Who's built a JTM45/100?

Guest · Post by **Guest** » Sun Feb 05, 2006 3:15 am

What value are you looking for?

John

Flames1950 · Post by **Flames1950** » Sun Feb 05, 2006 7:25 am

5.6K....you have the hookup on 'em?

neikeel · Post by **neikeel** » Sun Feb 05, 2006 7:51 am

I've gone for welwyns (wire wound) for reliability and longevity.

Did not really think that there was a tone issue here?

Flames1950 · Post by **Flames1950** » Sun Feb 05, 2006 7:53 am

I'm referring to the input grid resistor, not the screen grid resistor........

Guest · Post by **Guest** » Sun Feb 05, 2006 12:09 pm

Yeah, I have the nos red metal film in that value. But those aren't the right value for your amp, you should only have a 1.5k between the tubes(inner and outer). PI direct to inner tube and then through resistor to outer tube. I have both values e-mail me and I'll hook you up.

John

korina82 · Post by **korina82** » Sun Feb 05, 2006 12:14 pm

neikeel · Post by **neikeel** » Mon Feb 06, 2006 10:56 am

Oops

I thought the screen grid was 5k6.

I should read more carefully. It is my first 100watter my two other amps obviously did not have the input grid resistors slung between the pairs of valves

It is the input grid resistors (x2) 1k5 (which are missing in my kit by the way!). Did not realise these were vulnerable to pack up. How important is the choice of type of resistor in this position?

korina82 · Post by **korina82** » Mon Feb 06, 2006 10:59 pm

5.6k swamp resistors are standard on all 100's except the JTM 45/100's. I've never had one of these go bad...the 470 ohm or 1k 5w screen grid resistors go when a tube shorts.

John

neikeel · Post by **neikeel** » Tue Feb 07, 2006 4:45 am

I knew I should not post on these things without a schematic in front of me

.
At the risk of diverting Bashcoders thread (it's still relevant to his build.....)

I see the option of 470R or 5k resistors for these as screen grid resistors to help with catastrophic valve failure and as such are high rating wire wound at 5Wwhich value is correct/better with KT66s?

I also see 5k6 or 1k5 linking the pairs of output valves. I see these referred to as either swamp resistors and/or input grid resistor. Again what is the correct value and more importantly the rationale behind the different values? Not forgetting the rating 1W or 2W, carbon comp. or carbon film?

novosibir · Post by **novosibir** » Tue Feb 07, 2006 9:45 am

On the screen grids:

6L6GC, KT66, 5881 = 470/2W wirewound (best is Welwyn)

EL34, E34L = 1.0K/5W wirewound

On the control grids:

6L6GC, KT66, 5881 = 1.5K...2.2K

EL34, E34L = 5.6K...22K

No matter, whether they are CC's, CF's or metalfilm - 0.5W is enough.

They should be there, to prevent the tube agains RF oscillation! The resistor together with the input capacitance of the tube is creating a high-pass-filter, what prevents the tube.

Clear enough?

Larry

neikeel · Post by **neikeel** » Tue Feb 07, 2006 1:06 pm

novosibir wrote:On the screen grids:

6L6GC, KT66, 5881 = 470/2W wirewound (best is Welwyn)

EL34, E34L = 1.0K/5W wirewound

On the control grids:

6L6GC, KT66, 5881 = 1.5K...2.2K

EL34, E34L = 5.6K...22K

No matter, whether they are CC's, CF's or metalfilm - 0.5W is enough.

They should be there, to prevent the tube agains RF oscillation! The resistor together with the input capacitance of the tube is creating a high-pass-filter, what prevents the tube.

Clear enough?

Larry

white room · Post by **white room** » Tue Feb 07, 2006 4:59 pm

novosibir wrote:On the screen grids:

6L6GC, KT66, 5881 = 470/2W wirewound (best is Welwyn)

EL34, E34L = 1.0K/5W wirewound

On the control grids:

6L6GC, KT66, 5881 = 1.5K...2.2K

EL34, E34L = 5.6K...22K

No matter, whether they are CC's, CF's or metalfilm - 0.5W is enough.

They should be there, to prevent the tube agains RF oscillation! The resistor together with the input capacitance of the tube is creating a high-pass-filter, what prevents the tube.

Clear enough?

Larry

So, when I switched from EL34s to KT66s, obviously I should have changed the value of these resistors.

Does the 1k5W and 5.6k adversley effect the operation of the KT66 tubes in any way? Does it effect tone? Still learning here.

Billy Batz · Post by **Billy Batz** » Tue Feb 07, 2006 5:01 pm

No. You cant go the other way. You cant use EL34s without first subbing in a 1k Sg.

novosibir · Post by **novosibir** » Tue Feb 07, 2006 5:09 pm

white room wrote:Does the 1k5W and 5.6k adversley effect the operation of the KT66 tubes in any way? Does it effect tone? Still learning here.

You won't hear any difference between the 1.5K and the 5.6K grid stopper with KT66, because the cutoff frequenzy of both is far out of the hearable spectrum - even low intermodulations aren't present in the audio range.

But with a 470 ohms screen grid resistor on a KT66 (of course one on each tube

) you'd get a better touch response in comparision with the 1K

Larry

white room · Post by **white room** » Tue Feb 07, 2006 5:13 pm

novosibir wrote:
white room wrote:Does the 1k5W and 5.6k adversley effect the operation of the KT66 tubes in any way? Does it effect tone? Still learning here.
You won't hear any difference between the 1.5K and the 5.6K grid stopper with KT66, because the cutoff frequenzy of both is far out of the hearable spectrum - even low intermodulations aren't present in the audio range.

But with a 470 ohms screen grid resistor on a KT66 (of course one on each tube ) you'd get a better touch response in comparision with the 1K

Larry

Makes sense to me.

Thanks