value for bias splitter resistors JTM45/100

[Dom]

Hi to all JTM45/100 lovers and Metropolous friends,

Last week I received my package full with the chassis kit of the '66 JTM45/100 repro.
Trannies and sockets are mounted, boards are 95% finished so you see it goes forward.
I also ordered the Allen Bradley NOS CC resitor kit, and some reistors are a little bit out of tolerances.

Well, regarding bias resitors, the layout says 220k, but my 2 are in the
185k 190k range.

Any concerns?

Also the for the bias supply dropping resistor I installed one with 205k (150k in the layout)

Thanks for your replies.

Cheers

Dom

[neikeel]

I would use carbon films in that position for stability and to be within tolerances, same goes in the bias circuit.

I use carbon comps on inputs V1 cathode, V2 cathode, V1 and V2 plates and tone stack, whether all of these make any difference is a matter for debate but any drift here will contribute to individuality in your amp (good or bad) but in the bias circuit may cause some thing else to fail if they drift badly, although ABs are supposed to be good they are only silver band and are often at the extremes of the range (+/- 10%).

[Flames1950]

The ones I got from George for some work I'm doing for him were the same, in the low-180K range. I put in some of my stash instead, which measured around 230K (typically CC's read a little high.)

[Dom]

I contacted George and he me said every value above 150k is no problem.
So I let them in.
Tomorrow I will check the voltages, so we will see if they're in tolerances.

Cheers

Dominik

[dynaman]

Dang, if I caught a bunch of employees measuring resistors while assembling, I'd fire them. My tolerances are +/- 50%.

[novosibir]

I contacted George and he me said every value above 150k is no problem.

George is right!

Since you have the .1 PI caps to the bias splitter resistors, I'd even say, that any value above 100K isn't a problem. With a lower resistor value the power stage might be a tad tighter in the lows, but the low mids are slightly more pronounced (more 'ooomph') - and the amp isn't as much prone for blocking distortion, when dimed.

Larry (also from Bavaria)

[Dom]

Hi Larry, du fr

[Roe]

I contacted George and he me said every value above 150k is no problem.

George is right!

Since you have the .1 PI caps to the bias splitter resistors, I'd even say, that any value above 100K isn't a problem. With a lower resistor value the power stage might be a tad tighter in the lows, but the low mids are slightly more pronounced (more 'ooomph') - and the amp isn't as much prone for blocking distortion, when dimed.

Larry (also from Bavaria)

does .022uf pi coupling caps demand 150k or higher?

[novosibir]

does .022uf pi coupling caps demand 150k or higher?

A .022 to the bias splitter of 220K is already on the tight side, so it demands 220K or even a tad higher (+ tolerance).

Larry

[Bluesgeetar]

This raises a good question. What values on a amp build can you fudge on? how far north or south can you go on say 470k, 100k, 68k, 10k, 8.2k, 1m, 820r, 15k etc. etc.? What can be fudged and what can't?

[novosibir]

This raises a good question. What values on a amp build can you fudge on? how far north or south can you go on say 470k, 100k, 68k, 10k, 8.2k, 1m, 820r, 15k etc. etc.? What can be fudged and what can't?

There's no simple answer, the situation is too complex and you can't watch onto the grid leak resistors isolated from the rest of the circuitry.

Go lower with the value of the grid leak resistors, then you're cutting bass response due to another time constant with the previous coupling cap. And keep in mind, that the grid leak resistors AC-wise are in parallel to the PI's plate resistors - so reducing the value of the grid leak resistors is reducing the PI tube's gain.

Go higher with the value of the grid leak resistors, then you'll get blocking distortion, when you drive the power tubes into saturation - or even already before saturation, when you have output tubes with a bad vacuum.

This especially with 100W amps, where each two tubes are sharing only one grid leak resistor - where the grid leak current of two tubes has to flow through only one resistor.

After the data sheet of i.e. an EL34, the max. grid leak resistor's value is 700K - but only then, when NOT operating in saturation!

For two EL34's this means, that max is 350K (two 700K's in parallel). But you have to add to the grid leak resistor's value the value of the swamp resistors and the entire resistance through the bias supply (47K or 56K & 25K trim pot)!

So each two output tubes in a 100W Marshall with a grid leak resistor's value of 220K actually are seeing about 300K to (real) ground! 600K each tube - not far away from the max. 700K for clean, not saturated operating!

And you now see, that's impossible, to tell a range like 'from xK to yK', where within your amp wouldt work properly.

Larry

[5150loveeddie]

Larry you rock, great info here!!