EM84 Magic Eye Indicator

293

02:14:47

I wanted to share my results with the EM84 magic eye indicator tube. I made a VU Meter with it to show the signal strength of my Vacuum tube amplifier. By the way if someone has an interest in these things, and has a few tubes in the junkbox i designed a pretty reasonable class A vacuum tube amplifier with very common tubes, and I'm willing to share the circuit. It uses EL82 (6п18п), which can be easily replaced with ЕL84 (6п14п) and a simple preamp with ECC88 (6н23п) or any other of the ECC8x series. That's the amplifier playing in the video, it has very deep basses and very sharp highs. The preamp revolves around the double triode ECC88 with one half bootstrapped to increase the linearity and the other working as an autobiased common cathode amplifier. The power amp is a normal single ended topology. Sometimes the power amp needs the cathode resistor readjusted to rebias it correctly because in the schematic I've shown you the tube's heat dissipation is at almost its maximum (12W), and it shouldn't exceed it. The power stage ia perfectly calculated. Keep it as it is. The only change you can do is switch the operating mode from pentode to triode by connecting the second grid to the anode plate via a 150ohm resistor. If you have the right transformer you can play with the ultralinear mode but i don't have such a transformer and i couldn't verify how it works. 

Here's the link for the Magic eye indicator video - https://youtu.be/k3axWpAJOIY

Here's the circuit for the Class A power Amplifier - http://s10.postimg.org/ws6gautft/DSC_0444.jpg

published 9 years ago

tpfohl

9 years ago

Nice circuit. Have not seen the video because I'm in a lecture. I have built a similar amplifier but with a buffered adjustable filter as a preamp in stead of a cascode circuit. One thing you can do to increase linearity in spite of the lack of a proper transformer is make a negative feedback from the secondary of the transformer over a adjustable resistor to the kathode of your input triode. Beware if you put the feedback on the wrong side of the secondary the amp will oscillate. What do you think?

hurz

9 years ago

First video is "private" :-(

thebugger

9 years ago

There you go now it's public, i haven't noticed, thanks. The cascode preamp has quite a low output impedance, so it should fight the miller effect of the power tube. I don't like negative feedback in tube amplifiers for several reasons. First the output transformer, from which I'll be taking the NFB, may phase shift some frequencies, making them essentially a positive feedback while having a negative feedback for others, thus making the frequency response chart uneven and in spite of its apparent function it may cause instability. It'd personally take the negative feedback from the second tube's anode, through a large cap, where the phase shift is lower. Second the NFB totally ,,kills'' the warm tube sound leaving a transistor like sound, by negating some of the even order harmonics, and that's exactly what I'm looking for in a tube amplifier. A well designed vacuum tube amplifier will have a few local feedbacks but not a whole one according to me. One huge thing i noticed. Leaving the secondary of the transformer ungrounded severely deteriorated the sound quality. As soon as i grounded it, the warm, deep sound was back. Removing the ground on the transformer or adding a completely NFB or putting the power tube in a triode mode, caused a vary stale, unrich sound which i really disliked.

thebugger

9 years ago

But, there is a but :D Some people like the cleared by the NFB sound better, so i guess it's just a matter of opinion. The NFB removes some of the second harmonic (even order) which is pleasent for the ear (kinda filling), but on the other hand it also removes the third order harmonic, which is an odd number and it's unpleasant. So i guess it's about how well designed the amp is to not have a high third order harmonic in the first place, that will determine the need for a feedback for me.

tpfohl

9 years ago

Don't misunderstand me, I like the cascode design better than mine.

thebugger

9 years ago

I'm new in tube amplifiers, but if the same rules apply as transistor amps then using 1 half of the tube as a constant current source for the other (more like a bootstrap), should really give good results. Anyway, the preamps are not as important with tubes, as the power amplifiers (pentode,tetrode) . Triodes have much less distortion than pentodes and tetrodes, so it's more important to devise a good power amp stage for best results. I tried ECC88 ECC85 ECC83 6н23п 6н2п for the cascode preamp and it pretty much sounds the same with ECC83 sounding a little better and with higher gain than the others. On the other hand i started with 6п14п (EL84) on the power amplifier, and when i switched to 6п18п (EL82) the difference was huge in favour to the EL82. It sounded less chiming when overdriven, with warmer, richer sound.

tpfohl

9 years ago

It's just less gain with a triode i think. The distortion comes from the fact that pentodes more resemble transistors in the transfer diagram and thus have sharper turns at the ends of the linear section. I had my em84 display high rather than low frequencies and it was like it was flapping lips to the vocals.

thebugger

9 years ago

Triodes have less gain due to the grid plate capacitance, but offer lower distortion due to its simplicity. Pentodes are much more complicated with a lot more grids, so they offer more gain, but due to complexity the electrons keep bumping into each other thus increasing the distortion. They're less efficient too. I think beam tetrodes are better than pentodes as far as efficiency and distortion is concerned.

tpfohl

9 years ago

Pentodes are more efficient. The screen-grid shields the negative control-grid from the field of the anode and accomplishes a very low miller-effect. The braking-grid prevents the secondary current from anode to screen-grid from distorting the anode current. Thats why Tetrodes have their own kind of nastiness because they have a lot of secondary current.

tpfohl

9 years ago

But efficiency is not necessary if you are interested mainly in signal quality. Otherwise you wouldn't use class a because it has a maximum of 25% efficiency.

thebugger

9 years ago

I'm talking about beam tetrodes. Their screen current is 5-10% of the plate current, while pentodes use 15-20% of the plate current. Normal tetrodes are not good for amplifiers, they have a nasty kink of negative resistance in their characteristic, which makes them unstable. In oscillators they are good, but not in amplifiers. Beam tetrodes or kinkless tetrodes on the other hand are said to have lower distorion than pentodes and higher efficiency too.

martinlb

8 years ago

I'm new here so I hope I'm using terminology that makes sense. Could you put your class A amplifier circuit into the database here so it can be used here? Thanks!

add comment in editor