Plush P1000s on the bench

[Danglin' Fury]

I really want to fix this amp for this guy but when I turn it on, all I get is a farting out of the speaker. A very LOUD farting with all the controls at zero. I managed to measure at the rectifier and I have about 525 Volts. It seems to quiet down the more pre amp tubes I pull out but even with the last one pulled I still hear some noise. Any Idea where to begin with this beast?

[Enzo]

I hate to get into a discussion of the word "farting," but I need to know what you mean. TO ME, farting means an extra sort of noise accompanying sounds. Not a continuous noise. In other words like a blown speaker. Farting to me is o=something that only happens when a sound gets amplified.

If you have a loud continuous BRAAAAAAA or loud hum, then I'd call that a hum or at least a continuous noise.


The reason I dwell on this is that in my use of the word, and amp can't really fart with the controls all at zero. Well unless it is making a poot noise every few seconds on its own. And the nature of your problem is what will direct us down the proper repair path. A continuous loud hum might be worn or failing filter caps.

[Danglin' Fury]

It is definitely a loud, continuous hum/sound that sounds like somebody giving you a raspberry (tongue out, pursed lips, blowing air through it). Very loud. Somebody before me has replaced the multi-cap with a new one. The 2 large electrolytics after the rectifier have been replaced as well as more than a few of the smaller ones. Sorry for the farting reference. It seems even some of the non-electrolytics have been replaced. I have poked and prodded the circuit a bit. Not too long because of the loudness of the noise. But long enough to hit all the solder points and components. Could the bias pot be way out of whack? I have a set of earplugs now so I can measure more voltages but after the rectifier I have about 525 Volts DC or so. I will take an accurate measurement tomorrow. Thanks for your reply Enzo.

-Shawn-

[Enzo]

Now raspberry is a technical term I understand. The 'ole Bronx Cheer. Hey, it was just me, maybe no one else was confused.

A bias adjustment won't make a raspberry.

You may have a bunch of replaced caps, but are they doing the job they are tasked with? For example, you bring up bias. A new bias filter cap won't help much if it was installed backwards. That is not as rare as you might think. A bunch of perfectly good filter caps can't do their decoupling if their ground return path is broken somewhere.


A dummy load would help with the noise.

No scope?

Set your meter to AC and check where you got that 525vDC. I'd expect maybe 5v AC there, but hopefully not 50vAC o0r more. Likewise the bias supply, I expect pretty much no AC ripple there at all. But if we have some sort of oscillation going - motorboating - you can probably detect that signal with the meter too. And if it is on the B+, then that might explain a few things.

Any chance someone replaced the output transformer? If so they may have swapped the primary wires. See if you can find the negative feedback line from the speaker outs back to the phase inverter and disconnect it. ANy difference?

Got a presence control? Does turning it affect the frequency of the noise? (the note it makes)

[Danglin' Fury]

All the transformers are original, as in the power and output trannies say Plush P1000 on them. It does not have a presence control and when I disconnect the negative feedback line from the PI, it merely hums and is MUUUUCH quieter.

[Danglin' Fury]

I need to make a correction. At the rectifier, I have 566 VDC. I switch the meter to AC and measure at the same place and get 1.5 Volts AC. On the AC side of the rectifier I am getting 715 VAC.

[Danglin' Fury]

Also since removing the NFB from the PI, A faint, but distorted, guitar signal passes through the Vibrato channel. No signal will pass through the Normal channel.

[Enzo]

So if pulling the NFB quiets it down, I still have to think the OT primary is out of phase, or you have serious trouble in the PI.

So pull the PI tube, now all you have is power tubes and OT between you and the speaker. Now touch each power tube grid pin with a meter probe (leave the black wire not connected), or maybe a small screwdriver. Does that make a little hum come out the speaker? Or feed an actual signal to each grid through a cap. Does that come out? If not, you may have transformer issues.

Can you link a schematic? I have no idea what is inside one of these. For example, is the PPI a split load type or a long tail pair type?

If that works, put the PI tube in, but pull the earlier stage tubes. I will assume the PI is a long tail pair. Touch your hum source - wire or screwdriver to each grid pin of the PI. Or inject a real signal at each. Does either yield sound out the speaker?

You measured B+ and also found low ripple, but have you taken meaningful readings at the tubes? Is there B+ at BOTH plate and screen of every power tube? Is there good clean bias voltage at every power tube grid? Is the plate voltage present on both sides of the PI, and about even? Is the cathode voltage up to snuff?

[Danglin' Fury]

I would love to link you a schemmie. But I have none. Can't find one. Some people swear that it is a direct copy of a Fender Twin ala Bugera but I'm not so sure. A twin has 6 pre tubes this has a 7th for a slave and recording out. It would be pretty sweet to have a Plush schematic or if somebody could verify that, yes indeed, it is a Twin copy. I desperately need a schemmie.

[Enzo]

OK, so assume it is the same circuit until we run into something that doesn't fit. Certainly the power tubes and PI ought to be easy enough to follow.

[Danglin' Fury]

On the power tubes, on all the pin 5's, I have -52 ish Volts DC. On pins 3 and 4 I have around 475 VDC. When I remove the PI and just touch pin 5, there is a slight crackle but no hum. When I was measuring the voltages, as I touched pin 5, there seemed to be a slight hum with the black lead on ground.

[Enzo]

That is enough to think they are probably working. You can put a cap in series with a signal from somewhere and touch that to pin 5. Might be easier to hear.

So now move back to the PI stage.

[Danglin' Fury]

Soooo.... I've pretty much changed all the components in the PI stage along with the 12AX7 and that bastard "Bronx Cheer" noise will not go away!! AAAAARRRRRGGGGGHHH!!!!! What the hell is wrong with this turd?

[Gtr_tech]

Reverse the plate leads on the output tube sockets...just swap the blu and brn around. I bet it stops the air raid siren....