Junkbox Trainwreck

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gingertube
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Location: Adelaide, South Oz

Junkbox Trainwreck

Post by gingertube »

I had an old Australian AWV PA Amplifier with cathode biased 6CA7 (EL34) Outputs. One Sunday I ripped out the line output tranny and replaced it with a spare 50W Marshall (Raa 3k4) which I had on the shelf. Stripped the front end and rebuilt it as the A0 Tranwreck circuit (with mods as below).

It was good but very noisy (hissy) and I just put it aside. The local "guitar god" noticed it sitting in the corner when he dropped in to see me about something else and asked if he could borrow it to give it a whirl.
Well he liked it so much he has been using it for gigs ever since and doesn't want to give it back.

On the weekend I managed to get it back to do a full circuit trace of what I did. Schematics are attached. During the trace I noted that the resistor between the volume control wiper and the next stage grid was 470K. The A0 circuit showed this as 820 Ohms. Whoops - replaced it with a 1K and now the hiss has gone. It was all coming from that post volume control stage - too high an impedance to shunt grid noise effectively.
Not much special in the power amp - I did do a couple of HiFi'ish things, the shared 470 Ohm screen resistor and splitter anode loads adjusted for best balance.

Also note the "relaxed" operating voltages in the power amp - 35 Watts out rather than the 55W most try to "screw" out of a pair of 6CA7/EL34. I'm convinced this adds to its lovely sound.

Note also the parallel front end with a twist, dissimilar operating points for the paralled triodes - that was'nt in the A0 circuit, that is an idea of my own, for a bit of harmonic emphasis.

So for your possible interest - The "Junk Box Trainwreck" ala Gingertube.
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John_P_WI
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Re: Junkbox Trainwreck

Post by John_P_WI »

Cool Ian! Love the hand drawn schematics. How do you find the bass response / tonal balance on your build? Interesting take on the fixed power amp / no presence either.

See if your guitar slinger will record a clip or two...

Thanks,

John
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Reeltarded
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Re: Junkbox Trainwreck

Post by Reeltarded »

Junkbox Gingertube! That sounds like an amp title for something awesome!

I would love to hear that thing. I like the lowering voltage approach. I want to hear it too.
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
gingertube
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Location: Adelaide, South Oz

Re: Junkbox Trainwreck

Post by gingertube »

Guys,
Are the schematics readable.
I scanned them at 300 dpi and then used Irfanview to resample/resize them to 600 x 800 pixels. I can repeat this at higher resolution if necessary.
Cheers,
Ian
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overtone
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Location: 230V Frankfurt

Re: Junkbox Trainwreck

Post by overtone »

I can read your fine schematics well, ok, but I do zoom in a few times.

Thanks for posting this, I just got an Australian 2 x KT66 PA with a 16 ohm tap thrown in. I like the schematic printed on the backside of this one!
Still have to smoke test it.

Best, tony
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John_P_WI
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Re: Junkbox Trainwreck

Post by John_P_WI »

Ian,

Yes, the drawings are legible at screen resolution. Thanks.

John
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overtone
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Re: Junkbox Trainwreck

Post by overtone »

Ian,
could you explain "the hifi thing" with the single shared G2 resistor that you mentioned in your OP?
Usually I have the screen resistors on the socket, to double up as a stopper.

An amp that I just got in has 1k5 G2 stoppers plus a single shared 1k "before" them. (4 x KT77 circuit and I am wondering if the shared 1k is to be kinder to the screens of later EL34s)

So I am interested in the consequences of these different approaches... err, if any?

Best, tony
gingertube
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Re: Junkbox Trainwreck

Post by gingertube »

Tony,
The "ultimate" for HiFi Amps is a regulated supply to the screens. You have to remember that G2 (the screen) is a grid and the tube will exhibit some gain from signal at G2, so you don't want any signal at G2.

In a push pull amp the screen signal current will increase with tube current on one side, andt decrease with tube current on the other side, and then swap for the other half cycle.

The average total screen current from both sides of the push pull are therefore reasonably constant and a common screen resistor is used to keep the screen voltage constant.

When you start running tubes hard, particularly at higher voltage, it has been the "norm" to sacrifice this shared resistor scheme for higher value individual screen resistors in order to protect the tube. That, is screen voltage varies somewhat with signal, performance is sacrificed for tube reliability.

I'm running only 380V on the 6CA7/EL34 and not biasing them very "hot" so the shared screen resistor can be used.

Cheers,
Ian
Last edited by gingertube on Thu Apr 19, 2012 5:20 am, edited 1 time in total.
gingertube
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Location: Adelaide, South Oz

Re: Junkbox Trainwreck

Post by gingertube »

Tony,
I noted the Aussie 2 x KT66 Chassis you mentioned above. If you can read the transformer part numbers (power and output) there is a reasonable chance that I can find the data for them (if you would like), particularly if they are Furguesson or A&R Aussie transformers.
Cheers,
Ian

John,
The tonal balance is lovely so I had no desire/need to put a presence control on the thing. I admit that I originally left it off because there was no hole for it in the chassis I re-used and I was too lazy to drill a new one. Richard (the guitar god) uses it with a 12" Strauss Vintage Guitar Speaker in a home built box (2/3 open back cabinet). I test it with my low power "testbox", a vintage Aussie Etone 10" guitar speaker in a closed box with a very loose tune port (fancy name for an open 3" hole in the baffle below the speaker).
Cheers,
Ian
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da Geezer
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Re: Junkbox Trainwreck

Post by da Geezer »

gingertube wrote:Guys,
Are the schematics readable?
Yes they are.
I can even read them on my iPad without my magnifying glasses on! I sometimes have trouble doing that even on a full-sized screen!

Geezer
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overtone
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Location: 230V Frankfurt

Re: Junkbox Trainwreck

Post by overtone »

Thanks Ian for the info,

with regard to the shared screen resistor:
when one sees both a shared screen resistor leading to G2 "stoppers" would that possibly be intended to get the best of both approaches?

This Australian PA amp is an AWA Amalgamated Wireless Australia PA872. I looked it up on the OZ Valve amps site: http://www.ozvalveamps.org/awa.htm

There are no markings on the OT, but it I get 6k6 on the 16 Ohm tap.
The PT is marked 50483 in large stencil type letters low down on one of the bell covers - which are all perforated. There is also a small 46 legible. All printed, not stamped.
Injecting 2,4V in with all valves pulled I measure 6,05V across the HT, so the 290v PT output as per the schematic looks fine.

The previous owner was using it for harp and some parts have been changed out. Before I do anything with it at all, I would like try it "as is".
The Cathode resistor has me scratching my head, 1k8. It looks like the preamp valve heaters are running in series on DC taken from the output valve cathodes. Never seen that before.

Cheers, tony

edited 14th Dec. 2012 because I missed a comma in the 6,05V
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Last edited by overtone on Fri Dec 14, 2012 5:55 pm, edited 1 time in total.
gingertube
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Re: Junkbox Trainwreck

Post by gingertube »

The 1K8 is in parallel with the 2 preamp tube heaters which are wired in series.

The preamp tubes heaters (12A_7 series) obviously run at 12V @ 150mA.

That means that the output tube idle current (for both tubes) needs to be about 150mA or 75mA per tube and the bias voltage will be about 2 x 12.6 = 25.2 Volts. The 1k8 just trims that setting and gives a "safety" tie to the output tube cathodes for when you pull a preamp tube an break that heater chain. Using preamp tube heaters as the cathode bias resistor is a technique which was used a lot "back then".

One thing to watch with amps of this vintage is that they tended to run the output tubes in particular, really hard. It was cheap to just stick a new pair of output tubes in it every 6 or 12 months so they did'nt care about hammering them.

It saved the cost of a power resistor and gave "free" DC heaters for the front end preamp tubes.

The shared screen resistor should be the last thing before the common screen connection. With separate resistors following a common resistor you will still get individual tube screen currents affecting the screen voltage. As far as I know the shared screen resistor was a Mullard idea.

I looked at the transformer numbers and they don't stack up with the usual A&R or Furguesson tranny numbers. They were probably rebadged for AWV who built these things. AWV ran one of the 3 tube factories we had in Oz. The factory was in Sydney and they had a big applications engineering section, they published the "Radiotron" tube magazine and the chief engineer was Fritz Langford-Smith who edited the Radiotron Designers Handbook while working there.

Cheers,
Ian
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Timbo63
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Joined: Wed Aug 15, 2012 12:29 am
Location: Blue Mountains-Sydney

AWA PA1001

Post by Timbo63 »

I have taken some time to photo an amp I have been trying to bring back to life as a guitar amp. I am not an electrical engineer ,I have basic knowledge of AC-DC circuit design and application. I have tried to find some information on OTs with multiple output sections with no luck as the AWA OT was originally designed for 600 ohm speakers. I have attached a series of photos of the amp for all to see as there does not seem to be many examples around. My second thought was to modify the amp to a New zealand designed Fireball 120, do you think thats a mad idea?
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I have a Philips 957C -Moody BR17- Goldentone 1750 - and 3 Phillips EL36 Modified cinema amps and a AWA PA1001 amp and lastly CBS Arbiter L120 with support photos.
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Timbo63
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AWA PA1001

Post by Timbo63 »

I have just 2 more images of the multiple wiring sections of the AWA PA1001 OT, and the tone section.
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I have a Philips 957C -Moody BR17- Goldentone 1750 - and 3 Phillips EL36 Modified cinema amps and a AWA PA1001 amp and lastly CBS Arbiter L120 with support photos.
gingertube
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Joined: Mon Nov 14, 2011 2:29 am
Location: Adelaide, South Oz

Re: Junkbox Trainwreck

Post by gingertube »

Timbo,
The output tranny as shown in your pics above is strapped for a single 8 Ohm speaker. You can probably use the entire power amp section unchanged.
Cheers,
Ian
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