Enclosure and Power Supply for Tube Mic Pres,etc

[Scott S.]

I thought I would share a good idea (well, I think so anyway) for a cheap and easy platform for tube mic pres, etc.

Ebay is flooded with old tube based HP test gear with nice ventilated aluminum enclosures and well designed power supplies.

One of the best platforms is the HP 400D AC voltmeter

-250 VDC tube regulated power supply
-DC heater supply good for at least 1 amp
-High quality potted power trans

Its pretty simple to strip out the meter circuit, make a new front faceplate without the meter cutout (but with the jacks and controls of your new circuit), and then build the new circuit where the meter circuit was on new terminal strips. You might even be able to use the meter with the new circuit.

A search of completed auction show they are selling for $10 or less.

I recently built a bass guitar preamp/DI box using this concept. I actually used an similar HP oscillator that had freq tuning cap issues, but have since picked up a couple of 400Ds for similar missions.

I can take some pics if anyone is interested, also have the 400D schem.

 

[Consul]

I wouldn't mind seeing some pictures, please. I'm still agonizing over what enclosure types and styles I intend to use for my DIY gear, when I can get to building it again.

Your idea sounds like fun.

 

[MikoKensington]

Wow, thanks for the tip. I'm sure we'd love to see what you've cooked up.

 

[NewYorkDave]

As a person with a collection of HP 400Ds, who has repaired and uses these meters on a regular basis, I don't like this idea at all. Why butcher classic test gear when you can buy a variety of blank chassis, power transformers, etc. at reasonable prices from a number of manufacturers?

It's one thing if the meter has a lot of "issues" (and 400Ds do have some power supply issues, by the way, so you may be in for a surprise if you think you're getting a ready-to-go PSU for your preamp project). But to hack apart a working 400D, or one that only needs minor attention, is a shame, IMHO.

It reminds me of a few years ago when audiophiles were cannibalizing old Tek scopes for their "Bugle Boy" 6DJ8s.

I know it seems I'm being a grouch. But while a couple of guys hacking up a couple of 400Ds doesn't really mean anything in the grand scheme of things, these things have a way of snowballing, thanks to the Internet. Before you know it, every kid on the TapeOp message board will be looking for a 400D to hack and suddenly they won't be available for $10 anymore... And the world will be a little poorer of relics from the golden age of tubed test equipment.

 

[bradzatitagain]

[quote author="NewYorkDave"]As a person with a collection of HP 400Ds, who has repaired and uses these meters on a regular basis, I don't like this idea at all. Why butcher classic test gear when you can buy a variety of blank chassis, power transformers, etc. at reasonable prices from a number of manufacturers?

It's one thing if the meter has a lot of "issues" (and 400Ds do have some power supply issues, by the way, so you may be in for a surprise if you think you're getting a ready-to-go PSU for your preamp project). But to hack apart a working 400D, or one that only needs minor attention, is a shame, IMHO.

It reminds me of a few years ago when audiophiles were cannibalizing old Tek scopes for their "Bugle Boy" 6DJ8s.

I know it seems I'm being a grouch. But while a couple of guys hacking up a couple of 400Ds doesn't really mean anything in the grand scheme of things, these things have a way of snowballing, thanks to the Internet. Before you know it, every kid on the TapeOp message board will be looking for a 400D to hack and suddenly they won't be available for $10 anymore... And the world will be a little poorer of relics from the golden age of tubed test equipment.[/quote]

With you on this one Dave. Go for Eico crap, Eico 460 'scope has a monster of a power transformer in it, other doo-dads have enough current, filament and otherwise, to run two or four tubes instead of chopping HD gear that can run a bunch of tubes. Most Eico will give you the oppotunity to use 5Y3-GT rectifier, or other more saggy 5 filament volts rectifiers; Eico 460 can run a cathode ray tube, btw, so don't fiddle with that particular lead unless you like kissing 1000volts before you walk through the door of the next world. That or chop up your grandma's old tube tv, I have a Magnavox power transformer that's about the size of a friggin' grapefruit I'm trying to figure out what to use for. :wink: Try fixin the HD and using it, duh.

 

[NewYorkDave]

I have repair notes and tips for anyone who actually wants to fix up a 400D. If you wanna hack it, you're on your own. :green:

 

[gyraf]

Dave,

I totally agree.

I have a HP '400 that I use on a daily basis..

http://gyraf.dk/tmp/Main.jpg

It's there - just behind one of the lamps..

Jakob E.

 

[bradzatitagain]

[quote author="NewYorkDave"]I have repair notes and tips for anyone who actually wants to fix up a 400D. If you wanna hack it, you're on your own. :green:[/quote]

If they aren't copyright protected :roll: I'd love to compare your D notes to my uncompiled H notes. Got fresh tubes & caps for my power supply but haven't dug into it just yet--old Navy gear, noisy switches and not calibrated but does pretty good as is compared to readings from new DMM. Factory rack mount version! You're right: tons of projects, little tiny piles of time.

 

[Arrigotti]

Coming from a relative newbie with only a cheap RS DMM, what does this thing do? If I can pick one up cheap and make it useful, that would be really cool. Then you have everyone thinking you're some kind of brainiac when they see all this stuff in your garage.

 

[idylldon]

Heh! I have a 400H and if anyone wants to share info on it, I'd love it. While I use it, I have a feeling it would benefit from some in-the-know servicing. At the most sensitive levels, the meter suffers from random bouncing.

Cheers,
--
Don

 

[NewYorkDave]

What does this thing do?

The short 'n sweet answer is that it's a precision AC VTVM, a great tool for measuring signal levels, gain and loss, etc.

http://members.cox.net/dalehcook/radios/pages/hp-vtvm.shtml

 

[Scott S.]

I use classic tube test gear myself, a 200CD, Tek 561A scope, etc, so I know what you are saying, a lot of the tube test gear works great and is just as usable as newer stuff. I definitely prefer using a tube scope on tube electronics, I don't trust SS standing up to transients or other "events". For ac/dc volts, the ground isolated DVM is definitely nice, although I use a VTVM (RCA Voltohmyst) about half the time and the DVM the rest. For AC measurement I also use my 334A Distortion Analyzer (OK OK it is SS), that is a great piece of gear to have.

Think about the flip side, if a non electronics person lists one for say $10 and it doesn't sell (there are some of those in the completed auctions), its possible they just throw it away, which is much worse than turning it into a usable piece of audio gear.

How about this, anyone who is really interested can PM me asking for pics, in order to prevent a stampede.

 

[bradzatitagain]

[quote author="Arrigotti"]Then you have everyone thinking you're some kind of brainiac when they see all this stuff in your garage.[/quote]

Exactly!

400 measures AC peak to peak, aka RMS, good for tube gear at the outouts. 410 measures AC/dc and ohms, a multimeter, popular & has an expensive tube in the ac probe, 412 measures +/- DC good for poking around in a tube circuit between ins and outs. Radio guys would have needed this stuff when ham was huge, & well, they still need it even tho ham ain't so huge. Other "kewl" HP tube stuff is Disortion Analyzer, which has a built in meter circuit, and Wave Analyzer, both Baby Huey sized rack pieces about 8U high. Other handy HP things to have are pen and paper X-Y recorders (not tube things tho.)

You can certainly pick one of these things up cheap, as you say, but after recapping, rebuilding and retubing I think they qualify as an investment. Don't bother turning them on until you test the tubes, which would require, um, a tube tester (do we want to go *there*?) You should have a local electronics instrumentation calibrator shop if you live in a big or biggish city to tune 'em up. I think they can be somewhat easily calibrated using comparitive readings from a fresh and accurate DMM but I drop mine off. I also think it's fair to say these units used as test equipment are as fast/precise as the advances in circuitry allowed them to be in their time. The 400's and the 200 signal generators were produced well past the transistor take over however, so don't compare them to old Heathkit stuff that looks the same. YMMV

 

[MikoKensington]

I picked up a 400C and a 200AB in a crinkle finish louvered rack for $50 at the flea market.

What are the differences between all the suffixes?

 

[bradzatitagain]

[quote author="NewYorkDave"]

What does this thing do?

The short 'n sweet answer is that it's a precision AC VTVM, a great tool for measuring signal levels, gain and loss, etc.

http://members.cox.net/dalehcook/radios/pages/hp-vtvm.shtml[/quote]

 

[bradzatitagain]

[quote author="bradzatitagain"][quote author="NewYorkDave"]

What does this thing do?

The short 'n sweet answer is that it's a precision AC VTVM, a great tool for measuring signal levels, gain and loss, etc.

http://members.cox.net/dalehcook/radios/pages/hp-vtvm.shtml[/quote][/quote]

bama.sbc.edu

 

[mcs]

[quote author="MikoKensington"]I picked up a 400C and a 200AB in a crinkle finish louvered rack for $50 at the flea market.

What are the differences between all the suffixes?[/quote]
As far as I remember the 200AB is the low frequency version. The 200CD (that I have) goes up to about 600kHz, and the AB is the audio version.

Best regards,

Mikkel C. Simonsen

 

[mcs]

[quote author="NewYorkDave"]I know it seems I'm being a grouch.[/quote]
No, not at all. Don't destroy good and useful test equipment just because it's cheap...

Best regards,

Mikkel C. Simonsen