ST70 Line Voltage?

Hi everyone, Question regarding line voltage: Have an updated Stereo 70 and would like to know if I need/ should use a step-down transformer (bucking transformer)?

Since wall current is appx 120V and the 70 is 117V(?), is this a concern?

Also have a couple other pieces of 60s equipment which are below 120V. Don't think a variac would do.

Any advice and info on what to use, would be appreciated. Thanks.

I purchased a 5 amp variac per Bob's recommendations for $65.00 (Circuit Specialists - check Amazon) Keep my VTA ST-70 set at 119VAC. Line fluctuations here range 116-126 on any given day. In my experience, when it goes up so does bias. Cheap insurance and also nice to have for bench work. Check Amazon

Thank you for the reply. I wasn't /am not sure using a variac would hold the desired input voltage steady. Now I am not a tech and just trying to understand what I need.

Going by your note, I will look closer at a Variac. In general, was looking to get some direction/ knowledge about transformers. Thanks again, it is appreciated.

A Variac is the best inexpensive alternative. As mentioned, I get 10V drift in my house and just turn on the variac, set it to 119V, turn on the amp and done. If the reading on the variac shifts at all based on incoming house voltage, I turn the dial a bit. Other option are very expensive line conditioners and I could not see investing $4K+ for a decent one.

raudio44 wrote:Thank you for the reply. I wasn't /am not sure using a variac would hold the desired input voltage steady. Now I am not a tech and just trying to understand what I need.

Going by your note, I will look closer at a Variac. In general, was looking to get some direction/ knowledge about transformers. Thanks again, it is appreciated.

My understanding is that a variac is a proportional buck/boost device, not a voltage regulator.  Therefore, it would require monitoring and manual readjustment for a varying line voltage.  Someone please correct me if I'm wrong.

You are correct, if your line voltage goes up so would the preset variac voltage and on mine, the knob on the top would nee to be adjusted slightly. It has a readout for that purpose. In my rig, I have a a Furman SMP LIFT for preamp, streamer, Oppo. TT and RTR which is plugged in the wall and it shows wall voltage. For my amp I have a separate wall outlet with the variac in line so I look at the Furman for the wall voltage and adjust the variac accordingly. On a daily basis I rarely have to adjust it.

Thanks for the replies. I do not want to have something that needs to be monitored/adjusted. Looking for insight regarding the need for reducing wall line voltage to this old amp. Recently I installed a Furman PST-8 mainly for surge protection. Have not checked the voltage at the wall. in an older apartment complex. I was wondering if there could be something to install which could be set at desired input voltage and be set.

You may want to contact Furman, Audioquest, Niagra, PS Audio or check out Music Direct. They offer several solutions for power regulation.

Our line voltage runs 120 > 126. I added a Furman SPR-20i that regulates the voltage to 120vac +/- 1v. It shows the regulated line voltage with a digital display.

Then I got the 5AMP Circuit Specialists Variac w/ digital display. I dial it down to 116vac to run my Dynaco ST70 & FM3. I feel confident that those old pieces of equipment are not getting too much voltage that way, 115 > 117.

I can look and see what's going on easily.

Cheers,
Aguaazul

Appreciate the replies. Need to look into more when I am able. that Circuit Specialists Variac looks interesting.

Don't recall where, (probably have links save somewhere) but there were discussions of bucking transformers etc., which trew me off of thinking about a Variac. Again I was hoping to find something that could just be set it and done. Now I am not a tech so I am just looking for ideas. Thanks all.

I’ve used a variac but found a bucking transformer to be a more elegant solution.  All you need is a 5 or 6 volt transformer rated for the current of the amp.  I went a little higher and built several with 5, 10, and 20 Amp ratings just to be sure.  They’re pretty simple to build.  You just need to check your connections thoroughly.

Here’s the last one I built for my M125 monoblocks.

corndog71 wrote:I’ve used a variac but found a bucking transformer to be a more elegant solution.  All you need is a 5 or 6 volt transformer rated for the current of the amp.  I went a little higher and built several with 5, 10, and 20 Amp ratings just to be sure.  They’re pretty simple to build.  You just need to check your connections thoroughly.

Here’s the last one I built for my M125 monoblocks.

https://www.flickr.com/photos/79446895@N00/51819768697/

Nice - here is mine - I made for my M125s - uses the standard Hammond everyone recommends. Produced 109V and 116V (which I'll use)




Best way to go vs cheap reliable variac as I believe there is no such thing.

Thanks guys, nice works on both those builds. something like that is what I had read about and was inquiring about. thing is, I am not a tech and while maybe could put something together, I wouldn't trust it!

Are there any on the market I could check out? Thanks again.

There is a guy on Ebay with 2 offerings, one has a switch.

Without Switch


With Switch

Not sure what the switch does, either on / off or 3% / 7% is my guess.

Cheers,

Aguaazul

Do you have a local tech that can help? If your voltage is only at 120 - you should be fine w/out one based on my reading of this forum.

I was going to buy the parts and have my tech build it - then after reading up on them - it was so trivial to put together - I just did it. But my tech was totally on board to do it for me.

I built a bucker from scrap pieces of wood. No fuses, no light, no switch, no display, no soldering. 2 parts only, electrical outlet plus the 6.3v CT transformer. Works great, drops my line from 122 to 115-116.

aguaazul wrote:There is a guy on Ebay with 2 offerings, one has a switch.

Without Switch


With Switch

Not sure what the switch does, either on / off or 3% / 7% is my guess.

Cheers,

Aguaazul

It's an On / Off Switch

I built my own but if you look there is an E-bay seller - search-bucking transformer i think there about $85 or so which is not a bad price if you have to by the cord-transformer-enclosure-outlet and it's very well made with a stainless enclosure and a fuse.

Thanks, everyone, again, replies/info much appreciated. Just FYI, checked my voltage at the outlet used for the Equpment and it reads 121.8-9 to 122V so 122V it is.

Never thought of eBay, yeah, they look really good. seems like the one without the switch is sold out. Those seem to be what I was looking for. Thank you for those links.

Don't have a tech and not in a position to try to make my own. maybe should grab one or search around for other options. then again, these are what I need.

Just for conversation, besides the ST70, I have both a tube tuner and tube preamp that are from the 60s. Could one of these handle all 3 units or would I need a separate one for each?

Where are you located? I thought I'd not be able to find a Tech - sure enough I have an awesome one here in Prescott AZ.

Regarding your question about will it be big enough to run all your stuff - I created this thread at AK - and there is info in that thread on how to size the transformer using a kill-o-watt and measuring VA.

https://audiokarma.org/forums/index.php?threads/help-me-with-parts-needed-for-bucking-transformer-my-voltage-is-125-3.990870/

well never looked for a 'general' tech but when looking for someone to do work on some specific gear, really no luck but have more check out. Am in the Chicago by the way. That AK link was interesting thanks although most of it was beyond what I know about.

so, to recap, technically looking for a -5V drop for my ST-70 (seeing about 122V and need 117V). do I really need the transformer?

I'm a little lost here.  We seem to have gotten into the buck-boost vs. variac discussion.  The idea of voltage regulation has fallen by the wayside (rightfully so, if you ask me.  Spending more on your line conditioner than your amp does not make sense in my opinion).  

1)  Is the goal merely limiting the maximum line voltage your ST-70 sees, and accepting a 5%(?) drop from that number acceptable, or is tight voltage regulation necessary?  Maybe there are two answers, one for equipment protection, and one for equipment performance.

2) I have never done this myself, but for those who frequently monitor their line voltage,  what kind of swings do you get?

scrotuss wrote:2) I have never done this myself, but for those who frequently monitor their line voltage,  what kind of swings do you get?

During the summer months the wildest short term swings have been as great as 8V in only a few hours, which, while it's unusual when compared to the 9 year period I've been in this house, when it happens - lots of the wild swings happen during a multi-week period of time.  Max voltage at my house has been as high as 130V, minimum has been as low as 118V.

Since about a year ago the voltage has been much more stable. Most of the time during the non-summer months the incoming voltage is 119V, and can be a volt lower or up to 5 volts higher, but lately has been in a pretty tight range of only a few volts. Last summer the median voltage was around 124V. and varied about the same amount. That was the first summer with a smaller range of voltage swings, so maybe something got fixed/updated.

I've got two Panamax M5400-PM so it's easy to see the voltage, and it's really easy to hear the relays switching durning the summer when they can be pretty active. And no, no amps are plugged into them, the SS amps go straight to the wall and tube amps to the Variac.

I would guess where you live makes a huge difference - Mine is always at or very very close to 122V. With my bucking trans Iam at 115V. Measuring my 6.3v filament voltage for my tubes is right at  6.3v exactly telling me the power transformer in my ST70 was designed for 115V. Was running 6.7-6.8 with my 122V line. I believe it's worth it to drop it for what is was designed for and have the correct Voltage for your tubes!

Check for past threads this is frequently debated and covered. Variac is best per Bob. Yes it drops voltage proportionately and your output voltage changes with variations in your line voltage. But if your goal is to get it off the now-typical 123-126v down to 117 or 119v, set it and forget it. Any variation will be minor and again…your goal is to get it off that hot line and down to the 120 range. So what if it goes up or down a volt or two?  There are many purists who buy oxygenated speaker cables and precious metal connectors and you can spend thousands on new flabbergasters or do-dads… IF having the same exact precise voltage to your amp under any and all conditions is a huge thing to you. This is a really simple issue.  You will find a $70 Variac is a total solution but you can build stuff and buy expensive stuff if you’d like.