Phonoland

Was wondering if there are any tricks for using standard 8 ohm remote speakers with a seeberg shfa5 amp? There are low and high impedance output windings in power amp output transformer. Low output impedance is 16 ohms. I'm afraid to connect two 8 ohm speakers in series to the low impendance output strip for fear of lousing up the impedance match of internal jukebox speakers. Sounds like with connection to the high impedance a and b terminals, I will need a 70 volt matching transformer to connect to 8 ohms. Certainly don't want to damage the old girl.

If i recall correctly, those tube stereo amps have settings for the internal speakers of 1-4-8-16.

If you set the internal speakers to 4 watt you can connect one 8 ohm speaker to each channel also at the 4 watt tap and they'll each draw 8 watts.=16W total each channel

If you decide to connect two 8 ohm spks in series for each channel you would put both the machine speakers and external on the 8 ohm taps.

What is really involved in these calculations is the voltage an amp delivers into a given impedance. The wattage ratings are just a shorthand to simplify things. In theory any given tap on a transformer should be able to support the full output wattage of the amp.

Rob/NYC

Thanks again Rob. The box is in a fairly small recroom and limiting the internal speakers a bit shouldn't be a problem. I think your solution is much better than messing around with 70 volt speakers. They probably have pretty poor frequency response anyway running the audio through cheap transformers. I saw the power connections listed on the low impedance strip, but wasn't certain how that worked. I did think i read somewhere that the low impedance output was 16 ohms and thats why I was considering two 8 ohm speakers in series for an impedance match. Again thanks for the input.

I just remembered I already have earbud speakers. My 16 watts might be pretty tapped out. Might have to be 70 volt speakers after all.

Joe, going to 70volt won't actually increase available power from the amp. I rebuilt a "Q" amp for use in a 222 back in 1989. The 6973 outputs give approx 10-11 watts clean both channels driven into a 16-0hm/ch load. I believe that the amp is rated at 20W/ch? and that would be typical for the loose way amps were rated before 1975.

There is no great sin in putting a slight overload on the amp, loading 3-5 watts above rating shouldn't be a problem.

As for transformers, Radio Shack sold a very good 10watt that was up 1 db @50Hz and down 1-1/2 db @ 14kHz efficiency was about 89% and it cost about $7. I use about 50+ of them in locations. Unfortunately Radio Shack is becoming a 'housewife's' electronics store and I haven't seen them lately.

Most of the others I have tested (with the exception of original Seeburg transformers which are down 6-7db @ 12khz) have been very good as well so do not be afraid to use a 70V tran if that is more convenient.

Rob

That was great info Rob. I've gotten quite the crash course in jukebox over the last few weeks. You seem very Knowledgeable on this subject. How long have you been working on these machines?

One last thought on this subject. If we do accidently overload the amp running too many speakers, what is the chances of doing physical damage to the amp? Would we experience weak or distorted music long before the output transformer or something else is at risk?

Ron and Rob Merry Christmas and happy new year. Thanks for all the help in 2011