Ampeg B-18N Portaflex Question

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Ampeg B-18N Portaflex Question

Postby ZONBASS on Wed Feb 08, 2012 5:08 pm

Just a sound question on this amp (1967 vintage). I recently purchased it, looks very well cared for, installed all new tubes (matched power tubes) tightened up the speaker jacks,etc. The volume treble and bass pots are all quiet, not scratchy at all for a '67 amp...The speaker has been replaced, looks like a quality 200 watt 18" speaker....Tested the speaker using a newer amp, works perfect. My question is concerning the distortion level, from 0 volume up to 12 o'clock the amp sounds absolutely perfect, especially using a Fender P bass straight in, no preamps or FX....When I crank it up somewhere between 1-4 pm, I get the tube amp distortion that most of us dig...however at some point in those levels, I also get the distortion from the speaker, as if certain notes (A, F) cause the speaker to go into wobble mode, and those notes don't only sound distorted they sound like two or more notes going on at the same time cuz the speaker is bouncing like crazy.

The output tubes are glowing properly, not overheating, and the output transformer is cool to the touch. Power transformer just slightly warm.

I tried the amp on a different speaker combination and pretty much get the same effect. Of course if you back off the treble and bass controls on the amp, you can bring up the volume a bit more before going into speaker spazzing.

I love the amp at the lower levels, am I really just expecting too much from a 60 watt amp? Or does it sound like something not happening properly? Not really expecting to knock down the walls with it, I have other amps for that purpose, but I would like to get enough out of it to keep up in a studio situation...again, maybe I'm thinking too big....
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Re: Ampeg B-18N Portaflex Question

Postby PeterS on Wed Feb 08, 2012 7:39 pm

A couple of possible issues, one is power tubes, if you used new production tubes maybe they can't handle the high voltages in that amp. Not convinced that's important, it might be more an issue for reliability than sound quality.

The speaker cabinet is ported... the ported design is usually matched to a particular speaker, if you change the speaker (as has been done) the tuning of the port is not as it should be so you can end up with a bigger resonant peak than is desireable. Also many modern speakers are designed for sealed box only, and they are too floppy to use in a ported box...
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Re: Ampeg B-18N Portaflex Question

Postby Baron Von Machinenmann on Wed Feb 08, 2012 10:03 pm

Another possible issue is weak filter caps not providing the proper power to the OT. Does it hum at all when you are not playing even if the volume is down?
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Re: Ampeg B-18N Portaflex Question

Postby ZONBASS on Thu Feb 09, 2012 2:10 pm

Well, the speaker theory sounds interesting......maybe I'll try it on my 4-10" bottom and see how it sounds....
And, it does not hum much at all in the lower (6am to 12pm volumes) She does hum nicely once you approach 3pm settings...

If it was the filter caps, would it hum all the time? I also turned the hum balance just to see that it works, and it basically goes from no/little hum to good hum twice for the full rotation. So I just put it back to the low/no hum setting.

I also have an old West tube amp, (little known but an awesome amp) and I seem to recall that amp doing the same thing years ago. I had the amp re-done by the guy who built it (Dave West) and after that fix, it is the cleanest sounding tube amp you have ever heard. Sure if you push it it will give you the RIGHT distortion but believe me the walls shake in my house severely before you ever reach that point.
I'm trying not to compare them both based on that but the speaker bounce thing can really annoy you when you play an A and it sounds like you are getting a B flat mixed in with it.. and lucky for me I have enuff other gear that I can eliminate many of the other variables...

I'm also thinking that the output tubes are not the issue because I had noted the same thing with the old (date unknown) tubes.

hmmmmm....
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Re: Ampeg B-18N Portaflex Question

Postby ZONBASS on Thu Feb 23, 2012 5:27 pm

Well, Baron was on to the right path. turns out the power supply was missing parts and was mis-wired. Filter caps mis-wired by a previous owner.....a few new parts and the wiring as per the proper diagram and much better now!
The speaker was 100% as was the enclosure......
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Re: Ampeg B-18N Portaflex Question

Postby Baron Von Machinenmann on Thu Feb 23, 2012 11:15 pm

Got pics? :thumbsup: :cheers:
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