Plate dissipation

[Rob11966]

Hi Folks,

I have a question about plate dissipation.

I have a Shuguang 845 tube with the company data sheet. I have attached a copy of this.

It has been tested at 1250v with a plate current of 120mA. This corresponds to a plate dissipation power of 150W - not bad! I imagine that this is a maximum rating? What is the rule for dissipation at idle? I was thinking of setting the bias at idle for a power dissipation of 75% of max i.e. 100W.

Any opinions?

Cheers,

Rob

[SY]

I'd check with ShuGuang to see where their tube is actually rated. "Normal" 845 is 75W max, but in recent years, liberties have been taken with the type numbers of tubes; they are more like brands than they are actual type numbers, so who knows. Those numbers could be a short test...

Rated dissipation aside, it's important that you check the bulb temperature when running the tube close to its max. Even though the dissipation rating may not be exceeded, you can easily overheat the glass. That means forced air to keep things from failing.

[Rob11966]

Thanks Sy,

The problem with Shuguang is that it is difficult to get any real technical information from them.

I have emailed them in the past a couple of times without response.

The suppliers are always helpful but often seem to be in the dark as well.

I am not even sure that measuring the glass temperature would help as I don't even know what temperatures the glass envelope can tolerate!

With respect to this particular tube, it certainly has higher numbers than most, whatever that means. As I mentioned my other 845's are all graphite plate 'B' type tubes with ratings around the 100W (80mA at 1250v) mark. I have been running these at around 75W idle and this is where I got the '75% of max' number from. The new 845 C that I have definitely has higher factory ratings but what that means is unclear to me.

It's a good looking tube though.

Rob

[Gordy]

Originally posted by Rob11966
Hi Folks,

I have a question about plate dissipation.

I have a Shuguang 845 tube with the company data sheet. I have attached a copy of this.

It has been tested at 1250v with a plate current of 120mA. This corresponds to a plate dissipation power of 150W - not bad! I imagine that this is a maximum rating? What is the rule for dissipation at idle? I was thinking of setting the bias at idle for a power dissipation of 75% of max i.e. 100W.

Any opinions?

Cheers,

Rob

I have a 845 datasheet from Amperex that states (for a Class AB amplifier) the max anode voltage as 1250 V and the max current as 130 mA.

Surely common sense suggests that it does not mean 1250 V AT 130 mA !!!

The datasheet goes on to state the max plate dissipation as 75W.

[Gordy]

Here you go Rob:

http://www.pentalaboratories.com/pdfs/845.pdf

[Rob11966]

Originally posted by Gordy


I have a 845 datasheet from Amperex that states (for a Class AB amplifier) the max anode voltage as 1250 V and the max current as 130 mA.

Surely common sense suggests that it does not mean 1250 V AT 130 mA !!!

The datasheet goes on to state the max plate dissipation as 75W.

Hi Gordy,

That's in class AB. The tubes are 'idling' for some of the operational time. Operation in class AB will allow more current at any given voltage when compared to class B but the maximum plate dissipation will be the same.

In class A at 1250v, the maximum current is not given on the Amperex data sheet in your link but the plate rating is 75W. This equates to a plate current of 60mA. This is the maximum rating and obviously more than the 'typical' operation in class A.

Although the Shuguang 845's are often compared with the RCA/Amperex tube (in fact the company referred to the old Amperex data sheet for a while), I don't think that these tubes are comparable any longer. I think that Sy's point is very relevant. The tubes, even between models 'B', 'C', 'M', 'W' are different beasts with varying characteristics and tolerances. This may be even true with different production runs of the 'same' model.

Very confusing.

Rob

[Wavebourn]

That's why George, tubelab.com, tortures tubes to check their real max ratings.

[SY]

At these prices, George isn't torturing them.

That whole type number thing really cheeses me. I think it started to really irk me when I got some Russian 572s that weren't ANYTHING like the 572s that had been made for decades. Not plate ratings, not mu, not envelope, not pinout. Why did they call it a 572? Since then, it seems like tube peddlers will call any DHT a 2A3 if it's small, a 300B if it's medium, and an 845 if it's big without regard to what those type numbers mean.

Grrrrrrrrrr!

[Wavebourn]

Originally posted by SY
Since then, it seems like tube peddlers will call any DHT a 2A3 if it's small, a 300B if it's medium, and an 845 if it's big without regard to what those type numbers mean.

Heh... I do it already. I call GU-50 "My big tube", 6P15P -- "My medium tube", 6F12P - "My small tube".


Actually, I have some more: "Fat medium tube" is 12L6GT. "Green tube" is 6E1P.

[gingertube]

Rob,
I got a pair of those those Shuguang 845B (came in a nice box as a matched pair). Test sheets for each tube stated pretty much the same as you quote. These are maximum figures.
I've been running them in a Music Angel 845 SET at 900V and 83mA for quite some time and they have been happy at that. This is exactly 75 Watts Anode dissipation.
Why did I choose that setting? Well there is a 12 Ohm current sense resistor from the filament hum pot to 0V. Exactly 1 Volt across that 12 Ohm is 83mA which with the 900V is 75 Watts. I decided that the 12 Ohms was not chosen at random. 75 Watts is the figure which has been recommended many times in other threads.
Of more concern in that MA 845 design is that there are 2 off 470uF/450V caps in series for the 900V rail with 330K voltage share resistors across the caps. I changed those voltage share resistors to 100K as the second mod I ever did. The first mod was to wire the un-connected mains earth pin to the chassis.
Cheers,
Ian