JTM50 Tweaks

[Littlewyan]

My JTM50 lacks balls compared to the originals so I took it out of it's case tonight and took some measurements. Made a few interesting discoveries......

Pots
Volume 1 - 960K
Volume 2 - 960K
Treble - 260K
Middle - 26.4K
Bass - 933K
Presence - 4.19K

As the presence pot only measures 4.19k I won't have as much NFB as I should. Its not much but can still make a difference. More NFB would bring the bass down a tad whilst letting the highs through still.

I also have a PPIMV installed but I may remove it as I wonder if it changes the tone even when its turned all the way up. I never use it as it sounds terrible in an amp that relies on a lot of NFB!

So I'm thinking, remove the PPIMV and maybe replace the Volume and Presence pots? Also may change the tonestack capacitor from 250pf to 270pf.

Thoughts?

[neikeel]

I typically change V1 electrolytic to 10uF or less.

270pF is a value I have used and it works fine - not sure if there is much difference vs 250pF.

I quite like 27k NFB on the 16 ohm tap (i.e. stock values).

The secret on this is your settings.

Plug into high input channel 1 (I use 100pF tubular ceramic on the pot) don't jumper.
Volume 1 on 8 (unused vol 2 on 4)
Bass 0-2
Middle 10
Treble 8-10
Presence 6-8

Decent 4x12 and you are away.

Give the amp some time. In a circuit like this I have found the OT is very important and I have used the Marstran -139 and -128 (even more bass!!) to good effect. I built one with a cheaper OT I had lying around and it was awful, IMO. The B+ should be around 420v with that sort of voltage on the output plates. Only use 50uF mains filtering, 32PI, 16 screens and 32/32 preamp.

[Littlewyan]

I want to stick to stock values if I can as I know the stock values work well in other amps I've listened to. Plus its more of a challenge fine tuning then .

The settings I currently use are:

Volume 1 on 10 (unused vol 2 on 0)
Bass 0
Middle 10
Treble 8
Presence 10

I'm using a Classictone OT at the moment (40-18025). Have you ever used them?

I may move the NFB over to the 16Ohm tap, first though I'm going to see if I can get hold of a 5K pot that reads 5K or over. I did some calculations and as my pot reads 4.2K it will lower the NFB a bit and also if you think about it the presence cap won't be as effective. Same effect going from 270K to 470K .

I'm removing the PPIMV as well just in case that is affecting it somehow. I never use it anyway.

[Littlewyan]

5K Presence Pot is fine, when I took it out of the circuit it measured 4.9K which is close enough. The new one I bought is 5.1K so there isn't much point swapping them.

I removed the PPIMV and now the amp seems to sound better. I played it through my PC the other night (Load Box, Line Out, Focusrite, Virtual Cab Software) and it sounded good. Played it tonight through a real cab and it sounds much better. Maybe the PPIMV was killing the sound somehow?

I am tempted to try the 16Ohm Tap for the NFB. What I may do one night is put a resistor in parallel with the 27K to bring it down to 18K (same as 27K on the 16Ohm tap). I still have to keep the bass on 0 but I reckon that with the extra NFB I could probably turn the bass up or even jump the channels.