Welcome to Australia's only Vintage Radio and Television discussion forums. You are not logged in. Please log in below, apply for an account or retrieve your password.
Australian Vintage Radio Forums
  Home  ·  About Us  ·  Discussion Forums  ·  Glossary  ·  Outside Links  ·  Policies  ·  Services Directory  ·  Safety Warnings  ·  Tutorials

Special Projects

Forum home - Go back to Special Projects

 My Little General for 1976 Part three.
« Back · 1 · Next »
 Return to top of page · Post #: 1 · Written at 11:16:59 AM on 11 October 2019.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 563

Hi All, I have completed the project, wood bashed a little cabinet up and fitted the chassis in.
The part 3 has a description of the process, the final circuit used and a critique with the design short comings I can see.
There may be some among you with more valve knowledge than I that can comment on the Pentagrid type 6CS6 I used as apposed to the normal 6BE6. The 6CS6 appears to have been designed as a TV sync clipper/amplifier and I may have been pushing my luck using it as an oscillator/convertor!
I do not mind in the least any opinion or observation on the subject from anyone, particularly from any X radio/TV service people who had practical experience in the field, I am only guessing so even if you box me around the head it does not matter.
I will forward the usual PDF to Brad, who, with cutlass between his teeth fending off the pirates trying to board the ship, may be able to post it up.
Fred.

Little General 1976 - Part 3


 
 Return to top of page · Post #: 2 · Written at 9:46:04 PM on 11 October 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3685

The 6CS6 is a sync separator, both control grids a sharp cut off So with mixed results it may work, as you have found, similar to 6BE6 even that its construction is not the same.

Franks electron tube pages has them listed. http://www.tubedata.info/

Marc


 
 Return to top of page · Post #: 3 · Written at 10:05:17 PM on 11 October 2019.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 5491

Document uploaded.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 4 · Written at 8:05:22 AM on 12 October 2019.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 563

Thanks Marc, the oscillator runs strongly and that is clear from the waveshape but the mixing gain appears to be low and I would have to do a lot of research into the actual levels of modulation of the stream by setting up a lot of tests and as usual I am over the project!
I am clearly using a tube with unsuitable slopes, as you point out they are sharp cut off so I bet I have clipping (gee fred you are a genius, that's what a sync clipper does!!!) of the signal giving a low gain situation.
When I get back around to playing with it i'll do some back to back tests with a 6BE6 and see how much better it can be if I use the correct valve.
Fred.


 
 Return to top of page · Post #: 5 · Written at 9:15:39 PM on 14 October 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3685

I would think pin 7 has to be the signal grid with 6CS6 I know IBM made a modified version of 6BE6. My thought is that 6BE6 may be similar to 6SA7. With it one forgets everything they know about Pentagrids as it has a suppressor grid and no true oscillator grid: It is very different & behaves that way.

It is therefore a bit like an autodyne & has to use a tapped Hartley oscillator, or the recommended external exciter. I have serviced a Midwest autodyne where the exciter was a Colpitts. One may have to fiddle with voltages & signal levels to get a useable modulation & toy with an extra amp. Application to it of AGC may be an issue.

With a non tube test & play oscillator, the oscillator was 3/4 of a quad NAND gate (into a delay chain to get a modified square wave. The mixer was a dual gate FET. RF into one gate, Audio via pot, into the other to control modulation for a demo, where needed.

Marc


 
 Return to top of page · Post #: 6 · Written at 3:17:37 PM on 15 October 2019.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 563

Thanks Marc all taken on board, i'm onto a coin project at the moment (the varieties of the 1952 dated Perth Mint pennies) and will swing back to the radio when I get that out of my system. I was even doubtful if the tapped coil set up would actually work and was amazed when it fired up with my plain guess work coil turns. More work to do with the 6BE6 however.

Interesting tube the 6SA7 and on a different subject I found it worked so well as a convertor but even more so when using it as a discriminator in my syncrodyne experiments. It was a very "smooth" valve to use if I can use a very non tech term!

Fred.


 
 Return to top of page · Post #: 7 · Written at 4:53:41 PM on 16 October 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3685

Americans seem to have issues with it, it can be a swine to get oscillating, which is probably why the valve manufacturers say "external exciter". It appear to have similar features to a Pentagrid like 6A8, so an oscillator formed with grid one similar to 6A8, I would see as workable.

The problem of course is balancing the level of signal as mentioned, to sustain a decent modulation. As we know (and it is written by the prophets who make them) as the frequency increases with 6A8 and others (Not 6SA7) AGC serves to destabilise. That is why AGC is commonly discontinue on a SW band. However, if there is a TRF stage first, AGC can be applied to it, to help modulation.

All entertaining.

Marc


 
« Back · 1 · Next »
 You need to be a member to post comments on this forum.

Sign In

Username:
Password:
 Keep me logged in.
Do not tick box on a computer with public access.