KT 88 project request

[KevinOConnor]

Hi Guys

The 100k in series with the zeners is not required and does not contribute to protection. It's good to do this in signal clamping circuits to give compliance and a rounded clamp action rather than hard clip, as TUT shows in the Preamp Modifications chapter.

Most high-voltage zeners tend to clamp at their rated voltage PLUS the tolerance, so 110V rated plus 5% or 10% depending on the tolerance, 115v5 to 121v.

As Enzo stated, and I've stated a few times on other threads here, the cap voltage is working voltage and there is no need to derate the cap. A slight surge above this at turn-on is also fine. When the circuit is fully powered, the 500V cap node is likely loaded to below 500V, so having 500V worth of zeners (including tolerance) is fine to protect them, even if there is a preceding node of stacked caps. The intervening dropping resistor will limit current. The zener string should be rated to conduct the full load current of the lower-voltage node. The diodes will turn off once the tubes warm up.

Have fun

[catalin gramada]

Hello. Finally tone stack was moved between second voltage amplifier and cathodyne inverter.It work much better than before.
Found with treble pot near maxed open (ca.3 ohm series) some instability into inverter grid. I isolate the capacitance with 100 ohm resistor and problems gone. Although I tested this evening a 12ax7 in first stage. Using a voltage divider into the plate the volume control is pretty consistent.(or consistent enough to not ask for a cap compensation).
I think it.s done. The amp have 100mV input sensitivity for 24.5V rms into 6.2ohm load before clipping.means 97w crystal clean output. 400hz test. And it is damn quiet.cannot realize if is open or not at max.into 98db.speakers.
I did a sketch for 12ax7 first stage version. Now thinking between 12at7 and 12ax7 both very nice usable tone but different voicing. 12au7 was rejected as option.

Cheers. Catalin

[catalin gramada]

Hello.
Have a blip issue like in this old thread http://music-electronics-forum.com/t41214/
Finally it was identified. It is a tone stack issue.
The circuits are different but manifestation forms are identical. The circuit used here is a James tone stack. (in the old thread was a marshall tone stack.)

Here was simple to identify the source of problem splitting the bass and treble sections.
Each individual sections works well without problems but when are linked together with 120k resistor the output generate those 'blip'
Treble.

and bass

The amplification factor was scaled on the scope on each image to put better in evidence but is pretty clear this issue comes when both section are tied together.
The mistery was solved Now my question is how can I cure this problem?
It is a resonance problem? A phase problem? I have no idea. Can you help me please? Thank you. Catalin

have to mention the test was done at 385 hz at TS center frequency with bass and treble at max. this blip slide on sine and disapear when begin to turn the treble pot down

[nickb]

The tone stack is a linear network (see *below) and therefore cannot of itself cause non-linear issues. The distortion comes from elsewhere and the tone stack is simply modifying the harmonic content and therefore hiding the problem. You can put the most horribly distorted waveform through a sharp enough low pass filter and get a perfect sine wave out. What is the waveform going into the tone stack look like? Have you tried a different signal generator? Don't forgot to list voltage and timebase scales when posting scope shots and exactly which point(s) you are scoping.

*Real components are not perfectly linear. Normally the effect is small especially if you are using good quality film caps.

[catalin gramada]

Hello nickb.
Thanks for answer. I checked and the input is absolutely clear.
The blip is present just when bass and treble section are tied together and no trace of issue in individual sections.Did a sketch how I isolate the circuits for those three measurements.

The blip is present just in first situation

[nickb]

I ask again, where exactly are you were making the measurements and what are the voltage and time scales? As it is symmetrical, I suspect your signal generator.

Have you tried a different generator yet? Try feeding you signal generator directly into the tone stack as see what you get.

The tone stack can simply make a problem that exists on the input more obvious as it can attenuate the fundamental relative to the harmonics. It's very hard to see distortion on a waveform, you need to look at the frequency spectrum if you can.

[catalin gramada]

Hi. I disconnected the tone stack from the amp and connected directly from my generator. I have the same issue. But seems You have probably right. I scooped the output of my generator and the pick of the sine is not quite round but a little bit ogival shape like it have an angle in flexion point. It is very.very hard to observe on my scope but seems the pick of sine is not continuous round shape but angled into inflexion point. I will check with another generator and let you know the result. Thanks.