Marshall clone, what to do?

[Jazz P Bass]

It will work fine.

It will also throttle back the overall output.

[bob p]

the wires run under the board

Standard tech equipment: plastic dental mirror.

[Chuck H]

It will work fine.

It will also throttle back the overall output.

Well yes, I figured it would "work". I'm intrigued about any AC coupling of the screens all standing on a single resistor without a cap to ground.

[Chuck H]

I have the old school metal one. I barely use it so I'm not cavalier about seeing to it there's no voltage on the amp when I do. Even so, it just now occurred to me that I should shrink tube the handle.

[eschertron]

I'm intrigued about any AC coupling of the screens all standing on a single resistor without a cap to ground.

I'm thinking the net current* through that resistor would remain nearly constant for a PP arrangement, mitigating the need for the cap.

edit: * and so too the voltage at the point the four screen resistors branch off.

[pdf64]

It's a similar arrangement to that used by the JTM45 (1k shared feeding 2 x 470 ohm individual g2 resistors) http://raw-sewage.net/images/jtm45-readable.jpg
When I asked about it on here a while ago, it was said to be a hifi thing http://music-electronics-forum.com/t28094/
When compared on the scope to 1k individual resistors per g2, the large signal waveform is a little different, perhaps more rounded.
Yes I think there may be some local feedback going on with the shared component.

[Randall]

I have the plastic one, but there is just so much I can see with it.

I am still waiting on the builder, Ossie, to get back to me about that in line 1.5K screen feed resistor. I find myself at an impasse until he does.

[Chuck H]

I have the plastic one, but there is just so much I can see with it.

I am still waiting on the builder, Ossie, to get back to me about that in line 1.5K screen feed resistor. I find myself at an impasse until he does.

Why? The owner liked the amp as it is, until...? Changing the amp from what the owner liked on the word of the amp builder, who has certainly changed the design and has new ideals now, is a sidestep that can ONLY complicate the matter. The best you can hope for if Ossie says to change the design, and you do, is to then have to tweak the new DIFFERENT design into an amp that is like the amp it was before that the owner liked.

C'mon! Somebody back me up here?!? Am I the only one seeing this? Honestly, forget what the designer says. The amp is what it is and the owner liked it until he changed the power tubes. That only has to do with the amp. Involving someone with an amplifier design opinion is going to complicate the issue to unmanageable levels. DO NOT CHANGE THE AMP!!! FIX IT!

[dstrat]

Thing needs to be realized is each tube is different in lots of different ways - in other words one set may not bias the same or sound the same,
not a lot of attention is being paid to specs and they are not getting better either, seems to me.
Adding more resistance in the screen will change the sound yes, also will likely help the tube last more then hours or days.

[bob p]

I have a Blockhead 100 watt Marshall plexi clone on the bench. Guy says "it just doesn't sound as good as it used to when cranked up with my LP and 4x12 cab".

I've heard Pete Townshend says the same thing when he plugs into any amp. The problem is that nothing sounds the way it used to sound because he's gone deaf.

[bob p]

C'mon! Somebody back me up here?!? Am I the only one seeing this? Honestly, forget what the designer says. The amp is what it is and the owner liked it until he changed the power tubes. That only has to do with the amp. Involving someone with an amplifier design opinion is going to complicate the issue to unmanageable levels. DO NOT CHANGE THE AMP!!! FIX IT!

Long version:

The little angel standing on my shoulder says to tell him that new tubes sound different from old tubes, to send the amp out, and to re-evaluate it's tone after playing for a while. Chances are the customer will adapt to the tone and be happy.

The little devil standing on my other shoulder says fix the amp, and tell the owner that it's working to spec. And then tell him that if he doesn't like the way it sounds then the problem is that the honeymoon period has ended, and he should sell it and buy something else. Now where's that little devil emoji? We really need one of those. For now I guess I'll have to use this:

Seriously. Most musicians can barely afford to have an amp repaired, and very few of them can afford to have a circuit tailor-made to satisfy their ears. It's like buying a suit. Most of these guys need to buy off of the rack and can't afford having a suit tailor made for them. For this reason I fix peoples amps and put them back to spec, I don't offer custom tailoring services. That is, I don't offer custom tailoring unless someone is a special friend, or if that person is prepared to schedule a day that they'll come in, and put down a stack of $100 bills to pay on a time rate for me to change resistors and caps on the spot as they audition different circuit tweaks.

I find that it's really easy to get rid of whining tone hounds by telling them that the amp has been fixed, everything is in spec, and they're now at the point of diminishing returns in a tone quest, where they're going to have to put up money -- lots of it -- to have an amp custom built to suit their ear. In that situation they're better off spending lots of money on speaker swaps because speakers make a huge difference. And because they won't be paying for my time.

The problem with nitpicking tweaking is that if you invest enough time and money, then it's easy to get an amp/speaker to sound the way you want it to sound in the shop, studio or rehearsal space, but then it's going to sound totally different when it's in a huge hall full of people, or after the guitarist has had a few beers. I stay out of that type of work unless someone is willing to throw enough money at the problem to make it worthwhile for me... with the understanding that there are no refunds if they aren't satisfied because they're paying for my time in participating in their chase, with no guarantee of any desired result.

I'm with Chuck. Fix it and move on.

Short version:

Don't get lured into some guitarist's tone quest unless you're getting paid for your time.

[Chuck H]

That is, I don't offer custom tailoring unless someone is a special friend, or if that person is prepared to schedule a day that they'll come in, and put down a stack of $100 bills to pay on a time rate for me to change resistors and caps on the spot as they audition different circuit tweaks.

Funny. And accurate. I know because finite tweaks is sort of what I do whenever I take an amp project. I say "sort of" because while I do listen to what the customer wants from an amp, and I make it do those things, tone tweaking is done by MY ears. I can tell when an amp is working with the speakers to make the right frequencies play together and I can tell when the gain structures are lapping at their distortion levels to make the amp "feel" alive and grab notes instead of just making sound in a workman like way. So I tweak them until they're "right" by my standard. When I'm done I say the amp "has been Chuck'd". I do this for fun and experience. There is no profit or wage possible in it.

In light of my proclivity to take amps beyond simply repaired, and the fact that Randall did the same thing by taking in a repair based on "It lost something". Taking that repair is an agreement to try and put that "something" back. I'll go back to my suggestion in post #24. That would be as far as I would take it and it's probably the solution. The amp doesn't need to be "understood" by consulting with the designer. And it certainly doesn't need to have it's circuit altered on ANYONE'S suggestion. The customer already likes it the way it is. It "lost something". That loss corresponds with the power tube change. So make the new tubes behave more like the old tubes did. Biasing is the only parameter to control here since the tubes cannot be artificially aged. The new tubes were probably installed and biased up to some arbitrary spec. and are, perhaps, drawing less current than the old tubes were. Get the customer in there with HIS cabinet. Adjust the bias and have HIM play test the amp. If it works, you're done and the customer now has a current spec for the amp that puts the mojo back in.

[bob p]

Funny. And accurate. I know because finite tweaks is sort of what I do whenever I take an amp project. I say "sort of" because while I do listen to what the customer wants from an amp, and I make it do those things, tone tweaking is done by MY ears. I can tell when an amp is working with the speakers to make the right frequencies play together and I can tell when the gain structures are lapping at their distortion levels to make the amp "feel" alive and grab notes instead of just making sound in a workman like way. So I tweak them until they're "right" by my standard. When I'm done I say the amp "has been Chuck'd". I do this for fun and experience. There is no profit or wage possible in it.

It's totally different if you're tweaking your own design, or if you have a friend come in and play while you tweak. If that leads to the "Chuck" design that sounds great, then there is profit in it, a different sort of profit, but a profit nonetheless.

In light of my proclivity to take amps beyond simply repaired, and the fact that Randall did the same thing by taking in a repair based on "It lost something". Taking that repair is an agreement to try and put that "something" back.

I prefer not to take on "repairs" that are based upon imparting a totally subjective and ambiguous tonal standard upon a properly working amp. IME that's a fool's errand and nothing good could possibly come from it. Yes, if you really need the business then you might end up taking on those types of "repairs." If you don't need the business it's nice to have the luxury of refusing to take on jobs like those.

[bob p]

The customer already likes it the way it is. It "lost something"... Biasing is the only parameter to control here since the tubes cannot be artificially aged.

Yes, that loss does correspond to power tube change. And new tubes do sound different. So if you want to take the angel on the shoulder approach, bias the tubes and tell the customer to put off making a decision about the amp until he's had the opportunity to put some time on it. Chances are he'll be happy.

On the other hand, if you want to take the devil on the shoulder approach:

So make the new tubes behave more like the old tubes did. Biasing is the only parameter to control here since the tubes cannot be artificially aged.

Oh, but they *CAN* be artificially aged!

I have box full of meticulously aged, completely broken-in dead-sounding tubes that I'm willing to sell to anyone who really needs them to restore an amp that's "lost something." <-- surrogate devil emoji.

On a more serious note, we've often discussed the high failure rates of modern tubes in the tube reliability threads, and how it doesn't pay to stockpile tubes, especially considering that manufacturers won't warranty them for more than 20 minutes. The result is that you have to buy tubes on an as-needed basis, or you have to do your own screening when you buy in bulk, which is a PITB. I've mentioned this before, when I buy tubes I do burn-ins to cull the rejects and return them. It's a short hop between burning-in to cull the rejects and burning-in to age tubes, if you want to go that route.

I'm kind of funny in that I do a sine-drive burn-in on any amp that I repair, to be sure that it's not going to fail right after it goes out the door. If a customer is looking for that "special" sound of aged tubes, there's no reason that anyone couldn't pre-condition or "age" tubes for him by driving the tubes into a load for a week to pre-condition them. Sure, you're throwing away lifespan by doing that, but we live in a market where people pay premiums for brand new "distressed" guitars that look like they've been attacked with a belt sander. Pre-aging tubes could be a value-added service. <- the devil.

[Chuck H]

Well, ok but... Putting THAT much burn in on a tube is not really a reasonable thing to do when you consider that the old tubes probably sounded like they did because they needed replacing. So tht would mean artificially aging, say, 75% of the life out of the tubes and simply using four times as many tubes Not to mention all the snarky things that can foul that effort like one tube in a matched set failing prematurely, etc. But this does bring up a very noteworthy consideration...

Ear conditioning. Like air conditioning except that it's in your head when the space between your ears is empty. No, just kidding. Ear conditioning would be when you just get so use to the way something sounds that it sounds "right". I've experienced this with guitar tone in profound ways. Like being forced to use a really mediocre tone for so long that when you plug into something you remember as great, it doesn't sound right anymore. It's entirely possible that the weak sound of the old tubes was part of the lost mojo.

I really think heating up the bias will go in the right direction. It'll reduce headroom and mush up the overdrive sound. Not just like old tubes, but at least leaning that direction.