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Old 23rd October 2011, 01:58 PM   #1
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Default 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 θn 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,
Comments and suggestions are welcome.
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Old 25th October 2011, 03:45 PM   #2
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Looks like a nice design! GU-50 tubes sure look good for use in a bass amp
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Old 25th October 2011, 03:51 PM   #3
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Oh this is a lot of fun. I will be looking forward to watching your progress.
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Old 1st April 2012, 09:53 PM   #4
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Default Preamp for the 300W GU50 amp

Hi Juniorjux.

here is the preamp I promised you. I use it myself with a 69 Fender Jazz Bass and it sounds terrific.
My security device for Orcad Design went on strike some time ago so I have not been able to open the files for a while - it has now been remedied and I am publishing the preamp on the thread.

As for 1,000W I would suggest you use 12 x GU50's. The driver circuit from the 300W amp will drive 12 tubes or more without a problem, but you need an anode voltage of 900-1000V to get 1kW out of the thing. Remember - do not exceed 300V on the g2's.

Have a nice one,

The Basher
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File Type: pdf PREAMP.PDF (46.7 KB, 135 views)
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Old 3rd April 2012, 06:49 PM   #5
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The Basher,

do you know that these GU50 tubes do work so much the better, the higher plate voltage you choose? 1000 volts are possible, PP class AB specifications up to 800 V are available on WWW.

Best regards!

Btw, fine design!
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Old 3rd April 2012, 08:28 PM   #6
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Default GU50 anode voltages

In my experience GU50 is very forgiving. I have not used them with Va higher than 900V myself but some of my hamradio friends have used 1200V with no problems for transmitters even if the specs say 1kV max. rating. They drive app 85-90W out of a single tube at 14 MHz!

However, the g2 voltage is critical - *never* ever pass 300V!!! Immediate punishment, red anodes, fireworks and all and the tube is gone. IOW: No ultralinear unless you have a voltage regulator in the g2 circuit. I am currently working on an ultralinear HiFi version using MOS FETS as voltage regulators for g2. Still on the drawing board - so many ideas, so little time.

The Telefunken LS60 specs say, that the tube may be operated in triode mode with a max. anode voltage of 400V but I would not recommend it. A GU50 was built for penthode power at high voltages and should be applied as such.

In my design I use only 560V Va and expect the tubes to last the rest of my life - they are more or less idling at that voltage.

There are a lot of Russian articles around using GU50 (mostly HF stuff) driving them to the limit. Still they last for years and years. I am told, that the Russian GU50 is a lot better and sturdier than the original WW2 Telefunken LS50 they copied it from.

And thanks for the compliment - I can recommend the design even if it weighs 48kg - sounds beautiful with lots of headroom and power.

Someone pointed out the lack of a compressor - some people want it but I do not like it myself.
However, I have done one preamp for a buddy with a tube compressor, based on one 12AT7, and will publish it when I have applied the finishing touches. Attack and decay times are still under consideration.

Connoisseurs will notice, that I use a Baxandall-like tone stack design in the preamps instead of the usual Fender/Marshall/Peavey/HiWatt etc. misconception. I always hatred it and it basically does not work anyway. The mid-control is very useful even for guitar.
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Old 4th April 2012, 04:14 AM   #7
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Hi!
Let me add some comments, since I knwo Gu-50 tube, and first my tube amp I made using exactly Gu-50 tubes in 1974, being teenager. My neighbout found them for me and asked to forget where I got them. They were not for sale then.

Quote:
Originally Posted by thebasher View Post
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.
I did not try LS-50 for myself, but Gu-50 according to factory specs is more fragile. However, I have experience running Gu-50 with red anoder (bias failure problem) for long enough time to smell the problem, but after that tubes continued working as if it never happened to them.

Quote:
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.
No, they are not. They are different tubes and can't be replaced ditectly. Gu-50 wants higher alode voltage, higher load resistance, and higher voltage drive.
And they are so cheap because:
1) They were not advertized in Western world because were used in Cold War on opposite side of the fence, for military only purposes, and
2) because they look lmore like trash cans than like phaiilc symbols.

Quote:
However, they do have one weakness: You shall not exceed 300-320V on the screen grid g2.
They can actually. In triode mode screen grid can go as high as 800V, of corse together with anode. In pentode mode I would not suggest to use higher than 250V on G3, because when anode voltage goes below screen draws too much urrent since it is dencier than in other power pentodes. But you can apply like +12 V on G3, to shift curves a bit to higher currets on lover voltages, instead of increasing G2 voltage.


Quote:
Consequently they cannot be run as triodes (as can 6550 and KT88) and in ultra-linear mode only with special precautions.
In ultraliear mode it will be waste of tubes.


Quote:
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.
Also 6L6 - like tubes have anode near filament on a bakelite socket. A bit higher humidity and you risk to kill your amp, including output transformer. I used to repair some SVTs that were killed such a way.
Gu-50 does not have such problems, and being used in aircrafts and on vehicles it can go on the bumpy road for sure.

Quote:
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.
According to my experience it is impossible in class A or A1. At least 750V is neded, ot you need to drive them by cathode (or source) followers in AB2 region, like Ampeg did in SVT. But in any case, bass guitar amp must be built such a way so no "farting" distortions allowed, so no capacitive coupling from driver directly to grids of output tubes. So with 750-800V on anodes you can get like 360-420W from 6 tubes, with output transformer with primary like 2.5-3K P-P.


Well, congratulatins with your project! For bass guitar Gu-50 is much better tube than 6L6 cones that are awfully non-linear, that's why were so popular for solo guitar amps.
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Old 4th April 2012, 04:22 AM   #8
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Quote:
Originally Posted by thebasher View Post
Someone pointed out the lack of a compressor - some people want it but I do not like it myself.
It is easy, actully: LED in parallel with 100 Ohm resistor goes from screen grid supply voltage stabilizer to screen grids. LED senses screen grid current, and as soon as the tube approaches clipping schreen grid current goes up, LED flashes on LDR in input attenuator, an decreases gain.
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Old 4th April 2012, 04:36 AM   #9
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I openbed the schematic and looked a bit, is it drawing error, looks like R26 has wrong value?

Also it looks like waste of V2a, since 34V RMS is more than enough for EM84, even without voltage doubling rectifier.
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Last edited by Wavebourn; 4th April 2012 at 04:39 AM.
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Old 4th April 2012, 02:14 PM   #10
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Quote:
Originally Posted by thebasher View Post
Connoisseurs will notice, that I use a Baxandall-like tone stack design in the preamps instead of the usual Fender/Marshall/Peavey/HiWatt etc. misconception. I always hatred it and it basically does not work anyway. The mid-control is very useful even for guitar.
Well, this is one point of view. Another point is that if you plug your (bass) guitar into an amp with that Fender tone stack (or with one of it's clones), you can be sure that it always will sound good in any way. No need to jiggle and wiggle around with tone control knobs ;-).

Best regards!
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