1960s JTM 45 key changes

[London John]

I do believe the tube hats are original. I got this amp from Julian Marsh in 1990 (on trade) He found this amp (complete stack) in London as it is. The owner used it as a bass amp (from what I remember Julian telling me). The amp came with replacement black AC-30 pointer knobs which I replaced (using knobs from another JTM-45 that I sold). Your Pekalit knobs are original, not the repro Coopers??

If it's a Clapton knob '65 amp, it would have come with KT66s with the braid retainers. It looks like the tubes and retainers have been replaced, possibly at the same time the knobs were replaced. It's no biggie. Yes, knobs are original Pekalit, been in the parts drawer for 20+ years. Again, pot shaft will tell you what you need to know about which knobs were originally on the amp. Could be either in that transition period, but more likely to be Pekalit, particularly if the Scott knobs are tight.

 

[guitarzan2525]

If it's a Clapton knob '65 amp, it would have come with KT66s with the braid retainers. It looks like the tubes and retainers have been replaced, possibly at the same time the knobs were replaced. It's no biggie. Yes, knobs are original Pekalit, been in the parts drawer for 20+ years. Again, pot shaft will tell you what you need to know about which knobs were originally on the amp. Could be either in that transition period, but more likely to be Pekalit, particularly if the Scott knobs are a little loose.

This amp did originally come with the Pekalit (Clapton) knobs (Gold/Black block logo head).

 

[London John]

This amp did originally come with the Pekalit (Clapton) knobs (Gold/Black block logo head).

View attachment 155399

Extremely unusual to see a gold block logo attached with steel screws. Standard for silver/maroon, but I’ve never seen them on an unaltered gold logo head.

I don’t know what the picture is supposed to tell me about the knobs.

 

[TAZIN]

The witches hat tube retainers aren't original to this amp, they would have been the thin braid variety.

Do you mean this style?

 

[guitarzan2525]

Extremely unusual to see a gold block logo attached with steel screws. Standard for silver/maroon, but I’ve never seen them on an unaltered gold logo head.

I don’t know what the picture is supposed to tell me about the knobs.

These are stock oval Philips screws that came with the head. These are nickel coated (I assume). What other type of screw should have come with the amp?

The pic is showing the amp as an early '65 head that should have come with the Pekalit knobs. The potentiometer shaft's are solid shafts (if memory serves), not "D" shafts..

 

[London John]

Do you mean this style?

Yes.

I came across a box of 100 NOS years ago.

 

[London John]

These are stock oval Philips screws that came with the head. These are nickel coated (I assume). What other type of screw should have come with the amp?

The pic is showing the amp as an early '65 head that should have come with the Pekalit knobs. The potentiometer shaft's are solid shafts (if memory serves), not "D" shafts..

Across the original block logo JTM45 heads I've owned or seen in person, as a rule Marshall used brass screws with gold/black logo heads and stainless screws with silver/maroon logo heads. On cabs, brass screws with pinstripe, steel screws with Vynair front. I don't suggest that there might not be an exception to this, but I haven't seen a deviation from this pattern. Yours would be very much the exception, not the rule.

Given RS OT, it's probable that your head did originally have Pekalit knobs. Shaft shape and size differed between RS and AB, and (real) Pekalit knobs would need to be drilled out to fit the later shafts. There's nothing in your specific photo above, however, that indicates your head originally came with Pekalit knobs.

Anyway, enough from me, good luck with your amp.



On Pekalit vs the "no-brand" knobs referred to elsewhere, there are clear differences in knob height, molding pattern, silver disc machining and grub screw size.

Pekalit knob internal molding:



"No Brand" knob internal molding:



Pekalit silver disc machining (circular):



"No Brand" knob disc machining (straight):



Comparison of height difference and grub screw size:

 

[guitarzan2525]

Are these mounting screws solid brass or gold coated to match the gold logo?

 

[London John]

Are these mounting screws solid brass or gold coated to match the gold logo?

Solid brass and solid stainless respectively.

 

[guitarzan2525]

This mounting screw looks a lot like the stock brass one. How common are these mounting screws. Would it be easy to find exact replacements if the above screw is not stock to the amp?

What are the dimensions?

 

[London John]

This mounting screw looks a lot like the stock brass one. How common are these mounting screws. Would it be easy to find exact replacements if the above screw is not stock to the amp?

What are the dimensions?

They're both Posidriv screws. If you remove the screw, the untarnished shaft will tell you what it's made from. Yours looks like it had some paint or coating of some sort, with residue around the edge, although it could be the photo.

The screw you show has been out/in, as can be seen from the scrapes in the patina where the screwdriver gripped the inside of the screw, which begs the question of why would somebody remove the logo screws?

Do you have photos of the logo mounting holes from both outside and inside the head shell?

Replacement screws are relatively easy to find, they're BA imperial size (don't use metric if you want the right screw pitch). I'd have to use calipers to tell you exactly the diameter.

 

[guitarzan2525]

They're both Posidriv screws. If you remove the screw, the untarnished shaft will tell you what it's made from. Yours looks like it had some paint or coating of some sort, with residue around the edge, although it could be the photo.

The screw you show has been out/in, as can be seen from the scrapes in the patina where the screwdriver gripped the inside of the screw, which begs the question of why would somebody remove the logo screws?

Do you have photos of the logo mounting holes from both outside and inside the head shell?

Replacement screws are relatively easy to find, they're BA imperial size (don't use metric if you want the right screw pitch). I'd have to use calipers to tell you exactly the diameter.

Here are some pics of the mounting screw..

 

[guitarzan2525]

As you can see these mounting screws appear to be stainless steel so maybe stock to the amp. This amp is transitional as it does have the yellow/gold lamp instead of the red lamp that is common for the earlier block logo amps. London John, do your brass mounting screws look the same?