Selecting an OT for a 1 watt design.

[tubeamptech]

I'm going to build a 1 watt amp, which is going to use either a single, or pair of 12AX7, or 12AT7. I'm thinking of using the Hammond 125 push-pull series. The question is, how do you calculate what the primary impedance should be, first for a single tube, and then for a pair?

[Tooboob]

You can head here for starters:

http://music-electronics-forum.com/t13043/

[Alan0354]

I like to hear the result using the 12AT7. I read a lot about triode mode does not sound good.

[tubeamptech]

Tooboob, I went to the Valve Wizard link, but it's gone. I've read that I need to do the load line for the tube in question, therefore I need to know what my B+ voltage at the plate will be, right? I haven't selected the power transformer yet, but the fact I'll be using a 12 AX_ tube, B+ shouldn't be over 300V. Problem is, I've never calculated using load lines. I have a vague understanding. I usually tweak by ear. But this is a different animal.

[eschertron]

I built a small (call it one Watt) amp using a 12AT7 with both halves in parallel for a SE design. It was enough fun that I'd like to try a PP amp with the 12AT7. I went here Interactive Valve Data Sheets to get the idea for OT impedance. I've decided on 32k ohms plate-plate (without first checking to see if such an animal actually exists!) with a B+ of about 260vdc. Other configurations are possible, and the interactive chart is somewhat addictive as well.

[Alan0354]

Can you find an OT with 32K primary?

[tubeamptech]

I'm planning on an 8 ohm secondary impedance. According to the Hammond data sheet for the 125A-E series for 8 ohms out, the most I can wire up the primaries are for 22.5K. I can hook up the primaries for 27K, but I don't have any 4 ohm cabs. Without having done the calculations, I don't know if 22.5K will cut it. I'll check out the Interactive data sheets. Other than the Hammond, I don't know what else is out there.

5c0054.pdf

[NateS]

I've heard some good results from folks using the 6AK6 for a couple of watts - from what I've seen so far that's about the lowest power push-pull pentode that's commonly available, but you'll probably be making some compromises on the OT because the ratios are so high.

However... (sometimes I wish I could shut the wheels in my head off when I don't have time to explorer where they go):

It seems like the power is low enough you might could buffer your output tubes and broaden your OT selection quite a bit.

[tubeamptech]

Yes.. I still have to determine what the primary impedances should be for a 12A_7 family of tubes in a push pull config. Using one tube(using both triodes for PP) should yield a base figure for primary impedance. If the Hammond 125 can't be wired high enough for the correct impedance, then 2 tubes should effectively cut the impedance requirement in half, right? If so, then I should be able to wire up the available taps for the correct impedance. My question still stands on how to use load lines to determine what the primary impedance should be.

[Chuck H]

OR... You could use a 12au7 for the power tube. Possibly more wattage than you wanted but you could always reduce Vp. The au7 operates well with less than half the load typically used for an at7 and maybe a third of what you'd use for an ax7. Sorry I can't help with the load lines. I think I looked into it one and found it frustrating because the specs for those tubes focus on voltage amplification and not current.

[jazbo8]

Tooboob, I went to the Valve Wizard link, but it's gone. I've read that I need to do the load line for the tube in question, therefore I need to know what my B+ voltage at the plate will be, right? I haven't selected the power transformer yet, but the fact I'll be using a 12 AX_ tube, B+ shouldn't be over 300V. Problem is, I've never calculated using load lines. I have a vague understanding. I usually tweak by ear. But this is a different animal.

Load Line Plotter is still available at Valve Wizard. But the Hammond 125 series should work fine.

[J M Fahey]

In this (power) use, you need the opposite of what you'd need in a preamp, meaning here not so much gain but lots of current.
As power tubes: 12AU7 better than 12AT7 better than 12AX7

[nashvillebill]

This site Interactive Valve Data Sheets lists the 12AU7, 12AT7 as well as the 6SN7 in their interactive graphs, including PP output.

Now I'm not really good at interpreting the results, however.

[Tooboob]

Lookie here: http://www.drtube.com/datasheets/12ax7-rca1962.pdf

I like this datasheet because it has a nice plot of how mu and plate resistance change over the operating range. Anyway, go to the first plot. It shows a load line somebody sketched, and you can barely read "56K". They didn't need to write this.

At 0 mA, where the load line intersect at 225V, there's no currentin the load, and you see the full B+ voltage on the anode. It's 225V. Change the B+, and you move this end of the load line. At the other end of the load line, there's 0V on the anode (you can't really get here, but the load line is for the resistor), and the whole 225V B+ is on the load. The current is 4ma. R=V/I=225V/4mA= about 56K Ohms. If you need to draw your own load line, I = B+/R where the load line meets the 0V axis.

Once you get the load line in the graph, for a given grid voltage, you can follow the appropriate tube curve line, and where it intersect the load line, you'll find the anode voltage and current. It still doesn't tell you the impedance you should have, which determines the slope of the load line. That's controlled by the tube spec limits. You see that you can have 330V max anode voltage. In a push-pull class AB arrangement, you'll find that anode voltage goes well above B+ when the tube is off, and you should leave some headroom for this even though the tube doesn't seem to mind much. 250V should be very nice. The next spec to worry about is anode power dissipation which is 1.2 W max for each triode. At 0 current or 0 voltage, the anode power is 0 W, and the max is in the middle. On our load line, this occurs at 112.5 V and 2mA, or 0.225W, so the load line is conservative, even for DC drive near 0V bias. For AB push pull, you could aim for 60% of rated power at the bias voltage.

What's the cathode bias resistor? We can't get much past the 0V grid curve without grid current, so we want to pick a bias point on the load line that's half-way from the 0V grid curve to where the grid curves start bunching up for class A push-pull, or closer to the 0V grid curve for class A with more overdrive distortion, or farther from the 0V grid curve for class AB. For the curve shown, for single-ended class A, you'd want about 1V. Note this bias current will change if you substitute a different 12A_7 family member into the same circuit.

Now we know the bias voltage. If it's a Volt, on the drawing, we can read about 1.25 ma where the 1V grid curve hits the load line. 1V/1.25 mA = 800 Ohms.

It's just that simple, at least until you consider all the signal and tube-dependant nuances trying to maximize output power or bias current, and I don't think that's your goal.

Did you see that Musician's Friend had the Blackstar HT-1R on sale for $199 yesterday? You could have gotten a PT, OT, enclosure, tolex, handle, chassis, power entry module, grill cloth, caps, tubes, speaker, reverb module, etc. real cheap.

[tubeamptech]

Hey, thanks for that. I've been hitting the books on load lines, and starting to get it. The other concern now, is finding a PT that'll give me about 250 to 300VDC. I've got some small Hammonds kicking around that may do the trick.

I have a Hammond 125c, and 125E on the way. 8W and 15W respectively. They have multiple primary taps available. I still have to figure out which tap to use. I know it's based on the load resistance of the tube. For instance, the 6L6GC data sheet gives you a load resistance in a push pull AB config averaging 5K, depending on the B+voltage. That lines up fairly well with a typical Fender style OT.

The problem is, the 12A_7 family of tubes don't give you that value. They only give you the plate resistance spec. A 12AX7 is listed as 62.5K. My gut tells me that is going to be too high, even when paired in a PP config. The 12 AU7 is listed as 7.7K. That sounds about the right value, after I do the calculation, which I haven't figured out yet. I know the Marshall 1 watts series are using the 12AU7.

Blackstar, $199? Sounds good. Why bother? Oh I know why, the wife caught me staring at some 1 watters online. "Your NOT buying another amp!". Sound familiar?