Grid screen resistors questions, JTM45.

[passerby3141]

Hello all, I've got a JTM45 RI which I bought used. Some previous owner did some mods to it and I just realized it has 2K2 5w grid resistors on the power tubes. I am not even sure what exactly these do, but from my own reading it seems this amp should have 470 5w resistors in it. Are these a problem? Should I switch them out? And does anyone know why someone, or even possibly Marshall, would use such a large value here?

Thanks for reading.

[Roe]

1k or more can be an advantage with el34s. do you use the flying resistor?

[passerby3141]

Hey, thanks for the reply. I was getting worried that my question was even too "newby" for the beginners forum. I have done some reading on the grid resistors and it seems it has more to do with protecting the tubes when the amp is dimed, and the OT if the tubes burn out, but electricity is a bit beyond me still.

If you mean the swamp resistors, no, I am not currently using any.

I am thinking of replacing these with the "correct" value, the ones in there now seem like they may be on the way out, the amp is nearly 20 years old. I plan on running NOS 6l6's as they are cheaper than NOS KT66's, but still hundreds of times better than anything current production. Really can't afford the NOS KT66's, I mean I could, but I'd be eating ramen for a month or two, the wife wouldn't be happy about that! Been on a bit of spending spree lately as it is.

[Roe]

original would use 470 ohm plus a 1k flying resistor. that would be like using a 2.5k without the flying resistor

[thousandshirts]

Unless it sounds bad (super stiff or 'non-organic') or the voltages on the screens are wonky, I'd leave them as is. If it is a bit too stiff, or the voltages are way low, I'd leave the 470 ohm resistors for some other amp, and just install two 1K resistors. As for the flying resistor it certainly works (and you use less wire, too), but it isn't essential. The flying resistor is a particularly nice touch in vintage recreations and restorations but its purpose can be achieved, as Roe points out, in other ways.

[passerby3141]

Thanks guys.

[Structo]

I'm kind of new to Marshall amps, what do you mean by flying resistor?

Are you talking about a grid resistor that is soldered to the tube pin?

[thousandshirts]

The B+ for the screens on a JTM 45 actually comes off the B+ line on the preamp board (in the traditional layout) so instead of running a wire from that preamp board turret to the tube socket area, the tube sockets happen to be quite close, and instead you can bridge the distance with a resistor instead. This is one way to do it, anyways.

In the following picture, note how that extra carbon comp is headed from the board down to pin 6 of V4. And though it's hard to see, a yellow wire then heads off to pin 6 of V5. From each pin 6 to pin 4 (of V4 and V5), we can also see that individual screen grid resistors are installed. Thanks to MWM for the excellent example:

[Roe]

that's a good pic of the flying resistor

[passerby3141]

Oooooh, I hadn't read about the flying resistor before. Thanks for the info. My amp does have a bit of a stiff feel to it. I think I may have to check out my grid resistors. One of the solder connections looks like it may have taken too much heat, it's a bit discolored. My voltages seem quite stable though, just had some Philips 6l6GC tubes bite the dust in the amp. They were in there when I got it, so who knows how long they had been in use. Nothing spectacular, just a slow loss of power, the amp got quieter and quieter. I just replaced them with some other NOS, and am just checking everything to protect my investment. Would the 1k alone be enough for 6l6? I don't think I will attempt the flying resistor, my soldering is not even close to pics I see around here. Does the screen grid resistor have a large effect on the feel of the amp. I may do some mods to the circuit at some point, but for now I want to get it as close to original spec as possible and live with it for a while before I go all out on another modding binge! 1k vs 1.5k vs 2.2k a big difference?

Also I noticed in the store there are no 1.5k 5w or higher resistors to be had. If I want the value of 1.5k to check out this arrangement would using a wirewound for the flying resistor be "kosher" (if I decide to bite the bullet)? Where else could I look, or do you think Metro would have some if I emailed a request?

Thanks again.

[Arclight]

The choice other than the 470ohm screen grid resistor would be a 1K ohm 5watt fireproof resistor. Which will work for EL34's.