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]

Originally posted by SY
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