5E3 circuit. Output power too low

[TATS]

I am building an amp based on the 5E3 circuit, but modified it with a larger voltage supply and the fixed bias voltage at the PI grid as I saw in many posts on this forum. Thanks a lot for this. It really helped me building the amp I want. I am not looking for the specific 5E3 sound, which is great, but I want more more headroom and changed the tubes from 6V6 to 5881. Power is a secondary issue.

Despite all the experiments and calculations I cannot get the output power above 6-7 watts. The sound is what I am looking for, but a bit more power than 6-7 watts is preferable, especially with 5881 tubes.
I suspect the output transformer. I order them from someone who’s profession is winding transformers and he assured me that the impedances are right and also the wattage. The transformer is a P/P double C (about 20 watts) with 10k primary and 8 ohm secondary (close to the 9K from Radiotron’s datasheet)

I’ve tried different transformers (same manufacturer) with different impedances (7K,8K,10K,17K primary) and wattages (up to 100watt) and also changed output tubes, 6V6, 5881, 6L6. As well as the cathode resistor from 270ohm up to 470 ohm.The power output however remains the same. The power transformer is supposed to be 200mA with 270v output, enough to drive a pair of 5881’s. The rectifier is a solid state bridge.

Some measurements taken with a multimeter and a scope (volume of bright channel at max.):

Ua=360 volt, Uscreen=345 volt, Upi=285 volt, Upreamp=245 volt (165 volt at anodes).
Output load: 8 ohm resistor, Input: 100mVpp, 1000Hz
Cathode resistor: 318 ohm (15W), -28,5 V
Max voltage swing PI: approx. 25Vpp
Max voltage swing Anode: approx. 140 Vpp
Max voltage speaker: 7,5 Vpp (!) in a 8 ohm load.

I measured the speaker voltage against the primary transformer voltage, by turning up the volume slowly and wrote down the voltages. The funny thing is that the speaker voltage is proportional up to a certain point, where the graphics goes flat and the speaker output voltage doesn’t follow the input anymore. This looks like saturation. That is why I suspect the output transformer and called the manufacturer, but he told me the impedances and power are right.

Are my calcultions right? What am I doing wrong here? I try to tackle this problem for a while now, but I am out of ideas.

Thanks a lot for your help!
I’ve attached a quick drawing of the circuit.
Chris.

[loudthud]

Change the 2.2 meg resistor that biases the concertina to 4.7 meg. You might want to increase the volltage at the C node. Change that 15K resisitor between B and C to 4.7K. Take a look at this thread: http://music-electronics-forum.com/t5837/

[TATS]

Thanks Loudthud. I will try this. Sounds logical.
I didn't try this with the 2.2M resistor, but what I have done in an earlier stage of the project was increasing the supply voltage at the PI and noticed that when I go beyond 300V, and the voltage at the cathode of the PI goes beyond 100V, the amp stops working and I get no sound at all. 100V at the cathode is probably the limit.

[TATS]

I changed today the 2M2 resistor to 4M4. I hadn't a 4M7 resistor at hand. This simple change resulted in an increase of the voltage swing at the concertina output. From 25Vpp to 35Vpp. And the voltage swing at the anodes was 165V against 140V with the 2M2. So this is a useful hint (thanks!) and I will certainly use it in future amp projects.
However, the output voltage at the speaker still doesn't exceed the 7,5Vpp. I didn't increase the supply voltage at the PI as Loudthud suggested, but I will later on. What would be the max. voltage without entering the danger zone?
First I tried something else. I was thinking maybe it could be a saturation of the OT iron core. The OT is a double C, silicon steel and it has no air gap and what I've read about double C cores is that they are notorious for saturation especially in SE amps, because of the DC current. In P/P this shouldn't be so much the case, but to be sure I disassembled the OT and inserted sheets of paper.
Also no success here. The same output voltage. I tried an EI transformer, but the same result.
So what's next to try?

[Bruce / Mission Amps]

Is the signal coming out of the OT secondary "clean" at 7.5vpp... which is awful by the way.
As viewed on an O'Scope ... a dead stock 6V6 5e3 with a 5Y3 rectifier and lowish voltages will do about 18-20vpp across an 8 ohm power resistor at 100Hz. Yup I do know this.
And by clean, I mean clean... no obvious distortion.
I'd like to see what the "signal form" looks like at the grids of the power tubes.
10K for a 5E3 is a bit high... and more then just a bit high for a pair of cathode biased 5881s!
I'd use 5K push pull OTs with those 5881 amps.

Oh, by the way, you can not have -28.5 v at the cathode of the power tubes....check that again.

Also, where is this NFB loop attached and how does that effect your 7.5vpp at the output?
Since you modded the phase inverter to be biased like that, where are you inserting the NFB!?

[TATS]

Thanks Bruce for your help. I really appreciate it. I tried to gather the information you asked for as detailed as possible, because I know it is frustating to troubleshoot without having enough details.

First of all I wrote the wrong value for the cathode resistor. It is not 318 ohm but 328 ohm. I made some changes in the supply voltages as Loudthud suggested. The PI supply is now about 310 volts and the preamp section 270 volts. I still measure a high voltage at the cathode resistor of the output tube. Since the changes of the PI supply voltage it has increased to 29 volts (when the volume was at 12, the DC cathode was 45 volts).

You asked about the feedback. I took as an example the 6g2 schematics. There the feedback resistor is 56K and is connected to the cathode resistor of the last stage of the pre-amp or first stage of the PI tube, however you want to call it. When I tested the amp I stroke the strings of the guitar very hard to see how the amp would react. There was a glassy, nasty distortion, exactly on the attack of the strings. I increased the feedback resistor from 56K to 68K and the problem was gone. I also added a 4,7uF bypass cap, but I don’t hear a difference in the sound, so maybe I will remove it. The signal looks the same with and without feedback. I can close the feedback loop by an on/off switch. I like the sound more when there is feedback. Without feedback there is a lots of bass and with the feedback it ‘smoothens’ the frequency bandwith. It is also more touch sensitive, but that can be my imagination.

I attached some pictures of my measurements on the o’scope. The signal looks ‘clean’ to me, but I would like your opinion. The signal is clean till ‘8’ on the volume knob, Then distortion comes in and crossover distortion.
I also plotted the voltages of various voltage in- and outputs. You’ll notice that from the volume of 9 the output doesn’t follow the input anymore. If this would be proportional then I would have 10-11 volts at the output, still way too low, but much better than it is now.
The voltages at the grid of the output tubes are pretty clean and distortion of the PI starts at the volume of 10.

All these tests were done with the OT 10K-8ohm. Then I made a test with a OT 2.5 K - 0 – 2.5 K and saw worse results. The voltage output at the speaker was max. 4V and the signal was heavily distorted. All tests were done with an 8 ohm resistive load.

Ah, yes, I forgot. Whenever I monitor the signal from the anode of the output tube on the o’scope, the signal is ‘wobbling’. It is never steady, like there is a ghost signal. I don’t know if this is important. I don’t see this when I measure at other points of the amp.

That’s about it. I know I ask much, but if I would know the answer I wouldn’t have to ask, so thanks again!

Chris.

[Chuck H]

Your phase inverter seems to be badly imbalanced. And on the side that isn't clipping it's not even putting out enough swing to clip the power tubes. For all I can tell it may not be clipping the power tubes on the clipped side. Since the clipping looks to be analogous for the PI and the power tube. There is also excessive crossover distortion on both sides. Which is odd because only one side is clipping. I expect your 5881's aren't happy with the load impedance and are also biased cold.

You mention the OT primary as a 2.5k-0-2.5k. I've never seen it put that way but I'm not sure that works out to 10k. I think there is some exponential math for how that works out. But I'm sure someone will post about it.

With 5881's and higher Vp you'll need to swing pretty hard at the PI to get full power. After finding out why it's imbalanced you could try to get the voltage for the PI circuit up if possible.

If your OT is indeed 10k primary you can half your speaker load to reflect the more appropriate 5k primary if you decide that's what you need to do.

[loudthud]

Any noise or fuzzyness you see on the plates of the power tubes is power supply ripple. It is in phase on both plates to it doesn't transfer to the output. Slow you generator down to two times the power line frequency (100Hz?) and you will see a sawtooth wave crawling across the sine waves.

[frus]

Also, where is this NFB loop attached and how does that effect your 7.5vpp at the output?
Since you modded the phase inverter to be biased like that, where are you inserting the NFB!?

I can see it goes to the cathode of the gain stage before the PI, and is then shunted to ground by the cathode bypass cap

[TATS]

Any noise or fuzzyness you see on the plates of the power tubes is power supply ripple. It is in phase on both plates to it doesn't transfer to the output. Slow you generator down to two times the power line frequency (100Hz?) and you will see a sawtooth wave crawling across the sine waves.

We have indeed 50Hz here. This is what I see when no signal is attached and the volume to zero.

[TATS]

I connected the 8 ohm load from 8 ohms to 4 ohms and kept the primary 10K, so now it is 5K. And there is an improvement! The output climbs up to 9.4 volt. Normally I wouldn't be happy with these results, but every step forward means I am on the right track. Thanks to you all!
I wouldn't know at the moment how to improve the PI balance, but it distorts at the volume of 10 and higher. If the voltage swing of the PI us too low, I would like to know what a 'normal' voltage swing is for 5881. The grid voltage at the 5881's is -29 volts. The max. voltage swing of the PI is 35 volts. Shouldn't that be enough? I could increase the 56K plate and cathode resistor of the PI to 68K.
2,5K-0-2,5K is what is written on an old EI OT I have. This is a test transformer with various impedances on the primary. I use it for test purposes. '0' is the center tap.
In a 5K load, 360 volt plate voltage, 345 screen voltage and a 330 ohm cathode resistor, what would be 'safe' anode current?

[TATS]

That is correct. But I don't hear a difference with or without the bypass cap, so I will remove it. It was based on a 6g2 schematic.

[Chuck H]

I wouldn't know at the moment how to improve the PI balance, but it distorts at the volume of 10 and higher. If the voltage swing of the PI us too low, I would like to know what a 'normal' voltage swing is for 5881. The grid voltage at the 5881's is -29 volts. The max. voltage swing of the PI is 35 volts. Shouldn't that be enough? I could increase the 56K plate and cathode resistor of the PI to 68K.

The imbalance at the PI could be a resistor value or circuit error. With a bias of -29V you need the PI to "swing" about 60V just for max clean (corrections accepted on this). Increasing the voltage at the power supply node feeding the PI will help. As will changing to 100k plate and cathode resistors. You can then adjust the PI bias for best performance.

A four ohm load at 9.4 volts is just over twenty watts. So your getting closer for sure.

You mention that at a certain setting the amp is clipping. But it's only clipping one side. How much power ISN'T the other side making?

[Bruce / Mission Amps]

The heck with more of the art work and scope stuff ... how about also showing us your layout from the board to the sockets and pots..?
You could have something wired wrong.... as simple as the cathodyne driver's cathode and plate reversed or the triode's grid is connect where the cathode is supposed to be..... etc etc.
However, with 360vdc on the power tube plates and a cathode biased pair of 5881s and 5K output tranny.... you really are not going to get much more then 20-24 watts out of this amp!

[TATS]

The heck with more of the art work and scope stuff ... how about also showing us your layout from the board to the sockets and pots..?
You could have something wired wrong.... as simple as the cathodyne driver's cathode and plate reversed or the triode's grid is connect where the cathode is supposed to be..... etc etc.
However, with 360vdc on the power tube plates and a cathode biased pair of 5881s and 5K output tranny.... you really are not going to get much more then 20-24 watts out of this amp!

OK, I understand the message. I overdid it.
Here is the layout of the amp as it is now, without the transformer and rectifier wiring. I had a switch inside so I would have the possibility to switch from fixed to cathode bias, but I don't use it and disconnected the wires.
20-24 watts would be above my expectations. It would be more than enough.
Thanks for your help, your information is very useful. I won't bother you again.
Chris.