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tube amp damping / weird bass response, speaker movement

Hi folks , I especially wish those members that commented on my other tube changing thread could also come here.

So in general I hear and read that tube amp bass response is not as tight as their semiconductor counterpart assuming both are good build quality.
and even between semiconductor models I have noticed slight difference which is especially audible at lower frequencies , when more powerful amps with higher damping the bass is more tight and feels stronger while I remember some older models especially at louder volume literally moved the speaker to much length but there was not much juice to be felt.

heres my problem , I got a valve amp now and it doesn't feel as strong at the low end , even though it's a pretty strong unit for tubes , 50w per channel.
the Russian priboi.
I'm quite fine with playing more mids through it but something doesn;t give me peace.
whenever I put the volume louder I notice this one thing , at bass kicks the speaker cone vibrates at the bass kick but then after the kick is gone the speaker cone kind of slowly moves a bit out then pulls in and only then rests in the middle position.and this happens everytime on every kick the cone kleeps repeating this thing.
I changed the output tubes from the original 6R3S-1 to GU19-1, adjusted the balance of the idle current so that both tubes would have the same idle current and also adjusted the very idle current itself.
about 40mA per tube.

Please tell me is this a phenomenon of tube amps in general or have I done something wrong , or maybe the idle is too high and the traffo is saturating ? even though since it's a push pull output the idle current would cancel out any static flux saturating the core but who knows.
any ideas?
 

rayma

Member
2011-04-29 8:37 pm
at bass kicks the speaker cone vibrates at the bass kick but then after the kick is gone
the speaker cone kind of slowly moves a bit out then pulls in and only then rests in the
middle position.and this happens everytime on every kick the cone kleeps repeating this thing.

The amplifier might be marginally unstable at low frequencies. You'll need a scope and
signal generator to check further.
 
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Ok folks this might be a bit my own fault , I will check more tomorrow to determine whether it's really so because it's late here and silent.

the thing is I have used this preamplifier for a while since i changed the tubes , it's a behringer " tube-ultragain 1953" I have already done some reading about it and it seems very cheapish and low end quality.
it also seems it could be broken because as I now run the priboi directly from PC soundcard the bass seems more tight and the way it should be.

I got the behringer for free at some old house so as i said will check more tomorrow to see whether the preamp was at fault for spoiling the bass.


P.S. but still I assume tube amps have much lower damping factors than semiconductors due to the very nature of tubes themselves and also due to the fact that most of the design have a transformer at output.would that be correct?
 
ok i tested the priboi with a different preamplifier and it's fine , the behringer was to blame for the distortion.

in fact for a tube amp it pack's quite a punch , the sad thing is it needs a preamplifier because a pc sound card can't swing it up to it's full potential and the bass response is kind of weak.
 
heres my problem , ........
whenever I put the volume louder I notice this one thing , at bass kicks the speaker cone vibrates at the bass kick but then after the kick is gone the speaker cone kind of slowly moves a bit out then pulls in and only then rests in the middle position.and this happens everytime on every kick the cone kleeps repeating this thing.

any ideas?

Classic symptoms of Interstage coupling caps too high in value, also poor B+ decoupling between stages cannot be ruled out......NEED circuit diagram.

richy
 
No, motorboating is when the amp makes a loud sound that's bit like a motor running at high revs.

One thing that comes to mind is that the preamp could have a DC nulling servo at the output, that is not settling very well. For that to end up at the speaker, the power amp would need to have a very good bandwidth, but I guess it could be possible.
 
well it's a tube amplifier , the Russian priboi , but as I was observing the speaker cone it moved fast enough at the bass kicks but then it moved forth and back once until it settled and it happened so slowly , if I had to say that out with frequency it would be no more than 5hz it was really slow.

I too wonder how this could be because it has an output transformer and for 5hz to get through i mean is that even possible?
 

Koonw

Member
2013-04-09 9:37 pm
well it's a tube amplifier , the Russian priboi , but as I was observing the speaker cone it moved fast enough at the bass kicks but then it moved forth and back once until it settled and it happened so slowly , if I had to say that out with frequency it would be no more than 5hz it was really slow.

I too wonder how this could be because it has an output transformer and for 5hz to get through i mean is that even possible?

It's probably not an input signal, but it's oscillation translates into movement of your speaker under highly undamped condition, or simply ringing at very low frequency. Please see page 6 of this article. The problem is that your amp and speaker has to work as a pair, I quoted from page 4:

"From the foregoing we see that speakers vary
greatly in their requirements of source impedance
to critically damp the cone and achieve optimum
speaker performance. It also has been conclusively
shown with laboratory curves that best speaker per-
formance occurs with critical damping. No one value
of amplifier internal impedance can satisfactorily
match all speakers and enclosures. The missing link
has been found in critically damping the speaker."

So if the above critical damping of speaker is not met, you see it's oscillating or ringing, IMO.
 

elektroj

Member
2011-11-18 1:49 am
No, motorboating is when the amp makes a loud sound that's bit like a motor running at high revs.

No, at LOW revs. Quoting the same Wiki article:

'In electronics motorboating is low frequency parasitic oscillation (unwanted cyclic variation of the output voltage) that occurs in audio and radio equipment and often manifests itself as a sound similar to an idling motorboat engine, a "put-put-put.. " (emphasis mine)

'The term motorboating is applied to oscillations whose frequency is below the range of hearing, from 1 to 10 hertz, so the individual oscillations are heard as pulses. Sometimes the oscillations can even be seen visually as the woofer cones in speakers slowly moving in and out'.

Yes, that Priboi is unstable and has a tendency to motorboat. They all do to some extent. I already told the crazymechanic in the other thread that the amp is a dog and needs to be re-designed. (Been there, done that, have a t-shirt);) See the proof below.
Hint - too much OLG, too high T between the stages.
 

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