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Old 25th Nov 2007, 2:46 pm   #1
GrimJosef
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Default Eagle SA-80 do they come any simpler than this ?

Last week I picked up an Eagle SA-80 "8 watt" stereo amp from my local Freecycle (what do you mean I was robbed ?). After replacing the mains lead (the one it came with wasn't original and was only two core !) and gently reforming the power supply electrolytics it's now capable of making an impressively loud noise. Hi-fi it ain't, but for a circuit with what must be just about the minimum number of components I guess I shouldn't complain.

It's no surprise that it hums - half-wave rectification and single-ended output mean that huge and/or clever smoothing circuitry would be needed to stop that. My wife remembers her father saying that hum is how you know it's switched on and that if you turn the music up loud enough you can drown it out !

Any way it's more-or-less working but now I could use some help. My first problem is that both the output valves are drawing negative grid current (gas, I guess). One's just about OK (maybe 3 microamps) but the other one hovers around 9 microamps and is very temperature sensitive. If I turn the amp on its end the heat from the mains transformer warms the valve. The grid current quickly climbs to 14 micoramps and, I fear, would continue to rise if I left it that way. So I daren't try it with the lid on. I'm running it for a few hours to see if it improves, but it hasn't so far. So I'm really looking for one or two 30A5s or the European equivalent, HL94. I've contated RadioDaze in the US who list these on their web site, but I'd be grateful to hear of any a bit closer to home.

My second plea is for any information about this amp - I haven't turned up anything useful at all on the internet. I've traced the circuit (attached) and worked out what the connectors are and what the knobs do. But I'm completely in the dark about historical details. All I know is that it was made in Japan for export. When would this kind of thing have been on the market (clearly before Japanese industry became the world-leader in consumer electronics !) ? What would it have been used for ? How expensive was it ? Were a lot sold ? Are there any copies of literature (manuals, adverts etc) about the place. Is there a fan club (maybe only in Japan ...) ?

Any help or pointers would be very much appreciated.

Cheers,

GJ
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Old 25th Nov 2007, 3:05 pm   #2
Nickthedentist
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Default Re: Eagle SA-80 do they come any simpler than this ?

Not quite a Leak, is it?

But I admire its simplicity. It should be simple and fun to use
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Old 25th Nov 2007, 3:27 pm   #3
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Default Re: Eagle SA-80 do they come any simpler than this ?

I don't know much about the company, but they sold a wide range of budget electronics in the 60s and 70s. They made budget test gear as well.

It may be difficult to find those output valves at a reasonable price. I would consider changing the mains transformer to provide a 6.3V supply which will give you a range of options. EL91s are reasonably easy to find or you could even use EF91s if you don't need a lot of output power.

Don't forget to check the grid coupling caps before doing anything else

Paul
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Old 25th Nov 2007, 5:01 pm   #4
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Default Re: Eagle SA-80 do they come any simpler than this ?

I was about to suggest what Paul has outlined - but then I belatedly read his post! Another possibility is to use a separate heater transformer to power the EL91s, if you can find space for one.
-Tony
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Old 25th Nov 2007, 5:06 pm   #5
GrimJosef
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Default Re: Eagle SA-80 do they come any simpler than this ?

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Originally Posted by Nickthedentist View Post
Not quite a Leak, is it?
No it's not. But I believe most Leaks are push-pull, whereas this is single-ended. That'll be why this sounds so much better. (This is right isn't it ? I'm sure I've read it somewhere.)

Incidentally the sick valve seems to be getting better (fewer volts on the grid). I was going to swap them over to see if the difference followed the valves or stayed with the passives, which would have pointed the finger at the coupling caps. But for the time being I'm inclined just to let things cook. Thanks for the reminder about the caps though, Paul. They say 400V on them and they're only seeing 60 or so, but if they go then they can go completely of course.

GJ
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Old 25th Nov 2007, 5:35 pm   #6
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Default Re: Eagle SA-80 do they come any simpler than this ?

Are the smoothing caps really 150V as on your circuit? That's quite low HT for 8W output from class A. Unless of course it's 8W total, 4W per channel. Or not RMS.

If there's enough gain you could probably improve it a bit (if you wanted to) with some negative feedback. A resistor and capacitor in series between the two anodes (look at some radio circuits for typical values) would be a good start. Also try replacing the common output valve cathode resistor with separate resistors for each channel (double the present value) and not fitting caps across them. It depends if you are planning to use it regularly though, and if the sound quality as it is bothers you.
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Old 25th Nov 2007, 6:11 pm   #7
GrimJosef
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Default Re: Eagle SA-80 do they come any simpler than this ?

Yup, the HT capacitors are rated at 150V, but in fact they only run at about 105V except at switch-on (since there's a semiconductor rectifier) when they peak at 140-odd. The 30A5s are very low voltage valves. There's a datasheet here http://datasheets.electron-tube.net/...133/3/30A5.pdf which says "typical operation" is with just 100V across them. The maximum output power is then 2.1W and to get that you have to put up with 11% THD.

I think Eagle were fibbing a bit when they printed 8 WATT on the front panel. When I saw it my first thought was "8 WHAT ?". Actually the heater chain (all valves in series) draws about this. Maybe that's what they meant ? It should draw more than 10W, but the supply only delivers 66V rms so all the heaters are running below spec. And even then the mains transformer is too hot to touch. We are talking "cheap and cheerful" here !

I'm afraid the awfulness of this thing is, in a way, also it's redeeming feature. I'm inclined to keep it "as is", or at least "as it was meant to be" if I can fix these valves or get some replacements. But thanks for all the suggestions. I do appreciate the interest,

GJ
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Old 25th Nov 2007, 6:36 pm   #8
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Default Re: Eagle SA-80 do they come any simpler than this ?

Quote:
Originally Posted by GrimJosef View Post
The 30A5s are very low voltage valves. There's a datasheet here http://datasheets.electron-tube.net/...133/3/30A5.pdf which says "typical operation" is with just 100V across them. The maximum output power is then 2.1W and to get that you have to put up with 11% THD.
They are intended for use in AA5 type AC/DC radios running from 110V mains. That's why they're such a rarity here.

Paul
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Old 25th Nov 2007, 10:42 pm   #9
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Default Re: Eagle SA-80 do they come any simpler than this ?

ISTR Eagle were one of the "IHFM peak music power" brigade with their specs. Also, there's two channels,so don't forget to add them together

The hum should be amenable to a beefy silicon rectifier and CFT (capacitor .... tremendous) approach, unless it's caused by poor layout. You could probably get a 470uF that wouldn't be much bigger than the present 50uF specimen.

Chris
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Old 26th Nov 2007, 12:49 am   #10
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Default Re: Eagle SA-80 do they come any simpler than this ?

The nearest thing in the UK for this is the HL92. Rare, but probably not unobtainable.

Got a heater voltage of 50v though. Apart from that, it'll probably do.

Cheers,

Steve P.
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Old 27th Nov 2020, 4:34 pm   #11
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Default Resurrected-Eagle SA-80 do they come any simpler than this ?

Searching the site I came across this closed topic from 2007!
I have just acquired a little Eagle SA80 stereo amp with a slightly amusing history.
The amp is about 2w per channel, single ended 30A5 ,each channel driven by half an ECC81. The o/p TXs are tiny as is the mains transformer which supplies an isolated 72v for the heaters and 110v for the half wave rectified HT.
The history goes like this: my long time friend( from school days) became manager of a coach station in Manchester, responsible for the whole site.
Someone in the firm had the idea of a PA system to provide information for passengers etc. Items were obtained for this which included a very nice Telephone Rentals desk mike, a large Reliance 100v line re-entrant speaker and the little Eagle amp. Obviously this latter was the weak link and the system was never installed, going into storage in my friends loft, probably 30 years ago. The set up was passed to me last week . I will do the usual checks on the amp and see if it works, though I don't know if it is much use these days. The speaker looks like it could make a bit of a din connected to my Philips EL6420 (70W)
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Old 27th Nov 2020, 7:40 pm   #12
Ted Kendall
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Default Re: Eagle SA-80 do they come any simpler than this ?

Judging by the styling, probably early 1960s. Somebody just gave £180 for a later, restyled, example on eBay. A fool and his money...
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Old 27th Nov 2020, 7:43 pm   #13
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Default Re: Eagle SA-80 do they come any simpler than this ?

Quote:
Originally Posted by GrimJosef View Post
We are talking "cheap and cheerful" here !

I'm afraid the awfulness of this thing is, in a way, also it's redeeming feature.
I can beat that.

I've got the transistor equivalent of that amplifier sat in front of me on the table as I type this. It's the Eagle Products model EG-1004, 4-transistor push-pull audio amplifier, complete with its original paperwork with circuit.
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Old 27th Nov 2020, 8:44 pm   #14
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Default Re: Eagle SA-80 do they come any simpler than this ?

I was just reading this thread and thinking about suggesting how 8W was arrived at when I came across post #9......

I then noticed the thread date
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Old 27th Nov 2020, 9:41 pm   #15
GrimJosef
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Default Re: Eagle SA-80 do they come any simpler than this ?

I think this was either my first or second posting on this forum. How time flies ...

Cheers,

GJ
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Old 27th Nov 2020, 11:48 pm   #16
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Default Re: Eagle SA-80 do they come any simpler than this ?

Déjà vu
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Old 27th Nov 2020, 11:59 pm   #17
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Default Re: Eagle SA-80 do they come any simpler than this ?

The date fooled me as well. If the thread was closed, how come it was added to, or did the mods work their magic? 🙂

Cheers

Aub
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Old 28th Nov 2020, 12:09 am   #18
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Default Re: Eagle SA-80 do they come any simpler than this ?

In one. The thread was still relevant, especially with the circuit diagram in post 1, seemed rude not to!
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Old 28th Nov 2020, 1:21 am   #19
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Default Re: Eagle SA-80 do they come any simpler than this ?

Quote:
Originally Posted by Aub View Post
The date fooled me as well. If the thread was closed, how come it was added to, or did the mods work their magic? 🙂

Cheers

Aub
The amp was sold in the US in the mid-60's. It was sold under a number of importer's names. The US version used two 35C5's, a 12ax7 and a 35W4 rectifier valve. No mains transformer, as it was for 120V mains.
I referred to it as the play-toy amp.
BTW, You can use 35C5's as outputs, that would be more available. 30A5's are scarce, here as well. I only have one Japanese AM-FM radio that uses one.
Dave, USradcoll1, too many radios!
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Old 28th Nov 2020, 1:52 am   #20
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Default Re: Eagle SA-80 do they come any simpler than this ?

I remember Eagle products from the early seventies, often promoted with pictures of Gerry Adler .

Wonder what became of him ?

Internet search produced only this

http://www.richardsradios.co.uk/eagle%20sg70.html
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