Are tube designs still evolving, or have all circuits effectively been invented?

[cnpope]

Yes, it all comes down to current delivery, at the most basic level, but current X voltage is the complete picture. 2.5A for the 6C33 is completely off the charts. I'm using the much more powerful 6P45 and the peak current is 1400mA. The 6C33 tops out at 875mA and that's with the control grid at -2V, running well outside the linear part of the curve and utterly useless for audio. At practical bias levels, the current limit is 450mA for this tube. You would need a half dozen of them on each half of the amplifier to achieve 2.5A output current, notwithstanding the efficiency factor and that some of that power is being burned up by plate losses. Plate resistance is 130?. At full conduction, it's like a transistor amp with a 130? resistor in series with each pass transistor. That won't deliver much power to an 8? load as most of it will be burned up in the plate resistance.

Servo feedback can only achieve the appearance of low source impedance at very low levels, up to the current delivery of the tube. I doubt much more than 450mA into the load. With push pull, double the voltage, but the current is still limited to 450mA.

I agree that the 2.5A peak current is "off the charts" in the sense that it goes way off the graphs in the tube datasheet. That figure of 875mA at -2V grid voltage you are quoting is, I think, what you are reading off as the largest displayed intersection of the plate current curves with the limiting value of plate dissipation; it is for a plate voltage of something like 65V. That is not at all like the actual voltages being used in practice (the power supply rails are typically +/- 160V or so). It is true that at the peak, the plate dissipation will be way above the rated value for the tube. Two things help to improve this considerably, however. If we consider a sinewave signal, then the rms current is only about 1.8 amps. Additionally, each output tube is only handling that large current for about half the time (one tube for the upper halves of the sinewave, whilst the other tube handles the current for the lower halves). So the average plate dissipation is a lot less.

It is still true that if one were to run the thing continuously outputing a sinewave into an 8 ohm load at 25 W then the tubes would be being overun and would quite likely register a protest after a while. But in practice, using it for music listening, the amplifiers functions perfectly well and reportedly achieve quite long tube lifetimes. I've been using mine as my everyday amplifier for two or three years with no problems at all. I think Hans Beijner reported using one of his for 10 years or so.

So, if you are saying that in continuous operation at full power (25 W) the tubes are being pushed beyond their designed limiting parameters, then yes, you are of course correct. But if you are saying that it is not possible to get the 2.5 amp peak plate current, then I must respectfully disagree. As documented in many places, including the articles by Hans Beijner and Tim Mellow, 25W into 8 ohms is achievable, and therefore a 2.5 amp peak current is indeed being passed by each tube. And I can vouch for this myself; I have indeed confirmed with my own measurements that these amplifiers can push 25W into an 8 ohm load. Hans Beijner reports 0.1% distortion at 1W and 1% at 25W, in an early version of his OTL before he stepped up the feedback. I think with his latest version he's getting something like 0.01% distortion at 1W, and 0.1% distortion at 10W.

As far as the reliability of 6C33C tubes is concerned, speaking from my own experience I have had one tube failure, which occurred quite early on. Other than that, I seem to have had quite good luck with them.

The totally transformerless OTL I've built is indeed very similar in its output and driver stages to the one you sketched. Mine is loosely based on the old Dickie and Makovski design from the 1950's.

Chris

 

[cnpope]

Question for Mark [Basspig] Weiss & Chris / cnpope & other knowledgeable people please join in.

Amplifiers that use the 6c33c tend to have means to set the bias. Some tubes even if matched to begin with quickly lose the exacting match once they are in use.
Therefore, should one bother buying matched 6c33c tubes?
Or, do not bother with "matched sets" & just break-in singles & properly maintain the bias?

Thank you for your assistance.

Respectfully,
zz

In most of the discussions I've seen, the advice seems to be that the concept of "matched" 6C33C's doesn't really exist, since as you say, even if a pair seem balanced at first, that can fairly rapidly change. So according to what I've read, one might as well just buy singles and break them in, keeping a fairly close eye on them in the early stages. That's what I've done.

Chris

 

[microstrip]

In most of the discussions I've seen, the advice seems to be that the concept of "matched" 6C33C's doesn't really exist, since as you say, even if a pair seem balanced at first, that can fairly rapidly change. So according to what I've read, one might as well just buy singles and break them in, keeping a fairly close eye on them in the early stages. That's what I've done.

Chris

The Graaf GM20 stereo OTL used 6C33 and was a pain to bias. I found that during the burn-in phase the tubes would change a lot, but after that phase the amplifier was stable and reliable, if used with adequate speakers. However, the stereo amplifier could be strapped for mono operation and here your trouble would start, as the mono's had very poor reliability - also perhaps because people used them with speakers needing more current.

I think that a key point in OTL use should be avoiding speakers with low impedance. Most pure ESLs have typically an impedance over 10 ohms in the bass zone and are a good match. Happily in the very high frequencies, where their impedance drops, music has usually lower power.

 

[MylesBAstor]

The Graaf GM20 stereo OTL used 6C33 and was a pain to bias. I found that during the burn-in phase the tubes would change a lot, but after that phase the amplifier was stable and reliable, if used with adequate speakers. However, the stereo amplifier could be strapped for mono operation and here your trouble would start, as the mono's had very poor reliability - also perhaps because people used them with speakers needing more current.

I think that a key point in OTL use should be avoiding speakers with low impedance. Most pure ESLs have typically an impedance over 10 ohms in the bass zone and are a good match. Happily in the very high frequencies, where their impedance drops, music has usually lower power.

Part of the reason Futtermans sounded great into Crosby Quads

 

[cnpope]

Mark; Here is an estimate of the average power dissipation in each 6C33C output tube when the OTL amplifier is delivering 25W into an 8 ohm load. In this calculation, I assume the supply rails are +/- 150V. Let us consider the upper 6C33C tube, whose anode voltage is therefore +150V. It will be conducting and dissipating power during the upper half of the sinewave, and essentially non-conducting during the negative half. The discussion for the lower tube will be analogous.

For a sinewave at angular frequency w, the current will be I = (5/2) Sin(w t) during the positive half, i.e. 0 < t < Pi/w, and the voltage on the cathode will be 20 Sin(w t), since 25W into 8 ohms means a peak voltage of 20V. Therefore the anode to cathode voltage is V = (150 - 20 Sin(w t)) and so the instantaneous power dissipation is

P = I V = (5/2) Sin(w t)(150 - 20 Sin(wt)).

Integrating this over the positive half of the cycle (0 < t < Pi/w), and adding zero for the negative half cycle, gives the total dissipative energy in the upper tube per cycle. Dividing by the period (2 Pi/w) then gives the average power dissipation in the tube; it comes out to be 25(30-Pi)/(2Pi) which is about 107W. This is indeed considerably more than the rated maximum dissipation of 60W, but it is not astronomically more. Tubes, unlike transistors, don't typically fail catastrophically if pushed a bit beyond their limits. And in practice, when used for listening to music rather than running continuously at 25W sinewave into 8 ohms, so that the amplifier is only intermittently being called upon to output the full 25W, the tubes seem to last pretty well, for a good few years.

If one really wanted to be conservative and stick within the rated tube dissipation even at continuous maximum power of 25W, then it would be quite sufficient to use two paralleled tubes for each of the upper and lower bank, not the six in parallel that you were proposing. And, as a matter of fact, since roughly speaking the maximum power handling grows as the square of the number of paralleled tubes, such an output stage with 2+2 6C33C tubes in total would be able to output roughly 100W for limited periods.

Chris