Help, My H.H. Scott tube amp died

I suppose this happens on rare occasions, but I have to say in my 50+ years using hi-fi and guitar amps, I have never once had that happen unless there was some other problem. I have tube amps and solid state receivers that are 50 years old that have never had the fuse fail. Not saying it doesn't happen, and it's certainly an easy and obvious thing to try before tearing the amp apart, but I personally wouldn't put too much hope in that one.

 

Rare. Worth an easy check with a meter or known good fuse. I have seen it more, perhaps, working in cable TV head ends and on the amps in the cable system. Any thin metal that warms and cools can fail, just like bending a coat hanger back and forth.

 

Because this amp was originally designed to be stacked, it has no power switch. I keep it on a switched outlet on my Brickwall power cube so that I don't have to unplug it every time. Once I got home, I did double check and it wasn't an outlet problem. Dropped by Radio Shack this morning and there was a "back in 15 minutes" sign in the window.

 

seed_drill,

Try the fuse replacement first, if that or the rectifier tube does not get it going, try calling Terry DeWick and see what he says. He knows HH Scott amplifiers well and is helpful and friendly.

 

Hmmmm.

(He's upset because the Hold button is not the traditional Fluke functionality but is "like those cheapo one hung low meters". Classic.)


I haven't bought a Fluke for many years because every one I have ever bought still works perfectly except my 867 which is an oddball unit.

It appears the several models they made in the $60-90 region are now obsolete and the cheapest new one is $120-140.

Here is some useful commentary:

http://tangentsoft.net/elec/meters.html

Having said all that: Fluke is the safe choice and the way I would go if I had to buy another one.

 

I have seen it several times. Ohm it out, but even before you take it out ohm across the power cord terminals.

 

Craig at NOS Valves is the man you want to fix it!!!!

 

Well, I do know that the fuse is bad, but I don't know why. I won't have time to pop in a new one to see what happens until tomorrow.

 

Check that you had a 2.5A slo-blo in there, should have a spring and a metal strip. If someone put a fast blo in there (single wire only inside), that could be the entire problem. 2.5A/250V is what you want. Hope it is just an old fuse that gave out per my posts above. Good luck.

 

Hehehee...

A good friend of mine in Jamestown, R.I.....Jane Scott, just happens to be H.H. Scott's daughter.

I've stayed with her several times, the entire 2005 summer in fact, at her modest estate.



http://www.jamestownpress.com/news/2008-09-04/news/016.html

http://www.wetherledge.com/


'spose she's got some vintage Scott electronics ?

 

It had a 3 amp fuse in it, not a 2.5. Anyway, I replaced it, and it turned on, and blew again about 5 seconds later. A bright flash went through the rectifier tube before it died. Thoughts? (I'll be taking it to Ken's Hi Fi in Hendersonville, regardless).

 

The rectifiers are notorious for doing what your amp's just did.

 

Agreed. Get a new rectifier and you should be good. They do often have that 4th of July failure mode since the voltage and current are substantial.

 

In my days on the bench at this point we would have pulled the rectifier tube out, replaced the fuse and started it up again just to check that it kept running and to check the unregulated voltages. If all was OK (as it probably will be) just replacing the tube should do the trick.

 

I'll agree with the others that it's your rectifier tube.

 

And it usually wouldn't take down anything else, but R. Totale gives good advice in firing it up without the rectifier and see if the fuse holds. Then re-tube.
Wish I still had one of those!

 

Well, almost a year to the day after the last "flare up" of my amp, it blew a fuse again today. Weird. The fuse was black, which is a little alarming.

 

Something is drawing excess current. Thankfully, the fuse is doing its job - sadly, not always the case in hi-fi. Has the power supply been renewed? (Electrolytics) Avoid ersatz 5AR4 they are not worth the trouble.

 

I believe it is the original power supply, but I don't remember what was done when I first bought the amp and sent it out west to make it operational. Years ago we had to replace the canister capacitors with cardboard ones because the voltage was slightly too high and we never could figure out why. But it had run rather problem free for years prior to last years fuse blowing. Seems like I swapped out a Chery for a Sovtek 5AR4 last year. For some unknown reason, my tube tester has no setting for the 5AR4.

 

As many of us have already recommended to you, that you need to send your amp to Craig at NOSVALVES.COM and he'll give it the new blood that it needs and you won't have to worry that you might burn your house down, using it!

 

Rectifiers are so simple an equivalent test can easily be subbed by using just about any other rectifier test setting (mind your filament voltage, of course) Try 5V4

Rectifiers can't really be effectively tested the way other tubes are though, not for emissions, and in any event the modern manufacture 5AR4 are junk and have no place in a fine amplifier, in my experience. They simply can't handle the current and voltage requirements like the old timers.

The NOS or good-used aren't any more expensive after adding up the price of all the junk tubes that went Roman candle. A shorted tube may take out much more expensive components as well.

The best way to tell rectifier health is measure (carefully!) B+ under load, in operation. The schematic will list acceptable voltages, a worn out, tired rectifier will not hit those numbers.

Also consider your line voltage from the mains. Tube gear in general does not play well with voltages above ~ 120+, and this is not uncommon. It also helps to bias outputs a little on the cool side. Have the amp checked out and the power supply renewed. Avoid cheap tubes, make sure the voltages are in the ballpark.

 

You're right about recent production Shuguang and Sovtek 5AR4's, they won't survive long in many vintage amps. The original tubes were already run very hard, but it didn't matter, as they were cheap to replace. There are some supposedly better quality 5AR4's being produced, which may take the extra current at turn-on without arcing. Or, simply have a couple of silicon diodes placed in series with the rectifier plates, and probably never have to replace the 5AR4 again.

jeff

 

New GZ34s won't come close to lasting as long as a 40,000 hour rated Mullard from the UK, or an Amperex from Holland or Belgium. Your amp was designed to use one of those British/Dutch/Belgian made tubes - it's worth paying more upfront and being assured your amp has a rectifier tube that's up to the task.

 

Actually, line voltage into my house is barely within tolerance (on the low side), unless Duke Energy rectified it (heh-heh).