Swapping 12AX7 with 12AU7

[Banjan73]

Hello!
Some years ago, I built my first tube amp. This was based on some schematics which I found on the net, but I modified it to have more gain. That amp was an 1 watter, the original schematics had one 12AX7, but I added one more. So two 12AX7 now.
As I built that one, I didn`t really know what I was doing. So my project now is to modify it/increase the gain (distortion) to make it suit more my preferences. And to make it more stable.
The transformers in that amp are not intended for this kind of use. They are just normal, small step-down transformers. Alle 3 trannies are identical (i will put out some pictures later). The updated schematics is attached. I haven`t started the physical modification work yet.
Anyway, a week ago I saw the schematics for the Hughes an Kettner "Creammachine", which is somewhat in the same direction as my amp. I noted there that they use a 12AU7 for the last gainstage + "power stage" instead of a 12AX7. I guess the reason for this is that the 12AU7 can deliver more current to the OT.
So, my question is the following:
I am going to use 2x12AX7 to start with now, as I think the amp sounds pretty good with those. But later, I will try to swap the last tube with a 12AU7. As I have understood, all these 12Axx tubes can be swapped without damaging anything, but the bias setting as it is in the schematics now will probably be a little awkward for the 12AU7. But do any of you have any opinions of what kind of result I will get by just putting in a 12AU7 in this circuit? Will it work "better" as a "power tube"?

[Banjan73]

Here is the the Hughes and Kettner schematics

[tubeswell]

Yes, you can use an AU7 in your amp for the last preamp driver (although the preamp gain will be less that if a 12AX7 triode was in that stage). For the PA stage, you will need to increase the cathode resistor for an AU7 triode to something between 4k7 and 6k8.


With an AU7 triode for the last preamp driver, you also might want to experiment with decreasing the plate resistor to between 10k to 33k (for more centre bias/ steeper load line).


Alternatively you could lose the 3rd preamp driver and just use both 12AU7 triodes in parallel SE output for the PA stage, with a shared 2k7 or 3k3 cathode resistor, and if you want more preamp gain, you could increase the 12AX7 plate resistors to 150k or 220k or 270k or 470k and decrease the cathode resistors to 1k or 820R.


But either way the gain characteristics will be somewhat altered. YMMV

[Banjan73]

Hmm.
Quite a lot of interresting factors there. Running 12AU7 in paralell havent crossed my mind at all, hehe.
Yeah, I realise that I need to rebias for 12AU7, but "as it is" now, it will work just not optimal, right?
Do anyone dear to try to describe the sound fram a 12AU7 vs a 12AX7 here?
Actually a lot more to consider than I first thought of here. I think I will calculate gain for some settings fr the 3rd stage with a 12AU7. I lean to believe that that tube is better as a PA tube.. (or maybe not..)
Anyway, thanks for nice input!

[PRR]

Front end of two AX7 stages will be distorting about all the time. (Input overload sensitivity about 15mV.)

I do not understand your R16 R17.

[Banjan73]

Front end of two AX7 stages will be distorting about all the time. (Input overload sensitivity about 15mV.)

I do not understand your R16 R17.

Hi.
Nice spotted! That schematics is actually already old. I was actually thinking the same as you here, so r16/17 is removed. Gain pot is also moved to after the first gainstage.

Drawing this schematics is an ongoing process. I will post the final schematics when its finished😉

[Banjan73]

I have some trouble regarding this amplifier.
There is a high frequency squeal when turning up the volume/gain.  Depending on the setting, it ranges from approx 9kHz to 20kHz. I have done a lot of things trying to get rid of it, that includes plate capacitor and increasing grid resistors. I have measured that the noise appears already after the first grid resistor, and after the first plate, it is off course amplified...

Anyone have any tips? I am clueless..

[Banjan73]

Btw, I suspect some kind of oscillation. But where is my loop here. I cant seem to figure that out...