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Old 6th November 2012, 09:28 PM   #1
Pano is offline Pano  United States
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Question 6V6 vs EL84 for output

If one were building an amp, Single Ended or Push-Pull, with 6V6 tubes or with EL84 tubes in the output position, what would the practical difference be? (Socket, of course!)

They are similar enough that I wonder what the practical implications would be.
  • Same heater voltage, similar draw
  • Can run on the same anode voltages
  • Rg1 might be different
  • Similar plate resistances, so similar or same OPT
  • Similar max power
  • Different transconductance
  • Bias?
.

What sort of things, from a practical standpoint, does one need to take into consideration if designing for one or the other? They seem similar enough that they could work in the same circuit, but I'd like to hear from some of you with experience in the matter what you would do differently with one or the other. Hi-Fi, not guitar amp.

Thanks
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Old 6th November 2012, 09:47 PM   #2
DF96 is offline DF96  England
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Different transconductance would mean either different gain, or different feedback fraction. Could affect feedback stability.

Quite different heater current, last time I looked.

You could buy the same socket if you use a 6BW6 instead of 6V6.
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Old 6th November 2012, 09:59 PM   #3
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I think one big reason for the EL84's popularity is the ease of driving it. Although its input capacitance is higher, required voltage is lower, for a net gain. Easy is good.

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Old 6th November 2012, 11:56 PM   #4
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Yeah, looking again I see the heater current is more different than I thought. The EL84 draws a good bit more. Not good if your heater circuit was designed with the 6V6 in mind.

More gain out of the EL84 could be an advantage. Any other differences worth mentioning? Or are these 2 tubes that will pretty much be used in similar circuits with similar results?
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Old 7th November 2012, 09:30 AM   #5
DF96 is offline DF96  England
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One is a pentode, the other a beam tetrode. I'm puzzled that you keep asserting that they are pretty much the same. I would regard them as quite different: heater current, screen current, gain, bias all differ. They really only have two things in common: both intended for audio output, both have similar power ratings. The difference in gain is enough to possibly require a different driver circuit, as the 6V6 will need much more grid swing so a better driver to avoid driver distortion.
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Old 7th November 2012, 10:54 AM   #6
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One big differance 90% of the used 6BQ5's i find are bad and 80% of the used 6V6GT's i find are good . 6V6's are much easer to come by in the continental US .
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Old 7th November 2012, 12:59 PM   #7
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Quote:
Originally Posted by DF96 View Post
One is a pentode, the other a beam tetrode. I'm puzzled that you keep asserting that they are pretty much the same. I would regard them as quite different: heater current, screen current, gain, bias all differ. They really only have two things in common: both intended for audio output, both have similar power ratings. The difference in gain is enough to possibly require a different driver circuit, as the 6V6 will need much more grid swing so a better driver to avoid driver distortion.
EL34's, 6CA7's and KT77's are constructed totally different with the first being a pentode and the others beam tetrodes. Nontheless they are interchangable.
Agreed, the EL84 and 6V6 are less similar than these, but you can build an amp that can run on either type.
Some decades ago I build some guitar amps that have sockets for both. Sure, there is a difference in gain, so you turn the volume knob a bit.
For audio it should be similar, as long as you take the requirements of both types into account: the lower grid leak resistor of the 6V6 and the higher filament current of the EL84. Should you use NFB, then you could put a selector to change the NFB to serve both types.
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Old 7th November 2012, 02:32 PM   #8
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Quote:
Originally Posted by DF96 View Post
One is a pentode, the other a beam tetrode. I'm puzzled that you keep asserting that they are pretty much the same.
Well if that's what I was asseting, it's not what I meant! Sorry. I mean that they can be used in very similar circuits, sometimes in identical circuits with similar results. (See post above).

For a couple of tubes that are not direct replacements, I see them used a lot in very similar circuits. What might be the advantages of either in a small power amp? In guitar amps they have different overdrive sounds, but we hope that won't be a factor in a Hi-Fi amp.
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Old 7th November 2012, 04:27 PM   #9
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In an old RCA radio receiver, I had 6V6s, in the same socket I tried 6F6, 6K6, 6W6 and all them sounded similar. This including a couple of metallic 6K6 which worked hotter than glass ones.
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Old 7th November 2012, 04:38 PM   #10
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I seem to recall that the typical 6BQ5 has an edge in linearity over the 6V6, and have to admit that in similar circuits I have generally preferred the 6BQ5 over the 6V6. I will say a 6V6G is more visually appealing than the typical 6BQ5 though. My next small SE amplifier will be a 6BQ5 based design, having done what I think is a pretty mediocre sounding 6V6 design a few years ago. One thing I did notice is that the 6BQ5 seems to sound a lot better than 6V6 in triode connection IMVLE.. I guess I would choose the 6BQ5 unless I was after looks..
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