SVT2 Pro - pre distorting too early

[babebambi]

Bought the amp pre-owned bout 3 weeks ago.

First week it played beautifully, playing for around 1-2 hr everyday on that amp. Then I got really busy with work, so I didn't touch it for the next two weeks.

Yesterday, turned it on - loud squeal when I take it out of stand-by, mute still on. Go back to stand-by, wait a couple minute then turn off. Let it sit for about 30 min, then try to turn on again. This time no squealing, but had a loud shudder from within the chassis when I take it out of stand-by. Bias light on back seems fine.

Un-mute, play a few notes then I notice immediately that the pre distort a lot earlier. For example, I used to have the gain and drive both at noon, peak light solid on the loudest note and still little to no audible over-drive. No input pad on the amp.

But now with the same bass, same setting on that bass, and I have to back the gain down to pretty much no peak light to avoid any OD. Just to give a better idea of the input level difference required to get the same clean input, I have to use the pad now, and jack up the master to get back the volume.

Repeat this a couple more times, and the shudder happened twice more, just randomly. Try the same today, and no more shudder, but still the early distortion. Also tried it with another bass, same result.

I worked on CRT TV, thus I understand the hi-voltage pre-caution, but I am a total nube when it comes to tube amp.

So, any suggestion as to what to look for first?
Or just take it to the tech right away?

Thanks for the patience reading this long-winded post.

[Rune Bivrin]

Sounds mostly like a bad solder joint somewhere.

My classic initial diagnosis method for any intermittent problems is to lift one end of the amp about an inch and then drop it. If that changes anything you know it's a mechanically based issue.

Now, I have zero knowledge about that specific amp, but the things that act up most often are heavy components that are only fixed to the circuit board through soldering, such as big filter caps and interstage transformers. Tube sockets can also do this quite easily.

Unless you feel comfortable mucking about inside the amp with the power on I'd say it's tech time.

[staindbass]

i would have the tubes checked before lugging the whole thing around. is there a "power amp input" jack ? .plug a bass in there. that might tell you where the wierd sounds are coming from (preamp or poweramp). when you plug into power amp input it shuts off the preamp from the circuit. johnny a.

[BassmanPaul]

IMO you should take the amp to a tube tech and have it checked out. There is a large difference between a TV and a tube amp.

[rickdog]

Originally Posted by babebambi

Bought the amp pre-owned bout 3 weeks ago ... First week it played beautifully ... I didn't touch it for the next two weeks ... Yesterday, turned it on - loud squeal when I take it out of stand-by ... Let it sit for about 30 min, then try to turn on again. This time no squealing, but had a loud shudder .. the pre distort a lot earlier

How old is this amp? The symptoms suggest to me that a power supply capacitor may have failed. They don't last forever, is this amp old enough for that to be credible?

Part of the job of those filter capacitors is to prevent the amplified signal at a later stage from feeding back into an earlier stage. If the capacitor isn't doing this, the amplifier becomes an oscillator leading to squealing or "motorboating" (low frequency oscillation, maybe what you mean by "shudder"). Also, it may drag down the power supply voltage, which would produce the early distortion you describe.

The electrolytic capacitors used for power supply filters slowly degrade with time, but are "reformed" when power is applied. If the amp had seen regular use before you bought it, that might have been enough to keep a marginal cap working, but two weeks without use might have let it slip past the point of no return.

But it may be a simple mechanical/contact issue. Before breaking out the meter and disassembling the chassis, just reseat all the tubes, and try some contact cleaner in the jacks (especially the effects return, if it has one). If that doesn't do it, it's probably tech time.

[babebambi]

Thanks guys.

Originally Posted by Rune Bivrin

Sounds mostly like a bad solder joint somewhere...lift one end of the amp about an inch and then drop it...

too heavy of an amp for me to do that, lol

Originally Posted by staindbass

i would have the tubes checked before lugging the whole thing around. is there a "power amp input" jack ? .plug a bass in there. that might tell you where the wierd sounds are coming from (preamp or poweramp). when you plug into power amp input it shuts off the preamp from the circuit. johnny a.

the distortion is coming form the pre; I used the different patch point to test it as you mentioned.

Originally Posted by BassmanPaul

There is a large difference between a TV and a tube amp.

isn't that obvious, lol
I merely mentioned my experience just so ppl will understand that I am aware of the high voltage danger (even unplug from wall) inside a of this device.

Originally Posted by rickdog

How old is this amp? The symptoms suggest to me that a power supply capacitor may have failed. They don't last forever, is this amp old enough for that to be credible?

Part of the job of those filter capacitors is to prevent the amplified signal at a later stage from feeding back into an earlier stage. If the capacitor isn't doing this, the amplifier becomes an oscillator leading to squealing or "motorboating" (low frequency oscillation, maybe what you mean by "shudder"). Also, it may drag down the power supply voltage, which would produce the early distortion you describe.

The electrolytic capacitors used for power supply filters slowly degrade with time, but are "reformed" when power is applied. If the amp had seen regular use before you bought it, that might have been enough to keep a marginal cap working, but two weeks without use might have let it slip past the point of no return.

But it may be a simple mechanical/contact issue. Before breaking out the meter and disassembling the chassis, just reseat all the tubes, and try some contact cleaner in the jacks (especially the effects return, if it has one). If that doesn't do it, it's probably tech time.

Don't know exactly how old, but it's one of the SLM unit, so not too new. And it looks very likely that it's the cap, because the more I play it, the more the distortion seems to back down. Re-seat all tubes too.

I figured out that the loud shudder (mechanical noise withing the chassis) is just the top cover shaking. This happens when I take the amp out of stand-by, not when playing; so it's not the speaker rattling the amp chassis. Don't know if this is an issue of itself, please let me know if any of you have experience such thing.