JTM45 filtering confusion...

[Mynameisfritz]

Hi guys,
I know this is really ridiculous, but can somebody enlighten me about the proper filtering and filtering options in a JTM45 please? I have this 1966 JTM45 sitting here for quite a while now, got a new PT from Brian Wallace and found some good old NOS tubes, but still I’m not able to fix it. I read a lot about filtering too, but to be honest, I’m still getting confused each time I’m trying to understand it properly.

Let’s start with the basics. Looking at the 1966 JTM45 schematic from Brian, there are two 16uf caps in the preamp section and two 32uf caps for the power amp section. Not too hard to understand… First I’d like to know how these caps are called. I’m going from left to right in the schematic now:



First 16uf cap – left to/after the 10k1W resistor, connected to V2b: Is this the preamp filter cap located on the board?

Second 16uf filter cap – right to/before the 10k1W resistor: ??? is this the screens/PI cap? Is this the cap located on top of the chassis? I have a single TAD 32uf cap sitting on the chassis. In George’s instructions there’s a dual cap mounted on the chassis. Why this dual cap? Because of the 16uf/32uf option?

First 32uf cap after/left to the choke, before the 10k1W resistor: How do you call that one? This ain’t no joke…

Second 32uf cap before/left to the choke, after the standby switch: ???

Both 32uf caps are part of the dual 32uf cap, mounted to the block end inside the chassis, right? But I have a dual TAD 16uf/32uf cap there…

Now I feel like 14 years old and back to school again. Aaahhh...

[neikeel]

The first 16uF to the left of the 10k is the preamp filter, the next one that is fed from the 16uF half of the internal 16/32 is the PI filter. The other half of that is the 32uF on the screens (before the 8k2 on the board but after the choke) . The topside cap is usually just a single 32uF of 40uF and is the mains filter (ie after the switch but before the choke). Simples

If you want to leave it there for a vintage spec JTM45 stop reading now! If you want more options:

Now if you have a dual 32uF 500 or 450v cap up top you can use half of this for mains, half for the PI and if you have a dual 16 inside you can either use half of it for 16uF in the PI or both halves for 32uF in the PI and the single 16uF on the board for the preamp. I believe that is what Georges kits used.

btw The JTM45 RI used 50uF throughout (too tight for vintage feeling 45, IMO). I blue printed my RI with dual 16 can for pre and PI and dual 32 for mains, screens - worked well.

Next complication. Some people swap the PI and screens around for 16 on screens (like a 12 series plexi) with 32 in the PI and 32 mains. Generally avoid more than 64uF after a GZ34 as it stresses them to much. 48uF is tightest to go with that IMO and that can be achieved with 32/16 tied together on top can and use another 16/32 inside - you can decide which one you prefer as higher screens or PI and single 16 on board.

PS that is not one of Brian's PTs for a JTM45:shock: