A single stage amp ?

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pre65
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#16

Post by pre65 »

It would be a lot better if a solution involving an output transformer with a more common rating, ie 5K, were available. :wink:

Why is using a cheap tube like ECL82 "cheating" and using an expensive step up transformer not ?

Surely a "spud" amp is a single tube ? :lol:
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#17

Post by Dave the bass »

...because the title of the thread 'a single stage amp'......?

Ha!

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#18

Post by Alex Kitic »

pre65 wrote:It would be a lot better if a solution involving an output transformer with a more common rating, ie 5K, were available. :wink:
14k and 16k transformers are not at all difficult to source. Also, keep in mind that a 3.5k into 4 ohms becomes 7k into 8 ohms and 14k into 16 ohms (approximately), thus combinations can always be made to suit the purpose.

Not to mention the fact that if you were to parallel 2 of these tubes per channel, you would get twice the power into a more usual load (and twice 6W is a very interesting proposition).

But this is why I did not mention the parallel version:
pre65 wrote: Why is using a cheap tube like ECL82 "cheating" and using an expensive step up transformer not ?

Surely a "spud" amp is a single tube ? :lol:
Because it would be too easy (two stage amp) and because it defeats the purpose (one stage amp).

I guess even paralleling 6JD5 to get a more usual primary impedance, and more power at that... is out of the question as far as the exercise goes.

That said, and staying in cheap Compactron domain, the 6LU8 comes to mind. This tube is great for the purpose because it consists of an ECC81 and a 14W beam tetrode in the same envelope - I guess by now everyone knows where that leads. I was going to present an RH SPUD in the near future, with 6LU8... but somehow I always get distracted (RH813 project) :D

Jokes aside, the 6JD5 amp is actually very real and very feasible. The only relevant pre-requisite is not being afraid of a 1000V B+ (which is something that goes without saying with all the GM70/845/211/212 etc. amp builders).

EDIT:
Now that I think of it, I am really tempted to try this amp. I have at hand a pair of humongous transformers that can be 7k into 4 ohms (thus 14k into 8 ohms), I have a pair of 6JD5 tubes... and I think I do have a transformer that could provide 1200V or more in voltage doubler mode (at 30mA per tube the voltage doubler alternative is very feasible).

The reason why I am not already getting my tools to make it is because I am strangely attached to the "RH feedback" and this is a one stage amp... but who knows, I might give it a quick try :) After all, it's a couple of sockets and a handful of parts (and yes, some hands on experience with 1000V B+)... and maybe I can add some Shade feedback to it ("real Shade feedback", i.e. voltage feedback from anode to grid) if I don't like the sound...
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#19

Post by Mike H »

Dave the bass wrote:...because the title of the thread 'a single stage amp'......?

Ha!

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#20

Post by Alex Kitic »

Now that we are at it, I can't get these 6JD5 out of my head... it is so simple to make... the most complicated part is the voltage doubler (actually, counting the caps, and choosing the ones that are adequate for the purpose)...

A very simple amp indeed... I might really try to make one (with a CCS under the cathode, probably LT1086 since the voltage drop matters a lot here... if bias is at least 2.2V I hope it would work). Or maybe I'll just use a resistor...
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#21

Post by JeanBeo »

What about an EC8020.
It was possible to find them a few years ago in France as they used to be kept for maintenance (they were used in TV transmitters a long time ago), but the supply might have dried out now or they may be expensive to source.

It is a triode with dream characteristics.
Gm=60mA/v, mu=55, Ia=40mA, Pp=8w.

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#22

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#23

Post by ed »

certainly looks good, but hasn't he grounded the cathodes through the 250v power supply...shorted the battery?
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#24

Post by Nick »

ed wrote:
certainly looks good, but hasn't he grounded the cathodes through the 250v power supply...shorted the battery?
Yep, plugging an input in will remove the bias to the valves from my reading.
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#25

Post by Mike H »

Assuming that the input ground terminal is connected to 250V minus and chassis, which there's no indication that it is.

Awkward though if it is also earthed at source, so then yes.

I would omit the link in the PSU from the 250V minus to earth, then the 250V minus would be elevated 1.5V positive due to the bias battery. Should be OK?

EDIT: but of course earth the input ground terminal, or earth lift resistor.
 
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#26

Post by Nick »

Assuming that the input ground terminal is connected to 250V minus and chassis, which there's no indication that it is.
I think that is a reasonable assumption that the input ground will be at earth potential, and the -250v point is connected to earth in the power supply.
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#27

Post by Mike H »

Ah, you are assuming. :D

I don't see the earth symbol on the amplifier schematic, only the PSU.

From which I deduce input terminals are 'floating'.

Or no, actually, –1.5V relative to chassis.
 
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#28

Post by Paul Barker »

If Phil builds it he needs to be aware of the discrepancy in the drawing. I would just ground the power supply not connect it to the cathodes, jobs a goodun.

Phil if you want a pair of these valves to play with I have some, which I bought as a driver but never got around to using.
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#29

Post by Nick »

Mike H wrote:Ah, you are assuming. :D

I don't see the earth symbol on the amplifier schematic, only the PSU.

From which I deduce input terminals are 'floating'.

Or no, actually, -1.5V relative to chassis.
So you are also making an assumption in that case, saying you have deduced it makes no difference, I am going to equally validly assume the input is referenced to 100,000v, lets see what happens then.

The point is, if someone built that amplifier to the published schematic on the web, and in the real world plugged an input into it, the valves would be at 0v bias and the battery would be shorted.
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#30

Post by Mike H »

I'm not disagreeing with you, it would. :D
 
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