-10dBm to +4dBu w/ 6111 tube my first tube design...

[Blue Jinn]

Well, this was inspired by another thread I started, and this is my first attempt at actually designing something from scratch. Used the calculator found here:

http://amps.zugster.net/tools/triode-calculator

and the duncan amps power supply calculator. PSU is inspired by the PRR Varimu psu, which uses the second primary of a dual primary transformer for the B+ supply. Being totally a newbie at this, I based the heater around the constant current model (.3A for the heater) and used what I thought was the total resistance of the tube load being the plate resistor and the follow on stage in parallel (?) for the B+ supply. I assumed the load to be 160k, based on driving a 10k impedance balanced input, (10k reflected back though a 1:4 transformer) which is where I got the 62k load for the B+ psu model. (100k || 160k)

I eliminated the output coupling cap as this is transformer coupled. Note that everything I have is pin 3 hot, so I re-inverted the phase by swapping the terminals on the transformer output (which I thought I read about somewhere else)

The gain is supposed to be right around 16 per the calculator listed above, and so the 4:1 transformer knocks that down to 4x or about 12dB, and should lower the impedance to around 250 ohms or so, again based on the output impedance from the triode calculator of about 4k. The zugster.net calculator differs somewhat from the datasheet for teh tube itself. B+ was chosen as a swag based on the 120v transformer. The plate resistor and cathode resistor were more or less from the datasheet and teh calculator kinda a compromise between teh two. I have yet to actually learn how to read a plate curve so I'm basing all this on the what I've gleaned from various websites.

One thing, not sure how necessary it is to have the input coupling cap, I've seen it omitted in some circuits, and not sure why it would be needed except as a precaution. If so seems .1uF is a pretty standard value.

Also, I had thought also of using this as part of the PRR, which has a regulated B+ of 100v. So B+ here is 100-150v.

I picked the 6111 because I thought a submini tube would require slightly less parts, (tube socket) the 6111 is pretty cheap, and the whole thing less transformers could be built on one or two of those radio shack terminal/perf boards. Sonically, I'm not sure how well the 6111 does, I've seen it used in a lot of guitar circuits which weren't intended to sound clean.

Here it is:

 

[Blue Jinn]

Here are the schematics inline rather than pdf:

 

[mshilarious]

Seems interesting. A few comments:

Dropping 6V across the 20Ω resistor works but won't win any style points. Note the power dissipation there: 1.8W. A better approach is a transformer with the proper secondary voltage and a voltage regulator. Although Vregs aren't terribly important on heaters, especially on a line-level circuit, but I might think about a Vreg on the B+ supply.

Speaking of which, I'd be tempted to go with a smaller R and a larger C on the B+ supply. Lowering supply resistance is generally good. You can do a simple Vreg with a high-voltage power MOSFET, a zener stack fed by a 1M resistor, then another 1M to a 1uF cap on the MOSFET gate. Set that at 120V and you won't have to worry about voltage fluctuations, and the following RC circuit will have a lot less ripple to deal with, so you can use a much smaller R.

Other component values: the input cap should probably be 1uF if you want 100K input impedance.

Be a bit careful about transformer selection. The PRR totally punted on that (as a design goal: no fancy transformers!) by using an opamp output. The danger of tossing any ol' 10K:600 on the output is that the primary inductance may be too low for your intended 4K output, and you'll lose low-end response. You may have been looking at something like the Jensen JT-10K61-1M, but they say 1K source. You might shoot them or whomever you were looking at an email to see if it will work.

It would be groovy if somebody made a line output transformer that was intended as a single-ended plate load, but I don't know if that's a common part. Probably not these days . . .

As for polarity, hook it up, if it reverses polarity, drink a beer, swap the secondary leads, drink another beer, problem solved

 

[Blue Jinn]

Thanks for the critique. The PSU's were just a swag, the duncan tool only allows rc lc an c filters.

I wanted to use the similar prr but with 6.3 rather than 12.6 secondaries and use them in series for the 12.6 output, I can prob just use a 317 regulator using the 12.6 volt winding and just get a decent heat sink for it. That's assuming I even need DC on the heater, might try both and see what happens. For the b+ I'm assuming you're talking something like the psu below, I wanted to keep the PSU simple, but that doesn't add but a couple of parts.


For the transformer, I'd thought of using the Edcor 2.5 watt (XSM serires) transformer (price is right) and got that idea from looking at the NYD 1 bottle, he has a decent freq response per the spec sheet, (doesn't qualify that spec with output trafo just lower gain, and the help thread seems to approve of sound of the edcor apparenlty as a mic/inst pre) The 12AV7 impedance in the one bottle is about 5K, only a little higher. He also has a 6SN7 design with about a 4k impedance and a 10k:600 transformer (don't know the specs on that design or on the trafo.) BTW figure I'll try the 6418 as a mic/inst pre only, rather than balanced amp.

 

[mshilarious]

Go larger on the MOSFET resistors, like two 68Ks. What you have now as 475R should go between the zener and 0.1uF (I would go larger, like 10uF at 68K, 1uF at 680K, etc.) You don't have to worry about MOSFETs as you would with BJTs and base current, so that part of the circuit can be nice and high impedance. The ratings are a bit tight on 2SK135 for my tastes; you do have capacitors in front of it but it could still see surge voltage of 168V or higher, depending on what the power company decides to send you . . . I'd look for a 250V rating. Also, you want enough series resistance to prevent inrush current into the filter caps from toasting the MOSFET.

Very good on the output transformer, give it a shot.