Question on WE310

[tim845]

Hi folks

Recently my friend had built a SE 300B amp which is only 2 stages. He used WE310 to drive China made 300B.

After the listening, we discovered that the machine's sound stage is very dim and the high range seem lower than expected.

We arrange a simple test for the gain level of certain frequency and found the following:

1) concentrate the testing on WE310 output, i.e. the milli volgage meter connected to the output cap. of WE310

2) WE310 fed by 310V B+ approx. and the plate voltage is 220V approx., the cathode is measured 4.6V accross the cathode resistor of 1.5K

3) WE310 is connected as penthode

4) the frequency is starting to roll off at 2Khz !!! when 12Khz applied to WE310, the gain down to 1/2 only as compared to 1 Khz

We wondered why frequency response of WE310 will start to roll off at 2 Khz !!

We tried it's identical - 6C6, same result !!!

Later, we tried to connect 6C6 as triode. The result is - only start to roll off at 23 Khz !!!! But the gain is also rolled off !!!!!

So, anybody has this experience in using WE310 connected as penthode ?

Any comment to play with this expensive tube ?

Thanks for anyone could teach me how to solve this big problem

[Palustris]

Have you got the screen bypassed to ground?

[Tom Bavis]

Do you know how much capacitance your AC voltmeter has? It may be enough to affect your measurements. A pentode has a high output impedance and triode is lower, so it will have less high-frequency roll-off for the same capacitance.

[pauldune]

Most standard multimeters arent capable of higher than 50 Hz measurements. Some work up to 1-2 kHz, but you'll need a true RMS multimeter, or better an oscilloscope to measure the frequency range correctly.

[tim845]

Thanks for all of you.


To Palustris : the screen was bypassed previously but it is connected to the cathode now. The result is only slightly better than directly bypassed to ground.

To Tom Bavis & pauldune : I'll check it and tell you.

Thanks again.

[tim845]

Hi folks

The mV meter is no problem and is catered for the measurement of precise reading for audio.

The measurement instruments are all for audio !!

So, we wondered that WE310 is a really legendary tube ?

Should we to clarify and to prove some legendary tube using testing machine ?

Even we directly plug in a 6C6 (also WE !!!), except we change the filament voltage, the result still better than WE310 !!!

[pauldune]

The only thing I can say after some thinking, is that or your circuit is faulty, the tube is broken, or you don't measure correctly.

I don't think there were ever tubes created that had a frequency range lower then 2 kHz.

What happens when you measure without the 300b? If the frequency range is a lot better, than maybe its the miller capacity of the 300b. Can't be shure without a schematic...

[astouffer]

Who makes the meter you are using to take the measurements? Bench meters can read well up to 100Khz but like pauldune says, handhelds aren't good above a few Khz.

Do you have access to an oscilloscope? That will make finding the problem much easier.

[Tom Bavis]

300B has a grid-to-plate capacitance of 15 pF and a amplification factor of 4, so there will be 60 pF of Miller capacitance to drive. The output impedance of a pentode stage is pretty much the same as its plate load - maybe 100K? So the -3dB point at the grid will be around 25 KHz. Your meter might have 30 pF input capacitance, and 5 feet of coax cable, another 150 pF. So it would roll off by 3 dB about 8 KHz...

As a triode, a 310 or 6C6 might have a 10K output impedance (plate load resistor in parallel with plate resistance) and so would have a -3dB point about 10 times the pentode's.

[pauldune]

Originally Posted by tim845

Hi folks

...The mV meter is no problem and....

Is this a typo? Or is it really a mV meter? Because you should measure 10s of volts.

If you want some real help, please draw a schematic of the amp, and how you are trying to measure... Then we dont have to guess...

greetz, Paul