JTM45 rectifier question

[Roe]

No, I meant CLC, assuming that the choke's DC resistance (e.g. 110 ohms) often compensates for the lack of a tube rectifier (with resistance of roughtly 60 ohms)

[Tony Bones]

Ok, so we're learning some things here. How about going to a CRC filter for the reservoir? I think a 32u-250Ω-32u might do it, and if that's not enough, the resistor could be increased to 500Ω at the expense of a bit more plate voltage. That resistor should be a 10W, BTW, and you could parallel a couple of 1k 5W to try that out. You have two two-section caps available, so a 32-32 with the resistor across the positive terminals serves as the reservoir and the other one (16-16) will serve the screens and the PI. The resistor will shield the rectifier from the second reservoir cap, but I would move the standby switch after the reservoir. The resistor also shields the plates from the first cap, and so this solution might minimize the sound and feel impact.

I don't understand the motivation to replace a 110 ohm choke with a 250 ohm resistor.

[SPeter]

I don't understand the motivation to replace a 110 ohm choke with a 250 ohm resistor.

I indicated that I want to get away with one choke for packaging reasons if possible. There is really no good way to place the second choke on the chassis without having to lift the whole circuit board.



@Martin, I actually got a second 32/32 so now I have two 32/32's and the 16/16. Also I have the JJ 16/16 on order which Sluckey recommended (higher voltage rating).
I also ordered a 1000 pack of 1/2 watt resistors LOL, if I understand your suggestion I need a 10W 250 Ohm or 500 ohm. I only have 5W 470 ohm which go over the power tubes No 250 ohms in that high rating. Edit: nevermind I reread your post and I can even parallel two 470R 5Watters for 250 10W... perfect.

Could you mark up one of the older sketches just to make sure I understand what you recommend here correctly.

[martin manning]

Like this, and yes paralleling two 470’s will be fine.

[SPeter]

Like this, and yes paralleling two 470’s will be fine.

Thanks!

The OT (plates) is the one we also called OT CT correct (the brown wire in my amp)?
What should be the rating of the 8k2 resistor between the two 16uF capacitor sides?
What is the reason you recommend moving the standby switch? Can I get away without moving it for the test only? I feel like it would make a wiring mess.

[martin manning]

/:^| ...

[martin manning]

Yes OT plates is the CT. The 8k2 is already in there I believe, on the board. Yes you can leave the standby where it is to test. The reason for moving it is that if the reservoir is allowed to charge slowly as the rectifier warms up, then the rectifier won’t experience a big surge in current when the standby is moved to play.

[SPeter]

/:^| ...

Sorry man, you are talking to a (not very well) trained monkey LOL I'm a metallurgist by profession and the most complicated guitar electronics stuff I've ever done before is a rewiring of an epiphone LP.
I truly appreciate everybody's efforts to help me solve this thing.

[martin manning]

Not st all; that frowny face is there because I double posted!

[SPeter]

So this is how I read your sketch (without moving the standby switch):

CRC filter idea.JPG

I need to get this right I don't want to burn down anything LOL

[martin manning]

Yes, correct.

[SPeter]

Done. It worked! No ghost notes everything dimed and neck pickup!!! I added two 470R 5W in parallel between the 32 caps (so a total of 235R resistance).
I measured the voltages and it is as follows:

CRC filter idea voltages.JPG

So the B+ went up from 465V to 480V at the standby switch. I'm not sure why. But it dropped a lot on the resistors as expected. It used to be 462V IIRC after the filter now it is 447V and 440 after the choke so the FT cap is safe for now LOL

Measured the preamp pins too:
V1
Pin1: 200 (used to be 216. metro spec 180)
Pin6: 200 (used to be 217. Metro spec 180)
V2
Pin1: 173 (used to be 173. metro spec 155)
Pin6: 310 (used to be 324, metro spec 280)
V3
Pin1: 231 (used o be 240, metro spec 205)
Pin6: 222 (used to be 234, metro spec 195)

I played a few riffs before I had to shut it down and it sounded great. More testing is needed but first impression: it's a keeper. I just need to figure out a nice lead dress and a positioning for the 10W resistor I need to order.

Thanks Martin!

[martin manning]

Great! Props to Tony for suggesting the CLC ahead of the plate node, which uncovered the root cause. When you tried adding more capacitance to the reservoir and didn't see any improvement I thought the problem must be elsewhere. There is no reason not to just keep the 2x 470Ω you have there. They will run quite cool (2.3W each at idle per your measured voltages), and a 10W will be about 2x the length.

[SPeter]

Now, regarding to the standby switch. I learned form the posts in this thread that the rectifier only sees the first 32 cap as the resistor decouples the second one the same way the choke used to do. So I would think no need to worry about overloading the rectifier tube... what am I missing?

[martin manning]

When you throw the standby switch and connect the already warmed up rectifier to the first cap it will see a large current spike, maybe 4-5 amps, because the cap looks like a short to ground. You can leave it as it is, but it's not considered good practice. See here: http://www.valvewizard.co.uk/standby.html