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Old 20th November 2005, 01:20 AM   #1
Sherman is offline Sherman  United States
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Talking Finished First Guitar Amp

It took longer than I planned but I finally finished my first scratch-built guitar amp. My previous excursions into guitar amps consisted of modifying tube console amps.

I decided I wanted to build a simple guitar amp from scratch and searched for a design. I liked the AX84 P1 but wanted a separate 'drive' control. The P1 Extreme had the tone stack and drive control I wanted but used EL34's and I wanted to use the EL84. So, I grafted the preamp/tone stack from the P1 Extreme onto the P1. Pretty easy requiring only separating the B+ into 2 voltages, 350 for the pre and 250 for the output.

When I finished wiring it up and tested the voltages all seemed well so I plugged in a guitar and a speaker and turned it one and turned up the drive then the volume. Nothing!

A quick look back inside revealed I had wired the input jack wrong. Five more minutes with the soldering iron had that little blooper fixed. This time when I fired it up the guitar sound blasted from the speaker. (OK, as much as five watts at half volume and half drive can blast. ).

A few hours later I had tried it on a 3 way PA speaker, louder, and a 4x12 cab, louder yet. I wired in a switch that changes it from pentode to ultralinear. I guess it was my hifi mindset kicking in. I don't know that I would do that again with a guitar amp, I think pentode is fine.

The drive control works as advertised getting the sound really dirty without having to have the master volume all the way up. A clean/dirty switch adds to the grunge. The tone stack also works well. All in all I'm pleased but with two guitar playing sons in the house I don't think I'll be getting to use it much.

Here are some pics.

The front (and top). The trafos and the choke all came from Uncle Ned at Triode Electronics. A great buy at $69 for the set. The pots also came from Triode. The knobs were in my 'misc knobs' drawer.

I'm especially proud of the fine labeling!
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Old 20th November 2005, 01:24 AM   #2
Sherman is offline Sherman  United States
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Default The Back

Here's the back. An IEC power entry (this one is also and EMI/RF filter, no particular reason other than I have about five of them), a fuse holder and a quarter inch jack for the speaker.

The chassis is just an aluminum project box from Fry's. I cut a hole in the bottom below the PS section and put in a piece of perforated aluminum for ventilation. Nothing seems to get very warm.
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Old 20th November 2005, 01:25 AM   #3
Variac is offline Variac  Costa Rica
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Very encouraging! Do you have links to the 2 circuits?
And I see you used as much stuff from your parts box as possible, but what would the total cost be from Triode Electronics?
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Old 20th November 2005, 01:31 AM   #4
Sherman is offline Sherman  United States
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Default The Guts

Here is the inside.

One the left is the main PS section. It is a CLCRCRC filter. The choke is mounted on top. That feeds both the large power resistor on the main board (which drops the 350VDC to 250VDC for the output section) and the decoupling filter at the bottom which supplies the preamp section.

I used perfboard and some insulated turret style terminals to create the main circuit board. I thought that with the long wires running from the terminals then under the board to the tube sockets and from the terminals to the tone controls would invite noise but the amp is really amazingly quiet.

Best of all it sounds great!
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Old 20th November 2005, 01:43 AM   #5
Sherman is offline Sherman  United States
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Quote:
Originally posted by Variac
Very encouraging! Do you have links to the 2 circuits?
And I see you used as much stuff from your parts box as possible, but what would the total cost be from Triode Electronics?
I'm attaching the schematic I used.

The original 2 circuits can be found on http://www.ax84.com. The output section is pure P1, while the preamp is pure P1 Extreme.

The cost should be under $115, including the caps and resistors if you shop carefully or use the stuff you have around.

The PTX, choke and OPT were $69. The shielded tube socket was $3. The five pots ran $2 each (I think). The 1/4 inch jacks came from Radio Shack. Maybe $5 for two. I had the resistors and big caps (47uF and 1 100uF 450V) but had to buy a few of the small value caps. They probably ran me $5 because I bought enough to build a couple more amps. I also had the IEC socket, fuse holder and power cord.

The case was about $12 at Fry's.
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Old 20th November 2005, 02:56 AM   #6
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A neat job, well done. BTW, where is the clean/dirty switch in your schematic? I can only see the triode/UL/pentode sw.
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Old 20th November 2005, 04:19 AM   #7
Sherman is offline Sherman  United States
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Quote:
Originally posted by ray_moth
A neat job, well done. BTW, where is the clean/dirty switch in your schematic? I can only see the triode/UL/pentode sw.
Thanks! If I did it again I might just put everything on one big perf board instead of three. I would also do that ground bus differently but this one works well and should be easy to mod in the future if desired.

Good question about the switch! I saw a schematic with a clean/dirty switch and added it to the amp but didn't add it to my schematic. I also noticed another small error. Both cathode resistors on the 12AX7 are 1K5, not one 1K5 and one 1K6 (not that there is any practical difference).

The clean/dirty switch goes in like this-

Tie in one end of a 1K resistor between the cathode of V2 and the 1K5 cathode resistor. Tie the other end to a SPST switch so it can be switched to ground. When the switch is in 'dirty' postion it parallels the two resistors to ground.

I think there is a diagram on the AX84 site somewhere that shows it in pictoral form.
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Old 20th November 2005, 04:20 AM   #8
Variac is offline Variac  Costa Rica
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Another case idea, but adds to your cost. It is a DJ's carrying case for vinyl records. Only costs $35

http://www.platinum-records.com/Blue...e-prod7220.htm

You could put some metal angles in the opening and cut an aluminum plate to fit. So when you want to us it, you flip open the lid, exposing the plate with a row of Fender knobs. the open lid would raise the front a bit to give a good angle for access to the knobs.
I guess leave a 1/2" slot between the top of the plate and the case to allow ventilation..

With the case closed you'd have great protection. I guess a bit too big
but that gives room for suspending the existing chassis with tension coil springs!! Also maybe a compartmentfor cords and stomp box? Too bad they don't make one with half the capacity.
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Old 20th November 2005, 04:28 AM   #9
Sherman is offline Sherman  United States
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One other thing isn't shown correctly on the schematic because I didn't have a 3 position rotary switch that would fit. That is the pentode/ultralinear/triode switch. I just went with a pentode/ultralinear switch using a submini SPDT. And if I were to build this again I don't think I would bother putting it in, I'd just wire it pentode.
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Old 20th November 2005, 04:34 AM   #10
Sherman is offline Sherman  United States
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Quote:
Originally posted by Variac
Another case idea, but adds to your cost. It is a DJ's carrying case for vinyl records. Only costs $35

...

but that gives room for suspending the existing chassis with tension coil springs!! Also maybe a compartmentfor cords and stomp box? Too bad they don't make one with half the capacity.

That's a cool idea! Separate the inside and keep the speaker and guitar cables in there, and maybe a couple of spare tubes and an extension cord as well. Or use the box to make a small combo amp with a 6" or even an 8" speaker.

I wanted a smaller chassis for this one because I'm thinking of making it a "convertible combo". I'm planning on building a cabinet for an 8" speaker with a shelf so that the amp can slide in. An opening on the front of the cab would allow access to the controls.
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