Help with Cornell Plexi 45/50

[lgehrig4]

Anyone familiar with these amps? I just picked up a used one here and fired it up for the first time. It worked but I wanted to check the bias like I do with all used amps I receive.

I connected my Amp Head dual bias probes that I use for every amp (except Louis Electric amps) and noticed that V5 wasn't glowing. I flipped the standby and did not get a reading on either tube so I turned off the amp and popped in a new set of tubes. Still got no reading so I turned off the amp and noticed some smoke.

First I thought the smoke was from the tube, then I turned it over and saw this. I already sent an email to Dennis but I wanted to check here as well. I can replace these myself but I don't know the value/brand of the large resistor.

I'd also like to know how to bias this particular amp. I think it was the probe that shorted something. Any input from the techs here would be helpful. Thanks!

[IMG:800:600]http://i43.photobucket.com/albums/e390/lgehrig4/P1040230_zps069a1cb2.jpg[/img]

[ampgeek]

Looks like smoked screen grid resistors.

Are you running EL34's? If so and the red leads from the burned resistors run out to pin 6 of the power tubes...you definitely have burned screen resistors.

Replace with new, equally rated resistors, whip up a light bulb limiter, remove all the tubes and fire it back up. Report back with what you discover for the next strategic move.

See the sticky-topic in the Technical section for the light bulb limiter gig. Give us a shout if there are any questions.

Are you comfortable poking around in live amps? If not, find a reliable tech in your area for consultation. Let us know where you live, someone right up the street might just be that reliable tech!

Have you used your bias probes with the same tube type in the past? If not, what were you using? That might provide a clue as to whether or not the probe caused a problem.

Good luck!
Dave O.

[ampgeek]

BTW: They look to be 560 ohm resistors. My guess is that they are rated for no less than 3 watts. You might be able to verify that after removal.

Cheers,
Dave O.

[martin manning]

Looks like smoked screen grid resistors.
Are you running EL34's? If so and the red leads from the burned resistors run out to pin 6 of the power tubes...you definitely have burned screen resistors.

The red leads would be going to pin 4 if the screen resistors are on the board instead of on the sockets ;^)

[overtone]

The first resistor in the power supply is seriously fried too and they look like they were vitreous enamelled wirewound Welwyn types.
Not sure what to make of "I'd also like to know how to bias this particular amp." in the OP.

[lgehrig4]

Thanks for the replies! I found out what happened. One of the EL34s had a broken guide so I just stuck it in the socket incorrectly. I usually go by feel unless I know the guide is broken so lesson learned to check from now on.

Never used a limiter but I've read about them when building my Express.

As far as me asking how to bias I meant where to take the measurements from and whether to use mV or mA. My Express turned out great because I followed the guide and info from you guys, but I don't have the knowledge to figure this stuff out on my own. I asked because I first thought that my meter caused this.

So can anyone guess what type of resistor that long fried one was? The others look like Welwyns, that I might have in that value. I can probably find the value of the long one from internet photos but I would like to know the brand and type too.

Thanks again!

[Reeltarded]

Replace the screens with 1k 5W and that WXX series is probably a 10k @ 7W..

Probably

Does it have biasing resistors (1 ohm) on the tube sockets to ground? In that case you measure DC and that will be in the mV range and we use Ohm's Law to simply 'see' it in mA. The probe is black to chassis and the red goes to the other end of that bias resistor from ground. There is a really easy pictorial guide or ten or even a hundred (hah) floating around on Metroamp forum if someone else hasn't thrown one up by... three.. two ... one...

now..

[ampgeek]

Yes...pin 4! My bad.

A bit too much sampling of my Christmas scotch to be providing troubleshooting advice I fear.

Good luck!
Dave O.

[lgehrig4]

Dave, forgot to mention that I live in Trumbull CT. Always good to know of reputable techs in the area. NY/NJ/CT area works for me.

[ampgeek]

I grew up in NH and now live in San Diego..but...I am certain that a few of our fine and esteemed members live right in your area.

Throw up an S.O.S. in the Garage Talk section with a link to this thread if you run into trouble and I am sure that someone will offer up some local help.

Rock on,
Dave O.

[rp]

For noise reasons I thought screen grid resistors were best right on the tube? I don't think I've ever seen them on a board. Plus as they do like to burn up mounting in free air on the socket at least they can't bake something next to them like a cap. Maybe best to mount on socket and jumper the board?

[lgehrig4]

Per Denis Cornell the large resistor is 470 ohm 10W and the smaller ones 560 ohm 7W