# The endless question about "proper" OT primary impedance....

Discussion in 'Shock Brother's DIY Amps' started by tweedy_woodpecker, Feb 14, 2020.

1. ### tweedy_woodpeckerTele-Meister

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Hi all!

I was searching the web concerning this topic and there are a lot of opinions on this as well as sometimes contradicting suggestions.

Here is my question: I built a Parallel SE 2x6V6 amp inspired by various threads, articles and the "Truth about vintage amps" podcast. I took the Angela Super SE amp schematic as a starting point but did my own tweaks to the preamp as well as a variable NFB and spring reverb. Luckily it turned out great and it sounds really nice.

Now here's the thing: I use a Hammond 125DSE (10W) transformer with an 8 Ohm speaker. The OT gives me a choice of 2.5k, 5k, or 10k for the secondary.

Since I used seperate cathode resistors (tweaked to give me just under 95% of max. dissipation) for the power tubes I have the option of using it either with one 6V6 or two 6V6s in parallel.

The Angela article suggests 2.5-3k OT primary for 2x6v6. That seems low to me, especially for higher plate voltages. (With my last 6V6 PP amp I used 6.6k like a Dleuxe Reverb uses.)

In Merlins article about SE output stages (http://www.valvewizard.co.uk/se.html) he gives the following equation for the right plate impedance value when center biasing biasing:

Z = Va^2 / Pa

My plate voltages are 373V for PSE and 398V for SE.
If I take 12W as maximum dissipation for 6V6s I get

(398*398)/12=13.2k for one 6V6
(373*373)/12/2=5,79k for two 6v6s

That would suggest that the 10k and 5k tap would be the appropriate match for my little amp.

Any objections? Or input?
Thanks!

2. ### SquawkerTele-Meister

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Aren't your Va values way out of range? 6V6GTA has Va(max) = 315V according to e.g. Tung Sol's datasheet.

At 315V, the datasheet specifies an 8k5 load, but note that is for Vg2 = 225V and the idle Ia = 34mA, Vk = 13V

By comparison, a Princeton 5F2 runs at approx. Va = Vg2 = 305V (according to the schematic)

Most schematics running more conservative Va and dropping little or nothing to Vg2 call out for 5-6k or thereabouts, and that is pretty much the rule of thumb for an SE 6V6.
So two of them in parallel would want to see 2k5-3k.

Neither you nor Merlin is wrong here. If you're cooking your 6V6's at >370V then the higher impedance seems justified. But it doesn't sound entirely healthy.

3. ### tweedy_woodpeckerTele-Meister

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Thanks, Squaker!

Yes, I know what the datasheets and schematics tell us, but in reality I have never seen any Fender guitar amp with less than 350V on the plates. Most BF Fenders show more than 400Va, the schematic of a BF Princeton shows 410 Va, and all of them seem to be doing fine.

https://robrobinette.com/AB763_Model_Differences.htm
(If you scroll down a bit you find a table with typical Fender plate voltages. My PRRI measures 426Va, my 66 BFDR over 430Va.) Also, I biased the tubes accordingly with higher cathode resistor values.

So even with moderate real world 5F1 values (350Va) the load resistance would be about 10k for a single 6V6:
(350x350)/12W=10.2k

Here are some OT specs from Classictone:
1x6v6:
Champ Original: 17k at 4 Ohm
Champ, 5F2a Princeton, Harvard, Vibrochamp: 5k or 8k
High Watt SE OT: 5 or 8k

2x6V6:
5E3 Deluxe: 8k
Deluxe Reverb: 6.6k
BF Princeton Reverb: Classic Tone 8.5k, Hammond: 8k

So, I never see a 2.5k specification for 2x6v6 tubes. Or am I missing something here?

Last edited: Feb 14, 2020
4. ### elpicoTele-Afflicted

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Fender famously ran them at 420V in the deluxe reverb, princeton reverb etc, which are still some of their most popular amps >50 years later. They've been reissued in modern times, with the current princeton reverb running 440V on the 6V6.

And it wasn't just fender, even outside the guitar amp world good 1960s 6V6 were widely accepted to be much tougher than the sheet suggested and capable of taking a least 400v.

BUT... those are push pull class AB amps. Single ended amps are a different beast. They have much less flexibility in plate voltage. Not because of ratings or damage, just that there's really only one plate voltage that will allow any particular tube to both idle at max power AND be center biased. It's not much more than 300V for 6V6. Raise the B+ above that and you'll be forced to push the bias off center towards the colder, curved side of the transfer function to keep the dissipation in check. The amp will still work, and if you like the sound then who cares right? But this is outside the range of what is considered by this next part:

So yeah, at 398V your single ended 6V6 is not center biased (and can't be) so that's one assumption of the formula out the window. Also keep in mind this thing is just for a quick ballpark and naturally can't take into account all the complexities of the output stage. The screen voltage and bias voltage are factors too and the whole thing is really interactive, you change a single one of those variables and the ideal value of all the others changes. But a couple things to note are: in cathode biased amps Va =/= B+. You have to subtract the cathode voltage from B+ to get the effective plate voltage, Va. That should knock around 20V off your Va numbers I imagine. And second I'd use 14W for Pa not 12W.

5. ### tweedy_woodpeckerTele-Meister

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Thanks Elpico, very informative. Yes, I could probably assume 14W max dissipation.

From the Angela Super SE amp article:

"In our prototype amp we used an Audio Note EXPERIMENTER 2.5K single-ended 15W output but you can use anything that fits, between 2.5K and 4K or so with good (but different...) results. Hammond and other suppliers make output iron that'll work great. Check out the NEW 125ESE, etc on our Hammond pages; compared to the old 125E, the much larger, air-gapped 125ESE offers better bass and more headroom but still gives up plenty of harmonic distortion when cranked, especially with a hot P-90 or humbucker. I used the "2.5K" primary tap on the 125ESE but you can experiment with the other taps and pick the one that works best for you. If you're really, really broke you can scrounge suitable iron from old radio/phono junkers or even use half of a push-pull transformer. Just remember that the primary impedance required is around half that of a Champ, since there are two output tubes in parallel. We've used a Fender Deluxe reissue power transformer, putting out 337VAC per side under load, resulting in around 370VDC on the plates of the 6V6GT output tubes."

So I am pretty spot on to his plate voltage in PSE mode.

6. ### FenderLoverPoster Extraordinaire

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Thank you @elpico for post #4.

I use Merlin's rule of thumb to ballpark. Rules of thumb are not precise, but reasonably accurate. Accurate enough. You are just going to buy the closest transformer off the shelf anyway, regardless of load line calculations.

Another point is that the Power (P) number used is for 100% class A dissipation, and that also has to be a number that the power supply can support. If you decide to swap 6V6's for 6L6's, it doesn't mean the B+ current is up to the task, and may not change the reflected impedance at all.

7. ### RottenTheCatTele-Holic

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I'll bugger up the works and point out that there is no such thing as a primary impedance, unless you also know the output impedance. That is to say, transformers have impedance ratios, not impedance values.

Nickfl, Paul-T and tubelectron like this.
8. ### tubelectronTele-Afflicted

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As you have a Hammond 125DSE with multiple wiring possibilies of varying the Z ratio, I'd suggest you to make tests with a generator and a scope and see what's best, assorted of course by listening tests in clean and overdriven modes.

For 2 paralleled 6V6, I'd say circa 3-4K should be suitable as a resultant Z on the primary. On old radios, the load of 6V6 was usually 5 to 7K.

For one of my projects, with one single ended 6V6, I came to the 5K/8R ratio with a Hammond 125BSE, with these results at idle :

Vp=334V
Vs=324V
Rk=470R 5W
Vk=17.8V
Ik=38mA
Pout=3.2Wrms clean tone (onset of clipping / 440hz sinus / 8R resistive load)

But it's me, OK ?

-tbln

Last edited: Feb 15, 2020
9. ### FenderLoverPoster Extraordinaire

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And that's why we use the term reflected impedance.

But that's how one would shop for a transformer with a (reflected) primary impedance value, because the output impedances are assumed, and a transformer cannot be used without a load.

We are not going to try and create a non-standard load to reflect 5K in the primary just because we decided to try and use a 6.6K transformer. We look for a 5K transformer that has the desired output taps.

10. ### RottenTheCatTele-Holic

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Guess I bugger'd up the works. Mission accomplished!

11. ### NickflFriend of Leo's

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Also keep in mind that the 5k primary impedance (and relatively low plate voltage) suggested for SE use in most 6v6 datasheets (and used by most old radios) was chosen for max clean power, for our purposes a higher level of THD is both acceptable and even desireable along with higher power output. Hence, higher plate voltages and higher primary impedances tend to work well in guitar amps, low to mid 300v range and about 8k have worked well for me in single ended 6v6 builds.

12. ### schmeeDoctor of TeleocitySilver Supporter

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There was a very long discussion on this topic many years ago over on the Weber Forums, and your answer is pretty much where it all came to end the end.

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Don't forget that your 6k6 OT for your push pull Deluxe is shared by opposite working 6V6s, while your parallel SE 6V6s are both working together. So don't be fooled by thinking that 6k6 is applicable in both cases.

The usual cop out is to say to read the valve wizard website articles, and I'm going to cop out exactly that way.

You'll see with a sketchy loadline or two that class A push pull with a commonly seen 6k6 or 8k OT would work out very close to those Angela suggestions of 2k5 to 4k for SE

14. ### tweedy_woodpeckerTele-Meister

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I found this thread pretty interesting...