using and biasing 7408 Tubes (Mil Spec 6V6)

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withmittens

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I have some Harmon Kardon 7408 tubes. These, in normal amps, are safe to use as a sub for 6V6 tubes as they are simply the mil spec version of that tube.

can anyone suggest what voltage these should run at and how they might be biased?

Thanks
 
biased them at around 20 mA each. I like the sound on my Fenderesque mod. I keep watching for a flare up but they seem steady.

20mA seems low and safe based on the
12Watt/plate voltage = current in Amp equation

is this safe? should I bias lower?
 
12/500 = 24 is max...

I would be biasing no higher than 18...but maybe your RM50 PV is not at 500V ... mine is sometimes even higher than that
 
withmittens said:
thanks for the response. do you know how I can test the plate voltage....easily?

thanks

Dunno, I just probed the back of the socket last time I had the amp out of it's shell, so I could check tubes loaded and unloaded PV / current.

I forget which pins it is, but you can probe unloaded in top of socket too....just be careful.

If you want to probe them loaded, need some kind of adapter....and at that point, might as well buy a bias rite or summat.
 
The MOD50 manual says use JJ 6V6 only. Can those tubes handle the plate voltage? It also says bias between 18-22, IIRC.
 
apparently old tubes of Military spec can handle high voltage, built better than new ones. I asked a tube guy and he feels that a lot of nos tube will handle higher voltages.

keeping my eye on them, really just wanted to see if I liked the sound. I'll go JJ if I want to stay with a 6v6
 
I figured an old MIL SPEC tube would do the job, but figured it was worth mentioning. I'd start with a low bias and see how it goes. Good luck.
 
they are steady and even though these tubes are most likely 40 to 50 years old they bias within ..1 mA of each other. kind of shows how well tubes were made back then. I guess it was worth it to build them well since tubes were ubiquitous

at any rate, using my Twinface in Deluxe flavor I get the distinct spongy attack with easy overdrive. very sweet sounding. push the volume and I get that great snap along with the other good stuff. not unlike an old MusicMan amp I used to play

I wonder how long they'll last
 
I get asked about old tubes enough in the shop that it's worth posting about it here. NOS tubes are a gamble, as are any tubes. Just because they were made 40 years ago doesn't mean they are any better than modern tubes as a rule of thumb. Nor does it make them worth a mint.

Tubes have always been manufactured to a set of design specs. This info is found in the data sheets. For example, here is a sheet for an old Tung Sol 6V6GT: http://www.tungsol.com/tungsol/specs/6v6gt-tung-sol.pdf

You can find everything you need to know from operating voltages to warm up time (note it's only 11 seconds, not the poor info you see regurgitated on forums like 5-30 minutes). Occasionally, if there was a specific set of demands for a particular design they will show up as well. Notice that there is typically no published spec for lifespan. That's because real world applications run tubes under different operation conditions. Some circuits are far kinder to tubes while others pummel them within an inch of their life (a Deluxe Reverb is an example for 6V6's, as would be a Class A SE amp).

For an example of a special spec, check out the 7320 (long-life EL84 variant) data sheet: https://frank.pocnet.net/sheets/128/e/E84L.pdf

Note the section on the top right about reliability (measured in how many of 1000 tubes fail over 1000 hours) and long-life (10,000 hours average life expected over 100 tubes). Typically "mil-spec" tubes were outliers on a bell curve that exceeded the normal production specs and met a standard set by the military. They were produced in the same batches as all the other tubes, they were just the top performers of the batch.

As for NOS tubes today, this market has now been combed over for 30 years. A lot of the best remaining stock was sold first at this point and a lot of what I see being sold recently tends to be leftovers. My observation based on the thousands of tubes I seen through the shop is that if an old tube is still functioning today, it was probably somewhere in the center of the bell curve of reliability. Do they sound better? Sometimes. Probably not quite 50% of the time even. Are they worth a mint that folks pay on the open market? Well things are only worth what the market will bear according to economics.
 
Thanks for the info, Rob.

I don't buy NOS tubes. Ii pick them up here and there in old equipment or yard sales, estate sales, etc.....it's just a fun hobby that sometimes works out
 
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