So, how does it sound?
A lo-cost HT-supply true transformer-coupled tube headphone amplifier - Page 4
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- 842 Posts. Joined 5/2011
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Sorry I had to take a break from working on this, I had some problems with the LT supply. There have been some other issues with access to tools and things 'cause of other work going on, I just stopped enjoying what I was doing since I felt it was kind of like work, but I wasn't getting paid for it. This is supposed to be recreational for me.
I'm taking a few days to update my website, which is another of my creative outlets, and which I have neglected of late.
I thought you guys were supposed to be looking over my shoulder.
In order to get ~8.3Vpk from a 6.3V winding, that's 2 Shottky drops taken into consideration, cause the peak would otherwise be ~8.9V, you need to think about the ripple. In order to get the ripple down under a volt you need a 22,000uF cap, not a 2,200uF. So I had it assembled with a 2200 cap embedded in some glue and in my impatience to get it out, I damaged one of the other caps, and you know how it goes from there, you can just get caught up in a cascade of increasing problems and damaged insulation. Stop.
Then I decided to build a low-dropout regulator to pass 1.5A out of components I had in hand, wait for the 2200uF cap to arrive and try and do something else constructive.
This seemed to simulate all right:-
The cap arrived, so I made a PCB, but there was a problem with it and I got bogged down when it wouldn't regulate off the LAB supply...
So a little vacation from this one is required, and I'll get back to you when I've got myself back into the right head-space.
In my experience with regulators for high current draw such as tube filaments, you need more than a 2V difference between V-in and V-out. The current draw will drop a few volts through the filter and secondary resistance even without the regulator. It sounds like your running out of available voltage and your regulators are dropping out. Been there, done that!
If you have another 6.3V winding you could series them for an unloaded voltage of around 15V and do a 12V filament circuit (series the two filaments). Half the current draw, less voltage drop, less ripple and less heat dissipated in your regulator circuit.
OR just hook the filaments up AC. Chances are your gain structure is such that you won't notice any hum if you run the (tightly twisted) wires carefully, and either a center tap on the 6.3V winding to ground or a virtual ground node via a resistor divider across the winding. I built a mic preamp with AC heaters, a high-gain circuit to be sure, and didn't have problems with hum. Microphonic 6922's yes, but no hum.
This was my fear with AC heaters - I have also had AC heaters run dead quiet with good tubes but "set off" microphonic tubes. I would personally say its a good reason to put the tube in a coffee can and crush it up with a brick, but in a commercial product the customer is always right and you built something that hums with the best tube (lolerskates) he owns...
Since this amp uses Russian tubes which I have found to be a bit more microphonic than their American/European equivalents I would be careful here.
As a different thought - would running the heater regulators in CCS mode instead of V-reg mode get them to light consistently? I still think this is crazy overkill considering to the lack of B+ regulation, but it may be worth a shot.
I would think that using a current regulator instead of a voltage regulator would work, I've seen a few articles on this before somewhere but never tried it. It might need more initial voltage than 8 point whatever volts to get it to work.
When I was first experimenting with this stuff years ago I had quite a frustrating time trying to design a DC filament supply that was simple, low ripple and at the correct voltage. It's not easy given the current draw, so I understand the use of a regulator for filaments. But a 6.3VAC winding won't get you 6.3VDC at the currents were discussing.
Waki, what are your plans now?
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