wiring 6SN7 instead 12AU7

[boroman]

I recently made my first amp which is a low-power version of classic 5F1 - with 12AY7 as preamp and 12AU7 for power tube. I have some 6SN7s here and data sheets of these two is almost identical. I thoguht I use it on a place of 12AU7, but AU7 is noval and 6SN7 octal. As I see it;s missing "heater center tap". How should I wire it without that one? My PT is 3,15v-0-3,15v for heaters.

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And second question - the same stuff with 12AY7 - I dont have them but have some 12AT7s. I know it has a bit more gaib and I can direct replace it (spec sheets looks very different in some parametres) but to make it behave like 12AY7, what should I change in the circuit?

[The Dude]

On the 6SN7 heater: It's just 6.3V from pin 7 to pin 8. Also note (says Mr. Obvious): There are other socket pins that are different. On using the 12AT7 in place of the 12AY7, I would just sub it in first and see how it sounds.

[Enzo]

That^^^^^

It might be helpful not to think of your heaters as 3-0-3. It isn't two 3v supplies. it is one 6v supply with a center tap. So there is 6v from pins 4/5 and 9. 6SN7 wants 6v from pin 7 to pin 8. Easy as that.

In your preamp, any of those 12A?7 types will function. I don't care if the specs are "wrong". Just try them in the socket and see what you think. It is just a little guitar amp, not precision circuitry.

You want to use a 6SN7? Go ahead. Wire up an octal socket as appropriate.

[The Dude]

Agree, and to elaborate: The reason I said to just try the different 12A?7 type without modifying anything is because the easiest way to control gain, IMO, would be to have a voltage divider between the first two gain stages. The volume control in this amp is just that- a variable voltage divider between the first two gain stages.

[J M Fahey]

"Tube rolling" is the WORST way to control gain,because besides gain, "everything else" also changes, speacillyn bias and working point.

A 12A(TYZVU)7 is NOT "a 12AX7 with different gain" but a wholly different tube.
Some are very close but then gain is also very close, so negating the Mod.

As said above, the versatile and precise way to change gain is to add resistive dividers as needed.

As a side note, that amp is very different in sound and character to a regular champ.

Hint : soft triodes instead of a sharp biting pentode.

[boroman]

That^^^^^

It might be helpful not to think of your heaters as 3-0-3. It isn't two 3v supplies. it is one 6v supply with a center tap. So there is 6v from pins 4/5 and 9. 6SN7 wants 6v from pin 7 to pin 8. Easy as that.
In your preamp, any of those 12A?7 types will function. I don't care if the specs are "wrong". Just try them in the socket and see what you think. It is just a little guitar amp, not precision circuitry.
You want to use a 6SN7? Go ahead. Wire up an octal socket as appropriate.

Still quite dont get it.. So I put one heater wire to pin 7 and second to 8, right? (instead of one to (4+5) and one to (9) in 12AU7) - am I thinking right here?

[Enzo]

Exactly.

Inside the 12AU7, there is a 6v heater for each half of the tube. If you wire 12v from pin 4 to pin 5, they are in series for 12v operation. If you connect pins 4 and 5 together and use the center tap of pin 9, then the two halves are in parallel for 6v operation. That is what we usually do, 6v operation. SO if you want to use 6SN7, just move the 6v wires over to the octal socket on pins 7 and 8.

[boroman]

Exactly.

Inside the 12AU7, there is a 6v heater for each half of the tube. If you wire 12v from pin 4 to pin 5, they are in series for 12v operation. If you connect pins 4 and 5 together and use the center tap of pin 9, then the two halves are in parallel for 6v operation. That is what we usually do, 6v operation. SO if you want to use 6SN7, just move the 6v wires over to the octal socket on pins 7 and 8.

Wonderful, thank you!

[mozz]

I would actually wire up the octal socket for the 6sn7. You can get a adaptor for 12au7/6sn7 that would sit on top if you wanted to experiment. Also, a 12at7 in V1 doesn't sound that great.

[eschertron]

boroman,
I misread your post about the 12AT7, and thought you were proposing to swap that in for the 12AU7 (instead of the 6NS7). Made me remember my first amp build - which was very similar to your design - and that used a 12AT7 in the power tube spot using a 200R bias resistor. I added a bias resistor switch to add 360R (for 560R total bias) to use a 12AU7 in the power tube spot. The 12AU7 gave a bit more clean headroom, so if you're looking for more distortion, maybe go the opposite direction and try a 12AT7 there? A much simpler mod. Keeping in mind the bias must be changed.

[boroman]

boroman,
I misread your post about the 12AT7, and thought you were proposing to swap that in for the 12AU7 (instead of the 6NS7). Made me remember my first amp build - which was very similar to your design - and that used a 12AT7 in the power tube spot using a 200R bias resistor. I added a bias resistor switch to add 360R (for 560R total bias) to use a 12AU7 in the power tube spot. The 12AU7 gave a bit more clean headroom, so if you're looking for more distortion, maybe go the opposite direction and try a 12AT7 there? A much simpler mod. Keeping in mind the bias must be changed.

Thank for that! But my goal was subsitute AY7 with AT7 (preamp), because I have some of AT7s that I will not use in the future.
And for the power tube, with aboslutely no changes to design, 12BH7 would be good too - 6sN7 as well but with octal setup.
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What I still dont understand, is the relation of tube to output transformer. For example - why 12AU7 needs 25k primary OT and EF80 needs 10k primary? Is there such a information which tube (working in a singe ended) amp or any rule I can set to calculate this?

[Helmholtz]

Suitable plate impedance is found from loadline construction with transformer (inductive) load.
For pentodes, see here:

http://valvewizard.co.uk/se.html

Always make sure that the plate dissipation limit is not exceeded at idle.

You can use this loadline calculator:
https://www.vtadiy.com/loadline-calc...ne-calculator/

[boroman]

Suitable plate impedance is found from loadline construction with transformer (inductive) load.
For pentodes, see here:

http://valvewizard.co.uk/se.html

Always make sure that the plate dissipation limit is not exceeded at idle.

You can use this loadline calculator:
https://www.vtadiy.com/loadline-calc...ne-calculator/

I tried to understand it but I can't... So, to calculate "perfect" match to choose output transformer I need only HT and max anode dissipation. That's weird - quote:

"there is a rule of thumb we can use to find a suitable transformer impedance:
Z = Va^2 / Pa
Where:
Va = Anode voltage.
Pa = Maximum anode dissipation."

So, if ECC82 and EF80 have about the same dissipation around 2.5W, and being in the same circuit it will need the same transformer -- which in this case is not right. They recommend 25k primary for ecc82 and 10k primary for EF80...

[Helmholtz]

So, if ECC82 and EF80 have about the same dissipation around 2.5W, and being in the same circuit it will need the same transformer -- which in this case is not right. They recommend 25k primary for ecc82 and 10k primary for EF80...

The formula works for pentodes - not triodes.
Conditions are:
- SE class A operation
- 100% plate dissipation at idle
- center-biasing.
- max. possible output.

Triodes limit plate voltage swing, so the formula doesn't work.

You can use the online calculator for both tubes. Check "reactive load".

For two triodes in parallel, plate impedance should be halved to achieve doubled output.


Note that with a B+ of 400V, the plate voltage will swing up to more than 700V at full output.
There's a rule (found in some good books) stating that with transformer load SE class A operation, B+ must not be larger than half the tube's plate voltage limit.

[loudthud]

Bottom line ... Unless you are going to wind your own OT, or buy one of everything that's available, buy one of those Hammond Universal OT's and try all the taps.