Variac advice

[John]

Hi fellas, I'm looking at 2 different variacs on ebay; 1 AC, and 1 DC (thought it would be handy someday for trouble shooting heater hum) (?) Anyway, do any of you have experience with the Elenco products. I'm comfortable buying "as is" on ebay, so far haven't been burned.

Anyway, question is, are these okay or are they not? Obviously, I don't use this stuff much so I wouldn't ever wear it out.   Thanks for any advice!

*edit* Sorry, I'm too dumb to shorten the links...
http://www.ebay.com/itm/Elenco-Precision-XP-650-DC-Power-Supply-0-20-and-0-40-variable-volts-/300631592444?_trksid=p4340.m1374&_trkparms=algo%3DPI.WATCH%26its%3DC%252BS%26itu%3DUCC%26otn%3D15%26ps%3D63%26clkid%3D4730884632533840091#ht_500wt_1156

http://www.ebay.com/itm/Elenco-Precision-XP-810-variable-AC-power-supply-600-Watts-leakage-meter-/260911516306?_trksid=p5197.m185&_trkparms=algo%3DSIC.NPJS%26itu%3DI%252BUA%26otn%3D12%26pmod%3D280785389797%26po%3DLVI%26ps%3D63%26clkid%3D4730843069871267047#ht_500wt_1287

[kagliostro]

Both aren't Variac

A Variac is a particular type of transformer, something that can seem as a BIG rehostat with wire windings

http://en.wikipedia.org/wiki/Variac#Variable_autotransformers



the first link is a DC Power Supply, the quality of instruments don't look to be very high so may be about the circuit

the second link will give you 0-150v AC, that seems too low for the use in breadbording amps

However I don't know that brand and can't say anything for sure about quality

Kagliostro

[John]

and 1 DC (thought it would be handy someday for trouble shooting heater hum)

Yep, I did notice the DC.

On the AC product, I thought the point was to vary the AC line voltage coming from the wall? I might be confused though,happens a lot.  Thanks for the reply!

[kagliostro]

I'm just a stupid

you live in a country with 110/120v line

so you can use a real Variac (if the max voltage is 150v) to vary the input of the amp

but I'm not sure that the circuit that is in that Elenco Precision AC Power Supply is adequate

also, if you reduce the voltage in input of an amp, the B+ will decrease in all the amp and ALSO the filament voltage will decrease and this isn't desired

with a variac + a HV transformer (say a 110v primary & 300v secondary) you can have a regoulated supply for breadboarding amps


Kagliostro

[John]

Ahh, now I understand what you meant. I always forget the Europe is 220!

[stingray_65]

The variable AC supply seems pretty rugged.

it is also isolated, a Variac is not.

looks to be an expensive item when new.

http://www.esssales.com/elenco/power-supplies.html

it features variable current regulation also, would be a fancy way to not use a lightbulb current limiter.

another neat feature is the soldering iron varible supply.

If I did not have all those things it could replace (variac, isolation xformer, variable temp soldering iron) I would be interested in it myself.

I'm not 100% but I seem to remember Elenco stuff was kits, like heathkit.

[stratele52]

This  AC variac is not very powerfull like high quality one. But it ( write ) is able to deliver 3 amps need for most of the amp. I don't know the quality of this product.

AC variac is usefull to have.

The DC power supply is not very usefull for amp working, Ok if you want to replace battery on effect pedals on test or someting like that .

[HotBluePlates]

... 1 DC (thought it would be handy someday for trouble shooting heater hum) ...

The supplies you linked are worthwhile.

Many guitar amps (maybe most?) will only need 3A or less from the AC line. I'd personally look for something that had a 5A capacity, if I could find it cheaply, to be able to plug in most amps without paying a lot of attention to the AC line draw (look at the fuse rating if not printed on the amp somewhere).

The DC supply you linked is really for building/testing transistor circuits. However, it looks like it can supply 3A of d.c. (maybe more with the voltage turned down, maybe not due to heating of the control element). That's okay for a bunch of preamp tubes, but a pair of EL34's will draw 3A by themselves. It could be problematic to supply the preamp and power amp separately in an amplifier, given the existing filament wiring. You'll probably want a unit with a higher current rating, even if it only goes up to 15v (for 12.6v-wired 12A_7's).

[John]

Thanks for all the replies, much appreciated.

After I thought about it more, I decided that I didn't "need" the DC supply. After all, why would my heaters ever hum?    If they do it just means they're not wired right.

I am still considering the AC supply though, it looks like it'd be really useful down the line.

I really have to stop browsing ebay!

[HotBluePlates]

After I thought about it more, I decided that I didn't "need" the DC supply. After all, why would my heaters ever hum?    If they do it just means they're not wired right.

Or you have a tube with significant heater-to-cathode leakage, or you built a design that tends to exceed the heater-to-cathode insulation rating, or you want to rule out a 60Hz hum as coming from the heater circuit/wiring (it could come from a failing cap in a fixed-bias supply), or you want to verify the routing of your heater wiring.

So there are uses for it. But a lot of these things are "nice to have" rather than necessities. But if you get them cheap enough, then...

[John]

Hmmm.... maybe I do "need" that. And you're right; if it's cheap enough it'd sure be a nice-to-have item.

Building amps is like doing your own home improvements. You don't save any money, but you end up with lots of cool stuff!

[HotBluePlates]

Building amps is like doing your own home improvements. You don't save any money, but you end up with lots of cool stuff!

Until you already have the tools on hand. Then (just as if you're equipped like a pro contractor), it can be much cheaper to build. That's dependent on how pricey the amp you're copying is.

I'm working up an amp now that would cost me $3k (modern clone) to $5k+ (original) to buy. Tools aren't a factor, and I have a very large percentage of the parts already on hand. Sure I paid for those parts at some point, but it's fun fiction to think they're "free" and this build might only cost me $4-500.

[John]

 I've got somewhere between $700 and $1000 in my HoSo, but that also includes a lot of spare stuff I bought along the way. I never really kept track of what went in only that. I honestly don't think I could have bought an amp that sounds as good for that, and for sure not one that's as easy to work on and tweak.

Never would have attempted it without this place though!