New SE With Triode Mode KT120/150

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Cressy Snr
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Re: New SE With Triode Mode KT120/150

Post by Cressy Snr »

Nick wrote: Sun Feb 21, 2021 7:53 pm Remember that G2 looks like more like an anode than a grid and it's driven positive. If it's not driven by a low impedance distortion will be increased.

Also in terms of how easy G1 is to drive. It's not running as a pentode so there will be some miller between G1 and G2.
Thanks Nick. I’m aware that Miller on Ultralinear outputs lies somewhere between the triode and pentode values.
Wasn’t aware of the possibility of higher distortion caused by g2 not being driven from a low impedance though.
There seem to be lots of ways that one can inadvertently introduce distortion into one’s design.
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Re: New SE With Triode Mode KT120/150

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Nick wrote: Sun Feb 21, 2021 7:54 pm If you want to drop the G2 voltage maybe a zener would be lower impedance.
I have plenty of 5V zeners. They’d probably work quite well.
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Re: New SE With Triode Mode KT120/150

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Put a 1n4007 (pr similar) in series to prevent current if the zener(s) become forward biased.
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Re: New SE With Triode Mode KT120/150

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Job done.
I did the resistor move and value change as Paul suggested, and the zener, plus anti reverse current diode mod to the screen as suggested by Nick.
Screenshot 2021-02-22 at 17.56.03.png
I ended up with a 47V zener, which meant that I could use a GZ34 in the power stage supply. This bumps the anode up to 349V, whilst leaving the screen at 302V, so putting a little bit of distance between it and the anode. I'm not about to quibble with the static screen voltage being 2V over the maximum.

It sounds cleaner overall, with a bit more insight into the background details; probably due to a drop in distortion. Quite a useful little set of mods then. Thanks extended to Paul and Nick for the tips. :)
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Re: New SE With Triode Mode KT120/150

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Cressy Snr wrote: Mon Feb 22, 2021 6:04 pm It sounds cleaner overall, with a bit more insight into the background details; probably due to a drop in distortion. Quite a useful little set of mods then...
Hmmm...So....Right then.....

Cleaner indeed it is but at switch-on and switch-off there was a hiss occurring through the speakers, that sounded like air being let out of a tyre. This faded away at switch on as the 807s came up to voltage, and when the amp was switched off, the hiss appeared as soon as the power was taken off and faded out slowly until everything had ceased amplifying.

I suspect it was the fact that zeners need a minimum amount of standing current through them before their noise will fall to the correct level, and the higher the voltage/wattage spec, the noisier they are, and the higher is the minimum current level requirement, before they will shut up.

Anyway, to cut along story short, for each 807 screen grid, I made up a composite zener out of some 5.6V, 500mW jobbies I had left over from building a couple of MJ statistical regulators. Using these, silenced the start-up and power down hiss-problem pretty handily. I was also able to adjust the screen voltages in 5.6V increments by strapping out zeners until I got what I wanted, ie the ability to set the screen/anode relationship very easily.

I wanted to use expensive rectifiers such as my GEC U52 or Mullard fat bottle CV378 (GZ37) without reducing the standing current in comparison to a GZ34. This worked great. Below is a composite zener, with a couple strapped out to give 290V screens with 317V anode:
compositezener.jpeg
compositezener.jpeg (77.89 KiB) Viewed 244 times
Fat bottle GZ37 in use:
latestrocky.jpeg
latestrocky.jpeg (90.67 KiB) Viewed 244 times
I also redid the input stage power supply, rectification: taking out the ridiculously over-specced GZ34 and replacing it with a far more appropriate for the job EZ80. 600mA heater vs 2A for a GZ34 will save a bit little of juice too.
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Re: New SE With Triode Mode KT120/150

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It would have been interesting to look on a scope, the hiss could have been a side effect of some oscillation as the voltages wandered about. Anyway, sounds like its sorted.
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Re: New SE With Triode Mode KT120/150

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Nick wrote: Thu Feb 25, 2021 8:53 pm It would have been interesting to look on a scope, the hiss could have been a side effect of some oscillation as the voltages wandered about. Anyway, sounds like its sorted.
Yes it was a weird one. I’ve tried to find some info about oscillating zeners. There isn’t much about and no one seems to really know why it sometimes happens apart from poor performance below 1mA and some speculation about quantum tunnelling effects.
https://forum.sparkfun.com/viewtopic.php?t=12830
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Re: New SE With Triode Mode KT120/150

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Yep, maybe a cap across them would make it better or worst. But again, 5.6v zeners are very different from 47v ones.
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Re: New SE With Triode Mode KT120/150

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Nick wrote: Thu Feb 25, 2021 11:30 pm ....But again, 5.6v zeners are very different from 47v ones.
Aye, did a bit of reading of the venerable MJ yesterday and learned that pure zener action only occurs up to 6V. After that the avalanche effect becomes more dominant and the noise levels from the devices increase. I should probably have made up composite zeners in the first place.

But I’ve used them as voltage references with cap bypasses with no issues. Must be something to do with the application in the screen feed for ultralinear, which let’s face it, is not that common a thing to do.
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Re: New SE With Triode Mode KT120/150

Post by Mike H »

I've had exactly this happen. Years ago built an amp - probably cobbled together from various magazine articles, as I was wont to do - all hunky dory but always had a persistant annoying hiss. Finally tracked it down to supply smoothing Zeners, they were right on the edge of their avalanche Voltage. Can't remember what I did but I did something that changed the input Voltage (increased it) an it all magically went quiet. Later still, I learned about making white noise from a Zener. :D

There was one you could buy via the magazines, expressly for the job.
 
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Re: New SE With Triode Mode KT120/150

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Mike H wrote: Mon Mar 01, 2021 6:59 pm I've had exactly this happen. Years ago built an amp - probably cobbled together from various magazine articles, as I was wont to do - all hunky dory but always had a persistant annoying hiss. Finally tracked it down to supply smoothing Zeners, they were right on the edge of their avalanche Voltage. Can't remember what I did but I did something that changed the input Voltage (increased it) an it all magically went quiet. Later still, I learned about making white noise from a Zener. :D

There was one you could buy via the magazines, expressly for the job.
Glad somebody else has had the same thing happen. So I inadvertently created a white noise generator for a few seconds every time I switched the amp on and off. :)
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Re: New SE With Triode Mode KT120/150

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Well it's revision 5 the amp is on now.
Some may think it is a bit of a backward step, but it is not: far from it. :)

So here is the circuit revision 5:
DCCECC83807SEUL.jpg
DCCECC83807SEUL.jpg (108.81 KiB) Viewed 75 times
The input and driver stage is now an ECC82.

Glow shot below:
IMG_0589.jpeg
IMG_0589.jpeg (85.74 KiB) Viewed 75 times
I made a couple of adapters up, so that the top plate wouldn't be subject to any hacking about. The adaptor was fixed from below, and uses the bolts that held the octal socket and before that, the B5.
Also, the coupling caps have been changed from the Jupiter Astrons to Russian green PIO caps. The mods were done in stages: first the caps, then the front end.
Both changes have added up to make a great sounding setup, when combined with the single driver speakers, which themselves have been changed from sealed to aperiodic (leaky box)

All in all, a nice fat, saturated, tonally colourful, listen. If you like that sort of thing it's lovely. I like it.
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Re: New SE With Triode Mode KT120/150

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I think with the ECC82, I’ve been guilty of dismissing something without ever having heard an amp that uses them. It started right at the beginning, with MJ in ‘Valve Amplifiers’ being pretty snotty about them, then developed from there, so that by the time I’d read all the dissing in DIYAudio and the Asylum around 2007-8, I’d written the little blighters off as a complete POS, only fit for radiograms. Except that with ECC82s in the system, it does voices with my Fanes, like nothing I’ve heard: beautiful. Treble is refined, and liquid, the mids are detailed without hardness or glare and the bass is both warm and fast at the same time, which is something I am liking a lot.

It’s ironic to me that such a good sound is coming from an amp, made up of a cheap transmitter valve, dismissed, for producing nasty high order distortion products, an input and driver valve regarded as a poor, non-linear relation to the 6SN7, and with coupling caps in it that 10 years ago people couldn’t give away. Either I’m easily pleased, or 95% of what’s discussed about the ECC82 on the web is a complete load of bollocks.
Last edited by Cressy Snr on Thu Mar 04, 2021 5:20 pm, edited 1 time in total.
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Re: New SE With Triode Mode KT120/150

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Mind you I can’t be that easily pleased, given the amount of scrap metal and electronic audio waste I’ve generated over the past 16 years. Sheesh!
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