Inexpensive Lightweight Can of Whoopass

[printer2]

Started a thread on another site about this amp, thought you guys might find it interesting.

The conception of this amp was a 'building an inexpensive tube amp feasible?' question that occasional comes up calling for a lightweight amp to hang with a drummer. Looked over stuff I had laying around and came up with the main components being Hammond transformers, a 125E is a multi-tap 15W output transformer and a 30VA 186D120 line voltage transformer. I will be using a voltage doubler to get the voltage up around the 360-370V range. With no heater supply I am using a 12V switching power supply since I am using a couple 12AB5 output tubes. They are a 6V6 variant in a 9-pin bottle with the same ratings as the 6V6. I am using a smaller power transformer as a choke, hope it works. Made up a Tweed type aluminum chassis from one I had in mind for a head.

[img650]http://i406.photobucket.com/albums/pp14 ... 3ffcdc.jpg[/img]

Did a board out of counter top laminate, brass nails and copper tubing. Yeah a little unconventional but stuff I had, did a Champ type of amp with it before and had no issues yet. Also showed a couple of future chassis made out of an old desktop computer lid. Another cheap build in the making?

[img768]http://i406.photobucket.com/albums/pp14 ... 46ff06.jpg[/img]

Isolated the input pentode with some silicone tubing, hope it works.

[img:793:695]http://i406.photobucket.com/albums/pp14 ... 57a145.jpg[/img]

Probably should introduce the circuit.

[img514]http://i406.photobucket.com/albums/pp14 ... cef4ab.jpg[/img]

Pentode with a morph control to adjust the gain. First time I'll be using this circuit and can't wait to see how it works. I have a Mosfet follower after it that is not on the schematic. The tone stack can go before or after the next gain stage with a cathode follower. Then just normal LTP PI and a fixed bias output.

Oh right, first it was to be cathode biased, changed my mind. Also will not be using the multitap OT as it currently resides in a different experimental project. Yeah I have too many things going on at once. Also changed to a post PI master volume. But the rest should really be close to the schematic. Except the power supply resistors, I am going to try different values, tuning it by ear.

Picked up a Fender 12" Lightning Bolt speaker for $25. Going to give it a shot in a 5E3 sized pine cab or use a WGS Veteran 10". The 12" should come in under 25 lbs or the 10" at 20 lbs. Did a weigh in with the major components.

[img368]http://i406.photobucket.com/albums/pp14 ... 572665.jpg[/img]

[printer2]

Just have to wire up the input jacks. I do not have the power supply resistors in as I want to adjust the voltages to taste. Hope it all works.

[img464]http://i406.photobucket.com/albums/pp14 ... 950434.jpg[/img]

[xtian]

Great build! I love "beaks and feet" builds, from stuff you have in the house.

[Smokebreak]

That looks like so much fun! Great work!

[printer2]

Well did I really think it would fire up first go round without problems? No, but it would have been nice. Not much hum out of it, power supply voltage dropped quite a bit. One tube has a nice red spot on the plate. Adjust the bias, up to -38V and still can not stop it. Swap out the tube and try to find a tube better balanced to the other one. Still two high on that side, swap tubes between the sockets, same socket running too hot. Bias voltage high, check at the pins, 0V on the offending tube. The bias voltage was shorting out on one side of the dual pot. Bend the terminal and all is fine now.

No signal through the input tube. It would help if I had a cathode resistor don't you think? Left the spot open so I could experiment with different values. Add resistor. Dropping resistors for the filter caps are too high, knew they would be, put in values to get closer to the voltages I want. First section voltages still not right. Might as well add a ground to that filter cap, otherwise it is not doing a heck of a lot.

Still need to lower the output tube bias range. Thought I would be running at higher voltages, might be hard to hit 15W. The preamp section does not have much gain, not much more signal than what is going into the input. Swap out the input tube, no difference.

On the bright side nothing melted. The switching supply is holding the heaters at 12.3V. A signal can go from input to output. A good way to start debugging tomorrow.

[JazzGuitarGimp]

Progress! That's good news, Printer!

[dorrisant]

I'm pulling up a chair for this one...

[printer2]

Getting funny voltages on the input pentode, when troubleshooting a new build and I do not know what is going on I simplify the circuit I can. I disconnected the Mosfet, still no good.

Hit myself on the forehead time, I was meaning to use a 6AK5 (12V version) but was could not remember the tube number. Decided to stick in a 12AU6 instead. Forgot the 6AU6 does not tie the suppressor to the cathode as the 6AK5 does. Jump the pins together and wonder of all wonders, there is my gain. It did not seem to matter switching the tone stack before and after the second stage, might just forget about the Mosfet. The simpler the amp the better.

Since I have the oscillator and scope out I might as well make my way through the amp. Adjusted the PS voltages some. Second stage seems to be working fine. On to the PI, can get 80V clean out of it, I'll have to decide on that later. Drop the input voltage to the power tubes to get a clean sine wave, barely any AC voltage on the plates.

What the heck is going on? Poke around some, voltages seem right, got a little increase in output moving the output jack. Play with the jack, have the switch shorting in case the jack is pulled. The switch seems alright, BOY THAT IS LOUD! Well what do you know, the output jack was shorting out.

As much as I appreciate a good sine wave my guitar is within arm's reach. Turn it up and see what it can do. Sure is loud, going to have to grab another speaker as my little 8" test speaker is not liking its current job. Well the amp works. Now on to wasting away countless hours swapping resistors and deciding what sounds best. Also need to rescale the bias supply. Looks like it might be a good day.

[printer2]

Don't think much of the morph control. On the fence regarding the pentode in the first position. I will try it again later, other than a lot of gain I am not getting a lot out of it. Found the tube I thought I was going to use, 6V heater, not worth the trouble.

Thinking maybe going with a 12AX7 instead.

[matt h]

(deleted)

[printer2]

Well I tried it with some boost before the pentode and its not bad for distortion. The part that bothers me is playing clean I am getting a lot of attack on the notes. When I ran the tube as a triode I did not hear it as much.

It's back in pentode operation, I dropped the supply voltage to the tube and raised the cathode resistor to get some more headroom on the input. It seemed to help. Probably should live with it for a while, maybe put it through a quieter speaker while I'm at it.

[matt h]

(deleted)

[printer2]

I have a WGS ET-65 that I was never to crazy about, seemed like it was more suited for larger powered amps. I thought what the heck, I took some acetone to it and removed the dope on the cone. Hooked it up to the amp. Together they have a rip-your-head-off kind of sound. It would definitely cut through a mix. Forget about the cleans, with 15W not a lot.

Might try to clean it up some, thought the tone stack switch to put the stack after the first stage would do it. It does a little, but the amp has more than enough attitude to laugh it off. Not sure what I am going to do yet, don't want to loose the attitude though.

[printer2]

Really drops more voltage from the power transformer than I expected. Open circuit the Hammond put out around 138V, rectify it and double it without a load you get 386Vdc, I was thinking it would drop into the 340V range, more like 310V driving the amp hard. So with 310V and a 6.6k transformer that gets you, 310*310 / 6600 = 14.5W. You think I would be happy with that call it a day. But the output transformer is a Hammond replacement for the Deluxe Reverb and is rated for 20W. I want my extra 5W. Will not make it a whole lot louder, just make me feel better.

So I rearranged the power transformer. It is a 120/240V to 120V transformer. If instead of hooking up the two primary windings to the wall I put them in series and hook up the 120V secondary to the line I get 208V AC out. This is because Hammond winds their secondaries a little hot, recall I was getting 138V out, so with the 120V going into it I get less than the 240V you would get if transformers were ideal devices.

So take 208V AC, rectify it which gives us 291V, this is going into a doubler so multiply by 2, 2*291 = 582V dc. That is just plain scary. But since I had some significant losses hooked up normally, what the heck let's try it. I have a variac and I can stop turning up the voltage if there is too much voltage. Keep in mind my capacitors in the doubler are 250V so I should be good for 500V.

Well turned it up and without the tubes conducting it got up to 500V. Which is ok for the output section, will have to look at the preamp capacitors later (I forgot about them, I was just watching the output tube current in case they were about to melt). When they started to conduct the voltage went down to 420V, 415V on the plates. So let's see what that should give me for power. The tubes do not swing down to 0V, maybe down to 40V, so 380V. 380*380 / 6600 = 22W. With transformer losses maybe up to 20W.

I had the bias voltage turned up all the way just in case, going to have to adjust the resistor values as I had 30mA per tube at idle. At 410V*0.030A = 12.3W. The tube plate dissipation is 12W, say 2W of that is going to the screen, about 85% dissipation. A little hot for fixed bias but should be fine until I change the resistors.

I really have to order some 500V capacitors to replace the 350V ones. No room to double them up, maybe I can stack them, worth looking at. Other measurement I took, playing at the edge of breakup the voltage drops to 370V, down to 355V with the amp feeding back and gain at a point where more really does not make much difference. All warmed up the plate voltage is now about 400V. No red plating so maybe the tubes will live. Pretty sure I got the most out of the power tubes. Next is to tweak the preamp. and I have yet to add NFB.

[JazzGuitarGimp]

Merlin says you take the amount of swing, square it, then double it, then divide by primary OT impedance. I have a feeling your estimation of the tubes pulling down to 40V is generous. Also, to think you'll have a B+ of 420V at full tilt is not realistic. I imagine you'll have a B+ of about 385V at full tilt, and the tubes will pull down to about 85V. So, 300V of swing squared is 90,000, times 2 =180,000, divided by the OT impedance of 6.6K gets you about 27W, assuming the tubes you're using are good for that much power.