Lee Jackson Metaltronix Amplifiers

[shear625]

Just picked up a VL-1002 that was smoked. The guy said he ran it with no speaker connected and is sorry about that. The power amp board was burnt where a grid resistor exploded and there are signs of arcing between the traces. Smelled kinda funny. The amp came with 4 GT 6550's and no preamp tubes.
I repaired the damaged traces, cleaned off the carbon from the fire, rewired the heaters, removed the humbalance pot, grounded the 6.3 VAC ct. Put in some 12ax7's from the junk box and turned her up. Bias was good @ -55 vdc. and the other voltages seem fine too.
One of the 6550's is bad w/ an open heater but the other 3 are good. So I played around with it for an hour with only the outer two 6550 in. It sounds really good.
I have a set of EL-34's and some new 12ax7's on the way. But in the mean time....

The question is: Can I do any harm running with only 2 power tubes?

Is there a half power mod that someone has tried to turn off two of the power tubes.

Thanks for reading. Also thanks for the videos on youtube. That's the way to do it.

Pauly

[Doink]

Odd that someone that can repair all that other stuff has to ask this question. You're obviously quite skilled in electronics and amp teching.

It should be perfectly fine to run 2 tubes, either the 2 inner slots or the 2 outer slots. You will have to double your impedance load. I don't know what the VL 1002 has for impedance options, but if you select, for example, an 8 ohm output from the amp, you'll have to use a 16 ohm load.

Good luck resurrecting this thing!

[shear625]

Thanks, Odd has always been my MO. I regretted asking the question as soon as I hit submit. There is certainly a wealth if information on this topic available for the reader. e.g. Half the power tubes does not produce half of the perceived 'loudness' of the amp, more like only 1/10. So 'half' power mods aren't recommended by most builders.
My A.D.D. has prevented me from thoroughly understanding impendence and that was my question. Not getting enough Z's (get it?)
The cab I was using is 8 ohm so I'll select the 4 ohm option on the back of the amp until the new tubes arrive.

In the meantime I am loving this thing. Lot of iron and glass for the small price I paid.

Thanks again,
Pauly

[Doink]

Better to ask than to fry your amp.

You also might notice the power tubes "breaking up" a bit earlier and more than with 4. This is usually a good thing.

[rdmahurin]

There are no stupid questions. If it were me and I'd done all that work, I'd just be patient and wait for all of my tubes.