The transformer is 2.5K:8 as there are four parallel pairs [10k/4 : 8] I will check the OT tonight on my signal generator... I spoke with Merlin Blencow, the well know valve design author, and we have been through the design in detail and he and I are both at a slight loss as to why the power is so low. I am thinking it might be the opt. so that is the next test...
Thankyou everyone for your contributions, please keep them coming!
I have been involved in several discussions related to output power, often prompted by the disbelief of some of the power measurements I have made on tubes. Recently I got well over 100 watts from a pair of 13GB5 / XL500.
Look at it this way. When the amp is driven to max voltage excursion, the plate voltage at one output tube is near zero, and the OPT CT is connected to B+ so the other plate must be near twice B+. Look at the OPT CT as the pivot point in an equal length lever, or See Saw. On the next half cycle the polarities reverse. This shows that the voltage excursion at each plate goes from near zero to twice the B+.
This can be verified with scope measurements. If your scope (or its probe) can't eat 600+ volts, make up a voltage divider with resistors....that can handle the voltage. I use five 100K 2 watt resistors in series since each is only rated for 300 volts. Attach one end of the resistor string to the plate, and the other to ground. Connect the scope across the single resistor at the ground end. Multiply all readings by 5.
Given the Philips example, each plate will swing from 50 volts to 450 volts, for 400 Vpp on each tube with the other tube being 180 degrees out of phase. This does agree with the claim of 7 watts into 10K assuming OPT losses.
It should be possible to get 40 to 50 watts into a 2500 ohm load from a 300 volt supply assuming that the output tubes can adequately drive this load impedance REGARDLESS of what tubes they are. The type of tube used will affect the absolute saturation voltage, which does affect the output power, and too small a tube (or combination of tubes) will lead to distortion as the tubes approach saturation. Adding additional tubes in parallel increases the drive capability and helps lower the distortion, but once the tubes are saturating properly will not increase the output power without lowering the load impedance.
A higher impedance OPT will give LESS output power given no other changes.
One thing that hasn't been discussed here is the cathode resistance. Do you have it bypassed with a capacitor. If not, try it. I have seen similar effects in pentode P-P amps with unbypassed cathodes.
It would seem that measuring the OPT is the next step. I use a simple method. Load the secondary with the proper resistance (8 ohms on the 8 ohm tap). Plug the full primary into the wall outlet. Measure the secondary voltage and calculate the ratio and primary impedance.
We have 120 volts at 60 Hz in the US, so this works with small OPT's. You seem to be in a 240 volt 50 Hz country, so if the OPT is 2500 ohms, you will see about 23 watts on the load resistor during this test (don't test small OPT's). Your OPT's are rated for 40 watts at 18 Hz so this is safe. You should see 13.55 volts across the load resistor.
Some Sowter OPT's have multiple windings. Are they interconnected properly? Do these OPT's have multiple secondary impedances (4,8,16 ohms). If so try all available taps and see if you get more power on a different tap.
There you go!!!
The voltage ratio in a transformer equals the turns ratio.
The impedance ratio is the square of the turns ratio.
Have a second look at our combined brain stormings, please.
You will find, that the Philips (10k) OT has 35.4, your OT should have a turns ratio of 17.7.
So, you Need to use your OT with two and two in parallel and those pairs in series.
Your 4 in parallel would yield a ratio of 8.85.
The voltage ratio in a transformer equals the turns ratio.
The impedance ratio is the square of the turns ratio.
Have a second look at our combined brain stormings, please.
You will find, that the Philips (10k) OT has 35.4, your OT should have a turns ratio of 17.7.
So, you Need to use your OT with two and two in parallel and those pairs in series.
Your 4 in parallel would yield a ratio of 8.85.
I spoke at great length with Merlin Blencowe today and Brian Sowter and the design is sound according to both so I am not going mad ( though some would disagree 
)
The cathode resistors are fully bypassed, the OPT does indeed have multiple taps, and I have arranged them as per Brian's notes to get 8 Ohms, as described above :
Here is the spec:
Its 2.5k Raa
And 120mA per side max
Drop throught style (half shroud)
40W at about 18Hz
43% UL taps
There are four windings on the secondary 1,2,3,4, Brian suggests 1,3,4 in series and 2 in parallel with 3 (as above!)
I will test the OPT now with my signal generator, don't fancy shoving it across the mains, even though I have a 2KW capable variac, if I do decide to do the mains test and you don't get another post from me after this one, well then the test did not go to plan
I'll report shortly...
)The cathode resistors are fully bypassed, the OPT does indeed have multiple taps, and I have arranged them as per Brian's notes to get 8 Ohms, as described above :
Here is the spec:
Its 2.5k Raa
And 120mA per side max
Drop throught style (half shroud)
40W at about 18Hz
43% UL taps
There are four windings on the secondary 1,2,3,4, Brian suggests 1,3,4 in series and 2 in parallel with 3 (as above!)
I will test the OPT now with my signal generator, don't fancy shoving it across the mains, even though I have a 2KW capable variac, if I do decide to do the mains test and you don't get another post from me after this one, well then the test did not go to plan
I'll report shortly...
Hi Ian, yes checked this, but I will buzz out the dc impedances to see if they are labelled wrong.
7.3 turns ratio gives a primary load of 426 ohms between anodes.... anyone tell me what the power would be with such a mis match...
Genius observation - I owe you a beer at least!
6.6v ac to 0.37 vac or 17.7:1
spot on now, connect across the "UL" labelled taps!!!!
Nice thread
Here is the prototype, running on one of my vintage Heybrook HB1 speakers that I have had from new. (30 years!)
EL95 Valve Amp Prototype - YouTube
and some pictures:
http://www.togethia.tv/amp/amp1.jpg
http://www.togethia.tv/amp/amp2.jpg
http://www.togethia.tv/amp/amp3.jpg
Truly impressed by the amount of willingness to help here! Thanks to you all!
That's a cool amp! Great pics too, I bet the reg tubes look pretty in the dark.
jeff
That's a cool amp! Great pics too, I bet the reg tubes look pretty in the dark.
jeff
The great bit is, a pair of 350B pentodes has the same anode draw and Raa as 8 paralleled EL95 tubes... I feel some more prototyping coming....
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