High gain, excellent PSRR when utilized correctly, such as using a stable reference voltage at the screen, and a CCS tail load.
Some resistively loaded Pentode can pick up noise or hash if they aren't given a healthy current. Capacitance can be an issue, especially at lower currents.
6AU6 are a very nice Pentode that is woefully underutilized these days. The 6V6 makes a great beefy Pentode, and has only a .45A filament when you want to push a good dreal of current. The D3A and EF86 are very nice too, in fact for many the D3A is *the* Pentode.
Some resistively loaded Pentode can pick up noise or hash if they aren't given a healthy current. Capacitance can be an issue, especially at lower currents.
6AU6 are a very nice Pentode that is woefully underutilized these days. The 6V6 makes a great beefy Pentode, and has only a .45A filament when you want to push a good dreal of current. The D3A and EF86 are very nice too, in fact for many the D3A is *the* Pentode.
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I'd rather use it for a ~3-6w push pull output stage, but it would probably do well in a LTP as a driver. Common screen grid restricts you to true Pentode operation, but would work well with a VR tube screen supply.
An externally hosted image should be here but it was not working when we last tested it.
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The only "advantages" are aesthetic: glowey bottle coolness or the desire to avoid solid state for nostalgic reasons. Otherwise, it's all downside: another hole in the chassis, a high negative rail voltage, poorer performance.
For active tail loads, I like a cascode of BJTs: no extra holes required, and the negative rail doesn't need to be all that high, and could be derived from a heater winding. The gm is much higher, cascoding reduces Early Effect, and therefore this circuit comes closer to perfection than does a pentode, with a significantly higher effective impedance for the tail that improves on AC balance.
True, but I don't think this is what the OP asked about. I interpret the question as whether using pentodes as the amplifying devices in a long tailed pair has any advantages. In that sense, I think it has. Excellent performance can be obtained from sharp cutoff IF pentodes. Advantages are high bandwidth, high gain, high output voltage capability at low output impedance, low input capacitance and low distortion.
Because these sharp cutoff IF pentodes are often TV tubes and radio tubes, they must be bad (disregard the advantages given above, they don't count because they are not triodes), and therefore they cost little compared to well known audio types when bought NOS. Being insensitive to this kind of BS I am getting good results from the Russian 6Z38P, but the "lowly" EF184 and its Russian equivalent 6Z51 can also be made to perform very good. Example:
I am currently getting 94 VRMS (plate to plate, so that's 47 VRMS per plate) at a gain of 95 times and an output impedance of 20k (again, plate to plate, so the individual plate resistors are 10k), from a pentode based LTP at a B+ power supply of 250 V. The tubes are 6Z38P-EV, with a tail current of 25 mA and a shared and unbypassed G2 resistor to B+ of 36 k. The THD at this output level is about 0.7 %. -3dB is at 800 kHz. Not too shabby for NOS tubes that cost about 2 Euros a piece! This is a first test, so I bet there's room for improvement. But as it currently is, this is a rather decent driver for a power stage, especially when you consider that it manages to do this from only 250 Volts.
Now you can get some of these performance figures from a triode based LTP, but not at the same time. You'll have quite a challenge getting a gain of 95 times alone from it, but with the other properties as well... Forget about it.
I thought he was asking about active tail loads, oh well.
As for using pents as LTPs, all of that is true. I used cascoded VHF triodes (6BQ7) for the same thing, and got quite excellent results: AC balance, distortion performance, bandwidth, and more than enough gain to eliminate an extra gain stage and still have enough for gNFB.
In audio use one 6360 (QQE03/12) delivers >10 W with Ua = 300 V, Ug2 = 200 V and RL = 10k.
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The Quad II uses a pentode (EF86) LTP as the driver. It is interesting to look at the screen grid biasing: for DC they are individually biased, but for AC they are tied together with a capacitor. This tends to straighten out the Ia/Vg curve under these circumstances, reducing distortion. Just google Quad II schematic, they're all over the web.
The Quad II uses once common pentodes, and it was designed to tolerate tube mismatch without a significant performance penalty. It is an interesting design to look at, and within the constraints of its time, IMHO it was excellent.
The Quad II uses once common pentodes, and it was designed to tolerate tube mismatch without a significant performance penalty. It is an interesting design to look at, and within the constraints of its time, IMHO it was excellent.
kinda OT, but please also look at the Rathford topology....Vintage Audio Repair Dordrecht The Netherlands Restoration of a Radford STA-25
The Radford has an interesting phase splitter, where the left side of the LTP is a pentode to take advantage of the low input capacitance and thus to obtain good bandwidth from the input stage, which is a high impedance ef86. The other side could just as well have been a pentode, but I think it was a cost and availability issue: triode/pentodes are more common than dual small signal pentode. Two separate pentodes would mean an extra socket, and thus higher cost.
indeed, there are 9-pin triode pentode tubes available, most are tv tubes....
this is the article....New Phase Splitter
Thank you for all the very informative discussion. Can we talk screen supply for a bit?
I know that a current source tail is the best approach for a number of reasons but for the moment let's assume a resistive tail. My thought is that a separate well filtered and stable screen supply is desirable. Thinking of a completely independent supply (with its own PT even) we can reference it to any thing we want. Would we want it to be referenced to ground or perhaps to the cathode (since current through the tail will move Vk around somewhat). Assuming that it is stable with respect to screen current and remains proportional to line voltage is it really necessary to have it truly regulated?
If my assumption that a separate supply is desirable is wrong convince me that a derived supply (voltage divider or dropping resistor) is adequate or better.
I know that a current source tail is the best approach for a number of reasons but for the moment let's assume a resistive tail. My thought is that a separate well filtered and stable screen supply is desirable. Thinking of a completely independent supply (with its own PT even) we can reference it to any thing we want. Would we want it to be referenced to ground or perhaps to the cathode (since current through the tail will move Vk around somewhat). Assuming that it is stable with respect to screen current and remains proportional to line voltage is it really necessary to have it truly regulated?
If my assumption that a separate supply is desirable is wrong convince me that a derived supply (voltage divider or dropping resistor) is adequate or better.
i think that the primary considerations for design of tube amps is that of cost,
i suggest that you read up on PRR's, posts this is clear....
where parts are in the thousands, that makes a lot of sense...
i am of the opinion that such is not the case for diy'ers making a one of an amp...
you can make it as xrazy as you like....
still at the end of the day, whatever course you take, it still all
boils down to the construction practices you actually put into it...
i suggest that you read up on PRR's, posts this is clear....
where parts are in the thousands, that makes a lot of sense...
i am of the opinion that such is not the case for diy'ers making a one of an amp...
you can make it as xrazy as you like....
still at the end of the day, whatever course you take, it still all
boils down to the construction practices you actually put into it...
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