KT88 amp

[catalin gramada]

Any other wild ideas?

Yes. Forget you 360V supply. Use 12at7, supplied at 400V or more, stuck 47k resistors on the plates and 470 ohm shared cathode (or separate bypassed 1k better) Imagine a 10k or some series/100 ohm shunt nfb network, supply the screen grids at 500V but don't forget some hefty 1k stoppers for each tube, and be happy with that...

[Helmholtz]

For what it's worth, the sound level difference between 70W and 60W is less than 0.7dB.

[hylaphone]

Ok, this makes more sense.
Both PI signals get clipped by grid conduction just below 40V, the value should correspond to the bias voltage at full power.
So that's normal. It seems the PI provides enough signal for the KT88s.
You may still try to lower bias voltage (i.e. making it less negative) and see if this helps.

The different PI output waveshapes (and changed duty cycle) are quite normal for a clipping paraphase, which is far from being a perfect PI.

Still wondering about the rounded positive tops of one of the grid signals - and that PI balance would depend on frequency.
Does increasing the values of the 22µ and 22nF caps in the PI make a difference?

The reason for not getting the theoretical ~ 70W clean doesn't seem to be the PI.
Other possible reasons for lowered output might be:
- OT losses of maybe 5W
- Higher "saturation voltage" of your KT88s compared to original GEC types.

Ok, well that explains why the MOSFET followers didn't really help

I raised the 22nf cap to 220nf, and paralleled the 22uF cathode cap with a 100uF. No change in balance.

I guess I'm running out of options to reach the target power... I'm surprised at such a big discrepancy, as I did do a lot of head scratching prior to building this and all the math all pointed to 70W+.

Any other wild ideas?

[Helmholtz]

Sorry about that, I had re-centered the traces as the waveforms clipped. Here is another picture just as you asked.

Ok, this makes more sense.
Both PI signals get clipped by grid conduction just below 40V, the value should correspond to the bias voltage at full power.
So that's normal. It seems the PI provides enough signal for the KT88s.
You may still try to lower bias voltage (i.e. making it less negative) and see if this helps.

The different PI output waveshapes (and changed duty cycle) are quite normal for a clipping paraphase, which is far from being a perfect PI.

Still wondering about the rounded positive tops of one of the grid signals - and that PI balance would depend on frequency.
Does increasing the values of the 22µ and 22nF caps in the PI make a difference?

The reason for not getting the theoretical ~ 70W clean doesn't seem to be the PI.
Other possible reasons for lowered output might be:
- OT losses of maybe 5W
- Higher "saturation voltage" of your KT88s compared to original GEC types.

[uneumann]

Just tried this with 100K stoppers, stays balanced until roughly 55W output, but as it clips the inverter side can't keep up. Here is a picture.

OK - it does look more balanced than the pic in post 50. If you look at the PI outputs, rather than the kt88 grids, you'll likely see it's more balanced looking too.
The effect of the clipping is reduced as the grid stoppers get bigger. It's a "feature" of the self-balancing paraphase PI. If the PI output gets clipped, it impacts the balance. That's just the way the circuit works. To completely eliminate this problem you need to add a buffer (follower) between the PI and output tubes. The suggestion to use low-z output tubes (12at7) is also a good one for the same reason. It will be less impacted for the same load (grid stopper values).

[Jazz P Bass]

I would ditch the 6SL7 in favor of a 12AT7.

[hylaphone]

The balance issue is likely due to the feedback (through the 0.022u cap) used to stabilize the lower triode gain in the PI. Without the output tubes, that feedback keeps the PI balanced. Once you add the output tubes and they clip, the grid clipping on the lower tube is not isolated from the PI. It feeds back through the 1k grid stopper and the .22u coupling cap to the plate of the PI. Once that plate sees clipping, it impacts the feedback that is trying to stabilize the PI and it leads to asymmetric clipping. You can fix this (temporarily or permanently) by making those 1k grid stoppers bigger. Try 100k. I'm pretty sure the PI will remain more balanced since it will be less impacted by the clipping at the power tube grids.

That may not solve all your problems, but it can't hurt, and it could address the freq dependent behavior as well.

Just tried this with 100K stoppers, stays balanced until roughly 55W output, but as it clips the inverter side can't keep up. Here is a picture.

Also, issue of imbalance at different frequencies is there with the KT88 removed... not sure if I was clear about that

[hylaphone]

One more test, as some of your previous posts were a bit contradictory:

Assuming the signals in the pic were taken directly at the KT88 grids using DC coupling, using the same settings: Are you getting zero lines centered to the screen when you reduce the grid signals to zero?
I need to make sure there is no DC offset involved.

Sorry about that, I had re-centered the traces as the waveforms clipped. Here is another picture just as you asked.

[uneumann]

.... the PI signals were perfectly balanced before installing the KT88s

The balance issue is likely due to the feedback (through the 0.022u cap) used to stabilize the lower triode gain in the PI. Without the output tubes, that feedback keeps the PI balanced. Once you add the output tubes and they clip, the grid clipping on the lower tube is not isolated from the PI. It feeds back through the 1k grid stopper and the .22u coupling cap to the plate of the PI. Once that plate sees clipping, it impacts the feedback that is trying to stabilize the PI and it leads to asymmetric clipping. You can fix this (temporarily or permanently) by making those 1k grid stoppers bigger. Try 100k. I'm pretty sure the PI will remain more balanced since it will be less impacted by the clipping at the power tube grids.

That may not solve all your problems, but it can't hurt, and it could address the freq dependent behavior as well.

[Helmholtz]

Ok here we are, I did double check the PI signals were perfectly balanced before installing the KT88s

One more test, as some of your previous posts were a bit contradictory:

Assuming the signals in the pic were taken directly at the KT88 grids using DC coupling, using the same settings: Are you getting zero lines centered to the screen when you reduce the grid signals to zero?
I need to make sure there is no DC offset involved.

[hylaphone]

Replace 100k grid resistors with 220k, 22nF coupling in pi stage to second pi grid is way to small, make it as large as practical at least 100 nF . Stuck 2x 470k resistors on the PI plates and add another series resistor till perfect balance is done .Hope it help. Cheers

Thank you for the tips. I tried these changes and the imbalance is still there, relative to frequency.

Can you scope the signals over the PI plates without power tubes ? Apply 1Vpk at the PI input and report what you get over the PI plates please. I think you get too much voltage into non inverted side. Make the feedback built resistor as well to reduce the second plate voltage as large as the first stage or less. You can make individual cathodes, it doesn't matter as time it is bypassed. It is a simple gain stage circuit, should work as simple as it is.There are lots of Acrosound and Heath amps who use it.
This is identical circuit as yours but built with a different tube. Is not perfect but good enough for a guitar amp

Probes on the PI plates, correct? Sorry, I don't understand "Make the feedback built resistor as well to reduce the second plate voltage as large as the first stage or less." can you clarify?

[hylaphone]

Wait, this should work better:

Set both scope channels to DC coupling and connect probes to the KT88 grids with no or very low PI signal (same 2V/div setting as before).

Adjust zero lines to center of screen, increase PI signal until clipping starts and post scope pics.

Ok here we are, I did double check the PI signals were perfectly balanced before installing the KT88s

[catalin gramada]

Can you scope the signals over the PI plates without power tubes ? Apply 1Vpk at the PI input and report what you get over the PI plates please. I think you get too much voltage into non inverted side. Make the feedback built resistor as well to reduce the second plate voltage as large as the first stage or less. You can make individual cathodes, it doesn't matter as time it is bypassed. It is a simple gain stage circuit, should work as simple as it is.There are lots of Acrosound and Heath amps who use it.
This is identical circuit as yours but built with a different tube. Is not perfect but good enough for a guitar amp

[catalin gramada]

Late edit: you'll get large voltage output from you 100w amp so can use 16ohm tap for feedback from the output and may reduce the 220 ohm shunt resistor more like 100 ohm. This will help to better balance too.
If you're not happy with kt88 sound, and have doubts you will due to poor driver capabilities just stuck a pair of el34 on it, think you'll get 70w around from a pair in you project.
Better to use a noval 12at7 as inverter if stuck with kt88 in you paraphase project; better drive, good voltage excursion and still 6db of global nfb available in you circuit.
And btw. 220nF coupling caps are huge. You will not have the possibility to apply a lot of feedback to have an effective speakers control in low freq range, so try to roll off as much with an eye on scope to not compromise the amp stability when feedback apply.... 47-100nF in conjunction with 100k grids or 22-47nF/ 220k...or so...if you wanna use it for bass geetar.
I use also a paraphase to drive 4×kt88 at 650v in one of my project, Hefty clean 240w but use a 12bh7 instead. With 6sl7 as driver el34 seems more tailored for....just my thoughts...

[catalin gramada]

Yes you're right, sorry about that. I'll take some new pictures.

The cathode cap is good.

Regarding PI balance - symmetry at 1kHz is far different than at 5kHz, or 400Hz, etc.. Since this is a bass amp, the most critical frequencies are all under 1kHz... so I tried the pot at 400Hz, it balanced perfectly at 430K/470K which is the same ratio as the original 470K/510K! But at 1kHz we're back to the 20-ish% difference in amplitude... should I keep it there and run all further tests at 400Hz?

Replace 100k grid resistors with 220k, 22nF coupling in pi stage to second pi grid is way to small, make it as large as practical at least 100 nF . Stuck 2x 470k resistors on the PI plates and add another series resistor till perfect balance is done .Hope it help. Cheers