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crazyfrog 22nd March 2013 04:43 PM

45 amp with 6N7
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I'd like to know what interstage is adequate for a 45 tube instead of a 46. I found an interesting schematic at Vinylsavor blog

VinylSavor: 46

Also, I guess since it's not dual grid, The OPT is directly wired at the the anode.



Vinylsavor 22nd March 2013 06:26 PM

Hi crazyfrog,

The same interstage and output transformers can be used for the 45 or any other similar types. Reduce the B+ a bit for the 45 to 275V. Yes OPT wires to the plate only for the 45

Best regards


crazyfrog 22nd March 2013 06:36 PM

Thanks Thomas,
So 5k would be probably a good value between the 6N7 and 45 tubes.

Vinylsavor 22nd March 2013 06:39 PM

Hi Pierre,

since the schematic shows an unloaded interstage transformer, it does not have an impedance of it's own. the impedance values given by transformer manufacturers are a guideline for the usage. You would want something like 20k for the 6N7. What is of concern is the primary inductance. You want a value well above 100Hy for the 6N7.

Best regards


crazyfrog 22nd March 2013 09:12 PM

I'm a bit confused...
I wanted to give those values to Jack Galiano from Electra-Print and this was his answer:

If these Lundahls were 20K, the response on them at 13khz would be down 3-4db. 10ma 6n7’s may be lower than that.

You have to measure this number first. The 6SN7 is 10K

An impedance of 20K is too high for full bandwidth, too capacitive, really can’t be done by anybody realistically. Also, the inductance of 100H is not a workable number, if the trans sec is unloaded, then it would be near that, but if loaded correctly,

which it has to be to work full bandwidth, inductance would be around 20H. Transformers are not chokes. Operating them unloaded makes them a capacitor

Did I miss something?

Vinylsavor 22nd March 2013 10:02 PM


As I wrote above transformers don't really have an impedance of their own, they merely reflect the secondary load. The spec'ed Lundahl works nicely with the 6N7 and has a good bandwidth. In the meantime I get the gapped for 7.5mA for the 6N7

Best regards


crazyfrog 22nd March 2013 10:11 PM

Ahh! Thanks Thomas!

BTW I know terminations are necessary. But for that stuff, I usually refer to experts to avoid bad surprises...

Good night,


crazyfrog 22nd March 2013 10:25 PM

Made a mistake on name: it's Jack Elliano, not galiano. Sorry.

Vinylsavor 22nd March 2013 10:30 PM



Originally Posted by crazyfrog (
I know terminations are necessary.

Not always. In that amp I left the secondary of the interstage unloaded. Works beautifully. If you load it down to much, many advantages of interstage coupling get lost

Best regards


45 22nd March 2013 11:01 PM

1 Attachment(s)

Originally Posted by Vinylsavor (
it does not have an impedance of it's own.

It's not true. Every transformer has a finite (frequency dependent) impedance.
For example this is a 8K/6R PP transformer. As you can see it does have a finite impedance when the secondary is open.

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