Vox (Thomas Organ) tube Berkeley without screen resistors

[fazeka]

Hi there,

Have an early Thomas Organ Vox Berkeley I'm working on.

Have recapped the electrolytics and am using a 150-ohm/3W cathode resistor.

Plate dissipation is coming in around 8.25 (317 plate voltage - 10.3 cathode voltage = 307 x 27 mA) and 10.75 (316 - 10.3 = 306 x 35mA) watts.

I noticed the amp does not have screen resistors on the EL84s. Both screens just go straight to the 240V power supply node.



Although this amp is absolutely bone stock, I'm thinking it could be better to install screen resistors.

However, this is a learning experience for me so I'd like to know what y'all think? What is the disadvantage of leaving the amp without scren resistors? Advantage?

Thanks!

[Latole]

We can see often old amps without screen resistors.
Screen resistor are used to protect tube.
I will put 470 ohms

[sluckey]

Vox AC-15 power amp is almost identical to the Berkeley except it has two 100Ω screen resistors. I like to keep old amps stock. EL84s are cheap. If I were building a clone I would use screen resistors.

[Latole]

I see the 470 ohms on a AC30

[sluckey]

I see the 470 ohms on a AC30

Maybe on some of the newer models but the old AC-30s use 100Ω.

[Latole]

I see the 470 ohms on a AC30

Maybe on some of the newer models but the old AC-30s use 100Ω.

IMO 100 ohms is a minimum and 470/ 500 ohms is a maximum.
If fazeka like the tone with no resistor, I'll go for a 100 ohms.

[sluckey]

There are 18 AC-30 schematics in the library. All use 100Ω screen resistors except one unpopular model that uses 470Ω. I suppose Vox prefers 100Ω.

[Latole]

I have my own library ,"new Vox use 470, old one 100 ohms

 those shematics show 470 ohms
AC10 C1
AC15C1
AC30C2
AC4C1
AC4 HW1
NT15H

[sluckey]

That's nice, but only one AC-30 with 470Ω. I have a butt load of Fender schematics with 470Ω screen resistors. But we seem to be drifting farther and farther away from the original post.

[fazeka]

But we seem to be drifting farther and farther away from the original post.

[sluckey]

You don't think so?

I was just engaging Latole with a bit of trivial banter while having my first cup of coffee. And we ended up with a wheel in the ditch and a wheel on the track.
 
So, rolling back to the beginning, I'll repeat my first post...

Vox AC-15 power amp is almost identical to the Berkeley except it has two 100Ω screen resistors. I like to keep old amps stock. EL84s are cheap. If I were building a clone I would use screen resistors.

[Jalmeida]

I have my own library ,"new Vox use 470, old one 100 ohms

 those shematics show 470 ohms
AC10 C1
AC15C1
AC30C2
AC4C1
AC4 HW1
NT15H

Vox has only made loose tributes to a few of their original AC30. The rest are anything but the models they call them.

[fazeka]

So, rolling back to the beginning, I'll repeat my first post...

Vox AC-15 power amp is almost identical to the Berkeley except it has two 100Ω screen resistors. I like to keep old amps stock. EL84s are cheap. If I were building a clone I would use screen resistors.

Thank you.

[Latole]

I have my own library ,"new Vox use 470, old one 100 ohms

 those shematics show 470 ohms
AC10 C1
AC15C1
AC30C2
AC4C1
AC4 HW1
NT15H

Vox has only made loose tributes to a few of their original AC30. The rest are anything but the models they call them.

You may not wrong.

[PRR]

If you find Horizontal Sweep tube ratings, they often suggest no screen resistor for low-volt low-power plans and increasing resistors as you get closer to the tube Maximum ratings.

This model is WAY below a EL84's ability.

IMHO, at 250V plate-cathode (230V screen-cathode!), the EL84 is fine without a screen resistor. At 300V I might use one; 330V for sure.

[PRR]

A bigger tube working as TV sweep. Note that sweep duty is FULL power ALL the time, much more severe than any music amplifier.

[fazeka]

IMHO, at 250V plate-cathode (230V screen-cathode!), the EL84 is fine without a screen resistor. At 300V I might use one; 330V for sure.

Thanks Paul. Seems I'm right on the edge: plates 311V, cathodes 10V, screen 287V.

[pdf64]

What’s the V AC across the heaters? As your V DC are so much above nominal, it may be that the 6.3V is too.
Possibly resulting from the mains it’s being fed being higher than it was intended for.
Hence a V AC reading for the mains would be useful too.

[fazeka]

What’s the V AC across the heaters? As your V DC are so much above nominal, it may be that the 6.3V is too.
Possibly resulting from the mains it’s being fed being higher than it was intended for.
Hence a V AC reading for the mains would be useful too.

Filaments: 6.67VAC
Mains: 123.5VAC

[pdf64]

What are the actual measured values of R25 (120R cathode resistor) and 42 (1k2 HT dropper to screen grid node)?

[fazeka]

What are the actual measured values of R25 (120R cathode resistor)

10.5 - 10.75VDC (original cathode resistor of 125 ohms replaced with one that is 140 ohms).

and 42 (1k2 HT dropper to screen grid node)?

I measured this initially when I first got the amp but not since the replaced power supply electrolytics and related power supply resistors. My notes say it was somewhere between 316 and 319VDC (EZ81, pin 3/R42/C26A junction). As of yesterday, the amp is back together again and unfortunately I'll admit I'm not terribly inclined to pull it all apart just to measure it, post the component replacement.

[pdf64]

What are the actual measured values of R25 (120R cathode resistor)

10.5 - 10.75VDC (original cathode resistor of 125 ohms replaced with one that is 140 ohms).
…

A higher value cathode resistor will reduce the current drawn by the output valves.
That will reduce the load on the HT supply, which will increase its voltage.
So yeah, change the circuit and the schematic noted voltages won’t be applicable.

The above effect illustrates why the tinkering with the cathode bias resistor value to alter (cool) idle dissipation can be somewhat counterproductive.