My desktop amp died (and misplaced it during move). I would like to build a single ended EL84 amp. Trying to find a design that uses a common voltage amp tube like a 12A_7 that can be easily sourced. In addition, I would like for it to use plate to plate feedback, because I do not have equipment to tune GNFB.
I can build from a schematic if plate and cathode voltages are specified.
Open to other suggestions as well. The Mullard 3-3 would be okay, except that the GNFB would not necessarily work due to different output iron.
Roger
I can build from a schematic if plate and cathode voltages are specified.
Open to other suggestions as well. The Mullard 3-3 would be okay, except that the GNFB would not necessarily work due to different output iron.
Roger
There's always the RH84. It's not UL, but that doesn't mean you couldn't do a UL version. Tons of threads spread across the internet.
Thanks, that looks doable. The ps looks like it will only do one channel with a 50ma choke. I suppose one could use two chokes. Thanks for the tip.
Roger
Roger
I suggest to check the circuit even you use standard GNFB or the Schade, mainly for this one
It is simply with a good sound card and Multitone software
And the circuit on post 2 is not perfect; the Vac for filaments haven't the central tap tha must be connected to ground,
The Ra of Ecc81 is low and the cathode is not bypassed .
There aren't values for voltage in the anode of 81 and on cathode of 84, it help to understand the bias point.
The 250 ohm on cathode is also low, if the Ia is around 5 mA ( can be good) the Vk will be 1,25 volt that can be low if the source is a dac
About power supply the use of 5U4 is strong due the filament current that is 3A, not so good for this simple amp.
Probably a GZ34 will be better considering to have a less Vac on trafo ( around 10% less)
If you need a stereo you can use one 20 H chocke but the rate on current will be 70-100 mA so you can drive both channels
Walter
Maybe that was the original intent, and never got updated. You'd probably want at least 125ma for both channels.
jeff
I agree with waltube in his post #4. I also advise against the schematic in post #2.
The plate resistor of the 12AT7 is 22K which is only about 2 x the internal resistance of the 12AT7. That ratio is already about as low as you should go with a triode when wanting to prevent unnecessary extra distortion.
But on top of that, the plate to plate feedback will reduce the input impedance of the EL84, causing the 12AT7 to work into a much lower impedance than the 470K grid resistor of the EL84. This results in an AC load for the 12AT7 which will be quite a bit lower than 22K (because for AC, so for signal, the plate resistor of the 12AT7 and the grid resistor of the EL84 are in parallel).
See for an explanation of this effect pages 396 to 399 of this book:
Fundamentals of Radio Valve-Technique, Deketh, Philips, 1949
The plate resistor of the 12AT7 is 22K which is only about 2 x the internal resistance of the 12AT7. That ratio is already about as low as you should go with a triode when wanting to prevent unnecessary extra distortion.
But on top of that, the plate to plate feedback will reduce the input impedance of the EL84, causing the 12AT7 to work into a much lower impedance than the 470K grid resistor of the EL84. This results in an AC load for the 12AT7 which will be quite a bit lower than 22K (because for AC, so for signal, the plate resistor of the 12AT7 and the grid resistor of the EL84 are in parallel).
See for an explanation of this effect pages 396 to 399 of this book:
Fundamentals of Radio Valve-Technique, Deketh, Philips, 1949
so here is what I think I would start with.
Chassis 10x6x2 I have used these several times, and it would be nice to have 'matching' chassis (phono, pre-select, 6LU8 ...).
I would like to stick with Edcore iron for a few reasons, 1. They come wired for my line voltage @120vac, whereas the Hammond's come with either 115 or 125, makes for more guesswork on getting an accurate B+, 2. they are economical. So likely a power transformer XPWR106 550V@125Ma, and use a 6CA4 rectifier. I've only done solid state rectification, so tell me If I need higher HT to offset the voltage loss from the rectifier. for output iron, I would go with GSXE15-15W, 5K.
So, are these the components that would be good for an SEUL EL84 with 6CA4 rectifier? The power tranny has a single 6.3vac winding at 4A, is that sufficient? There are other PT options, but hoping this will do the job.
Roger
Chassis 10x6x2 I have used these several times, and it would be nice to have 'matching' chassis (phono, pre-select, 6LU8 ...).
I would like to stick with Edcore iron for a few reasons, 1. They come wired for my line voltage @120vac, whereas the Hammond's come with either 115 or 125, makes for more guesswork on getting an accurate B+, 2. they are economical. So likely a power transformer XPWR106 550V@125Ma, and use a 6CA4 rectifier. I've only done solid state rectification, so tell me If I need higher HT to offset the voltage loss from the rectifier. for output iron, I would go with GSXE15-15W, 5K.
So, are these the components that would be good for an SEUL EL84 with 6CA4 rectifier? The power tranny has a single 6.3vac winding at 4A, is that sufficient? There are other PT options, but hoping this will do the job.
Roger
That PT is more than adequate if you're going to use solid state diodes, but you'll need a different one with another 6.3V winding for a tube rectifier.
jeff
This is my small SEUL amp using a pair of 6BM8 tubes. Sounds nice and is easy to build. Used a pair of the 10W GXSE OT but the 15W would have better bass. https://www.skunkiedesigns.com/6bm8-amp
Stephek, I built that amp and loved it. Unfortunately it died, and was misplaced in a recent move. My thoughts were to try an EL84. There are a lot of EL84 amps out there, I am trying g to find a design that will work for me and my capabilities. I want a design that relies on current production tubes. Just because, I guess, I know NOS tubes abound.
Roger
Roger
The RH84 can be easily converted to driver cathode local Fdbk to get lower distortion. Here is a DIYaudio simulated version from some while back. Unfortunately I don't have the author information saved. Maybe they will see this and drop in.
5K SE OT, Fdbk resistor 130K.
5K SE OT, Fdbk resistor 130K.
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We used 10mH 100mA Hammonds.
The RH84 (it is not UL, not sure what happens when you add UL — another feedback loop at least) is where i would start, quite good as i, can be tweaked for a bit better performance.
Remember that the last cap in the PS is in the signal path and a poly cap is suggested.
dave
Edit:: i note that Stepe's ECL82/6BM8 looks to have both UL and Scade feedback
There are a lot of good sounding amps using UL and Schade feedback.