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 Western Electric 717A pentode sub for 12SK7 in tube 'AA5' radio
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 Return to top of page · Post #: 1 · Written at 12:04:06 PM on 20 June 2023.
Wa2ise's avatar
 Location: Oradell, US
 Member since 2 April 2010
 Member #: 643
 Postcount: 830



Unusual tube substitutions in an AA5. Here is a Western Electric 717A where a 12SK7 would usually be. It's a sharp cutoff pentode. The heater current is spec'ed at 175ma at 6.3V but on 150ma in the heater string it ran at 5.5V. a bit low.. I then added resistors to the series string to make up the extra current for the 717A beyond the string's 150ma. See diagram. The values I used ended up being more resistence than I calculated, to get the 717A's heater to 6.3V. Seems my tube wants about 160ma, not the rated 175ma. In a parallel wired heater system, this wouldn't matter, so it seems the manufacturer didn't worry about it As Aussie radios were mostly if not all transformer powered, with parallel heater feeds, this heater current issue does go away.

The military designated as VT 269. It is a 6AK5 laid on its side It is used to improve receivers. It took the place of the 6SK7, 6SJ7.and the other pentides on the receivers at that time. As it's a sharp cutoff pentode, it has less AVC action, but you can get less signal distortion in the final IF stage on AM stations. It changed the transconductence from 675 to 1,440 uohms. You may want to touch up the IF alignment. Try it you will be amazed, and besides, it looks cool on the radio chassis!


 
 Return to top of page · Post #: 2 · Written at 4:12:22 AM on 21 June 2023.
NewVista's avatar
 Location: Silver City WI, US
 Member since 10 May 2013
 Member #: 1340
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0.03 cap in audio output would cause positive feedback?


 
 Return to top of page · Post #: 3 · Written at 6:46:41 AM on 21 June 2023.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1267

The 0.03 is definitely a feedback loop.
The cathode is not directly bypassed so has an AC level.
That causes one change to the sweep response of the stage.
Then the plate feedback causes another change.
I am guessing that set up would provide a mid range boost then roll off.
Also the cap provides a damper across the primary of the OPT.
You would have to sweep the stage between say 10cps and 50Kc and see what the response is.
Always interesting to see a designer employ regeneration in a circuit.
Fred.


 
 Return to top of page · Post #: 4 · Written at 3:02:02 PM on 21 June 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
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The other odd ball thing distinguishing it as a little bit different, is the delayed AGC by splitting the two diodes. Don't like their reference to "ground" that really should indicate chassis.

Marcc


 
 Return to top of page · Post #: 5 · Written at 6:47:29 PM on 21 June 2023.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2405

Fascinating tube! Thanks for bringing it to our attention!

6AK5, eh? I can imagine it being used to upgrade VHF omnirange receivers used in planes.

The 717A is not cheap tho, just checked. US$22.95.

I seem to remember reading somewhere a long time ago that that OP circuit was designed to get a doubling of the bass notes so that a small speaker could reproduce them better. Does it work like that?


 
 Return to top of page · Post #: 6 · Written at 8:52:08 PM on 21 June 2023.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1267

Marc, something about that AGC circuit.
The AGC voltage is a derivative of the IF level and the Audio level?
Am I reading the circuit right?
There are two source points,
One, the negative DC caused by the top diode.
Two, The AF component, picked off the volume control.
I am a bit confused because in a valve "diode" circuit there is the "contact potential" from the electron stream as well as any DC rectified level.
Is the lower diode just creating the "delay" pedestal for the varying voltage to sit on?
Fred.


 
 Return to top of page · Post #: 7 · Written at 9:43:45 PM on 21 June 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

Fred:

If you look at 6B8 data sheets (Franks electron tube pages http://www.tubedata.info/) There are several ways of doing things. In a large proportion of radio's with delayed AGC the signal for it, is taken off of the plate of the last IF tube and fed to one of the diodes whilst the other one takes care of the audio. It is set to cut in / become effective only after a certain voltage is attained.

That is usually done to stop direct shunting of the diode circuit. Taking the lower voltage from the primary also stops shunting and actually improves the other diodes behaviour. All of that headache material is in the Radiotron Handbook.

The reason they did it that way in this set, albeit differently has got some merit.

As an aside 12SA7 works completely differently than the other Pentagrids and is a true BC band tube. It is much more voltage stable & will tolerate AGC on SW however, above 6MHz it is likely to need a separate exciter tube. It has no true oscillator grid: That is why a tapped Hartley configuration is preferred.

Marc


 
 Return to top of page · Post #: 8 · Written at 5:00:39 AM on 22 June 2023.
Wa2ise's avatar
 Location: Oradell, US
 Member since 2 April 2010
 Member #: 643
 Postcount: 830

QUOTE: 12SA7 works completely differently than the other Pentagrids and is a true BC band tube. It is much more voltage stable & will tolerate AGC on SW however, above 6MHz it is likely to need a separate exciter tube. It has no true oscillator grid: That is why a tapped Hartley configuration is preferred.


You could think of the 12SA&'s cathode, grid 1 and grid 2 as a triode oscillator circuit. Grid 2 like a triode plate. This makes a pulsating flow of electrons that then passes through grid 3, the antenna input. At this point, the pulsating flow mixes with the antenna signal,, creating the IF frequency.

QUOTE: The 717A is not cheap tho, just checked. US$22.95.


I picked up mine at a "hamfest" (amateur radio fleamarket) For a dollar. From someone looking to unload stuff.


 
 Return to top of page · Post #: 9 · Written at 12:20:41 PM on 22 June 2023.
Brad's avatar
 Administrator
 Location: Naremburn, NSW
 Member since 15 November 2005
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What things go for depends much on what the seller knows about the product they are shifting and also how badly people want it. I once bought an AWA Radiolette, model 500M in jade green for $80.00 from an antique shop in Concord, NSW. On an Ebay auction, the radio would have easily gone for up to $1,000.00 despite having a damaged dial pointer which took me another ten years to locate and I ended up paying more for that than the radio itself.


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

 
 Return to top of page · Post #: 10 · Written at 4:34:40 PM on 22 June 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
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 Postcount: 5303

The thing is and noted by RCA; That if you are working with 12SA7, which can be cantankerous, anything that you learnt from the other Pentagrids, will not apply to it, as its construction is entirely different and its internal resistance is different, as it is the only one with a suppressor grid.

Marc


 
 Return to top of page · Post #: 11 · Written at 1:03:06 AM on 23 June 2023.
NewVista's avatar
 Location: Silver City WI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 977

QUOTE: 12SA7 works completely differently than the other Pentagrids

This made me wonder if I had any AA5's with 12SA7 and found an RCA 8-x-541 (w crumbling pwr cord) (and further economies like not using 2nd 12SQ7 diode for AVC, just decoupled audio) But It has secondary winding coupling from the Osc tank.


 
 Return to top of page · Post #: 12 · Written at 9:55:17 AM on 23 June 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

As noted. for 6B7 & 6B8 their data sheet from possibly RCA does show the several ways that you can wire it up. There are around five.

I did note why delayed AGC for one, was wired to the last IF primary and only used one diode leaving the other for the audio.

https://frank.pocnet.net/sheets/201/6/6SA7.pdf
https://frank.pocnet.net/sheets/049/6/6SA7.pdf
https://frank.pocnet.net/sheets/021/6/6B7.pdf


 
 Return to top of page · Post #: 13 · Written at 10:09:25 AM on 23 June 2023.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
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STC in the late thirties also did not use the second diode to provide delayed AGC even in their 8 and 10 valve sets. Often wondered why; can't be economy for such high range sets. Maybe no good for the SW bands.


 
 Return to top of page · Post #: 14 · Written at 5:07:00 PM on 23 June 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

Might have to look at what they are doing with the AGC. Some actually used 6H6. Albeit basically a charge, there may be a need for extra voltage for an eye tube. Probably need to have a look at the manual.

6SA7 is actually the only Pentagrid other than 6K8 that is not overly destabilised by AGC on SW. 6A8 being the horror tube, even admitted by RCA.

Not sure with the tubes with separate exciters like 6J8 & 6K8. The latter tends to suffer flutter & needs 8μF on the oscillator grid but is a better tube than 6J8 which is possibly why, with it being somewhat voltage stable, its in the BC-221-N Frequency meter.


 
 Return to top of page · Post #: 15 · Written at 8:25:49 AM on 24 June 2023.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1284

STC used to claim "For Tone it Stands Alone". So would perhaps be looking for the best fidelity that they could get. I seem to recall that on the transition from delayed to not delayed AGC there is a distortion problem; maybe avoided delayed AGC for that reason. Would need to trawl through Radiotron Designers Handbook to find that reference.


 
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