Is this good for anything?

[Stankfut]

Some one gave me a small transformer, marked Atlas/Soundolier HT-87. My research says its an audio matching transformer. It sure looks like a PP OT to me. The secondary side is marked 8 ohms with green, white and brown leads. The primary side is marked in wattage with a corresponding lead for each wattage, but no impedance numbers. It's supposed to be good for 8 watts. Can this be used for a tube amp?

Thanks!

[jojokeo]

No. I heard of some people using them for a small SE amp, but I wouldn't use it myself. They weren't made for SE use and have no "gap" either. If you did, it wouldn't have the freq response nor any clean headroom either. Each primary tap has a different wattage rating. They're made for home/office use for having multiple speakers in different locations away from the main stereo unit/amplifier if I'm thinking what you have correctly?

[HotBluePlates]

"Model HT-87 is designed for use with 70.7-volt lines."

If you are installing speakers in the ceiling of a restaurant, or in school classrooms, then you use 1 of these transformers for each one of your speakers. It is for matching a sound distribution network to a speaker, not matching a tube to a speaker.

[darryl]

PA line transformers certainly can be used as push-pull output transformers in low-powered valve amps.

There is some information here: http://www.ozvalveamps.org/optrans.htm  and there are further links from that article.

Here in Australia, we generally have 100-volt line transformers, whilst in North America 70-volts appears to be the norm.

I've used them in many amplifier builds.

[Stankfut]

I assumed it was unusable, but I had to hope!  Some people do well at scavenging, I really don't have much luck. I'll just stick with purpose built iron

Darryl, how do these things sound? Just abut curious....

[HotBluePlates]

PA line transformers certainly can be used as push-pull output transformers in low-powered valve amps.

...

I've used them in many amplifier builds.

Darryl, could we say that these would be usable, but the low volts ratio of the topic transformer (8.8:1, rather than 20-35:1 of typical guitar OTs) is the deciding factor that makes this one transformer not suitable for a guitar amp?

In other words, the larger 20-25w distro transformers will likely have turns ratios that allow performance for guitar OT duty?

[darryl]

Darryl, how do these things sound? Just abut curious....

They work remarkably well, but I'll leave it at that lest I be accused of blowing my own trumpet. 

Darryl, could we say that these would be usable, but the low volts ratio of the topic transformer (8.8:1, rather than 20-35:1 of typical guitar OTs) is the deciding factor that makes this one transformer not suitable for a guitar amp?

In other words, the larger 20-25w distro transformers will likely have turns ratios that allow performance for guitar OT duty?

I should have looked at the data sheet for that particular transformer before commenting. 

The turns ratio is 8.8:1, so the impedance ratio is 77.5:1. An 8Ω speaker on the 8Ω tap would produce a reflected impedance in the primary of 620Ω plate-to-plate. Even deliberately mismatching the output by using an 8Ω speaker on the 4Ω tap will only produce a primary impedance of 1240Ω - far too low to be usable in a low-wattage valve amp.

The line transformer I often use is the Altronics M1120: http://www.altronics.com.au/index.asp?area=item&id=M1120  Its measured turns ratio is 31.4:1 from primary to 8Ω secondary, producing a primary impedance of approximately 7900Ω plate to plate. This is a very useful value for 6V6 and EL84 push-pull designs.

Apologies for the potentially misleading information in my previous post. 

[HotBluePlates]

Apologies for the potentially misleading information in my previous post.

Not at all!! I appreciate you sharing the info, as I know I learned something new.

So now we can tell Stankfut maybe this one distro transformer isn't useful, but that doesn't preclude salvaging others which give the proper reflected impedance.

Cheers!!

[PRR]

Speaker line transformers over 70.7V are *rare* in US practice, and almost invariably way-too-low-Z for tube amps.

Note that the "best" impedance is not the maximum W tap (8W here). There's usually a 8W 4W 2W 1W sequence. 70.7V at 1 Watt is 5K. The center-tap of this is the *2W* tap (not 4W as you might think). So it's not totally absurd.

However the voltage ratings IS 70.7V rms. The peak voltage is 100V. Assume a good tube may pull 80% of B+ across its load. The maximum B+ is 125V. So this makes sense for 50L6, 35C5, 6Y6, and other LOW voltage tubes. Not our friends the 250V-400V tubes.

100V line distribution exists in US practice but not in our Class 2 wiring methods. So 100V only makes sense for BIG systems, typically taking much more than 8W per speaker. Edcor will wind you 100V speaker line transformers. I don't think they are any cheaper than a comparable plate transformer. They sure do not compete with the ultra-low-bid 8W Speco Atlas and Bogens.

> the larger 20-25w distro transformers will likely have turns ratios

No. The *Voltage* is limited. Higher power means lower impedance.

What you want to find for 6L6 even 6V6 is speaker transformers for 200V or 300V lines. 200V exists in non-US *BIG* system work, 300V is absurd (nobody has THAT much audio power to deliver so far away).

> how do these things sound?

The really cheap ones aim lower than a cheap guitar amplifier. They are pretty notoriously bad for shorting-out the bass; part of why you buy a better fixed-install amplifier is to drive all the bass-loading of hundreds of $4 transformers. The much larger ones are pretty darn good; they have the size, and their impedance is low enough to avoid trouble. Mid-range models like Darryl is using are probably fine for guitar.

[jojokeo]

Speaker line transformers over 70.7V are *rare* in US practice, and almost invariably way-too-low-Z for tube amps.

Note that the "best" impedance is not the maximum W tap (8W here). There's usually a 8W 4W 2W 1W sequence. 70.7V at 1 Watt is 5K. The center-tap of this is the *2W* tap (not 4W as you might think). So it's not totally absurd.

However the voltage ratings IS 70.7V rms. The peak voltage is 100V. Assume a good tube may pull 80% of B+ across its load. The maximum B+ is 125V. So this makes sense for 50L6, 35C5, 6Y6, and other LOW voltage tubes. Not our friends the 250V-400V tubes.

100V line distribution exists in US practice but not in our Class 2 wiring methods. So 100V only makes sense for BIG systems, typically taking much more than 8W per speaker. Edcor will wind you 100V speaker line transformers. I don't think they are any cheaper than a comparable plate transformer. They sure do not compete with the ultra-low-bid 8W Speco Atlas and Bogens.

> the larger 20-25w distro transformers will likely have turns ratios

No. The *Voltage* is limited. Higher power means lower impedance.

What you want to find for 6L6 even 6V6 is speaker transformers for 200V or 300V lines. 200V exists in non-US *BIG* system work, 300V is absurd (nobody has THAT much audio power to deliver so far away).

> how do these things sound?

The really cheap ones aim lower than a cheap guitar amplifier. They are pretty notoriously bad for shorting-out the bass; part of why you buy a better fixed-install amplifier is to drive all the bass-loading of hundreds of $4 transformers. The much larger ones are pretty darn good; they have the size, and their impedance is low enough to avoid trouble. Mid-range models like Darryl is using are probably fine for guitar.

Backing up pretty much what I was trying to say w/out the technical jargon.
Without the airgap, way mismatched impedance, etc. your freq response just isn't good, with any slight power applied you get instant tranny saturation, and all in all end up not very satisfied with the sound, response, playability, your money, time, & efforts are futile and feel all wasted. Just get the appropriate OT and be better off for it in the end. The OT is not something to scrimp a few dollars on.

[printer2]

When we gut areas to rebuild them at work we sometime toss the 8" speakers with matching transformer. Given that they are free, well how could I resist? I took one apart and restacked the laminations so the E's and the I's were all together and put a sheet of paper in between as the air gap. Breadboarded a 6AQ5 in a little Champ type circuit. It wasn't too bad, I mean it was bad, but not too bad. It worked but the bass saturated easily. It probably would have been good for a 1-2W amp, I wanted a little more.

I used one of the transformers for a P-P 6AQ5 amp running around 230V with no problem. I think it put out about 5-6W. It was my first completed amp after getting into tubes. I did run sort of SE in this amp, it is Class A so I turned off one output of the PI. Learned a bit with this little guy.

[jojokeo]

Nice improvising & creativity printer. Does it have one of those 8" speakers internally and an external output on the back? Good thinking inside the box.