhaps not) is that if I reduce the gain using alternate tubes like 12au7's then the thing becomes 75% controllable, I can adjust the preamp pots to make it go away or just turn the drive pot full.
I really dont need a voltage controlled occilator
for a back up although I find the sound interesting.
This is affected by all 3 Bass,Mid, and Treb which are acting as frequency controllers.
The sound will go into the pwramp and get louder if I turn it up. When using the lower gain tubes it all but disappears from the drive mode and only stays in about the first 50% of the moredrive mode.
I suspect the drive circuit is causing something in the bias of the preamp tubes but since the darned drive circuit is so oddly complex I really dont know where to start. Ive searched the net and found to check my coupling caps but they appear to be okay.
I was wondering if anyone has had a similar problem or has come across this phenome before.
Any assistance would be greatly appreciated.
Thanks in advance.
It sounds like one of two things. Does this circuit use negative feedback, with a bit of the speaker voltage feed back into the phase inverter? If this voltage is the wrong phase it becomes positive feedback and the circuit is prone to oscillate. Usually it's obvious with a continuous squeal but I've seen cases where it's happening too high to hear and when you adjust controls the frequency snaps into an audible range.
The cure is to flip the phase. Easiest methods are to flip the speaker wires at the jack or to swap the coupling caps feeding the output tubes, so that each side of the OT primary is now getting feed from the opposite side of the phase inverter driver stage.
More likely you have a simple but worse to fix PITA problem - lead dress! This term refers to how the parts are positioned and the wires run. Two wires running near each other will have a small but measurable capacitance between them. In it's simplest form a capacitor is merely two plates with a gap between them. Wires or parts can do the same thing. There is a lot less surface area than with plates so the cap value is much lower but it's there, nonetheless. Wires crossing at right angles will have only the crossover point to have mutual capacitance. Wires running alongside each other can have a whole bunch! Wires running over the tube socket can end up with capacitance between one 12AX7 half and the other.
This stray capacitance can be enough to couple signal from one stage back into a previous one. If the feedback is postive you've just made an oscillator!
If this is what's happening then that's exactly why you've got all this spooky stuff with the controls affecting it. High gain stages are much more prone to the problem. A little bit of stray capacitance in a regular circuit may not couple enough unwanted signal from one stage to a previous one, just like sticking a microphone in front of the speaker. Add tons of gain and there can be more than enough!
What's the cure? You have to change the layout of the parts and wires. If you're lucky you can find the problem areas by taking a wooden or plastic dowel (not a pencil! The lead is conductive and if you touch high voltage it will travel up the wood!) and gently pushing at parts leads and wires, to see if it makes a change in the oscillation.
Did you use a pcb, eyelet board or "scratch build"? Pcb's mean a proven, problem free layout. You only have to worry about the wires coming on and off the board. Eyelet boards usually are ok if you've followed a proven layout. Remember that the first amp with such a board that Leo Fender or Pete Traynor used probably had "bugs". They would have tried a number of arrangements before the final one.
Scratch builds are the most prone to problems, at least for beginners. When you're wiring up the amp you have to be conscious that you can't run wires willynilly like in a car frame. You avoid running wires in close parallel. Crossing at right angles is the safest bet. More spacing helps big time! Think of those two capacitor plates and what the cap value is as you increase the gap. It falls off not linearly but with the square root: 1, 1/2, 1/4, 1/16,1/256...
Every time you go through a grid-fed triode stage you flip the phase. Oscillation can't happen if the feedback is negative, or out of phase. So usually the problem is coming from 2 or an even number of stages ahead.
A look at "The Tube Amp De-bugging Page" at
http://www.geofex.com is always worth it.
One further point that has cropped up in recent years is the practice of running higher plate voltages on 12AX7's to flog every scrap of gain out of them. A vintage Fender or Marshall would run 160-200 vdc on the 1st stage plate. Some amps like Peavey or Booger will bump that to maybe 220. Some modern 12AX7 brands get problems when you do this. Small problems are tubes running microphonic. Big problems are squealing oscillations. Even those great new TungSols can't handle such situations. It's not the tube's fault. It's the designer's. Usually a less gainy tube like a 12AX7WA Sovtek fixes the problem but it's not the greatest for tone. A 5751 almost always works. It seems like screwy engineering to design for so much gain with the voltages that you have to use a lower gain tube when you're finished. Why not flog the poor tube less in the first place?
The reason they get themselves in such situations is that they don't want the added cost of an extra tube, socket and parts for more gain. They just torture a chain of fewer tubes made to run past their ratings!
You said you only had a multimeter. A 'Scope is big bread but if you could borrow one...
Getting a 'scope for such problems is like chopping wood all your life with an axe and someone hands you a chainsaw!:smile:
Keep us posted!
:food-smiley-004:
