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beamnet 1st July 2005 03:17 PM

Gu50 output problems
I was building a kt88 50 watt amp.

As my test quad of kt88 turned out to be toast, i needed new valves.

I was repelled by the ugliness of most kt88 and the price of others. (i have little money to spend)

Somebody offered me hit stack of GU50 pentodes for a reasonable price. I am not too bright with tubes yet, but after some advice from others i decided to alter the amp for the tubes

I made a 250Volt power supply for the G2 as the datasheet said.

I powered it up and something strange is happening:

I hooked up a tone generator, and the opt started to sing with the frequency applied...

i measured the current through the tubes, and one tube has a hard time stabilizing the current as the other seems to draw no current at all.

The opt worked fine with the kt88 tubes...

I have a B+ of about 500V, a g2 of 250 volt, i can regulate bias between -18 and -65 volts. The phase splitter is a long tail ecc99.

when i turn down the volume, the singing does too. But then at the background a "tick" apears. The tick varies in frequency when bias voltages are changed.

Does anybody have a clue what happens here?

I checked all the connections a 100 times, checked every solder joint.

i attached some of the datsheet


845 1st July 2005 04:35 PM


i measured the current through the tubes, and one tube has a hard time stabilizing the current as the other seems to draw no current at all.
You have bad (GU50) tubes?

beamnet 1st July 2005 04:55 PM

I really like getting help, but...that's the first thing i checked. I have about 20 tubes and none worked

SY 1st July 2005 07:19 PM

Bas, let's do some troubleshooting. A schematic would be very helpful, but I'll throw a few things out to try in general:

Pull the tubes. Using GREAT CARE, check the voltages at the sockets. Are the output tubes getting the proper bias voltage at their grids?

Disconnect the driver stage. If it's capacitively coupled, pull the caps from the driver end and ground that end of them. The idea is that you want the grids to be at AC ground but to keep the DC bias on the grids. Do things bias up OK with no singing? If so, you've eliminated the output stage and power supply. If nothing biases up properly, you've got some checks to do. Be suspicious of socket contacts.

If everything is OK to this point, verify that the driver stage is still working, then we'll assume you've got some positive feedback happening somewhere.

beamnet 1st July 2005 07:34 PM

2 Attachment(s)

I got it working now

some nasty odds here. I bought a lot of 20 tubes, supposedly good, new ones. As i discovered, some are in terrible shape! I changed the pairs 3 times. 3 times i got 1 bad and one good tube!

Now it is playing relatively good. I suspected some old stock couplin caps, so i changed them wit extremely cheap film caps of which i know the sound suffers terribly. I'll change them back and hope i get some decent sound.

I read somewhere that somebody made these tubes run in ultralinear. What would be the consequence if i hooke the g2 to the ultralinear tap?
I connected the g3 to the cathode.

I'm still very fresh on pentodes, but i'm excited i got them to work! (plus they look ultracool in their cast aluminium enclosures)

made a quick shot with my phone cam


martinab2 2nd July 2005 12:13 PM

What sort of tubes are you using for the first stage? From the pic they looka bit like the 7193/2c22 I'm using in my linestage - my favourite octal triodes :)

beamnet 3rd July 2005 05:57 PM

they are. This is my first amp i built, but i am faily shure these are fantastic matches to the GU50's

I have crappy speakers, the amp is still a little hummy, but i have NEVER heard such a detail in bass. The low end is do defined, that i can actually hear the amount of guitars playing in songs that i thought it was impossible to hear! (deftones)

My triodes are faily microphonic. Do you have an idea what a ood brand is? I ma planning on stocking up on different barnds to try out.


edl 3rd July 2005 07:03 PM

[Here, where I live a NOS GU-50 costs 1USD. Not so expensive...:) ]

beamnet 3rd July 2005 09:04 PM

That sounds cool!

I payd 125 euro's for 20 valves and 4 sockets. No bargain, but a good price concidering the sockets (5 sockets cost about $69 on ebay)

Do they sell the sockets too? I would be interested to buy some more.

beamnet 3rd July 2005 09:22 PM

2 Attachment(s)
By the way, i bodged a schematic together for the interested persons.

This is my personal adaption of a kt88 circuit, please feel free to comment

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