'68 Bass Specs vs '68 Lead Specs vs '70's & Up Lead specs

[neikeel]

Did you forget the short wire link on V1 socket pin 3 to 8?

[neikeel]

The shared cathode wire that you snipped off when you went bass to lead?

[Webby14]

Hi Neil,

Sorry about the delay in replying, but this thread has got so long, that I didn't realise your replies were on page two!

You are a star. I, on the other hand, am an idiot!!

I got so wrapped up in the differences between the two circuit boards (JMP 50 Bass Specs & JTM 50), that I completely forgot about wiring differences elsewhere in the chassis!

Yes, I did miss the link! In my defence, it was 8 months ago, so I had completely forgotten about that!

Before I do something else idiotic, and short out the National Grid (Lol!), is there anything else that I may have removed, and forgotten to replace, or anything extra I need to add (Valve Rectifier excepted!), to complete the transition from JMP 50 '68 Lead Specs to JTM 50 Specs?

Regards

Steve

[neikeel]

Hi Neil,

Sorry about the delay in replying, but this thread has got so long, that I didn't realise your replies were on page two!

You are a star. I, on the other hand, am an idiot!!

I got so wrapped up in the differences between the two circuit boards (JMP 50 Bass Specs & JTM 50), that I completely forgot about wiring differences elsewhere in the chassis!

Yes, I did miss the link! In my defence, it was 8 months ago, so I had completely forgotten about that!

Before I do something else idiotic, and short out the National Grid (Lol!), is there anything else that I may have removed, and forgotten to replace, or anything extra I need to add (Valve Rectifier excepted!), to complete the transition from JMP 50 '68 Lead Specs to JTM 50 Specs?

Post a board pic
Regards

Steve

[Webby14]

Hi Neil,

I replaced the link, and 'went for it anyway'. Everything working O.K!

First impressions: Sounds a little on the dull side tonewise with the LarMar down low, but gets brighter as you wind it up higher. I think I read on this forum somewhere, that the presence doesn't do a lot at low volume levels, as the LarMar sort of nullifies the effect of the NFB supply when below a certain level?

Anyway, I will wind it up to 'gig level' at rehearsal next week, and decide whether or not it is bright enough for my tastes. I can always swap the 27K NFB Resistor for 47K, or even 100K if necessary.

Thanks again!

Steve

[Webby14]

Hi Neil,

****Update!****

Realised I still had NFB Wire connected to 8 ohm tap, re-connected to 16 ohm tap. That tightened/brightened things up more to my liking. Wound LarMar MV up at work while boss was away.....

Wow! Sounds amazing! There is so much more Bass available than the '68 JMP design, that I actually have to roll it right back. Always had Bass flat out on the JMP! The other tone controls also seem to be more usable as well.

I did notice something while looking at differences in the layouts of the '68 Plexi/JTM 45/JTM 50. The JTM 45 & the JTM 50 do not seem to have the 5.6K Output Tube Grid resistors like the Metroamp JMP 50 layout has.

I have left mine connected, is that O.K?

Regards

Steve

[Webby14]

Hi Neil,

As well as the 5.6K Output Tube Grid resistors, I have also realised another difference between the JMP 50 and the JTM 50.
My JMP 50 has 2 x Dual 50uf Radial Caps mounted on the top of the chassis, whereas the JTM 50 would appear to use 2 x Dual 32uf Radials?

Is it O.K. keep the 5.6K Grid Resistors, and the Dual 50uf's?

Regards

Steve

[neikeel]

The resistors make no tonal difference and might help stop oscillation.

The filter caps will make it a little tighter in the bass which will be no bad thing.

I do prefer 32/32 in the preamp on one of these but would not rip out the 50/50 if that is what you have - the difference is very subtle.

How are you finding the amp? Do to turn your PPIMV lower than 75% - it will not sound very good!

[Webby14]

Hi Neil,

Thanks for your reply.

I thought that might be the case with the 5.6K Resistors (I'm all for stopping any unwanted oscillations!). I prefer a tight bass end as well, so I will probably keep the Dual 50's in.

Actually, it sounded pretty fantastic at work the other day, with the PPIMV on about 30%, and the Bright Channel on around 6 to 8. Very similar to Angus Young's tone ('Let there be Rock/Powerage' era), as long as I dial the Bass right out. I love the tone he gets, 'A nice amount of Gain, but with a bit of 'Chime' as well, is how I would best describe it.

I'll be rehearsing this Wednesday, where I usually set the PPIMV at 45 to 50% so I will let you know how I get on.

Just wondering why you would feel that the sound wouldn't be very good?

Steve

[neikeel]

Too quiet and your speakers are not being pushed, the presence not effective etc.

I find it can get a bit fuzzy too, but you might find a sweet spot?

[Webby14]

Hi Neil,

Too quiet?!! Where do you play? Madison Square Gardens!! (Lol!!).

With my PPIMV on 30%, it's pretty loud, and not at all fizzy (to my ears anyway). 50% and you would have to shout to be heard. I agree that the Presence doesn't do much, but it sounds about right to me anyway. Maybe I have found my sweet spot?

You must like it pretty loud. Lol!

As I say, I will let you know how I get on at Rehearsal on Wednesday.

Regards

Steve

[neikeel]

Hi Neil,
Too quiet?!! Where do you play? Madison Square Gardens!! (Lol!!).
With my PPIMV on 30%, it's pretty loud, and not at all fizzy (to my ears anyway). 50% and you would have to shout to be heard. I agree that the Presence doesn't do much, but it sounds about right to me anyway. Maybe I have found my sweet spot?
You must like it pretty loud. Lol!
As I say, I will let you know how I get on at Rehearsal on Wednesday.
Regards
Steve

I wish! I use an attenuator on -9db typically (no PPIMV) none of my amps have a PPIMV at the moment but will probably drop one in next JTM45 build as they only need knocking back a little.

[Webby14]

Hi Neil,

Oh I see!

Yeah, I have an Airbrake Clone, but I can't bring myself to use it (or any other attenuator actually!). I always compare running an amp flat out all the time, to driving a Ferrari with your foot hard to the floor all the time. It's bound to blow up before too long, it's inevitable!

Probably irrational, but there you go, that's just me!

Steve

[neikeel]

Not irrational, I had same feelings but the improvement in tone was worth it IMO

Try the airbrake on one click down (-4dB ??) and the PPIMV on 2/3 of max - again find the sweet spot where your power valves are working and the PI is being driven too.

[Webby14]

Hi Neil,

The amp sounded really great at rehearsal last night. PPIMV on 4, Bright Volume on 7 to 8, Presence on 10, Bass on 0, Middle on 10, Treble on 10. Not fuzzy at all. I guess that I have found my sweet spot.

If I was being super, super picky, I would still like a tiny bit more sparkle on the top end. But like you said, the Presence control is virtually ineffective at low PPIMV levels, so maybe I will go the attenuator route, or failing that, I could experiment with a 47uf or 100uf NFB Resistor.

As you can see, the Bass was set to 0. Where does all the extra Bass come from?!! I would never need more with a Les Paul, but I can see how having all that Bass available, would probably benefit a Strat player using standard single coils.

Bye for now!

Steve