Why would my screens be higher than my plates?

[frischmann]

I haven't tried the 2k2. I don't have anything with high enough wattage. I hope to get to the electronics store on Monday.

I'll get a few values so I can play with the string in general

I'll post back with my results.

I think the d'lite is based on a 325-0-325 PS, which is standard tremolux iron. It looks like tremoluxes had a bout 410v on the plates/

Do you think that's all I can get on the plates with this PT?

[JimiB]

you might get 420-430 once you get this problem solved but not much more. (i think i have close to 430 in mine with 6L6's)

[JimiB]

what OT are you using?
Also do you have another set of power tubes you could try?

[frischmann]

The OT is the standard, original d'lite PT. I think it's a heyboer.
The numbers on it are
PT650
HTS-8816
9960721

I'll have to see if I can get my hands on another set of 6l6's. I current have some RCA black plates in there I got used...they may a little tired.

The sound great though...

I'd be cool with 425 volts on the plates. I've kinda take what was going to be my bedroom/grab and go amp and turned it into one of my main amps.

It just sounds that good, especially with the choke upgrade.

Thanks for all the help btw...I really appreciate it.

[dogears]

I am confused. If you measure 426 on the plates and 426 on the screen node after the choke, how do you have higher screen voltage? Fwiw, with a 90ohm choke you may measure the same 426v on both nodes. Even though the screen side is a tad lower.

426v is totally normal btw.

How can you possibly measure less than the 426v on the plates when your main filter cap B+ is 426? Something is not computing.

[frischmann]

Hi Scott,
I'm getting like 426 ish on the screens and 10-12 volts less or so on the plates.

I'd always hear it was bad to have the screens higher than the plates.

['67_Plexi]

I am confused. If you measure 426 on the plates and 426 on the screen node after the choke, how do you have higher screen voltage? Fwiw, with a 90ohm choke you may measure the same 426v on both nodes. Even though the screen side is a tad lower.

426v is totally normal btw.

How can you possibly measure less than the 426v on the plates when your main filter cap B+ is 426? Something is not computing.

Measure the DC resistance of your OT primary winding. THat will probably give you the answer.
I've seen this a few times.

There's 4 different voltages to consider.

The HT supply node, the plates, the screen supply node and the Screens.
The Plate voltage will be lower than the HT supply due to the volt drop across the primary winding.
The Screen supply will be a few volts lower than HT Supply, but may be more than the Plate Voltage. There is a relatively small current flowing through the choke, so the volt drop will be only a volt or two.
At idle the screen current is fairly low so there's not a massive volt drop across the screen resistors. The combined volt drop from the choke and the screen resistors can be less than the volt drop across the primary windings.
The solution is to use a choke with a higher DC resistance or replace the choke with a resistor if you like that tone.
A different set of power tubes will give different readings.

[Structo]

What will that show?

Make sure you have the center tap of the OT well grounded.

[frischmann]

Interesting,
I would have never thought of that.
Do I measure Primary to center tap or to common ground?

I assume I have to disconnect the leads to verify.

['67_Plexi]

What will that show?

Make sure you have the center tap of the OT well grounded.

It will show that I took the time to answer his question.
I edited my message, re-read it.
I've seen this situation quite a few times.

['67_Plexi]

Interesting,
I would have never thought of that.
Do I measure Primary to center tap or to common ground?

I assume I have to disconnect the leads to verify.

Measure plate to CT on each leg, sorry. Using the the idle plate current values, you can figure out the voltage drop you can expect from the HT to the plates.

[rhinson]

yep exactly right. depending on the dc resistance of the output trannie pri. legs you can get a higher screen voltage than plate. it's not that uncommon in smaller amps like deluxes, etc. that have a relatively large winding resistance. the plates draw a lot more than the screens in 6l6's/6v6's, and if the winding resistance is substantial (a deluxe reverb for example is in the 225 - 250 ohms range per leg) you'll see a drop across the winding bigger than the drop to the screens. usually it'll be just a few volts. don't know what the dc resistance of the vibrolux/bandmaster/etc. sized fender type trannies are, but the winding certainly could be a factor. rh

[rhinson]

sorry double post

[Structo]

What will that show?

Make sure you have the center tap of the OT well grounded.

It will show that I took the time to answer his question.
I edited my message, re-read it.
I've seen this situation quite a few times.

Hey, I wasn't trying to be a smart ass in my question so I don't understand your answer...

I merely asked what that will show because you gave a vague answer.

['67_Plexi]

What will that show?

Make sure you have the center tap of the OT well grounded.

It will show that I took the time to answer his question.
I edited my message, re-read it.
I've seen this situation quite a few times.

Hey, I wasn't trying to be a smart ass in my question so I don't understand your answer...

I merely asked what that will show because you gave a vague answer.

OK no worries, my bad. I wear a teflon suit...lol

Here you go:

For example lets say you have a two tube amp with a 430V HT Supply and the DC resistance from CT to each plate is 80 ohms and the idle plate current is 40mA. The volt drop will be V=IR = 80x0.04=3.2V so the plates will be sitting at 426.8V.
If it was a 4 tube amp, you would need to add the idle plate current of both tubes on each leg.

Taking it a step further, lets stick with a 50W EL34 amp for the sake of an example.
In order to figure the volt drop across the choke, you need to know the combined screen current from both tubes and the current from all the preamp triode. Lets say it has three preamp tubes (six triodes). The idle current from those will be somewhere around 6mA. Say the idle scren current from an EL34 is 2mA, then the combined idle current through the choke will be 10mA. Lets say the choke has a DC resistance of 80ohms, then the Screen node will be at V(HT)-(80x0.01) = 429.2V
Now taking in to account the screen resistor volt drop using a 1K screen resistor then the V(Screen)=429.2-(1000x0.002) = 427.2V

In this example the Vp = 426.8V and Vs=427.2V
Therefore the screen voltage is higher.