12AX7 basics, some questions on wiring

[jmilch]

Greetings.

I've decided to gut and completely rewire my preamp section on my recently acquired PA amp converted by someone else to a guitar amp.

I have just a few basic questions about the preamp 12AX7's specifically. I know this info also applies to other vairants...



Can a single 12AX7 be wired in series for more gain? (Can both sides of the tube be wired in series?) If so is this what is referred to as "cascading" ?

I am looking for a bit more gain out of my amp, thus the reasoning for the question. My iron can handle the additional stress of another 12AX7, so if a single 12AX7 cannot be wired in series, can I just add an additional preamp tube at the beginning of the signal?

Does each additional tube added increase the gain? Or is there a point where the preamp tubes no longer provide an increase in gain?

I read that my second 12AX7 is purely the phase inverter, and does not increase the gain at all. Is this true?

Any and all help would be greatly appreciated, and I have no problem with links if you can find the answers elsewhere. I have scoured the net for the past few hours and I cant seem to find these answers...

Thanks again

 

[jmilch]

I have done some more research, and found that it is possible to wire a single 12AX7 in series, and it is usually called SRPP.

http://www.freewebs.com/valvewizard1/srpp.html

I am still reading about it, but my general question is what is easiest way to increase the gain on my setup?

I have (2) El84's, and before I started messing with the preamp, the sounded much louder. After I began cleaning up the wiring, it seems like I have lost some output, which is my reason for tearing down the entire preamp and starting from scratch again.

 

[rotaspec]

I have done some more research, and found that it is possible to wire a single 12AX7 in series, and it is usually called SRPP.

http://www.freewebs.com/valvewizard1/srpp.html

I am still reading about it, but my general question is what is easiest way to increase the gain on my setup?

I have (2) El84's, and before I started messing with the preamp, the sounded much louder. After I began cleaning up the wiring, it seems like I have lost some output, which is my reason for tearing down the entire preamp and starting from scratch again.

You didn't miss the cathode bypass capacitor did you. Common cathode stages in single-ended preamps will lose a lot of gain if it is left out or substituted for one that is too small (you generally need 100uF or more).

I missed them when I reassembled my Chinese amp after a tidy up, and wondered where all the gain had gone.

Gary

 

[jmilch]

You didn't miss the cathode bypass capacitor did you. Common cathode stages in single-ended preamps will lose a lot of gain if it is left out or substituted for one that is too small (you generally need 100uF or more).

I missed them when I reassembled my Chinese amp after a tidy up, and wondered where all the gain had gone.

Gary

I am checking into it now. Thanks

 

[Arnulf]

Can a single 12AX7 be wired in series for more gain? (Can both sides of the tube be wired in series?) If so is this what is referred to as "cascading" ?

Yes and yes. Note that "SRPP" is not a cascade. A two stage cascade is just two stages, one after the other (and a coupling capacitor inbetween if they are AC coupled).

I am looking for a bit more gain out of my amp, thus the reasoning for the question. My iron can handle the additional stress of another 12AX7, so if a single 12AX7 cannot be wired in series, can I just add an additional preamp tube at the beginning of the signal?

In theory: yes, in reality: depends on gain of your existing amplifier (that extra stage might drive the following stages into clipping as it is).

Does each additional tube added increase the gain? Or is there a point where the preamp tubes no longer provide an increase in gain?

Yes. This is how solid state people do it

There are practical limits though. There is no point in driving tubes into clipping on normal levels. Noise is also amplified along the way so immensely large gain means immensely large amplified noise from the input. One can use the extra gain for some negative feedback.

It makes more sense to design backwards from your load (speaker) - first determine your OPT, then output stage swing into OPT, then swing of the previous stage, then check whether you get that kind of swing from your input. If not, adding an extra stage is an option.

I read that my second 12AX7 is purely the phase inverter, and does not increase the gain at all. Is this true?

Possibly, there are phase splitters with and without gain out there. This question is pointless without a schematic.

 

[pauldune]

Are you intending to use it as a guitar amplifier, or reverting it back to hifi?

Because the 12ax7 has already a max gain of 100x, (depending on how its connected) which is plenty. The reply from rotaspec makes sense; did you check this already?

 

[jjman]

The best approach would be to use a proven schematic as a guideline. There are many variables involved and adding an additional stage could be bad news w/o considering the big picture.

 

[jmilch]

The best approach would be to use a proven schematic as a guideline. There are many variables involved and adding an additional stage could be bad news w/o considering the big picture.

I realized the SRPP setup in the preamp section was not "cascading" and that it had no real practical use in guitar amps. But I had real trouble finding a decent explanation of cascading. It essentially ties both halves of the 12AX7 together with a capacitor in between correct?

I do agree guys it is furstrating to ponder and answer questions without a schematic. I tried my hand at drawing a schematic, and the chances of me omitting or misrepresenting data is very high, so I do not think the hour or two it may take me to do it would be worth the effort.... if I made one mistake, the whole effort would be useless.

I went ahead and stripped the entire preamp section. Everything is bare, all pins are cleaned and read to be re soldered. I will however do my best to draw a diagram of the output stage, so someone can chime in and let me know if it looks ok for use with (2) 12AX7's.

I will draw it out tonight and post it.

My other option is to completely strip the entire amp, and just scratch build it to my liking, off a schematic of choice.

I have looked at a variation of the 18 watt marshall amp, called the 18 watt Lite II. It has the same tube setup, same iron requirements. So I am considering just ordering all the caps and resistors for this schematic, and just stop putting wasted effort into solving my issues with the current topology, whatever it may be.

Should I draw up my current topology, or just order parts for the 18 watt marshall variant?

I have had the chance to play a nice 18 watt marshall clone, and it was a very nice little punchy amp.

But if all I had to do was rewire my preamp, in order to use this amp as it is, I would likely just do it.... and buy new iron for a 18 watt project.

Perhaps I have answered my own question. I may end up just trying to draw out the topolgy as it stands, without the preamp section. Perhaps then I can re design a preamp section with the help from the forum, and have a unique sounding fun amp to play on I am not chasing any particular sound, so I am not stuck on running after that classic Marshall tone etc.

 

[jmilch]

Ok I have done my best here, so this is my makeshift schematic...

Just a few notes.. (might not be clear in my drawing)

V1 - 12AX7 first preamp tube
V2- 12AX7 second preamp tube
V3- EL84 Output tube
V4- EL84 Output tube

Pin #2 on V3 connects to pin #8 of V2 through a .01 capacitor, as well as is grounded through a 470K resistor.

Pin #2 of V4 connects to pin #6 of V2 through a .01 capacitor, as well as is grounded through a 470K resistor.


Diagram 1

An externally hosted image should be here but it was not working when we last tested it.

Full size image available here

And there is a component I cannot identify, or I do not understand what its purpose it. I have it on the diagram in the bottom half. Here is a picture of it

An externally hosted image should be here but it was not working when we last tested it.

It looks like a large variable resistor, and it connects to pins 4 & 5 of all my tubes except the EZ81, since it originates from pins 4 & 5 of my EZ81. Does this just simply vary the voltage going to my heater pins?

V3/V4 ----> Pin 4 ---->Component part 1 ----> Pin 4 EZ81
V3/V4 ----> Pin 5 ---->Component part 2 ----> Pin 5 EZ81

If you need additional information, I will try my absolute best to provide it. I am very grateful for the help so far.

Any suggestions now on how I can wire up my preamp section?

 

[rotaspec]

Ok I have done my best here, so this is my makeshift schematic...

And there is a component I cannot identify, or I do not understand what its purpose it. I have it on the diagram in the bottom half. Here is a picture of it

Variable resistor to balance/minimise hum in the tubes.

 

[jmilch]

Variable resistor to balance/minimise hum in the tubes.

Is it good taht I never experienced any real hum? I have adjusted it and realized the heater voltage moved accordingly, but I never had any hum at all. I could go between 5.5V and 6.3V and it didnt make any audible difference. I figured it was "hum control" as some older amps have.