Vox (Thomas Organ) tube Berkeley without screen resistors

[seven]

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 for each tube.

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!

[mozz]

Voltages don't seem too high for el84s. That being said some Vox's use 100r. I would actually leave it the way it is if there's no problems.

[Chuck H]

Voltages don't seem too high for el84s. That being said some Vox's use 100r. I would actually leave it the way it is if there's no problems.

Agree. That said, and depending on how the amp will be played, I'd have been more inclined to go lower in value from the schematic for the cathode resistor rather than higher. If I had, say, both 100r and 150r resistors on hand I would have chosen the 100r. It's much more typical to see el84's biased hotter rather than cooler. Especially with cathode bias because voltage will rise on the cathode resistor when the tubes are conducting which further cools the bias. And el84's tend to have more audible and bad sounding crossover distortions than some other tubes so the usual goal is to eliminate as much of it as possible. A cooler bias will mean more crossover distortions. So...

If you play the amp clipping at all and you notice unpleasant buzzy tone you might try a 100r cathode resistor rather than the 150r and see if that helps.

[Jon Snell]

If you fit other than Sovtec or NOS Mullar EL84s in it, 150R is more suitable as the new cheaper built JJ and Chinesium EL84s cannot stand the heat.
Screen grid 100R 1/2W Fuseable resistors are always a good idea for safety and to avoid destroying more than just the valve that flashed over.
When I service AC30s, I fit Sovtec EL84s or replace the 50R cathode resistor with 60R for reliability with other makes.
Watch out for broken wires on the valve base pins.

[Chuck H]

If you fit other than Sovtec or NOS Mullar EL84s in it, 150R is more suitable as the new cheaper built JJ and Chinesium EL84s cannot stand the heat.
Screen grid 100R 1/2W Fuseable resistors are always a good idea for safety and to avoid destroying more than just the valve that flashed over.
When I service AC30s, I fit Sovtec EL84s or replace the 50R cathode resistor with 60R for reliability with other makes.
Watch out for broken wires on the valve base pins.

I certainly appreciate your position and regard your experience. Considering the VERY conservative voltages in this amp do we still need to be concerned about the suitability of JJ and east Asian tubes such that the amp must be biased at 70% dissipation even with cathode bias? Are these tubes utterly failing in every other normal amp when installed?

The question is half tongue in cheek but half honest. Because I have no experience at the bench with modern JJ or east Asian el84's. But it's hard for me to imagine that Tiny Terror owners are installing JJ tubes only to watch them instantly incinerate.

My own experience with el84's (that are NOT JJ or east Asian) is simply that they sound best when biased hotter than colder and nearly all manufactured amps from respected brands seem to agree. JJ has been a popular el84 for a long time. I can't imagine, though I admittedly don't know, that they are prone to failure in all extant amplifiers that run the power tubes MUCH harder than the amp currently being discussed.

I'm still enjoying a stash of great old Reflektor el84 tubes. Maybe enough to last my lifetime since I don't really play that much anymore. But are the current offerings on the market really as bad as you say?

[Jon Snell]

Reflektor 6p14p-er are excellent and equivalent to a 7189 or the very ole British EL84, the same as Sovtek.
They can take the heat as they can hapilly dissipate 16Watts.
Go for it.

[Chuck H]

They are VERY good. I don't have the "ER" suffix tubes. Mine are the "K" suffix. Supposed to be steady against vibration. I chose these specifically for that reason. My combo amp, designed to be cranked for overdrive, would render a pair of Sovtek's rattly in a couple of months. So I was looking for a more durable tube more so than a longer life tube. The ER suffix is rated for a 10,000 hour life!!! Which is really remarkable. The K suffix is rated for a more nominal 3000 hours but has held up in a 1x12 combo cab under heavy clipping for at least a thousand hours with no sign of microphony. I couldn't ask for more. As a bonus they sound great too. Big bottom end (for el84's) and still some of that characteristic el84 chime and intermodulation. Pretty much the exact perfect tube for my own amp. I parted with a few for a customer that has a similar amp but that's the end of it. I'm keeping the last three pairs for myself and this amps future.

But what I was saying above was that I imagine that at 270Vp the Vox Berkeley above might not be so hard on the common JJ's that they couldn't stand up to a 90% or 100% bias. Which is really where el84's sound "right". Below that they buzz like bees with crossover distortion. Much worse than big bottles at cooler bias settings. My point is that if the amp will be overdriven at all the circuit is basically useless unless the user will never clip the amp or the available tubes can handle the 270Vp with a hotter bias. I suppose screen resistors could be fitted but at the lower voltage it's probably not a concern? Then again, as stated, I have no experience with modern el84's.