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Ampeg SVT-CL hums hums hums...

boroman

boroman

Apr 27, 2009
374
149
boro
www.dustblow.com
#1
I have on my workbench early 00s SVT-CL that was given to me in the "original" shape. I'm sure it wasn't even opened once. The amp has steady 50/60 cycle hum that is not the "end of the world" but it's two or three times louder than my other CL and Heritage I have here. This hum is not related to the preamp controls. It's constant. In the last few days, I have checked everything - from replacing tubes to solder joints, checking bias, resistors, grounds, replacing one 100uf cap on the mainboard and one input jack.

What I've found is:
- when I plug in to SLAVE OUT hum goes away.
- When I plug into PREAMP OUT hum DOES NOT goes away.
- When I join together PREAMP OUT & POWER AMP IN I hum goes away by 50%, the same as plugging only to POWER AMP IN.

So I assume the hum is coming from the preamp area? Tapping on the board components does nothing. I'm still wondering if there's anything out I could have missed here? Again, solder joints tested, tubes tested, measured and replaced (2 times to be sure).
 
#2
Have you checked/measured voltages? Power supply ripple (throughout)?

There are many possible causes, including ground loops, noise induced into wiring, noise leaking into sensitive nodes (via a leaking defective cap), etc,

Measurements along the signal path will help narrow down the cause.
 
#4
@beans-on-toast hmmm... That's nice to know about heatsink on the VR, but these are not on the CLs.
@agedhorse Thanks for the tips. All seems in specs or close to, I am not seeing anything odd.

I may try to desolder R49 on preamp and install a jumper there. It's one of the AC heater / 220 Ohm resistors. I've read that It may cause ground loops on some CL's?
 
Last edited:
#6
When you insert a jack into Power Amp In, it lifts the signal from the preamp. If the preamp were the source of the problem I would expect this to kill the hum instead of a 50% reduction.

When you insert a jack into the Slave Out, it does not lift the signal from the preamp. The mystery to me is why this kills the hum. Are you shorting out the signal? Perhaps this is a grounding issue and connecting the Slave Out to another device provides a good ground to the power amp. I believe the ground would be localized to the chassis through the jacks, so this seems unlikely.

I am not that familiar with the modern SVTs. On the vintage amps, the Pre Out and Power Amp In jacks serve as the central grounding points for the power amp and preamp modules. Does inserting an unterminated dummy plug in the Slave Out also kill the hum?

How about inserting a dummy plug into the Power Amp In, to break the connection with the preamp; then checking the signal on the Pre Out to check if the hum is coming from/also in the preamp.
 
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#8
Wasnex said:
When you insert a jack into Power Amp In, it lifts the signal from the preamp. If the preamp were the source of the problem I would expect this to kill the hum instead of a 50% reduction. When you insert a jack into the Slave Out, it does not lift the signal from the preamp.
Click to expand...

I think they both disconnets the pre:
Slave Out vs. Preamp Out on SVT CL?

The mystery to me is why this kills the hum. Are you shorting out the signal? Perhaps this is a grounding issue and connecting the Slave Out to another device provides a good ground to the power amp. I believe the ground would be localized to the chassis through the jacks, so this seems unlikely.
Click to expand...

Yes, I have "shorted" 1/4" cable, sometime (if I don't, I just connect it to bass guitar instead). I have checked all the grounds I could see (spend like 2 hours on that). All fine. Between the preamp chassis screws/chassis and main chaiss ground, Also checked IEC socket soldering to be sure, cleaned/deoxided all the 1/4" connections.

Does inserting an unterminated dummy plug in the Slave Out also kill the hum?
How about inserting a dummy plug into the Power Amp In, to break the connection with the preamp; then checking the signal on the Pre Out to check if the hum is coming from/also in the preamp.
Click to expand...

Will check that today...

The R49 mod:

"(...) amp still had hum that I traced to the front end. I suspected it to be leakage / eddy current from the filament transformer. The front end heater string is DC. The diodes and cap in the DC supply were good. Each side of the heater AC input had a 220 Ohm resistor to ground. This was to act as an artificial center tap to cancel hum. But shunting one of 220 Ohm resistors (R 49) made the amp dead quiet. There was little measurable current - about 10 mils - across the resistor, so I took it out and put in a jumper. Eureka - all problems solved. What a huge job this POS was. At least now it sounds great - puts out rated power - and is quiet as a mouse at idle."
 
#9
boroman said:
I think they both disconnets the pre:
Slave Out vs. Preamp Out on SVT CL?
Click to expand...

Per the schematic neither the Pre Out or Slave Out interrupt the signal coming from the preamp. When you plug into the Power Amp In, the signal from the preamp is interrupted.

The difference between the Pre Out and Slave Out. Pre Out has a 22K resistor so the level will be padded down a bit lower than the slave out, and the jack is before the Power Amp In.

The Slave Out is after the Power Amp In jack and the signal is not padded down.


boroman said:
Yes, I have "shorted" 1/4" cable, sometime (if I don't, I just connect it to bass guitar instead). I have checked all the grounds I could see (spend like 2 hours on that). All fine. Between the preamp chassis screws/chassis and main chaiss ground, Also checked IEC socket soldering to be sure, cleaned/deoxided all the 1/4" connections.
Click to expand...

If you plug a shorted cable into the slave out, you're basically shorting the preamp signal to ground. If you plug a shorted cable into the Pre Out you are shorting the preamp signal to ground through a 22K resistor. This would explain why there is some residual hum when you plug into the Pre Out.

When you plug a shorted cable into the Power Amp In, the preamp signal should be interrupted and input circuit to the power amp shorted to ground. I would expect the hum to go away just like plugging a shorted cable to the Slave. But the Pre Out should still be live. You could then run the Pre Out somewhere else to see if it is the source of the hum.
 
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#10
boroman said:
The R49 mod:

"(...) amp still had hum that I traced to the front end. I suspected it to be leakage / eddy current from the filament transformer. The front end heater string is DC. The diodes and cap in the DC supply were good. Each side of the heater AC input had a 220 Ohm resistor to ground. This was to act as an artificial center tap to cancel hum. But shunting one of 220 Ohm resistors (R 49) made the amp dead quiet. There was little measurable current - about 10 mils - across the resistor, so I took it out and put in a jumper. Eureka - all problems solved. What a huge job this POS was. At least now it sounds great - puts out rated power - and is quiet as a mouse at idle."
Click to expand...

I'm assuming 12.6VDC heaters. If I am reading the circuit correctly, the current draw across R49 should be 6.3/220=29mA. If you jumper R49 the entire 12.6V is placed across R50, so 12.6/220=57.27mA. Keep in mind that 220*(0.057.27^2)=722mW). I have no idea what power rating the resistors carry.

If the current in R49 is actually only 10mA, then I would suspect a partial short to ground somewhere in the heater circuit. Perhaps a tube with a bad heater element could cause this?
 
#11
Will be back to "hum" topic shortly but now.....

OK. Now I am completely broken... Yesterday after I posted this I removed the Power Amp board again just to check everything again, I have cleaned it once again too. Today I set everything up and the amp after getting off the standby - produces SUPER LOUD star-trek oscillation noises. I MEAN SUPER LOUD THAT CAN SCARE YOU. So loud I need to turn the amp down after half second...!!! I reassembled it three times already just to check If I have missed something. I even opened my other SVTCL and connected my other CL preamp (yes, back to the main hum topic, it would be probbably the best way to check if something is wrong with the pre or power amp). What's going on? :( I hate these amps, really...
 
#13
beans-on-toast said:
You missed something simple, finding it isn't always easy.
Click to expand...

Yeah.. I did re-assemble it 4th time now, and I smoked R14 resistor on the main board near the slave out... I have already replaced it but amp behaves still the same way. Bias is super high (both R/G lights on and tubes glow bright blue). It does not happen when the power tubes are off the board.
 
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#14
boroman said:
Yeah.. I did re-assemble it 4th time now, and I smoked R14 resistor on the main board near the slave out... I have already replaced it but amp behaves still the same way. Bias is super high (both R/G lights on and tubes glow bright blue). It does not happen when the power tubes are off the board.
Click to expand...

Re-inspect the power amp section, especially where any work was done. Trace through the power supply. Look for shorts on both sides of the PCB. All it can take is a small solder bridge or a stand of wire. Inspect the tube sockets for a short between terminals.

Check all components associated with R14. That 220R resistor is not prone to blowing. Check the antiparallel diodes near the input of that stage.

When you get to examining voltages, do it without the tubes in place.
 
#15
I'll to that now. Give me some time...

By the way, what's that alu box labeled "T1". I dont see it on any of my CL schematics. That part is also not covered in the "parts list" for CL? I think I smell burnt something from that but it may be the resistor that blew (it's on the left side of T1)
Zrzut ekranu 2020-01-13 o 15.40.34.png
 
#19
boroman said:
@beans-on-toast Thanks so much! Is there any way to bypass that? (simple unsoldering the unit away from the PCB will do the job?). Amp is still blowing the R14. I checked nearby (and other) diodes and all seems OK, main caps too, other reistors too...
Click to expand...

It appears that you successfully removed it. It's somewhat delicate 4:1 winding with very fine wires inside. Desoldering could destroy it. You can test the primary and secondary sides by examining the schematic and seeing where to probe with your ohm meter. The resistance should be very low, but you can test for an open. This is not a high current circuit. Test all the resistors in that circuit as well as the ore/post switch.

Just to be clear, the R14 that has the issue is on the first tube going into the power amp. It is going to ground on one side and to a 1K cathode resistor on the other side. Is this correct?
 

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