Noisy Ampeg SVP preamp hummmmmm~

[oc disorder]

Anyone have any experience with this Pre-Amp?
With the master turned down (and with the preceding valve removed)
there is too much hum in the output.It sounds mainly 2'nd harmonic dominated
which suggests the power supply but no ripple observed with the cro
and the main filter caps seem all good ..low esr measured with esr meter
and all within the ball park.
This valve is mounted near the power transformer and was wondering if
there is some sorta induction going on.
It does seem unusual to have the master pre the 2 halves of the 12AX7.
As the power amp in jack has no volume control on its input (a separate
power amp with solid state pre not used) the final valve in the preamp
is constantly feeding the pwr amp with this hum.
Its not an earth loop either as have tried all manner of hookups including
the non approved ones !
Have read other forums where it is said to be noisy with suggestions
relating to the lack of full rails for the IC's( ie it is +/- 8 volts instead of 16)
but am not even looking that far into it from the back end.
Maybe I should call a gastroenterologist !
I have noticed as soon as the power is cut the hum goes but it is still
amplifying as the signal dies away (can tell by putting my fingers on the valve
to make it hum (that's proper hum) as I switch it off.
Was even thinking of jigging the power transformer out of the case away
from the valve but seems a bit radical.
I am hoping someone has been here before as surely it is not supposed to be
like this even if Loud in their infinite wisdom have discontinued it.
Attached are some simplified diagrams of the power supply and final post master
stage.Any help gratefully received.

[Enzo]

I have that exact amp in here now with that exact complaint.

I have not torn into it yet, other than popping the lid and pulling the drawings out of the files.

I think it is not a matter of poor filtration, you are not measuring excess ripple. I suspect a ground issue. The output circuit shares copper in the ground return with some charging currents of the main filters. That is my working theory.

The theory is also consistent with your finding that the hum stops as soon as power is cut while the amp still amplifies on the cap charge as it fades. The charging curents stop the instant power is removed.

I'll keep you posted on what I find. Does this description make sense to you?

[oc disorder]

Hi Enzo
Yes I agree..I have put it aside but mulling it over I was going to start
by resoldering, as some of the connections look a bit dull and some with a few minuscule remains of air bubbles(holy soldering).
Secondly I was thinking of playing around with the earthing as that was my gut reaction to the noise hence the initial anti earth loop experiments.
Not only does your description make sense, it's succinct eloquence..!
Likewise if I stumble on anything I will post.
Thanks for your reply.

[Bassman3]

Hi Enzo
Yes I agree..I have put it aside but mulling it over I was going to start
by resoldering, as some of the connections look a bit dull and some with a few minuscule remains of air bubbles(holy soldering).
Secondly I was thinking of playing around with the earthing as that was my gut reaction to the noise hence the initial anti earth loop experiments.
Not only does your description make sense, it's succinct eloquence..!
Likewise if I stumble on anything I will post.
Thanks for your reply.

I have some experience working on these. I have seen hundreds of these units come through SLM Electronics service dept. And 90% of all complaints was HUM. It may seem simple but the problem is 3 things.

1. These things have no hum balance pot.
2. There are 5 tubes.
3. The pre amp has a lot of output.

The specific problem is, every tube has a slightly different filament spec, a hum bal pot usually fixes this "mismatch".

If the hum is always present when the Master Vol is all the way down, then it is the last stage.
If it hums only when the Master Vol is up & gain is down, then it's the middle stage.
If it is gain dependant only then it's the first stage.

Having worked in factory service for SLM, I would take a box of tubes & start plugging them in one by one, by first locating the noisy stage & starting there. Repeating this process until it quiets to an acceptable level.

Once you get it to quiet down, you can reduce the hum further by reducing the Master Vol slightly, then make up the difference on volume by turning up the gain. Like I said these things have a lot of out put, so reducing the Master will also reduce noise & will still have enough output to drive your power amp.

I have never found anything other than tubes that are the problem, so ground problems, or transformer induction or bad solder would not be the place I would start, unless you suspect a real electronic failure. It won't hurt, but it won't fix it either.

I know most people do not have access to hundreds of tubes to sift through, but you can try the old "shell game" with the the 12AX7s that are in it, sometimes getting them in the correct order can help a lot, then locate the stage that is still giving you problems & go buy a few more tubes to see if that helps further.

Another note: EXPENSIVE DESIGNER SUPER TUBES WILL NOT FIX IT. DO NOT PAY $100 FOR 3 TUBES & EXPECT THAT TO MAGICALLY FIX IT!

Tubes are tubes, they still all work the same no matter how much you pay for them.
Most of the time the Groove Tubes were the problem, & installing a sovtek would be much quieter. I had much better luck with sovteks than GTs. And ther're $6.00 a pop.

[Rick Erickson]

What exact model is this? I have worked on many SVT's over the years, but don't recall any hair-pulling (if I had any) hum problems in the newer ones. The originals had some grounding issues, as well as a (imho) rather dangerous grounding technique in the preamp, which can result in the preamp chassis charging to the supply voltage if and when the tiny signal cable drain wire breaks.
Other than that I don't know of any design issues with these amps.
It is very possible to burn a trace if connectors are swapped, which can happen very easily in some newer models. Look for a couple of two conductor pc connectors on the preamp unit, one with white wires, the other with blue wires. If these are swapped you will lose a trace - probably a ground. Without the model # of your amp I can't be certain these connectors even exist on the unit you are servicing.

RE

[Rick Erickson]

Nevermind - I just re-read the title "SVP". My bad eyes saw this as SVT...

[Enzo]

On the one here, there is the common hum, and then when the EQ is engaged there is a serious increase in buzz.

[oc disorder]

Well I'm part way into it..Bassman3 is correct regarding the choice of valves.
This one had a Groove tube 12AX7R(a Sovtek AX7WA) in first position
and the rest were Chinese (the ones with that sideways bit of metal that resembles the Greek letter pi )
Well putting the GT into the 5th position (final valve) really increased the noise!
However it was much quieter microphonicaly not that it matters much at the end of the chain.
In the TalkBass forum "Schwinn" passed on a comment from Lord Valve

LV said that there is a design issue with the preamp, but that the right tubes can only minimize it.If you crank the volume you will hear noise, but it shouldn't be audible while you're playing.

I also tried removing the 2 x 220 ohm hum canceling resistors that go between the heater supply and gnd and fitting a 470ohm hum balance pot but didnt seem to make much difference.
So as the user hasn't a input volume on his power amp am going to make
a simple "pot in a box" to go between the SVP output and his power amp.
I wonder though..why the cheaper valves hum less than the tested ones.
If I had time would be interesting to hard wire a twisted pair heater supply
direct to the transformer.
Ah well another of lifes little mystery's.......

a P.S. Also I noticed the hammertone paint is non conductive...
on some of the pots if you remove the knob and grasp the pot nut from the front it also increases the hum.The lid doesnt seem to connect to the chassis either so I scraped some paint away to make sure it did.

[Bassman3]

I wonder though..why the cheaper valves hum less than the tested ones.


Because Name Brand tubes are over rated & over priced, after changing out tubes for 20 years, I find the cheapies are quieter [generally] than GT, or Mullards or whatever. Sovteks seem to be the quietest. Or I have the most luck with them.
Of course some will argue, the tone just isn't there, but I don't buy it.

When SLM first started carrying GT's, I immediately installed one in my amp, needless to say there was no difference in tone, & the amp hummed a little more, so I yanked that out & put back in the nice quiet Sovtek. When your recording, it makes a difference.

If dropping $35 a pop on 12AX7's makes you feel like your amp sounds better, by all means.

[David Schwab]

My Mesa 400+ has Sovtek preamp tubes and Ruby power tubes, instead of the Mesa or GT tubes the amp came with. They sound fine.

Tubes do sound different though. I have an old TubeWorks BlueTube pedal, and I have played around with swapping out the 12AX7 it came with for different brands, and they each have a different vibe. For distortion I preferred an old British Sylvania ECC83 tube I pulled out of something or another. It just had a nicer tone than the stock tube the pedal came with.

[Bassman3]

My Mesa 400+ has Sovtek preamp tubes and Ruby power tubes, instead of the Mesa or GT tubes the amp came with. They sound fine.

Tubes do sound different though. I have an old TubeWorks BlueTube pedal, and I have played around with swapping out the 12AX7 it came with for different brands, and they each have a different vibe. For distortion I preferred an old British Sylvania ECC83 tube I pulled out of something or another. It just had a nicer tone than the stock tube the pedal came with.

Im sure there are difference. Rock on!

[oc disorder]

Disregarding the "cool" hyped up marketing campaigns we have two hypothesis both of which could apply.

The output circuit shares copper in the ground return with some charging currents of the main filters. That is my working theory.
The theory is also consistent with your finding that the hum stops as soon as power is cut while the amp still amplifies on the cap charge as it fades. The charging curents stop the instant power is removed.

The specific problem is, every tube has a slightly different filament spec, a hum bal pot usually fixes this "mismatch".

I could say that GT,Ruby, Mesa or whover are full of b/s and don't test them at all which would fit my finding that a pre tested Sovteck 12AX7WA and relabled a GT 12AX7R hums more than a bulk untested 12AX7.

But as I understand it the new wave of Chinese and Russian valves particularly a few years ago were inconsistant which forced Marshall to alter their screen resistor from 1k to 2.2K to avoid so much valve failure under warranty.

The valve/tube testing era arose from a need.

Heck in the old days the first 50w marshalls (45's) didn't need a screen resistor the valves were so tough wonder how many still have no screen r ?

I guess they dont test the two halves of the heater for current draw but are more interested in gain,
transconductance and microphonics etc.

I would surmise the design fault in the SVP is something other than what is tested for which fits with the inconsistency of the "name brands" .

I buy mainly Sovteck and Electro Harmonix and occasionly JJ's from a wholesaler and not the holy grail gurus.

The wholesaler also sells magntrons eht transformers fluro lights and would think transconductance is something out of a sci-fi book and that gm had something to do with food crops.

But some times I buy pre tested ones from another supplier purely because the odds are better of the valve working and not being microphonic .

I have to pay freight and going throught all the bullshit of sending them back getting approval and then getting replacements adds about a week and a half to the job and I get sick of being surrounded by half finished chassis getting in the way.

I send less back of the pre-tested ones than the bulk ones.

I gather also Fender have got Groove Tubes to pre test all the valves as the odds are better of less amp failure and consistant bias which could reflect on the amplifier.

Some people havn't a clue about amps they just buy them and plug in.

If it plays up or is noisy its a faulty amp and has to go back to the shop.

These weekend stars help keep the price down for those in the know...!

But I do agree the boutique valves are a little over the top and that valves have different tones within a brand and between brands.
Im still undecided about the cryogenic treated ones though.
A lot of the facts make sense but it would take a lot of time and money to prove or disprove the supposed advantages.
Improving a circuit electronicly doesn't mean it sounds better.
I don't think CBS intended to make Fender sound worse by making technical improvements to the circuit.
Sometimes simple basic imperfect circuits or whatever just sound good in the musical smorgasboard.
Technicans have spent years trying to remove distortion and guitarists have spent years trying to put it back in !

Anyway back to the SVP.
I bought 3 Sovtek LPS bulk to try (thinking the spiral filament should help as well) The first one hummed a bit the second one didn't and they were both from the same batch according to the numbering.

I guess had I bought some branded ones at 6 times the price I could have got the same result.

The only noticable noise was hiss which I thought was well in the ball park considering the gain of the pre-amp.

I realised later that the first valve has a D.C. filament and the C.T. earthed at a different point to the others.

Crossed my mind to try levering up the transformer heater supply legs and inserting a bridge rectifer to make the lot run on DC but didn't have enough time.

I removed the test hum balance pot and replaced the 220 ohm resistors which did an identical job.

So in conclusion I think its got something to do with the ct earth of the ac heater supply.

I am interested to know what Enzo has found if he has had time to work on the one he has.

Now does any one know where I can get a Grove Tubes magnatron so I can mod the microwave and make my reheated pizza taste better ?

[Mars Amp Repair]

to concur,
my notes on this preamp indicate that I had the identical problem & could only resolve it by swapping around the preamp tubes for the typical filament issue.

My notes also indicate that the filament dc offset trick did not work on this model.

glen

[ETR]

What exact model is this? I have worked on many SVT's over the years, but don't recall any hair-pulling (if I had any) hum problems in the newer ones. The originals had some grounding issues, as well as a (imho) rather dangerous grounding technique in the preamp, which can result in the preamp chassis charging to the supply voltage if and when the tiny signal cable drain wire breaks.
Other than that I don't know of any design issues with these amps.
It is very possible to burn a trace if connectors are swapped, which can happen very easily in some newer models. Look for a couple of two conductor pc connectors on the preamp unit, one with white wires, the other with blue wires. If these are swapped you will lose a trace - probably a ground. Without the model # of your amp I can't be certain these connectors even exist on the unit you are servicing.

RE

I have an SVT here now with a bad hum that I think is a grounding issue. If you don't mind could you tell me what the grounding issues are with this amp. It looks like the main filter caps are grounded at the speaker jack onthe back panel. Some of the grounds seem a little odd to me.

[Enzo]

This thread is about hum in the SVP, not the SVT. SVP is a one space rack tube preamp. The preamp is part of the SVT series, but it is not in an SVT head. So the power supply, circuit boards, layout and everything else is different from any SVTs.