Low voltage Marshall inspired amp for voltage challenged individuals.

[printer2]

The thread 'JCM800 Downscale'

http://music-electronics-forum.com/t30501/

created a fair amount of interest a while back. The idea was to build a Marshall type of amp but there was a limit to the voltage that the amp could run at. It was for a university thesis and for safety sake they limited the voltage to 40V. I proposed using the 6AU6 as the gain stages rather than using triodes. I thought they would give more gain at low voltages and may sound better that the 12AX7 running at low voltages. I drew up a quick schematic as a starting point but it never got farther than that as I had other amps to build.



Well I got the other amps built and returned to this idea. The OP of the originating thread has gone on to bigger and better things so I had a blank slate to work with. I wanted to keep the voltage under 48V, in lot of jurisdictions that is considered a save voltage that you can not really hurt yourself badly. I wanted to use one transformer for the heaters and the power supply so I selected the 25L6 for the output tube which heater needs 25V. The 6AU6 also comes in a 12V version and putting a couple in series would allow a person to run them off the same winding. Another reason for using the 6AU6 is that I won an auction for 50 of the tubes at a low initial bid and have no real application for them.

I took an old chassis that I tried making a 100W SS amp in when I was in my teen's and mounted sockets, transformers, pots and a breadboard



I used a bridge rectifier to get about 34V from the 24V transformer. Threw together a bias circuit for the output tube and did a input stage going to a LTP PI then to the output tubes. I originally thought of using a 70V line matching transformer (for intercom speakers in hospitals, schools, and the like) as a 10W one is suppose to have down to a 500 ohm winding when used in P-P. Putting together the test circuit I used a Hammond multitap transformer I had instead, thought it went down lower but for an 8 ohm load the lowest impedance it has is 3k. Good enough for a start, got the circuit working enough to see if it was worth bothering with.

Well it was pretty much not worth bothering with. Just not enough voltage to get enough volume out of the amp. While I had it set up I decided to try it at 50V. Passable for someone that had an efficient speaker and lived in an apartment block. I thought about using 35C5's which would get the dc voltage up to 50V, then thought what if you used a 12L6 and a voltage trippler? Could also use two 6AU6's in series or a 12AU6 directly across the transformer. See, it is feasible, good enough for me to try the full circuit.


With the test circuit I found to get the most volume out of the 25L6 I had to run the bias voltage around 3V, not a lot of headroom. I also found that the normal method of supplying the screens of the preamp tubes voltage with a resistor off the supply and with a capacitor to ground did not change the operation of the tube much and that I could just connect the screens to the supply. Makes for a lower parts count and complexity. Thought what is good for a preamp tube might be good for a power tube so I ran those off the supply also. Going to be hard getting to the screens dissipation limits running at these voltages. I also thought I could simplify the circuit by going cathode bias on the output tubes. Loosing three volts will not matter much and it lowers the complexity. I did give each tube their own cathode resistor as I wanted to know what each tube was drawing. I have a bunch of used tubes and found some are not all that healthy.

After spending some time getting the circuit to work I came up with the following values.



Not a lot of gain as compared to a Marshall but then again we don't have a lot of headroom to work with. I tried a higher plate resistor on the input tube but it sounded a little ratty. I did not have the tone stack working, I think I have a bad treble pot. In place of the stack I put a 1M pot in as a volume control. As little the amount of gain it is easy to overload the PI without any attenuation. Probably will lose the mid pot when I get back to looking at this project and putting in the tone controls. Too many other things I should be doing at this time of year.

[exclamationmark]

I also like to fiddle with sub 50 volt amplifiers! I think the best investment I made for boredom killing was a $20 SMPS that did 48v @ 1.5 amps.

I found a major problems with these types of amplifiers is the lack of suitable output tubes. I've made many acceptable sounding tube pre-amps at this voltage, but I never really came across an output design that I liked. Recently I've had the idea to use some output tubes at near 0 bias, but feed them with a direct-coupled cathode follower so I can really crank up the current - still working on that though . Pentodes seem to require rather large screen voltages, so I think some triodes might be better suited to this. I tried this with a 6as7g, but it just required too much drive voltage and I couldn't even force the thing into clipping - put out a decent amount of power though.

Another idea might just to be to run the output of the output stage into a solid state amplifier. I recently made an all jfet amplifier that had a tiny push-pull transformer output stage (though it still put out something like 1/4W) using a 1:1 audio isolation transformer about as big as my thumbnail. It was partly an experiment to see if I could hear saturation distortion, but I never did find another bigger transformer to A/B test it. I could clearly see the saturation on the scope, though I couldn't identify by listening. It just sounded really mid focused, as was expected with a limited bandwidth OT.

One of the really cool things about low voltage designs is that you can purposefully add in resistance/capacitance to the node feeding the center tap of the OT to control dynamics, without requiring 20W+ resistors. If you bias your amplifier close to cutoff and use a really big dropping resistor, you can get some serious sag when you actually play and it can sound really (artificially) responsive and bigger than it should. As mentioned before, you can then run the OT into a resistive load (or reactive one), then further pass that on to a big bad class-D amplifier or something.

The little jfet amplifier actually sounded pretty good recorded, albeit only putting out about 1/4W. I actually had to cut the bass a little whilst mixing it due to cab resonances(!). The tone is quite mid focused, but that's probably just due to the lack of a tone stack and the crappy OT. https://www.dropbox.com/s/0ppug2zuslk4jp1/dat80s.wav

[jazbo8]

Printer2 - thank you for (re)-starting the thread, it is an interest topic for sure. I don't have much to add now other than to say if we can try to come up with some designs without using SS output stages, which is already done by many of the "tube" modeling amps on the market...

[printer2]

Printer2 - thank you for (re)-starting the thread, it is an interest topic for sure. I don't have much to add now other than to say if we can try to come up with some designs without using SS output stages, which is already done by many of the "tube" modeling amps on the market...

I wanted to use a big TV tube that swings more current but decided to try the 'L6 tubes. I would like to try a transformer with a lower primary impedance to get the power up some more. With the current Hammond 125E and the amp cranked it will get loud enough to drowned out an acoustic guitar. So there is potential here. I was thinking of sending the output from this amp into another to boost the level so I can hear the edge of distortion area to see what tweaking I can do.

I like the idea of the low voltage Marshall but I might drop the CF stage and see how simple I can get the amp, maybe just with a volume and tone control. I know there are people out there who would not mind making a tube amp but they are scared off by the high voltage. And once they get a low voltage amp up and running, well their amp can get louder with some extra voltage and a few more parts, right? By that time we will have them hooked.

[exclamationmark]

Now that you mention it, some sweep tubes from the PL500 family might be a good choice. IIRC a PL500 tube can swing something like 100+mA at around 50v plate + screen. The only annoying thing about these family of tubes is the weird heater voltages and the fact that they nearly all have anode caps. They're not averse to a bit of grid current either Most of these tubes take something like 25ish volts for their heater at 300mA, so you can just wire a couple of them in series (at least in my case, where I feed everything off the SMPS). They're dirt cheap as well.

Another advantage is that you can probably just use a cheap transformer, because the low effective impedance of these tubes allows you to use something with low primary inductance (power/line transformers and the like).

[printer2]

When we were looking at the idea way back when I found a 25DN6 in my stash of tubes. I have not had any luck finding one locally and while the tube is cheap the shipping makes it a $20 tube so I went with the 'L6 tubes. I Played with the circuit again trying the 70V transformer again. Still low output. When I hooked up the Hammond again for some reason I stepped through the impedances rather than using the 3K. I got an increase using the 6.8k tap. Measured a clean sine wave and I was getting a whopping 0.25W. It is getting pretty close to practice amp status! Tried again with the 70V transformer, really wanted it to work as it can be gotten cheaply, the real OT works better. Might have to revisit the preamp tubes now.

[Tooboob]

Kevin O'Connor has documented the technology you need. He calls it GMX, and he envisions it as a way to make a super-powered tube amp. You'll apply it to an amp that matches up well with O'Connor's power scaling solution.

Your problems are low gm in the power tubes, requiring high drive, and very little ouput power.

Parallel the power tubes with a FET. Drive for the FET gate comes from an opamp. The tube and FET have un-bypassed low value current sense resistors to ground on their cathode and source. The voltages on them drive the opamp (tube current goes to the positive terminal. The tube's gm and the output power are multiplied by the ratio of the sense resistors (there's a +1 in there too). The tubes just see a higher impedance, and control the output as they normally would, with all their lovely non-idealities, and everything is behind the transformer. You can keep your push-pull architecture.

You'll need a single-digit negative bias voltage for the power tubes, and the output of the PI will need to be scaled to the proper level. You'll need a lower ratio output transformer to compensate for the low B+. The tubes will still see a high impedance, since they are only sinking a fraction of the transformer input current.

[printer2]

Sounds kind of complicated for someone starting out. As far as getting more power out of a tube amp I am more of a traditionalist, high voltage, good iron, good tubes.

[jazbo8]

Sounds kind of complicated for someone starting out. As far as getting more power out of a tube amp I am more of a traditionalist, high voltage, good iron, good tubes.

+1, that's how tubes should be used, and again - throwing SS into the mix for this exercise defeats the whole purpose since it has already been done... There are quite a few 12V SE designs at Sopht amps - all tube 12v amps. The soundclips all sounded a bit thin to me... could be just the recording though. Another tube of interest is the 26A7GT, the datasheet shows 0.5W output into 2k running off 26.5V, anyone has tried the tube?

Wattkins has a similar thread going on, and this simple design seems to be meet all the LV safety requirements.

[printer2]

That is what I need to work on with my circuit, sounds thin in the same way.

I am going to try a lower impedance speaker to see if I can get more power. I wonder if biasing the tube positive might work, have not thought it out yet.

[exclamationmark]

I think I saw a thread over at ax84 where there was a low voltage el84 operated at either zero or positive bias. I also did some napkin math and found you can get roughly 3-4W out of a pair of PL500 (though the lack of curves made me extrapolate a bit), or you can even get about 10W from a pair of 6lw6/26lw6 @ 48v. This was without class A2/AB2 operation. AB2 operation may not be a 'safe' idea unless you have a B+ of 30v or lower, due to the high voltage differential between B+ and negative rails, which is near 100v.

Getting the right transformer ratios is somewhat important. Most sweep tubes need extremely low ratio OT's which can be hard to find.

[printer2]

I would like to get up to one watt with the 12L6 tubes, at 50V supply and zero volts on the grid they are suppose to pass 50mA. I read somewhere about injecting current into the grid and running positive, wish I could remember more about it.

[jazbo8]

Found another LV design using the 12AL8, he even made some recordings, unfornatly the link no long works, so no way to judge if it is even worth pursuing...

[jazbo8]

Found another one, still no soundclip...

[jazbo8]

iccaros has a great thread here with many clips - I thought they sounded better than the Sopht's. Too bad he does not seem to hang around the forum anymore...