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Old 18th Jun 2008, 9:06 pm   #21
Herald1360
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Default Re: MATE AM transmitter

In the dim and distant when EF80s were one of the few 6.3V valves you could salvage from old TVs I tried using one for audio- it was horribly microphonic!

Maybe they are not ALL like that but I acquired some EF37As shortly after which were not surprisingly much better!

Chris
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Old 24th Jun 2008, 8:30 pm   #22
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Default Re: MATE AM transmitter

Hm, I didn't know that, I'll bear it in mind and try several EF80's as well as the others with the same pinouts or, better still, I'll just use one in the final stage. Thanks.
BTW, I was told once that the EF37A is a low noise version of the EF37 and an octal version of the EF86. I could be wrong though.
Cheers de Pete.
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Old 24th Jun 2008, 9:00 pm   #23
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Default Re: MATE AM transmitter

Quote:
Originally Posted by Tractorfan View Post
BTW, I was told once that the EF37A is a low noise version of the EF37 and an octal version of the EF86.
The EF36, EF37, EF37A and EF39 are all related valves. The EF36 was originally developed as an RF pentode but its shortcomings in this role led to the development of the EF39. The EF37 was a development of the EF36 supposedly optimised for audio use and the EF37A is a further (postwar) development of the EF37.

The EF86 was designed to be a B9A replacement for the EF37A. There is a special quality version, the E80F, which was designed to replace the EF37A in industrial applications like computers. The E80F and EF80 are unrelated.

I haven't found EF80s to be particularly microphonic but they're certainly not a low noise valve. This shouldn't matter if they're being fed by a line level signal in a noncritical application, but they wouldn't be suitable as, say, microphone or magnetic pickup preamps. (There are several phono preamp designs based on the functionally similar B7G EF91 though).

Paul
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Old 28th Jun 2008, 11:00 pm   #24
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Default Re: MATE AM transmitter

On the subject of EF80's.......

A HMV TV of the mid 50's ( can't recall the model) used an EF80 as the audio output!

I also intend to build one of these units as soon as time permits. With such a simple design - are harmonics much of a problem??


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Old 28th Jun 2008, 11:18 pm   #25
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Default Re: MATE AM transmitter

I haven't built one (yet) but I would expect there to be quite strong harmonics at multiples of the fundamental frequency, given the simplicity of the design. This shouldn't be a problem given the low RF output though.

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Old 29th Jun 2008, 11:09 am   #26
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Default Re: MATE AM transmitter

It'll be fine, after all it was designed in Invercargill
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Old 1st Jul 2008, 10:02 pm   #27
Tractorfan
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Default Re: MATE AM transmitter

Maybe, but don't harmonics work differently in the northern hemisphere?
Cheers de Pete.
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Old 4th Jul 2008, 7:18 pm   #28
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Default Re: MATE AM transmitter

EF80s tend to be OK for audio work if they are connected as triodes, although the output isn't too great, and the level input is reasonable. I have a commercial 'replacement' radiogram chassis which uses one as an audio driver feeding an EL84. As output valves I've come across one or two designs, claiming that they can give 500mW output.
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Old 12th Sep 2008, 10:20 pm   #29
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Default Re: MATE AM transmitter

Here is an old post of mine with an AM modulator which was a little bit OTT,
Specification
HT Voltage 25V
HT Current 10mA
Heaters 10V@5A DC and 6.3V 0.3A DC
Frequency range 0.5MHz to1.8MHz
Power output < 1.5mW into 50 Ohms
O/P filtered to remove harmonics to <-60db
Screen Modulation of 813
Modulation 70%
Subjective quality of signal V good
Range with 1 metre whip antenna limited to less than 20m
This Modulator should not raise any concerns regarding interference or for that matter illegality when operated internally to buildings.
The same cannot be said for the majority of valve and transistor modulators which have been portrayed on this site. Valve modulators utilising 100s of volts will have outputs in the tens of milliwatt region. The various solidstate modulators inc the commercial kit jobbies have outputs that are even higher and suffer from IPs from local radio stations meaning that the outputs are anything but what they are supposed to be.
While any unlicensed transmission out side of the permitted bands is illegal, transmissions which do not go outside the domestic walls at normally recievable levels are normally overlooked.
It was with this in mind and the spirit of the Valve pioneer that this unit was bunged together.
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