Pentode noise in phono stage: power tube vs. A. F. amplifier tube

[Walter66]

Hello,
planning to use a triod wired-pentode in the second stage of a MM phono preamp, Wondering which tube would make more sense concerning tube noise:
-E80CF: A. F. amplifier tube, pentode in triode mode operation
-ECL84: output pentode in triode mode operation

Both can drive the LCR network for RIAA equalisation via a 20K-600 Ohm transformer.
Will there be any restriction for not using a power pentode in a phono stage due to more noise compared to the E80CF?
Is the E80CF the tube better suited by design for such a phono unit, because its general low level A. F. amplifier tube?

Both can be run @ 8/10mA and 180V anode voltage, triode mode for low output resistance.
Both show linear operation with the simulation attached.
Which one to be preferred, please?

 

[ruffrecords]

The noise of the overall circuit will be dominated by the noise of the first stage. You should design this stage for maximum gain and minimum noise. The noise level of subsequent stages then has very little effect on the overall noise so you can concentrate on the need to drive the output.

Cheers

Ian

 

[merlin]

ECL84 has twice the gm which usually results in lower white noise, but in a phono stage 1/f noise is exaggerated, and is not possible to predict! As Ian said, it doesn't really matter anyway because the first stage will dominate. I'd probably use the E80CF simply because it needs less heater current.

 

[Walter66]

ECL84 has twice the gm which usually results in lower white noise, but in a phono stage 1/f noise is exaggerated, and is not possible to predict! As Ian said, it doesn't really matter anyway because the first stage will dominate. I'd probably use the E80CF simply because it needs less heater current.

Thank you. Yes, E80CF needs less heater current. But it will work near max. plate current while ECL84 can work very relaxed and maybe have more authority in sound. The concept is quite interesting, as it replicates the line input combination of a tube radio. First tube EF86 or triode, second one power output tube Pentode and then speaker. I was afraid a power pentode will induce more hum into a phono stage but glad it wouldn't.

 

[Walter66]

The noise of the overall circuit will be dominated by the noise of the first stage. You should design this stage for maximum gain and minimum noise. The noise level of subsequent stages then has very little effect on the overall noise so you can concentrate on the need to drive the output.

Cheers

Ian

Thanks Ian! What do you think will produce more noise, a low noise triode (6072) in a cascode circuit vs. a special longlife pentode like EF804S?

 

[ruffrecords]

Thanks Ian! What do you think will produce more noise, a low noise triode (6072) in a cascode circuit vs. a special longlife pentode like EF804S?

Hard to say. A pair of triodes in cascode will be noisier than a single triode but has more gain. A single pentode (wired as a pentode) will in general have more noise than a single triode but it will also have more gain.

Having said that, provided you can achieve about 30dB gain in the first stage, the noise of subsequent stage will have very little effect. So I would go for the highest gain single triode I could find.

Cheers

Ian

 

[Rusan]

How about the 6GK5, a 7-pin triode with a mu of 78 and 15mA/V Gm? Kinda like a 6922 that's been bodybuilding.

A buddy of mine once built a great sounding phono stage using a pair of them. He later built one using the 6C45 (mu 52, 45 mA/V Gm), which sounded amazing overall. But, it often had a faint metallic pinginess at certain midrange frequencies, that we suspected was probably due to intermittent oscillation at some stratospherically high frequency. I joked that it probably oscillates sitting in the box, before you ever plug it in.

 

[merlin]

Thanks Ian! What do you think will produce more noise, a low noise triode (6072) in a cascode circuit vs. a special longlife pentode like EF804S?

Probably about the same as each other. I'd probably choose the pentode because they don't get enough love

 

[Walter66]

Probably about the same as each other. I'd probably choose the pentode because they don't get enough love

Yes, I like studio gear very much, so a low noise pentode would fit the bill exactly. Otherwise, a triode has a lovely sweet sound that no pentode can create...will try both.

 

[Rusan]

It might just be my ears/brain tricking me, but to me the cascode circuits I've built have had a subjectively "harder" sound than a grounded cathode stage using the same triode type. FWIW, cascodes leverage the triodes' transconductance into voltage gain, so a high Gm type such as 6DJ8/6922 will translate into greater net gain (if needed) than a grounded cathode stage using a much higher mu but lower Gm type, plus relatively low noise.

In addition to the E80CF you mentioned, the E810F/7788 might be a possibile contender, if you can find 'em for a reasonable price. If you can ever lay hold on one, many models of old Tek scopes had the 7788, plus lots of Bugle Boy 6922s. They're a grab bag of awesome tubes.

 

[Walter66]

It might just be my ears/brain tricking me, but to me the cascode circuits I've built have had a subjectively "harder" sound than a grounded cathode stage using the same triode type. FWIW, cascodes leverage the triodes' transconductance into voltage gain, so a high Gm type such as 6DJ8/6922 will translate into greater net gain (if needed) than a grounded cathode stage using a much higher mu but lower Gm type, plus relatively low noise.

In addition to the E80CF you mentioned, the E810F/7788 might be a possibile contender, if you can find 'em for a reasonable price. If you can ever lay hold on one, many models of old Tek scopes had the 7788, plus lots of Bugle Boy 6922s. They're a grab bag of awesome tubes.

Thanks for the hint. My approach would be 6072 in cascode vs. EF804S pentode phono input stage. As both will have high gain, no need for the output trioded pentode to be extreme values of Gm. It just serves as the impedance transformer to feed the equalitsation network. It's an interesting pentode for special purposes, I've included it in the pool of circuit variation tubes.

Some highend companies like Audio Note (M7) has taken the route of cascoding 6072 and it sounds good to me. It may sound "harder" than a simple triode circuit, but a pentode will beat it by far. So it may be for the subjective preference of the different sounds which way to choose. Pentode is studio sound, they just needed accuracy and technical excellence.

 

[bruce seifried]

It might just be my ears/brain tricking me, but to me the cascode circuits I've built have had a subjectively "harder" sound than a grounded cathode stage using the same triode type. FWIW, cascodes leverage the triodes' transconductance into voltage gain, so a high Gm type such as 6DJ8/6922 will translate into greater net gain (if needed) than a grounded cathode stage using a much higher mu but lower Gm type, plus relatively low noise.

In addition to the E80CF you mentioned, the E810F/7788 might be a possibile contender, if you can find 'em for a reasonable price. If you can ever lay hold on one, many models of old Tek scopes had the 7788, plus lots of Bugle Boy 6922s. They're a grab bag of awesome tubes.

Please do not strip Tek scopes of their tubes. That inevitably results in those scopes being parted-out or tossed in the trash, which is a crime.

 

[Rusan]

Please do not strip Tek scopes of their tubes. That inevitably results in those scopes being parted-out or tossed in the trash, which is a crime.

If I find a Tek 500 series somewhere in good condition, it's going home with me. But, it may not keep all its original tubes, even if they're still very strong. No matter what it is, if it's a good, functioning piece of tube gear and has some primo quality old audio types in it that test strong, I'll pull and replace them with new production or "lesser" grade old stock. That doesn't alter its function or diminish its worth, it just gives me some top-notch tubes at low cost for my audio gear.

I don't condone needless disposal or destruction of any sort of classic gear, that's either in working condition or can be restored. I'm talking about parts-only units, broken and busted gear pulled from dumpsters, stuff that's been stored in old out-buildings with leaky roofs and is rusted and deteriorated, etc. Things that fall into the category of "non-restorable" are what I'm referencing. I'm quite persnickety about preserving pieces of history intact, and for example have refused several times when asked to do mods to classic guitar amps that involved drilling or punching holes in the chassis.

 

[rock soderstrom]

I don't condone needless disposal or destruction of any sort of classic gear, that's either in working condition or can be restored. I'm talking about parts-only units, broken and busted gear pulled from dumpsters, stuff that's been stored in old out-buildings with leaky roofs and is rusted and deteriorated, etc. Things that fall into the category of "non-restorable" are what I'm referencing. I'm quite persnickety about preserving pieces of history intact, and for example have refused several times when asked to do mods to classic guitar amps that involved drilling or punching holes in the chassis.

This is a difficult subject and there are no absolute rules IMHO.

I see it more like Rusan, if something is already in the trash and in very poor condition, I think it's better to use this device as a parts donor and/or basis for an upcycling project. Restorations don't always make sense (for various reasons) and so I think it's better from a resource point of view to use at least parts of the old device for something new or repair.
As I said, there are no rules, it all depends on the device in question and the situation. But common sense will get you a long way here but there will probably always be different opinions on this question.

Personally, I definitely favor upcycling over disposal.