Really hot grid resistors!??

[shiner555]

Hey All,

Wanted to post an issue I've been having here that I can't seem to figure out. I'm near then end of a total re-build on this amp

http://www.schematicheaven.com/barga...uperguitar.pdf

I've re-capped the whole thing and I've also converted the amp to run on EL34's instead of 6L6. So in doing that conversion I've changed the resistors on pin 5 to 5k 2W, and the resistors on pin 4 to 1k 2W. The voltage on pin 4 is around 460, and I've biased the amp to 30mV. Amp sounds great, however, twice now I've melted the 1k Grid resistors. When I say melted, they seemed to get so hot that the solder on them re-flowed and they would fall off.

I've been poking around on this issue for a while, and can't seem to figure out what's going on..I've been checking for shorts, etc, etc.

Any ideas on what to check out??

Cheers,
-Jason

[olddawg]

If it sounds good and you have no other issues. (I am assuming that it takes some time for this to happen) Have you considered just replacing the resistors with 5 watt equivalents and see what happens? Quick and dirty.

[shiner555]

If it sounds good and you have no other issues. (I am assuming that it takes some time for this to happen) Have you considered just replacing the resistors with 5 watt equivalents and see what happens? Quick and dirty.

Hey Olddawg...

Not a bad idea, I had just figured that the resistors getting that hot was an indication that something else was wrong. I take it, that's normal??

Was it common for other amps using EL34's to use 5W grid's?

Thanks,
-Jason

[olddawg]

Frankly Jason, I'm not sure. My tube theory is actually pretty weak. They didn't mention tubes at all when I got my degree. Hopefully Enzo, Bruce, or one of the other tube pros will probably know. You can calculate the grid current by measuring the voltage drop across the resistor and decide if it is acceptable. You could also look at a schematic of a similar amp using EL34s and see how it is set up. I would probably do that if it was mine. If you are playing the amp for long periods of time, it sounds great, nothing else is stressing, and the tubes aren't red plating I wouldn't worry too much about upping the current spec of those resistors. I'm kind of an "if it works don't f..K with it" kind of guy, but that doesn't mean it shouldn't be done right.

[tubeswell]

EL34 screen current draw is higher than 6L6s (so yes you should use at least 5W resistors on the screen grids in this case. Smaller resistors won't dissipate the heat so well - so they fry). Remember that Watts = Amps x Volts, and Volts = Amps x Resistance. So if you increase Resistance you drop more Volts for a given amount of Amps, and if you increase Amps and Resistance, you drop even more Volts. So when you tote up the increased voltage that you are dropping across the screen resistor (which you have made bigger) that is attached to a tube that draws more screen current than the earlier tube you had in there, you need a more powerful resistor (i.e. more wattage) to dissipate the energy that the screen grid resistor now needs to lose as heat.

[loudthud]

Did you ground pin 1 of the EL34s ?

[shiner555]

ampzzz

EL34 screen current draw is higher than 6L6s (so yes you should use at least 5W resistors on the screen grids in this case. Smaller resistors won't dissipate the heat so well - so they fry). Remember that Watts = Amps x Volts, and Volts = Amps x Resistance. So if you increase Resistance you drop more Volts for a given amount of Amps, and if you increase Amps and Resistance, you drop even more Volts. So when you tote up the increased voltage that you are dropping across the screen resistor (which you have made bigger) that is attached to a tube that draws more screen current than the earlier tube you had in there, you need a more powerful resistor (i.e. more wattage) to dissipate the energy that the screen grid resistor now needs to lose as heat.

Hey Guys,

Many thanks for the replies. Seems like to me that all I really need to do is try 5Watts. The reason I put in 2W was based on a book I have about working on tube amps. However, I now realize that when they made this recommendation about the wattage, I don't think they were taking into account the grid voltage!! I could see with a lower grid voltage, a 2W being ok..possibly.

Thanks again for the help...I'm actually a EE by trade, but my area is more in DSP and acoustics...been a while since I tried to use any network theory.

Cheers,
-Jason

[shiner555]

Did you ground pin 1 of the EL34s ?

Yep,

pin 1 is connected to 8 and then to ground.

-Jason

[black_labb]

i had put a 1W 1k resistor into my homemade amp, because i calculated that at max grid dissipation (as per the datasheet, 8w from memory) the resistor would be dissipating ~0.4w, so i thought i would put in a 1W resistor, because i'd prefer the resistor to go than a tubes grids.

i havent blown any of them yet (finished a few months ago), and the amp is running close to full output once a week for a couple hours. the grids are running at roughly 390v. any insight into why i'm alright, and some people's die regularly?