2550 Oscilation

MacG
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« on: September 12, 2010, 06:09:07 PM »

Hi guys (first time caller long time listener)
I've just finished my 2550 build and have a bug. In 50 watt mode on the normal channel i get a high pitch squeal if my presence pot is less than halfway. With the presence over half the oscillation goes away. Any tuning tips? Thanks in advance. MacG


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cmoore
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Re: 2550 Oscilation

« Reply #1 on: September 13, 2010, 01:32:34 AM »

Usually, oscillation gets worse when the Presence is increased. Make sure you have the NFB/Presence circuit wired correctly. If it looks like you have done it all proper, I would swap the OT wires on your power tubes. Just unsolder the wires on Pin #3 of your power tubes and swap them, one for the other. Brown goes to Blue, and Blue goes to Brown..... or whatever color you used for the Pin #3 hook up.
Good Luck


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MacG
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Re: 2550 Oscilation

« Reply #2 on: September 13, 2010, 02:22:09 AM »

All seems correct on the feedback circuit. All good with feedback loop unhooked. I'm thinking I need to switch the OT pin 3s also. Sad

Thanks and I'll let you know how this turns out.


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MacG
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Re: 2550 Oscilation

« Reply #3 on: September 14, 2010, 04:23:06 AM »

Swaped my OT leads and that took care of my problem. Thanks Hired Hand.
And Thanks Nik, GREAT STUFF


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cmoore
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Re: 2550 Oscilation

« Reply #4 on: September 15, 2010, 01:24:13 AM »

MacG -
Your pictures made me laugh....!
If I had seen these first, I would not have suggested you check your wiring; I would have just said to swap your OT wires. That is a real square build job! It looks like one of Nik's guys put it together. Very clean, tight, tidy, and neat.
Just curious.... are your heaters green? It is hard to see through all of the FX and speaker jacks, impedance selector and bias jacks, etc.
I am impressed, there is a lot going on in those chassis'. As the old saying goes, It looks like a good neighborhood to live in.
Good Luck


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MacG
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Re: 2550 Oscilation

« Reply #5 on: September 15, 2010, 01:48:44 AM »

Thanks HH.
Yes my heaters are green & most of my Gnds. Used Weber Trannies
Just put in a set of TungSols and had band practice. WOW this amp sounds really good!!!!
Top notch Nik


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MacG
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Re: 2550 Oscilation

« Reply #6 on: September 23, 2010, 11:54:10 PM »

Quote from: ToneQuest on September 23, 2010, 11:04:36 AM

I'm waiting for my 2550 (fully assembled). I'll probably replace the preamp tubes with Tung-Sols, too. For my taste it's the best current production 12AX7. Probably a Sovtek LPS for V3. Have you kept the JJ power tubes?
More importantly: what speaker cab and what speakers do you use?

I like the TungSols, for EL34s I also like Shuguangs. I put a NOS JAN/Phillips 5751 in V2 and it tames things down nicely. This amp has tons of gain. Played through Weber Silvers & EVs. Kinda bright through the EVs. Thumps through the Silver Bells


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