So... I have been posting a few things about a guitar amp and I'm right about to start building the one in my attachment (for the most part).
The first stage is one 12AX7 in parallel with itself (both triodes, like some matchless amps).
I also plan on putting caps in parallel with each rectifier diode, and a large valued drain resistor at the first filter cap to drain the caps when it is off.
Critiques or any opinions are welcome.
The first stage is one 12AX7 in parallel with itself (both triodes, like some matchless amps).
I also plan on putting caps in parallel with each rectifier diode, and a large valued drain resistor at the first filter cap to drain the caps when it is off.
Critiques or any opinions are welcome.
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Most guitar amps with 1 EL84 only use 1 12AX7. You can get sufficient drive and gain.
You need a good reason to depart from the 'accepted wisdom' embodied in existing circuits like the Fender Champ.
w
You need a good reason to depart from the 'accepted wisdom' embodied in existing circuits like the Fender Champ.
w
Caps in parrallel with the rectifier diodes are always a good idea. But, it is better to build an AX84 P1. I have built one and it is a highly recommended design! It also includes the draining caps, in a smart way.
AX84.com - The Cooperative Tube Guitar Amp Project
AX84.com - The Cooperative Tube Guitar Amp Project
I dunno if I buy into saying it is better to build something, especially because everyone else is building it.
AX84 is a great site. The the projects are very well documented and supported. Some of the biggest problems for inexperienced builders is layout. The AX84 site has layout diabrams for the chassis and for the turret boards to make an amp that will help guarantee success. These are the reasons to consider the designs on the site.
I agree its a great site and set of projects, but just saying it's good because lots of people build it is not a valid argument. I am not going to completely change my whole project at this point, I only wanted some ideas on any small improvements from experienced builders.
I did not think anybody actually said to build an AX84 because "lots of people build it". Costis_n suggested to build an AX84 because he built one, was obviously happy with it and it is a "highly recommended design". He also mentioned some design features of that amp that you might be interested in. Have a look at the member ID section on the left of the posts. People from all over the world contribute to try and help each other out. For many, English is not a first language. Even those who do speak English, local 'turns of phrase' may not translate as expected. Unlike many online forums, I have found that just about everyone on this site is genuinely interested in offering constructive help, not trying to prove a point...
In any case, I think you have a recommendation not to parallel the first 12AX7 sections if it is not what is on the schematic of your preferred design. Costis_n mentions the bleed resistor setup on the AX84. It is not primarily a bleed resistor, however it does perform this function secondarily. He is talking about the resistor divider from B+ that is tied to the centre tap of the heater winding that elevates the heater voltage reference. I will let you look up the advantages of this. Secondly, as I stated above, layout is critical in vacuum tube circuits. The AX84 site has chassis and turret board layout diagrams. If you are not going to use one of their designs directly, I highly recommend you adapt one of their layouts to suit your schematic.
Also, as I do not know your experience level, I would recommend the following article regarding grounding. I found it really helped me understand how to prevent hum. It will be useful for you when deciding how to lay out your components. Star Grounding
Last bit of advice: plan your layout and wiring before drilling any holes or soldering any components. It will save much grief!
In any case, I think you have a recommendation not to parallel the first 12AX7 sections if it is not what is on the schematic of your preferred design. Costis_n mentions the bleed resistor setup on the AX84. It is not primarily a bleed resistor, however it does perform this function secondarily. He is talking about the resistor divider from B+ that is tied to the centre tap of the heater winding that elevates the heater voltage reference. I will let you look up the advantages of this. Secondly, as I stated above, layout is critical in vacuum tube circuits. The AX84 site has chassis and turret board layout diagrams. If you are not going to use one of their designs directly, I highly recommend you adapt one of their layouts to suit your schematic.
Also, as I do not know your experience level, I would recommend the following article regarding grounding. I found it really helped me understand how to prevent hum. It will be useful for you when deciding how to lay out your components. Star Grounding
Last bit of advice: plan your layout and wiring before drilling any holes or soldering any components. It will save much grief!
It's cool to work out a design for yourself, particularly with say, some cheap Russian tubes (take a look at the 6N6P) that there's no well-known existing design for, but with valves as well known as these, although going through the design effort yourself is still good practise, they really have been pretty much done to death.
Your amplifier will work, and if it doesn't you can tweak it (particularly if it's wired point-to-point), if that's what you are concerned about. It's not economical though, you've got gain and drive going to waste.
You'd be better using the gain from your first stage to drive a spring reverb, but again, take a look at the Fender '63 Reverb with a 12AT7, 12AX7, 6V6GT (not a million miles away from an EL84, which is a bit more sensitive but with a bit less dissipation) to see the general arrangement. The schematic's on the Fender website, and the ones that aren't are on Dr.Tube. The specialist in the repair and modification of tube amps. Sale of tubes and components.
w
Exactly, the argument is that i built it and it sounds sweeeeet, to everybody that listened to it. Whatever ypu build, don't skimp on cabinet and speaker. A 12 inch low-power Jensen was very good for this one, I am sure that fitting something inferior would not do it justice.
I built a small guitar amp using some old TV tubes I had laying around. PCF80 and PCL82. Tried to give it a lot of gain. As it turns out the sound sucks at high gain but it's one very sweet sounding amp at low gain.
My point? If you want to get it right off the bat, and spend more time playing the guitar, go for the AX84. If you want to experiment, do your thing and be prepared to not to get the sound you expect from the first try.
That said I don't see anything wrong with your schematic. What is C10 for??
Keep it fun!
Kenneth
My point? If you want to get it right off the bat, and spend more time playing the guitar, go for the AX84. If you want to experiment, do your thing and be prepared to not to get the sound you expect from the first try.
That said I don't see anything wrong with your schematic. What is C10 for??
Keep it fun!
Kenneth
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I have an extra Celestion V30 that I'm going to use, and if I don't like the sound I'll get a Jensen, but I love V30's. It can handle 60 watts, so I won't be pushing it at all, but I think it'll be okay for this project. The cabinet is also an expertly crafted one. Solid as a rock, made from great plywood, open back 1x12 combo.
Of course I like playing guitar but I do enjoy the engineering and tinkering a lot, so this is a good project. I'm pretty confident in the design as well, the amp it is based on is a great sounding little amp. I've played it myself! I know for a fact that the transformers I'm using are better than the original, so hopefully mine can't sound any worse.
Of course I like playing guitar but I do enjoy the engineering and tinkering a lot, so this is a good project. I'm pretty confident in the design as well, the amp it is based on is a great sounding little amp. I've played it myself! I know for a fact that the transformers I'm using are better than the original, so hopefully mine can't sound any worse.
Then you are set! Mine has a Jensen c 12R. Some old amp tech (the link eludes me) says that the lower the wattage, the better guitar speakers sound. It has a warm all around sound.
I'm looking at the schematic for the basic AX84, and I see the 4th pin of the EL84 is connecting to the 3rd pin (heater to cathode), is this the elevation of the reference you are talking about? What happens if I have a center tapped heater winding? I have two green wires and a green with yellow stripes.
Is this advisable: "A typical way to apply the DC reference is by connecting the centre tap (real or artificial) to the cathode of a power valve, providing the power valve is cathode biased of course. The bias voltage of most power valves is usually more than 5V, and this will be 'added' to the heater voltage." - http://www.freewebs.com/valvewizard/heater.html
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Heater to cathode? Looks like a typo. I built mine using the Theory document rev.10, which describes it nicely. Basically, the HT+is divided by a 47K / 100K resistor divider, and then smoothed by a capacitor, and this feeds a +40V to the center tap of the heaters secondary. It does make a difference in hum. So go withthe theory document.
I think it is a viable way to do it but I'm not sure. There are a few ways to do it, that I am sure of. Has anyone else done either of these methods? It seems pretty simple to tie the center tap to the EL84 cathode.
I would not do that, because the cathode is also a signal input. Seems like connecting it to an AC voltage (even as a center tap) is not the best way to keep the signal clean....
I agree, it does seem strange. Ill probably try to do the method from the AX84 theory document and using my center tap to elevate the heater DC.
Yes, that's exactly what is meant, and it works perfectly. And no, it does not mean that you are adding AC to the EL84's cathode circuit, because no current flows from the heater to the EL84 cathode. It only creates a voltage reference point, nothing more.
Kenneth
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