JTM 45/100 build

[David B]

With the holidays upon us and some time off from work I have begun my first JTM 45/100 build using Marstran iron & chassis, had some questions from those experienced with this model.

First is the choice of screen grid resistors, I'll likely go with 1k 5 watt resistors as on most original amps though there are some with 470r, was there a reason for that or just random or aftermarket replacement?

Also originals use one 1.5k control grid stoppers for each pair of output tubes, is this usually sufficient reducing oscillations or better to use one resistor per tube?

Thanks & Happy Holidays

David

[neikeel]

I used 470R 3watt welwyns on my first 45/100. They worked well and I was pretty sure my GECs could handle the screen current.

5w are quite big and 7w too big. I know for sure that the JTM45 twin pots came with 470R originally.

For my Hendrix clone I have a choice of 3w grey RS ones or the ceramic 5w ones (as per that amps present incarnation) again at 470R - more GECs

The 470R is supposed to sound a little more open, I have not tried a swap with same specs. If you are using new tubes or think you might use EL34s then I guess 1k make sense. I suspect that if you did a back to back you would probably not notice a difference.

With a JTM45/100 the single 1k5 per side is fine even when front end heavily goosed (A tweaked high voltage early 70s Metal panel is different!)

[David B]

Hi Neil,

What secondary voltage tap is your PT using 490 or 560?

I only have a small stash of GEC so for now it will be new production KT66 hence the 1K 5 watt cement resistors, actually I like the idea of the 3 watters, some fusing protection for the OT...I'll try the 470r after the amp is up and running correctly..

Yeah those 7 watt Welwyns are big suckers & if a tube goes south I don't think the thing will open?

Good on the 1k5, gotta find some with long leads ..

Many thanks for the advice..

David

[dai h.]

hey David this is a bit of info I have on the asymmetrical grid stopper wiring (I don't think this is on the web any longer) :

[Weber Kits Q&A] [FAQ]
2328
Date: April 28, 2005 at 15:40:28
From: ToneVision
Subject: Ted: error in 5F8 layout?

Ted, I think there's a little error in the 5F8 (hi-power twin) layout.

Look at the power tube 1K5 gridstopper wiring. The schematic is OK but the layout shows wrong wiring. It is kind of obvious but I might be worth changing while less experienced might not trap this error themselves?


Responses:
[2329] [2331] [2332] [2334] [2333] [2337]

2329
Date: April 28, 2005 at 16:30:21
From: earl
Subject: Re: Ted: error in 5F8 layout?

right, shouldn't the differential signal go through the 2 1.5Ks and then couple to the pin 5's of the tubes? The layout shows direct couple to one pin5 and then 1.5K over to the other pin 5 for each pair?
Earl

Responses:
[2331] [2332] [2334] [2333] [2337]

2331
Date: April 28, 2005 at 16:35:04
From: Earl
Subject: Re: Ted: error in 5F8 layout?

Although the original Fender layout shows it the same way as the weber drawing. what I'm I missing?

Responses:
[2332] [2334] [2333] [2337]

2332
Date: April 28, 2005 at 16:46:49
From: Bruce /Mission Amps
Subject: Re: Ted: error in 5F8 layout?

I don't see anything wrong.
Without going into too much detail, the "grid stoppers" are there to control high freq ultrasonic oscillations.
It's not totally uncommon to see only one on each pair in a four power tube amp.

I'll look them over better with a little time but I don't think you missed anything.

Bruce

Responses:
[2334] [2333] [2337]

2334
Date: April 28, 2005 at 20:18:12
From: ted
Subject: bingo...

spoken like a true ham.

For everyone else...

The input capacitance to the tube is low enough that the 1.5K in series isn't of any consequence as far as rolling off the frequency response.
The original term for that resistor was 'losser resistor', or 'grid losser'. The purpose was to lower the Q (thus 'loss' of Q) of the drive circuitry by introducing resistance and that would dampen oscillations, just like the oil or gas filled shock absorbers on your car.

So... having them on one side, or being asymetrical is no big deal.

Had I made them symetrical, someone would have said "hey, you're not being true to the original! " ((-;.. just kidding..

Ted

Responses:
None

2333
Date: April 28, 2005 at 16:53:03
From: CSB
Subject: Re: Ted: error in 5F8 layout?

Nothing really looks "wrong" as far as how the circuit will perform in most situations, but he is correct that the layout does not match the schematic.

CSB

Responses:
[2337]

2337
Date: April 28, 2005 at 23:49:10
From: Bruce /Mission Amps
Subject: Re: Ted: error in 5F8 layout?

... but he is correct that the layout does not match the schematic.
******************************
Why are you guys beating around the bush?
Where is he correct that it doesn't match 'cause I can't find it.

Bruce


an old thread on AGA newsgroup:

https://groups.google.com/forum/?hl#!to ... R4RAHU2YIA

http://www.univox.org/pics/schematics/u1061.gif

(To be honest I don't quite get it 100%. (Not clear, but I guess the grid stopper combined w/C of the tube's electrodes shunts a slight bit of high freqs to AC ground? "Damping" is a bit of levelling out of a peak, so I suppose if there is any peaking in the high freqs, and that is damped, that should help decrease risk of oscillation by decreasing the peak amplitude a bit and radiation (plus capacitive coupling nearby also?) Also re: the 100 ohms series Rs on the Univox output: I've done that with a 5532 op amp (to successfully quell oscillation) but not quite sure what I did also (series R increases the Z the ouput works into and there is a bit of loss and damping from the output stage not having to work as hard (as hard as with the previously lower Z?)?

[neikeel]

Hi Neil,

What secondary voltage tap is your PT using 490 or 560?

I only have a small stash of GEC so for now it will be new production KT66 hence the 1K 5 watt cement resistors, actually I like the idea of the 3 watters, some fusing protection for the OT...I'll try the 470r after the amp is up and running correctly..

David

My Metro was an early one and ran at 530v on UK tap (240vac). Not decided with the JH amp but probably go high as I like the whump that it gives to these for cleaner tones with a strat and rely on a fuzz for the gain tones.

Interesting to read the thread about grid stoppers. Not sure how they can be shunting signal to ground, surely just slightly increasing the impedence on the grids to provide the 'damping 'mentioned as of course this is a push-pull system of each pair of outputs 180 degrees out of phase. I use 4k7 or 5k6 on most builds for each socket.
I need to think (well go and have a quick read up )

[David B]

Hi Neil,

Yeah good reading, Dai thanks for digging that up, good stuff..this is interesting.

The input capacitance to the tube is low enough that the 1.5K in series isn't of any consequence as far as rolling off the frequency response.
The original term for that resistor was 'losser resistor', or 'grid losser'. The purpose was to lower the Q (thus 'loss' of Q) of the drive circuitry by introducing resistance and that would dampen oscillations, just like the oil or gas filled shock absorbers on your car.

So... having them on one side, or being asymetrical is no big deal.


Ah 530 so a bit more forgiving, I'll be running 490 for now maybe wait to see how your JH likes the 560 think I'll try those JH tone stack values as well but leave the filtering at 16uf for now..

On my personal 45 & 50 watters I have gotten away with no grid stoppers but it might be wise to slap a 1.5k on them...

Time for some eggnog spiked with something good

David

[dai h.]

hey Neil check this out (originally from Aiken Amps' Tech Pages which don't seem to be available at present) :

http://web.archive.org/web/201308271829 ... chInfo.htm

Merry Christmas & Happy Holidays Everyone!

[neikeel]

Thanks, the reasons for use I knew (PO and blocking distortion) but the behaviour of the resistor in conjunction with the capacitance of the grid and internals of the tube make sense to create the low pass filter ( ie more than just a high impedence node) makes sense.

[Stu]

I used these in mine, similar size to the old RS ones, but a bit skinnier.: http://uk.rs-online.com/web/p/through-h ... s/1996564/

[neikeel]

I used these in mine, similar size to the old RS ones, but a bit skinnier.: http://uk.rs-online.com/web/p/through-h ... s/1996564/

Do you think screen resistors at 6w might not act as a high value fuse like 3w should a valve go? Or should we not kid ourselves that this is not a useful function?

[Stu]

Probably not. I've always used 5W or better - I hadn't come across the "screen resistor as fuse" idea until recently. I'm not sure I like the idea of a resistor burning up inside the amp, and depending on how long it takes to fail, would it realy offer any protection to the transformer?

[David B]

I had one burn up in a Deluxe Reverb due to a faulty Stovetek 6V6 shorting out, I caught it early so nothing other then the tube & resistor were damaged. Now a have a good supply of RCA & GE...
Unfortunately the Deluxe has no HT fuse but if it had as in a Marshall I would think it would have blown before the resistor smoked?

These Dale ww look like a good choice as well..

6.5 watt
http://www.mouser.com/ProductDetail/Vis ... x2SjI4s%3d

5 watt
http://www.mouser.com/ProductDetail/Vis ... p7ehyys%3d

[neikeel]

Probably not. I've always used 5W or better - I hadn't come across the "screen resistor as fuse" idea until recently. I'm not sure I like the idea of a resistor burning up inside the amp, and depending on how long it takes to fail, would it realy offer any protection to the transformer?

My old Leak Stereo 20 had one, it was a 3w 100R that was supposed to be fitted in specific lugs (not wrapped around the turrets) so that if it was subjected to overheating it would desolder and protect the mains transformer, akthough I can remember leaving mine on for days by mistake with no such occurrence!

I ran a guitar through it (pair of Wharfedale RS12DD speakers!) it had yellow label Mullards throughout!

[David B]

I believe SVT's use a fused screen scenario,one tube fails it shuts down keeping the rest running to get through the gig...Did the Leak have an HT fuse?

[David B]

Schematic for the SVT, has 22-ohm/1watt screen resistors & 10 ohm / 5watt plate resistors...
Question is in a typical Marshall circuit will a 3watt screen resistor shut down the tube faster than the HT fuse?

http://www.webphix.com/schematic%20heav ... p6146b.pdf