Vacuum Tube Nostalgia - or a glowing, overheated nightmare?

[Redwolf]

I have a question for all of you active and former electronics people. Especially those of you who have worked with Vacuum Tubes.

Basically, I listened to Izotope Ozone for Winamp, and liked the sound of the tube emulator. I talked to my dad, and he says that tube amps are hot as hell and have to 'warm up', but the audio quality is incredible. He and two of his brothers (now deceased, unfortunately ) used to be big audiophiles when they were young, and he has fond memories of Heathkit tube amps and home-made speaker cabinents. So now that got me convinced.

I wanna try!

I'm fairly experienced with electronics (good at sodering, better since the time I butchered that Voodoo 3, which wasn't really my sodering skills but more my skills of obvious observation ; have built kits before ; never played with AC tho, since I prefer to keep my hair the way it is thank you very much!). I also believe I'm saftey concious enough to get to the next level, playing-*errhem* working with AC current. To do what you ask? Why to run a homemade tube amp of course!

So I gets to looking around for some plans, kits, anything. The kits are WAYYYY out of my price range, even though they are VERY nice. The plans are completely free though, which is very nice.

I find this guy's website, Bob, and an amp that I think I've settled on:

http://www.geocities.com/bobdanielak/darling.html

I plan to build the original one, the one with the blue-dotted schematic. (I would use the 'latest-version' one, but it seems to have a few glaring errors in it, while the other one seems okay). It uses 2 types of tubes, the 1626 and the 8532. I go to another site:

http://www.vacuumtubesinc.com/

which has some great prices on old 'new' tubes. The 1626 is $4, the 8532 is $5. So I start doing research on vacuum tubes to learn what I can. I started 2 days ago, FYI. I also looked up the two tubes in question. The 1626 is an RCA triode:

http://hereford.ampr.org/cgi-bin/tube?tube=1626

The 8532, however, is not on that website, nor any other tube databases or tube cross-references. So if anyone can dish up information on that model, I'd greatly appreciate it!

So here's my MAIN question. What do you think of the amp design I chose? Is there anything special I should know before starting? (Like any specific wire-gauge I need to use, problems you see with the plan, saftey hazards not normally associated with high-voltage electronics?)

FYI, I plan to plug the input into my sound card's line out (that is okay, right?). I will use it to power it's own speakers, although if I can do it safely I want to use it between the sound card and my Sony STR-D365 (100 watt) amp, since that has the good speakers on it and I want it for the 'tube sound' instead of actual amplification really.

I only plan to make 2 modifications. Don't worry, they're simple. First, I plan to replace the power switch with either a DPDT switch (connected to both power wires) for safety or a relay, so I don't have to be dealing with AC directly (it would be a relay designed for DC on the coil and AC on the switch of course). The second is that the input would be converted to a mini-phono jack (the kind used for small headphones and sound cards) and the output would be in multiple jacks for a variety of speakers (only using a few jacks at once though, I may use a rotary switch to keep me from using more than one jack type at once).

I've worked with tubes for many years. My purposes are RF amplification, not audio.

The design is rather simplistic, so no major problems are foreseen. However, beware discharge from tubes, as the power range could kill in you in certain situations. These are not low-voltage DC digital parts! While your amp is a different situation, RF amps can require several thousands of volts from the transformer.

8532 can be used a switching tube to build a keying circuit for an RF amp. It could be listed as a 6J4 or 6J4WA at some sites. Shouldn't cost you more than $10, usually around $5 from surplus dealers. A 7137 or 7245 can be substituted, but you may have to rewire the schematics, as I am unware if pin differences exist between the models.

http://hereford.ampr.org/cgi-bin/tube?tube=6J4

Ham Tubes has it listed, though I have never dealt with this vendor:

http://www.hamtubes.com/tubes/list5.htm

Also here, again never dealt with:

http://www.elexs.com/5Rcvg.htm

When all else fails, RadioShack offers tubes via its express parts service.

[Redwolf]

Thank you SO much Rob!

I understand the dangers involved ( from a) the fact that the thing is AC, b) the fact that it uses over 100 volts! and c) the fact this isn't exactly battery powered ). I won't be making on-the-fly connections to it anytime soon, and will definately have it encased so that fingers and dog tounges don't find their way into the circut. I'll definately not neglect to ground the thing either.

Oh, and I don't think I'm going to trust Radioshack I'll probably get the rest of the parts shipped from Mouser or someone like that. The place I'm looking at buying from has some pretty good shipping costs ($6 for up to 6 tubes, $.25 a tube after thatm $.50 for big ones) and has prices similar to the sites Rob posted.

Oh, yet another question. I'e heard of Microwave repairmen using screwdrivers with plastic handles to 'short out' the ratehr large caps in a microwave so that they don't hurt themselves. Doesn't sound like a good idea, but I've never worked with caps that large. Should I do this with the large cap in this project, or not risk it?

[John Prophet]

There are other ways to discharge a capacitor such as "bleeder resistors" which "bleed" a little of the charge away after the circuit is shut down.

You could make a discarger device with a resistor that would discharge the cap fairly rapidly without shorting it with a screwdriver...of course the resistor would have to be pretty burly or it would just burn thru itself...so it would have to be a large wattage resistor.

I am not sure whether discharging a cap with a screwdriver is in any way supposed to be harmful to the cap...I think it is more a matter of ruining screwdrivers by arc welding the tips etc. After all, TV repairmen short the charge in the tube with a cap and its way more than a radio circuit has...at least voltage wise.

The reason that is given for tubes sounding better is that when they distort they produce "even order" harmonics.....as ooposed to "odd order" harmonics as produced by solid state stuff. Evidently the even ordered harmonics blend more naturally and sound more pleasant.

One of the main uses for tubes as far as audio is for high powered guitar amps. I have a 100 watt Carvin tube amp for my electric guitar. Most pros wouldnt be caught dead using a transistor amp for guitar and they could identify transistor vs tube in a double blind test any day of the week.


Anyway, yes be careful working with tubes. DONT EVER put both hands inside the equipment you are working on....this includes leaning on the case of the equipment or resting the loose hand on the case or on any ground.....when I was in the USAF they used to make fun of me saying that I always tried to find a ground with my loose hand, lol. The old advice is to keep one hand in your pocket and work with the other hand to prevent making a circuit thru your body.

We used to like watching the new guys work on the old tube radio transcievers...they had like 900 volts plate voltage, which was in a cage so you couldnt really get to it..but they also had 400v dc and other voltages just sitting exposed on all these tubes sockets and loose components in a hand wired chassis....so it was only a matter of time before the new guys got "tickled", hehe.

I would be in trouble now if I worked on a tube radio because working on comps allows you to have many bad habits and do many "no-no's" without getting shocked....you can put your hand basically anywhere in a comp and never pay for it....but take those habits to a tube radio...hehe....make sure your insurance is paid up!

Have fun, JP

[John Prophet]

As far as hurting yourself by shorting a cap?? I wouldnt sweat that at all....get you a big fat screwdriver..put on a rubber glove or leather work glove..and go for it, lol.

JP

[Redwolf]

Hmm...I might get a resister like what you talk about. Mouser sells Xicon/Arcol power resisters. I guess 15 watts would be enough (they go from 10-300, but I'm not paying $15 for a resistor).

And yes, my health insurance is paid Although Life Isurance is another story I'll be sure to keep one hand in the pocket, and I'll probablly work on the thing outside, with the case hooked to a metal pole in the ground. That way, it won't be ME who get's to ground the stupid thing.

-edit- I wasn't thinking about hurting myself really. And I have plenty of gloves that I can use, although I don't think I'll have to use the 'big gloves' (old pair of gloves used by the CP&L guy who hooked up our house).

[Redwolf]

This is going to sound smart, but yes, I forgot. For the transformer, it doesn't matter if the amperage is over what the circut requires, as long as the voltage is correct and the amperage is not under the requirment, right?

Correct. Voltage levels must be accurate, but any amperage above the desired rating is perfectly fine.

[John Prophet]

Check on the resistor thing cuz I just more or less threw that out there.....obviously a little ohms law work would help.....in other words a low valued resistor would bleed the circuit faster but also would allow more current flow thru the resistor therefore it would need to be a big wattage....in general "bleeder" resistors are large values....they stay hooked up all the time and since they have such large values they dont really affect a working circuit much until the power is off..then that tiny current flow thru that huge resistor "bleeds" or you could say "drains" off the charge.

Basically a screwdriver is like a shorted resistor.....whereas a megaohmmed resistor is almost like an open circuit.....so somewhere in between there is a resistor value that would drain the cap without blowing the resistor and without taking all day....or maybe you would use some sort of resistor to drain the charge quickly to like half of what it normally is..THEN pop it with the screwdriver, lol.

I am sure that google would turn up many many discussions and how to's on this very subject.

And I think I have seen discharging tools with some sort of resistor built in....then again in the service we had a tool that was basically some sort of ceramic handle with a metal hook shape on the end that shorted caps quite nicely.

JP

If you are working on any tubes with open connectors, then you may need to discharge the tube array as well (though the charge is generally stored by the caps). Bleeder resistors are great for delicate work, but I just use a long handled screwdrivers for most RF equipment, as the devices I generally work on are built for massive abuse.