GU50 in a bass or guitar amp?

[imaradiostar]

I've searched here and on google- does anyone know of any practical examples of GU50 pentodes being used to make big power, as in 100 watts per pair? I find lots of examples of hifi use and people trying to make 10-30 really pretty watts but that seems to fall so far short of what these suckers should be capable of. I'm too poor to buy a bunch of KT88's so these things seem like an interesting option.

jamie

[Steve Conner]

You mean the big things with metal lids that look like coffee pots?

No, I don't know of any examples of MI use, but they should surely work great, it's just a question of choosing the right screen voltage.

Edit: The datasheet: http://www.mif.pg.gda.pl/homepages/f...043/l/LS50.pdf

You can see that for Class-B push-pull (Gegentakt-B-Betrieb in German ) they recommend 300V screen voltage, 1kV plate voltage, 25mA per tube idle current, and 90mA per tube full power. They have less than half the transconductance of 6550s, KT88s or EL34s, so they need a higher impedance load, hence a higher supply voltage to make the power.

[imaradiostar]

Thanks Steve. I had a hunch I'd land at around 800 volts on the plates. I have a few transformers from tube scopes around that are rather large- around 15 pounds of iron and copper. I have some similarly large 4k:4/8/16 potted output transformers. I was originally planning to parallel up some EL34's but I figured four GU50's into 4k with between 600 and 1000 volts wouldn't even be working hard.

I wonder how durable the screens are. Do you think I could get away with running the screens at half of plate voltage with a doubler or a split supply? I'm picturing screens at 350 to 400 volts and plates at twice that.

The other though is to use an antek inc toroid with the screens at about 320 and plates at 640- probably not the best option for power but I could stand to parallel up the valves and get a solid 150 to 200 watts without the tubes working hard.

All speculation, of course.

jamie

[GainFreak]

You should search russian sites and forums. They are more familiar with this tube. Just google it in russian (ГУ-50). Also you can add to it the word "усилитель" which means amplifier.

[imaradiostar]

Thanks for the tips! I added "бас-гитара" (bass guitar according to google) and it helped get more results that apply to musical instrument amps.

I'm finding a lot of people that say these things are remarkably similar to 807's or 6146's or 1625's. I seem to recall having some 1625's. Maybe I'll have to experiment with them.

jamie

[Steve Conner]

Hmm... Yes, the 6146 might not be too far off as a "modern" data point, except that the GU50 has almost twice the plate dissipation, but not much more cathode current or screen dissipation capability.

Maybe you could take some inspiration from the original 6146 version of the Ampeg SVT.

[imaradiostar]

Hmm... Yes, the 6146 might not be too far off as a "modern" data point, except that the GU50 has almost twice the plate dissipation, but not much more cathode current or screen dissipation capability.

Maybe you could take some inspiration from the original 6146 version of the Ampeg SVT.

that's more or less where I was headed. I was thinking a quad of them should make more than enough output. I may have to get creative and stack secondary windings to get the voltages I want but I'm sure it'll work out fine.

jamie

[mrbthree]

Anything new happening with the GU-50 bass amp?

[imaradiostar]

No forward movement- too many other projects. I intend to do something one day though.

jamie

[thebasher]

Real bass amps have tubes – semiconductors are musical director trainees

I have seen in this and other fora queries as to who has any experience with the Russian GU50 power pentode for instrument amplifiers. I do.

GU50 is rugged, powerful, forgiving and has a very long life. It is a copy of the famous Telefunken LS50 developed during WW2 as power output stage for tank transmitters and mechanically designed for that extremely hostile environment.
The Russian copy, manufactured for the very same purpose, is reputed to be superior to the original but I cannot confirm it or the opposite since I never laid hands on an LS50.

All I can say is: Why spend a zillion on a NOS KT-88 or 6550 when you can use GU50 at $3.50 each (100 off price from a Russian wholesaler)? GU50 is close enough to both types to drop in as a more or less direct replacement. However, they do have one weakness: You shall not exceed 300-320V on the screen grid g². Consequently they cannot be run as triodes (as can 6550 and KT88) and in ultra-linear mode only with special precautions.

There is an ample supply of these military grade beauties available. The socket is something else, but these too are available at $7-10 for a military type. One thing is for sure: Once inserted it takes brute force to retract it.
In other words: GU50 is the ideal tube for road use. 6L6’s and 6550’s have been known to crack their sockets when the gear is bumped around and do flashovers between the pins. Never happens with GU50.

I am currently re-assembling a monster bass amplifier using 6 x GU50 as power tubes under the project name Continuo - from 'Basso Continuo' and the fact, that even if this is the 3rd issue it is undergoing continuous improvements èn route.

The anode voltage is a lot lower than the max. value (1000-1200V) but I found, that running six tubes at a cosy and comfortable 560V – idling, really – produced some 265W as opposed to the theoretical 300W RMS i.e. 0.6dB lower output power.

Re-assembling? Yes – the first design was done on an aluminium chassis, but the transformers were so heavy they buckled the chassis and finally the OPT ripped the fastening bolts through the top during a road trip causing twelve kinds of mayhem on its way. The new design is done in 1.2mm reinforced steel with thick brass rod supports and mounted in a commercially available really rugged 19” case and is not likely to fall apart like the first model did. The total weight of the amp is some 75 pounds but it is worth every ounce. Don’t tell our roadie I said so.

You may ask: When GU50’s are so cheap why not 8 x GU50 in PPP instead of 6?
Elementary, dear Watson: 6 tubes will fit nicely across a 19” rack chassis whereas 8 will not. And with good speakers (I use 6 x FANE Crescendo 10" + a horn) you will have all the dB's you need.

I am attaching the schematics for the power stage and PSU of the amp. Connoisseurs will notice some resemblance to the Ampeg SVT – guilty as charged. I call it inspiration. The output transformer was bought as NOS on eBay for app. $300. It is the original transformer for the Fender 300W amp employing 6 x 6550 with 6L6 driving an intermediate transformer. I bought this from Rissom (look him up on eBay) and he may still have some left.
Else a similar transformer is available from Hammond (1.65K primary, 4 + 4 ohms secondary) at a reasonable price - as ‘reasonable’ goes. This is not a cheapo project. If that is what you are looking for buy a Chinese CrappoDeLuxe with Coughing Dragon tubes in it.

As you will notice there is a reference to a preamplifier on the attached circuit diagram. All data, schematics, pictures and comments plus some other designs using GU50’s will soon be available on my website, Bergqvist Family Homepage.
Comments and suggestions are welcome.
300W.PDF

[Ronsonic]

Basher, that is serious looking. I like the alternative tube coolness and stealing ideas from the SVT is only a good idea.

Well done.

Would love to see pic's of the build.

Thanks for sharing.

[imaradiostar]

Awesome build. Only problem is it's not wired for 120 volt operation!

Have you had any problems running 300+ volts on G2?

It would seem the GU50 is at its best with higher plate voltages. One day I'll try them. I bought some KT120's but don't have output transformers large enough to take advantage of them.

jamie

[thebasher]

No problem wiring it for 115VAC. The transformers will do both 230 and 115. Doesn't take much to rewire them. The next build will end up in Florida with my nephew anyway - hence the double windings on all the primaries and the 115VAC fan.

And yes, exceeding app. 300-330V on g2 draws a lot of current on the screen grid and the tube lights up nicely (attempted SE triode mode with a 600V supply - not recommended). The g2 supply must be reasonably steady and should never be taken from the Va supply via a resistor. The g2 current drain varies from 0.9 to 10 mA and supply must be stabilized.

However - and here is where the ruggedness comes in - the tube measured 100% even after the abuse. This tube was made for Soviet grunts to mistreat it and still keep working.

GU50 is closer to the KT90 than to KT88 but will of course not match KT120 according to the spec sheet - but then throw in a few more GU50's. At $90-100 for a Tungsol KT120 there is a long way to go from $3.50 for a GU50.

Drawback with the GU50 is, that it was only designed to run as a pentode - not triode mode, not ultra-linear. And the filament voltage is 12.6V which can be a little awkward sometimes. You can push the tube very hard with anode voltages like 1kV and a g2 voltage at 300V max. and easily get 120W at 800V or 150W at 1kV out of a pair. The tube will sustain it for eternity even if it is beyond the specs.

However, 120W compared to 100W is only 0.8dB more power - you will not be able to tell the difference, so in the current design I decided it was not worth the trouble using very high voltages. Remember: Just to notice an increase in sound level requires 3dB additional power which means doubling the power output. Better use bigger magnets on the speakers if you are short of SPL.

Still, the present 560V 1A supply will definitely kill you.

With the actual bass amp design I cheated a little: The 8 ohms output is loaded with a 5.33 ohms speaker array (6 x 8 ohms units in serial/parallel) in order to reduce the primary inductance of the OPT, thus drawing more current at a lower anode voltage and still obtain the 265-300W as a result.

1kV+ is cumbersome and dangerous to deal with, capacitors get very bulky if at all available, suitable power trannies are hard to come by and the tubes don't mind the extra current anyway. Reducing the primary by loading the secondary and the other way around is the oldest trick in the book.

Basher

[imaradiostar]

I was kidding about the voltage- that is only a minor concern and easily fixed.

You're totally right on output power. I really only need around 125-150 watts for any bass gigs I'll ever do. I've gigged with far less.

I've been planning to use (but haven't yet ordered) a large Edcor transformer with to use with an impedance mismatch with my KT120's. I have a few old oscilloscope power transformers with a number of lower voltage windings that can be rectified, filtered and stacked in various ways to get voltages between 600 and 1000 volts. Along the way I get screen taps of 250 to 400 volts depending on how I arrange things. The oscilloscope transformers have a lot of 6 volt windings that can be connected in series for 12 volt filaments so I've not abandoned the idea of using GU-50's.

I'm glad to see the GU-50's being used- one day I'll build an amp for them.

I'm confused- how are you generating bias voltage?

It looks like you're using about 100 volts peak to peak to drive the outputs to full power- is that correct?

What is the minimum grid leak spec for a GU-50? If I'm only driving a pair of them I'd imagine I won't need as substantial a grid driver.

jamie

[thebasher]

Hi Jamie.

I have been looking for old Tektronix power transformers on eBay myself. Unmatched quality and very conservatively rated. Good idea, but the good ones usually disappear before I manage to respond. Must be other tube afficinados out there.

I understand your confusion as to how the bias voltage is generated: There is a link missing: The VAC off-page connector should be connected to the yellow wire on one of the 400V windings. Stupid omission - must update. C19 is used as a drop resistor (reactance) and DC trap biasing the regulator tube sufficiently to ensure a stable bias. One might alternatively use stacked zeners but I happened to have a pile of OD3's around. Looks better too: The optical signal-to-noise ratio is equally important.

100V p-p driver voltage is about right: The GU50's are not quite as steep as f.ex. KT88. I primarily use the cathode follower to negate the combined Miller effect from the 2 x 3 grids in parallel. The voltage swing is more than adequate for driving the PA tubes and ensures very good transient handling as opposed to the classic 6SN7 anode-fed driver stage with 47k anode resistors.

I believe grid leak current is about 0.15mA i.e. you should not exceed 300K resistors for self-biasing. Grid current at full power is about 13mA per tube, so even ½ a 12AU7 would easily drive a single GU50. I admit the driver circuit employed is belt and suspenders but I did not set out to minimize component count or cost. There is much to say for using two ECC82/12AU7 in parallel instead of the single 6SN7. I have done so in the past with 807's in PPP with subjectively good results.

However, the input capacitance of each GU50 is ~14pF so be wary of RC constants with high feeding impedance. Driving GU50 as a pentode evidently enhances the Miller effect.

This circuit could be reduced to 2 output tubes and then run at 800V anode voltage and make a very good HiFi mono block too. The phase splitter circut works very well (thanks, Ampeg), a lotter better than the 'usual suspects.'

Basher