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Question for 18w Marshall Users.

Teahead

Member
Messages
1,375
I have a 2006 Cornell 18/20 Plexi Combo... link

A great sounding amplifier indeed, but so damn choosy about the first position preamp tube! I'm not saying it only sounds good with certain valves, far from it, the amp sounds great, if a little different, with both NOS and current production.

The problem seems to be with any tubes that are even slightly microphonic. I can use tubes in the PI position of the Plexi, or anywhere in my AC30, that cause lots of problems when used in v1 of the Plexi. These problems are usually odd noises like whines or crackles.

My question is, are all 18w Marshall type amps so choosy about the first tube? If so, why? Is it something to do with the amp having more gain than my Vox? I've been told short plate nos ECC83s would help, any opinions there?

Thanks.
 

Steve Dallas

Member
Messages
8,317
They are a little picky about V1. This is because both triodes of that tube are biased extremely hot compared to other amps. This little bottle is really cookin' in 18W amps, which is part of the sound.
 

andrekp

Member
Messages
5,626
They are a little picky about V1. This is because both triodes of that tube are biased extremely hot compared to other amps. This little bottle is really cookin' in 18W amps, which is part of the sound.
Not sure I entirely agree with that statement. They (18 watt amps, if that's what you meant) are biased pretty normally it seems to me (my Valve Jr. biases the preamp hotter...). I think a more critical part of the 18 watt sound is the parallel preamp and the relatively lower gain, but that's just my view.

I'm more concerned with the statements on the Cornell site:

"Complementary push-pull class "A" output has been used in many renowned amplifiers throughout the history of guitar amplification."

Maybe this is just British terminology, but an amp is either class A or push pull, it's not both at the same time.

Then:

"The problem with this basic circuit, which has remained common to all manufacturers with some minor differences, was one of unequal current flow through each output valve and unacceptable crossover distortion."

I agree about the crossover distortion, but I would necessarily characterize the other part as correct as a rule; assuming what they actually MEAN is the unequal or unbalanced outputs from the PI. Lots of folks, me included, think the unbalanced PI is a critical part of the rich sound of these amps. These are amps, not Hi-Fi systems, they SHOULD have a certain number of "problems" as a part of what makes them sound as they do.
 

DavidH

Member
Messages
2,367
No, Class A amps are push-pull too (if they have more than one power tube and they're not parallel single-ended, which is rare), the difference being that in class A push-pull there's still some signal passed by the 'off' tube when it's the other one's turn in the cycle, due to the way it's biased there's still current left on the grid. In AB the valve shuts completely at some point in the cycle. That's like the definition of the two classes of operation, at least in my very basic, non-techy way of understanding. Edit: In other words, in Class A both valves stay open all the time at least to a small degree, in AB there a moment where the valve shuts. Is that right?
 

Steve Dallas

Member
Messages
8,317
Not sure I entirely agree with that statement. They (18 watt amps, if that's what you meant) are biased pretty normally it seems to me (my Valve Jr. biases the preamp hotter...).

:huh

V1 in an 18W is biased with an 820R, which is hot compared with most amps (1.5-3.3K). When run in parallel mode, it is biased to 410R, which is extremely hot, which is why you get that fatter, richer tone, but not much more output when you plug into input 1.


I think a more critical part of the 18 watt sound is the parallel preamp and the relatively lower gain, but that's just my view.
I didn't say it was "the critical part" of the sound. I said it was "part of the sound." Indeed it is.

Maybe this is just British terminology, but an amp is either class A or push pull, it's not both at the same time.
Completely untrue. Class A is a mode of operation. It applies to push-pull and single-ended amps alike.
 
Last edited:

Teahead

Member
Messages
1,375
... V1 in an 18W is biased with an 820R, which is hot compared with most amps (1.5-3.3K). When run in parallel mode, it is biased to 410R, which is extremely hot, which is why you get that fatter, richer tone, but not much more output when you plug into input 1...
Thanks again for the info guys.

Can I ask, what is this parallel mode? Is this a feature of the circuit rather than a user selectable option?
 




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