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

Tech Talk

Forum home - Go back to Tech talk

 AWA PA449Y radio tuner kit option in 1960's PA amps
« Back · 1 · Next »
 Return to top of page · Post #: 1 · Written at 10:00:59 PM on 22 July 2020.
Trobbins's avatar
 Location: Melbourne, VIC
 Member since 11 July 2012
 Member #: 1179
 Postcount: 42

AWA had available a MW radio tuner module that could be fitted to their public address amplifiers of the 1960's, with a model number PA449Y.

The tube complement in the module was 6BA6, 6BE6, 6N8, OA2 and EM81.

A topside photo is in the link: https://www.dalmura.com.au/static/AWA%20PA449Y%20tuner.jpg

So far I haven't come across a schematic for that module. Kevin Chant's website has quite a few AWA radio schematics, but I'm not sure where to start searching to see if I can identify similar circuits, and then cross check to see how close I can get to the PA449Y.

Anyone able to provide some direction, or is it a case of trawl through all the schematics ? Unfortunately the 449-MA receiver doesn't align.

My final aim is not to completely restore the radio module, but more about providing a complete set of data for the PA amps I have, and possibly to modify the circuitry to use the EM81 somehow.

Ta, Tim


 
 Return to top of page · Post #: 2 · Written at 11:45:31 PM on 22 July 2020.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6013

If you don't get the answer here, suggest contacting John McIlwaine, President of NSW Branch of HRSA. John worked for AWA for a long time and is a font of knowledge on most aspects and is in contact with other AWA veterans.


 
 Return to top of page · Post #: 3 · Written at 10:07:57 AM on 23 July 2020.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4345

This is on Kevin Chants site. https://www.kevinchant.com/uploads/7/1/0/8/7108231/pa1001_advert_awa.pdf

They did make a 449 radio in 1952 era with a different valve compliment.

It is not unusual to have a variant of a radio, or valve appearing elsewhere. I suspect 6M5 appears as the pentode in other valves like 6CK6 where the only apparent difference is the wiring of the suppressor.

Most unusual was an AWA R301 variant. It shares the same parts list as the R301, but if you look care fully there are parts not in the production model. I have never heard a radio like it. "Golden handshake job... No expense spared.

No Hum at full volume. (two filter chokes)

Add on Push pull amp built on chassis with a 6A6. Original OP tube left but back bias abandoned for self bias.

Bigger transformer, retained bandpass filter. 3 position a.v.c. switch but still had delayed AGC. 3SR from Wangaratta needed a.g.c. to be increased to step 2. to cut back the signal.

If you get your hands on one of the 449Y's: Reverse engineering is going to get a circuit.

Marc


 
 Return to top of page · Post #: 4 · Written at 1:08:04 PM on 23 July 2020.
Trobbins's avatar
 Location: Melbourne, VIC
 Member since 11 July 2012
 Member #: 1179
 Postcount: 42

Thanks guys - I will shoot John McIlwain an email now.

The circuitry is pretty simple, as I've gone through the magic eye and powering section, and there are a few 3 valve radios using 6BE6, 6BA6 and 6AV6 in Kevin's collection of schematics/manuals that must be pretty close (like the B15, but it only has a 2-section tuning gang) - although I found it odd that I haven't come across any that included a magic eye.


 
 Return to top of page · Post #: 5 · Written at 9:23:55 PM on 23 July 2020.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4345

Magic eyes were more common on up market units. 3 Gangs will often be a set with an RF Pentode as a TRF stage ahead of the 6BE6. OA2 is also a bit unusual. However, at a minimum it will likely be stabilising the voltage to 6BE6.

So we are one tube short in your count. 6N8 has diode plates and is likely last. 6BA6 is most likely for the IF amp. 6BE6 is of course the pentagrid. The missing one will be identified by the pins, 6N8 has 9pins, 6BA6 7pins if you find a bare chassis, the tubes will be a different diameter.

Much rarer outside a TRF is the 4 gang tuners some Midwest's have, as they use a Colpitts oscillator (like many signal generators) and it requires two gangs just for the oscillator.

All fun.


 
 Return to top of page · Post #: 6 · Written at 11:25:49 PM on 23 July 2020.
Trobbins's avatar
 Location: Melbourne, VIC
 Member since 11 July 2012
 Member #: 1179
 Postcount: 42

The OA2 is likely used due to the supply B+ feed from the PA amplifier varying due to output power - which may have degraded tuning performance?


 
 Return to top of page · Post #: 7 · Written at 2:49:51 AM on 24 July 2020.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4345

Any variation in voltage can upset most oscillators, but one would need to see the voltages. I have a Philips "Line Amp" here & the 6CM5's class "B" PP are running at 300V 2x100mA Plate, so its add on amp would need to have it volts dropped to 250 or less.


 
 Return to top of page · Post #: 8 · Written at 9:28:39 AM on 29 July 2020.
Trobbins's avatar
 Location: Melbourne, VIC
 Member since 11 July 2012
 Member #: 1179
 Postcount: 42

It seems the main influence on B+ variation for the radio circuitry would be the EM81 and any dropper resistance (that would have been the alternative if an OA2 regulator was not used) to the main B+ in the amp.

The EM81 supply current varies with indicator level by about 1-1.5mA, and the amp's 420VDC main supply is used for the radio module feed.

If they had fed the radio module from a lower 'dropped' voltage within the amplifier circuit then the EM81 current demand variation would have also caused gain variation within the amplifier circuitry, so they pretty much had to take the radio feed from the 420VDC raw B+ of the amp. So this indicator option required not only an OA2, but also two 10W resistors.


 
 Return to top of page · Post #: 9 · Written at 11:35:05 AM on 29 July 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1910

I think you are missing the main reason for the OA2.

The amp's output stages (typically a pair of KT88s in AWA amps) ran in class AB2, which meant that, particularly when heavily loaded, the amp would pull down the 420v B+ quite substantially on peaks and sustained tones, possibly by anything up to 100v. Especially if the amp had a thermionic rectifier and it was getting a little tired. No wonder the tuner B+ needed to be regulated. The effect of the EM81 variable demand would be quite insignificant by comparison.


 
 Return to top of page · Post #: 10 · Written at 4:47:52 PM on 29 July 2020.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4345

Voltage will vary the EM81 a bit, however I would stick to the theory that the only way the radio part is going to stay stable, especially the oscillator is by using a regulator to maintain a constant "B" voltage to the critical parts of the radio section, or all of it. The "Radiotron hand books" (mainly) will attest to that.

This chassis was not a "happening" it was thought out. 0A2 is 150V. 6BE6 will run at 100V: http://frank.yueksel.org/sheets/049/6/6BE6.pdf

Marc


 
 Return to top of page · Post #: 11 · Written at 4:57:40 PM on 29 July 2020.
Trobbins's avatar
 Location: Melbourne, VIC
 Member since 11 July 2012
 Member #: 1179
 Postcount: 42

Ian, that is what I initially presumed in post #6, however with my restored amp the 420VDC only droops to 410V when output is grossly clipping.

That seeming lack of droop could be assisted by a few aspects:
- UL operation with individual cathode bias.
- ss rectifier with quite a stiff power transformer (8 ohm primary and 4 ohm secondary DCR).
- I am running 6L6GC rather than KT88, and have increased the screen stoppers a bit.

Link to PA 30BZ schematic: https://www.dalmura.com.au/static/AWA%20PA%2030BZ%20schematic.JPG


 
 Return to top of page · Post #: 12 · Written at 8:31:29 PM on 29 July 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1910

Yes but what happens if the amp is running into a too-heavy load, as commonly happens? That's the scenario AWA would have been addressing. And with cathode bias you'd be running in Class A or maybe AB1 when pushed hard. That, and the stiff power supply explains the lack of droop.

With RC coupling the bias gets increased with heavy drive due to grid current flowing on positive excursions so that also limits things.

A higher than nominal load would also defeat the neon - CDS limiter.

On reflection I think the varying tuner current draw due to AGC compounded by the necessarily large dropper resistor would be the main reason for the OA2.


 
 Return to top of page · Post #: 13 · Written at 12:00:14 AM on 30 July 2020.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4345

I would expect with a 10V fluctuation in "B" Voltage that the tuner would be absolutely unstable. Bias, & AGC would fluctuate & as many pentagrids are voltage sensitive, its oscillator frequency would be unstable as well: Nightmare.

Beware I have a HMV 5101, benched & stalled at the moment (currently has a chainsaw in front of it). It is supposed to use KT88. As noted some time ago, it has a metal 6L6 in it. Pin one was used (factory) as a tag of convenience. Pin one of a metal 6L6 is its body so the shell was alive, instead of being grounded: Scary.

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.