Mikeb
10 posts
Jun 04, 2014
2:05 AM
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Sorry I posted this a few hours ago but then realised the subject line was incomplete
Hello tech gurus. My '59 Bassman RI was made in 2000 and a couple of weeks ago it has developed a nasty habit. When I play a high note I can hear another tone at the same time which is approximately one octave lower. It almost sounds like a built-in octave generator, except the tones are not exactly an octave apart so it actually sounds quite discordant
The "ghost note" seems to kick in about half a second after the main note. I can make it softer by turning the treble way down, but the amp is still obviously doing something it's not supposed to do
I've replaced all the tubes one by one and no luck. I've disconnected each of the 4 speakers one by one and the ghost note is still there. I've used different inputs on the amp, I've played through different mics, I've even plugged it into a different wall outlet. No luck
Some guys on harp-l have suggested that it might be bad power caps. But I took it to my amp guy today and he said he thinks it doesn't really sound like that, and he's concerned that changing the caps might not fix the problem. But then again he's not a harp guy so maybe it's just something he's never come across before. And he certainly doesn't have any alternative suggestions
So does anyone have any advice/recommendations/suggestions before I commit to spending a fair bit of money pulling it apart and replacing the caps in the hope that the problem will go away?
Jason from Squeal Killer has suggested that while the amp is being worked on I should also reduce the filtering in the preamp filter cap from 44uF to 8uF, or something like that. Does that sound like a good idea?
Any advice would be much appreciated, as I'm totally clueless about this stuff
Cheers
Mike
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5F6H
1782 posts
Jun 04, 2014
4:34 AM
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I'm not being funny, but do you hear this ghost note at the same frequency with other harps? Record yourself playing the same notes acoustically, is it still there? Before you do anything to the amp eliminate the possibility that it's somehing before the amp.
If you are then sure that it is definitely the amp, can you record it?
If it is something failing in the circuit (cap/resistor), changing tubes & speakers won't make any difference. My first ports of call would be R14, R28 & R29. But checking these out means probing high vdc (potentialy lethal), best left to a tech. He needs to take into account any preamp tube subs as these will throw out the schematic voltages.
Are there any other changes to the sound, generally, that you have noticed (assuming this is a new issue)?
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www.myspace.com/markburness
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Mikeb
11 posts
Jun 04, 2014
7:42 AM
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Thanks for the reply, and the answer is yes, the same thing happens if I play the same note on a different harp. So I'm as sure as I can be that it's the amp and not the harp or player.
I can't really identify any other general changes to the sound. I'm now using another amp so I guess I've stopped listening now :)
Thanks for the tip re R14, R28 and R29, I will tell the amp tech (don't worry I'm not going to test them myself!). And I haven't done any preamp tube subs, now that I'm using a Squeal Killer I've gone back to all 12AX7's
Cheers
Mike
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Greg Heumann
2734 posts
Jun 04, 2014
7:53 AM
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Also want to be sure - you're playing a clean single note, right? Any time two or more notes sound there will be "difference tones" down an octave or so..... they are real and they're coming from the harp
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***************************************************
/Greg
BlowsMeAway Productions
See my Customer Mics album on Facebook
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BluesJacketman
145 posts
Jun 04, 2014
8:51 AM
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My Pro Junior did that when it needed a new tube.
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Albey
42 posts
Jun 04, 2014
1:24 PM
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I have a tweed Tremolux that did the same thing! I described it as playing along with itself out of tune. I played it for my tech and he said he thought it was caps leaking. This was along time ago but he found the problem and fixed it.
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Thanks,
Albey Scholl
The Shadows
Blind Willies Blues Club
Atlanta, GA
http://www.blindwilliesblues.com/
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dougharps
632 posts
Jun 04, 2014
2:43 PM
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1) microphonic tube(s)? Have you swapped them ALL out at the same time?
2) mechanical resonance??? Have you tried this with the amp in different rooms? Sometimes something in a room can resonate. How about trying it with the amp chassis separated from the cabinet?
3) drawing too much current? I have had odd tones (not cone cry) when driving too much bass with my 2x6L6 Stromberg-Carlson head. I get sag, and beyond that, some odd pitch at the same time. If I cut back bass some, it goes away. It seems to be a case of trying to drive the bass louder than the amp can manage.
Does it do it without the Squeal Killer?
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Doug S.
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tmf714
2550 posts
Jun 04, 2014
3:00 PM
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Its the caps more than likely-I had it happen with my RI as well-I replaced the caps and the ghosting went away-
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Mikeb
12 posts
Jun 04, 2014
5:20 PM
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Thanks everyone, just woke up and great to see all these replies! Answers:
Greg H, yes it is happening when I play a single clean note
Albey, "playing along with itself out of tune" is a good description. But only on high notes
Dougharps, no I haven't tried swapping all the tubes at once, only one at a time. I've tried it in different rooms and the problem is still there. I haven't tried separating the chassis from the cab, sounds a bit difficult to me. And yes it happens both with and without the Squeal Killer
tmf714, I hope I get the same result as you!
Cheers
Mike
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Moon Cat
412 posts
Jun 04, 2014
5:22 PM
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Ive had this issue! It can be a simple the swap but can also result from putting pedals on top your amp!
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www.mooncat.org
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Barley Nectar
404 posts
Jun 04, 2014
5:24 PM
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Check all the fasteners holding the boards. A loose screw may cause this. Might be a loose speaker also. It may make sound weather it is hooked up or not due to cab vibrations. Electronic parts can become microphonic. Might be a PITA to locate. The number 1 problem with PCB Fenders is bad solder joints. Reflow anything that looks suspect. Let us know what you find. Good Luck...BN
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Mikeb
13 posts
Jun 04, 2014
6:08 PM
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Thanks Jason. Tube swaps haven't worked and there are no pedals on top of the amp
Thanks also BN for the suggestions. I hope your warning that it might be a PITA to find doesn't turn out to be true!
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