power resister problem

pappadoyle

Member
2012-12-05 4:58 am
Hi, well this is my first post. I am a begginer and I am sorry if this sounds trivial. here goes. I have a heath a9c. I just bought it on ebay. I was trying to check it out. I installed just the 5v4g and turned on the power. the tube started to glow I was thinking great. but just as I reached to turn it off the tube flashed and burned out. as I was trying to check for the problem. I noticed that the power resister on the 5v4g tube was labelled 10a 400w. now I have a schematic that says the power resistor in that position should be 350 0hm 10 watt . I think that someone must have placed the wrong resister. also if I replace this resister. I have found 350 ohm 10 watt resisters in ceramic and also 390 or 400 ohm 10 watt resisters in wire wound. could I get a reccomenddation on what type of resister would be best. and thoughts on my analysis of the wrong resister. thanks from a newbeee
 
Maybe the resistor is 400 ohms and 10 watts, though the a and w make no sense. We can be pretty sure it's not 400 watts! Measure it with a DVM. If it's 400 ohms that's not far enough away from 350 ohms to matter much.

Something else is wrong, very likely bad capacitors. You need to measure every part to know what's going on. Old filter caps can be bad and if it has any of those black tubular paper/oil caps, they're notoriously prone to failure. Carbon composition resistors also rise in value, crack or go open. Get a DVM and measure the resistance of all the resistors. You should also get a rising reading across caps as they charge up. You need to know more and do more, but at least that's a start.

Be sure to discharge the caps with an insulated resistor, say 100 ohms, before touching anything!
 

pappadoyle

Member
2012-12-05 4:58 am
ok thanks. I guess I was jumping to conclusions there. the a and the w threw me off. thanks for the reply. I was just checking to see if the power switch worked. I heard somewhere. you couldd check with juyst the rectifier. I am glad I didn't blow all my tubes. then again maybe the reason it blew was I didn't have all the tubes in. anyway now I have to wait till I get another rectifier. I am new at troubleshooting. I bought a harmon kardon commander 35. a couple months ago. and I got lucky it worked from the git gfo. this one is gonna end up teaching me a few things I guess.
 
ok thanks. I guess I was jumping to conclusions there. the a and the w threw me off. thanks for the reply. I was just checking to see if the power switch worked. I heard somewhere. you couldd check with juyst the rectifier. I am glad I didn't blow all my tubes. then again maybe the reason it blew was I didn't have all the tubes in. anyway now I have to wait till I get another rectifier. I am new at troubleshooting. I bought a harmon kardon commander 35. a couple months ago. and I got lucky it worked from the git gfo. this one is gonna end up teaching me a few things I guess.

If you plan on doing any troubleshooting I would highly recommend building a dim bulb tester (DBT) to safely power up your amps (especially tube amps).
Just be aware tube amps have high voltages (300-400V), so just be careful in what your touching. My suggestion is not touch anything with the power on.
Working on live circuits inside old equipment like that should only be done by someone who is qualified and safely understands the potential risks.....if you do then that's fine..
Always a good idea to hook up a meter to your test points with power off first and then switch it on. Make your measurements and then power off. Move on to the next test points and repeat the process.
If you are new to this stuff and not familiar with high voltage circuits like old tube amps it's a good idea to just not do it as there are all sorts of shock hazards that can be fatal in an instant. Old electronics is usually not grounded.
I always hate to think of some kid sitting in his bedroom dead on the floor....maybe I'm being paranoid but it still bothers me on forums like this where you have no idea who your talking to!...:wave2s:
 

pappadoyle

Member
2012-12-05 4:58 am
thanks for the reply . I understand what you are talking about. I am a newbee but not a young one. I am 67 and I am a retired truck driver of all things,ha ha. anyway. I have done a lot of reading on the subject and I know a little. I know a little knowledge can get you in a lot of trouble. but I am trying to learn a little. after I retired I started building guitars I have built over a hundred in the last couple of years. now I am starting to get into amplifiers. and I hope I can pick up on how to troubleshoot the problems. I am aware of the high voltages and the safety measures, like discharging the capacitors. my problem is I dont know much about troubleshooting electronics. I have a little problem reading schematics. but the big problem is that looking at the wiring and trying to figure out where things are is kinda hard because of the way things are installed doesn,t look like the schematic. I know that they make pictures of the layout sometimes but its still confusing. anyway thanks for the help I just ordered the manual for the heathkit. cost me 20 bucks but I guess it will make it easier to work on.
 
Hehe pappadoyle....good to know...I'm 60 so a little below you there but have years of working on electronics. Sounds like you are wise and careful.....

I attached a schematic of the amp below.
It has voltages shown in various locations....which will help you find bad components with a meter and a few readings. They are the ones in "ovals" on the drawing.
Just trace out the wiring using the schematic you should be able to check each component for wear and tear. Even check that someone before you hasn't modified it or hodge-podged something together.
If there are original capacitors in there, you will need to desolder the wires and measure them out of the circuit with a capacitance meter to make sure they are not bad or for the few there are....just replace them with new ones.
They dry out over the years and just become a problem. Resistors generally last a lot longer but they too over time become brittle and will crack and start falling apart. Especially those wirewound types that take a lot of hot and cold cycles.
Those are cheap to replace too...
Then of course there are the tube sockets that get dirty and corroded with age...and then the tubes which can be microphonic or just dead. Those are probably the most expensive part to deal with..
Anyway looks like a fun project and I hope that helps you on your way to guitar tone nirvanna!...good luck :)
 

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hey thanks for the schematic. I will get busy checking out the caps . the amp looks in fairly good shape. there is one big capacitor that is about three inches long and an inch in diameter. kinda looks like it has stains on it. is that leaking. anyway. I appreciate the help I will get back on here if I run into more problems. have a good one.