The AX84 Guitar Amp Revisited - UK Vintage Radio Repair and Restoration Discussion Forum

Jim - G4MEZ · 6th May 2007, 9:27 pm

Getting on for seven years ago now I took up learning the electric guitar. Getting on for seven years later I still can't play it

My dream of being a Hank Marvin Impresario never materialised

Back then, I bought an Encore Strat copy that came with a horrible (semiconductor) amp that sounded like a wasp in a jamjar. That had to go and it did, very quickly.

I built an AX84 amp to replace it, a simple ECC83 and EL84 circuit from a design published here:

http://www.ax84.com/?pg=legacyprojects&project_id=p1

It was Ok although nothing special. I got a Jensen P10R speaker to go with it and built a reflex cabinet for it from a Babani design. Since then a lot has happened and the little AX84 has languised, largely forgotten after the aquisition of a vintage '62 Strat and a Fender Champ amp.

Recently, a reorganistation in domestic arrangements called for a little amp to complement one of my Leak Troughlines. I dug out the AX84, dusted it off and found that whilst it worked, it sounded terrible for music or speech. Not bad on guitar but for general Radio 2 and Radio 4 amplification duties it was terrible. It was noisy and very "toppy", lacking in any bass and sounding very uneven in frequency response.

I've been playing today and compared the circuit as originally built to the latest AX84 incarnation. There were a lot of detail differences around the second pre-amp stage and after modifying it to the latest circuit it was better but stilll not quite right. The "toppiness" was still there although the bass response was a good bit better. It was still a bit noisy (hissy).

An idea struck me. This amp has no negative feedback. I added some and experimented with component values and found a nice combination that makes this little amp sound great

I ended up taking the cold end of the gain pot to chassis via a 1K resistor and took negative feedback from the secondary of the output transformer and applied it to the junction of the 1K resistor and the cold end of the volume pot via a 4700pF capacitor and a 15K resistor. The reduction in gain is of no consequence in practice.

It still performs very well as a little guitar amp too

Today I have enjoyed a very valuable lession in the practical application of negative feedback.

daviddeakin · 7th May 2007, 11:01 am

Good to find you're learning eh? Yes, you can't expact a guitar amp to perform very linearly at all! Maybe one day you'll convert it into a real hifi mono-block? Who knows!

Jim - G4MEZ · 7th May 2007, 7:43 pm

Quote:

Originally Posted by daviddeakin

Good to find you're learning eh? Yes, you can't expact a guitar amp to perform very linearly at all! Maybe one day you'll convert it into a real hifi mono-block? Who knows!

Yes, David, that is precisely what is happening!!! I've just increased the value of the second pre-amp stage cathode decoupler to 47uF and added a 1000pF capacitor between the EL84 anode and chassis. Anotehr improvement.

It'll soon not be an AX84 at all...

daviddeakin · 8th May 2007, 7:52 pm

I'm a little confused now, what is it you actually want this amp to do? Do you want it to be a guitar amp or not? I think if you really want it to approximate hifi then you'll want to completely overhaul the circuit design, but it's a great guitar amp, why mess with it?

Jim - G4MEZ · 8th May 2007, 9:25 pm

Quote:

Originally Posted by daviddeakin

I'm a little confused now, what is it you actually want this amp to do?

A good question. Both jobs, to be a reasonable guitar practice amp and a reasonable little amp for the Troughline. A fair compromise really as it'll never be hi-fi as it is driving a guitar speaker and has a very tiny output transformer rescued from a KB VC52 TV Chassis.

I seem to have found a satisfactory compromise for now.

The main thing is that in a couple of evenings of experimentation I have learned more than I ever have from reading a whole pile of textbooks on the subject

A good grounding for my next audio project.

cantankerousbhr · 14th Jun 2007, 4:29 am

AX84 website is a good place to learn basic tube theory. Plus the amps sound great with tinkering, ive got a modded p1 extreeme that i gig with, woks great.

6th May 2007, 9:27 pm	#1
Jim - G4MEZ Tetrode Join Date: Feb 2006 Location: Newport Pagnell, Buckinghamshire, UK. Posts: 95	The AX84 Guitar Amp Revisited Getting on for seven years ago now I took up learning the electric guitar. Getting on for seven years later I still can't play it My dream of being a Hank Marvin Impresario never materialised Back then, I bought an Encore Strat copy that came with a horrible (semiconductor) amp that sounded like a wasp in a jamjar. That had to go and it did, very quickly. I built an AX84 amp to replace it, a simple ECC83 and EL84 circuit from a design published here: http://www.ax84.com/?pg=legacyprojects&project_id=p1 It was Ok although nothing special. I got a Jensen P10R speaker to go with it and built a reflex cabinet for it from a Babani design. Since then a lot has happened and the little AX84 has languised, largely forgotten after the aquisition of a vintage '62 Strat and a Fender Champ amp. Recently, a reorganistation in domestic arrangements called for a little amp to complement one of my Leak Troughlines. I dug out the AX84, dusted it off and found that whilst it worked, it sounded terrible for music or speech. Not bad on guitar but for general Radio 2 and Radio 4 amplification duties it was terrible. It was noisy and very "toppy", lacking in any bass and sounding very uneven in frequency response. I've been playing today and compared the circuit as originally built to the latest AX84 incarnation. There were a lot of detail differences around the second pre-amp stage and after modifying it to the latest circuit it was better but stilll not quite right. The "toppiness" was still there although the bass response was a good bit better. It was still a bit noisy (hissy). An idea struck me. This amp has no negative feedback. I added some and experimented with component values and found a nice combination that makes this little amp sound great I ended up taking the cold end of the gain pot to chassis via a 1K resistor and took negative feedback from the secondary of the output transformer and applied it to the junction of the 1K resistor and the cold end of the volume pot via a 4700pF capacitor and a 15K resistor. The reduction in gain is of no consequence in practice. It still performs very well as a little guitar amp too Today I have enjoyed a very valuable lession in the practical application of negative feedback. __________________ Jim G4MEZ

7th May 2007, 11:01 am	#2
daviddeakin Hexode Join Date: Apr 2007 Location: W Yorks, UK. Posts: 407	Re: The AX84 Guitar Amp Revisited Good to find you're learning eh? Yes, you can't expact a guitar amp to perform very linearly at all! Maybe one day you'll convert it into a real hifi mono-block? Who knows!

8th May 2007, 7:52 pm	#4
daviddeakin Hexode Join Date: Apr 2007 Location: W Yorks, UK. Posts: 407	Re: The AX84 Guitar Amp Revisited I'm a little confused now, what is it you actually want this amp to do? Do you want it to be a guitar amp or not? I think if you really want it to approximate hifi then you'll want to completely overhaul the circuit design, but it's a great guitar amp, why mess with it?

14th Jun 2007, 4:29 am	#6
cantankerousbhr Retired Dormant Member Join Date: Jan 2007 Location: Detroit Michigan Posts: 0	Re: The AX84 Guitar Amp Revisited AX84 website is a good place to learn basic tube theory. Plus the amps sound great with tinkering, ive got a modded p1 extreeme that i gig with, woks great.