Marhsall 100watt 1959 SLP Oscillation?

[kniqht152]

Hey guys hope everyone is doing well.

I've got this Marshall 1959 SLP from ebay that was just completely miswired with cold/loose solder joints and I believe the output transformer was even miswired originally. It also had a few bad parts in it and to put it generally, it was in really bad shape.

Anyway my brother and I have been working on this amp on and off these past few months just swapping things out and cleaning it up trying to get it into a working condition. As it stands now it sounds fine and works fine except for one problem, the output volume is considerably lowered. Now my brother has a working 100 watt SLP so I have a pretty good idea as to how loud this guy is supposed to actually be. In my opinion it seems that this ebay SLP at a volume of 6 is equivalent to the working SLP's volume of ~2. BIG DIFFERENCE (although the tone is good). Another symptom to note is that with the volume on zero and no guitar signal applied we would get a light motorboating sound coming out of the speakers.

I think we've narrowed this down to a problem with oscillation after the phase inverter tube before the presence knob. The reason I say this is because if we were to disconnect the NFB loop then the tone changes (obviously) but the volume jumps back up to appropriate levels and the amp would motoboat even with the other pre-amp tubes removed.

We tried measuring the voltage on the PI plates and they seemed to be okay (note: 1 side stayed a constant voltage while the other would oscillate ~4 volts every 4 seconds)

We also swapped out the PI snub cap from a 47pf to a 100pf. This made the motorboating go away but didn't touch the volume issue, if anything it made it slightly quieter. I'm starting to think we have a bad PI socket. What do you guys think/suggest? Anything we can try to help narrow down the source of our oscillation?

FYI:Multiple sets of tubes where used (power and pre) and the problem stays with the amp, we also don't have an oscilloscope =( .

[olddawg]

I don't have as much experience troubleshooting tube amps as many on this board, but the first thing I would do is look at (for symmetry) and measure the output with a scope on a load bank with at 1khz sine wave on the input just under clipping. If you don't have a scope use an AC volt meter. At least you will have a reference and know what the output actually is. If the problem is in the PI you may only be amplifying half the waveform.

[kniqht152]

Thanks for the reply olddawg it's much appreciated. I don't understand though, let's say for the sake of argument that we are only amplifying half the waveform shouldn't it have no effect on volume? The NFB loop doesn't add volume right? It only smoothens out the sound by cancelling out with the PSB?

You guys always teach me so much.

I'll make the measurements today after work!

[olddawg]

Thanks for the reply olddawg it's much appreciated. I don't understand though, let's say for the sake of argument that we are only amplifying half the waveform shouldn't it have no effect on volume? The NFB loop doesn't add volume right? It only smoothens out the sound by cancelling out with the PSB?

You guys always teach me so much.

I'll make the measurements today after work!

I've seen people gig with amps with only one tube conducting. It sounds funny with reduced volume. If the amp is oscillating at a frequency above your range of hearing, you will see it with a scope on the output or using an AC voltmeter with no input to the amp. But a problem like this is difficult to troubleshoot without proper test equipment.

[Gtr_tech]

Motorboating is often caused by screens wired common with no series resistor on each. It can also come from the lack of grid stoppers on the control grids. Look into this....

[Jazz P Bass]

And or faulty filter capacitors.

[kniqht152]

Sorry Olddawg I thought I could find a oscilloscope to use but no dice. Gtr_tech I checked and the power tubes each have their own screen resistors and both V1 and V2 have grid stoppers. Jazz P this amp is built on a PTP board from what i think is a reissue chassis (else it probably would have cost me a ton more) so I highly doubt the filter caps are bad. Yet I am running out of ideas to try so if push comes to shove....

I read online that if you lift the NFB loop and the problem goes away then it could be reverse wired primaries, but I swapped those around and then it started screaching like a banshee so that's not it either. I also bit the bullet and drilled out the rivets holding the PI socket in and replaced the darn socket. Unfortunately that didn't solve the problem either.

I found another symptom to add to this puzzle. With the amp turned (volume on 0) and no guitar signal running through it the power tubes will oscillate with a blue glow, although interesting thing to note is that the first pair and the second pair oscillate asynchronously.

Any ideas?

EDIT: I forgot to let you guys know that the tube oscillation and motor-boating goes away with the PI tube removed, removing the tubes from V1 and V2 had no effect.

[kniqht152]

So I've been thinking.... instead of sleeping. Which is part of why I was late to work this morning

What if I'm going about this the wrong way, what if the volume drop is not a symptom of oscillation but the oscillation is a symptom of the volume drop?

This is a used amp, now I can't check on it currently cause I'm not anywhere near it nor will I be for a good while, but WHAT IF the NFB resistor was actually a 47 ohm resistor instead of a 47k ohm resistor?

Could that be causing my massive volume drop?

Someone please correct me if I'm wrong here but could it be that such a small NFB resistor is basically letting TOO much NFB in and cancelling out all of my sound?

[Gtr_tech]

NFB resistor should *not* be 47 ohm. 47k is the correct value for plexi era amps. With a 47 ohm the amp would be very quiet indeed....

[pdf64]

Have you got any good spare high voltage electrolytics, eg 47 or 22uf 450V?
If so just try tacking it across the PI B+ supply node and / or the screen supply node.
Pete

[kniqht152]

Good News people!

I had my brother swap out the NFB resistor (even tho it was color coded okay) and everything seems to be in perfect working order.

Maaaaaan it's also some resistor I'm tellin ya...

Either way thank you everybody for your input and advice and the suggestions, it is highly appreciated and I can honestly say you all rock. Knowing this amp I'll probably be back here sooner rather then later.

Until then!

[kniqht152]

Hey guys I'm back!

Now that didn't take too long did it?

Let me tell you whatsup here. At band practice yesterday I was rocking out all fine and dandy with this plexi when out of no where my volume went to zero. I immidiately checked my cabling and determined it's not the guitar and then dived toward the amp to turn it off. Point to note when turning off the amp I noticed that the two power tubes closest to the phase invertor were red plating really bad, I also caught a wiff of smoke. I hadn't had a chance to really work through the amp yet but I did open it up last night and I didn't see anything burnt or broken.

Anyway I was wondering if you guys have any ideas of what could be causing the issue/things to try?

Thanks.

[Jazz P Bass]

Both tubes red plating sounds like you lost the grid bias voltage.