So a friend gave me this cigar box. I had to do something with it...
It is a line stage using ECC99 dual triodes. It runs from a 12V external power supply.
I designed a non-isolated flyback converter (switching power supply) to generate a 200V B+ supply for the circuit. There's been some discussion about such a thing here lately, so here's a working example (albeit low power).
Also, I built a muting relay circuit to keep the outputs off until the tubes warmed up, to eliminate the big "thump" you can otherwise get when the output caps charge up as the tubes heat.
All the details are on my website Pete Millett's DIY Audio pages.
Pete
"Also, I built a muting relay circuit to keep the outputs off until the tubes warmed up, to eliminate the big "thump" you can otherwise get when the output caps charge up as the tubes heat."
What thump? The valve warm up takes up to 15 Seconds. No need.
What thump? The valve warm up takes up to 15 Seconds. No need.
In a perfect world, perhaps.
Try connecting a preamp like this to a solid state subwoofer amp. There is a thump... Though it is worse with a resistive plate load or dissimilar triodes.
It's particularly nasty if you power off then back on while the tubes are still warm. Or worse yet if the mains flickers off and on.
Thump
The first scope shot shows the line stage output and B+ at startup with cold tubes. You can see a ~1V output thump about 10 seconds after power on. It is low frequency, so it may be acceptable if your power amp has limited LF response. But it makes a subwoofer go nuts.
The second plot shows what happens if you power on with warm tubes (turned off for 10 seconds, then back on). A 20V pulse. That will pop your woofer...
That is why a muting delay circuit is a good idea.
Pete
The first scope shot shows the line stage output and B+ at startup with cold tubes. You can see a ~1V output thump about 10 seconds after power on. It is low frequency, so it may be acceptable if your power amp has limited LF response. But it makes a subwoofer go nuts.
The second plot shows what happens if you power on with warm tubes (turned off for 10 seconds, then back on). A 20V pulse. That will pop your woofer...
That is why a muting delay circuit is a good idea.
Pete
Attachments
Yep, Klipschorn's woofer went off axis, needed replacement. My tube preamp also killed a set of mini monitors via a chip amp. Not bad for a small preamp - killing big boys much taller than him.
You really need a mute circuit or, if you're lazy, a mute switch.
BTW so little comments....sexy name, sexy preamp, me not understand. Oh my, Trump is gonna win. Hehe.
I also have a relay mute,
On my Aikido line stage because its fed from an inverter running app 300V via Maida reg fed from a laptop PSU.
And the thump is very real. I have mute on power up and power down.
Power down is because the inverter winds down after power off so I don't get any strange noises etc.
Nice to see an inverter for tubes..🙂
I had to put a filter on the B+ to remove the switching ripple.
I did power down mute by just using a normally closed relay grounding the outputs and powered the relay from a double pole switch one pole fed the PSU the other fed the relay so the PSU didn't hold the relay up after power off.
The use of an "off the shelf transformer" is a big plus its one of the problems with building a SMPS for tubes. They can be noisy when you wind them and even potting has little effect to keep them quiet.
Regards
M. Gregg
On my Aikido line stage because its fed from an inverter running app 300V via Maida reg fed from a laptop PSU.
And the thump is very real. I have mute on power up and power down.
Power down is because the inverter winds down after power off so I don't get any strange noises etc.
Nice to see an inverter for tubes..🙂
I had to put a filter on the B+ to remove the switching ripple.
I did power down mute by just using a normally closed relay grounding the outputs and powered the relay from a double pole switch one pole fed the PSU the other fed the relay so the PSU didn't hold the relay up after power off.
The use of an "off the shelf transformer" is a big plus its one of the problems with building a SMPS for tubes. They can be noisy when you wind them and even potting has little effect to keep them quiet.
Regards
M. Gregg
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Thanks very much for the design. Had this on the back burner for a few years d/t health issues. I used a tube rectified power supply and c core transformer. Running 6n6p and or 6h30 tubes. Have adapter for 7119 / 5687 tubes. Should be fun to compare tubes. Sounds really nice with first watt m2 and very low noise as well. The bass is noticeably better than the grounded grid pre, I had been using?
I am needing to increase gain from my phono stage.
What is the best way to increase gain to 20 or so without increasing output impedance significantly? Using various ampifiers w 10k to 100k input impedance. Any ideas would be appreciated.
What is the best way to increase gain to 20 or so without increasing output impedance significantly? Using various ampifiers w 10k to 100k input impedance. Any ideas would be appreciated.
Thanks for the response. Unfortunately, the reason I built this project was to use tubes on hand and I would prefer to not buy more tubes. I was thinking of reducing the feedback resistor as an option.
You said 5687, which along with the Russian 6H30Π (6n30p), do exhibit lower amplification factors than the ECC99.
Perhaps you are tackling this problem in the wrong "place". Does your phono preamp employ passive RIAA EQ? If it does, switching from resistive loading of the 2nd gain block to constant current source (CCS) loading can increase net phono preamp gain.
Perhaps you are tackling this problem in the wrong "place". Does your phono preamp employ passive RIAA EQ? If it does, switching from resistive loading of the 2nd gain block to constant current source (CCS) loading can increase net phono preamp gain.