Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: Steve_P on February 12, 2014, 11:07:38 pm
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Anything I should look for or brands that I should stay away from?
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> Need a VTVM
For what?
Most "VTVM" work can be done with DMMs, in less bench-space.
Given the vast number out there, if you truly need a VTVM, I would steer to HeathKit. The manuals can usually be found, the inside construction is User-Friendly, and they don't get the big bucks of H-P.
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> Need a VTVM
For what?
Most "VTVM" work can be done with DMMs, in less bench-space.
Given the vast number out there, if you truly need a VTVM, I would steer to HeathKit. The manuals can usually be found, the inside construction is User-Friendly, and they don't get the big bucks of H-P.
Thanks PRR!
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I have 4 Heathkit IM-18's. (Also 2 Eico's that need a rebuild for a long time now.) I love them but they are a bit quirky.
Maybe I'm a bit paranoid but I like to monitor several voltages at the same time in an amp under test. Analog meter readouts are best for this - You can see at a glance if the needle is where it should be. Digital readouts are visual overload for this purpose. However a modern SS analog meter should work fine.
Also I have 3 Fluke 8000A's -- SS DMM's, and a cheap Radio Shack handheld DMM. The Flukes are great too, but I think I use the Radio Shack DMM the most!
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You could always make one. Without looking there might be circuits in RDH4.
There are circuits for RCA's VoltOhmyst in the back of RCA tube manuals.
That said, you do not want to build one yourself. Just like building amps, it always costs more to build yourself than to buy.
And since all VTVM's have been replaced by digital meters with many more functions, you can buy a complete, working VTVM for the cost of a single parted-out meter movement. Range and function switches also tend to be specialized, multi-wafer units.
Looking back through my records, I was a motivated buyer when I picked up an RCA WV-77E for ~$24. I've averaged ~$53 for the HP 412A, though I have picked up one for as little as $14.60.
Radiomuseum says the WV-77E (http://www.radiomuseum.org/r/rca_voltohmist_wv_77.html) retailed for $46.95 in ~1959 (which one calculator says would be ~$375 today), while the HP 412A is listed in the 1960 HP catalog at $375 ($2951 in today's money). Given how relatively cheap these items are today, and the fact they're easier to fix for the home hobbyist than a DMM, I have a fascination with acquiring them. The downside is the bulk of most units, though the RCA WV-77E is a pretty small full-function meter.
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You should be aware of the input impedance issue, which has appeared before on this Forum. It's an artifact of the good ole days. :icon_biggrin:
VTVM's have a hi input impedance, usually 10M, up to 15M or 20M. That's good, because it doesn't load down the device under test. The first digital meters had a fault: a lo input impedance, which is common to transistors. That's bad because it does load down the device under test. I.e., the load introduced by the SS meter draws down the voltage being measured in the device under test -- so you get a false voltage reading! This problem with SS meters has long since been corrected, but clunkers are still produced. So it's best to check the spec sheet of the DMM to make sure that it's input impedance is at least 10M.
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VTVM's have a hi input impedance, usually 10M, up to 15M or 20M. ...
This is one reason I like the 412A (even though it only does d.c. volts/amps & resistance): the input impedance for ranges 300mV and up is 200MΩ.
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You'd have to be certifiable to want to build a VTVM from anything other than a Heathkit and I suspect you can't even buy such a thing any more. The number of complex switching circuits and the need for 1% resistors would kill you on cost of procurement. There are some, like HBP who like VTVMs and they have definite use. But at the same time, with the general age they are, they can get drifty. I appreciate the desire for an analog meter for monitoring circuit action in use. In those situations, you are monitoring volts in 90+% of cases. You don't measure ohms under power. My suggestion would to get some NOS panel meters for $10 each and use those....or you get some cheapo DVMs for under $20 and have something a lot more useful, that's a digital device, of course.
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Just like building amps, it always costs more to build yourself than to buy.
Say what?
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Assume you have to buy the cabinet, chassis, speaker & electronic components retail. Then if you assemble an amp yourself, it will cost you more than a similarly-powered amp at a music store, much less one that is second-hand.
Where the cost equation flips is when you are building a copy of a collectible vintage amp; then you may be able to build for much less than you can buy (assuming you could find the collectible amp in the first place).
Think from the standpoint of someone new to this hobby, not a person that's acquired mounds of bits & pieces over the years. I still recall that it cost me about $700-800 all told to build my first 5E3 amp, and I could have bought similar-powered (but not vintage or replica) for less money.
And old VTVMs are, by and large, not collected like guitars & amps and tend to have cheap prices.
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Assume you have to buy the cabinet, chassis, speaker & electronic components retail. Then if you assemble an amp yourself, it will cost you more than a similarly-powered amp at a music store
I... find that hard to believe.. especially in America where your stuff always looks so cheap... I guess your commercial amps must be really cheap too! :icon_biggrin:
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Maybe the difference is that we're not paying VAT on every little thing we buy.
It still costs more to build than buy; I bought a Valve Jr in a pawn shop not terribly below retail price because it still cost less than building a Champ-style amp.
And don't forget that when I say "retail" I don't mean the fake "list price" or "MSRP". Those are always falsely inflated to give the dealer room to "discount" and give the buyer the impression of a good deal.
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Just like building amps, it always costs more to build yourself than to buy.
Say what?
Yes, a new enclosure alone would likely cost more than an old VTVM for rebuild.
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You could always make one. Without looking there might be circuits in RDH4.
There are circuits for RCA's VoltOhmyst in the back of RCA tube manuals.
That said, you do not want to build one yourself. Just like building amps, it always costs more to build yourself than to buy.
And since all VTVM's have been replaced by digital meters with many more functions, you can buy a complete, working VTVM for the cost of a single parted-out meter movement. Range and function switches also tend to be specialized, multi-wafer units.
Looking back through my records, I was a motivated buyer when I picked up an RCA WV-77E for ~$24. I've averaged ~$53 for the HP 412A, though I have picked up one for as little as $14.60.
Radiomuseum says the WV-77E (http://www.radiomuseum.org/r/rca_voltohmist_wv_77.html) retailed for $46.95 in ~1959 (which one calculator says would be ~$375 today), while the HP 412A is listed in the 1960 HP catalog at $375 ($2951 in today's money). Given how relatively cheap these items are today, and the fact they're easier to fix for the home hobbyist than a DMM, I have a fascination with acquiring them. The downside is the bulk of most units, though the RCA WV-77E is a pretty small full-function meter.
Not true! I can build a Champ at a fraction of the cost to buy one! Seen them in horrible condition at 2k as recent as Christmas! Many people have built VTVMs and they dont cost too much to build. However, RCA Voltohmysts can be had for cheap so buy one on ebay!
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Sure, that's why I pointed out the scenario changes once you're building a copy of a collectible vintage amp.
Bottom-line, it's not worth the hassle of building a VTVM when so many old full-function meters can be had for a song. And some are cheap even though they have technical capabilities that would be near-impossible for a home-builder to duplicate (again, the HP models come to mind here).
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> Need a VTVM
For what?
Most "VTVM" work can be done with DMMs, in less bench-space.
Given the vast number out there, if you truly need a VTVM, I would steer to HeathKit. The manuals can usually be found, the inside construction is User-Friendly, and they don't get the big bucks of H-P.
And DMM is safer to use as it is not connected to 120 volts wall outlet and..... more usefull.
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> You could always make one.
Here's one. (FET-VM, just as good, safer.)
http://www.americanradiohistory.com/Electronics%20_Illustrated_Master_Page.htm (http://www.americanradiohistory.com/Electronics%20_Illustrated_Master_Page.htm)
January 1970, 10MB PDF file, page 45/47.
Switching is just 2P5T (could be 1P5T plus a SPST power switch, but 2P6T are common) and a 3P3T (will be a 3P4T or 4P3T).
The meter movement is NOT something you can buy naked today. Hunt hardware stores for a needle-meter VOM which says "20K/V" in small print, or has a "50uA" scale. This is also sweet because you get the voltage marks, tho if there isn't a 0-5 range you may have to mark-up or do mental math.
The FETs specified are long obsolete. Buy 10 of any good JFET, preferably "Amplifier" not "Switch". Socket Q1 Q2. Find a pair which zero-up with pot R6 near center.
1N34 have become collectible. They can be most-any small diodes with little error.
All part values can be +/-20% standard values--- you will have to calibrate all the ranges anyway. Oh, R1-R5 should be the "right" values at 2% tolerance or better.
Power this with a BATTERY. Not your pedal-power-pack. One 9V battery will run it for years at several hours a week, and totally-floating is MUCH safer than any connection to wall ground.
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I restored an old one from the 1950s.
The tube tester guys have had most of the ones like mine for the meter as it fits there cabinets.
The thing they do best is to show voltages that vary too much for a DVM to respond and the meter is usually less damped than on an ordinary moving coil meter as the tube circuit absorbs big surges making them quick.
Mine is like an AVO 7 with an input resistance of 11 meg ohms.
You just need to enjoy restoring vintage equipment in general and not just vintage amps and then it costs very little to do one.
Time and patients is the name of the game though.
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NICE! restoration
we're not paying VAT
yet.....
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I did the restoration back in 2012 and wrote it up.
http://www.forum.radios-tv.co.uk/viewtopic.php?t=3685&p=54384#p38618 (http://www.forum.radios-tv.co.uk/viewtopic.php?t=3685&p=54384#p38618)
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:thumbsup: :thumbsup:
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Great job on the AVO, a true classic.
I restored this 1947 Philco VTVM in 2011:
http://www.frontiernet.net/~jff/Philco7001VTVM/Restoring_a_1947_Philco_7001_VTVM.html (http://www.frontiernet.net/~jff/Philco7001VTVM/Restoring_a_1947_Philco_7001_VTVM.html)
It has a chrome and enameled steel gravure front panel like some of the great Hickok tube testers. 1947 list price was $105.
In 2012, I bought an HP-400D AC VTVM (~1960) for $20 (craigslist), and restored it. It was 90% replacing molded paper coupling capacitors - they all leaked. Strangely, the electrolytics were just fine. I did reform the electrolytics in the power supply first using an external variac / rectifier / filter cap at low current.
The cool thing is the HP has a signal output right before the meter rectifier, so you can listen to signals scaled by the VTVM's input amps/attenuators into 150 mV. Unlike the Philco job, I didn't take pictures of everything I did.
Jon
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> cool thing is the HP has a signal output right before the meter rectifier
Copied from the Boonton ACVM (may not have originated there either).
A milliVolt AC meter is mostly an AC amplifier. Perhaps a pretty good one. The meter network is fairly low impedance. The amplifier can drive other external loads. Cost may be as cheap as one binding-post. It IS a handy side feature.
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Yeah, the HP 400 is basically a wideband (4 MHz) amp. I recall some discussion on GroupDIY using one as a microphone preamp.
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I've got a HP 400E that's in excellent condition. It was used once a year by FAA until April 2013. I doubt I'll ever turn it on again.
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The "E" model is solid state (not that there's anything wrong with that), and has a 10+ MHz bandwidth.
I can see they day I when I will divest myself of all my HP boatanchors....
My younger daughter (grown) is a fine picker and shredder who loves classic guitars and amps, but she hasn't picked up a soldering iron yet.
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Ah, yes. I see now. My eyes glazed over and I totally missed the model "D". :embarrassed: