Ev'nin, y'all,
I'm new to the forum, and am just about to start my first build, but that rain must fall another day.
Right now I need your collective wisdom for this problem: The other picker in my band (www.burleymaple.com) has this late-model Ampeg combo with a pair of EL84s. The amp is perfect for the parties and lounge gigs we do, but the thing eats power tubes. He's had three sets of EL84s go microphonic in the last year. I think the cab/chassis layout of this particualr amp harbors heat and they just get too warm over the course of a three hour show.
I've got two amps that use 84's, and have never had a problbem, but the designs are much more open.
Has anyone ever tried the slip-on tube heat sinks? Should I rig his amp for a small fan, a la MB?
Thanks,
Rich
Amp eats EL84s
Moderators: pompeiisneaks, Colossal
Amp eats EL84s
Rich Gordon
www.myspace.com/bigboyamplifiers
"The takers get the honey, the givers get the blues." --Robin Trower
www.myspace.com/bigboyamplifiers
"The takers get the honey, the givers get the blues." --Robin Trower
-
VacuumVoodoo
- Posts: 924
- Joined: Fri Feb 17, 2006 6:27 pm
- Location: Goteborg, Sweden
- Contact:
Re: Amp eats EL84s
Some amps with EL84 run them at plate voltages way above max specs. More often than not this condition is also accompanied by too low screengrid resistors and insufficient cooling.
I would suggest checking screengrid resistors and replacing them with new ones in the range 470ohm to 1k.
Installing a small fan to aid cooling airflow is also highly recommended and more efficient than heatsinks.
I would suggest checking screengrid resistors and replacing them with new ones in the range 470ohm to 1k.
Installing a small fan to aid cooling airflow is also highly recommended and more efficient than heatsinks.
Aleksander Niemand
------------------------
Life's a party but you get invited only once...
affiliation:TUBEWONDER AMPS
Zagray!-review
------------------------
Life's a party but you get invited only once...
affiliation:TUBEWONDER AMPS
Zagray!-review
Amp eats EL84s
Thanks, Aleksander,
We may try the fan. I'm loathe to work on his amp because it's a printed circuit board amp, but I may give it a shot. I've worked on a lot of stompboxes, so I know how obnoxious those wee tiny copper traces can be.
I mentioned earlier that I was about to start my first build. I got some really nice iron (PT & OT) from a '61 Harmon Kardon amp, which ran a pair of 6L6s in cathode bias. So I'm gonna build a Matchless lightling with a 6L6 cathode biased output stage. I'm looking forward to the project.
Ciao,
Rich
We may try the fan. I'm loathe to work on his amp because it's a printed circuit board amp, but I may give it a shot. I've worked on a lot of stompboxes, so I know how obnoxious those wee tiny copper traces can be.
I mentioned earlier that I was about to start my first build. I got some really nice iron (PT & OT) from a '61 Harmon Kardon amp, which ran a pair of 6L6s in cathode bias. So I'm gonna build a Matchless lightling with a 6L6 cathode biased output stage. I'm looking forward to the project.
Ciao,
Rich
Rich Gordon
www.myspace.com/bigboyamplifiers
"The takers get the honey, the givers get the blues." --Robin Trower
www.myspace.com/bigboyamplifiers
"The takers get the honey, the givers get the blues." --Robin Trower
Re: Amp eats EL84s
Rich
What brands of EL84s has he been using? I just worked on a Boogie a while back running 400v on the plates and understand that boogie has their El84s special made. Check the b+ and the screen voltage and let us know what he is running.
What brands of EL84s has he been using? I just worked on a Boogie a while back running 400v on the plates and understand that boogie has their El84s special made. Check the b+ and the screen voltage and let us know what he is running.
-
skyboltone
- Posts: 2287
- Joined: Wed May 10, 2006 7:02 pm
- Location: Sparks, NV, where nowhere looks like home.
Re: Amp eats EL84s
I'm just a newby here, but could it be that the cathode/bias resistors have drifted or been designed wrong?. EL84s plates can dissapate only about 12 watts. If they are pushed hard; by improper bias or by hi plate voltage either one, the results are failure. Watch them in the dark while using the amp. If the plates are more than about bing cherry red, chances are they are running too hard.
.02 Your mileage may vary.
.02 Your mileage may vary.
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Re: Amp eats EL84s
You may want to check the screen voltages and resistors on the EL84s. IIRC they are quite sensitive to overvoltage and/or overcurrent.
Amp eats el84s
Dudes (& Dudettes, in case there are any in the forum)
Thanks for all your advice. Jimmy put new bottles in the Ampeg, though he couldn't remember what brand (he's whatcha call technically challenged), but they don't rattle like the last ones. Time will tell on these.
As for my project amp, it's under way. Everything's in the chassis, and I've commenced soldering. I reckon it'll be about another 4 - 6 hours of work (I go kinda slow). As I go along I can already see things I'd like to change on the next one.
You guys are probably way ahead of me on this, but I'm thinking that for my next build I'll use a really big chassis and build one into a combo cab with the chassis that can be opened on top. Basically, and amp purpose built for tweaking and trying out different ideas. "Tweakenstien" sounds like an appropriate name. Angela has a nice universal PT that has monstrous current capacity that could run both sane and insane tube complements. Anybody ever build an amp just for tweaks?
Hasta la pasta,
Rich (aka Gordo)
Thanks for all your advice. Jimmy put new bottles in the Ampeg, though he couldn't remember what brand (he's whatcha call technically challenged), but they don't rattle like the last ones. Time will tell on these.
As for my project amp, it's under way. Everything's in the chassis, and I've commenced soldering. I reckon it'll be about another 4 - 6 hours of work (I go kinda slow). As I go along I can already see things I'd like to change on the next one.
You guys are probably way ahead of me on this, but I'm thinking that for my next build I'll use a really big chassis and build one into a combo cab with the chassis that can be opened on top. Basically, and amp purpose built for tweaking and trying out different ideas. "Tweakenstien" sounds like an appropriate name. Angela has a nice universal PT that has monstrous current capacity that could run both sane and insane tube complements. Anybody ever build an amp just for tweaks?
Hasta la pasta,
Rich (aka Gordo)
Rich Gordon
www.myspace.com/bigboyamplifiers
"The takers get the honey, the givers get the blues." --Robin Trower
www.myspace.com/bigboyamplifiers
"The takers get the honey, the givers get the blues." --Robin Trower
-
skyboltone
- Posts: 2287
- Joined: Wed May 10, 2006 7:02 pm
- Location: Sparks, NV, where nowhere looks like home.
Re: Amp eats EL84s
Sounds like a heck of a good idea. Remember though, mucho lethal power in there. Keep the kiddies away. Maybe a locking pin at each end?
On old transmitters I've worked on, you lift the lid on the PA section and B+ drops out. A switch in series with the standby switch?
How about a little bitty board with snubber caps and a rotary switch to take them in and out? The switch itself would add some unknowns though.
Dogears, what do you think?
Dan
On old transmitters I've worked on, you lift the lid on the PA section and B+ drops out. A switch in series with the standby switch?
How about a little bitty board with snubber caps and a rotary switch to take them in and out? The switch itself would add some unknowns though.
Dogears, what do you think?
Dan
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.