Pro Reverb Voltages

[Mick Bailey]

Then I'm a bad tech - I don't like high power screen resistors. A resistor that holds out whilst a tube is pulling excessive current beyond what the manufacturer intended is not a good situation to me. By the time a 5W or 7W resistor has let go there's potential for damage to the amp. Even more so if the amp has blown it's HT fuse and the owner has replaced it with a higher rating to 'get by'. Fender (for one) fits relatively small, fusible resistors to screens in some amps as additional protection in case a tube shorts. Marshall fits small fusible cathode resistors for the same reason.

[jmaf]

Then I'm a bad tech - I don't like high power screen resistors. A resistor that holds out whilst a tube is pulling excessive current beyond what the manufacturer intended is not a good situation to me. By the time a 5W or 7W resistor has let go there's potential for damage to the amp. Even more so if the amp has blown it's HT fuse and the owner has replaced it with a higher rating to 'get by'. Fender (for one) fits relatively small, fusible resistors to screens in some amps as additional protection in case a tube shorts. Marshall fits small fusible cathode resistors for the same reason.

People really don't need to argue over Ohm's law. Peak Vg2 - Vp / (max stage impedance). That's your peak transient screen current. Just add a resistor twice as powerful for reliability. That's all. Chinese built amps use 2 watt resistors where 2 watts are consumed 100% duty cycle and they end up at the tech bench because of this.

Gum foiling fuses and other irresponsible practices are completely unrelated to the discussion of using sturdy components in critical areas. A good tech will know the difference between masking underlying problems by using sturdy components or when sturdy components are technically necessary. I'm talking about the latter.

[mozz]

"Gum foiling fuses and other irresponsible practices are completely unrelated to the discussion of using sturdy components in critical areas."

Not really, using a over rated fuse is a typical hack seen many times. I will use 1watt or 2watt screen resistors max. I've never calculated the actual screen resistor dissipation but they do have a surge rating.

[Tone Meister]

... Probes are consumables, they will fail eventually, they aren't family heirlooms.

Damn, take me to school Pete. I have probably 3 sets that need tossing out, now that you mention it. Never thought of it that way.

Thanks!

[g1]

The debate over whether screen resistors should be 'fusible' or not is a very old one. (my own priors go back to debating KOC about this back before his first TUT book ).
There are good arguments both for and against.
Best argument for: all those Fender amps out there. (2W screen R's)
Best argument against: all those Marshall amps out there. (5W screen R's)

[bobloblaws]

FWIW: If a screen resistor fails, the tube is going to conduct very very little, if at all, so there's little chance of harm to the tube.

Are you sure? This quote form jmaf seems to contradict that.

"The screen grid is a very fragile structure. The purpose of those resistors is to protect the screen grid from excessive dissipation (literally burnup). "

[J M Fahey]

Quote Originally Posted by The Dude View Post
FWIW: If a screen resistor fails, the tube is going to conduct very very little, if at all, so there's little chance of harm to the tube.

Are you sure? This quote form jmaf seems to contradict that.

"The screen grid is a very fragile structure. The purpose of those resistors is to protect the screen grid from excessive dissipation (literally burnup). "

Where do you see a contradiction???????

Those are two different statements and, as quoted, both are valid.

Pentodes practically stop passing current if screen connection opens ..... check.

Screen resistors limit screen current, so limit screen dissipation, so protect them ..... check.

[g1]

Exactly, resistors fail open. So a resistor opening toward 'infinite' resistance will offer near 'infinite' protection to the tube.

[bobloblaws]

Exactly, resistors fail open. So a resistor opening toward 'infinite' resistance will offer near 'infinite' protection to the tube.

Ok, that makes sense. So should I leave the screen resistors at a more fuse like 1W then? I've been led to believe it's a good idea to increase power capability to 2W or even 3W.

[dstrat]

Screen Resistors are set depending on the amp , kind of tubes , how much current the screens draw at max signal.

There's no one size fits all.

Fender used I believe 2w resistors for years for 6L6 amps and still work fine, well unless they were abused.

[The Dude]

......Fender used I believe 2w resistors for years for 6L6 amps and still work fine......

Agreed. There's no reason to redesign the amp. It's worked since 1972.

[Chuck H]

Screen Resistors are set depending on the amp , kind of tubes , how much current the screens draw at max signal.

There's no one size fits all.

+1

I've done a couple of projects where I had to monitor grid drive and screen current to find a happy, safe zone. Granted this was not with beam tubes like the 6L6. But compromizing between the safe zone and the tone zone put me at 5W rating for a 1k screen resistor using EL34 tubes. A 2W rating there would have meant replacing the resistors every other time the amp was cranked up (probably why most Marshall amps use a 5W rating) But beam tubes are gentler on the screens from what I've read here.

Bob, monitor screen current and decide for yourself.

[bobloblaws]

Agreed. There's no reason to redesign the amp. It's worked since 1972.

Ok, I thought they were 1W (per schematic) but maybe they are already 2W. I'll just leave well enough alone for now.

This may be the source of the opinions I had taken as gospel some time ago, I can't recall.

https://www.thegearpage.net/board/in...stions.274440/

For example from message #2

Screen grid resistors help keep the screen grid at a lower potential than the plates. On a 6V6 tube, the pin is 4.

You don't want the screen grid to act as a secondary plate which can happen if the plate voltage goes below the screen grid voltage.

And from message #3

2W is the minimum I would use, and preferably wirewound.

You don't want the resistor to burn if the tube shorts, contrary to a lot of advice you will hear. A resistor is NOT a fuse, and is not designed to destroy itself safely - it will quite likely melt, burn or actually set fire to nearby components and wiring.

Or do other bad stuff...

[Justin Thomas]

So, the "fire" thing may happen if you use carbon composition, as they are basically charcoal... But using a metal film or other ceramic-bodied 1W resistor should help avert the conflagration, no?

It's not the 60s/70s anymore - we have resistor choices now!

Justin

[Enzo]

Ok, that makes sense. So should I leave the screen resistors at a more fuse like 1W then? I've been led to believe it's a good idea to increase power capability to 2W or even 3W.

Or another option: Stop trying to overthink it, and just build it stock. Everyone on the internet will have some rationale for why they changed a part to some other value or rating, but really, the basic as-designed circuit has worked just fine for decades and decades.

In the long run it really won't much matter whether you use 2w or 3w resistors.