Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: billcreller on August 21, 2010, 08:27:23 pm
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The little Magnatone I fixed up a while back has a problem with the 6N7 type tubes, like a really bad shorts. I had a spare 6N7, and it is shorted also (??) When I turned it on a few days ago, I saw something arcing/sparking in the back, which is mostly covered with a back panel.
Today I decided to check it out a bit, and when I turned it on, no sound. after about 20-30 seconds, the 5Y3 rectifier started to red-plate. When I checked the other tubes, the 6N7 PI tube was really hot, which is why I tested it on my simple emission type tester, to discover it was shorted, and the spare tube was also bad.
Question is, can I wire it up for a different PI twin triode tube? Like a 6SN7, or maybe a 6SL7 etc. The 6N7 is an ancient tube (of course) and the NOS ones I bought are what crapped out. The amp has likely 4 or 5 hours time on it since I put it together.
6N7s are cheap, but don't seem to work for long.
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I bet this PI would work great in that amp. I have a 6SN7 in it now, but the 6SL7 should be fine too and it has the same pinout. I do have a 6SL7 in the preamp hole.
Schematic is at the bottom of this page...
http://home.comcast.net/~seluckey/amps/rocky/rocky.htm
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> The little Magnatone
Which little Maggie? So many amps, so few memory cells left.... (also I was snorting glue tonight.)
> can I wire it up for a different PI twin triode tube?
What is the 6N7 REALLY doing?
A PI stage, dead-short, will burn up its resistors, will NOT strain a 5Y3.
However 6N7 is marketed as "Power Amplifier Twin-Triode". Two triodes (one bottle) will make 10 Watts... a serious hunk of power. To get there it needs a heavy driver and grid transformer. And it runs on the ragged edge of meltdown.
It could be running as a PI too. When 6F6 and 6V6 deposed 6N7 in 10-Watt radios, 6N7 were a glut on the market.
Unlike what you might guess, 6N7 is NOT like 6SN7. Quite a different tube. The direct kin is probably 6J6, but that won't stand the strain of a 10W power stage.
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Which little Maggie?
This one...
http://home.comcast.net/~seluckey/amps/pila/pila.htm
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Thanks sluckey. I had better look into it a bit further as far as what got the 5Y3 so hot. Maybe more than one thing is wrong.
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I've got an old 6N7 floating around here. It came out of a PA Amp, in which I kept the bottom plate for it (and it's buried in my closet) because it had the schemo. I believe it was driving a couple of old 6L6GTs.
The 6L6s are RCA and the 6N7 is Sylvania.
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That PI won't cause smoke or red-plate.
Consider main filter caps, sick 6V6, 6V6 cathode cap, 6V6 grid caps. Get voltages; "normal" and "bad" if possible.
The linked schematic seems to be missing a filter-cap at power point "A". Since hum would be atrocious without it, I assume you have one here.
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You called it PRR. One of the caps in the filter can was doin' it. Replaced the cap can. The rectifier without filters made 400+ VDC, and now node A is 346, node B is 317, and node C is 301. 6V6 plates are 341. Maybe it will live a while. Guess I'll just get a couple 6N7s for now, to get it going again.
Thanks........
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6N7 is running in class A - Pdiss rating in class A is 1.1W max. PI is a paraphase. rewire for 6SC7 you might have a bit better luck. Rp and Rk are probably close enough to work. tweak 4.7 grid resistor to balance.
http://tubedata.milbert.com/sheets/093/6/6N7.pdf
you could 'borrow' from fender 5B3 deluxe
http://www.schematicheaven.com/fenderamps/deluxe_5b3_schem.pdf
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Yeah, I thought about the 6SC7, and I have some of those, since I have an old amp that uses one. Don't believe they are made now, are they? That's back to the NOS stuff that has been handled and tossed around for 60 years I'll give it a try.
Thanks for the responses.....
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Just a reminder, 6SC7's are NOT a direct repacement for 6SL7 or 6SN7. In case anyone was skimming! As an aside, the 6SC7 is my favorite preamp tube tonewise. Finding them is a treat, though. A few years ago, Tubes, Inc was moving from here in Cincinnati to Florida so was selling off some tubes. I got 20 NOS glass Tung Sol 6SC7's for $4 each. I still have about 5 of them left. I'm saving for.... something. That's my all time best tube deal.
Dave
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> One of the caps in the filter can was doin' it.
If it's broke, fix it.
> rewire for 6SC7
If it aint broke, don't fix it.
Is the input sensitivity OK? You say it has tone and power. By my figurings, you need a hot pickup or a strong arm to MONDO OVERDRIVE it, it isn't Hendrix or Young trademark.... but that's not the point, is it? Any normal arm and axe should be able to cruise at 5 Watts and peak at "20W", forcing the ~~14W amp into "flavor territory". I'm thinking of dozens of musical styles where this is just-right.
> Don't believe they are made now, are they? That's back to the NOS stuff that has been handled and tossed around for 60 years
"Not made now" is a good thing. A good day in Moscow 2009 is like a bad day in Owensboro 1962. The 60 year storage does no harm except you should power-up and bake a few days before judging the bias (not relevant) and hiss.
The real problem is "sexy" NOS tubes. The high-buck vendors buy a crate, test, sell the primo examples at inflated price, and re-sell the marginal bottles. By now all the golden-age 6DJ8 and 12AX7 have been picked-over multiple times. May as well buy new-Russian. However the 6N7 has NEVER been "sexy". But it was a fine workhorse in 1933 large-radio and also as utility hammer in radar etc. And IMHO it is a fine gain=20 paraphase.
> 6N7 is running in class A - Pdiss rating in class A is 1.1W max.
Schem says 4.5V on 1.5K cathode resistor, 3mA to two cathodes, equal split, 1.5mA each. (Check: 55K plate resistor has 280V-200V= 80V across it, 80V/55K= 1.45mA, 1.45mA==1.5mA for practical purpose.) The 1.5mA with 195.5V tube-drop is 0.29 Watts per plate. This is far-far below the apparently conservative 1.1W rating.
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My mind (what's left) is half made up. No more 6SC7s here. must have used them or gave them away. Guess I'll get more 6N7s. And the amp will at least remain as designed. It's more of an ornament than anything else, but it should at least work OK if it's gonna take up space.
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Yesterday my younger brother, who forgot more than I'll ever know about this stuff, came over to pick through my tubes, and discovered a 6N7G, the larger one, so the little amp is working again.
The local ham radio guys have a collection of tubes, to which I'm gonna donate a bunch more. Time to clean house again......
Thanks for the responses....