Help needed in restoring a 73' SA-212

[[PJS]]

http://www.facebook.com/album.php?aid=7 ... 8333&saved

Go check out some pictures at my Facebook -profile. I didn't have a better place for them so that'll have to do.

[[PJS]]

The project is moving along nicely...

I've got all the wood materials and I've ordered everything else exept the grill cloth and the speakers. Being a huge David Gilmour fan I'm propably going to get the Weber FC12 Fane Crescendo clones as the speakers. But should I buy the ceramic or the AlNiCo versions? I did some studying in the internet and didn't find a clear answer on whitch magnets were preferrable for Hiwatts and why.

Opinions...

[Dr.HI-TONE]

ceramic.

[[PJS]]

ceramic.

But why? What is it that makes the ceramic magnets so mutch better for the Hiwatt amps.

[Dr.HI-TONE]

lack of compression found with alnico speakers

[[PJS]]

Hi, heres some more silly questions:

I've decided to get a pair of the Ceramic Weber FC12 speakers but how about the impedance of the speakers? The external speaker output is 8 Ohms. But how about the internal speaker output? Should I get 4, 8 or 16 ohm speakers and how should I connect them for achieving the most authentic outcome...

Thanks allready.

[OldSchoolDave]

I've decided to get a pair of the Ceramic Weber FC12 speakers but how about the impedance of the speakers?

Not silly at all. Good choice of speaker, by the way. I was finally able to hear a quad of those at the Vintage Hiwatt Convention.

Order 16 ohm speakers and wire them in parallel. That's the way the originals were configured. That will also allow you to run an external 8 ohm load, if desired, with more-or-less equal power to both.

Dave

[[PJS]]

Just finished building the cab. It turned out to be pretty good and I think it's not that far from the original. A few things have been done a litlle different (propably) since I did'nt have any drawings or a real one to look at but who cares as long as it's good.

I Didn't do any of the roundings yet and I'm still looking for the cover cloth/grill cloth, but there's little work to be done there. The next thing I'm goin to do is to send the amp to my amp-tech for a complete check; A cap-job, a new power chord, possibly new tube sockets and a bypass for the useles and dangerous polarity switch/death cap. Once all that's done I'll hit the studio with my band and capture that sweet vintage hiwatt tone on our fourthcoming EP....

I'll post some pics and some "drawings" from my Hylight-style Combo-enclosure soon.

[[PJS]]

Hi!

So I connected the two 16 ohm speakers parallel and thus the Impedance should be 8 Ohms, right? Is it safe to run the amp on 8 ohms load or should the Impedance be 16 Ohms? The speakers are connected with a speaker cable from the external speaker output. The Amp seems to work just fine and the sound is so good it almost makes me wanna cry, but I'm still a bit scared about using it extensively because I'm not sure the impedance is right...

And should there be n Impedance selector switch in teh back panel or is that something that was only implemented in the later/other models?

[OldSchoolDave]

So I connected the two 16 ohm speakers parallel and thus the Impedance should be 8 Ohms, right? ... The speakers are connected with a speaker cable from the external speaker output.
...
And should there be n Impedance selector switch in teh back panel or is that something that was only implemented in the later/other models?

Two 16 ohm speakers in parallel equal an 8 ohm total load. There should be a pair of leads (on my SA212, inside a black jacket) coming from the output transformer. That's where the internal speakers should be connected. The external jack is left for any external cabinet you'd like to run. I don't know if running your amp with no load connected to the "internal" leads will cause any harm. Perhaps mark can provide some guidance there.

Lots of combos were built without an impedance switch, which is what makes me think the internal versus external leads may be connected to different taps.

Vinyde and grill cloth can be found at VHR.

Dave

[[PJS]]

Thanks!

I allready installed the vinyl and the grill cloth, a local DIY Amp/Electronics store had them in store. The Schematics (http://www.drtube.com/schematics/hiwatt/hwpwr50w.gif) say something about 8 and 16 Ohms, but I'm not sure if I'm reading them correctly since I don't know the first thing about electronics (that's what the amp techs are for, right)

[OldSchoolDave]

I'm no expert at electronic schematics, either. But, the reference to "Shorting jacks" implies to me that the internal speakers are fed from the 8 ohm tap and the external jack - clearly labeled 16 ohms on my SA212 - disconnects the internals while connecting to the 16 ohm tap.

I haven't tried mine to prove that theory, but it makes sense considering the lack of an impedance selector.

Are the internal leads present on your SA212 (indicated by red arrows below)?



Dave

[[PJS]]

No. The previous owner had removed them.

So it would seem, that I'm currently running the amp on an 8 Ohm Load (two 16 Ohm speakers wired in parallel) instead of 16 Ohms. Isnt that supposed to be a pretty bad thing. The External speaker output has a text: 16 (or possibly 15, hard to make out) Ohms written with a marker, but since the machine's been modded I'm not sure if thats correct.

I just realised, that I've been running the amp succesfully in to a 8 Ohm 2x12 Cab and it hasnt blown up so far so I think the current setup should be fairly safe. But would it be better if I connected the speakers in to a 16 Ohm load anyway?

[[PJS]]

And to futher add my confusion, the 1974 Hylight catalogue says the external speaker output is 8 Ohms. And if that's correct changing the wiring in to 16 ohms would blow the transformers where as running the 16 ohm output to a 8 Ohm speaker load would just blow the power tubes (If I understood it right). So it's kinda risky to try the 16 Ohm setup unless I can be absolutely certain about it.

[mhuss]

The topic of transformer output impedance is a wonderfully complex thing.

One thing to remember is the "8 ohm" tap, "16 ohm" tap, etc., all assume that they are the *only* tap connected. In other words, it doesn't work as designed if you hook up an 8 ohm load to the 8 ohm tap at the same time you hook up a 16 ohm load to the 16 ohm tap!

You should have your two paralleled speakers connected to the 8 ohm tap, and nothing else should be connected (including any shorting jacks).

Just to set your mind at ease, hooking up an 8-ohm load to the 16 ohm tap (or vice-versa) is sub-optimal, but not horribly dangerous. You will get less power output with a mismatch. In fact, many of the mid-period Partridge 50 watt OTs only had two taps, and the 4 and 8 ohm pins on the output impedance selector were tied together and connected to the same tap. See an example here: http://mhuss.com/Hiwatt/images/DR504_topR.jpg.

--mark