KT88 amp

[hylaphone]

First make sure that both scope channel knobs are in arrested CAL positon.

Ok, now adjust the PI input signal to get around 100Vpp at both PI outputs. (Does balance improve?)

Then lift one end of the NFB resistor and reinsert power tubes. What happens to the grid signals?

Yes on the Cal position I've made that mistake before

With 100Vpp on the inverting side, there is 80Vpp on the non inverting.

With NFB disconnected, it's making 55W clean - about 70Vpp inverting side, and 65Vpp non.

[Helmholtz]

With 100Vpp on the inverting side, there is 80Vpp on the non inverting.

Does balance improve if you lower the signal?
If not, the triode sections of the 6SL7 are not balanced. You could try to adjust the 470k/510k ratio for best balance.

With NFB disconnected, it's making 55W clean - about 70Vpp inverting side, and 65Vpp non.

Please post scope pics of grid signals. I'm looking for signs of grid conduction.

[hylaphone]

Does balance improve if you lower the signal?
If not, the triode sections of the 6SL7 are not balanced. You could try to adjust the 470k/510k ratio for best balance.




Please post scope pics of grid signals. I'm looking for signs of grid conduction.

Balance tracks the same, I bumped up the 510K to 560K which helped a little, at least below 1kHz. I'll play around with those values some more.

Here are shots with the KT88s in, NFB still disconnected.

Pic 1 is with 2Vpp into the PI. 75Vpp inverting side, 60Vpp non

Pic 2 is with 3Vpp into the PI. 100Vpp inverting side, 80Vpp non

Pic 3 is with 8Vpp into the PI. 100Vpp inverting side, 125Vpp non

[Helmholtz]

Sorry, this is confusing.
Sure you didn't mix up the order of pic 2 and 3?

Also I can't interpret your pictures without knowing where the zero lines are.


BTW, is the 22µ PI cathode cap good?

I suggest to first balance the PI. You may use a 1M lin pot between the PI plates to find the best ratio.

Also your power output should increase if you reduce the KT88 grid bias (i.e. using less negative bias voltage).

[hylaphone]

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?

[Helmholtz]

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?

If your amp corresponds to the schematic you posted, PI balance can't depend on frequency - down to at least 40Hz.
Check the 22nF and 22µ caps for capacitance.

Things will change when grid conduction starts.
So adjust balance at some medium level.

It may make sense to post a complete schematic including power supply.

[hylaphone]

If your amp corresponds to the schematic you posted, PI balance can't depend on frequency - down to at least 40Hz.
Check the 22nF and 22µ caps for capacitance.

Things will change when grid conduction starts.
So adjust balance at some medium level.

It may make sense to post a complete schematic including power supply.

Wish I could explain why the PI balance changes with frequency, but for now it's balanced at 400Hz and I'll use that frequency for further measurements.
The caps all are correct.

Today I resolved the "compression" of transients - the screen supply was also feeding the PI and preamp, and was sagging as the power tubes began to clip. I moved those nodes to the plate supply and it's much better. It's now up to 60W clean.
Here is a schematic of the PSU as it stands. Please disregard the extra primaries, I'm new to the software.

Can you describe what sort of waveforms would be most useful when looking for grid conduction? Onset of clipping? Full clipping? I don't want to clutter up this thread with more fuzzy and/or impulsive snapshots.

[Helmholtz]

Wish I could explain why the PI balance changes with frequency, but for now it's balanced at 400Hz and I'll use that frequency for further measurements.
The caps all are correct.

Frequency dependent PI (im)balance can ony be caused by (too low value) capacitors. Check at low to medium PI output, say 40Vpp.
In case of doubt, replace the 2 caps.
Of course it could also be a wiring error.

Can you describe what sort of waveforms would be most useful when looking for grid conduction? Onset of clipping? Full clipping?

Scope grid signals with KT88s in, no NFB.
Increase PI signal until the positive grid signal tops start getting flatter and somewhat square (like in post #18, pic 4.)
With scope use AC coupling and center the zero line to the screen.

[uneumann]

Can you describe what sort of waveforms would be most useful when looking for grid conduction? Onset of clipping? Full clipping? I don't want to clutter up this thread with more fuzzy and/or impulsive snapshots.

To better see grid current effects temporarily increase the grid stoppers to something like 30k-100k.
Scope both sides of the stopper and it is easy to see when the grid side flattens since the two waveforms differ at the top crests.
You will lose some peak power, but it will make the effect of grid current very clear while you're trying to sort things out.

[hylaphone]

Frequency dependent PI (im)balance can ony be caused by (too low value) capacitors. Check at low to medium PI output, say 40Vpp.
In case of doubt, replace the 2 caps.
Of course it could also be a wiring error.




Scope grid signals with KT88s in, no NFB.
Increase PI signal until the positive grid signal tops start getting flatter and somewhat square (like in post #18, pic 4.)
With scope use AC coupling and center the zero line to the screen.

PI imbalance tracks the same at lower amplitude. Since the coupling caps aren't in the feedback loop, I chose them on cutoff frequency with the grid resistors. Is that incorrect? I also now wonder about any loading issues with the 470K/510K?

Here are some new pictures. The red dot indicates the inverting side. First is as they start to flatten, second is full clip. Traces were readjusted to stay centered on the Y axis

[Helmholtz]

Looks better but not good.
At least the positive going signals don't clip below the bias voltage.
I don't understand why the grid signals don't clip at same levels.
Did you connect scope probes directly to the grids?

[Helmholtz]

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.

[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

[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]

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