KT88 amp

[g1]

It's custom from Hammond, with a separate winding for the screen supply. I couldn't find an off-the-shelf transformer that fit the bill.

But you must have specs for it?

[hylaphone]

But you must have specs for it?

Sure, the secondaries are 720VCT@.3A w/60V bias tap, 250V@.1A, and a few different filament windings

[Helmholtz]

So the measured sag seems to tell us it's not a PT issue.

A PT secondary can be seen as a stiff voltage source in series with some resistance.
This resistance (aka equivalent source resistance, needed for PSUD2) consists of the winding DCR plus the reflected primary DCR.
Sag is the voltage drop across this total source resistance and thus can be calculated from the resistance and the load current.
The core has no influence on sag.

BTW, a transfomer cannot saturate by load current.

[catalin gramada]

I just built a functional paraphase circuit using 12at7 you can easily drive you kt88. 6db global nfb applied. With 2.5 Vpk input sensitivity think is just enough to drive from tone control output. Slight tweaking may be necessary. You may share the bias cathode resistors linking together as in original circuit, it will not make any difference as time are full bypassed Please get inspired if you feel.


Late edit: series nfb resistor 1k was used from 16 ohm tap/ not 470 ohm. Common OT tap should be isolated from chassis.
I fast wired a B15 style preamp with -20db James tone control in between, using 12ax7 in front and definitely get some low end lovers may use.. Not too bad to push 300W rms with just two tubes in preamp...

[hylaphone]

I just built a functional paraphase circuit using 12at7 you can easily drive you kt88. 6db global nfb applied. With 2.5 Vpk input sensitivity think is just enough to drive from tone control output. Slight tweaking may be necessary. You may share the bias cathode resistors linking together as in original circuit, it will not make any difference as time are full bypassed Please get inspired if you feel.


Late edit: series nfb resistor 1k was used from 16 ohm tap/ not 470 ohm. Common OT tap should be isolated from chassis.
I fast wired a B15 style preamp with -20db James tone control in between, using 12ax7 in front and definitely get some low end lovers may use.. Not too bad to push 300W rms with just two tubes in preamp...

Thank you for sharing that -- how far up the audio band do the PI outputs stay symmetrical?

And can you elaborate on that OT common arrangement?

I may try something similar with the 6SN7, 6dB feedback is cutting it close but certainly worth a try.

[catalin gramada]

That simple. In original circuit you have 240 ohm shunt nfb resistor in series with first cathode. Because the cathode is not full bypassed but partial you will loose some of you precious gain rising in same time the internal tube resistance. You need to make the shunt resistor as small as practical, but take a look at circuit: the series nfb resistor is also the bias resistor for those cathode: the closed path from cathode run also from: cathode- series nfb- OT secondary-signal ground which is in parallel with cathode- shunt nfb- bias resistor- signal ground. You don't need to bias the tube through series nfb resistor and OT secondary winding. This is a unwanted consequence of the circuit as time you want to keep common OT tied to the ground. But there is no real reason to do that. (..allright there are some but not subject of our analysis). Close the nfb loop over nfb series/shunt resistor and bias the tube just from bias resistor not through series nfb in parallel with bias resistor-. The common OT wire should not touch the ground, you need isolated speaker jacks.
And btw be careful when you do the measurements with your scope, sometimes may create a loop path through ground probe clip- earthing wire..and depend what you measure you may involuntary run some currents in circuit through this path....that's can happen...sometimes..
The "floating" OT secondary will not harm anything.

[catalin gramada]

Better pic than my broked English instead:
Be sure you use a good quality bypass capacitor. 100u will be better and you may put a film 100nF in parallel over it.

[catalin gramada]

-- how far up the audio band do the PI outputs stay symmetrical?

I didn't found any difference in amplitude of both inverted/non inverted at least 20kcps. Anyhow found a slightly phase delay in between and I wonder myself where this problem come from and how I can align it ?

[Helmholtz]

Anyhow found a slightly phase delay in between and I wonder myself where this problem come from and how I can align it ?

In a paraphase PI the second triode follows the first one and inverts the signal. A slight phase delay is caused by the RC coupling, probably increasing at lower frequencies.

[hylaphone]

I didn't found any difference in amplitude of both inverted/non inverted at least 20kcps. Anyhow found a slightly phase delay in between and I wonder myself where this problem come from and how I can align it ?

I was able to fix the phase shift by bypassing the 470K side of the divider with 150pF. This also made lower frequencies more symmetrical, but nowhere near even to 20kHz. I'll fiddle around with that more this week

[catalin gramada]

I was able to fix the phase shift by bypassing the 470K side of the divider with 150pF. This also made lower frequencies more symmetrical, but nowhere near even to 20kHz. I'll fiddle around with that more this week

Be sure the signal not roll off at the input due to internal capacitance.

[hylaphone]

Be sure the signal not roll off at the input due to internal capacitance.

Looks like 6SL7 and 6SN7 are 3 to 4pf input capacitance... my source Z is 150 ohms, no problem

[Helmholtz]

Looks like 6SL7 and 6SN7 are 3 to 4pf input capacitance... my source Z is 150 ohms, no problem

Even taking into account the Miller effect, which increases input capacitance by the voltage gain, input capacitance won't cause a frequency dependent imbalance.
Reason is that input capacitance affects both triodes likewise.

[catalin gramada]

40 cps / 400 cps / 4 kcps / 40 kcps 1Vpk at input/ no nfb applied. Yellow-inverted, blue -non inverted

[hylaphone]

40 cps / 400 cps / 4 kcps / 40 kcps 1Vpk at input/ no nfb applied. Yellow-inverted, blue -non inverted

Yes that looks great --
In my last round of tests I discovered the reason for my wild PI imbalance - one of my scope probes was busted, the variable cap wasn't doing anything. Second major "senior moment" of this thread, thanks for bearing with me.
Basic 6SN7 version shows near perfect balance like yours, with low enough output Z to use 47K grid resistors for 6550. I look forward to trying out your feedback arrangement.