Hi All,

First time poster. I realise this is a radio site, but hoping someone may have information on vintage valve amplifiers.

I have a 1940s era AWA valve amplifier that is missing one valve. Despite my best Google-fu I am unable to find any relevant information. I've owned this amp since my teens in the 1970s. My recollection is that the glass broke and the label had rubbed off.

Ideally I would like a circuit diagram, but failing that suggestions for the missing valve would be appreciated.

Photos here https://photos.app.goo.gl/Fj3oeagxGcgKKoEw7

The missing valve is in the front right hand position. My guess it is part of the pre-amp stages. The valve behind was never installed and this didn't stop it working. The socket front left is for a vibrator allowing the amplifier to run off 6V DC.

The other valves, left to right are 6X5 (replaced by two diodes), 6V6GT, 6J7G, missing valve. The connectors across the front of the chassis are for various speaker impedances and inputs.

Regards,
Glenn

Welcome to the forum!

For the circuit, have a trawl through here, you may find something close.
https://www.kevinchant.com/awa-includes-thorn.html

I'd say your missing valve would be another 6J7.
If you can't find a 6J7, a 6U7 would work.

AWA used to use these in their PA hire business in the 40's 50's and early '60s.
They'd turn up at an outdoor venue such as a school sports carnival with horn speakers on a roof rack on a van - usually an FJ Holden.
Hence the 6VDC option.

Looks like it had an overhaul in the mid 60's. Probably won't need much work to get it running.

There is a white UCC "Hi-Qual" cap there. It is paper. Get rid of it!

Two UCC's and those electrolytics will be way past their use by date.

Here's a close relative from that era.

QUOTE: The 6J7 was for two decades (from the late 1930s) the international standard valve for audio amplifier input stages and audio systems generally.

Thanks Ian,

This amplifier came from the Commonwealth Bank in Manly. It wasn't working when I got it, and I took the valves down to the Tandy store in Brookvale to test them. The 6X5 had failed, and I replaced it with the two diodes. The system sprang to life after that. Unfortunately it has sat in my garage for the last 30 years and I don't recall when the valve broke.

The electrolytic caps in the cans look a bit suspect, so I'll try to replace those too.

Any ideas on the black capacitors (?), one near the volume control and one under the tag strip on the left?

I'll keep looking through the Kevin Chant site.

Regards,
Glenn

G7061 appears to be what it is. My advice is to toss every electrolytic The two UCC caps which appear to be paper and the two big black mud caps. One already has a cracked jacket, that is not to be tolerated, they're likely paper get rid of them.

Judging by the size of the transformer that may well be a push pull and its another 6V6? The HT may be 315V

What may make it difficult is that it could be classed as a Line or Monitor Amplifier.

One can always bite the bullet & reverse engineer it. Tedious but not uncommon here.

Marc

Hi Ian, GTC and Marc,

I was gently dusting off the chassis and miraculously the old valve labels appeared on the chassis. I've added some photos to the album above, and this confirms the missing valve is a 6J7G.

I had a much closer look in the chassis. I found another 3 electrolytic caps under the tag strip that look a bit dubious. And another 5 smaller black caps too. The 4 electrolytics above the chassis are 2 x 16μF and 2 x 8μF 525V Peak. So would 400V be ok to use or so I need to find a higher rating?

Can you recommend anywhere that I might purchase the valve and caps? I've seen a couple of 6J7G valves on eBay but there's no guarantee they even work. There's a Jaycar near my home, but wondering if there is a place that might have these higher voltage caps?

Thanks for your input so far. Things are looking promising.

regards,
Glenn

The output transformer looks just like the one in the 35W amp at the Powerhouse Museum in GTC's message. The list of impedances on the front panel is the same.

The 4 electrolytics above the chassis are 2 x 16μF and 2 x 8μF 525V Peak. So would 400V be ok to use or so I need to find a higher rating?

You cannot go below the peak voltage of the circuit, so you need caps with peak voltages of 525 or higher. These days the common voltage in that region is 600 volts and they are considerably smaller than back when that amp was built.

If you intend to mount them under the chassis -- as opposed to re-stuffing the cans -- be sure to disconnect the originals.

Glen you might find a 6J7G hard to come by, they were not normally used in radios, the 6U7G remote cutoff version was far more common.

I've found this US site very helpful in the past even to the extent of free shipping-paid replacements if the tubes were bad.

https://www.tubedepot.com/products

I notice that a search for 6J7 brings up the bottle-shape -G version as well as the -GT which is shorter and the no suffix metal version.
Any of these would work, I'd probably go for the metal tube because it's the cheapest!

They also have caps that you could include to share the shipping cost.

If you can get a 6U7G it would work too at the expense of a tiny (read "inaudible") increase in THD and maybe slightly less gain.

Just had a browse for caps on that site and I'll withdraw my recommendation. at least as far as capacitors are concerned. They seem to be catering for the "golden ear" cohort with rubbish like paper caps for outrageous prices.

The JJ can electros there are reasonable value, I've used them in TV for years.

Have a look at Element14. Jaycar are not big on high voltage caps.

Or Wagners.

Tube Depot still seems to be OK for tubes tho. Pity I only ordered some TV repair tubes from them a month back, could have added your 6J7 to the order and saved shipping.

When I need 'paper' caps, electros or valves I simply go to ebay type in 'vintage radio capacitors' or similar.
Forget about Jaycar or Element whatever they do not understand vintage valve radio stuff.
There are Australian ebay suppliers who understand capacitors need to be 600v rated and resistors need to be 2 watt or greater package sizes. There is also that big vintage historical radio club thing with member access to valves and parts.

Every 6 months or so I just order a 'kit' of caps and resistors of many sizes from suppliers who may or may not post on this and other web sites. I get a bag full at reasonable cost and use those on new builds and repairs.

If I wanted a 6J7 I would just try ebay first, there are a couple of sellers there who I have bought many valves from.
2 volt, 6 volt 4,5,6,8 pins of all types. Most suppliers stick them on a tester of some sort and stand behind the sale.
I have also bought 'pig in a poke' junk lots of valves, untested and dirty.
Odd thing is most are ok and work normally.

I do not buy from suppliers with silly prices like $100's for a golden valve.

Cheers, Fred.