Inexpensive Lightweight Can of Whoopass

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Yeah 40V seems low, I just took it as worst case for power dissipation on the plates. The datasheet gives curves at 250V but with grid voltages of up to +15V. I just looked where the knee was and picked 40V, with the knee on the graph being 60V. Actually in the previous post I said the supply sags down to 370V at the edge of breakup and down to 355V when making square waves.

I am sure Merlin knows a heck of a lot more than I do. I just used a formula that others say gives a rough indication of the power. The transformer that I used was a replacement for the Fender Deluxe reverb which is said to put out about 20W. Also Harman Kardon put out a receiver back in the 50's that used these tubes running at 440V and it was rated at 20W at 2% IMD. With the sag we'll see if I get close to 20W, no way could I see more than that.

I did not want to wait for higher voltage caps to come in by mail, doubled up on the 350V ones and added balancing resistors. Not to say it was a neat layout before but it is looking a little more disorganized now. Just amazed I was able to fit them in at all.

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Finally sparked her up again. Capacitors are functioning fine, half the voltage across each of the pairs. Time to track down some hum. Lot of hum when I put my hand by the input pentode, take some aluminum foil and wrap it and grounding it, helped a little. Then I realized the suppressor grid was not tied to the cathode again. I removed the jumpers when I was soldering in the capacitors and forgot about this one.

It helped a lot again, still a little hum lingering. Then I thought maybe the switching dc supply for the heaters, I thought the heater dc is not tied to the circuit or the chassis, maybe try it to ground? A little concerned about just shorting it to ground as I do not know how the switching circuit is arranged, took the safe route and shorted it to ground using a 1k resistor. No hum. Now that is a relief. So with the master volume turned up all the way and the preamp volume control turned up I just get hiss coming off the pentode.

So now that I have a quiet amp I can do some fine tuning. Well maybe at least put it in a cabinet.

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Now that is a tight chassis!

Is your circuit board made of wood?

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No, countertop laminate. Epoxied two pieces together and am using one as an insulator. The stuff is tough enough and was cheap, scrap from work. Was an inexpensive amp. The power transformer was under $15, output transformer at $40, Old aluminum chassis I cut down and bent to shape. Speaker was used $30, pots, sockets, jacks, resistors and capacitors maybe $50. Bought a bunch of the 12AB5's at about $4 each. Cabinet about $20 in wood, 12v supply was $3 from a thrift store. Assorted other stuff, a bit over $150.

I always seem to squeeze more than I should into my amps. 'It looked like lots of room when I started.' One day I will learn. Tried it out today without the neighbors around, quite happy with it, doubt I will do any changes for now. Should update the schematic I guess.

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Posted this build on another site that had an amp challenge that I took part in, the person that started it sent me the 12AB5's that I used in this amp. This amp follows the general intent of the challenge in making a guitar amp with a minimum of cash. I will just cut and paste what I wrote there as it explains some of the circuit

Some of what I learned during the challenge made it into the making of this amp. I could have done things a lot easier, buying bits and pieces but I wanted to show there are other ways to do things when you are short of cash. Here is the schematic. The nice thing is the 12AB5 tubes can be had for about $5 each, the 12AU6 also, the two 12AX7's I used as I had them but the dual triode's out of Russia are another way to go shaving a few bucks.

The fixed bias supply is a little more involved than usual as I was using a bridge rectifier and the 0.68 uF cap was needed to isolate the dc, the 22k and 8.2k resistors were needed to get the right range. Since I used two 120V windings in series for the secondary there was no reason to take the full voltage, I grabbed the AC across one winding. I still might adjust the tone stack values although they seem to work for now. The switch moving the tone stack before and after the gain and cathode follower stage was to give a Fender Blackface vibe when before and a Bassman when after. The point was to maybe give a little more headroom sonically by scooping out the midrange before the next gain stage. In practice there is an audible difference but, also some roll off of the highs after the pentode, not surprising. There is not a large a volume loss as I was expecting having the stack after the pentode, part of the reason I got rid of the Mosfet.

The NFB is taken off the 4 ohm tap of the OT, the terminal strip I used only had two positions so I only used the 8 and 16 ohm taps of the transformer. When running one or the other I just move over the wire from the output jack to the right terminal. I have all the capacitor, other than the first one, grounds running to the voltage doubler using a bus ground and then to the chassis. The input section capacitor ground is returned through the input jack which is grounded on the chassis. It is dead quiet. One side of the heater supply is also grounded to the chassis. I had a hum that I could not find the source of and then thought the heaters were just floating.

The Morph control, the 1M pot on the input pentode is subtle in its operation. It gives a little compression to the sound when dialed in. If I did not like the sound of the pentode up front I was going to use this pot for a volume control and stick a 12AX7 up front and convert this amp into a Plexi. I think that about covers everything.

The updated schematic. Just finishing up the cabinet, otherwise this one is good to go. It has been an interesting project.

[IMG542]http://i406.photobucket.com/albums/pp14 ... dd1007.jpg[/img]

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Well came in at 24.4 lbs, still need a handle. The standard back panel off shot.

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And the sort of front-top shot.

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I want to live with it a while before I label the controls. The amp worked fine on the bench but in the cabinet I had something that sounded like a bad connection. Bad output jack for one, did not think these second rate jacks were as bad as they are, no wonder I got them cheap. After that still had some crackling. Poking around on the joints I saw this.

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I have a couple of resistors in the power supply filters that are unsoldered and just wrapped around the turret. I was not going to solder them in until I was sure I had the voltages I want. Guess I can't do that. The above joint was continuously arcing. I have seen arcs in equipment before but not one that sat there lit up like a LED.

Once I soldered all the resistors in I made a little noise. Ears as still ringing a bit. And I had it cranked for 10 minutes tops. I am going to have to roll off some bass. With the bass control at zero I had enough playing clean and distorted with my Tele. I removed the NFB, I did not have very much but the amp sounded more lively without it.

With the master volume turned down I can still get a good sound out of it. I can't say what other amp this one sounds like, can't place the distortion yet. Sounds good with the Fender speaker, I'll have to try it with a few others. Going to be hard getting a sound file of it. The guy I had checking out my other amps just gets away with playing my little 5 watters at home. Don't know if he can crank this one and not have to sleep on the couch.