60Hz hum in soldano sp77 guitar preamp

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

okgb

Well-known member
Joined
Oct 18, 2004
Messages
5,884
Location
Winnipeg Mb Canada
I'll start another thread if anyone thinks that better , but I have a grounding issue as well
A friend bought a soldano sp77 guitar preamp  [ made in japan version ] that has a ground type hum
bought it used this way , hum rises with master volume mostly independent of gain control , way too much
to be normal , I separated the psu and signal ground point from the pcb and that helped , it is a.c. 6v c.t.  filaments
soldano's answer was to send it in  and no one on that newsgroup had any answers , could it be a problem with
the xfmr ?  any help is appreciated , hopefully an easy solvable [ even if stupid ] problem .
tia regards Greg
 
forgot to say I changed the psu filter caps just in case , but they weren't very old and it may have made a little difference
but certainly no solved the problem .
 
Not much there apart from the PSU filter caps. Decoupling is not likely to be a problem if it once worked and you have since changed caps. Possibly the bridge rectifier diodes?

Similar with ac  heater - if it once worked right, not likely to be causing any issues unless some wiring has come unstuck.

I would guess the grounding has some resistance/impedance somewhere - check with a cro and look for ripple.
Or use a probe on the ac coupled parts of the circuit and look for the noise.

If that all checks out, could be a tube - I've had high gain tubes go noisy, fixed by replacing.
Possibly tone stack caps - had one of them go south once (the pf mica) - intermittent hum, starting soft getting loud over time. Even the coupling caps  I guess.

The switching circuit could contribute some psu issues if the cap is going south too.

Then, maybe the transformer. ?

Good luck  :)
 
yep tried changing all the tubes , even with only the last stage tube it hums and gets a little worse
as you add the other tubes back in . I didn't however change the cap in the switching circuit , worth a shot !
 
Isolated the exact origin point in circuit?  Inspected solder joints on socket pins?
 
How is it being use?  Are you going into a power amp. ( Solid state / Tube) 

The soldano preamp has lots of gain and can hum from a single coil pickup but also if running the preamp into the input of a Guitar Amp like fender/ marshall,  there can be gain mismatch that brings up hum as well as loop between the preamp and Guitar amp.  Gain on the fender for instants with high gain from the soldano.   

I know when running 2 guitar amps and switching between "a" amp and "b" amp, there can be a loop between the 2 amps via cables.  very difficult to get rid of with out Transformer isolation. 

Many amps used in very high gain mode (metal style)  need a noise gate to get rid of hum thats always there when the guitar is not being played.

You may have thought of this but just bringing it up to locate where the hum is created from.

One other thing.  Check the diodes on the bridge rectifier and make sure that they all drop .7volts when checked and are not shorted.  One diode can cause hum if shorted.  usually a fuse will go out under that condition but in a preamp it may be working still but with 120 or 60 hum.
 
Good point there, regarding high gain pres and how they are used exacerbating the noise problem, including using none too quiet pickups.

I've changed most of mine to some form of noiseless ones, either emg/other active or fender scm. And a nice active Seymour Duncan on a rick bass.

Some great guitars which can be all but useless direct recording with stock pups when you are doing high gain stuff in a studio chain, particularly diy!

Gates obviously can help, but best without them. Digital ones are best I find, and the best of them is the Finalizer multiband expander, which makes an awesome last link in the high gain direct guitar chain  8)
 
Last time I tried it was directly into a mixer with no input , that or
compared to another preamp [ CAE 3+ dead quiet ]  still hum
 
check the input jacks, they are self shorting so when the chord is unplugged, you ground out the grid thru the input resistor, sometimes drunken musicians wiggle the guitar chord to hard getting it into the slot, which can bend the jack so that the plate no longer shorts out the input,

you have a circuit?

i could  rip into this problem 100 times better with the schemo,
 
Thanks for digging that up Alex , I can tell you what is grounded where
the input jack is isolated and grounded at the pcb standoff close to the first tube
there is a kind of ground trace running along the pcb  [ I have split the preamp ground
from the psu on this trace and grounded the psu by the pwr xfmr , that helped ]
two output jacks ran through one shielded wire are ground at the back of chasis , one through
a resister , isolating this has no affect on hum .
switching circuit is grounded from pcb standoff in the psu area , separate from psu but also
grounded from switching jack & shielded wire , isolating this has no affect .

I may have gone in circles but still contend [ without having heard another one ] that something must
actually be wrong because I can't imagine it being used or sold with this much hum
thanks so far for all the ideas




 
Is it my imagination or is that one seriously weird schematic? Looks like the signal gets split and sent down two separate amp chains then combined at the output. Also the foot switch seems to do nothing but light a couple of LEDs.

Cheers

Ian
 
someone has redrawn that one  , I'll try to find a version I have
the ldr's are at most of the switch points to change channels
with common stage at input as well as output . the o/d looks to use
the first clean tube as additional gain
 
What that person drew as switches are the resister parts of the ldr's
except the bright switch, so some parts get grounded out , here another ver
of the schematic .
sp77.jpg
 

Attachments

  • sp 77.jpg
    sp 77.jpg
    465.4 KB · Views: 35
ok get the thing into hummmm mode, then pull out the first tube, v1a/b, then the next, until the humm stops,

report back immediately your results
 
because the hum get worse as the master is turned up and relatively unaffected by the gain controls
I have already pulled all the tubes  [ tried different ones ] and worked my way back from the output
with the last tube in, it still hums and gets a little worse as the rest are added back in one at a time
 
okgb said:
because the hum get worse as the master is turned up and relatively unaffected by the gain controls
I have already pulled all the tubes  [ tried different ones ] and worked my way back from the output
with the last tube in, it still hums and gets a little worse as the rest are added back in one at a time


. . . have you located exactly where it starts?  You can do this by grounding the grid of each stage.

It sounds like it's towards the front, which is common.  Pull V1 of one split stage out and ground the grid of V1 for the other. What happens?
 
I replaced the electro in the switching supply and while I' was at it changed the 1uf electro
on the cathodes to poly film [ I know , I did just to say I'd changed every electrolytic ]
cut another trace and grounded the switching at the the HT & heater ground , strangely enough
a hard tap to the front panel got rid of the last of the 60hz hum  , and tapping on anything didn't
do it [ nothing is grounded on the front panel ]  the hiss is now louder than the hum which seems to be
higher than 60hz of what is remaining
 

Latest posts

Back
Top