UL to triode

[kathodyne]

Hi!,

My amp runs in ultralinear (7027a beam tetrodes)
Yesterday i put it in triode mode (g2 via an 100ohm resistor to anode)

The primary of the opt is 6k, the UL winding is about 60 to 70% of this....
Which would be best for my end-tube? because in triode mode it has a different rA.....

- just use the main connection for anode??
or
- use the tapped winding?

Thanks for any comment

 

[Agent327]

Hi,
I would not recommend using the UL taps.

If your UL tap is 60% then the resulting impedance would be 0.6 x 0.6 x 6K = 2k16, if i am not mistaken.
I am not really sure if this 60 % is turns ratio or impedance ratio...

That is a very low value for a triode connected push-pull (i assume) 7027A. The RCA datasheet suggests it would rather see 8K. Your 6k is not so far-off.
The 7027A triode curves indicate Ra is about 2K. For PP these are in series and it is not uncommon in triode amplifiers to choose a 3 or 4 times higher anode load, so 12K would not hurt, distortion-wise. You could use the 4 ohm output tap with an 8 ohm speaker to get close to that.

Did you already listen to it or measure something?

Regards

 

[SY]

6K anode to anode is an excellent load for 7027s in triode. With 450V or so, you should be good for 15 watts.

Just tape up the ends of the UL tap wires.

 

[kathodyne]

thanx for the info

i'll let it run on the anode taps

i'm now running at 380v 62,5ma idle per tube
There is a possibility within the amp to set this bias-current.....

it ran like this when it was still used as beam-tetrode (push-pull)
so could i turn the current up a bit?? (70ma) to get a little more output??

or is this not good for the tubes? because the operating point will shift....

The sound is real nice, better then before

 

[SY]

Turning up the current will not get you a whit more audio power. It will create more heat.

 

[kathodyne]

okay....
because b+ is the main factor now; it's a triode....

it's hot enough as it is... so i'll leave it like this, it's very gentle now!
will switch back to penthode-mode(beam-tetrode) for power if needed...

thanx SY and others

 

[Johan Potgieter]

Kathodyne,

This is not what you asked, but . . . why go to triode if you were in UL? I do not want to stir up feelings among triode-preferring folks, but if you look at the tube data regarding output and distortion vs. the two modes, you will find that UL has about 85 - 90% of the characteristics of triodes, but quite a higher max. output, which means lower distortion at the same output. This is the beauty of UL.

I can only find my data for KT66s now for comparison, but it will be similar for 7027. In triode mode a a pair will give 13W output at 3% total distortion (Va = 400V), while in UL mode for the same conditions the output will be 28W at 1.8% distortion (GEC data sheets). I have comparative data for KT88 which I cannot find now (is that not always the case?!) but as I can recall the r(a) is 1.8K-ohm for triodes, rising to only 2.5K-ohm for UL (43% taps) while the maximum available output (push-pull) is 40W in UL and only 22W as triodes, or something similar (same H.T. voltage). The point is that, despite the belief of some, one gets very much the same characteristics for UL as for triodes, but with a significantly higher ceiling or, inversely, lower distortion at the same output.

As said, this not to confuse you, but do consider - I do not know what your reasons for change are. I have been using 6L6GCs (equivalent of 7027) now for years (I have been in hi-fi for a half century) in this way with great success, and measurements confirmed the above. If you do change, I concur with the advice previously given by others.

Johan

 

[SY]

Johan, have you considered sensitivity to load? Speakers do not generally look like nice 8 ohm resistors- they have impedances that vary with frequency and level, not to mention reactance. When you take that into account, the small power loss going from UL to triode is no big deal compared to the reduction in load sensitivity.

 

[rdf]

When you take that into account, the small power loss going from UL to triode is no big deal compared to the reduction in load sensitivity.

Is triode less sensitive to load than UL? I was under the impression local feedback gave UL the advantage here.

 

[cerrem]

Here are some usable numbers to check out...

Using a pair of 7027 tubes in Push-Pull wired in TRIODE....
Using a 6K plate load....

Your "cut-off" voltage is -60...so you "could" bias into Class A by adjusting till yo get to -30 @ 74mA .... WHich is 28W plate dissipation...
Or just stay in Class AB 1 run cooler...

The Power Output is 12.6 Watts ..Clean RMS Watts...at 6K plate load..
Plate resistance AVERAGES over the cycle at roughly 1500 ohms....

If you use the 4 ohm tap into 8 ohm load, as another post suggested, you will get 10W output with a 12K plate load...

Damping Factor at 12K would be about .5 ohm "open loop" ....then you need to calculate the DF after the feedback loop is closed....

Since the output transformer was not really intended for TRIODE operation...Frequency response need to be evaluated..since leakages and inductance now contribute differently to the TRIODE's lower plate resistance... Most likely the high frequency response should be OK...The inductance may be too lw and you may have a soft low frequency...but no way of knowing until evaluating...

Chris

 

[SY]

Is triode less sensitive to load than UL? I was under the impression local feedback gave UL the advantage here.

Well, if you want to use a local feedback model, the triode (strapped pentode- think of a UL with 100% taps) has more feedback than the 43% UL, right?

There are some very illuminating charts in Crowhurst's "Understanding Hifi Circuits" for the 5881, comparing UL to triodes with variations in load impedance with a reactive component included. As always in engineering, there are tradeoffs between the two modes; triodes have greater max power falloff with load change, but their distortion curves are much flatter at loads straying high. The UL does a bit better with loads straying low.

 

[ray_moth]

In triode mode a a pair will give 13W output at 3% total distortion (Va = 400V), while in UL mode for the same conditions the output will be 28W at 1.8% distortion (GEC data sheets).

I understand that the way the pentode or UL distortion is made up - its spectrum, if you like - is different from that of triode mode. Triode distortion is mostly 2nd harmonic, which will be mostly eliminated by PP, and 3rd harmonic. Pentode and UL modes also produce 5th, 7th and 9th harmonics, which are unmusical and objectionable.

 

[rdf]

Well, if you want to use a local feedback model, the triode (strapped pentode- think of a UL with 100% taps) has more feedback than the 43% UL, right?

(Too) obvious once I think about it that way. Thanks for the Crowhurst tip, I'm finding UL resources difficult to come by.

...Pentode and UL modes also produce 5th, 7th and 9th harmonics, which are unmusical and objectionable.

I'ld be interested in seeing measurements. I compared triode to UL on an EL84 using Audiorightmark. Without a scope the peak output levels were unknown but within the 120 dB limits of an Audigy I didn't see this behaviour. There was a noticable increase in 4th or 5th compared to triode but nowhere near what's often suggested. Anyone know of data?

 

[kathodyne]

@Cerrem....

wow.... you're right!...high frequency is really nice but i was already wondering why there was a little less bass then before...impedance

so 6k would be a little too low....just as a test i'll listen what it does when using the 8ohm taps with 8 ohm speakers (6-8 ohm they have written on them)

if i understood you people correctly there should be even less sound!

the other sort of distortion ánd because the mid/high is soo nice now i'll be looking around to build a real triode amp.
if just the ship with money came sailing in....

For now i'll just install a switch so i'll be able to switch the two operating modes (when the HV is down ofcourse-----wonder what happens but don't dare to try )

- don't work on amps with friends nearby ------ they'll find a way to get your soldering iron INTO your circuit-

 

[SY]

I compared triode to UL on an EL84 using Audiorightmark.

Did you have a load that was reactive, i.e., a reasonable speaker model?

You're right, hard comparisons between UL and other modes are difficult to come by.

 

[rdf]

No, the load was simple R. Even at that I don't completely trust my results having found significant discrepencies between two of my computers making the same measurement. It's something I want to revisit.

 

[PRR]

> switch the two operating modes (when the HV is down ofcourse

You can switch triode/UL/pentode with power on and signal flowing.

It would be best to use a snap-action non-shorting break-before-make switch. Break the screen wire and the tube goes dead safely.

If you used a shorting make-before-break switch, that was not snap-action: if you linger in the in-between positions, the output transformer is shorted. This does no immediate harm, but if you also have signal flowing and crank it up because nothing is coming out, the tubes will overheat.

It really does want to be a 600V switch. With small tubes and low B+, switching at low-signal moments, a lesser switch will work for a while. But with big signal power the switch will arc enough to burn the contacts of a $1 50V switch after a few operations.

> the way the pentode or UL distortion is made up - its spectrum, if you like - is different from that of triode mode.

Not the mode but the tube. OK, if you swing a pentode right into the low-volt high-amp knee you will find a lot of 7th, 8th and 9th, but that vanishes when you run a few dB lower.

If the grid were perfect, high-order harmonics would be vanishingly small, with or without a screen grid.

Grids are never perfect.

The classic audio triodes like 2A3 and 300B were laid-out with coarse grid pitch so that minor variations in pitch due to grid lathe wobble would cause only minor kinks.

The "hot" pentodes were not intended to be used for "good" audio without feedback. With feedback, or for "bad" audio (cheap radios), what really matters is gain. They use a close-pitch grid that gives more gain, but is also more sensitive to grid-lathe wobble.

I'm thinking of 1930s production. 1950s grid lathes were good and old 6L6 are pretty linear for a tube that was not made to be super-linear, though some of the low-price tubes like 50L6 may have been assembled on worn-out lathes to keep costs down. 1990s production is small-market low-tech, and some grid lathes still in use may not even be good enough for a 2A3 to show really low high-order harmonics.

> wondering why there was a little less bass then before...impedance

Still wondering. The triode's low Rp should give better bass, because it is not loaded by OPT inductance so much. However hi-fi p-p OPTs usually have way more inductance than they "need", to keep distortion down. Maybe what you are hearing is not less bass, but less bass distortion? That could happen if the OPT is not oversized enough.

 

[Johan Potgieter]

Johan, have you considered sensitivity to load? Speakers do not generally look like nice 8 ohm resistors- they have impedances that vary with frequency and level, not to mention reactance. When you take that into account, the small power loss going from UL to triode is no big deal compared to the reduction in load sensitivity.

You are quite right about the atrocious loads that loudspeakers present. But: The power output for UL could be a little higher than for straight pentodes because the screen is now also contributing somewhat to output power, while the triode output under similar conditions would only be about 30% of the above. At about 35% taps the r(a) [plate resistance; I cannot do subscripts] on the other hand would only be about 10% - 15% higher than that for a triode. Since that is what makes a stage "load insensitive", you have almost the same load tolerance as for triodes, but with a distinct nett advantage in sensitivity and more important, output power and distortion. The power difference is not so small, while the other advantages exist.

I am so sorry that I cannot like a decent chap back this up now with figures; I will really hunt for a very illustrative GEC graphs for KT88s that I have here somewhere and give figures as soon as possible. Meanwhile you will have to accept the figures quoted from memory in my earlier post. To sum up, with UL (and proper design!) one has most of the advantages of pentodes (tetrodes) and most of the advantages of triodes. That is because, as the screen is moved from B+ towards the anode, the available output power decreases slowly (after even an initial increase as explained above) and only more rapidly after a 50% tap, while the r(a) deceases rapidly and flattens out later.

To venture a further observation: Certain amplifiers give a switchable option between triodes and UL. As far as I know only the screens are switched between UL taps and anodes and folks witness to hearing the difference (in favour of triodes). If such switching is true, this is an over-simplified comparison. Neither the load nor the current is the same for optimal conditions in either mode.

I would say again: I do not disparage the use of triodes (have done so myself), but I truely cannot see myself preferring those in favour of UL. The type of tube also plays a role, but only slightly so. But let me see if I can come up with figures (might even be able to display the graphs here).

Johan

 

[Johan Potgieter]

I have read post #17 only after I posted (#18) earlier.

Good post by PRR.

Just to remember that pentodes were initially made not primarily for low distortion but high frequency (RF) (small pentodes) with realisable advantages for audio (bigger pentodes). High fidelity as we know it only came later, I think with the Williamson. The beam tube principle (a little more linear it would appear) was also later, although (way back!) I think the 6L6 was already there before 1940.

Yes, UL graphs are difficult to come by (I mean distortion-wise). How I wish I had the spectrum analysers that my work could afford! I am at present busy with a 100W amp using 4 x 6L6GC. Perhaps I can endear myself at the local university and score a few hours on their equipment.

Johan

 

[PRR]

> High fidelity as we know it only came later, I think with the Williamson.

That's when it was called high fidelity. But the 2nd-generation movie sound systems, early 1930s, were very faithful reproducers, within their design goals (dispersion is "bad" when you have limited power and over-live acoustics). Fantasia was played on these systems with stunning effect. These were often hot push-pull triodes. The rise of Pentodes was the decline of audio, what the hi-fi nuts of the 1950s were fighting against.

> pentodes were initially made not primarily for low distortion but high frequency (RF) (small pentodes) with realisable advantages for audio (bigger pentodes).

Pentodes happened because triodes and tuned circuits don't play well together. And not just because triode Rp damps the tank. If the input and output tanks are not right on the same frequency (they never are), you get over 45 degree phase-shift in each, which with the plate-grid feedback (1/Mu) and the tube's inverting gain gives everything needed for oscillation.

A second RF-grounded grid kills the feedback path. If left at DC ground, it also kills current, because the grid-cathode interface feels the extra grid voltage more than the plate voltage, and electrons have no reason to wander across. If taken to or past plate voltage, it sucks all the electrons. But kept at a high voltage, lower than plate, with AC grounding, you get a good tube that can give high RF gain without instability.

In audio, the tetrode separates two functions: urging the electrons away from the cathode, and collecting electrons to heat-up the load. Plate voltage can drop while current is rising. Gain can be high.

Problem is that the screen has to sit at a high voltage and the plate has to stay above that voltage or the screen will capture ALL the electrons. Another problem is that electrons that hit the plate knock secondary electrons loose and flood the screen.

The fix for that new problem was a third grid, near DC ground. Screen (G2) is run positive enough to pull electrons away from the cathode and throw them through the negative region to the plate. Secondary electrons are low-energy and the negative region turns them back to the plate. G3 also adds another layer of shielding for incredibly low feedback. However, G2 current tends to be a significant fraction of total current, reducing Gm a little, complicating wiring, eating power, and causing huge extra noise.

> The beam tube principle (a little more linear it would appear) was also later, although (way back!) I think the 6L6 was already there before 1940.

Beam "tetrode" tube is just a different way to build a pentode, apparently to avoid certain patents on 3-grid power tubes. Instead of a third grid near DC ground, they get essentially the same effect by forcing the electrons into a thin beam. The electron's own charge gives a sufficiently negative area to act as a G3, without looking anything like the patent drawings for power pentodes. At a high level, you could say that RCA just replaced the several-turn horizontal electrode with a 2-element vertical electrode structure, and the electrons don't know the difference. To cover their rears, RCA found several other small improvements and wrapped them up (around 1937) as the 807/6L6, and provided extensive write-ups to show its novelty. In many ways (not so much for fidelity) it was a very superior power tube. But if you compare the early 807/6L6 curves to Euro Power Pentodes and then to the later 6L6GC, you suspect that RCA finally abandoned the extreme beam-forming proportions, and OTOH the Euro tube makers adopted the 6L6's aligned-grid trick.

> I wish I had the spectrum analysers

A PC with a sound card can do spectrum analysis. I was doing it on 386s, though a hot 486 is a much better engine. There are several packages, some freeware. One (RightMark?) is primarily designed to test soundcards with a loopback cable, but you can put anything you want "in the cable" if you can get the overall gain to unity. Working with Power Amps, you need some care to attenuate the signal so you don't smell smoke from the PC. You are of course limited by sound card performance, but I've had $19 no-name USB sound-boxes show THD below 0.05%, which is good enough for most power tube work. (I got the $19 USB-sound because an on-board audio claimed 24-bit but actually delivered 15-bit, as if the top bit of the ADC was stuck.)