Marshall clone, what to do?

[J M Fahey]

If you don´t have 15 minutes to check biasing circuit, and what is actually feeding the screens (hey, only 3 (three) options: regular screen resistors / MosFet / wirewound pot) then I´m not sure we should be cracking our brains miles away just to guess for you

the wires run under the board

Now that´s evil, and makes amp impossible to service.
Unless you remove the board and take a peek, that is

He also says it don't sound too bad thru my cab, which is a bass cab with the horn turned off.

Then he´s already deaf. Not a surprise in a guy who plays 100W Marshalls and 4 x 12" ful blast regularly.
Not an insult, I mean *clinically* lost hearing, where the treble sensitive cells (which are the smaller ones) are the first to die, and the World becomes dull muddy.
A woofer only Bass cabinet is fine to test that an amp *works* , period, but absolutely no reference for Tone.

In a nutshell: you do not need to "wait for a schematic" on an amp that to begin with is a clone is a very well known one, perhaps with minor tweaks, and which worst case involves 4 tubes, 4 resistors (or whatever´s there and which we would LOVE to know, there´s the key of the problem) , 4 grid resistors and (maybe) 4 cathode sensing resistors.
How long does it take to look at and draw that?
I´m baffled by why this simple drawing was not here since day 1 .

[Randall]

I found the guy who built this amp 15 years ago, and he is gracious enough to help me fix it. He tells me the screens sitting about 40v below B+ is indeed a problem, that they should be only a few V's below plates. OK, good. While I am tracing my way through the screen supply I find the feed from the choke goes to a turret that connects a filter cap, a 10K node resistor and a 1.5K resistor that goes to pin 6 of the first output tube, which feeds the four 1.5K screen resistor for the four tubes.I disconnect the filter cap and the node resistor leaving only the choke and the five screen resistors. I am dropping 34v across the first resistor and ~ 8v across the four on the sockets. So my question would be, how would it be possible to think the screens could run a few Vs below plates when the 1.5K R feeding from turret to first socket is dropping 34Vs? Why is it even there? I don'see it in the Marshall schems I've looked at. I am awaiting his answer, and wonder if someone has modded this.

I have subbed the 1.5K and the tubes, same result.

[Chuck H]

The screens CANNOT be only a few volts lower with that 1.5k series resistor in there. I assume that the 1.5k resistor was there when the guy liked the amp? The person you're working with now may ask you to bypass or remove it. You should probably resist because it will move the design away from what the owner was happy with. It seems like an odd design the way you've described it. Is there anything we don't know here? The customer had the amp and liked it. Then it lost some mojo. Now it's with you because of that. Is that it?

[Randall]

Thank you for confirming that Chuck. I am trying to get to the bottom of if that series resistor is original and if so, WHY? Also, the customer is, shall we say, a bit murky in is descriptions, in fact I almost didn't take the job because he seemed so vague. My next order of business is to pin him down on exactly how his chain of events occurred. I don't care for these, "It doesn't sound right to me, it has lost, I don't know, SOMETHING. Can you fix that?"

And fellow will, when asked a specific question with more short questions to follow, give you his life story, show you pics on his phone of all his gear, and tell how he met his wife.

[Chuck H]

Because the screen grid current of all four tubes is going through that single 1.5k resistor it's roughly like haveing individual 6k screen grids resistors. With the exception that there is some interaction between the screens WRT any AC present on top of that resistor which isolates the screen grids from the filter cap on the HV node. I've never seen it done like this and I probably wouldn't think to do it either. But whatever is happening, the customer liked it. Now he doesn't. How old are the tubes??? I inferred earlier that worn out power tubes would drop LF and LF definition. Since the guy heard the amp through a cab that likely has a higher LF balance than his own, and thought the amp "didn't sound too bad", it could be that it's just time for new tubes!?! But how old are the tubes? I don't even care if they still have useful service life according to any tube tester. If they're the least bit weak he may need new ones now to get his mojo back. Some professional guitar players that crank their amps replace power tubes every couple to a few months!!! If he plays cranked A LOT then this may be the issue. Try not to let him side track you with how his cat is doing after surgery

[Randall]

He says the tubes have about 6 hrs on them, and they all test strong and very close to each other.

[Leo_Gnardo]

I found the guy who built this amp 15 years ago, and he is gracious enough to help me fix it. He tells me the screens sitting about 40v below B+ is indeed a problem, that they should be only a few V's below plates. OK, good. While I am tracing my way through the screen supply I find the feed from the choke goes to a turret that connects a filter cap, a 10K node resistor and a 1.5K resistor that goes to pin 6 of the first output tube, which feeds the four 1.5K screen resistor for the four tubes.I disconnect the filter cap and the node resistor leaving only the choke and the five screen resistors. I am dropping 34v across the first resistor and ~ 8v across the four on the sockets. So my question would be, how would it be possible to think the screens could run a few Vs below plates when the 1.5K R feeding from turret to first socket is dropping 34Vs? Why is it even there? I don'see it in the Marshall schems I've looked at. I am awaiting his answer, and wonder if someone has modded this.

I have subbed the 1.5K and the tubes, same result.

That 1K5 resistor in line with the screen grid feed is indeed a curiosity, definitely not a standard feature in any Marshall I've ever seen. Hard to know if it wasn't added by some well meaning solder slinger, but WHEN in the history of the amp? Maybe just before the tone started to suck ??? Hm wouldn't that be a coincidence... Also the 1K5 screen grid resistors. This non stock screen grid rigging won't necessarily cause the amp to sound bad but it sure won't be exactly what it was before it was modded.

And yes I'm all too familiar with those - luckily rare - customers who are all loosey goosey with the facts, can't remember what sounded how, and when, and who worked on it, and what they did. I had one particular stupid git who came in with his Twin Reverb basket case, and after I set it to rights claimed I ruined his "special tone." Hell two output tubes didn't even light up because they were jammed cattywampus into their sockets, all sorts of other mayhem, there's no way he could have played it on those high falutin' jazz gigs he claims he does. All that's not enough, he dug into his guitars and boogered up the wiring in them too. Special sound indeed, the sound of a loser with his head up his bum. I hope he's given up calling me, forever.

[Chuck H]

Leo... You should find that guy and smother him in his sleep with a pillow. The anxiety you feel will be LESS than what you have now with the apprehension that every time the phone rings, it could be him. JM2C

[Chuck H]

He says the tubes have about 6 hrs on them, and they all test strong and very close to each other.

Ok. Now we're cookin' with gas. Here's my take...

He got new tubes and was happy for it, cuz, "NEW TUBES!!!", right?. It took him a bit to realize the amp wasn't doing what it use to do. Who put the new tubes in and who biased them? Anybody? What was the bias current BEFORE the tube change? Christ guitar players are ignorant to the things that actually matter.

He doesn't like the new tubes NOW. Nothing wrong with the new tubes, but they're biased differently from the old tubes. Have him come in to your shop and set the bias by listening. If the mojo comes back and the customer is happy then you check the bias to see that it's in a safe operating condition. If it is then that is this guys sweet spot for this amp. (<period)

That's my advice right now.

[Leo_Gnardo]

Leo... You should find that guy and smother him in his sleep with a pillow. The anxiety you feel will be LESS than what you have now with the apprehension that every time the phone rings, it could be him. JM2C

I have plenty of anxiety when the phone rings Chuck. Could be this guy, or the Microsoft Official Malware tech in Bangladesh, some geek trying to sell me aluminum siding, etc. I got a million of 'em. Let's not forget the guy who dropped his amp off and it isn't getting fixed fast enough for him.

Well Mister Dodgytwin did have a heart bypass operation last year. I figured, hey maybe his hearing was affected by the lack of oxygen to his brain while his blood circulation was comprimised. Alas no, he still claims his guitar sound is for shit, and he's just as obnoxious as ever, getting his heart fixed didn't brighten his personality that's for sure. While I don't wish him any harm, what concerns me is he's running his mouth telling anyone who will listen how I "wrecked his sound." Since then I've found he's also made himself unwelcome at several music stores, and local musos consider him to be the Joe Btfsplk of guitar. "Who dat?" In Al Capp's L'il Abner comics of yore, Joe B was the guy who was constantly walking around with his own personal thundercloud raining on him. A sharp observer of human nature was Al, yes some people are like that.



Now, let's get back to Randall's wacky Marshall, already in progress .

[Randall]

Makes sense Chuck. I'll give that good consideration after following up with him and the chap who built the amp. This is a slow turning wheel.

[Mick Bailey]

The resistor feeding the screens is coincidental to a situation that I just encountered with a Fender Blues Deville. A 10K resistor was inserted in the screen supply and then 4.7uf capacitor to ground. This then connected to the 2x470R screen resistors. Puzzling why someone would do this.

[g1]

Maybe we'll turn up a "ghetto VVR" article somewhere on the 'net?

[eschertron]

The resistor feeding the screens is coincidental to a situation that I just encountered with a Fender Blues Deville. A 10K resistor was inserted in the screen supply and then 4.7uf capacitor to ground. This then connected to the 2x470R screen resistors. Puzzling why someone would do this.

Someone's been stealing my secrets!

I have put a similar arrangement onto the screen (singular) of a champ-amp. Seemed to make the SE distortion a little less abrupt, a 'bit' smoother. Tried the same trick on a princeton-y build this winter. Snatched out the dropping resistor and capacitor as fast as the soldering pencil could heat up!! I'm guessing the high value "current sense" on the screens creates some NFB, maybe a bit like a hybrid triode-mode? Anyway, while I like it on the SE amp, it was a nasty tone-sucking mess on the PP amp.

edit: And yes, made the amp quieter too. Volume control, indeed!

[Chuck H]

What Randall described was that there'a the main B+ filters, a choke, then a 10k to another node (I assume the PI), but after the choke there's a universal 1.5k resistor that then feeds all four individual 1.5k screen grid resistors. No filter cap. That's just weird and I can't say how well (or not well) it would work out.