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Old 10th March 2007, 06:06 PM   #1
phn is offline phn  Sweden
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Default SE EL84 question

I have modeled a simple, no feedback single-ended EL84 in LTSpice. I have the Valve Amplifiers book, but that's too abstract for me. I need a more hands-on approach. Anyway, after trying the EL84 as pentode and triode-strapped, I have this question.

Say I want 500mV sensitivity. I will then need a higher gain driver if I use the EL84 triode-strapped. (Figuring I actually know what I'm doing.) This, as I understand it, is a good thing.

In order to make this clear here, I'm ignoring any subjective improvement a triode-strapped pentode might have. I'm only talking signal swing here. And as I understand it, there should be some real-world benefits driving the EL84 with higher voltage. I tried the same design with the 2A3 and needed even more gain to drive the 2A3 into clipping.

Now, whatever benefit higher signal swing has, it might be offset by the amp being driven harder and thus producing higher distortion. Any thoughts on this?
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Old 10th March 2007, 08:03 PM   #2
EC8010 is offline EC8010  United Kingdom
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A 2A3 is likely to need about -45V of bias whereas an EL84 is likely to need about -11V. Any valve needs a swing of twice the bias voltage, so the 2A3 needs 90V whereas the EL84 only needs 22V (in my example). Once you drive beyond these limits, you get distortion. If you only drive a little way beyond (and from a low impedance) you may be able to go into Class A2, and extract a little more power. You need very little more grid swing for that, but lots of current.
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Old 10th March 2007, 09:16 PM   #3
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Phn,

Even when triode wired, EL84s are not difficult to drive. 500 mV. I/P for full power is what you want. A CCS loaded 5965 (mu of 44) should be darn near perfect. Ib = 3.5 mA. and 2 red LEDs in series for bias.

Living in Sweden, the E180CC/7062 might be easier to source than the 5965. As if Mullard made E180CCs are junk. Hmmm, a Mullard made E180CC into 2X Russian 6p14p-ev/EL84Ms should work just fine. JJ's EL84 is the other good current production EL84. The Russian tube is a 7189 equivalent and that allows for a high B+, should you so choose.



Edit: fixed typo
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Old 11th March 2007, 06:33 AM   #4
phn is offline phn  Sweden
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I was probably "misthinking" anyway.

I was thinking in terms of easy to drive tubes, like 2A3 contra 300B. But the problem with my thinking--if my thinking was even right to begin with--is that I will trade output tube distortion for driver tube distortion.

I did try a Western Electric 417A model, gain close to the 5965. Perfect. But, of course, totally overkill.
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Old 11th March 2007, 12:28 PM   #5
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Phn,

Having the cake and eating it too is possible. 6V6 family tubes are competetive with 6SN7s in the linearity dept., when triode wired. Drive requirements are a tad greater than an EL84. The 6AQ5/EL90 would be a distinct possibility using the mid 40s mu driver. Tie g2 to the plate with a 1 KOhm resistor.
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Old 11th March 2007, 10:48 PM   #6
phn is offline phn  Sweden
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The 6V6 looks interesting.

While I fiddle with spice mostly to learn and understand, I got the idea for a low-power SE amp when I saw that Lundahl has these small 8W OTs. I seriously do not want more power than needed. (I think 2W are enough for my 97.5db Altecs.) I don't like to turn my room into a hothouse. I have this love-hate affair with tubes. My love actually goes for the likes of the 2A3. (The EL84 is a skinny-looking thing.) But then I will need some serious iron. It ups everything.
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Old 11th March 2007, 11:27 PM   #7
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Quote:
Originally posted by phn
The 6V6 looks interesting.

While I fiddle with spice mostly to learn and understand, I got the idea for a low-power SE amp when I saw that Lundahl has these small 8W OTs. I seriously do not want more power than needed. (I think 2W are enough for my 97.5db Altecs.) I don't like to turn my room into a hothouse. I have this love-hate affair with tubes. My love actually goes for the likes of the 2A3. (The EL84 is a skinny-looking thing.) But then I will need some serious iron. It ups everything.

2 WPC is about right from triode wired 6V6s. The screen grid in the 6V6 is tougher than that in the 6AQ5. 100 Ohms is safe to tie g2 to the plate. The Lundahl trafos should be FINE. New Sensor is marketing 2 decent 6V6 variants made at Saratov, Russia. In particular, the TungSol "reissue" deserves consideration, but the EH is not guano.

If you can source a pair of NOS RCA 6VGs (ST bottle), you'd get the max. look factor. Of course, RCA 6V6Gs sound good, but the price may be too high.
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Old 12th March 2007, 04:41 PM   #8
phn is offline phn  Sweden
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While I'm at it. I have been playing around with a couple of pentodes (I have a limited number of pentode models; unlike triodes, I can't make any myself), and they sport the same problem. This may be old news, but it's new to me.

First. While I'm starting to get a hang on these things (on a VERY low level), there are some things that are way beyond me. Impedance. From what I gather: you design an output transformer by starting with the load, say 8 Ohms, and then wind, say 3kOhms, from there. So if I hook up an 8Ohm resistor on secondary the signal will "see" 3k on primary.

As I understand it, a SE topology is "sensitive" and will interact with the speakers, or impedance thereof. I have no problem with that.

When I use a triode or triode-strapped pentode as output tube, the frequency response differ somewhat when I have a resistor connected to the secondary of the OT and when I don't have one. No problem. The frequency response is pretty much ruler flat 20-20,000 regardless.

The problem starts when I use pentodes strapped as pentodes. Without the resistor on the secondary the frequency response looks like an arch. This cannot be good.

After that I just had to try a pentode as driver and use an interstage trafo. The 6V6 isn't ideal since it likes 100 or so Henry. But this is spice. I can use any value I like to. To get a reasonable frequency response I need a grid resistor of about 5k!

How do I interpret this? That pentodes (strapped as pentodes) are more sensitive to load than triodes? That interstage trafos are unsuitable for pentodes?
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Old 12th March 2007, 07:51 PM   #9
zobsky is offline zobsky  India
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Quote:
Originally posted by phn
[B
After that I just had to try a pentode as driver and use an interstage trafo. The 6V6 isn't ideal since it likes 100 or so Henry. But this is spice. I can use any value I like to. To get a reasonable frequency response I need a grid resistor of about 5k!

How do I interpret this? That pentodes (strapped as pentodes) are more sensitive to load than triodes? That interstage trafos are unsuitable for pentodes? [/B]


It's been hammered into my skull that pentodes (drivers or outputs) don't like high valued plate loads or interstages.

My $0.02
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