6N7

[Mats]

I have tried my 6N7:s in my, 6N7 6V6 SE-amp.
I have thought about my next breadboard amp.
This time test a low power Triode amp  and try a PP with my 6N7 as Powertube.
Initial idea, use 3 6N7:s, for preamp, PI and a cathode bias Powertube.
Have seen this tube used in peamps and PI (Paraphase).
Have thought about to have 250-260 on the plates of the output tube.
Any suggestions, for the tube as Power tube ?
How to bias it ?
Perhaps use a Long tail PI ?

Mats

[PRR]

6N7 _IS_ a Power Amp.

But a Class B(2) power amp. Strongly optimized for that role.

In class A(1) it will make about a half-Watt. It just won't pass much current while the grid is negative.

When you _force_ the grid positive, then it flows good. One bottle P-P can do 10 Watts, 20 times more than class A.

BUT none of the conventional drivers will force power tube grids significantly positive. Certainly not to 42V and 22mA. Another two 6N7 as direct-coupled cathode followers won't do it. A 12AU7 might do it, though adds a lot to total B+ current and spoils the simple zero-bias feature of 6N7. The "sensible" plan is transformer-coupled driver.


Practical Amplifier Diagrams (5MB PDF file) page 21 of book (page 49 of PDF) is an all-6N7 power amp with enough gain for guitar. Ignore the "6V power" (unless you have a 1942 Plymouth) and build conventional power supply.

A couple other plans in that book use 6N7 as a super-cheap (at that time) driver. I have seen a 20W P-P-P 6N7/6A6 monster.

BTW: that book has a lot of plans which are just dangerous. All the "AC/DC" plans are shock-traps.

BTW: 6A6 6N7 and '53 are the same guts with base and heater differences.

BTW: 6N7 is one of the few once-common 7-pin/8-pin tubes which was never re-issued in Miniature. 6J6 does similar work but nowhere near as powerful as 6N7.

[PRR]

> any place a 12ay7 is used

With reservations. 6N7 is one cathode for two triodes. This really fouls stage-chaining such as guitar amps are full of. Bias will be different. Maximum plate current without grid current is quite limited, less even than tame 12AY7.

[Mats]

Thank you for replies!
PAD, PDF-file is very helpful, and all other amps there.

I read that 6N7 has a maximum of 300 V on plates.
Looked at Hammond 124- interstage transformers, would a 124A be alright to try ?
I have one 124A here.
About rectification, I have some 5Z4- metal to test in this circuit or just SS, have not decided yet.
Perhaps an all metal-tube amp.

Interesting input, 6N7, at that amp (5 tube 10 Class B), One triode input and two triode input.

Have to look more into Class B amp.
I have just tested cathode-bias amps.

Mats

[birt]

6N7 _IS_ a Power Amp.

But a Class B(2) power amp. Strongly optimized for that role.

In class A(1) it will make about a half-Watt. It just won't pass much current while the grid is negative.

When you _force_ the grid positive, then it flows good. One bottle P-P can do 10 Watts, 20 times more than class A.

BUT none of the conventional drivers will force power tube grids significantly positive. Certainly not to 42V and 22mA. Another two 6N7 as direct-coupled cathode followers won't do it. A 12AU7 might do it, though adds a lot to total B+ current and spoils the simple zero-bias feature of 6N7. The "sensible" plan is transformer-coupled driver.

and would it be possible to go a bit further and use a small powertube as a driver for the 6N7? something cheap  triode/pentode like an ECL82, ECL84 or ECL86?

[Mats]

Thanks for interesting replies.
Yes I want to keep it all octal.
The interstage transformer is an important part in this, I think.
Keep it close to PRR:s, amp in PDF.file, 5 tube Class B amp

[HotBluePlates]

The diagram shown in the practical amps book uses a transformer as a phase inverter.  From what little I can tell Gibson and Fender both used transformers as phase inverters. But I am wondering if you couldn't just use a 6N7 as a phase inverter in this circuit? ...

No, you can't.

Why?

6N7 _IS_ a Power Amp.

But a Class B(2) power amp. Strongly optimized for that role.

...

BUT none of the conventional drivers will force power tube grids significantly positive. Certainly not to 42V and 22mA. Another two 6N7 as direct-coupled cathode followers won't do it. ... The "sensible" plan is transformer-coupled driver.

...

A Class B output requires grid current as well as voltage, which means it needs grid power. As PRR mentioned, it needs ~42v and 22mA.

The transformer doesn't just provide phase inversion, though it does do that. It also takes a large voltage swing-small current and transformers it to a smaller voltage swing-big current, which is what the class B output stage needs to drive its grids.

You could do it without a transformer, but its wasteful of tubes and power. You'd need to make a phase inverter, have differential power tubes following that (maybe 2 6N7 sections), and then differential cathode followers made of some tube with bigger current capability and easier drive requirements than the 6N7. PRR says 12AU7 won't get it; 12BH7 (or something even bigger) might, but we'd need to check how much current they can flow with a reasonable driving voltage and at the supply voltages present.

The interstage transformer is much easier. Bigger class B amps usually use a single-ended power tube (like a 6V6/6L6/EL34/KT66) to drive a transformer which then provides the phase inversion and might step down the drive voltage and step up the drive current. But that's typical for big output.

[PRR]

> The transformer doesn't just provide phase inversion,... It also takes ...

And it _also_ gives low DC resistance in the grid circuit.

You can't capacitor/resistor-couple a grid which draws current only over part of the cycle. Same reason over-driven amps "grid block". While AB1 amps are designed to make "full power" without grid current, in stage-amp abuse we do try to slam the grids positive. Instead the coupling-cap charges negative, and holds the tube near off, muting or mangling everything after the initial transient. In some AB2 amps you can get to half-power before this happens. With 6N7 you can't get to 1/10th power without grid-current, cap charge-up, blocking.

The advantage of transformer coupling is that at one time a lump of iron was cheaper than a vacuum tube, and transformer-coupled circuits can be 10%-50% efficient compared to 6% or less with RC-coupled circuits. In 1933 this plan made a ton of sense. When fads change and distributors clear-out obsolete parts, as in 1947 when Robin and Lipman published, it still may make sense.

The Hammond 124A is a step-UP transformer for grids which do not draw grid current. Read the 6N7 data: it wants plate load over 20K (at over 6mA IIRC). The secondary should be under 8K and center-tapped.

You might try a 480V:240VCT 5+W power transformer. That's not an easy find either, and at these voltages is likely to have hardly any treble.

I don't recall any Fender with iron driver except the oddball little bass amp and the humongous 300. Gibson probably did, they did about everything and going back to times when driver iron was still in style; however few or none drew grid current.

The transformerless way needs two cathode followers. But this upsets the gross simplicity of a ZERO-biased amp. Also the peak grid current is 1/4 of the peak plate current. This means the driver is a quarter or more of the total power but its efforts do not feed the load. While RF power amplifiers sometimes must run with only 4:1 power-gain per stage, in audio we can usually do far better, 1000:1 or so power-gain per stage.

[Mats]

Thanks for replies

PRR
I looked at Hammond 124
Can I use an 124D Interstage and use Secondary for Primary 15800 Ohm CT
and Primary for Secondary 7000 Ohm CT ?
Or is this a no way to hook up ?
Mats

[TIMBO]

Hi guys , Just picking up where Mats has left off.
A mate has a truck load of both the 6X5 and 6N7's.

Found this on the link PRR provided and looks like a fun build.



The interstage transformer https://www.evatco.com.au/assets/brochures/124series.pdf
Is there a math or rule of thumb when it comes to these transformers???

[PRR]

> Is there a math or rule of thumb