The 6550 Experience

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

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niknik
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Re: The 6550 Experience

Post by niknik » Tue Apr 03, 2018 12:00 pm

I finally found all the parts to perform the mosfet screen regulator for the Major amp, and I have a little doubt about the capacitor in parallel with the 100m res, in the lower right of the schematic posted by Dave.
Is it an electrolytic cap? The value is .5uf?
Sammy who?!

daveweyer
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Re: The 6550 Experience

Post by daveweyer » Tue Apr 03, 2018 9:40 pm

That's a film cap. Be sure to use a film cap there because electrolytics have too much leakage. The impedance there is extremely high--the 100 meg just slowly leaks the charge off the capacitor if nothing else does, because you can't leave a high voltage on the gate of a mosfet when everything else has gone back to zero. The gate is protected by the zener from the source, but the drain can do whatever it wants. Anyway, yes, a .5 film cap. That, and the series resistor set the length of time it takes to charge the gate to 408 volts.

niknik
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Re: The 6550 Experience

Post by niknik » Wed Apr 04, 2018 9:40 am

Thanks Dave,
I'm gone with a .47uf film cap and all seems good! With 530v b+ I'm measuring 394v at the + side of the 20uf electrolytic :thumbsup:
Now I'm going to to disconnect UL tap from OT, and connect screens to this new supply.
when done, I let you know how this first mod affects the tone of the amp.
I also ordered a kt120 pair, and next step will be to change the OT. I have got a spare 100w 2k2 Marstran OT ready.
Thanks! :D
Sammy who?!

daveweyer
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Re: The 6550 Experience

Post by daveweyer » Wed Apr 04, 2018 6:46 pm

Is there any way you can share the sound of the new unit with the forum?

niknik
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Re: The 6550 Experience

Post by niknik » Wed Apr 04, 2018 7:21 pm

Ok, I did the first test with the regulated screen supply mod, but here in Italy is late night, and I run the Major heavily attenuated by a thd hotplate, so probably this test is not so reliable. Tomorrow will be better.
First impression is pretty good, the amp now seems less clean, a little more "Marshall" then before.
In the high range of dynamic playing, the amp is pretty good, but a strange issue occurs when the playing is soft or when a note fades. The effect is like when you turn off an amp when still playing, the notes die suddendly with an innatural tone.
When I'm playing hard this issue goes away, it appears only when I'm playing soft notes.
hope you understand what I mean, my english is not good. :oops:
I think it is normal at this stage of the work.
I think that before changing OT and power tubes, I have to upgrade the driver section, and go with a 12at7 and a 5687, is this correct Dave? Is this the problem, meaning the power stage now is not adequately driven?
And yes, of course, when the amp will be in order, I'll be happy to post some clips :D
Sammy who?!

Tek465b
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Re: The 6550 Experience

Post by Tek465b » Wed Apr 04, 2018 7:40 pm

Did you re-bias after changing the screen supply voltage? that could be the problem.

daveweyer
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Re: The 6550 Experience

Post by daveweyer » Wed Apr 04, 2018 8:38 pm

Tek has that right, you now have too much negative bias and are in full class B mode, which means cutoff at low levels. Adjust the bias for maybe 15 to 20 ma of idle current per tube and get back to us.

niknik
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Re: The 6550 Experience

Post by niknik » Wed Apr 04, 2018 8:44 pm

Ah, ok
Tomorrow I'll check the bias, and maybe I'll swap two tube.. now I'm going with four kt88, probably too much with this PT
Thanks, I let you know :thumbsup:
Sammy who?!

niknik
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Re: The 6550 Experience

Post by niknik » Thu Apr 05, 2018 8:13 am

Yes, the bias was very very low!
I changed some res in bias supply, and pulled out two power tubes.
Now all seems ok, no cutoff when playing soft.
The amp now is way better, it is good like before in the lower range of volume pot, meaning clean tone is very big and tight but not sterile or cold.
The positive difference is in the high range of the volume, now the amp crunches like classic Marshalls (stock Majors are very clean amps, low gain), and also is very silent, zero hum!
Wow, and this is just the beginning!
B+ is 552v,
screen supply is 397v (maybe I could raise a little this),
Bias is -52v, with about 20ma in pins 1/8 of tubes.
OT is an original Dynaco A-431, and I'm going with two KT88s.
I am still waiting for KT120s, so next step will be the driver mod, changing the 12au7 with a 12at7 and a 5687, and I think I have to swap the primary of the OT with this mod, is this correct?
Thanx Dave, and thanx to you all :D
Sammy who?!

niknik
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Re: The 6550 Experience

Post by niknik » Sun Apr 08, 2018 1:46 pm

Hello guys,
I just performed the driver mod in my Major, I changed the 12au7 with a 12at7 and a 5687, and I have to say this is a great improvement! The tone is big and tight, focused but not cold or sterile, every mod in Dave's schematic really is enhancing my amp tone!
Also I disconnected presence control, and connected negative feedback to the 12at7, between 1k8 and 470ohms resistors, in the cathode section of the first triode.
Now eq controls are way more effective, like Dave said, negative feedback in the preamp section mess up the tone of the Major.
I would like proceed with the 6kz8 tube mod, and I have some questions about:
Can I leave out the rfc, which I can't obtain for now, and connect pin 7 directly to 150k res, Or I'm wrong?
In the schematic, this tube is connected to wave compressor module, but for now can I connect the cathode follower directly to the tone stack?
This is an awesome project! :D
Sammy who?!

daveweyer
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Re: The 6550 Experience

Post by daveweyer » Sun Apr 08, 2018 5:10 pm

You won't need the 6KZ8 unless you do a wave compressor; it will raise the gain of the circuit too high, the whole purpose of it was to get enough gain to drive the compressor, which can drop the AC voltage in the circuit back to a couple volts. Using the 6KZ8 without a choke on the screen will probably make a little RF transmitter out of it, it tends to oscillate.

niknik
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Re: The 6550 Experience

Post by niknik » Sun Apr 08, 2018 6:03 pm

Thanx Dave,
I'm really into this really exciting project!
May you tell us more about this wave compressor module? In the video you posted, looks like it is a custom device, I think it is your own design, so I would like to know how or where I could buy one of this. I hope this is possible :D
For the time being, I could try to arrange the second triode of 12ax7 v2 left free, like a classic cathode follower, to drive the tone stack, and see what happens.
Sammy who?!

daveweyer
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Re: The 6550 Experience

Post by daveweyer » Mon Apr 09, 2018 1:11 pm

You could do that, or maybe use the extra stage as a cascade; use two 12AX7 input stages in series to get a lot of preamp gain. Then get your master volume control in there somewhere to control it.

niknik
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Re: The 6550 Experience

Post by niknik » Mon Apr 09, 2018 6:13 pm

Ok, I will try first the cathode follower arrangiament, so basically the preamp will be almost identical to a stock super lead or bass circuit, up to the tone stack.
Thanx Dave for all the awesome lessons you've shared with this forum, it is priceless to me.
Please excuse me if I have been so tedious about asking about wave compressor module, I just wanted to know more, 'cause I think your amp in your video is really amazing, a perfect amp for me, and I wish my amp could go near to yours. No problem if this device will remain not disclosed in this place. Again sir, I have to thank you for all.
And also thanks to you all guys, for technical support and advices.
As soon as I hope I can return the favours :D
Sammy who?!

Tek465b
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Re: The 6550 Experience

Post by Tek465b » Mon Apr 09, 2018 8:36 pm

Am probably wrong on this one, but i think that by wave compressor he mean an audio dynamic range compressor/limiter?
https://en.wikipedia.org/wiki/Dynamic_range_compression

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