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50's Aussie PA amp head

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Chris C
Aug 03, 2010 3:06 AM GMT

Hi all,

I bought a vintage Australian PA amp head off e-bay last week. It works and sounds great for harp. A bit like yours Drew I guess. I think you may have inspired this. I may have GAS.I have not found the brand or model yet but I think it is anAWA the classic aussie radio manafacturers. I also think itis early50's by some of the components in it. It has 2 x 6V6 and one 6A6 (similar to 6V6) power tubes, an 83V full wave rectifier tube, 2 x 6J7 and a VT99 input (preamp) tubes. It also hasan added on output transformer with two sets of output wires. I found this transformer specs and it handles 4,8,and 16 Ohms. I just dont know which wires are what impedence. Does any one know how to test?

Also looking at a cabinet forit. I have4 vintagePleesey speakers. 2x8' at 4 Ohms and 2x10' at 8 Ohms. I was thinking two cabs. A 2x4' and a 2x8' for bigger gigs. Any suggestions here on series/parrallel setups?

Thanks Chris.



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Aug 03, 2010 3:17 AM GMT
Chris C Replied:

just a correction in my last post.

Also looking at a cabinet forit. I have4 vintagePleesey speakers. 2x8' at 4 Ohms and 2x10' at 8 Ohms. I was thinking two cabs. A 2x8' and a 2x10' for bigger gigs. Any suggestions here on series/parrallel setups?

Thanks Chris.



Aug 05, 2010 10:18 PM GMT
OrthodoxBlues Replied:

I found a link that talks about measuring it, but it says that it is dangerous! I'm thinking that it may be better to take it to an amp tech. Here's the link:

http://sound.westhost.com/impedanc.htm

I also recall that you can use a higher number (let's say 16 ohms) with a lower number amp (let's say 8), but you can fry the amp if you use a 4 ohm speaker with an 8 ohm amp. More information here:

http://wiki.answers.com/Q/Will_16_ohm_speakers_work_with_an_8_ohm_amplifier

Here's a link on series vs. parallel:

http://www.termpro.com/articles/spkrz.html

Please don't quote me on any of this stuff, because I get it confused often, and I don't want to say anything that leads you to fry your amp.

I'm glad that I may have inspired this cool purchase. Many old PA amps are good for harp and guitar. My guitar friend was inspired by my old Dukane PA, and now has an old Thordarson PA head, as well as a Bassman clone in an old PA head box.

I run my Dukane PA into a 16 ohm speaker, so I know that I'm okay as long as the amp's impedance is 16 or less, and I'm fairly sure that the previous owner wired it at 16 ohms.

I'd love to see more photos of it.



Aug 06, 2010 5:18 AM GMT
Maka M Replied:

Chris,

Optimum performance is when the total impedance of the speakers matches the impedance expected by the amp. Speakers of higher total impedance than the amp is expecting are less likely to damage the amp than speakers of lower impedance.

Not sure how DC literate you are, so this is pretty basic. A 'two speaker' example - one 4 Ohm (S4), one 8 Ohm (S8) to make it interesting.

If you wire red (R) from amp toR on S4 then black (B) on S4 to R on S8 and then B on S8 to B on amp, you've wired the speakers in series. Easy arithmetic: Total Resistance Rt=R1 R2. In this case Rt=4 8 = 12 Ohms

If you wire R on amp to R on S4 and on to R on S8, then B on S8 to B on S4 to B on amp you've wired in parallel. Harder arithmetic: 1/Rt = 1/R1 1/R2 = 1/4 1/8 = 3/8 So Rt = 8/3 = 2.67 Ohms

If you had an 8 Ohm external speaker jack on your amp, neither of these setups would be ideal. However, you are less likely to damage anything using the series (12 Ohm) wiring setup than the parallel wiring set up. Better 'two speaker' configurations for an 8 Ohm jack would be two 4 Ohm in series or two 16 Ohm in parallel.

I've got an old tube amp from a 60's electric piano. Mono output at 4 Ohms. I've set it up torun it into two old PA speaker towers, which I could wire in series or parallel. Each tower has six (yep 6!) 4 ohm speakers in it, which I can wire in any sort of series/parallel mix. I've got lots of sheets of paper trying to figure out all the options. I finished up doing each tower as 3 parallels of2 series speakers (giving 2.67 Ohms per tower) with the two towers in series, giving a total of 5.33 Ohms.A bit more than the 4 Ohm jack, but more is better than less and I reckon that's pretty close.

Speakers and mics are relatively easy for amateurs like me to play with. Amps are a bit trickier - capacitors carry high voltage charge even after they're turned off, so be very careful with amps.

Cheers

Maka



Aug 06, 2010 5:19 AM GMT
Maka M Replied:

Just to make the previous post totally indecipherable, its dropped all the 'plus' symbols!

Sorry about that!

Maka



Aug 06, 2010 5:14 PM GMT
OrthodoxBlues Replied:

I'm DC illiterate, but I followed your post, Maka, and it makes sense. Do you know how Chris can tell what wires have what impedance?



Aug 07, 2010 12:53 AM GMT
Chris C Replied:

Thanks Drew and Maka, Thats all good info. I was lucky in my searching. I found a an old catalog which laid out the wire colors and the impedences. Just maid a speaker box for the two 8' speakers. I put them in series to make 8 Ohms. It sounded pretty good while it worked say ten min. It then died. I think I have killed the output Transformer. I had tried anouther speaker and I think the speaker wires did not connect properly. So I think I may have run it without skeakers connected. Sin of all sins.



Aug 09, 2010 5:24 AM GMT
Maka M Replied:

Chris, Its worth the effort to make sure you get the speaker resistance right. After my reply to you I decided to figure out if I could get my two 6 speaker towers to 8 ohms total so I could use them as an extension cab for my tube amp or for my mono PA. I figured out 20 (!) different ways you can wire up 6 speakers (there may be more, but I hope not!). As version 1 in tower 1 could be coupled with version 20 intower 2, I ran the 20 versions again themselves in series in a 20 x 20 matrix - 400 options. And another 400 options for the towers in parallel. 800 ways to wire two 6 speaker towers!

Fortunately there werethree ways in series and one way in parallel to make exactly 8 ohms. Unfortunately it can't be done with both towers in the same configuration, so I'm now in the proces of wiring them so the tower pair in series witll be 8 ohms. Looking forward to hearing how it sounds as an extension cab for my tube amp.

The golden rule is golden for a reason - not having speakers plugged in can do nasty things to OTs and tubes!

Cheers

Maka



Aug 09, 2010 5:32 AM GMT
OrthodoxBlues Replied:

Chris --

Sorry if you fried the OT. For what it is worth, I hooked up an old 5 inch speaker to my Dukane PA amp after I bought it, not knowing anything about ohms, risks, etc. and got totally lucky because it didn't fry the amp. I hope that it is not too expensive to resurrect the amp.



Aug 09, 2010 1:29 PM GMT
walterharp Replied:

i suggest running your set up by the guys at the lone wolf harp amp forum.. .they are pretty savy about this stuff and willing to help.




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