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12HL7-triode SE Headphone Amp - I finally built something!
12HL7-triode SE Headphone Amp - I finally built something!
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Old 12th July 2018, 03:12 AM   #51
rongon is offline rongon  United States
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12HL7-triode SE Headphone Amp - I finally built something!
In case anyone is following this build...

I picked through my collection of 12HL7s and found two of identical construction that look like they're made by GE, but one is labeled GE, the other RCA. They have their getters on the top of the bottle, 'powder coated' grey plates and square 'windows' with shiny metal structural plates visible inside. I have these two in the amp and it sounds well balanced between left and right, with matching subjective sound quality. (I like the sound of this type of 12HL7 in this circuit.)

The Raytheon 12HL7 I was using before has a smooth plate of a shiny, dark brown-to-black metal. I found a Philips ECG with matching construction. I tried the two in the amp and they match in 'tone quality' but are a little mismatched in level (channel balance is off). I find the tone quality is somehow 'softer' and less 'present' than that of the powder-coat grey plate GE type. This type doesn't sound 'right' to me. I don't know what to make of that. Perhaps this type has a tendency to draw grid current at grid bias of -2V?

There are two other types of RCA 12HL7s which I haven't tried in the amp yet. One has a 'clear top' bottle with its getter on one side and a black 'carbon' plate material. The other has the getter on the top of the bottle, also has a black carbon plate, but has a large shiny metal plate on one entire side of the tube's construction. It's a pentode construction type I haven't seen before.

I'll try to listen to the clear-top RCAs and report on what those sound like in this amp.

It's interesting that different examples of this tube type 'sound' so different from each other in this circuit. I guess their characteristics vary widely for a tube.
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Last edited by rongon; 12th July 2018 at 03:15 AM.
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Old 13th July 2018, 04:42 AM   #52
stocktrader200 is online now stocktrader200  Canada
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you probably need another tube gain stage to balance the tube distortion.
I found a tube gain stage driving the volume pot sounds better then solid state driving it alone.
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Old 13th July 2018, 07:16 PM   #53
rongon is offline rongon  United States
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12HL7-triode SE Headphone Amp - I finally built something!
At this point I'm not so worried about the sound quality as I am about channel balance. It's pretty distracting if there's an imbalance left-to-right and you're listening in headphones.
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Old 19th August 2018, 03:22 PM   #54
rongon is offline rongon  United States
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12HL7-triode SE Headphone Amp - I finally built something!
Default What about Miller capacitance?

OK, I got the channel balance thing worked out. Like any SET power amp with no NFB loop applied, you just have to match pairs of tubes. It's a pain, but that's life.

Now for the next question. What about Miller capacitance?

There are no triode specs available for 12HL7, at least that I can find. The data sheet says the following, for pentode:

grid1 to plate = 0.15pF
Input (grid1 to h+k+g2+g3+i.s.) = 15pF
Output (plate to h+k+g2+g3+i.s.) = 6.0pF
_____________________________

To find Cmiller for a triode,

Cg-p + Cg-k * (mu+1)
_____________________________

In the 12HL7 data sheet, the C from grid1 to plate is measured with the 12HL7 in pentode. This means the screen grid shields the grid from the plate, so the capacitance is much lower. Once I've triode-strapped the 12HL7, won't that make the Cg-p go much higher, now that the grid is no longer shielded from the plate by the screen grid? By how much will the capacitance rise?
_____________________________

Looking at a 5842, which is an RF triode with similar gm to the 12HL7 (about 20mA/V), it says its grid-plate C is 1.8pF. Its grid-cathode C is 9.0pF.

So let's say the Cg-p and Cg-k of the 12HL7 in triode are the same as in the 5842.

Cg-p + Cg-k * (mu+1)

1.8pF + 9.0pF = 10.8pF (round up to 11pF)

mu = 30, mu+1 = 31

11pF * 31 = 341pF
_____________________________

Now, if I use a 100k volume pot, which has a maximum output impedance of 25k ohms, I've set up an RC LPF between the pot's Zout and the triode's Cmiller, correct?

RC = Rseries = 25k ohms, Cshunt = 340pF
F3 = 18.7kHz

Really? -3dB at 19kHz? Wow. That sucks.
_____________________________

Did I do the above correctly? Are those the correct capacitances to use for calculating Miller effect on frequency response?
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Old 19th August 2018, 10:22 PM   #55
Wavebourn is offline Wavebourn  United States
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12HL7-triode SE Headphone Amp - I finally built something!
No, you should not multiply grid to cathode capacitance by amplification factor. Only capacitance from first to second grid is in the feedback loop.
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Old 20th August 2018, 02:13 PM   #56
rongon is offline rongon  United States
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12HL7-triode SE Headphone Amp - I finally built something!
Quote:
Originally Posted by Wavebourn View Post
No, you should not multiply grid to cathode capacitance by amplification factor. Only capacitance from first to second grid is in the feedback loop.
Thanks.

Unfortunately, there is no spec given for g1 to g2 in the 12HL7 data sheet. The closest I can see is g1 to p, but that is so small (0.15pF) that it hardly matters.

For 5842, the Cg-p is 1.8pF. I could add a couple pF to account for strays and get to about 4pF.

4pF(mu) for the 12HL7-triode would be 4*30 = 120pF.

Does that seem more realistic? If so...

If a 100k pot is used:
LPF f3 for Zout = 25k and Cshunt = 120pF = 53kHz. That's pretty low. Approx. -0.1dB at 5kHz. I wonder if that's audible as a slight attenuation of high audio frequencies...

If a 20k pot is used:
LPF f3 for Zout = 5k and Cshunt = 120pF = 265kHz. That's good for -0.1dB at 26.5kHz, so that's just dandy.

Do you think I'd hear the difference?
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Last edited by rongon; 20th August 2018 at 02:18 PM.
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Old 20th August 2018, 03:39 PM   #57
smoking-amp is offline smoking-amp  United States
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I just measured a GE 12HL7 with a digital LC-200 meter, grid1 to g2 & plate. Using difference between lead touching pin and just not touching pin, I get 6 or 7 pf. (lowest resolvable digit on meter, so average reading)

I also have a C-350 meter that resolves another digit for capacitance, and that gives 6.5 pf for grid1 to grid2 & plate.

Then an Extech LCR meter gives 6.8 pf for the same grid1 to grid2 & plate.
Those are all for a cold tube. Space charge would increase that slightly, can't recall what %.

I just checked with my old Heathkit Digital LC Bridge and I get 6.5 pf from g1 to g2 only. (fancy Kelvin clips will only grip one pin at a time) Couldn't read anything on g1 to plate.

Last edited by smoking-amp; 20th August 2018 at 04:08 PM.
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Old 20th August 2018, 04:51 PM   #58
rongon is offline rongon  United States
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12HL7-triode SE Headphone Amp - I finally built something!
Hey smoking-amp, thank you very much for going to that trouble.
Interesting results you got there.

Quote:
Couldn't read anything on g1 to plate.
That makes sense, since the GE data sheet says 0.15pF. That's basically zero.

I have one of those cheap handheld digital LCR meters. I'll try measuring a 12HL7 tonight; see if I get something similar.

For now, it looks like an estimate of 7pF would be reasonably accurate for calculating Miller capacitance.

Let's say 7pF(30+1)for a total triode input ("Miller") capacitance of about 220pF, then. Does that sound reasonable?

If so, then a 100k pot with 25k max Zout will result in f3 of 29kHz. That's too low for comfort.

I think I'll be putting something like an 18k resistor from input to ground on the pot for each channel. That should make the pot look like 15k. 1/4 that would mean a max Zout of 3.8k, giving f3 of about 190kHz. That would put the pot's rolloff well above the OPT's rolloff.
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Old 20th August 2018, 05:05 PM   #59
smoking-amp is offline smoking-amp  United States
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For the two lead LC meters I clip one lead to the grid2 and plate pins (adjacent for 12HL7) and clip the other lead to a tooth-pick, then just touch and un-touch the other clip to the grid1 pin (holding the other end of the tooth-pick). Take the difference in readings. That avoids disturbing the leads or hand capacitance effects.

Last edited by smoking-amp; 20th August 2018 at 05:08 PM.
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Old 20th August 2018, 05:22 PM   #60
piano3 is offline piano3  United Kingdom
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rongon, the only problem with lowering your input impedance is that you might want to use a source whose output cap would prefer to see 100k. Why not put a couple of transistor followers after the pot instead (or a couple of sub-miniature tubes which don't demand much power, if you don't like semiconductors)
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