Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: RicharD on October 20, 2011, 10:26:03 pm
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Here's another little bread board experiment. I started it using an old transformer set which originally was a 6V6 SE amp. This OT has a feedback winding that I did not use. I was intending to go further with this project, but the PT is just barely powerful enough to supply what's connected. When I crunched the numbers on the 6V6, I came up with Rk = 400 ohms and a bias of about -12V. In reality I ended up with 800 ohms and -27V. Not sure what I did wrong there. The 2 gain stages are pretty much copied from the RCA resistance-coupled amplifier chart, 300V lowest gain (AV=67). I was surprised that the screen voltage is higher than the plate voltage. The single gain stage (clean) setting is quite wonderful. The OD is just OK. I fooled around a lot with coupling caps and cathode bypass caps. I ended up happiest with Ck of V1 being full range and the coupling cap being just slightly into the lo frequency cut region. V2 has a much smaller Ck and a switch between a lot of lo cut and none on it's coupling cap. All pentodes. Stuck with the tube recto to keep the volts down a bit although I calculated a little more B+ than I'm getting. The PT has just enough umph and not really a mA more. Check it out, slap me around. How did I miss so badly on the 6V6 cathode resistor?
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how does it sound? looks like a cool little ckt.
--DL
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> How did I miss so badly on the 6V6 cathode resistor?
Show your work.
At a glance:
Vp-k Ip Rl and Pdiss are reasonable ratios.
G2 voltage is WAY higher than it needs to be. Therefore Vg1 must be huge to compensate.
> surprised that the screen voltage is higher than the plate
You asked for low gain. Its a pentode. Plate can pull down to around 1/5th of Vg2. There's plenty of swing. If you'd asked for max Gain, Vg2 would be set lower.
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>Show your work.
Max Pd for a 6V6 = 14W
14 * .7 = 9.8W
9.8/400 = 24.5mA
From the cut sheet, Vbias = -13V.
My assumed B+ = 415V
13/.0245 = 530 ohms
B+/Zpri = 400/12000 = 33mA
13/.033 = 393 ohms
>G2 voltage is WAY higher than it needs to be. Therefore Vg1 must be huge to compensate.
This is where I strayed because the cut sheet shows G2 to be much less than B+.
>Its a pentode.
This is the chapter in my edumucation that I'm struggling with. I can take any ol triode and it's curve sheet, calculate a circuit, and my measured results are always right on close. Power pentodes I'm usually pretty close & I can measure fast enough and make changes to prevent any damage. Pentode preamps I'm still copying from the RCA charts.
>how does it sound?
The clean (single gain stage) is beautiful. The OD is meh..... not awful, but the mother-ship didn't land either.
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From the cut sheet, Vbias = -13V.
at what Vg2?
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> 14 * .7 = 9.8W
"70%" is for fix-bias class AB.
In self-bias class A, 100% is a perfectly good idle.
Assumed 400V and 33mA is spot-on 12K and 13W, a fine op-point.
> From the cut sheet, Vbias = -13V.
At what Ip?? 25mA? 33mA?
And at what Vg2?
For -13V, at Vg2=250V, I read 45mA-48mA.
(http://i.imgur.com/ZVcoA.gif)
> I can take any ol triode and it's curve sheet, calculate a circuit, and my measured results are always right on close. Power pentodes I'm usually pretty close
OK, so how did you "miss so badly on the 6V6 cathode resistor?"
You can compute a pentode static bias with the pentode's "triode strapped" curves. This should be near-exact when Vp is nearly equal to Vg2, as it is here. Find desired current. Line-across. Find expected Vg2. Line-up. The intersection gives Vg1 for that current, that Vg2, and plate voltage fairly-near Vg2.
(http://i.imgur.com/yB4Tp.gif)
30V, 27V, rounding-error and tube-tolerance.
When you don't have triode curves, change both Vg2 and Vg1 in the same proportion.
Vg2 from 250V data to 370V amp: 1.5 times higher.
Vg1 for 33mA at 250Vg2 is -16V, times 1.5 is -24V. This fails to account for the roughly 15% rise in Ip due to Vp also going up 1.5X. If you fudge by making Vg1 1.15 times more, you get -28V, which agrees with smoke-bench.
It is good to know how Ip rises with Vg2. A 2X change from 100V to 200V gives a 2.5X change in current. This data is for Vg1=zero, but the proportion is fairly constant at other points. If you change Vg2 from 200V to 400V you expect Ip to rise another 2.5X, over 200mA.
(http://i.imgur.com/Tvxvx.gif)
So 400Vg2 allows 200mA to pass, and 370V will be mighty close to 200mA.
What do we really want? Class A swings from idle to zero then to 2X idle current and back. 12W and 400V suggests 33mA idle current. We need 66mA peak current. NOT 200mA.
In a "shouldn't clip" amplifier, reducing Vg2 so that max current is 66mA-70mA will give best sensitivity and least bias-loss. It seems that in guitar amps we favor raising Vg2 so that the plate will slam while Vg1 is still negative, so overdrive does not instantly start grid-blocking. 100mA ought to be ample. You don't need, and maybe don't want, Vg2 over 250V.
In an ideal world, you don't need/want a load as high as 12K, nor a B+ which threatens 450V electrolytics. 6V6 will drive 5K load. 250Vp and 50mA is 12.5W and 5K. There is a sweeter spot near 244V plate-cathode, 260V-270V total B+ including bias and filter losses. Yeah, in your lab that means all new iron; just saying that another project could as well be started from the 250V zone.
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Thanks for the study guide. I've read this about 10 times now. It's starting to sink in. Lots of great tidbitz I've been overlooking. Must spend some more lab time now.
Thanks!
-Richard