Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dwp on October 05, 2010, 11:31:45 pm

Title: telephone troubleshooting a 5E3 Mission Kit
Post by: dwp on October 05, 2010, 11:31:45 pm

I'm trying to help a friend out via long distance. He put together a Mission Kit, Iron says USA, they use good stuff, yes?
Says the power transformer is running really hot, even says the output transformer is getting hot.
says the tone sounds great initially, but may be breaking up alittle after prolonged play.
 I had him use the Weber bias calculator, he comes out 13 watts plate dissapation, 37 MA plate current. This OK for 6V6's in cathode bias, yes?
358VDC pin 3
328VDC pin 4
335VDC pins 3 to 8
21.5 accross the 270 ohm cathode resistor.
sound normal?
to reduce the MA, would we lower the Cathode Resitor to 200-170ohm? or raise the value?
Information much appreciated. Any additional thought will be helpful.
thanks in advance, dwp

Title: Re: telephone troubleshooting a 5E3 Mission Kit
Post by: mresistor on October 06, 2010, 05:36:57 am

Looks like that amp is running pretty hot. I get 39ma per tube, which is over 13 watts. It should be inside 11-13 watts.  I wonder if the 270 ohm k resistor is still 270 ohms? To lower the current you would want to increase the cathode resistor closer to 300 ohms or reduce the plate voltage. 280-290 ohms in that amp with those tubes would do nicely, I would think.

I read that Bruce says not to go over 300 ohms on the cathode resistor, he recommends reducing the plate voltage to fall within the 11-13 watt range on this 5E3 when inclined to go over the 300 ohms.  What 6V6's are in it. Maybe try a different set of 6V6.

Title: Re: telephone troubleshooting a 5E3 Mission Kit
Post by: Geezer on October 06, 2010, 05:51:54 am

Have you (or your friend) talked directly to Bruce for suggestions/troubleshooting?

sales@missionamps.com

Hmmmm......looks like they make you send it back to them, pay shipping both ways and $75 for them to troubleshoot it.

http://www.missionamps.com/policy.shtml

Title: Re: telephone troubleshooting a 5E3 Mission Kit
Post by: dwp on October 06, 2010, 07:59:53 am

thanks for the replies. I wasn't sure 1 watt would make that much difference, but I guess the Output tubes are hotter than the rectifier. I think they might have even sent him an additional resistor for the cathode. They use giant metal oxide, not the sandbox types. Will pass it on.
Thanks again, dwp

Title: Re: telephone troubleshooting a 5E3 Mission Kit
Post by: tubenit on October 06, 2010, 09:30:50 am

I typically use 330 - 430 ohm range cathode resistors on my 6V6 builds. I have done quite a few builds that way & it's worked out fine for me.

The original Carolina Blues Special has some similarities to a 5E3 power section and that's what I used on all of those.

With respect, Tubenit.

Title: Re: telephone troubleshooting a 5E3 Mission Kit
Post by: phsyconoodler on October 06, 2010, 10:17:20 am

It's also normal to have the transformers run hot on those amps.They are operating in an unvented cabinet that uses the tubes like a convection oven. 5E3's run hot.That's a given.
  How hot are we talking here?Hot enough to burn your hand?Yes? Then it's to hot and you will see stuff melting out of the transformer.Not quite hot enough to burn you but hot enough to be uncomfortable?Then that's normal.
  You are spot on with the bias and do not need to run it cooler.They work best when cooking at 13 watts.
A 5E3 will get hot enough to get the control panel quite hot to touch.Again that's normal for this amp design.
  If you are worried about it,maybe a small fan inside the cabinet will cool things down for you.
either way the amp is likely fine.