AX84 Amp - P1

[KimSeiji]

Hello guys!!


As my new tube amp project, I decided to make a 'HH Scott 240', but it was too much complicated, and requires a certain expertise.

So, instead of that, I am building the P1 amp from AX84, It's a very simple amp to get start with Tubes.

I just got started, here are some photos. And no doubts for now!
I have a power transformer for the filaments, and I made a test circuit for the other voltages. Next week I buy the output transformer.

This amp will be completed!!

[Paul Barker]

You know it is a guitar amp don't you?

The power transformer schematic you have drawn shows that you are half wave rectifying the mains. It is much safer to use an isolation transformer for HT. But perhaps in your country what you have drawn is the normal way of doing things.

[KimSeiji]

You know it is a guitar amp don't you?

Yes!

And I have already made a Overdrive with a 12AU7! (search 'vavecaster') I'm using it on my Peavey Blazer 158.



The Scott 240 was to other application.


xD

[Paul Barker]

OK that is great.

I have just built my first guitar head amp I based it on the AX84, but I decided on a Fender feedback tone control in the first stage. I chose to use an alternative valve in place of the 12ax7 because that meant I could buy original Western world valves without paying stupid prices. I decided to drive the output valve (6550 in my case) with a cathode follower. I have found that the 6em7 makes an excellent voltage amplifier and cathode follower in a huge number of applications. the reason I added it here was in case I want to use a mre beefy output valve like the 813.

[Paul Barker]

http://www.audio-talk.co.uk/phpBB2/view ... 5496#85496

[KimSeiji]

The power transformer schematic you have drawn shows that you are half wave rectifying the mains. It is much safer to use an isolation transformer for HT. But perhaps in your country what you have drawn is the normal way of doing things.

I was doing some tests on rectifying the HT voltage. The 'AC in' was plugged in series on 3 - 60W lamps, in case something goes wrong! haha

The main transformer will be equivalent to the original on the schematics.

There too many crazy people in Brazil! haha That's too bad.....
xD

OK that is great.

I have just built my first guitar head amp I based it on the AX84, but I decided on a Fender feedback tone control in the first stage. I chose to use an alternative valve in place of the 12ax7 because that meant I could buy original Western world valves without paying stupid prices. I decided to drive the output valve (6550 in my case) with a cathode follower. I have found that the 6em7 makes an excellent voltage amplifier and cathode follower in a huge number of applications. the reason I added it here was in case I want to use a mre beefy output valve like the 813.

I'll search for more info about 'Cathode Follower', 'feedback controll'... there are somethings I can't figure out. I studied Eletronic in 2007~2008 but those thing with tubes, they never teach us.

Nice amp! I'll check the video later!

[Paul Barker]

cathode follower is similar to an opamp. It has a high input impedance and low input capacitance, but a low output impedance.

With valve the input capacitance creates the high frequecy -3db roll off look at an RC filter, the R is the output impedance of the previous stage the C is the input capacitance of the next valve. Whatever the inter electrode capacitances may be all of the stages in your AX84 P1 are what is known as common cathode voltage amplifiers. Common in this case refers to ground, you could call them grounded cathode but for some reason that is not the name which has stuck. In a voltage amplification stage like this the input capacitance is increased by the gain. You may know the Miller effect from opamp theory in which is it the stage gain plus one times the feedback capacitance. In triode theory it is the stage gain (actual gain not the mu of the valve) plus 1 time the capacitance between the anode and the grid.

Why I am telling you this is because the 12ax7 with a mu of 100 has a very high stage gain in that amp it may be 60 I haven't actually calculated it. So the miller capacitance is very high.

When you use a cathode follower as a buffer there is no stage gain , therefore no miller effect.

the other benefit is that it has a low output impedance which means it can pass decent amounts of current into the grid of a power valve. A further advantage is that, when choke loaded it's voltage potential is nearly at zero, so you can direct couple it to the output valve if you use autobias on the output valve with a slight addition to the cathode bias resistor. Or you can fixed bias the output stage with a small negative supply on the cathode follower.

The other benefit of the cathode follower is it has a single stage feedback so is more linear.

You will notice that the versions of the AX84 which ara progressions of your P1 circuit use a cathode follower buffer stage to power the tone stack. But I feel they would be better to add a further buffer to drive directly the output valve.

I decided whether rightly or wrongly not to include a tone stack. Maybe I was wrong, time will tell.

But all I was after was the tone of '60's '70's telecaster blues players. I wanted to build an amp which sounds bluesy and vintage. I don't really want it to drift too far into the Prog Rock era. I hope that an amp and guitar calculated to sound like a blues system will do that job better than the more flexible and manipulateable stratocaster into a rock amp wi th a whole range of tones. i may miss the mark most certainly for this first attempt. The 6550 may be the worst choice. Perhaps I should have used a pair of el84's? But I have one thought that if the tone and distortion is achieved in the earlier stages, then a pure output valve may be worth a go, such as the 211. The 211 in itself has the right kind of pure tone, it may well make a very good output valve for a bluesy distortion provided by the alnico pickups in a telecaster, the simple tone control of a Fender amp of that same era and the overdrive in the amp.

[KimSeiji]

Huummm, this explanes a lot of things...

When you use a cathode follower as a buffer there is no stage gain , therefore no miller effect.

So by adding a second Pre stage, we can 'amplify' the frequencies that were affected by miller effect on the first stage?

the other benefit is that it has a low output impedance which means it can pass decent amounts of current into the grid of a power valve. A further advantage is that, when choke loaded it's voltage potential is nearly at zero, so you can direct couple it to the output valve if you use autobias on the output valve with a slight addition to the cathode bias resistor. Or you can fixed bias the output stage with a small negative supply on the cathode follower.

Wow! That's amazing!

The other benefit of the cathode follower is it has a single stage feedback so is more linear.

You will notice that the versions of the AX84 which ara progressions of your P1 circuit use a cathode follower buffer stage to power the tone stack. But I feel they would be better to add a further buffer to drive directly the output valve.

I'll try the tone stack before the amp is complete, maybe copy some Marshall tones or Fender ones.....

And I think maybe a third stage after the tone stack would be there to drive more current to the power valve, as you said.

In the past I build a simple passive 'EQ', five frequencies bands, and two FETs (MPF 102), for general audio, it worked perfectly! But I never tested it with a Instrument.


Thanks for the Info!!

[Paul Barker]

Yes I am frustrated, I can't test mine with an instrument either yet.

At least I now have a speaker and an amp.

[Mike H]

Why I am telling you this is because the 12ax7 with a mu of 100 has a very high stage gain in that amp it may be 60 I haven't actually calculated it. So the miller capacitance is very high.

Yes probably about X50, taking following load into account.

Incidentally as with most tone controls, V4A (2nd 12AX7) will be significantly loaded for treble frequencies by the tone stack, hence a cathode follower here is an option that I have frequently seen.

[KimSeiji]

Yes I am frustrated, I can't test mine with an instrument either yet.

At least I now have a speaker and an amp.

And about the speaker? Any sugestions?

I would like to buy a Celestion... Or any other good brand.

Why I am telling you this is because the 12ax7 with a mu of 100 has a very high stage gain in that amp it may be 60 I haven't actually calculated it. So the miller capacitance is very high.

Yes probably about X50, taking following load into account.

Incidentally as with most tone controls, V4A (2nd 12AX7) will be significantly loaded for treble frequencies by the tone stack, hence a cathode follower here is an option that I have frequently seen.

Guys, the 'P1 High Octane' and the 'P1 SEL' has a cathode follower, so, they drive more sound quality (tone) and gain? Is that correct?

tks!

[Mike H]

Yes like that, with preferably ECC82 for V3A so that R11 can be half the value at least for more drive current.

[KimSeiji]

Here is the output tranformer.

[Paul Barker]

I came at my AX84 based project from a non pro musician angle. There are so many stages and so many components that take away from the delciacy and tonality of the natural sound that compared to what I am used to hearing it sounded veiled.

I then tried a hybrid combination of the Fender Champ and a small VOX Se design.

I used for the first valve from VOX except I changed to 6sj5 pentode isntead of the EF86 pentode because I prefer to use Octal valves, and I used the 6V6 from Fender Tweed instead of the EL84 which Vox use for the same reason.

It was much better sounding that the AX84 clone. I hope this does not upset you. The Timbre was so good, the AX84 by comparrison had no Timbre to provide the sound of an acoustic instrument.

I know guitar players are looking for something different to me. I am the wrong sort of person to build a guitar amp.

But for me the simple ealry Tweed or Vox is the way to go. But what do I know about the guitar world? I speak as I find as they say round here. Not with the intention of upseting anyone, purely to provide more information from which people can make an informed decision.

[KimSeiji]

I came at my AX84 based project from a non pro musician angle. There are so many stages and so many components that take away from the delciacy and tonality of the natural sound that compared to what I am used to hearing it sounded veiled.

I then tried a hybrid combination of the Fender Champ and a small VOX Se design.

I used for the first valve from VOX except I changed to 6sj5 pentode isntead of the EF86 pentode because I prefer to use Octal valves, and I used the 6V6 from Fender Tweed instead of the EL84 which Vox use for the same reason.

It was much better sounding that the AX84 clone. I hope this does not upset you. The Timbre was so good, the AX84 by comparrison had no Timbre to provide the sound of an acoustic instrument.

I know guitar players are looking for something different to me. I am the wrong sort of person to build a guitar amp.

But for me the simple ealry Tweed or Vox is the way to go. But what do I know about the guitar world? I speak as I find as they say round here. Not with the intention of upseting anyone, purely to provide more information from which people can make an informed decision.

Huumm, I see. You had a very 'Clean' sound.. I'm looking for some 'crunch' sound, a little 'dirt'.

What I want is something like VOX AC30, and I know the AX84 will not give it to me right know. But, as my first tube amp, is everything all right! I may try to do some experiments you said, it will be Nice!