Next the sound IF stage, plus!

[AdrianPH]

Well it sort of works, but not that I could actually use it.  I tapped off the 38.15 MHz IF from my first sound trap in the vision stages by adding a loose coupled single turn, this then feeding via some co-ax to the input of the sound IF.  I do not have enough signal doing this and the best I can get it also provides plenty of frame timebase noise on the audio as well.  So back to the idea's board, see what I can improve on.  Perhaps pick up the IF a bit sooner, or via a small cap on the anode of the first stage, and maybe put a vision trap in the sound IF.

Other thoughts are filter the audio after the demodulator.  Think I need to check a few TV circuits before I do much more.

It's still fun.

Adrian

[Amie]

Would a narrowed BW help? More gain and let less muck in?

Just comments from the peanut gallery.



Amie.

[Ed_Dinning]

Hi Adrian, not sure if I recommended it but the Philips Tech Library series has a couple of good books on the design of RF and IF/ det stages in radio and tv sets. They are available to download from the web.
There is also the Mullard ads from WW in the 50's and 60's on designing with valves, in the style of an application note

Ed

[AdrianPH]

Not sure why, perhaps it is googlemail but I rarely get notifications from this site on any responses to thread, so I apologise to Amie and Ed for not being back sooner, I just did not know.

I have been working on the project in the background and had some success.  I am still using the same if transformers as before only with a small series cap to to the grid of the next stage, but then having to add a 100KOhm grid resistor to the EF81's.  It may be a little narrower in response, but very little.

What I have done is changed where I take the sound IF from, I am now across the output of the tuner and not from a trap on the video IF.  This gives better results, not 100% perfect, but reasonable.

I have done a video that can be watched here: -  https://youtu.be/UHnmOmn_gPw

I go on a bit sorry, as I show a few things such as what I use to generate a 405 signal and a bit of the TV.  There seems to be a copyright issue with the music from the BBC interlude so I may have to edit it at some point for the 60 seconds worth of Interlude playing on the TV!

Adrian

[Kalee20]

I've just watched the video Adrian.

Congratulations! A good setup! And I like the construction techniques for the IF amp under discussion.

It's a good project!

One detail - the double-diode as detector: there existed both an EB91 and an EAA91, they had identical characteristics, and both were completely isolated double-diodes (separate cathodes), but they had different envelope heights. Dunno the CV reference, it could be the equivalent to the shorter.

[Murphyv310]

Hi.
The 6AL5 is usually much smaller than the EB91 it's a direct equivalent.

[AdrianPH]

I actually got a notification.

Thanks for the comments.

In some military chassis gear I picked up when starting out the detectors were listed as CV140, but I found CV4007 as well as the CV140 being fitted. frank.pocknet.net site lists CV140, 6D2, CV4007, 6AL5, D77, DD6, CV4025 and EB91 as all similar enough to be used.

The CV5377 is the last Mil version of the EF91 that I can find and are still cheap to buy at £3 each, they seem to have less of a lightbulb moment with the heaters when power is applied.

Adrian

[Amie]

The CV5377 is the last Mil version of the EF91 that I can find and are still cheap to buy at £3 each, they seem to have less of a lightbulb moment with the heaters when power is applied.

Adrian

Hi Adrian, super job.

Heater flash as it is called by the audiophool community is seen as something desirable instilling some kind of magically audible improvements to the sound quality. Fortunately the likes of the EF91 have been largely ignored so far.

[ppppenguin]

I really wounld't worry about EB91 lighting up brightly at switch on. They rarely fail. Along with the EF80, when the human race is extinct, the cockroaches will do just fine with EF80 and EB91.

As for the 'phools, I've got a load of PC86 and PC88 here. (UHF TV tuner valves for those who aren't aware of them) Ideal for really pure high frequency response

[AdrianPH]

Hi Amie,

I class myself lucky I am not part of the 'audio-fool' crowd and thank the lilitary that they want valves that have long service life and can deal with shock and vibration. I have seen on Ebay matched pairs of the EF91 being sold.
I did build a stereo amp that has EF81, ECF82 and TT15 per channel, I was told I should not use the EF91 as it is microphonic, noisy etc. Works fine for me, no problems.

Adrian