Mesa Boogie 50 50 heater resistors toasted

[buford]

I just bought a Mesa Fifty Fifty untested but supposedly good. Seller seemed ok. I figured I'd open it up and check it out as well as clean it up before power up. I removed the power tubes to install TAD 6L6GC-STR's. Instantly noticed two burnt resistors which turned out to be 100 ohm 2 watt carbon comp resistors. Assuming that filament resistors have no affect on tone, I was thinking of trying metal film ones and would like to know if 5 watt ones would be overkill or a safer bet for the long run. The metal film resistors are much cheaper and easier to come by than the Carbon Comp ones. The only CC's that I can find are by Allen Bradley ? I also plan on keeping them up off the board a bit for better air flow and to protect the board. Any thoughts would be appreciated.

[pdf64]

Heater circuit ground reference resistors can take a hit in the event of a power tube short.
CC doesn't seem to be a good choice, carbon acting as a fuel load. Check that the pcb isn't charred; if it is, the discolored material is best ground out.
Flameproof / retardent metal oxide / film / wirewound are more suitable.
With a high voltage rating, eg about the same as the VB+; some wirewound may only be 100V.
As high a power rating as can fit, mounted off the board to allow air circulation and avoid pcb damage if they do blow.
5W types may be too big to fit, physical constraints including the pcb hole diameter.
The addition of B+ fusing would be a great idea.

[g1]

Heater circuit ground reference resistors can take a hit in the event of a power tube short.

Also if the locater pin in the base of a power tube is broken off and the tube is inserted incorrectly into the socket.

[Mick Bailey]

The higher the power rating, the more heat they dissipate in a fault condition before opening, and the more chance they have of scorching the PCB. Most amps use 1/2W CF or MF resistors in this position - dissipation under normal operating conditions is minimal.

[buford]

Are there any tonal effects from heater circuits? If not, I think it might be best to use 2 watt metal film non wire wound ones set off the board a bit. Hopefully the transformer is ok. The fuse is still good but the seller may've just changed it. I would think it would've blown a fuse. I think the resistors did show roughly 12 ohms across them but it might be from the rest of the circuit. I'll have to try and carefully snip one end to see if they measure anything. I would at least think if they're open then the amps would have no output.

[g1]

The amp will work without them. Their effect is mostly related to hum reduction, they will not affect the tone.

[buford]

The amp will work without them. Their effect is mostly related to hum reduction, they will not affect the tone.

thanks. Would metal film resistors be a good choice then as far as hum is concerned.

[deci belle]

I always use those brown 100Ω 3w IRC metal resistors cuz they're not any bigger than the blue 2w ones.

[buford]

I always use those brown 100Ω 3w IRC metal resistors cuz they're not any bigger than the blue 2w ones.

thanks. I think those are wire wound. I'm not sure if wire is good or bad if these are strictly for hum. I found a couple of nice 75 ohm 2 watters in my collection. Wish I could use those. I think ebay is my best bet for metal film 2w. Most are a couple of dollars for ten sent directly from China.

[deci belle]

hi buford~ they're metal film (1% or 5% available). Mouser and Digikey have them, but I usually buy them from Handmade Electronics.

[buford]

hi buford~ they're metal film (1% or 5% available). Mouser and Digikey have them, but I usually buy them from Handmade Electronics.

thanks so very much deci belle for the tip. I like the fact that these are made in the USA ( looked at their website, very nicely made part ) and can handle high voltage even though the circuit is 6.3v. Handmade is sold out so either one of the others. I think Digikey lists them as 1% where all others show 5% so I'll have to make sure they're not the wire wound ones if I buy from them. I could get 10 for a buck of 3 watt metal films from China through ebay but I've had bad tolerance's from those China made ones before and besides, This Mesa Amp is USA vintage with balls galore and I think the Brownies you suggested are the ticket. Thanks again and I will update when I fire it up. Hopefully without the fire.

[Wes]

https://www.tubedepot.com/products/1...power-resistor

Why make a dollar out of 15 cent?


Shipping....doh.

[buford]

thanks, shipping is reasonable. Prices are good too. They have a good selection as well. They might be easier to order from than Mouser/Digi. Not sure if they are USA parts like IRC.