Why would my screens be higher than my plates?

[Structo]

Thanks for the detailed answer.

It makes me think back to when I thought I had blown my OT.

But if I recall, my DC resistance measurements of the primary were way lower than those examples.

[frischmann]

O.k.
I think I'm going to have to read '67's post a few times to understand it..

I measured the DC resistance from the center tap to the other 4 wires on the primary side of the OT.

I did not unsolder anything,
The power tubes were pulled (didn't make a difference)
the amp was off.... Should it be?

From the CT to the
Plate (pin3) of V4= 324.6ohms
plate (pin 3) of V5= 306.5 ohms
screen pin 4 of V4=1.1K
screen pin 4 of V5=1.097K

Does this tell you anything?

[rhinson]

O.k.
I think I'm going to have to read '67's post a few times to understand it..

I measured the DC resistance from the center tap to the other 4 wires on the primary side of the OT.

I did not unsolder anything,
The power tubes were pulled (didn't make a difference)
the amp was off.... Should it be?

From the CT to the
Plate (pin3) of V4= 324.6ohms
plate (pin 3) of V5= 306.5 ohms
screen pin 4 of V4=1.1K
screen pin 4 of V5=1.097K

Does this tell you anything?

yes you have an output trannie that could give you higher screen than plate voltages under "normal" fender/dumble type power section conditions (ie--470 ohm- 1k screen resistor and lets say a fender twin type choke). that's quite a good size resistance on that winding---some old marshall's/hiwatt's etc may only measure 20-40ohms per side for example. and don't worry about the side-to-side mismatch---depending on how the trannie is wound/interleaved it'll take more wire ( hence more resistance) if keeping the turns the same on an outer layer----you'd need a few other specimens of this trannie at your disposal to check against and see if anything is amiss in that regard. rh

[Structo]

How do you have the primaries of the OT connected?

Do you have a switch on the two different primaries?

The solid blue and brown are 8K

The white striped blue and brown are the 4k winding.

The 8K is used for 6V6 and the 4K is for 6L6 use.

Pin 4 of the power tubes should be the screen resistor to the filter board or choke in your case.

I think Allen was just talking about the pin 3 to ground resistance.

[frischmann]

O.k.
I think I'm going to have to read '67's post a few times to understand it..

I measured the DC resistance from the center tap to the other 4 wires on the primary side of the OT.

I did not unsolder anything,
The power tubes were pulled (didn't make a difference)
the amp was off.... Should it be?

From the CT to the
Plate (pin3) of V4= 324.6ohms
plate (pin 3) of V5= 306.5 ohms
screen pin 4 of V4=1.1K
screen pin 4 of V5=1.097K

Does this tell you anything?

yes you have an output trannie that could give you higher screen than plate voltages under "normal" fender/dumble type power section conditions (ie--470 ohm- 1k screen resistor and lets say a fender twin type choke). that's quite a good size resistance on that winding---some old marshall's/hiwatt's etc may only measure 20-40ohms per side for example. and don't worry about the side-to-side mismatch---depending on how the trannie is wound/interleaved it'll take more wire ( hence more resistance) if keeping the turns the same on an outer layer----you'd need a few other specimens of this trannie at your disposal to check against and see if anything is amiss in that regard. rh

Does that mean it's O.K. to leave it like this or am I going to have to fix it?

[frischmann]

This is actually the original 22 kit OT.
So there are only the 8k primaries and I am running 6L6's.

I was seriously thinking of upgrading the OT to a bassman/super OT, the Dynaco 470 clone or..a twin OT (running an 8 ohm cab)

I wonder if that alone would help?

[rhinson]

This is actually the original 22 kit OT.
So there are only the 8k primaries and I am running 6L6's.

I was seriously thinking of upgrading the OT to a bassman/super OT, the Dynaco 470 clone or..a twin OT (running an 8 ohm cab)

I wonder if that alone would help?

i would ask moss if he has any samples left to measure and just compare yours to theirs. that would be the best way to verify. rh

[rhinson]

This is actually the original 22 kit OT.
So there are only the 8k primaries and I am running 6L6's.

I was seriously thinking of upgrading the OT to a bassman/super OT, the Dynaco 470 clone or..a twin OT (running an 8 ohm cab)

I wonder if that alone would help?

any of the larger trannies you listed will give you an increase in headroom and a bit of loudness too. get the general replacement bassman sized trannie with multiple taps from magic parts/ruby tubes- i believe these have been seen in dumbles a few times. made by lenco or so i've been told. rh

[Fischerman]

This is actually the original 22 kit OT.
So there are only the 8k primaries and I am running 6L6's.

Reading 67_Plexi's posts and this...couldn't this be it...or at least a big contributor?

If we had a 4K OT designed for 6L6s...then wouldn't they have to add more wire to get the 8K primary Z for 6V6s. More wire increases the DC resistance which then increases the voltage drop across the primary. And since you're running this OT at higher current than designed (because the 6L6s put out twice as much power as 6V6s and you bias them much hotter)...that just exacerbates the problem?

As a quick fix...could you just add in a 'shared' screen grid resistor? So off of the screen node you'd add say, a 100ohm-470ohm/5W resistor to get your screen voltage down? It would work as far as getting your screens below your plates but I dunno how much it would affect the tone.

[Fischerman]

This is actually the original 22 kit OT.
So there are only the 8k primaries and I am running 6L6's.

Reading 67_Plexi's posts and this...couldn't this be it...or at least a big contributor?

If we had a 4K OT designed for 6L6s...then wouldn't they have to add more wire to get the 8K primary Z for 6V6s. More wire increases the DC resistance which then increases the voltage drop across the primary. And since you're running this OT at higher current than designed (because the 6L6s put out twice as much power as 6V6s and you bias them much hotter)...that just exacerbates the problem?

As a quick fix...could you just add in a 'shared' screen grid resistor? So off of the screen node you'd add say, a 100ohm-470ohm/5W resistor to get your screen voltage down? It would work as far as getting your screens below your plates but I dunno how much it would affect the tone.

[frischmann]

I think I get what your saying.
Just add a small value resistor between the wire and the pin going to the screens to knock off a little voltage.....right?

BTW,
I still have the 1K/5w in there insteak of the 470ohm...should I switch it?


I always thought this was a standard tremolux OT but I guess it wouldn't be with an 8k primary...

We'll off t0 magic parts...

[rhinson]

This is actually the original 22 kit OT.
So there are only the 8k primaries and I am running 6L6's.

Reading 67_Plexi's posts and this...couldn't this be it...or at least a big contributor?

If we had a 4K OT designed for 6L6s...then wouldn't they have to add more wire to get the 8K primary Z for 6V6s. More wire increases the DC resistance which then increases the voltage drop across the primary. And since you're running this OT at higher current than designed (because the 6L6s put out twice as much power as 6V6s and you bias them much hotter)...that just exacerbates the problem?

As a quick fix...could you just add in a 'shared' screen grid resistor? So off of the screen node you'd add say, a 100ohm-470ohm/5W resistor to get your screen voltage down? It would work as far as getting your screens below your plates but I dunno how much it would affect the tone.

no. a trannie doesn't have an imp.----just a turns ratio. your bandmaster/pro/vibrolux sized trannie is a 4k/4ohm trannie in it's historical fender and clones usage. but this is also equivalent to 8k/8 ohms----your amps usage for 6v6's. for guitar amps, it's a bigger sized 6v6 trannie and a somewhat smaller sized 6l6 trannie (it's the same lam/core size and stack as an 18watt marshall trannie). the rest of your statement is more complicated, having to do with bandwidth, etc.

yes you can drop them a bit by adding an extra series resistor from the choke point to your individual resistors on the tubes. try 1k/5w ala the orig. jtm45 amps----6l6's don't draw much screen current at idle--- but because of this it won't be much of a drop. i wouldn't worry about it a lot---it happens depending on amp circumstances. once you start playing the amp and pushing it, the plate voltage drops down a lot and the screens take over. rh

[Fischerman]

no. a trannie doesn't have an imp.----just a turns ratio.

This much I understand.

So...take two different OTs for example. We have a 6L6 OT that has a 4k:8R turns ratio and a 6V6 OT with a 8K:8R turns ratio. Wouldn't the 8k:8R OT have more turns...i.e. more wire...i.e. more DC resistance?

So then we decide to use the 6V6 OT with 6L6s instead. So we 'halve' our speaker Z to reflect half the primary Z...so we are at 4K for the primary. Great. But we STILL have all that wire that's causing the DC resistance.

See what I'm saying? Yes...the reflected Z is the same (and we can do things to keep it the same...like changing the load/speaker Z)...but one has way more wire and thus way more DC resistance. And we can't change that...the wire/DC resistance is fixed. This is what I'm saying may actually BE the problem. But I'm just guessing...trying to learn.

[Normster]

Sorry, late to the party. I've had a few D'Lite builds with screens slightly higher than (or equal to) plate voltage, even with the 500R "choke resistor." Never more than 5 volts though. In most cases, swapping the power tubes for a different brand brought the voltages in line. FWIW, I've never been too concerned with the screens being lower than the plates in this circuit. Haven't had any tubes failures and the amps sound good either way.

[frischmann]

Sorry, late to the party. I've had a few D'Lite builds with screens slightly higher than (or equal to) plate voltage, even with the 500R "choke resistor." Never more than 5 volts though. In most cases, swapping the power tubes for a different brand brought the voltages in line. FWIW, I've never been too concerned with the screens being lower than the plates in this circuit. Haven't had any tubes failures and the amps sound good either way.

Hi Normster,
Did you mean the screens being Higher than the plates?

Thanks,
Paul