Biasing mti svt, am I doing this wrong?

[Wasnex]

Hello everyone for the next installment of this saga

It's been to 3 techs, none of whom could fix it. There was a 4th that I called, who didn't really have a clue either. I remember seeing the offending noise on the negative side of the wave which really made me suspect caps. I also found out the input was 10 ohms above ground, which really didn't make any sense.

It went to tech #4 in Greensboro while I was down there, and he immediately suspected the caps, as I did. It's been 3 weeks now since I've gotten it back and I'm happy to report it's FINALLY working correctly. It's been retubed with kt88s and it's been working beautifully.

I'm tube rolling in it now trying to get a feel for it, it sounds much different than my classic, very mid forward. I'm not yet gonna form a tonal opinion on the kt88s as I haven't had a chance to REALLY drive it but they're very middy sounding.

It's incredibly noisy which is why I assume the ground was lifted, not that you'd notice when it's working, it sounds like an svt should now.

Tl;dr it's fixed, I made a good guess, and I know what tech to take them to now.

The is from the Rev D power amp schematic. Notice the 10-ohm resistor between circuit ground and chassis ground. Yes, I believe this is intended to reduce noise. Some amps have the resistor, and some do not.

I would not expect a huge tonal difference with KT88. I have an amp that is running Svetlana 6550, which I believe are the same as Winged-C. For fun I swamped in JJ KT88, and I did not hear any noticeable difference.

In another amp I replaced some JJ KT88 with Shuguang GEKT88 labeled as Ruby KT88V-STR. I noticed some difference between these two KT88 variants. By fiddling with the bias, I was able to get them to sound more alike.

I previously owned an MTI. I do remember it had some residual hum, but I would not say it was extremely noisy.

I currently own a 69 and an 87 Skunk. The Skunk is the quietest of the three SVTs I have owned. The 69 is the nosiest, but the hum depends somewhat on the position of the mid control. When the mid control is straight up (0 cut or boost), the amp is relatively noise free. As the control is turned up or down, the hum increases.

Every vintage SVT I have ever played had some residual hum. Supposedly this can be corrected if you eliminate the ground loops in the preamp, but the mod is complicated and invasive, as it involves cutting traces on the circuit board and redoing the grounding scheme...and of course you have to know how to do it.

My observation is the minimum hum is determined by both the gain controls and also the hum balance. In other words, the ideal adjustment for the hum balance depends on where you have the gain set. If you always run the amp at approximately the same volume, I suggest adjusting the hum balance at that particular gain setting.

 

[beans-on-toast]

The 10 ohm offset was intended to shift the signal ground away from the chassis ground. In order for it to work, the power amp jacks have to be isolated from the chassis. They did it wrong.

It's incredibly noisy which is why I assume the ground was lifted, not that you'd notice when it's working, it sounds like an svt should now.

The amp shouldn’t be “incredibly noisy”. I would look for a ground loop. First determine if it is in the pre or the power amp. Then continue refining the search for the source of the hum from there.

 

[Wasnex]

The 10 ohm offset was intended to shift the signal ground away from the chassis ground. In order for it to work, the power amp jacks have to be isolated from the chassis. They did it wrong.



The amp shouldn’t be “incredibly noisy”. I would look for a ground loop. First determine if it is in the pre or the power amp. Then continue refining the search for the source of the hum from there.

I can't say for certain, but all four of the jacks on the back panel of my skunk SVT appear to be insulated. It looks like they have some sort of vinyl shoulder washer.

Although my 69 SVT has a bit of hum, almost all of it is coming from the preamp. I shorted the signal side of the "EXT. Amp" jacks to ground and the vast majority of the noise went away. So the power amp appears to be pretty quiet.

 

[beans-on-toast]

I can't say for certain, but all four of the jacks on the back panel of my skunk SVT appear to be insulated. It looks like they have some sort of vinyl shoulder washer.

Although my 69 SVT has a bit of hum, almost all of it is coming from the preamp. I shorted the signal side of the "EXT. Amp" jacks to ground and the vast majority of the noise went away. So the power amp appears to be pretty quiet.

Some of these amps had the washers, others didn’t. What’s important is that if you are offsetting the signal ground, the other chassis ground points have to be eliminated or you are nullifying the offset.

 

[agedhorse]

There is no offset of the ground. It’s a common method to steer currents in the direction that the designer intends. Just about every designer I know of has used this scheme.

In a tube amp, the most common way to establish the chassis reference on a modern tube amp is the combination of the AC safety ground and the grounding/bonding of the output transformer secondary. This insures that the highest currents are referenced to the chassis ground, and is also done to protect the player in the event that there is a primary to secondary short on the output transformer.

In designs where the chassis has no safety ground, bonding the power supply common and speaker jack common to the chassis for much the same reason, but artificially referenced the chassis to the AC line neutral through what’s today called a Y cap. Because some AC circuits were not polarized, a polarity switch changed which pole of the AC plug was the reference.

Note that Y caps are still used today, but for different reasons, and these caps are specially designed to fail open, whereas X caps (used across the line) are not designed to fail in the open condition.

The topic of grounding and shielding is unbelievably complicated. Grounding theories have evolved with experience, and also with technology. In general, today’s amps are a LOT quieter than the good ‘ol days IMO.

 

[poorbassist15]

The hum identified itself tonight at the gig, bad preamp tube in the first channel. I'm not sure which one yet, I'll check them tomorrow, but the power amp was as quiet as a mouse. I know this bc I finished the gig with the power amp in. Lesson learned I suppose, don't trust a shoebox of tubes you got at the flea market. I do notice a slight bit more bass extension with jj kt88s, what I did notice between this and the tung sols that were in it were a large physical weight difference. The jjs are much heavier than the tung sols. Thought that was interesting.

This amp never fails to fail lol

 

[poorbassist15]

Hello everyone and an update on the amp that never fails to fail(or stay fixed)
Amp has failed several times since my last post, however I’ve been learning and fixing things(several other amps and even a radio) and have fixed the last 4 failures on the amp. However, the oscillation has reappeared like an ancient nemesis and I’m still struggling in finding it. I’ve checked every component in the power amp and found no faults, swapped tubes in case it was a tube issue, and done load resistor testing. The issue only appears with a bass plugged in with the amp plugged into a cabinet, it does not do it with any frequency of signal generator. Going to next attempt removing pairs of outputs to see if it follows the socket or tubes. Current appears to be equal between the trio of tubes on each side, all are the same temperature more or less with an infrared gun. Amp was completely recapped about 2 years ago.

this is honestly just becoming so frustrating it’s funny, this amp just does not want to live.

 

[agedhorse]

The first thing to do is verify that there were no mistakes made in the previous "repairs", "updates" and "re-cap" work. I see this a frustrating number of times.

 

[Wasnex]

The issue only appears with a bass plugged in with the amp plugged into a cabinet, it does not do it with any frequency of signal generator.

If you insert a jack into the Power Amp In, the signal from the preamp should be disconnected from the Preamp. Try shunting the connection (ground to tip of the 1/4").

Next, plug in your bass and monitor the output of the Preamp with an O'scope to see if it breaks into oscillation. If you don't have an O'scope, send the amp to someone who does.

Another possibility is the amp is oscillating at a frequency that is beyond your hearing range. When you plug in your bass, it pics up the RF and transposes the problem to a frequency you can hear. AFAIK this is something you should be able to see on an O'scope.

The input jacks on the Preamp should have switching jacks that shunt tip to ring. When you plug in a bass, the shunt is broken. This could have some impact on how the preamp is interacting with itself

 

[poorbassist15]

If you insert a jack into the Power Amp In, the signal from the preamp should be disconnected from the Preamp. Try shunting the connection (ground to tip of the 1/4").

Next, plug in your bass and monitor the output of the Preamp with an O'scope to see if it breaks into oscillation. If you don't have an O'scope, send the amp to someone who does.

Another possibility is the amp is oscillating at a frequency that is beyond your hearing range. When you plug in your bass, it pics up the RF and transposes the problem to a frequency you can hear. AFAIK this is something you should be able to see on an O'scope.

The input jacks on the Preamp should have switching jacks that shunt tip to ring. When you plug in a bass, the shunt is broken. This could have some impact on how the preamp is interacting with itself

the preamp is completely clean with either a signal generator or bass, I have shunted and bypassed the Jack to isolate it to the power section, I’ve even completely removed all the tubes from the preamp to eliminate that as a variable. It does show clean on a scope as well. I will find a picture of the last time it did this as a point of reference.

 

[poorbassist15]

The first thing to do is verify that there were no mistakes made in the previous "repairs", "updates" and "re-cap" work. I see this a frustrating number of times.

I had noticed several poor quality repairs that had been done at some point in the past, and repaired those correctly the last time I had it apart. For the two gigs it worked, it never sounded better. On this opening of Pandora’s box I did check the quality of the recap work and it does appear to have been done well, the guy that did it is probably the best tech we’ve got in the area so I wouldn’t expect less than. But this time it’s even got him stumped.

 

[agedhorse]

I had noticed several poor quality repairs that had been done at some point in the past, and repaired those correctly the last time I had it apart. For the two gigs it worked, it never sounded better. On this opening of Pandora’s box I did check the quality of the recap work and it does appear to have been done well, the guy that did it is probably the best tech we’ve got in the area so I wouldn’t expect less than. But this time it’s even got him stumped.

This is why most techs fear working on previously repaired gear, you really never know for sure what kind of mistakes might have been made. It can take a lot of time, and sometimes you never find the cause.

 

[Wasnex]

the preamp is completely clean with either a signal generator or bass, I have shunted and bypassed the Jack to isolate it to the power section, I’ve even completely removed all the tubes from the preamp to eliminate that as a variable. It does show clean on a scope as well.

Okay...I guess I misunderstood your comments.

You wrote:

the oscillation has reappeared like an ancient nemesis and I’m still struggling in finding it

The issue only appears with a bass plugged in with the amp plugged into a cabinet, it does not do it with any frequency of signal generator.

Not sure how plugging the bass in could cause an oscillation after pulling all the preamp tubes.

 

[poorbassist15]

Okay...I guess I misunderstood your comments.

You wrote:




Not sure how plugging the bass in could cause an oscillation after pulling all the preamp tubes.

I’m running straight in to the power amp, via the power amp in jack. It doesn’t make any sense to me or anyone else that’s touched it lol

There’s a few things left for me to try but I’m not hopeful and the amp may just be a mystery.

 

[poorbassist15]

Update, problem appears with 4 power tubes in different sockets, not the output tubes nor does it appear to be connected to a specific socket. Removed the feedback rc filter setup completely, and that made issue worse, both cap and resistor show fine. I replaced the new screen resistor I replaced with another new one to eliminate my work as the cause, no change. All tubes changed for phase inversion and cathode followers. No change. Amp still tests at over 300w before I can see distortion. I’m fresh out of ideas lol. I’ll have to find what I did with my differential scope probe, I’ll clip on the small tube plates directly with a scope and see what I get, though I reckon I’ll again see nothing unusual since the issue only appears driving a cab so I wouldn’t expect to see anything there.

 

[Wasnex]

Update, problem appears with 4 power tubes in different sockets, not the output tubes nor does it appear to be connected to a specific socket.

I am sure you understand exactly what you mean...but I have no idea what you are trying to tell us.

 

[beans-on-toast]

Go back to post 96 (?) where ferrite beads are used instead of plate resistors on the power tubes. Every modern SVT uses these for a reason. It cures oscillation issues but doesn’t solve the cause of the problem.

A spectrum analyzer will identify offending frequencies. It helps with the troubleshooting process. Remember, a capacitor might test good at low frequencies but changes at high frequencies.

This may not help solve your problem but this multi-part video series describes design measures that are taken to ensure stability in simple RF amplifiers. Very interesting stuff.

 

[poorbassist15]

I am sure you understand exactly what you mean...but I have no idea what you are trying to tell us.

I have moved the tubes around while also having removed a pair, attempting to see if it follows a tube or stays at a particular socket. Neither is the case.

I’ve also removed the negative feedback loop to see if that made a difference, and it did, but not a positive one.

I’ve completely retubed the 12dw7/12bh7 section of the power amp to see if my problem was there, no change there either.

with load resistor testing I get over 300 watts and though I don’t have a spectrum analyzer to measure my thd it’s about 330 watts before I can see clipping, so obviously all 6 power tubes are working.

 

[Wasnex]

I have moved the tubes around while also having removed a pair, attempting to see if it follows a tube or stays at a particular socket. Neither is the case.

I’ve also removed the negative feedback loop to see if that made a difference, and it did, but not a positive one.

I’ve completely retubed the 12dw7/12bh7 section of the power amp to see if my problem was there, no change there either.

with load resistor testing I get over 300 watts and though I don’t have a spectrum analyzer to measure my thd it’s about 330 watts before I can see clipping, so obviously all 6 power tubes are working.

So no problem driving a load resistor, but it acts up when pushing a speaker?

Is the speaker 4 ohms, and have you tried another speaker?

Also, If I understand, the preamp is bypassed and you are injecting the signal in the the Power Amp input.

 

[poorbassist15]

So no problem driving a load resistor, but it acts up when pushing a speaker?

Is the speaker 4 ohms, and have you tried another speaker?

Also, If I understand, the preamp is bypassed and you are injecting the signal in the the Power Amp input.

I have tried my other av212 cab, and it still does it in that one too, so I can rule the speakers themselves out as a cause. Yes, the cab is a 4 ohm, and I’ve completely bypassed the preamp and have performed all my tests into the power amp in.