The 6550 Experience

His guitar slung across his back, his dusty boots is his cadillac.

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daveweyer
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Re: The 6550 Experience

Post by daveweyer » Wed Feb 07, 2018 2:10 pm

Roe, the 2K2 OT will work great, it would really be super with KT120s because of their cathode current capability.
The Major I just did has about a 2K2 primary winding, and I found it would crank out almost 200 watts with plate voltage in the 550 range--you should come close at 530 volts, if the PT regulation is good. Some are, some aren't, you have to load them and test them to see the percentage of drop at rated power. I just use a resistor bank, simple and quick.

I got the KT120s for $30 each on ebay, that seemed like a very reasonable price--they were new, matched at 8000gm, and in boxes. They were made in 2017 in Russia of course.

You don't have to use 500uf caps as the main PS caps, I just had a bunch here from other jobs. I would go as high as possible though, it really helps keep that ghosting hum out of the amp at high volumes.

niknik
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Re: The 6550 Experience

Post by niknik » Wed Feb 07, 2018 6:35 pm

Ok Dave, thanks!
I will try to find a 650ohm 25w res, or put some 10w in parallel to get the value.
I also have got somewhere a marstran 100w 2k2 ot, so I could try a kt120 quad when all will be ok :D
Thanks
Sammy who?!

niknik
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Re: The 6550 Experience

Post by niknik » Fri Feb 09, 2018 9:28 am

Dave,
In the Major screen supply, is the 27ohm after the mosfet, a fusible resistor? Is the wattage of this res important?
Thanks, and please excuse me if I am tedious, I'm really excited about this project, and I want to be sure to do things correctly.
Thanks again
Sammy who?!

daveweyer
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Re: The 6550 Experience

Post by daveweyer » Fri Feb 09, 2018 1:17 pm

The 27 ohm 1/2 watt resistor on the source of the HV mosfet has two purposes; it acts like a fuse, and will blow if there is a short on the output screens somewhere, AND, mosfets tend to oscillate if there is a capacitor directly on the source, and this varies from mosfet to mosfet. You could run the mosfet without a capacitor on the source in most instances, but for screen supplies the mosfet has to be protected from a phenomenon called negative resistance, something that occurs on the screens during switching in which the screens actually become a generator and try to push voltage back into the regulator. The electrolytic absorbs the pulses.
And also remember that a mosfet will oscillate if there is a capacitor directly on the gate, so be sure to use a resistor there--a 150 ohm 1/4 watt works fine in this situation.
Again, it varies from device to device. Be sure to use a 1000 volt mosfet, they are cheap and available at the big electronic suppliers. Try to find a mica insulator, and be very careful in mounting because there will be 600 volts on the drain and you don't want to short that to ground because there was a small filing accidentally on the insulator.

niknik
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Re: The 6550 Experience

Post by niknik » Mon Feb 12, 2018 8:39 am

Yesterday I did some works in my Major.
I installed 4 power tubes, and raised a little main filters.
like Dave said, I was going with 6550s, but unfortunately i found that my quad has one very weak tube :(
So, I went with a mix of kt88s and 6550s, I changed a bit the bias circuit with separate bias regulation, one each tube, so I was able to level the tubes mismatch.
B+ is 530v, bias is -69v, and I'm reading about 42mv each tube with 1ohm cathode res.
At the moment this amp is pretty stock specs, and it is very very loud and very very clean, the low E of my guitar is very big and tight.
Next step will be to disconnect UL wires, and go with the mosfet screen supply.
I ordered some components, but I can't find a 420v zener.. :shrug:
Dave, what diode are you using in your amp?
Thanks
Sammy who?!

Tek465b
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Re: The 6550 Experience

Post by Tek465b » Mon Feb 12, 2018 9:48 am

niknik

You can have Zener in serie(ie: 4x 100volt Z) OR replace the zener with a Vbe multiplier/amplified diode(make sure to use a transistor rated for that voltage!).
You you replace R2 for a potentiometer, now you have adjustable screen voltage ;)
Or use R1= 1M and R2 = 1.66k for 422 volt. Make sure R1 is 1/2 watt resistors.
Image

niknik
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Re: The 6550 Experience

Post by niknik » Mon Feb 12, 2018 11:43 am

Thanks Tek465b!
I miss some electronic basic :oops:
I have got some 100v zener diodes, so I'll go with serial arrangiament :D
P.s. I just received the pcb of Dave fuzz,
Many thanks!!
Sammy who?!

Roe
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Re: The 6550 Experience

Post by Roe » Tue Feb 13, 2018 8:05 am

Thanks, I just received the PCT too - looks great. Please do tell if you know of any suitable transistors that are available
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Super 100 amps: 1202-119 & 1202-84
JTM45 RS OT JTM50 JMP50 1959/2203/34/39

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