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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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18th Jun 2008, 9:06 pm | #21 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,535
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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 |
24th Jun 2008, 8:30 pm | #22 |
Dekatron
Join Date: Mar 2008
Location: St. Frajou, l'Isle en Dodon, Haute Garonne, France.(Previously: Ellesmere Port, Cheshire, UK.)
Posts: 3,183
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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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"Hello?, Yes, I'm on the train, I might lose the signal soon as we're just going into a tunn..." |
24th Jun 2008, 9:00 pm | #23 | |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,947
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Re: MATE AM transmitter
Quote:
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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28th Jun 2008, 11:00 pm | #24 |
Heptode
Join Date: Apr 2003
Location: Cornwall, UK.
Posts: 989
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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?? SimonT.
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28th Jun 2008, 11:18 pm | #25 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,947
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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.
Paul |
29th Jun 2008, 11:09 am | #26 |
Dekatron
Join Date: May 2006
Location: Invercargill, New Zealand
Posts: 3,457
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Re: MATE AM transmitter
It'll be fine, after all it was designed in Invercargill
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1st Jul 2008, 10:02 pm | #27 |
Dekatron
Join Date: Mar 2008
Location: St. Frajou, l'Isle en Dodon, Haute Garonne, France.(Previously: Ellesmere Port, Cheshire, UK.)
Posts: 3,183
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Re: MATE AM transmitter
Maybe, but don't harmonics work differently in the northern hemisphere?
Cheers de Pete.
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"Hello?, Yes, I'm on the train, I might lose the signal soon as we're just going into a tunn..." |
4th Jul 2008, 7:18 pm | #28 |
Retired Dormant Member
Join Date: Mar 2003
Location: Birmingham, West Midlands, UK.
Posts: 785
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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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12th Sep 2008, 10:20 pm | #29 |
Pentode
Join Date: Jun 2006
Location: Malaga, Spain.
Posts: 235
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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. |