Low output with ECLL800 power amp

I'm currently building a small power amp using an ECLL800 and this schematic (only the ECLL800 part, no feedback):
http://www.jogis-roehrenbude.de/Verstaerker/ECLL800-Schaltung.jpg

The amp generally works but I only get about 1V RMS @ 8 Ohm with quite heavy clipping.  The anode voltage is about 280 V and the  voltage above the cathode resistor (180 Ohm in the linked schematic) is about 5.5 V, so the current through each tube is about 15 mA. This is less than specified in the schematic but I think should rather lead to crossover distortion than early clipping.

When measuring the anode voltages it can be seen that, although the anodes are at about 280 V, the maximum voltage swing is about 15 V RMS and clipping at top and bottom, resulting in the output voltage of said 1V RMS.

I tried two different tubes and also two different output transformers. The output voltage is a bit different with different parts but it generally is around 1 V RMS.

Anyone any idea what might be at fault here?


Thanks,
Chris

What is the transformer primary impedance?

Sorry, I forgot to post that.
The first transformer had a measured primery impedance of about 16k, the second is a Hammond 125C where I used a 17k output.

I got the first transformer along with an ECLL800; the Hammond transformer is newly purchased as I though that the first transformer might be at fault.

15V rms at the anode with 1V rms into 8 ohms on the secondary implies a 15:1 turns ratio which implies the 8 ohms will be reflected to the anode circuit as 1800 ohms. 15V across 1800 ohms is an rms current of about 8.3mA which has a peak values of nearly 12mA. This is very close to the idle current so it is not really surprising that you are seeing clipping. I think you need to raise the idle current.

Cheers

Ian

chr1s said:

...The amp generally works but I only get about 1V RMS @ 8 Ohm with quite heavy clipping.  The anode voltage is about 280 V and the  voltage above the cathode resistor (180 Ohm in the linked schematic) is about 5.5 V, so the current through each tube is about 15 mA.....

Click to expand...

Are you sure that tubes  are OK? The anode current for Va=280V, -Vg=5,5V should be much  higher.

ruffrecords said:

....15V across 1800 ohms is an rms current of about 8.3mA which has a peak values of nearly 12mA. This is very close to the idle current so it is not really surprising that you are seeing clipping.....

Click to expand...

Here we are talking about PP (B or AB class) amplifier. For  EL95 (half ECLL800 pentode) there is also B class setting :
Va=250V, Ia=2x8mA, P=6,5W (at THD=3,5%)

moamps said:

ruffrecords said:

....15V across 1800 ohms is an rms current of about 8.3mA which has a peak values of nearly 12mA. This is very close to the idle current so it is not really surprising that you are seeing clipping.....

Click to expand...

Here we are talking about PP (B or AB class) amplifier. For  EL95 (half ECLL800 pentode) there is also B class setting :
Va=250V, Ia=2x8mA, P=6,5W (at THD=3,5%)

Click to expand...

Agreed but you will only get that into the load it the transformer ratio is right.

Cheers

Ian

Cheers

Ian

Thanks for your replies.

The voltages above were only estimates and also taken with a heavily distorted output. When checking with a more or less clean signal I get 34.2 Vpp at both anodes and 1.4 Vpp at the output. The voltage of both anodes need to be added as Raa is of interest, resulting in an imedance of about 19k which is a bit more than the 17.6k set on the output transformer, but I suppose this might be due to the ongoing distortion and maybe also as the load impedance isn't exactly 8 Ohms.

I just noticed that Raa should be 11k instead of 16k which I somehow remembered, but this is rather a side note; changing the output transformer impedance doesn't change much.

I inserted a resistor to measure the suppressor  grid curren and also measured all anode currents which leads to both tubes being at fault here.  The pentode's anode currents are about 5 mA which - as already stated by moamps - is by far to little.

I purchased the second tube quite recently and it was said to be tested so I assumed that it worked and didn't question much the tubes' states. I'll have a look for getting another one and let you know.


Thanks again,
Chris

chr1s said:

I just noticed that Raa should be 11k instead of 16k which I somehow remembered, but this is rather a side note; changing the output transformer impedance doesn't change much.

Click to expand...

The higher-than-recommended p-to-p impedance should in fact help increasing the plate voltage swing.

I inserted a resistor to measure the suppressor  grid curren and also measured all anode currents which leads to both tubes being at fault here.  The pentode's anode currents are about 5 mA which - as already stated by moamps - is by far to little.

I purchased the second tube quite recently and it was said to be tested so I assumed that it worked and didn't question much the tubes' states. I'll have a look for getting another one and let you know.

Click to expand...

You may want to check how the idle current changes when reducing the common cathode resistor. Just to confirm your diagnosis.

You may want to check how the idle current changes when reducing the common cathode resistor. Just to confirm your diagnosis.

Click to expand...

I did this and both ECLL800 I have won't vary much in anode current if I put an extra 100 Ohm resistor in parallel to the 180 Ohms. With a cathode voltage of about 2.3 V and 1.3 V the anode currents are only about 6 and 7 mA instead of the  ~3.5 and ~5 mA  when the 180 Ohm is used. So I'm quite certain it's the tubes.

chr1s said:

You may want to check how the idle current changes when reducing the common cathode resistor. Just to confirm your diagnosis.

Click to expand...

I did this and both ECLL800 I have won't vary much in anode current if I put an extra 100 Ohm resistor in parallel to the 180 Ohms. With a cathode voltage of about 2.3 V and 1.3 V the anode currents are only about 6 and 7 mA instead of the  ~3.5 and ~5 mA  when the 180 Ohm is used. So I'm quite certain it's the tubes.

Click to expand...

Looks like the cathode has dried out...

I now have rebuilt the power amp using two ECC81 and the Hammond OT which supplies about 1 W. It's less output power than I could get from an ECLL800 but on the other hand, ECC81 are better available and I don't need that much power. The amp I'm building this for is a guitar amp for practicing, so 1 W is more than enough.

Thanks again for your input.