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Philips Radiogram

Wharfedale EVO4.1 Bookshelf Speakers

venkatesh_k_s

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Hi,

I am new this site and looking to be an active member. I have laid my hands on an old philips radiogram F7X15A.

The issue is all the 4 speaker coils are charred and I have a task on hand to put together replacement speakers or introduce output transformers. I am not sure where to get these speakers from.

It has Speakers of 800 ohms (approx) 4 nos. - AD 3700AM - 2nos; AD 3690A - 2 nos.

Any suggestions/ideas on the above would be highly appreciated.

Thanks in advance.
Venkatesh
 

aks07

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The relatively easier way would be to find a pair of AD 3700AM. The OTL stage can be, in a manner, redesigned but it could be extensive and the performance certainly wont be as per original specs.

Your radio must be using EL86 tubes. These were specifically developed by Philips for OTL amplifiers. The purpose of the OTL design was to eliminate the shortcomings of the OPT, gain wide bandwidth and lower distortion. This is the reason they sound so nice. Thus no point in changing that. The other set of speakers must be driven by EL84 in a conventional OPT design therefore you can just get it reconed.

AD 3700AM in good condition should sound extremely nice. But they were very delicate. It could be difficult to find one in good condition. Almost all of these high imp Philips speakers I used to see were damaged. And this was about 35 years ago.

You may wish to contact our respected FM Shri Pandu Rajan for further suggestions. He is the last word on Philips valve radio restorations.
 

venkatesh_k_s

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Hi,

Thanks for your inputs.

This radiogram is with 2 x EL84 driving 4 speakers through 2 output transformers. As you said the AD 3700AM speakers are not easily available. Hence I need to go with modification of the output stage to suit conventional speakers of say 4 or 8 ohms.

Any more suggestions would be appreciated.

I am also contacting Sri Pandu Rajan for help.

Regards
 

pksnathan

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Friend,

Welcome aboard.I request you to kindly send images of your Philips radiogram.

Thanking you,

pksnathan
 

venkatesh_k_s

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Here are 2 pics. The cabinet has got damaged. I will have to re-build it.


 

venkatesh_k_s

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Hi All,

Today I had a detailed discussion with Shri Pandu Rajan. Very kind of him to share his knowledge and wisdom on the subject. He gave me a lot of information/tips on the restoration. He explained the following options :

1. Source the replacement speakers AD3700AM and AD3690A from Dutch forum. Alternate for 3690A would be 3800A or 3800AM.

2. Source output transformers from old scrap radios. Possible models would be philips B6CA-67A/77A; B6CA-37A/37A/01; B6CA-58/57; 15RB667; 15RB708/008. Here the ratio would be to suit 5 ohm speakers. The existing speaker shells could be used for new 5 ohm voice coils.

3. Add power transformer to the existing transformer to get a ratio from 800/5 (ohm ratio).

I shall post further updates once I start the restoration.

Thanks again to Shri Pandu Rajan and all.


Regards

Venkatesh.
 

aks07

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Dear Sir

I apologise for not reverting earlier as I was away. But good to see that meanwhile you have contacted Shri Rajan.

The easiest option will be retrofitting the output transformer. Philips was notorious for making complicated or unconventional output stages. They would typically have several taps/winding in both primary and secondary to achieve myriad functions.

If you are unable to source a junk philips radio then you can perhaps try the DELTA Single Ended OPT's as below:



Model OP-13 is a conventional replacement unit for Radios. Single primary and single secondary. This is most suitable for brands such as Murphy, EKCO etc with a straightforward output stage.

Model OP-241 is much better in sound quality and can also be used for home audio. This has two separate secondaries (not just taps) which is helpful if one wants to experiment with things like cathode feedback and such.

Both the OPT are suitable for type EL84 valves. These OPT's are made by DELTA and should be available if you ask them.

The 3rd option suggested by Shri Rajan should only be used if everything else fails. This will work but the audio quality may to be upto expectations.

Best regards

aks
 

Pandu Rajan

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Dear Mr. Venkatesh

The delta transformer will not work here because all the Philips output transformers have an additional HUM compensation winding. In an output transformer both DC and the audio AC flows through the same winding. The flow of DC current tries to saturate the core. To avoid this the transformer has an additional winding where the dc current through the hum compensation winding flows in the opposite direction to the current which flows in the main primary winding. This avoids the saturation of the O/P transformer core plus it also reduces the HUM. An ordinary O/P transformer does not have these features and so cannot be directly replaced. Your set is slightly different. Here the O/P transformer has been used for the EL84 valve but the secondary impedance is suitable for 800 Ohms speaker. There is no push pull amplifier.Since it is a stereo set there are two EL 84 valves. So the power amplifier is 1X EL84 for each channel. The audio pre-amplifier is 1X ECC83 for each channel.
In my opinion the best option is to search for old Indian Philips radio transformers where the primary is suitable for 1X EL84 valve and the secondary is suitable for 5 Ohms. All these transformers have the HUM compensation winding. The 4 existing speakers can be rewound for 5 ohms. This will be a permanent solution. Because even if you are able to get the 800 ohms speakers now again in future you may find it difficult to get them.

Regards
P.Rajan
 

venkatesh_k_s

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Dear Sir,

Yes I remember you explaining (over phone) about the standard output transformers not being suitable for this application. I thought the delta transformers would come with this additional windings and hence enquired in their Delhi office and also got a quote today.

Anyhow I will have to hunt for these trans in old philips radios. Will go around this weekend and let you all know if I am lucky.

Thanks for the suggestion Sir.

Regards
Venkatesh
 

reubensm

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This thread is hardly a page long but I've already learn't so much from it. Thanks all.

Would be great if you can source the original spares. Ebay.de is a good place to hunt for old European spares. Dad used to always discuss philips designs with me back in the day and he would explain how different their design philosophy was, from conventional stuff. We had the world's first ever boombox, the Philips radio recorder 22RR482, from 1969 at home and its design was so very different from all the Japanese ones that came, years after it. What was more important, the Philips stuff of those times, sounded very special.
 
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