50L6 guitar amp

[Wavebourn]

Some wasteful but highly effective brute-force hum reduction could be to run the heater string off of DC from a simple zener-referenced mosfet follower type of filter. You'll need a yuuuuge heatsink, but those can be found and/or repurposed easily. Nothing like a pant-load of standing current to keep the mosfet in a happy place. Run enough tubes in series and you'll reduce waste heat too

Or, a MOSFET current source, also to save lives of heaters.

 

[Gnobuddy]

...make sure that the amp chassis was properly grounded via the green wire of the line cord, and a suitable line fuse is used. Make sure that one side of the OPT secondary (black wire) was connected to this ground.

I should have mentioned that I do this with all my valve amp builds.

Using one of those IEC inlet plates that combines fuse, switch, and inlet into one component makes things simpler and safer: 4304.6071 Schurter Inc. | Connectors, Interconnects | DigiKey


-Gnobuddy

Run enough tubes in series and you'll reduce waste heat too

I'm not a fan of linear series regulators (and I know you were at least half joking).

But: four 12AX7 heaters in series adds up to 50 volts (close enough).

48V DC switching power supply modules are readily available and pretty cheap.

Add a second channel using two more 12AX7s to the Super (not so very)Lite, and running all heaters off a DC switcher becomes a quite attractive option. Four 12AX7 heaters all in series, and each 50L6 wired directly across the PSU.

Even simpler, use 25L6s instead of 50L6s, and a 24V DC switcher. That'll work perfectly with the two 12AX7 heaters in series (and, of course, the 25L6's wired individually across the 24V supply.)


-Gnobuddy

 

[Tubelab_com]

But I worry about filaments of 12AX7 in an amp for guitar pickup sensitivity with overdrive. I did not try that, but I am afraid of hum.

Here is the schematic for the little 4 tube amp shown in the previous post. It is a gain MONSTER since the first tube is a pentode with it's plate bootstrapped such that it's plate load is adjustable with the "saturator" pot (1 meg ohm). It is effectively in series with the parallel combination of R6 and R7 at AC or roughly adjustable from 250K to 1.25 MEG. Even a wimpy pentode working into a 1.25 megohm load makes a big bunch of gain (about 1000X). The second stage is an 18FY6 which is an 18 volt 6AV6, which is half of a 12AX7. It alone makes up for the tone control loss with gain left over. Hum isn't the issue, microphonics and hiss are! It took about a dozen input tubes to find a quiet one.

Note the unique power supply. Diodes are cheap, so I used 5. The input comes from a Triad N-68X isolation transformer. It is FWB rectified, but unfiltered, and fed to the series connected heaters. The heater string sees pulsating DC with the same RMS heating power as pure AC, and the power transformer sees a 360 degree conduction angle for this current so it stays happy, due to less peak current. Diode D5 allows for charging C5, the main B+ cap only on peaks and provides a 165 volt B+ voltage. R32 adds some sag. C15 was added later to kill a tiny bit of hum that was present when operating the input stage at maximum gain. It must be rated for 150 volts (preferably 200 volts) for the case of someone like me yanking one of the first two tubes on a live amp.

Note the 2.7K resistor R30. It is in series with the screen grids of the output tubes. This large value limits the screen current when the blonde with the guitar dimes all the knobs and turns all 4 watts into a full metal racket. Up until clipping the screen voltage runs from 160 to 150 volts. Once the amp starts to clip hard the screens suck a bunch of current and the voltage will dive down to as low as 110 volts on a full on feedback induced scream. This drops the preamp voltage reducing the gain and the drive, and allowing for some preamp distortion. The preamp could be fed from the hot side of the screen resistor if this effect is not desired, or the resistor could be split in two and the preamp fed from the middle.

Tweaking the screen resistor, the plate load on the second stage, the cathode and screen resistors on the input stage, and the bias resistors on the mosfet PI controls the entire character of how and when the amp distorts can be changed. The values shown here with these tubes make for an easily controlled "edge" that goes from clean to bluesy, to full metal racket by rolling into the guitar's volume knob.....or switching pickups on a guitar with separate volume knobs like a Strat.

At one time I had this thing stretched out across the workbench for several weeks with pots in place for each of these resistors. I would play with it for a while until I found a combination I liked, then play it a different day in a different mood, or with a different guitar, and start over. This went on for a while until I started taking pots out and was left with only one. It's now the "saturator" knob on the front panel.

I have been dreaming of a new amp with more tonal control like the bench full of pots, only this time they will be resistors and relays, controlled by an Arduino......I did this back in the late 80's with silicon and a PIC chip. That dream quickly became a nightmare, so I am hesitant to go back there just yet.

There is no reason why these concepts couldn't be applied to 12AX7's and 50L6's.

In fact my first pass at this was in the original HBAC and used 12AX7's and 50C5's. It took a different evolutionary path leading to 26AQ6's (UCC85's) and 45B5's (UL84's). It wound up much like the 18watt lite schematic. It' power supply used even more diodes to generate pulsating DC for the heaters, 165 volts for the output screens, and 330 volts for the main B+ producing 20 watts through a tortiodal OPT.

 

[Printer2]

I was using a no-name 100VA, dual 115V secondary toroid for power, with a single winding for the heaters (all strung in series, with the 50L6 filaments on either side of the 12AX7s, and then grounding the center tap of the 12AX7 filaments with them wired 4/5 and pin 9 for 6 volts, sorry if it sounds complicated, as it's not really) and a full wave voltage doubler for around 300V raw supply voltage with the amp idling. The doubler sagged a bit under high power, and was a nice way to go for a cheap build, with similar results to the sag from a tube rectifier.

Sorry that I don't have more details, as it was practically a sweep-the-floor build with parts-box grade parts, and a chassis from inside an old record player. None of the specifics are all that critical

300V would have suited me fine for an answer. I just was wondering if you were running a rectified 120V or higher.

Yesterday I touched 450V by left hand when holding the chassis by the right hand. Through the heart? I did not feel that, but I felt it going through the mouth, because of sour taste.

50L6 are tubes of my choice (I have a stash of 12L6). They are great for hifi amps. Very clean tubes. But I worry about filaments of 12AX7 in an amp for guitar pickup sensitivity with overdrive. I did not try that, but I am afraid of hum.

A little lost are we? Not used to seeing you in the Instrument section. If you are looking for a change of pace we would gladly entertain whatever you cook up for a guitar amp.

Here is the schematic for the little 4 tube amp shown in the previous post.

You always have some goodies in your circuits that make the wheels spinning. I made a Bassman styled combo amp with a 12AU6 on the front end. Maximum gain required you to keep hitting the strings otherwise the tube would result in mechanically feedback.

 

[Tubelab_com]

12AU6 on the front end

The 18FW6 that I used IS a 12AU6 with a 18 volt 100 mA heater. For those that want to tinker with 100 mA series heater strings, there is a 12AX7 with a 100 mA heater. It is the 20EZ7 and it is quite rare. I have two.

Microphonics are the big issue with the extreme gain. It also gets worse with use/abuse. I have learned not to leave the amp on top of the speaker cabinet. I have a good tube in there now where I can dime all knobs, but I have to stay away from the speaker and shut off the AC powered LED shop lights. They emit a high frequency from their power supply that pulsates at a 60 Hz rate. The guitar CABLE picks it up. Even the cable becomes microphonic at full gain. You can hear the "sound" it makes as it hits the floor or gets dragged around.

My big box of 6DG6's arrived yesterday. They are 6.3 volt 1.2 A heater versions of the 50L6. I have zero 50L6's and only 1 good 25L6. I have a receipt for a dozen 12L6's that I purchased before moving twice, but I can't find them now.

 

[Gnobuddy]

dual 115V secondary
<snip>
full wave voltage doubler for around 300V

Got it, and thanks.

Just to make sure I understand: with the trioded connection you described earlier, you were feeding the screen grids (through a 3.3k or 4.7k resistor) three hundred volts?

I've followed all Tubelab George's relatively recent exploits with these and similar little radio tubes, and AFAIK he's always reported that while you can feed 300V, maybe even 400V, to the anodes, the screen grids have to be kept at much lower voltage - almost invariably below 150V. Otherwise things start to blow up. We all know that, other things being the same, it's the screen grid voltage that controls current through a pentode, not the anode voltage.

In a normal pentode output stage it's easy enough to feed the anodes one voltage and the screen grid a much lower voltage, but wired in triode mode you don't have that option, unless I'm missing something?

-Gnobuddy

 

[Tubelab_com]

I've followed all Tubelab George's relatively recent exploits....even 400V, to the anodes, the screen grids have to be kept at much lower voltage

Yes, I have blown up two 6W6's by running them in triode mode on voltages at or above 300 volts. The 6W6 IS RATED for 300 volts in triode for use as a TV vertical sweep amp, which is essentially a class A linear audio amp optimized for one frequency, 50 or 60 Hz. The difference is that the TV set sees full output all the time, there is no volume knob on the vertical sweep. ALL of my failures in triode wired sweep tube amps have occurred at idle, which is worse case for any class A amp.

Now that I have a big box full of expendable tubes, I will revisit the triode mode at higher supply voltages. It may be possible that inclusion of enough resistance in series with the screen is all that's needed to avoid meltdown.

 

[Printer2]

Got it, and thanks.

Just to make sure I understand: with the trioded connection you described earlier, you were feeding the screen grids (through a 3.3k or 4.7k resistor) three hundred volts?

Right. That was the reason I was asking in the first place, in triode. When I read 300V I just thought pentode and it did not seem odd to me.

 

[Gnobuddy]

When I read 300V I just thought pentode and it did not seem odd to me.

I did the same slow double-take. Wait, what? 300V in triode mode? Huh?


-Gnobuddy

 

[Tubelab_com]

The 50L6 is the same tube as the 6W6 except for the heater. Compare the data sheets and curves. I have seen both 12L6 / 12W6 and 25L6 / 25W6 tubes with dual markings. There are no 6 volt or 50 volt dual marked tubes since the 6L6 type number goes with a much larger pentode, and the 50W6 never existed. The 35L6 is a different tube with smaller guts created for radios that needed more than the usual 5 tubes wired in series.

The 6W6 /12W6 / 25W6 has a clearly specified maximum screen voltage of 150 volts and a 10 watt plate dissipation. However, the same document shows a second set of maximum ratings for TV sweep tube use. Here 300 volts in triode connection is stated in the data sheet, but the plate dissipation drops to 7 watts and the screen dissipation drops to 1 watt. The "typical operation" data is at 225 volts.

I have blown up one tube, and sent a second tube into runaway by letting them idle at 300 volts and 10 watts in triode with a 100 ohm resistor in the screen grid connection. Perhaps they will live at 7 watts with the screen resistor selected to keep screen dissipation in spec? I don't know, but I may test some ugly looking tubes. The problem is that it takes a long slow cook to provoke failure, often several months.

 

[Printer2]

25L6's loaded from my experiments at doing a Marshall inspired 50V amp. I guess I grabbed the breadboard to try out another circuit. The L6's are on a 24V transformer, the little guys on 12V. They are 12AU6's, could swap in 6AV6's. Maybe over Christmas.

 

[Lingwendil]

Got it, and thanks.

Just to make sure I understand: with the trioded connection you described earlier, you were feeding the screen grids (through a 3.3k or 4.7k resistor) three hundred volts?
-Gnobuddy

Sorry, late reply, busy weekend


The screen is fed via the plate, with the resistor in series, and the supply voltage is 290~300 or so after a bit of power supply filtering. This in conjunction with cathode bias dropping some voltage means that the overall voltage across the tube is 260~270 volts or so depending on bias and other details. Run them leaner and you'll increase the voltage lost across the cathode resistor.

 

[Gnobuddy]

Sorry, late reply, busy weekend

I hope you've recovered from your weekend. I hate it when your "day of rest" turns out to be busier than your work days!

Thanks for stopping in to answer our questions!


-Gnobuddy

 

[hex69]

Hi Folks!

I was away but not so far, here I am for sharing some results about this lil 50L6 amp!

The original schematic I used can be found HERE, if anyone else is interested by this amp please DO NOT forget, the first 220 UF has been drawn with its POLARITY REVERSED (ref: greinacher doubler circuit), thanks again Circlotron for pointing this out!

Okay, first attempt for me at 'breadboarding' a tube amp, started with a couple of homemade PCB sockets then hooked all the power supply parts and tube heaters, as safely as I was able.

The big resistors in parallel were to simulate about 50ma of DC load at B+, the tubes were 50C5/12au7 with the heaters connected in series, it's always a good thing to keep some bad tubes with defects, HK leak or broken pin.. even if they are not use anymore they could be good candidates for tests, I don't want to messup with the NOS 50L6/12AX7 for now.

Voltages:
Primary: 120 VAC
Secondary:
Unloaded
69.8 VAC -- B+: 189 VDC
Loaded
61.7 VAC -- B+: 121.6 VDC

After running this setup for almost an hour the multi primary tap transformer I was talking about stayed cool all the way so I thought it can be good for this project. The power supply parts have been soldered all together and even if everything's holding quite well that's not an option for me to work such a way at higher voltages.. need a breadboard! Here it is

|
V

As basic as this may seem it took some time to design and build.. 2 identical 12.6VAC transformers are used for the main 6.3 VAC, in respect with the coil polarities I put the primaries in series and secondaries in parallel. Transformers are still mysterious to me, never really sure of the results using the maths, althought they can easily handle 2A of filament current with only a 0.15 voltage drop. I have some other transformers that can be used to provide auxiliary voltages, like 12.6 or 5 VAC, when needed along with the 6.3 VAC, then only a good industrial 600/120 step down transformer (that will be pri/sec reversed then powered by a variac) is missing, I can have them for cheap however still not sure about the VA rating/size I would need, surely don't want it too heavy.. would it be better to use a dual pri/sec 240/120 transformer? Still working on that. BTW Thank you Mr. Tubelab for sharing all this information on your web site, how precious it is for guys like me!

Finally here's the little 50L6 amp breadboarded.. Ohhhh man how better it is to work on this bench! Not perfect and probably never will but for now it's more than enough for my needs.

The good news, it worked on the first try and sounds great, clean until the half then slowly break up in a dirty way, the high/low inputs and the tone control are very effective, and its amazing how HUM free it is, NO HUM at all.. just a 'very' slight 'zzz' when cranked at max, I mean I can barely hear it but maybe that's because of the messy wiring on the breadboard, for sure nothing compared to other similar amps I have, even by tweaking them with isolation transformers, by adding new and/or bigger filter capacitors, 3 prong and clean up the wiring as much as I can..

Voltages (loaded):
Primary: 120 VAC
Secondary: 60 VAC
B+: 122.7 VDC
B++: 117.6 VDC
B+++: 97.7 VDC

50L6
plate: 118 VDC
cathode: 8 VDC across 184 ohm resistor

12AX7
plate 1: 80 VDC
plate 2: 72.4 VDC

Am I correct saying the bias is on the cold side.

Surely needs some tweak because the breakup is little 'confused' when cranked at max, when I hit the strings the volume drops a little then come back, I tried to swap 70V transformer with a true OT (coming from another working 50C5 amp) and its the same, however by dialing back the amp volume just a little it can be corrected, not sure where to look.

Have to setup the mics and take soundtracks.

HUGE THANKS must go out to all of you people, for your time and the good advices!

 

[PRR]

> Am I correct saying the bias is on the cold side.

Assuming 2.5k load(??), and that's all the B+ you have, the 50L6 bias looks appropriate.

You may just have too much gain for the power. But try 47k series "grid stoppers" on various stages.

 

[Gnobuddy]

...from my experiments at doing a Marshall inspired 50V amp...12AU6's, could swap in 6AV6's.

I recently found this: tubebooster - jonandersonmn

There is a schematic and an audio clip. It's a 6AU6 with a 36-volt B+. The audio clip isn't bad-sounding.


-Gnobuddy

 

[hex69]

> Am I correct saying the bias is on the cold side.

Assuming 2.5k load(??), and that's all the B+ you have, the 50L6 bias looks appropriate.

On the 70V line transformer I'm using COM/2.5 wires at the primary.. 5000V/2.5W = 2000 ohms

The other output transformer I tried, which have been taken from a working 50C5 amp, based on the results I got by injecting low AC voltage at primary, I should use a 3.2 ohm speaker to get near 2k5 at primary, just like the little speaker that was originaly connected to it, so by using a 3.8 ohm speaker like I do, I'm a little over 2k5, I guess.

I saw in the datasheet that at such a plate voltage the 50L6 can dissipate as much as 10W, by doing the maths I'm getting a bit over 5W in my setup, and that's why I thought it was biased cold.

From the result I got, the 50L6 would draw 43ma, that's for both the plate and the screen, however since I'm never sure with the maths I connected the meter in series with the plate, then with screen and it's exactly what I got ... 40ma for the plate and 3ma for the screen. Maybe I have some more ma to play with? I dunno it's the first time I deal with this tube.

You may just have too much gain for the power. But try 47k series "grid stoppers" on various stages

I first tried to put a 47k res at grid pins of every stages, didn't change anything, then I thought why not to use a 500K potentiometer as a variable resistor. Now I can hear the differences as I dial up the resistance, I would say that about 200K the blocking distortion goes away, by playing with the knob from 200K to 500K.. Well I would say it's a matter of taste but for sure, the grid stopper really is the trick! I found that at the grid of the second 12AX7 triode to be the best spot.

Thank you very much for the good advice PRR, think I'm gonna have lots of fun with the breadboard

 

[hex69]

 

[Gnobuddy]

...I would say that about 200K the blocking distortion goes away...

I had the same experience. I had to get the grid stoppers well above 100k in my little 6AK6 amp to get blocking distortion under control. I think I ended up with 120k.

For what it's worth, these small power pentodes and beam tetrodes only have a few picofarads of input capacitance, and 120k (or 200k in your case) should be perfectly fine with this. With 200k and, say, 10 pF of input capacitance, treble roll off only starts at 80 kHz - easily ten times more bandwidth than we need for guitar.

It's a different story with triodes, which can easily have have ten times the input capacitance. One has to watch the value of the grid stopper with those.


-Gnobuddy

 

[PRR]

When the series resistor is 1/2 the to-ground resistor, grid blocking is minimized. So 100K-250K is quite reasonable for typical grid-ground resistances.

As Gno says, this rolls-off treble, and often not enough to hurt guitar, and will take the edge off of distortion from earlier stages.

hex69 suggests a resistor box, right concept, but high-gain wires run out and back tend to pick up radio and squeals. A pot on short wires may be better. I have not done this in decades.... a bag of misc resistors and a hot iron can try many values in short time.