Indians For Guns

I would like to restore a Philips BX998A valve radio I have. Unfortunately the original speakers are missing. I am hoping to replace them.

This is what is known as a Bi-Ampli model with separate output stages for bass and treble. The bass amplifier is of SRPP type which acts as a push-pull amplifier using two PL 81 valves. The treble amplifier is of common single ended design using an EL 84 tube.

The loudspeakers are a 9758A (24 cm 700 ohms) for bass and a 9768M (15 cm) for treble.

I would be glad to hear from someone who is willing to part with these speakers or knows of someone who does.

This was the top model in the Philips range in the nineteen-fifties with motorised band selection and an FM tuner. My BX998 A54 is most likely a 1954 model. I feel it is worth restoring.

Hmm...I'm not sure if you can find specifically those speakers but valve amps are still very much in vogue in Guitar music and you can try if any one fits the specifications.

BowMan wrote:Hmm...I'm not sure if you can find specifically those speakers but valve amps are still very much in vogue in Guitar music and you can try if any one fits the specifications.

Hi Bowman,

Thank you very much for your valuable suggestion. I will act on it.

Regards.

I have not seen everything in this world, but the idea of a 700Ω speaker surprised me. I confess that I've never run across one. Where one might be found could be problematic. Have you considered trying to purchase another radio -- a non-working one -- for parts?

timmy wrote:I have not seen everything in this world, but the idea of a 700Ω speaker surprised me. I confess that I've never run across one. Where one might be found could be problematic. Have you considered trying to purchase another radio -- a non-working one -- for parts?

Timmy,

You may recall that valves amplify voltage and usually include a step-down transformer in the output stage to bring down the voltage and amplify the current to match it to a loudspeaker of low impedance in the 4 ohms to 8 ohms range.

Philips insisted on doing things their own way in their valve radios. One of their fads were transformerless output stages on the audio frequency side. Many of their radio sets had speakers of 800 ohms. The BX998A was peculiar in having one of 700 ohms (rather than 800 ohms) and 10 inches diameter on the bass side of the Bi-Ampli section. Bi-Ampli radios had two amplifiers, one each for the bass and the treble audio frequencies.

I purchased the set in a non-working condition as part of a complicated deal which included the frame of a Triumph 3HW 350 cc motorcycle.

I am exploring all options in attempting to restore this radio which was the top model in the Philips range of radios in the nineteen-fifties and relatively few were imported into India. My particular set is believed to have been ordered by the eldest son of the Nawab of Rampur in 1954 or 1955. At some stage it fell into inexpert hands resulting, among other things, in the loss of its speakers. While the treble speaker is of low impedance and can be replaced by a modern speaker, the absence of the larger, high impedance bass speaker is posing problems.

One way out would be to introduce a transformer in the bass output stage to drive a low impedance speaker with a matching power handling capacity.

Here is a link to one of these radios

Regards.

Another option is to try add high wattage resistors like wirewound in series with speaker. Not really recommended due to obvious reasons, but midrange won't suffer much. Or better yet, custom wind one. The elec shop guys can point you to those who make repair coils.

miroflex wrote:

timmy wrote:I have not seen everything in this world, but the idea of a 700Ω speaker surprised me. I confess that I've never run across one. Where one might be found could be problematic. Have you considered trying to purchase another radio -- a non-working one -- for parts?

Timmy,

You may recall that valves amplify voltage and usually include a step-down transformer in the output stage to bring down the voltage and amplify the current to match it to a loudspeaker of low impedance in the 4 ohms to 8 ohms range.

Philips insisted on doing things their own way in their valve radios. One of their fads were transformerless output stages on the audio frequency side. Many of their radio sets had speakers of 800 ohms. The BX998A was peculiar in having one of 700 ohms (rather than 800 ohms) and 10 inches diameter on the bass side of the Bi-Ampli section. Bi-Ampli radios had two amplifiers, one each for the bass and the treble audio frequencies.

…

I confess that it has been many a moon since I have messed with tubes (valves) and I don't have any experience with these European tube types. My recollection of power amplifier tubes is a plate resistance of ~5KΩ, making even 700Ω a pretty bad mismatch. Getting rid of an output transformer does have advantages, however. Let us know how things work out for you.

EL84s can be swapped for 6BQ5. I know this because these are some of the most popular tubes used in Guitar Tube/Valve Amps.

To be honest all guitar speaker cones are 4 or 8 ohms impedance so might not help you much.

Found this resource though;
http://www.vintage-radio.com/repair-res ... akers.html

And this...
http://www.vintage-radio.net/forum/show ... hp?t=15873

There is a guy recommending taking out the 800 Ohms unit and replacing with a 8 Ohm ono...

What you need to do is convert it to a standard system. I've never done it, but I think it will go something like this:

1: Remove and discard 800 Ohm speaker.
2: Remove R1.
3: Remove 50uF cap, and fit a 32/32.
4 Link the output of the UY85 to the first of the two caps.
5.Link the Transformer Primary here.
6. Connect R1 between the two capacitors.
7. Link the rest of the set to the second capacitor.
8. Put your 8 Ohm speaker on the secondary of the output transformer.
9. CHECK YOUR WORK.

Then switch on and hope!

Cheers,

Steve P
__________________

estousandy wrote:Another option is to try add high wattage resistors like wirewound in series with speaker. Not really recommended due to obvious reasons, but midrange won't suffer much. Or better yet, custom wind one. The elec shop guys can point you to those who make repair coils.

Hi Sandy,

As you have yourself said, this option is "not really recommended for obvious reasons".

A high wattage resistor in series with a low impedance speaker will provide a largely resistive load. A high impedance speaker will provide a greater inductive load as well as a resistive load. The results will be different in the two cases.

Moreover the high wattage resistor of nearly 700 ohms will dissipate a large amount of the power output of the amplifier as heat, which would otherwise be available for driving the speaker.

Thanks for your suggestion anyway.

Regards.

BowMan wrote:And this...
http://www.vintage-radio.net/forum/show ... hp?t=15873

There is a guy recommending taking out the 800 Ohms unit and replacing with a 8 Ohm ono...

What you need to do is convert it to a standard system. I've never done it, but I think it will go something like this:

1: Remove and discard 800 Ohm speaker.
2: Remove R1.
3: Remove 50uF cap, and fit a 32/32.
4 Link the output of the UY85 to the first of the two caps.
5.Link the Transformer Primary here.
6. Connect R1 between the two capacitors.
7. Link the rest of the set to the second capacitor.
8. Put your 8 Ohm speaker on the secondary of the output transformer.
9. CHECK YOUR WORK.

Then switch on and hope!

Cheers,

Steve P
__________________

Hi Bowman,

Thank you very much for your kind suggestions and advice. I have carefully considered the option of introducing a transformer and low impedance speaker in the output stage of the bass amplifier.

I consulted Mr Viren Bakshi, the well known designer and manufacturer of valve amplifiers and the proprietor of Lyrita Audio, regarding the feasibility of introducing an output transformer and a low impedance speaker in the bass amplifier section of the radio.

He advised that this would entail a redesign of the output stage of the bass amplifier and that it would be easier to have the voice coil of the speaker rewound.

I invite advice and suggestions from you and other members regarding the purchase of a modern 10" speaker and having its voice coil rewound to 700 ohms.

Regards.

Hi all,

You'll all be glad to know that I've been able to find a pair of original speakers plus a new dial plus a new back panel, all of which I needed, in Holland. This has been possible due to the efforts of Mr Pandu Rajan of Bangalore, a keen valve radio enthusiast and expert restorer of Philips radios.

Now the job is to safely bring them over.

Regards and thanks for all the advice.

That's great! I think that, failing to obtain real parts, your only good recourse would have been to have someone wind a new speaker coil for you. Winding a speaker is no small task, as there is more to it than wrapping wire around a form. I'm sure Philips had a special process that ensured the best fidelity, and I don't think it could be certain that a homespun speaker coil could have duplicated this. However, matching the impedance to the final output would be critical for obtaining good fidelity too, so I think that the hand-wound option would have been your second best option.

Getting the right parts is the best way to go, and congratulations on finding the right stuff! Now you know it will be "right."

Thanks a lot Timmy for your continued advice and support.

I was also advised by Mr Viren Bakshi, a well known designer and manufacturer of valve amplifiers and proprietor of Lyrita Audio, to have the voice coil of a 10 inch speaker wound to 700 ohms by a manufacturer of loudspeakers. There are several small scale manufacturers of loudspeakers in India who could have been requested to help out.

All's well that ends well.

Regards.