A 6V6 that sounds similar to an EL34?

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10thTx
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Re: A 6V6 that sounds similar to an EL34?

Post by 10thTx »

3w was not needed. I think it was simply what I had on hand.
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Littlewyan
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Re: A 6V6 that sounds similar to an EL34?

Post by Littlewyan »

I tend to use 5W resistors for screen grids. Simply because they can get fried if the valve blows.

Ill experiment this weekend with increasing the resistor value. I played the amp last night and it sounded ok but does need more compression. It's a bit spikey at the moment.
teemuk
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Re: A 6V6 that sounds similar to an EL34?

Post by teemuk »

Can one make an apple taste like an orange?

As was acknowledged earlier, EL34 is a true pentode tube while 6V6 is a beam tetrode. They will have somewhat different characteristics of operation discussed pretty well in various literature. This site is pretty good:
http://www.r-type.org/static/grid14.htm

In summary, beam tetrode, due to its aligned screens, draws less screen grid current than a true pentode tube and also produces lesser amount of third harmonic distortion, which pentode tubes produce a plenty. The same characteristic also makes beam tetrode slightly more efficient than an "equivalent" pentode tube, which shows in "harder" knees of the characteristic curves, and yes, as "harder" clipping too. Aside the kink characteristic curves of beam tetrodes resemble a lot those of generic bipolar transistors.

Yes, despite their nickname, beam tetrodes are not completely "kinkless" either, which ultimately reveals itself as weaker stability in comparison to true pentode tubes.

Pentode tubes require much less grid drive voltage than equivalent beam tetrode tubes. Thus biasing and drive voltage requirements will be different with these two tube types. In closed loop circuits switching from tube type one to another will alter the gain structure within the loop: Amount of gain will be distributed differently amond gain stages enclosed to the loop.

Then we encounter different internal plate impedances. In same circuit, without properly matching reflected load to tube's internal impedance, we are bound to heave differences in source or output impedances of the amplifiers. With reactive loads these will manifest as differences in amp's frequency response. These can be very audible in some systems. With loudspeakers that have higher impedance at higher frequencies and at the (lowish) resonant frequency the differences are heard in either bass & treble frequencies, or in mid-range frequencies (depending on one's point of view).

So we are talking about tubes that not only have different sensitivities, but different overdrive characteristics as well, and very often - in practical circuits - different frequency responses to reactive loads due to different plate impedances and output Z's.
IMO, this is a genuine apples to oranges comparison.

We can make these two tube types sound the same by applying a lot of negative feedback in order to minimize effect of all circuit and device variances, and after that, by never overdriving the amps because that would throw the differences of beam tetrode vs. pentode tubes right into one's face.

---With guitar amps in question I don''t see much point in that.
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Littlewyan
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Re: A 6V6 that sounds similar to an EL34?

Post by Littlewyan »

What's responsible for the lesser amount of third harmonic distortion? The lack of screen voltage sag from lower screen current draw.

I think the sensitivity of the 6V6 and EL34 valves is similar going by the data sheets. Obviously again we're almost comparing oranges to apples as the 6V6 is a lower wattage valve. I understand that a 6L6 needs a lot more voltage to drive it.

I have designed the output stage around the 6V6 though. Lower B+, higher impedance OT etc. Also increased NFB to match that of an EL34 (lower output obviously equals less voltage going back through the NFB circuit). It's interesting that you say to increase the NFB a lot as sluckey did that in his 6V6 Plexi.
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