marshall 1987x hiss reduction possible?

[blindboybenton]

Hi All.

Got a marshall 1987x Customer brought it without seeing it or hearing it. In good shape. New JJ's all round. The amp has too much hiss for the customers liking

Ive swapped out the tubes and checked/changed node resistors.

Ive tried a few 12AT7 - 12AU7 in V1-V2 but customer don't want to go down that route if possble

Anybody had any success with trying to tame the hiss on these beasts



have a good day

BBB

[Leo_Gnardo]

Ive swapped out the tubes and checked/changed node resistors.

Having swapped in some different 12AX7's you may have found the JJ's to be the least offensive in terms of hiss. I found that to be the case with Ruby HG+ selected JJ's. No joy thru swapping tubes? - here are two options. First is to limit hi frequency response by placing small disc caps, 100 to 470 pF, across preamp plate resistors. That will roll off hi frequencies in the pre stages, reduce hiss, and it's up to you how much noise to remove without affecting the signal you want to hear. (The old sound engineer maxim about bandwidth: the wider you open the window, the more s#!t flies in.) Second, if there's an effects loop, get a good gate and learn to use it. Short of that, the customer will have to "learn to enjoy" the steam-radiator effect. Just play loud enough and his ears won't be able to sense that hiss. Especially if accompanied by a drummer.

[blindboybenton]

Hi Leo.. I know about JJ's not my fav tube. I used Tung sol Premium selected 12ax7 12au7s. I will have a look at the caps, But it as you said it may affect the freq some what..
play loud and you wont hear it LOL. Any hiss on stage, blame the drummer

BBB

[big_teee]

Using good shielded wire between the input jacks and the preamp helped mine.
I redid it a couple of times, before I got it to my liking.
YMMV,
T

[blindboybenton]

Looked at the Big tee. all good. noise is worse from V2B onwards cant really see anything really before that

BBB

[J M Fahey]

Work backwards.
Ground the PI input grid, then the tone stack one, etc, going towards the input jack.
If grounding the first or second grids (high gain or normal inputs) does NOT lower hiss, then you have a real problem to solve, BUT if hiss seems to come mainly from the first triode (as heard through the speaker) then it´s simply a reality of life and you´ll have to live with it.
All you can do is try different 12AX7 as V1, picking the best.
Substituting a lower gain tube or restricting frequency range is not an option, the amp won´t be a 1987 any more

[blindboybenton]

Many thanks people. The owner is going to send it back for a refund, no matter what i do to the amp to sort of the hiss its going to change the tone/behavour of the beast,I Just goes too show , that its never a good idea to buy something used without hearing it first or knowing what they sound like. Some good may come out of this. I have a Marshall MK2 Master model Lead 100watt in the workshop, that after a full recapp and revalve its up for sale,, so that owner says

[stormbringer]

how about swapping the board-mounted 68k input grid stoppers with 10k metal film Rs. Ideally these would be mounted directly on pins 2 and 7.

[blindboybenton]

Thanks But this amp has long since gone

[danman]

I see that this is an older thread but I wanted to mention that clipping the large (5000pf) bright cap on the volume pot in my 1987 took away almost all of the hiss and gave a much better taper to the volume.

[Pedro Vecino]

That issue takes priority over a new wiring with shielded cable and grid resistors directly on the sockets.
4n7 (5000pf) value as bypass capacitor in bright volume aims to give the feeling of infinite volume and makes it very difficult to control it. 100pf is a more appropriate value.

[J M Fahey]

Both answers are Technically perfect but that huge 5000pF bright cap is an essential part of 1987 behaviour.

Take it out, it's anything else but.

100pf is a more appropriate value.

says Miss Wiggins:

[Pedro Vecino]

Why not try -better than wasting time finding a picture like that in google- to explain the different effect with or without capacitor (that value or approx.) when you place the volume to maximum and gradually lowered? Maybe concludes that the transition with respect to the full harmonic content is much more natural.

[MarkusBass]

I think that this is a simple mistake with interpretation of old schematics. On very old schematic this bright cap had value like: 0.0005 - even without the units. One can guess that it was in microFarads. Now, if you have three zeroes after the decimal point and before the first significant digit, you can easily make a mistake and say that this is 5 nF. But if you think for a while, you will see that this is 0.5 nF (ten times less). Many reissue amps were assembled with 4.7 nF capacitor. The amps sound bad because such a big capacitor simply shorts the pot for high frequencies. Very simple fix is to replace the capacitor with 470pF. If you insist that 4.7nF is essential part of the amp, you can install a switch that switches between 100pF, 470pF and 4.7nF. I did this several times in this amp and I can tell you that users never used 4.7nF position. And they say that finally they can play the amp and are very happy with the sound.

Mark

[dai h.]

5nF is not a mistake as far as Marshalls go. In reality, they could have measured lower due to loose tolerances of the ceramics used (have seen reports of lower measured values for this cap in old Marshalls). As far as the sound, I suppose it could be a preference thing. Some don't like it, some do, and some like a smaller value. The originals varied here depending on model (and whatever the circumstances were at the moment--lots of variations with old Marshalls). Could be no cap, 100pF, 470pF, (seen 2n2 (some Parks?) reported but haven't confirmed with chassis pics myself), 5nF (early 5F6-A circuit leads, later split cathode leads (which sometimes didn't have one). To me 5nF seems okay (as well as LIN volume pot taper).