TAB 174a 220V
- Thread starter embee66
- Start date
Help Support GroupDIY Audio Forum:
Michael Tibes
Well-known member
You might check your actual mains voltage. Here in Berlin I have very few volts over 220, so nothing to really worry about. The seemingly simplest way to reduce the voltage would be to insert a resistor into the mains line - but the effort to maintain electrical safety might be considerable and I don't know whether this affects the psu source impedance (probably not relevant here with this class a circuit and the psu design anyway?). The obvious, safe, and more expensive solution would be a mains step-down transformer.
Michael
Michael
Your location and local mains voltage?
Harpo
Well-known member
stepdown from what average level ?embee66 said:
From datasheet of a comparable V74, mains current draw will be about 50mA.
Dropping exceeding voltage above 220VAC is (your_mains_voltage - 220V) / 0.05A = x ohms.
This x ohms resistor needs rating of at least (your_mains_voltage - 220V) * 0.05A = x watts. Double rating to play safe.
For your maybe 230V AC mains at Andromeda or whatever galaxy you are (please update your profile with your location data), this 10V to drop resistor would be 200R/1W.
Easiest/cheapest way of changing mains voltage levels is probably finding a large-enough power transformer with a number of primary taps (for different mains voltages), and using it (just the primary) as an auto-transformer.
Jakob E.
Jakob E.
You basically have 3 options, two of which have been mentioned before:
- getting any power xformer with 220 / 230 / 240 taps (or something in the range) on the primary, with minimum power rating of 12VA (240V*0,05A=12W). Like Jacob said, you just use it as an auto-transformer, so the secondary remains open and thus the secondary voltage won't matter.
- getting an ready-to-use product which comprises an auto-transformer in a case, with frontpanel-switchable taps, power sockets and (if you're lucky) a meter for o/p voltage. Those thingies are often called VariAC in English. That's really the easiest method, you insert it between power socket & V74, dial in the right voltage and there you are. They can be had quite cheap second-hand, and you'll be able to power more than one unit if you should score some more.
- the dropping resistor method works quite well for individual units. A friend of mine didn't even bother to rack the thing but got a soap-box, barrier strips, tape, and now the thing is nicely dangling off the rear connector... But I figure you might just as well fit the two R's inside the V74 itself, there's some room right behind the connector, you'd just have to be careful and use tubing for any open leads as not to cause shorts between them or other metal parts.
BTW. It's not critical to match 220V exactly. IIRC, according to a tube related paper by Telefunken, a common miniature tube's life will be extended the most if the filament voltage varies by +5% / -0% (!), so you just want to get the same +5/-0% of 220V on the AC input.
- getting any power xformer with 220 / 230 / 240 taps (or something in the range) on the primary, with minimum power rating of 12VA (240V*0,05A=12W). Like Jacob said, you just use it as an auto-transformer, so the secondary remains open and thus the secondary voltage won't matter.
- getting an ready-to-use product which comprises an auto-transformer in a case, with frontpanel-switchable taps, power sockets and (if you're lucky) a meter for o/p voltage. Those thingies are often called VariAC in English. That's really the easiest method, you insert it between power socket & V74, dial in the right voltage and there you are. They can be had quite cheap second-hand, and you'll be able to power more than one unit if you should score some more.
- the dropping resistor method works quite well for individual units. A friend of mine didn't even bother to rack the thing but got a soap-box, barrier strips, tape, and now the thing is nicely dangling off the rear connector... But I figure you might just as well fit the two R's inside the V74 itself, there's some room right behind the connector, you'd just have to be careful and use tubing for any open leads as not to cause shorts between them or other metal parts.
BTW. It's not critical to match 220V exactly. IIRC, according to a tube related paper by Telefunken, a common miniature tube's life will be extended the most if the filament voltage varies by +5% / -0% (!), so you just want to get the same +5/-0% of 220V on the AC input.
You have 240V-250V, you want 220V.
Resistor is not a great idea because the preamp current is not known and may vary.
If you have a 240V:24V transformer, and wire the secondary in series with the primary, at 244V across the primary you get either 220V or 268V out depending on connection. So you need a voltmeter to be sure it is connected right. And you need to keep this line-voltage finger-safe. And you better fuse the line power just in case something goes wrong.
The 240V:24V transformer only carries 1/10th of the total load (the rest passes-through). Assuming a preamp might take as much as 20 Watts (?), you need at least a 2VA or 24V 0.08A transformer. 24VA 24V 1A transformers are common and cheap. Use anything in this range which is cheap yet reliable and can be mounted and wired safely.
FWIW: I have never heard an amp sound worse on lower voltage. Sometimes they wake-up, as when I ran a 117V mixer on 160V. True, it exploded in 20 minutes, but boy did it sound fine. An 11% over-voltage should not "blow up". The tube heaters will age 4 times faster, but small quality tubes have heater life of 5,000-10,000 hours. If this becomes 1000-2000 hours, assuming non-exotic tubes, you can still get some work done.
Resistor is not a great idea because the preamp current is not known and may vary.
If you have a 240V:24V transformer, and wire the secondary in series with the primary, at 244V across the primary you get either 220V or 268V out depending on connection. So you need a voltmeter to be sure it is connected right. And you need to keep this line-voltage finger-safe. And you better fuse the line power just in case something goes wrong.
The 240V:24V transformer only carries 1/10th of the total load (the rest passes-through). Assuming a preamp might take as much as 20 Watts (?), you need at least a 2VA or 24V 0.08A transformer. 24VA 24V 1A transformers are common and cheap. Use anything in this range which is cheap yet reliable and can be mounted and wired safely.
FWIW: I have never heard an amp sound worse on lower voltage. Sometimes they wake-up, as when I ran a 117V mixer on 160V. True, it exploded in 20 minutes, but boy did it sound fine. An 11% over-voltage should not "blow up". The tube heaters will age 4 times faster, but small quality tubes have heater life of 5,000-10,000 hours. If this becomes 1000-2000 hours, assuming non-exotic tubes, you can still get some work done.
Yes i was thinking that also with the resistor idea, the 240V:24V transformer sounds like the way to go. Thanks for your suggestion.
Out of interest has anyone here modded one of these.... Rumours are that they can be modded to achieve mic pre specs with up to 30dB gain.
"If you take a peek at the schematic you will see a 200K
resistor wired from the top of the I/P trannie's secondary to the control grid
of the first tube. Replacing the 200K r with a piece of wire (or shorting out
the little bugger) will give you more gain and get you closer to that of a real
pre."
Anyone tried this out?
Out of interest has anyone here modded one of these.... Rumours are that they can be modded to achieve mic pre specs with up to 30dB gain.
"If you take a peek at the schematic you will see a 200K
resistor wired from the top of the I/P trannie's secondary to the control grid
of the first tube. Replacing the 200K r with a piece of wire (or shorting out
the little bugger) will give you more gain and get you closer to that of a real
pre."
Anyone tried this out?
