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Steve Conner

Sun Dec 02 2007, 12:42PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hi all,

I've not been doing any HV projects recently, but don't worry, I've not suddenly got a life or anything!

I've been working on another tube guitar amp. The metalwork is all done now, and it's time to start wiring up!

The cabinet is a modern repro of a 1950s Fender Deluxe:

But the innards are from a totally different 50 watt amp from the late 70s. I made the chassis myself out of 1/8" aluminium and altered the cabinet to get it to fit.

Finn Hammer

Sun Dec 02 2007, 02:19PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Valves are the way to go, and your guitar amp will keep you (and the neighbours) awake.
At the foot-end of my bed, I have this one, which I built in previous life.

It serves the purpose of putting me and my sweet-heart to sleep.

Cheers, Finn Hammer

ShawnLG

Sun Dec 02 2007, 04:29PM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
Location:
Posts: 399

Thats nice steve! How does it sound?

Hazmatt_(The Underdog)

Mon Dec 03 2007, 01:55AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Are you going into Blues territory? Because if so 50W is going to be too much clean power before things get interesting. If you're wanting to get into tube distortion you'd want a small ~20W amplifier that you can crank up all the way without making your ears bleed.

I've had some plans on the table for a push pull RCA 6973 amp, but I would have to do all the chassis work too and that can be a real bother, so I haven't done it yet.

I'm sure it'll have a nice fat sound and you'll probably want to bias it for idle of 35mA/tube.

Coronafix

Mon Dec 03 2007, 06:05AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Damn Steve, that is nice! Got to be the only amp I've ever seen with a cathode current meter.

I must say I'm very jealous, and you know the only way to play a tube amp is at 11.

Well done!

Steve Conner

Mon Dec 03 2007, 11:16AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hi guys,

Finn, I remember seeing (and hearing) your 300B stereo amp when I was over in Denmark, and I've wanted one ever since. :P Is the chassis actually made of solid copper?!

I'm not sure how it'll sound until it's finished. But the prototype @

was sounding pretty sweet until components started to go bad.

Hazmatt, your point is taken. Cranked up full, it might well just deafen everyone and shred its speaker. But on the other hand, more power means better clean tones, and the dirty channel can generate a fair amount of distortion from its preamp tubes at any volume. I also have a little 3 watt amp that I keep around to abuse.

The prototype ran with 6L6s and a larger screen resistor, and it only made about 30W, instead of the 50 that I got with EL34s. I left it that way because I thought the lower power would make it easier to crank.

"The blues is all about a man, a woman, a broken home, and a broken amp. Or at least one that sounds like it's broken."

Finn Hammer

Mon Dec 03 2007, 01:17PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Steve Conner wrote ...

Hi guys,

Finn, I remember seeing (and hearing) your 300B stereo amp when I was over in Denmark, and I've wanted one ever since. :P Is the chassis actually made of solid copper?!

As solid as it gets. And 2,5mm thick too! (that`s 100mills to US-Guys)
I estimate it weights around 50kg. (that`s 100pounds to.....)

Now stepping out to avoid threadjamming

Cheers, Finn Hammer

Steve Conner

Mon Dec 03 2007, 02:22PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

No, go ahead, it's not a thread about me, it's a thread about tubes! :P I'm fascinated to know how you made such a good looking chassis out of copper. Did you screw it together, braze it, or what?

I have some really big, heavy transformers to make a 300W tube bass amp, but I've been stuck on the problem of making a neat-looking chassis that won't bend under the weight.

Finn Hammer

Mon Dec 03 2007, 08:09PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Steve Conner wrote ...

No, go ahead, it's not a thread about me, it's a thread about tubes! :P I'm fascinated to know how you made such a good looking chassis out of copper. Did you screw it together, braze it, or what?

I made this chassis about 12 years ago. At that time, I had really few tols, but a electric jig saw and a belt sander and a drilling machine made it. All the bigger holes are filed out with half round files. No problem, this is the kind of thing toolmakers are trained for

I used the saw to cut the plates into shape. Sawing btwn 2 scribed lines makes it relatively easy to get things accurate within a millimetre or so.
Then belt sand to as close as possible. About 1/10th millimeter is fine.
Then I soft soldered the plates together using a propane torch and plumbing tin.
It was surprisingly easy.
The surface finish was made with one of those rotary-vibrating grinders. They make a random pattern which really showcases the nature of the copper.
Today I have a milling machine and could do the parts to 1/100th of a millmetre but that makes little difference.
This chassis is an interpretation of the Audio Note "ONGAKU" amplifier, although the original was built by 211 tubes.
At the time, I was scared of the 1200V B+ that these tubes require to really sing, but that shyness has since faded considerably.
There was an audio rag called "Sound Practices" back then. Authored by Joe Roberts. This magazine, dedicated to cheapskates and dumpster divers

contained a wealth of information on tubes. Seweral guys even made comprehensive step by step tutorials about tube amp design.
If I was going over this again, I`d build an amp using the tube #45, in pure class A goes without saying. It wold have 1 W output.
"What the world needs is a good 0ne watt amplifier" as Paul Klipch once said.
Class A may be lossy, but it is "blixtschnell" und "supergeil"

Steve Conner wrote ...

I have some really big, heavy transformers to make a 300W tube bass amp, but I've been stuck on the problem of making a neat-looking chassis that won't bend under the weight.

I have some 3mm copper plate just waiting for something like that, so I could probably make a top plate for you really easy.
Are you thinking about a domestic amp or one for the road. the style differs much.
A classic domestic amp has wooden plinth with a metal top plate, easy to do.
What tubes do you have in mind? 833`s?

Cheers, Finn Hammer

Hazmatt_(The Underdog)

Mon Dec 03 2007, 08:33PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I think the EL34's are a good choice for your project and for those I think the bias is about 42mA each.

You might want to play with some single ended Class A 12W drivers too. A single 6V6, 6L6 or EL34 with a well made speaker makes a superb amp.

Another problem you're going to have, as I have noticed, is that with high power (and in my case Celestion V30's) is that you get too much punch and not much mid gain from an open back cabinet. At higher power levels you need closed back cabinets to get the 'breathing' from a setup. "Too Roling Stoned" by Robin Trower is a good example of this. So in my case, I don't yet have the cash to shell out for 4 Celestions to fill my crummy cabinet.

As Steve already knows, the phase inverter is really troublesome because the stock schematics usually recommend a 12AT7, but that's killing the amp's soul, so it has to be swapped out for an 'AX7.

I was playing with overloading my gain stages too trying to get a distorted sound. I was using an 'AX7 for all the pre-gain stages for guitar input, then dumping that into an 'AU7 because of its lower mu to achieve softer distortion, that then fed the 'AX7 phase inverter. It worked okay, but I'm switching over to what Carvin does which is to use a resistor network and electrolytic capacitor to boost the output by switching it 'in' on the Cathode side. It's the same for transistor circuits too, but has the effect of 'boosting' when the normal cap value is 1uF and then you switch in a 10uf cap.

Of course, I have to reduce my rig from 4x 6L6 down to at least stereo and use maybe one half of the amp because its too bloody loud.

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