Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Madison on November 21, 2011, 07:43:33 am

Title: Plexi screen resistors- 2 watt okay?
Post by: Madison on November 21, 2011, 07:43:33 am

Can I get away with using these 1K/2W on the EL34s?
I installed them but just want to be sure.
I don't have any 1K/5W here.
I do have 1.5K/5W.

Title: Re: Plexi screen resistors- 2 watt okay?
Post by: sluckey on November 21, 2011, 08:51:03 am

I think so. A 1K resistor would have to see 45ma to dissipate 2 Watts, or 31ma to dissipate 1 watt.

Title: Re: Plexi screen resistors- 2 watt okay?
Post by: jim on November 21, 2011, 09:27:03 am

I would use the 1.5 K 5W.  It is safer and the amp won't know the difference.  Jim

Title: Re: Plexi screen resistors- 2 watt okay?
Post by: Madison on November 21, 2011, 05:39:42 pm

>> think so. A 1K resistor would have to see 45ma to dissipate 2 Watts, or 31ma to dissipate 1 watt.

Thanks!
I'll probably try set the bias so it can't adjust that high.
I am estimating about 400 volts on the plates with this amp.
42ma would be the max dissipation a guy would want to adjust to, I think?

>>I would use the 1.5 K 5W.  It is safer and the amp won't know the difference.  Jim

Doesn't the increased value reduce overall output?
And/or tone?

Title: Re: Plexi screen resistors- 2 watt okay?
Post by: Cups on November 21, 2011, 05:59:17 pm

I don't think you'll be able to hear the difference in volume or tone. Could be wrong though. Why not try it out? Those are robust resistors and it's an easy enough swap.
I know I'd stick with the 5W but I'm cautious that way.

Title: Re: Plexi screen resistors- 2 watt okay?
Post by: sluckey on November 21, 2011, 06:12:26 pm

Quote

I'll probably try set the bias so it can't adjust that high.
I am estimating about 400 volts on the plates with this amp.
42ma would be the max dissipation a guy would want to adjust to, I think?

You're thinking plate dissipation. Screen dissipation is much less, typically only about 10% of plate dissipation.

Fender used 470Ω/1W screen resistors on most all of their amps, big and small. 'Course I've seen several scorched screen resistors in Fender amps.  :icon_biggrin:

Title: Re: Plexi screen resistors- 2 watt okay?
Post by: LooseChange on November 21, 2011, 09:54:18 pm

The standard screen resistor for an el34 is 5 watts. Use the 1k5. You'll never hear the difference.

Title: Re: Plexi screen resistors- 2 watt okay?
Post by: Madison on November 21, 2011, 10:31:28 pm

Thanks, LC.
I think I'll go for it.
I can't jeopardize my magic smoke.

Now that darn Sluckey.
That guy, he just left me hanging!
I mean good golly, I had to go out and read pages and pages about screen resistors and learn something new!

Oh......my head

Thanks again Gents. :notworthy:

Title: Re: Plexi screen resistors- 2 watt okay?
Post by: bigsbybender on November 21, 2011, 10:37:43 pm

I tried 2w with EL34's with a plexi-ish amp I built and they turned brown, so I switched to 5w resistors. I have ever since on any amp I build that can take EL34.

Title: Re: Plexi screen resistors- 2 watt okay?
Post by: Madison on November 21, 2011, 10:41:25 pm

Quote from: bigsbybender on November 21, 2011, 10:37:43 pm

I tried 2w with EL34's with a plexi-ish amp I built and they turned brown, so I switched to 5w resistors. I have ever since on any amp I build that can take EL34.

I figure the draw of 6L6s must be at the borderline for 1 watts?

Swapping them out right now.