would this work? SE EL84 circuit

[bikerhifinut]

Ok back online after a power cut since 9 am (Planned and warned).

Any way I try and cut this particular cake I can't work out how this particular stereo EL84 amp (Kontakt S110 if anybody has a clue about it, dutch brand but definitely made in Japan somewhere.) works with an 85mA transformer. Ok its only a 285V trannie so HT will be compromised with the EZ80 rectifier but It's got to be working on its limits. I'm going to have another look and see if I read the writing on it correctly.
I'll admit I'm trying to avoid butchering my testbed PSU which has the ex Armstrong Stereo radiogram mains TX in it, but I think I'll have to do it that way. I don't feel too guilty about stripping that "Kontakt" out, it doesn't seem to have any intrinsic interest or value. I suspect its a very cheap and cheerful bit of 60's consumer electronics. Got some concentric Noble pots in it though..........................................

Thanks for the felicitations Richard, I'm a whole lot better today but if this Flu type bug is heading your way, best of luck. It literally took 3 days out of my life and I don't get "manflu". Anyway Mrs Nut has sanctioned my return to the garage, I think she'd like some peace!

Yeah Philip strangely enough as I was reading up on the Pentode SE stage it slowly dawned on me that power pentodes can pass a fair bit of g2 current so my calculated current on the back of an envelope (I don't smoke so can't use the traditional fag packet!) was probably about right, did seem a bit low for an ECC82 type though, but I have found a couple of other circuits that are similar.
Trouble is its a nice day now and I just know the sleeping 955i sprint ST will drag me across to clean it and give it a double coat of looking at. Then the Royal enfield will get jealous lol. Must stay disciplined!

Andy (Unbelievably improved by 'eck those drugs are good!)

[Black Stuart]

I well remember listening to Steve Poskitt's brilliant little SE EL84 in 2001, he used two Maplin alu boxes, sensibly separating the PSU from the signal circuit - looked far better than a lot of commercial stuff.

Unfortunately he left this forum some time ago,otherwise he would gladly have provided details of what he had done. I'm not sure that he posts on forums any more - too busy making guitars for well known bands and had completed a prototype silent PC and that was 13 years ago.

Intermittingly we email each other I could see if he can oblige with the circuit.

[bikerhifinut]

Originally Posted by Black Stuart

I well remember listening to Steve Poskitt's brilliant little SE EL84 in 2001, he used two Maplin alu boxes, sensibly separating the PSU from the signal circuit - looked far better than a lot of commercial stuff.

Unfortunately he left this forum some time ago,otherwise he would gladly have provided details of what he had done. I'm not sure that he posts on forums any more - too busy making guitars for well known bands and had completed a prototype silent PC and that was 13 years ago.

Intermittingly we email each other I could see if he can oblige with the circuit.

That would be very kind of you Stuart.
I will probably go for the Mullard 3-3 circuit when i get started on it soon.
I am going to use the japanese output transformers for now and the mains Tx from the old armstrong stereogram.
I realised that I could use the jap mains transformer in my Valve rectified PSU that I use for preamp projects etc.
the plan is to solid state rectify the 3-3 and also use a 10H choke in the HT.
I'f I can manage to build that neatly and get it sounding ok I'll feel ready to embark on the 5-20 project. That particular one is scaring me to death.

A.

[Alex Kitic]

I would like to draw the attention of all those interested in SE amplifiers with EL84 tubes to my own RH84.

The amplifier has met world-wide acclaim during the last more than a decade as a very good one, and many claim on various forums that this is "The" EL84 SE amp to build.

I have published on my blog a revision 2 of the RH84 with a couple of tweaks that further improve the sound of the amp. It can be successfully built with salvaged output transformers, but those who have built it with serious iron testify of its quality.

I invite all interested to check out my blog and try this simple yet performing amp.

[bikerhifinut]

Originally Posted by Alex Kitic

I would like to draw the attention of all those interested in SE amplifiers with EL84 tubes to my own RH84.

The amplifier has met world-wide acclaim during the last more than a decade as a very good one, and many claim on various forums that this is "The" EL84 SE amp to build.

I have published on my blog a revision 2 of the RH84 with a couple of tweaks that further improve the sound of the amp. It can be successfully built with salvaged output transformers, but those who have built it with serious iron testify of its quality.

I invite all interested to check out my blog and try this simple yet performing amp.

Had a look Alex,
There's no real info I can see with reference to Voltages and current draw on the valves, which would be essential for knowing if it was working within its design parameters.

I'm still trying to find an EL84 SE that would work with an 85mA 285V transformer for a stereo pair. It must be possible as the amp the transformer came from was an EL84 stereo SE. I could I suppose reverse engineer the output stage from a physical examination.

Yes folks........... I am about to embark on the SE EL84 amp, have to say that it still looks like a "Mullard 5-6" or "4-6" depending on transformer choice.

[Alex Kitic]

Originally Posted by bikerhifinut

Had a look Alex,
There's no real info I can see with reference to Voltages and current draw on the valves, which would be essential for knowing if it was working within its design parameters.

I'm still trying to find an EL84 SE that would work with an 85mA 285V transformer for a stereo pair. It must be possible as the amp the transformer came from was an EL84 stereo SE. I could I suppose reverse engineer the output stage from a physical examination.

Yes folks........... I am about to embark on the SE EL84 amp, have to say that it still looks like a "Mullard 5-6" or "4-6" depending on transformer choice.

You have obviously not read the RH84 rev2 post on my blog carefully enough, because you would have read about current draw and various explanations of design choices, and two proposed power supply solutions.

The EL84 amp you are looking for would operate the tubes rather sparingly, with low voltage and lowish current. You could apply your underpowered transformer to the classic schematics of the RH84, and it would work but power would probably be halved. The values you propose are simply too low for a stereo performing amp with EL84 tubes.

I do not think that even the vintage Mullard circuits mentioned would work with such an underpowered transformer option. Last but not least, with no pun intended, those circuits are no match for the RH84, at least in terms of sound and performance.

But please do not take my word for it, do some research, Google the term "RH84" and read what other DIYers who have built it commented on the result of their build. Of course, only if you feel like doing it would be worth the fuss.

[bikerhifinut]

Originally Posted by Alex Kitic

You have obviously not read the RH84 rev2 post on my blog carefully enough, because you would have read about current draw and various explanations of design choices, and two proposed power supply solutions.

The EL84 amp you are looking for would operate the tubes rather sparingly, with low voltage and lowish current. You could apply your underpowered transformer to the classic schematics of the RH84, and it would work but power would probably be halved. The values you propose are simply too low for a stereo performing amp with EL84 tubes.

I do not think that even the vintage Mullard circuits mentioned would work with such an underpowered transformer option. Last but not least, with no pun intended, those circuits are no match for the RH84, at least in terms of sound and performance.

But please do not take my word for it, do some research, Google the term "RH84" and read what other DIYers who have built it commented on the result of their build. Of course, only if you feel like doing it would be worth the fuss.

I have read your blog, and there are no voltage or current values posted on the circuit or in the text that accompanies it. That in itself doesn't cause me any great concern, but it's always nice to know what one should expect to measure across devices when testing with a meter.
I've got another transformer which would be well up to the job in hand, but It would mean using a separate heater transformer if I used a Valve rectifier, I also have one of those in the toybox. The issue really is how much room in the chosen case work.
Mainly I am baffled as to why this SE EL84 amp I have had mouldering away in the Garage seems to be rated at 85mA when most EL84 SE seem to draw in excess of 50mA per Valve, and that again seems to be running them a bit hard as I would have expected 48mA to be the maximum current for an EL84, unless I am not accounting for g2 current.
It does look like I will have to find another use for the smaller mains transformer. I do like the simplicity of the RH84 if indeed it does outperform the 3-3 which has a very good reputation.

A.

[Alex Kitic]

Here's a couple quotes from the text:

"As the anode current is approximately 40mA, the voltage drop lies between 8 and 11V: thus it is safe to assume that with a 21V drop from B+, the resulting Ug2 will always place the 2nd grid on a lower potential than the anode".

"Fixing the current draw of the tube with a simple circuit (LM317 + current setting resistor) allows a constant current draw regardless of the tube inserted..."

"The current setting resistor is chosen for a standard value (27 ohms)"

1.25 V / 27 ohms = 0.04629 A or 46.3mA per output tube.

"The amplifier draws (both channels) slightly less than 100mA and the voltage foreseen at the B+ point (actually, C2 voltage) is approximately 315V."

A further explanation is given along with a few values on the power supply schematics, as guidance for necessary secondary winding values respective of rectifier tube used.

I usually publish a simulated schematics with lots of data, but this time I did not include it since the sim resulrs and reports of DIYers building it are common all over the various forums. The amp is more than a decade old, so I taught everything was already known.

Besides, the DIYer who is building it needs only to observe the components values, and since several power supply alternatives are proposed, if there have been no mistakes in the wiring the amp will work flawlessly and automatically without any need for adjustment besides checking the B+ and the heaters voltage values.

Indeed my newer designs are explained in more detail, including the parallel RH84 version with two alternative small signal pentodes used as drivers: nothing forbids the DIYer to build the pentode driver version without parallel output tubes (also explained in the text of that post).

The RH84 easily outperforms more coveted amps than the 3-3. Do some research: there is even a thread on one forum where a comparison between those two amps was inquired about...

[pre65]

Originally Posted by Alex Kitic

The RH84 easily outperforms more coveted amps than the 3-3. .

I'd agree with that, Alex has done a lot of work and got very good results.

[bikerhifinut]

Thanks Alex.
I apologise for a lapse of concentration, I found it a bit hard to follow for some strange reason.
The more I look at your circuit, the more I find to like. It certainly is a simple one and I would hope easy to wire up neatly.
The solution for me is simple, I shall use the transformer from the old Armstrong stereo radiogram that I have as I know that will be good enough for a pair of SE EL84. I will hope that I can find room for the 6.3V 1A filament transformer underneath as well as the 10H 100mA choke I have waiting.
The transformers I will pull out have the desired 5k primaries so it all looks quite good so far.
I have just done a bit of googling the RH84 and it does have a good worldwide following.
And I've just unearthed a few LM317's, I think it's an omen.

Mind you I am still puzzling as to why this SE amp in my collection used a weedy 85mA transformer, it just doesnt make sense to me. the output stage uses a 150R with a 47uF bypass on the cathode and when I crunch the numbers I get a current pushing 50mA which multiplied for 2 EL84 is over 20% more than that supplied by the transformer.
I had a crazy idea that maybe the labelling on the transformer was 85ma per 280V winding, in which case its well specced at 170 mA?


A.