# Single-Ended 6V6 Amp -- 5K or 8K OT Impedance Answered

Discussion in 'Amp Tech Center' started by K Teacher, Aug 3, 2021.

1. ### K TeacherTDPRI Member

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Greetings,

This was asked in several tube amp forums and generated a lot of opinions but no concrete answer.

So here it is: THE OT IMPEDANCE VALUE DEPENDS ON THE POWER SUPPLY VOLTAGE.

That means, for SE/ Class-A operation, higher voltage requires higher OT impedance to maintain the operational point reasonably centered over the load-line and BELOW maximum plate dissipation.

In this case, the primary OT impedance value can be approximated as:

Zp = (Eb ^2) / Wb

Where:
Zp = OT primary reflected impedance,
Eb = Plate to cathode voltage,
Wb = Plate dissipation

From the "Electron Tube Manual":

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So, for Eb = 250V

Zp_250 = (250^2) / 12

Zp_250 = 62,500 / 12

Zp_250 ~ 5000 ohms (closest off the shelf value)

And for Eb = 315V

Zp_315 = (315^2) / 12

Zp_315 = 99,225 / 12

Zp_315 ~ 8000 ohms (closest off the shelf value)

Looking at the 6V6-GT "Characteristics and Typical Operation" data from tube manual:

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Again, this is for SE/ Class-A operation.

Now, the PS (or B+) voltage, for cathode-bias, will be the bias voltage, plus plate-to-cathode voltage, plus voltage drop on the OT primary winding (which for a typical 6V6 OT is approx. 12 VDC).

So, B+ for 5K OT is:

B+_5K = 12.5 + 250 + 12

B+_5K ~ 275 VDC

To get that using a 5Y3-GT rectifier, the PT-HV needs to be approx. = 520 VAC, CT or 260-0-260 VAC.

B+ for 8K OT is:

B+_8K = 13.0 + 315 + 12

B+_8K ~ 340 VDC

Using a 5Y3-GT rectifier, the PT-HV needs to be approx. = 600 VAC, CT or 300-0-300 VAC.

WARNING!!!

I have a couple of replacement PT for FENDER CHAMP/ PRINCETON (different brands!) that were sold as 650 VAC, CT or 325-0-325 VAC.

However, they put out about 7% more than specified and this pushes the B+ to 380 VDC. That is too much for the poor 6V6 and also, it will punish the 1st filter capacitor… You cannot find 600 VDC filter caps anymore!

They work great for the PRINCETON PUSH-PULL that has higher current pull from the PT, but are too much for SE 6V6 amps.

Hope this helps…

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2. ### Phrygian77Friend of Leo'sGold Supporter

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The best thing to do is draw the load line to see for yourself.

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Also if you take from the typical characteristics chart the plate voltage/plate current (max or idle), you get something close to the load resistance, within 1K anyway.

4. ### andrewRneumannTele-Holic

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Ok I'll bite. What do you make of this chart?

To me, it shows that at least for Eb = 250V, I have a range of load resistances I could use. Each has its own power output, distortion, plate and screen current maximums. If you were to ask me to design a sweet sounding Class A amp with this tube and Eb = 250V, I could make an honest case that 8,000-ohms is a reasonable choice. More harmonic distortion (usually better for guitar amps) and ever so slightly more power. But maybe the real kicker for me would be the screen current. With more screen current in play, I can play around with screen grid stoppers and have some more control over dynamics and overdrive. This is not to say 5,000-ohm doesn't work, or is a bad choice, only that 8,000-ohm could work equally well or and maybe sound better even at Eb = 250V.

The formula Rload = (Eb^2)/(Wbmax) is simply the inverse of first the derivative of Wbmax = Ib*Eb (P=IV). In other words, it just gives you the inverse of the slope of the maximum dissipation curve (which is resistance) at any point of the curve. If you use that rule and set your bias for 100% dissipation, the loadline will never exceed Wbmax at any point. (Someone will point out the loadline is an ellipse in actual operation... but you know what I mean...) That is useful to keep from melting anodes and extending tube life, but there is no rule in guitar amps, not even in Class A, that the loadline can never exceed Wbmax at any point.

I hope my words haven't exceeded my knowledge here, but I'm happy to hear if they have. We are always learning.

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5. ### K TeacherTDPRI Member

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Hi Andy,

The distortion shown on the 6V6 graph you posted is total harmonic distortion. One thing that this chart does not show (for plate voltage of 250V), is that the distortion for loads less than 6K is mostly 2nd harmonic and for loads higher than 6K will be 3rd harmonic distortion. There is a chart similar to that for the 6L6 that shows the harmonic distortion content broken down instead of total harmonic distortion.

I would like to point out that most of the teachings about tube amps are for music reproduction equipment and they target the least amount of distortion. However, this is not the case for guitar amps where, sometimes distortion is desired to add some "color" (or harmonic content) to the sound, but I think these design rules are a good starting point to maintain tube integrity.

So, if it sounds good and you are not melting parts, it is good!!

Last edited: Aug 3, 2021
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6. ### King FanPoster ExtraordinaireAd Free Member

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Cool calculations. I’m really only familiar with our popular 5F1/5F2a SE amps, where if I’m right, P-to-C voltages are usually above that ‘8K’ value of 315V. Maybe there are other SE amps that run P-C at 250, and B+ at 275? Or, wait, does speaker impedance change the deal? Is there something about 5F1s originally having 17K OTs? All I know is I built my 5f2a around an 8 ohm speaker with an 8K OT. P-C voltage even with 114 knocked-down wall volts is around 330, so I like your 8K ballpark a lot.

7. ### andrewRneumannTele-Holic

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I think you are right there.

I would be curious as to how screen sag affects the THD situation. I assume the graphs were generated using a power supply with impeccable regulation. I personally like to have my loadline intercept Vg=0 below the knee, and then plan to have screen sag bring the knee down to the loadline.

As far as I understand, the typical operating conditions listed on the datasheet are a compromise between efficiency and the least amount of distortion. Neither are a top priority for me.

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8. ### andrewRneumannTele-Holic

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Hrrrmmmm... the fact that is 2 x 8,500 makes me wonder if you are remembering something from a push-pull amp.

9. ### K TeacherTDPRI Member

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For some reason, modern 5F1/ 5F2A amps work at much higher B+… but I do not see the advantage on doing that… Perhaps amp manufacturers need to use current production, higher voltage PTs (!!?)

That can be done, if the primary OT impedance is increased enough to prevent the plate dissipation from going beyond maximum design value. However, looking at the 6V6 SE performance graph posted by Andy, one can see that the output power plateaus at certain point and then starts to decrease…

Another issue is, if the OT impedance is increased to compensate for higher plate voltages, the load line will pass below the ZERO VOLT control grid trace "knee"; thus, if the stage is overdriven, the screen grid needs to be protected from over-current (over-dissipation) by a current limiting resistor. The Valve Wizard SE page explains this in detail…

10. ### King FanPoster ExtraordinaireAd Free Member

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Actually, original 5F2a voltages, per Fender, were 360V B+ and ~330 plate-to-cathode. Check out this schematic (as you know, the 6G10 was identical to the 5F2a, and with the same PT). As I say, maybe there were other SE amps that ran B+ around 275V, but AFAIK the Champ and Princeton didn’t.

11. ### King FanPoster ExtraordinaireAd Free Member

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Um, no. Classictone actually made a “teeny Champ OT” (the 40-18110 or something) with 17K. When someone questioned that, they said, “The ratings for the 40-18110 are correct since they were from the vintage output transformer that we reverse engineered from an actual vintage 5F1 1958 Fender Champ.”

This isn’t definitive, of course, but Champ threads thru the years have had 17K mentioned by smart folks, and several folks who built 'em that way have noted it really increased the articulation etc. That's all I was saying.

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12. ### andrewRneumannTele-Holic

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Fascinating! I guess it goes to show what a range of options an amp designer has, even after the power tube has been selected.

13. ### BendyhaFriend of Leo'sSilver Supporter

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Which brings us back to question whether to use 12W or 14W as the plate dissipation figure for a 6V6.

14. ### LowerleftcoastFriend of Leo's

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Smarter folks can opine on this, but AFAIK some of the early champish offerings had PT's with low amperage ratings and supposedly the high 17K primary worked with those low ~40mA PTs where a 8K primary would stress the PT more.
Just crank them 'till they red plate, then back off some. That'll give you the figure. hah!

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15. ### andrewRneumannTele-Holic

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Hey, you guys are pretty smart. No doubt about it. Maybe you could answer this question about using high load impedances—like the 17K in the old Champ. So yeah, you’ll push less peak current, but you’ll also have much higher peak voltage.

Do the B+1 supply caps need to withstand that peak voltage under max signal conditions? I’ve always planned for them to withstand the peak PT supply voltage, but never considered the voltages developed between the power tubes and the OT at maximum signal. My theory is a bit weak on the signal between OT and B+1. Hopefully that makes sense enough to generate an answer….

16. ### LowerleftcoastFriend of Leo's

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@andrewRneumann
Interesting question. I have not considered this before. IDK.

The signal does not seem to be lost/reduced through the filter caps. I assume the AC current (or the bulk of it) does not flow with the B+. Most or all of it must be used in the OT and speaker.

17. ### andrewRneumannTele-Holic

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I was thinking a little more about it myself. I remembered the supply caps are all so big they should basically be considered short to ground for any and all AC signals. So they shouldn’t have any AC voltage drop across them. I guess the OT must drop all AC voltage across itself, and the supply caps only need to be rated for DC voltages. ??? Sometimes I get my brain wrapped around backwards on itself thinking about this stuff. And I’m sorry I feel like a thread hijacker.

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18. ### King FanPoster ExtraordinaireAd Free Member

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Nah, Andrew, you’re doing well. If I mused out loud about how electrons work, I’d get locked up. I sometimes know what smart folks say *does* work (like 8K for a 5F1 OT) but I don’t know enough theory to derive the reasons why. You go. And @Lowerleftcoast can reign you in if you get to far afield…

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19. ### K TeacherTDPRI Member

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The 5E1 Champ Tweed and earlier versions ran at lower (approx. 300 VDC) B+ voltages (see below). The same goes for 5E2 Princeton Tweed and earlier versions. Also, 50’s Valco amps had B+ around 275 to 300 VDC.

================================================================

================================================================

================================================================

For some reason, amp manufacture’s increased PT-HV voltages on early 60’s, perhaps to use readily available off-the-shelf parts to lower mfg. cost (!?)

IT DOES NOT MAKE SENSE to abuse the 6V6 beyond its max. design plate dissipation…

Just to put things in perspective:
1. SE/ Class-A operation, produces max. power output (Po) that is a little less than half of the plate dissipation. So, with Wb = 12W, you get Po = 5W. If Wb is increased to, let’s say to 14W, Po will be around 6W (at expense of the 6V6 longevity).

2. If the output power is doubled, from 5W to 10W, which is an increase of +3dB, it cannot be perceived by most people – this explains why there are very few 6L6 SE/ Class-A amps – higher cost to make and no perceivable loudness increase.

3. What was describe on item 2) is due to the human ear non-linearity. In this way, to get double of the loudness of a 5W amp, one needs a 50W amp (+10dB increase).

Last edited: Aug 5, 2021
20. ### 2L manTele-Holic

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Perhaps power supply suffers but power tube and OT are in serious danger! Symptoms approach what happens when loadspeaker cable detach while playing.

Test few loadlines using this following link excellent calculator. Problem of too high impedance is "flyback current" which can destroy tube, OT or both when primary voltage spikes come very high. On loadline this is what happen on right. It is because first bias current store energy to the OT and if it does not transfer to the loudspeaker it exixts as high voltage back on OT primary.

When A-class amp is driven using sine wave signal and using an oscilloscope, watching power tube anode voltage, there can be almost double peak to peak voltage signal what the B+1 is. But when power stage is driven very hard there come distortion spikes which increase pp signal way more than double the B+1.