50/50 High Plates Voltage

[Dontiki]

Hello,

I have a 2Fifty with Mesa 6L6's on the bench that work ok, but when I measured the plate of each tube I get 474 volts, which seems to be a little high, and the screen is also at the same voltage with the 1k screen resistor.

Does somebody know if it's odd or not, since the schematic shows 450 volts for the plate and 436v for screen but with 470 ohm screen resistors?

Ps: I measured the current with some bias probe I get roughly 30 mA on each tubes for the 474 plate voltage

I'm in EU so the power transformer is for 230V

Cheers, Don

 

[John Micka]

Came up with this link:
Bias mod on a 50/50 : how to do
A few years back the pictures were available but appears not any more.
I'm interested but a little slow to get started doing this.
Last year a plate resistor burned out, had to change tube and resistor.
Can't remember any values.
Hopefully someone will chime in.

 

[Henz]

Greetings. 470 ohm 1 watt . a 2 watt wud be a lil better. Tho it probably came with carbon comps, metal oxides take the blows better. Blessings.

 

[John Micka]

Henz thanks for the reply.
I'm trying to wrap my head around why possibly I would use a 2 watt and 470ohm?
The manufacture puts in a 1 watt and they worked no issue's for the last 30 years plus in this amp.
The screen appears really only uses milliamps on that line, no where near the 1 watt.
The 1 watt I pulled out had a burn hole right in the middle and I think it was designed like this to blow if there was a screen plate issue, to prevent anything happening further down in the circuit like protecting, looks like a capacitor and other resistors caps rectifiers etc...
Makes me wonder, I'm a newbie.
Seems to me new tubes nowadays are no where near the quality of tubes I had when I was a young man.
I found my resistors and did replace with the 1 watt metal film oxide.
Also if a 2 watt was installed and a screen faulted or failed what would possibly happen to the rest of the circuit?
How bad could this impact other components, in my thinking the momentary circuit jolt could do much more damage if that tube screen faulted at 2 watts.

 

[Henz]

You're welcome! I was basing the 470 ohm of off the 50/50 schematic I've found. It does say 1 watt. I follow this amp tech on YT ( Psionic Audio) smart as a tack, and he always bumps up the watts on screens. To be honest, you know, I'm not sure. I need to comment him and ask him. Your theory may be true. The screen may act like a fuse but the screens job inside a tube is to keep a little less positive volts on the tubes screen to help attract some negative electrons. The resistor should make pin four read just a few volts lower than the plates , pin three. Though in my MKIIC+, with 470's , it barely does anything. Many years ago ,it did lower it 2 to 3 volts. I can't figure out why. And yes! Today's tubes suck! Its very difficult to find good ones without spending your mortgage payment on them. Ugh! As far as the fluctuating voltages, eh, I wouldn't be too concerned about that. It all depends, in part, of wall voltages. Many years ago my C+ always measured around 470vdc B+. Plus or minus 5% . today they measure 500 + volts. My house voltages are a little higher now. I need to ask that tech that question as well. Let me know if I can help in any other way. Blessings, Jim.

 

[joey_truelove]

Hello,

I have a 2Fifty with Mesa 6L6's on the bench that work ok, but when I measured the plate of each tube I get 474 volts, which seems to be a little high, and the screen is also at the same voltage with the 1k screen resistor.

Does somebody know if it's odd or not, since the schematic shows 450 volts for the plate and 436v for screen but with 470 ohm screen resistors?

Ps: I measured the current with some bias probe I get roughly 30 mA on each tubes for the 474 plate voltage

I'm in EU so the power transformer is for 230V

Cheers, Don

I'm in a similar boat with my Fifty Fifty. The thing is closer to 500V on the plates with this fellow, a former US model converted to european voltages.

I talked to a norwegian friend and amp tech on The Gear Page about this and he said this was pushing it but the valves should handle the voltage just fine, so I have to trust him on this.

 

[austin sonar guru]

The wattage prevents it from burning. It's the resistance that's key to the circuit as long as the resistor will physically fit. 1 to 2 watts is a tiny change that will not impact the circuit but will last longer as long as the amp is healthy. Very similar to Voltage rating for a capacitor.

 

[Jay Omega]

A lot of people used double wattage carbon comps for stability purposes, the resistance value is more constant under temperature rise. Screen current increases dramatically in a Class A/B power amp when loaded, looking at DC dissipation won’t give the full story.

Most small wattage metal oxides aren’t rated for voltages seen in screen circuits, so it’s not uncommon to see higher wattage than needed there too.

My IIB has a mix of wirewound and the original carbon comp screen resistors, I’ve seen other old Marks that have been retrofitted with metal oxides. I’m unsure what was done by the factory vs other folks though