Speaker Impedance

[PhilR]

I've just been given a Philips bi ampli radio model B5Z97A. It has inputs for phono and tape and outputs for 800ohm speakers. I would like to attach my mission 8ohm speakers to this.

What problems will I have? Do I need 2 output transformers and if so will a torridal transformer do the job.
Can I attach a CD player to this thru the tape input.

The output valves are 2 x El84's.

Thanks in advance.

[dhaen]

Are they EL84's or EL86's?
Yes a transformer is required. As long as there's no trace of DC, the a toroid is a possibility.
Another idea is to use audio transformers designed for 100V line. They are cheap, and sound quite good. RS components do a range. I think you'll have to pay about UK£6.50 each for them. Cheaper than buying equivalent toroids! 210-6526 is the part number for the 5W model.
You'll have to mess with the taps to optimise the matching.

[PhilR]

Thanks dhaen for your reply.

They are EL34's.

I found RS components here in New Zealand, do you think the 100v line transformer would be better than their audio output transformers ie part no. 210 6419?

[dhaen]

Do you have a schematic?
I'm concerned that the details are not adding up. A photo of the internals might help, alternatively.

For matching 800 ohm to 8 ohms, a transformer such as I mentioned would be OK. However If there is any DC current, it would prevent it from working properly.

[PhilR]

I'm sorry I don't have a schematic but shall post a photo of the internals soon.

I have found the following information:

http://www.vintage-radio.com/repair-...speakers.shtml

But as I lack any real knowledge on valve amps this has added to my confusion.

My plan is to remove the amp from the cabinet and improve/restore it as a learning project before embarking on building a valve amp from scratch.

Thanks for your time.

Regards Phil

[PRR]

> 800ohm speakers

The Philips hi-Z speaker radios are, as noted, real odd-ball tube designs, a dead-end in tube amp evolution. There are good reasons loudspeakers are wound 4 to 16 ohms, and not 800 ohms.

> Do I need 2 output transformers and if so will a torridal transformer do the job.

If you just want to hear it "work": The logical ready-made transformer is a Distributed PA Line To Speaker transformer marked 70 Volts, 10 Watts, with appropriate speaker impedance taps. They are inexpensive ($15) but often low quality (poor frequency response).
http://www.pagingsystemequipment.com...PBTRD&PID=1212
http://www.lashen.com/vendors/CSISpe...ansformers.asp

> My plan is to remove the amp from the cabinet and improve/restore it as a learning project before embarking on building a valve amp from scratch.

Sorry: This radio is a uniquely bad subject for this plan. Not only does it depend on a highly unusual output stage and load impedance, it may have no power transformer which is dangerous for DIY.

Get an old Dynaco or Eico hi-fi amp, much better subject for study.

[fdegrove]

Hi,

The Philips hi-Z speaker radios are, as noted, real odd-ball tube designs, a dead-end in tube amp evolution. There are good reasons loudspeakers are wound 4 to 16 ohms, and not 800 ohms.

Maybe, but I really doubt the Philips designs were as bad as you paint them.

The 800R speakers may be a little fragile but they were a good match for their EL86 design.

Hell, I've known these as a kid and I actually bought an EL86 Philips OTL ( it wasn't a real OTL) and hooked it up to 8 Ohm speakers.

Boy, I was amazed by the bass coming out of these babes...

Mind you that was 20 years after my kid experience.
I could give a list of well respected SS amps that will not even come close all things considered.

Sorry: This radio is a uniquely bad subject for this plan. Not only does it depend on a highly unusual output stage and load impedance, it may have no power transformer which is dangerous for DIY.

The output is often a user choice, either a 1:1 OPT or a 8 µF cap, but that doesn't tell you anything...The circuit is so cleverly designed it can get away with it and still provide good bass performance into low impedance loads.

There's alot to be learned from these Philips engineers, I reckon.

Cheers,

[SHiFTY]

Is it an ugly brown and cream looking thing with gold metal trim and the speakers on each end? I had one of these, sacrificed it for its power trannys and tubes. If you have a pic of it I might be able to help. Also are they EL34s (big suckers) or EL84s?

If I were you, I would grab the power transformer, tubes and scrap the rest. Buy some new tube sockets and capacitors from Jaycar Electronics and build a small stereo tube amp. I have heaps of bits if you need anything...

[PhilR]

Yes Shifty, its really ugly, that's why I wanted to get rid of the cabinet.
They are EL84's. After reading some of the comments on this radio your idea of salvaging the valves and power transformers might be a good idea. any ideas of a suitable circuit.

Thank you everyone for your time.