Motorboating returned, and this time it's a problem

[Wakculloch]

I'm still pondering your non-motorboating scenario, and can't help but think that it may return when least expected.
If the power supply is inadequately decoupled then an inadvertant feedback loop could be set up to one of the preamp stages which would easily provide enough phase shift to oscillate.
The marshall choke and 10K's decoupling scheme should be fine though.

[octal]

Yeah, i think thats what steve was saying, and it does make sense. But like i said to him i had put them back to 22uf on both tho, but now that i think about it i may have done that AFTER it motorboated and not before. But it's 22uf/22uf now, so if it doesn't do it again i'll know thats it. On a related note, the reason I can't stop tweaking this thing is that every time i make a change it seems like a number of other things much then be changed and before i know it the thing is worlds different and i end up going back to square one. (that being how it was a few weeks ago before i started phase to of my endless journey) But in doing this i learn a lot of valuable lessons in how various things relate to each other.

I was actually more concerned about the small size of the cathode caps than the inequality. Try something like 100+ uf on each output tube's cathode resistor. As I said, if the size of the caps is changing the tone, that means that they're too small! This is not the right place to be rolling off the bass in an NFB type amp. The frequency response of the output stage of the amp needs to be essentially flat down to subsonic (or is it infrasonic) frequencies for it to be very stable.

[daz]

I was actually more concerned about the small size of the cathode caps than the inequality. Try something like 100+ uf on each output tube's cathode resistor. As I said, if the size of the caps is changing the tone, that means that they're too small! This is not the right place to be rolling off the bass in an NFB type amp. The frequency response of the output stage of the amp needs to be essentially flat down to subsonic (or is it infrasonic) frequencies for it to be very stable.

ok, will do. I have a couple 50v 100uf caps in my hand this moment. I only used those smaller ones because at the time i was having issues with low end which are now gone. I'll throw em in when i get home. Thanks.

[daz]

Well, i put the caps in, tho i ended up using 220uf since you said 100uf or more. The amp seemed to sound healthier. Then i went back to the 220k plate on V1A. Thats when i knew that is a value i need to always have there. You lose perspective after changing a lot of things. I played one note and realized why i went to 220k there in the first place. With that back and the new bigger PSU caps and the 68k grid leaks on 2 preamp stages the amp just screams with tone now. I'm going to pray to god for strength to help me leave it as is !!

[Steve Conner]

I was actually more concerned about the small size of the cathode caps than the inequality. Try something like 100+ uf on each output tube's cathode resistor. As I said, if the size of the caps is changing the tone, that means that they're too small! This is not the right place to be rolling off the bass in an NFB type amp. The frequency response of the output stage of the amp needs to be essentially flat down to subsonic (or is it infrasonic) frequencies for it to be very stable.

Yea, +1, that's what I was thinking too. It's the small size of the caps that adds the unwanted phase shift. But I assumed Daz tried various sizes, and settled on 22uF for tonal reasons, so he would be reluctant to change them for bigger ones. He just did that though, so problem solved I guess.

Daz, remember the 12 step program!

[Enzo]

I still have my guts telling me this is a loss of a connection. If the circuit were really on the edge of instability, it should be fairly consistent and not that hard to push into it. But this one seems to work for a time , then not work.

If the solder work is all OK. I might look for a component with an intermittent lead connection inside. Try grabbing each filter cap and flexing it side to side. Does that make the thing start doing it or stop doing it? A resistor is less likely but still possible. A cap for example could look perfectly fine, but if you pulled on the + lead it could come right on out. I see that more on radial caps, but you never know. Certainly happens on can caps, internal lost connections.

[daz]

I did steve. i tried bigger ones, but you have to remember that was a different time in my amps life. And if i have learned one lesson about this process it's that changes in the circuit make for wildly varying effects that a given component will have. When i tried it before a cap the size of the ones i just used made the amp too muddy because i already had a low end issue. Instead of sounding better as it did this time, it sounded works.

Enzo, i'll look at that but i think it's all good. But i'll chopstick all the components and see if i can start the motor.

[daz]

Enzo, you may have hit the nail on the head. there certainly was a bad connection. the V2B cathode where the tone stack taps off of. the wire from the tube socket to the tone stack components was loose. I can't say that was why the motorboating took place because like i said it hasn't been doing that for a while. But i went to change slope resistors and when i tried the amp i got no sound. So i chop sticked it and found the problem. Soldering in that resistor i apparently moved that wire.

[Enzo]

You could probably disconect the wire and see, but if it is working, let it be.

[daz]

It WAS for all intents and purposes disconnected after i did that resistor swap. But theres no sound at all then. Thats what cause me to notice it. I figured if it was the issue it was one of those scenarios where the contact it made was occasionally right on the edge of not making contact and any juice at that point was not flowing freely and setting up the MB. But theres another issue that i'm thinking may be the more likely reason. As i was playing it last nite i was getting some single "pops" like you hear with motorboating, but single pops instead of a string of them. I found it seems to be the variable NFB pot, so that would make even more sense right? I cleaned it with deoxit and rotated it ad nausium and it helped, but it still happens a bit. it's kind of a funky old pot i got at a elec surplus place. I'll replace it soon as i can get another 50k.