[sdiy] Seeking Marshall 1987 guitar amp potentiometers (vintage...1975)

[Mike Pepper]

> I just bought a vintage Marshall model #1987 guitar amp head
> and found there is one of the inside 1meg log pots
> that is damaged. (Vol I channel). This is a vintage 1975 amplifier and I
can't find any source..
> The pot was made by CTS and it has a 1/4" plastic shaft
> with a lenght around 3/8".
> The shaft travel is specific to this amp since
> the pot's back has 2 notches which shorthen the overall shaft travel.
> See the picture here:
> http://www.arcenson.com/public/Marshall_1meg_pot.JPG
>
> This means I cannot use a standard CTS pot with one notch.
> Is there anybody that would have some in hand ?
> I'd need two of them.
>

This was the standard CTS pot of the day, as sold by RS components. I still
have 1 NOS 1 Meg Log one it you need it. Sadly, all others are 25K and 50K.

It might be worth putting a pointer knob on a new one and measuring the
rotation. I seem to remember that the internal stop was made bigger in the
new design, giving the same rotational angle, which is what counts. I did
reworked a few of the 50 watt heads for studio use, fitting the now
unpbtainium Holco low noise resistors, re-doing the heater wiring and
fitting conductive plastic pots. The 'Marshall Sound' survived intact, but
with a lot less background noise. Perhaps the most interesting thing I
discovered at the time was to make sure that the input ECC83's were the
orginal Mullard 'Long Box' type: (the anode casing is noticably bigger than
most clones), which made the sound better in a way that is not easy to
describe but was noticable. You can roll your output valves and bias to your
own preference.

Oh - and put some decent quality 1K screen feed resistors in whilst you are
in there - they are famous for either going high in value, or feeding some
much heat to the solder joint on the valve base that they go open circuit.
If the EL34 Anodes show a red glowing patch this is the most likely culprit.
There is a mod issued by Marshall to convert to 5881 output bottles. I have
the servce bulletin if you want a copy.

Contact me off list about the pot.

Mike