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Old 11th Jun 2021, 5:40 pm   #1
bobhowe
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Default Bush DAC90 FM conversion.

Hi been to FERNSEH shop this afternoon to look at the Bush radio that is on another thead & while i was talking to David about doing another conversion DAC90 FM only as i have a spare chassis . Now this would have to be a mains only set and the output valve would have to be a EL34 as David likes octal valves & the DAC90 has octal valve bases . What circuit should be based on this conversion it must have a valve tuner any ideas ? kind regards Bob
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Old 11th Jun 2021, 6:29 pm   #2
rambo1152
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Default Re: Bush dac90 fm conversion

Octal valves, yes but let's keep it transformerless AC/DC, so why not keep CL33 output valve. I suppose the front end has got to be Octal too otherwise it's going to look odd (what am I saying ) but nothing immediately springs to mind, perhaps some obscure CV types from the war.
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Old 11th Jun 2021, 7:06 pm   #3
Ed_Dinning
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Default Re: Bush dac90 fm conversion

Hi Bob, EL34 is far too big and heater current greedy for that set and there are lots of octal op valves in GT bottles, many with high voltage heaters id you go AC/DC.

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Old 11th Jun 2021, 7:37 pm   #4
bobhowe
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Default Re: Bush dac90 fm conversion

Hi Graham would there be an issue with heat if we go AC/DC route ? or do we use an auto transformer ? . Hi Ed would there be enough room for an auto transformer in side the set or do we go for an external power supply with various voltage tappings and have a male octal socket on the back for power like the Leak Point One Preamp . What valve would you use in the FM section ? kind regards Bob
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Old 11th Jun 2021, 8:46 pm   #5
kalee20
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Default Re: Bush DAC90 FM conversion.

With the constraint of octal throughout, DET20 might be a consideration for the front end.

But it's going to be a cramped chassis - at least two IF stages, then there's (?) 6H6 for the discriminator, then AF voltage amplifier... Unless you slip in a bit of reflexing to keep the valve count down...
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Old 11th Jun 2021, 9:12 pm   #6
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Default Re: Bush DAC90 FM conversion.

Suggested circuit for the FM only DAC90.
Kolster-Brandes FB10FM circuit but with a different range of valves.
ECC85, EF89, EF89, EABC80 and EL84.
Use a junk DAC90A chassis to permit the use of B9A base valves.

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Old 11th Jun 2021, 9:17 pm   #7
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Default Re: Bush DAC90 FM conversion.

Hi.
Is the DET20 Not a transmitting valve with two top caps?
Suitable octals for VHF FM could be difficult. I think David's idea of a DAC90A is the logical way to go. I'd plump for an ECC88 though.
So ECC88, 2X EF89, germanium discriminator diodes ECL82, EZ80.
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Old 11th Jun 2021, 9:21 pm   #8
bobhowe
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Default Re: Bush DAC90 FM conversion.

Hi Kalee20 like the look of the DET20 valve will look at home in a 1940s DAC90 now we need to find i circuit round it kind regards Bob
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Old 11th Jun 2021, 9:26 pm   #9
bobhowe
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Default Re: Bush DAC90 FM conversion.

Hi David got a spare DAC90A & DAC90 chassis as well so could we do 2 versions ? one B9A valves & one Octal valves kind regards Bob
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Old 12th Jun 2021, 10:00 am   #10
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Default Re: Bush DAC90 FM conversion.

Hello Bob and David
Have a look at the attached PDF book on Practical FM circuits. It is a bit old being published in 1955 and I am not sure how practical it will be now. However it does give component lists for each circuit and winding details for the coils.
I think it is an interesting book and starts with the theory of FM and then to simple circuits up to quite complex ones. The circuits are mainly based on B9A valves but some of the output stages use octal 6V6 valves.
I hope you find it interesting.
Regards Stan.
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Old 12th Jun 2021, 10:26 am   #11
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Default Re: Bush DAC90 FM conversion.

You might think about a Jason FM tuner. There are many second hand ones about and you’ll have the coils readymade. It can be taken apart and rebuilt on your chassis. Add an output valve and power supply. Takes away all the problems of winding/finding suitable coils.
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Old 12th Jun 2021, 10:52 am   #12
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Default Re: Bush DAC90 FM conversion.

Quote:
Originally Posted by Electrical View Post
Hello Bob and David
Have a look at the attached PDF book on Practical FM circuits. It is a bit old being published in 1955 and I am not sure how practical it will be now. However it does give component lists for each circuit and winding details for the coils.
I think it is an interesting book and starts with the theory of FM and then to simple circuits up to quite complex ones. The circuits are mainly based on B9A valves but some of the output stages use octal 6V6 valves.
I hope you find it interesting.
Regards Stan.
Thanks for posting this link, Stan. Agreed, an interesting text for both theory and circuits.
Mike
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Old 12th Jun 2021, 11:10 am   #13
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Default Re: Bush DAC90 FM conversion.

Quote:
Originally Posted by murphyv310 View Post
Hi.
Is the DET20 Not a transmitting valve with two top caps?
I believe it was used in the front end of the VHF tuner of the 19 set amongst other things. Postwar, lots were sold on the surplus market and they were used for all sorts of hobbyist projects like signal generators and one valve radios.
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Old 12th Jun 2021, 11:53 am   #14
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Default Re: Bush DAC90 FM conversion.

That's my understanding, too. It may be a low power transmitting valve, but often used below its full capabilities in receiver circuits.

I only made the suggestion, to comply with the all-octal constraint. Remove that, and the wonderful world of PC97, PC900, EF91, 6AL5, 6AQ5, 6X4 opens up in B7G - or the 'standard' ECC85, EF89, EABC80, EL84, EZ80 in B9A.

My favourite DAC90a conversion (although internally there's not much DAC90a left), is the DAB90a
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Old 12th Jun 2021, 2:22 pm   #15
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Default Re: Bush DAC90 FM conversion.

The Pye P117, the "FM-AM Piper" employs only three valves in a very clever circuit.
The valve functions on FM are: ECC85 reflex RF and IF amplifier, EBF89 IF amplifier, (the diode sections are used for the AM detector and AVC) and an ECL82 for the audio amplifier and output.
Omit the band switches and make an FM only receiver.
I believe the circuit design is of Italian origin because a receiver employing a similar circuit was made in Italy. Of course it could be the other way around and Pye designed the receiver.
The circuit can be found in the 1958/59 Radio and Television servicing book.

DFWB.
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Old 12th Jun 2021, 3:25 pm   #16
murphyv310
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Default Re: Bush DAC90 FM conversion.

Hi.
Again another suggestion and I'm happy to be shot down in flames.
I never rated the ECC85, failures are common and they are getting harder to find, the ECC88 is pin compatible and gives a welcome boost to sets with even a good ECC85. Why not consider the ECC189 or is it not suitable for VHF?
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Old 12th Jun 2021, 4:33 pm   #17
bobhowe
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Default Re: Bush DAC90 FM conversion.

Hi Stan interesting reading Hi Kalee20 surprised that DAB90A did not go into production like Bush TR82 Hi David the PYE 117 looks like a good candidate for the DAC90A conversion now its now to find DAC90 conversion with octal valves for you David
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Old 12th Jun 2021, 5:31 pm   #18
FERNSEH
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Default Re: Bush DAC90 FM conversion.

Murphy310 wrote: "Again another suggestion and I'm happy to be shot down in flames. I never rated the ECC85, failures are common and they are getting harder to find, the ECC88 is pin compatible and gives a welcome boost to sets with even a good ECC85. Why not consider the ECC189 or is it not suitable for VHF?"
Hi Trevor,
In 1958 Pye employed the PCC88 as the cascode RF amplifier in TV tuners before adopting the PCC89 in later models.
ECC88 and PCC88 mu is 33, gm 12.5mA/V and ra 2,640 ohms.
ECC189 has similar characteristics: mu 33 12.5mA/V and ra 2,500ohms.
ECC85 has characteristics similar to the 12AT7. ECC85 has intersection screen.

How about the 6AK5 pentode for the RF amplifier and mixer-oscillator?

DFWB.
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Old 13th Jun 2021, 7:49 am   #19
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Default Re: Bush DAC90 FM conversion.

I wonder if there is anything lurking in the Practical Wirelesses of the 1950's... I'd be surprised if there wasn't a constructional article or two for an FM broadcast receiver as it was the latest thing - and back then, constructors had an almost endless supply of cheap war surplus octal valves! Might be worth a trawl through...

An interesting project, good luck with it!
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Old 16th Jun 2021, 9:44 am   #20
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Default Re: Bush DAC90 FM conversion.

Good day to all,

This Australian site has a wealth of information on simple super-regenerative FM receivers.
Be sure to read about the Fremodyne FM receivers. Amazing performance on a shoestring budget.
Scroll down a bit on home page :

https://www.cool386.com/

Photo of relevant sections below.

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