The KT61 has a "drop" socket to give it headroom. I wonder why they changed the output valve so often, it's hard to believe they would be developing valve technology so rapidly at the time to justify changing specs so often in such a short period of production.

It would have been just as easy to use a 6V6G as that valve was probably the most popular of all output valves and AWA was already making that valve themselves so the fact that they used a few different ones is hard to reason.

My all-octal 429MA does have a dropped socket though it is fitted with a 6K6GT which is slightly shorter than most GT valves. I seriously doubt it is the correct valve for this receiver.

I am thinking that AWA changed from octals to baseless miniature valves shortly after production of the 429MA began and this would explain why the first version isn't covered in the AORSMs. This radio was also made at a time when factories were not only converting from octal to baseless valves but also from Bakelite cabinets to the use of oil-based plastics. The 429MA is therefore a bit of a bitser - Bakelite cabinet with plastic control knobs and a mix of octal and miniature valves for most of its lifecycle. It is a good performer though probably aimed towards the budget end of the market.

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A valve a day keeps the transistor away...

Radiomuseum gives it as N78 or KT61:

http://www.radiomuseum.org/r/amalgamate_radiola_429ma.html.

Also mentions AORSM volume 8.

6AQ5? How confusing! Radio Museum shows the 6AQ5 is the miniature equivalent of a 6V6GT. I haven't seen the circuit for the later 429MA but does this mean a 6V6GT would be a suitable substitute for a KT61 in an octal chassis? Would a 6AQ5 substitute for an N78?

I think I need to learn how to interpret valve data!

Warren

Having a valve data book has helped me a lot in the past. They were published by AWA and Philips, and possibly others too. Watch Ebay for them as they do come up occasionally.

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A valve a day keeps the transistor away...

The 429MA amended version, page 73, volume 10, AORSM, was well spotted, Brad.

There are, according to the circuit, two minor changes. R9 (500K) changed to 1M & R8 (75ohm) changed to 175ohm. These changes would alter the back-bias and the grid voltage to suit a 6AQ5 or 6V6GT.

It now doesn't matter whether you have a KT61 or N78 version of this chassis. With the circuit change you can now either use a 6AQ5 or 6V6GT.

The only thing not mentioned is if there is a difference with the speaker transformer primary impedance, 5K or 7K.

Scraps, I've sent you a copy of page 73, Vol.10 of the AORSM so you can make a comparison with Kevs copy.

And the good news is that the 6V6GT is still in current production in China & Russia and are cheap!

Cheers.

It's a shame that the pressing equipment that AWA and Philips used to make all our valves would be long gone. Imagine being able to make popular valves on an ad-hoc basis for the Nation's vintage radio collectors and servicemen...

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A valve a day keeps the transistor away...

You should see the prices commanding for KT61's these days. They are highly sought after by HI-FI nuts.

In fact I'd like a matched pair myself for a Leak TL10 mono-block I have. I'm just waiting to luck out on finding some in a mixed lot of valves.

There are a lot of discussions in the HI-FI forums as to whether you can use a 6V6GT as a substitute. The general agreement is that a 6V6GT would probably work but you would have to make some circuit changes.

The closest valve I know to a KT61 is a 6AG6G. This valve was made by Brimar (STC) in the UK and are found in some Australian made STC radios. The 6AG6G is probably even more scarce than the KT61.

Back in the late 1940's, (maybe due to post-war shortages) AWV started importing some Marconi Osram Valve types from the UK and re-branded them Radiotrons. The types that come to hand are: KT61, N78, X61M & X79. AWV eventually manufactured the X79 themselves as type 6AE8/X79.

I guess this is why we have a few oddball valve types in some late 1940's AWA radios. It would be interesting to see if the AORSM's have mods for other AWA models using M/OV valves types.

Cheers.

What a strange radio this is. Thanks for the copy of the Amended circuit. I've noticed a few other differences apart from the two resistors. R7 and C16 as well as the cathode no longer go to ground, the cathode's now connected to R6 (volume control) through R7 and C18 as well as the speaker transformer secondary with the other side of the secondary going to ground (back bias? I don't know enough to understand the significance of this change). C23 has also been removed from the list but for the life of me I can't find it on the original circuit anyway.

I think the 6V6GT likes about 5000 ohms so I can rig up a power supply and measure the existing transformer, fingers crossed it's in the ballpark. I take it that N78/KT61 need a different impedance.

All in all, because the 6V6GT is so readily available, it's worth making the change.

Cheers,

Warren

I think the 6V6GT likes about 5000 ohms so I can rig up a power supply and measure the existing transformer, fingers crossed it's in the ballpark. I take it that N78/KT61 need a different impedance.

From a quick look around, running Class A single ended mode with a Va of 250:

N78 likes 7K and KT61 likes 6K.

Scraps,

Here's a couple of photos of the Radiola. This model also has a nickname - the Champion. Not sure where the nickname comes from though.

I think all this talk about this model is making me ponder on restoring the other one I have. I've always liked this model, mainly for its looks.

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A valve a day keeps the transistor away...

I like them too, I'm torn between the simple 500/516 and these - the last of the AWA bakelites. The AWA's from the later 50's don't appeal in the same way.

Let me know your details and I'll get the payment done. There will be a great radio that comes from all this!

Cheers,

Warren

I think the good thing about this model is that the bezel around the tuning dial is part of the Bakelite moulding rather than plastic as per other models made around the same time. Sadly, due to this and later models having backs on them, the top of coloured versions are highly susceptible to heat damage and are seldom offered for sale without cracks on the top even when the rest of the receiver is in good nick.

This radio was dropped in 1992 by one of my mates acting the goat. Unfortunately these things just don't have the same flexibility as a soccer ball.

I'll send you an e-mail tomorrow arvo.

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A valve a day keeps the transistor away...

Ouch, is he still a mate? When I first saw the 429 I wondered about putting a very small computer fan on the 6 volt heater supply. They're very quiet and don't draw much current, has anyone tried this?

Warren

Ouch, is he still a mate?

Yep, but I was definitely capable of murder that night! I think the fact that I have two of them is what saved his bacon.

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A valve a day keeps the transistor away...

Hi Warren.

Most Australian radios using 6V6GT/6AQ5's generally have 5K on the primary of the O/P transformers. But this appears not to be a hard and fast rule. I have seen 7K transformers used with 6V6GT/6AQ5's and this has been part of the original circuit. Also the Kriesler Beehive with a hundred million variations uses a 12 K transformer. I am not super-dooper technical when it comes these matters, but fellow member, Marcc, recently pointed this out:

"The OP transformer impedance on 6V6 is also dependent on the voltage applied to the plate."

I dont have the 429MA circuit with me at the moment so I dont know what the plate voltage is for the output valve. I guess you'll know once you fire up your radio whether it is the right transformer or not. It probably wont sound quite right if it is wrong.

Cheers.