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Biasing mti svt, am I doing this wrong?

#101
rickdog said:
I think current SVTs all have a protection circuit that detects a tube that's drawing too much current and shuts off the B+. That means they don't need a plate resistor to act like a fuse in those circumstances. So in an older SVT without the protection circuit, wouldn't you still want the plate resistors?
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Despite the protection circuit, power tubes blow, screen resistors opening are common. The system isn't perfect.

A plate resistor will act as a fuse. A 10 ohm plate resistor rated at 5W will eventually open with sustained 1/2A. In '77 they used a 5 ohm 5W plate resistor, 1A sustained. There is also a 1 ohm 5W shared cathode resistor. When these blow damage is done, often to the tube and the PCB. The fusable screen resistor also offers some protection. High frequency oscillations are capable of damaging an amp as well. Ideally a plate resistor and a series connected ferrite bead would be good to have.
 
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#102
beans-on-toast said:
Despite the protection circuit, power tubes blow, screen resistors opening are common. The system isn't perfect.
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I can attest to that. (Fortunately I have the skills to diagnose an open screen resistor and replace it.) At least the protection circuit helped keep the shorted tube from causing more serious damage (like the output transformer).
 
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#105
20190922_184805.jpg

This is bias on k1 after a playing period to see if it would blow up
20190922_185306.jpg

This is between k1 and k2 at somewhere around 27v
20190922_183932.jpg

And the camera doesn't like the CRT scope so this is the best waveform picture I have but much better than it was.
 
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#111
I suppose it's time to resurrect this thread. The amp has finally gone to the tech, though not the one I initially anticipated taking it to. The issue with the output section was resolved, I'll get him to tell me what he did for posterity's sake in case someone else has this issue. We're currently looking for a bad ground loop that neither of us can seem to find. I'm hopeful as this is the best it's ever sounded, barring the incredibly loud 60 hz hum
 
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#112
poorbassist15 said:
I suppose it's time to resurrect this thread. The amp has finally gone to the tech, though not the one I initially anticipated taking it to. The issue with the output section was resolved, I'll get him to tell me what he did for posterity's sake in case someone else has this issue. We're currently looking for a bad ground loop that neither of us can seem to find. I'm hopeful as this is the best it's ever sounded, barring the incredibly loud 60 hz hum
Click to expand...


I believe the original preamp design has multiple potential ground loops. Every one of the original style amps I have ever played had some residual hum. Usually it is barely perceptible over the roar of the fan though ;). Disconnect the fan and the hum is very noticeable IMHO.

On my 69, somebody disconnected the ground from the channel 2 input jack to the preamp card. The ground was lifted on the card side, but not on the input jack. The ground on the channel 1 input jack remained, and both jacks are bonded together through the front panel

Lifting the ground on channel 2 actually made an improvement with the original caps, which were shot. When I did a cap job it changed something and the leads of the power transformer started injecting sort of a wiry hum into the circuit. The signal wire running from channel 2 input jack to the card would be microphonic at low to medium gain ranges. If I really cranked the gain, the amp would make a poof sound, the hum would disappear, and the cable would no longer be microphonic. Replacing the cable did not help. Re-establishing the ground on both sides of the shielded cable corrected the problem, but raised the level of hum in channel 1 slightly. So if I did not want to be able to use channel 2, it would make sense to lift the ground and turn channel 2 gain to off. The output section in this amp is pretty much dead silent.
 
#113
Wasnex said:
I believe the original preamp design has multiple potential ground loops. Every one of the original style amps I have ever played had some residual hum. Usually it is barely perceptible over the roar of the fan though ;). Disconnect the fan and the hum is very noticeable IMHO.

On my 69, somebody disconnected the ground from the channel 2 input jack to the preamp card. The ground was lifted on the card side, but not on the input jack. The ground on the channel 1 input jack remained, and both jacks are bonded together through the front panel

Lifting the ground on channel 2 actually made an improvement with the original caps, which were shot. When I did a cap job it changed something and the leads of the power transformer started injecting sort of a wiry hum into the circuit. The signal wire running from channel 2 input jack to the card would be microphonic at low to medium gain ranges. If I really cranked the gain, the amp would make a poof sound, the hum would disappear, and the cable would no longer be microphonic. Replacing the cable did not help. Re-establishing the ground on both sides of the shielded cable corrected the problem, but raised the level of hum in channel 1 slightly. So if I did not want to be able to use channel 2, it would make sense to lift the ground and turn channel 2 gain to off. The output section in this amp is pretty much dead silent.
Click to expand...
That's what he said, he jumped the power switch in the molex and he said the hum of the power amp would be would be indiscernible over the fan noise but when the preamp is plugged in all hell breaks loose.
 
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#114
Hello everyone,
Basically updating this for everyone's amusement. Ground loop gone. High frequency oscillation has returned. The tech has thrown in the towel, I went over and helped him today, given he has a probe capable of handling the voltage. Oscillation is present regardless of input patch location, whether power amp in or preamp. It's manifesting itself as a horrendous farting noise on high power transients. What was interesting is that in the power amp in, when the standby is off, the signal from the bass is perfectly fine on a scope but is immediately "farted" all to hell the instant hv is on, before it passes through any component. Looked at the power supply, caps, diodes and all, b+, bias voltage, plate current, feedback loop, all manner of stopper resistors, even the tech couldn't figure it out as none of the things to create oscillation were happening, the power supply investigation was just a hail mary.

Back into the shed it goes until I can find another tech that wants to give it a shot. I'll have enough money in this thing to have bought a new cl before long lol. Oh well, tl;dr don't buy someone else's problem.
 
#115
This is why a well qualified tech will systematically test and test and test to determine what is not measuring as expected, and why.

For example, how was the AC and DC current sharing between the tubes?

Were the results different when the global feedback is opened up and the input level is reduced to account for the increased open loop gain?

What does the phase shift look like across the audio spectrum (from input before the NFB injection to the output at the point that the NFB is taken off from?

Are the values in the feedback network correct and accurate?

These are all things that can affect the stability of the power amp and must be investigated before arriving at a "solution". Sometimes, things are not quite as they seem in spite of the assumptions that are made. This is what I used to teach in tech classes, it's the basis for a process called "structured troubleshooting", and it involves diagnostics to rule out entire sections of the amp (differential diagnostics) and diagnostics to confirm problems with a specific area. Negative global feedback amps use a special kind of troubleshooting that considers that for every action there is an equal and opposite reaction. This is the entire premise of NFB, so for example if an amp offsets positive, a stage may in fact being offset negative and NFB is applying a correction to drive it negative to get back to the target value. Sometimes looking for (or recognizing) the opposite of what you are seeing is what's really the cause of the fault inter-stage. That's why understanding how the entire circuit is SUPPOSED to work is critical to troubleshooting and identifying odd behavior.
 
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#116
With problems like this you can’t assume that what’s there is correct. Is the wrong value resistor in place for the NFB. Don’t just look at the color codes, test the components with a meter. Also, was the primary leads of the OPT ever removed and installed backwards. It affects the NFB.
 
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#117
I'll try to answer on memory, he has notes I'll ask about. He did check the current sharing, and he didn't tell me precisely what it was but he seemed to think it wasn't an issue. A lot of this wasn't done with me there so I can only go on what I saw while I was there. I'll be honest with you guys, this level of repair is further than I've ever had to go for fixing an amp, still learning here.

I can personally vouch for the NFB components, I took those out of circuit and checked them when this escapade started, and I believe he checked as well, but neither of us ran the amp with it unhooked so that'll be something to try, I can't honestly answer why I didn't think to do that as it seems pretty obvious now lol.

I don't think that the opt was ever switched, I have no reason to suspect it was at any rate, but I'll check that too. With the amount of repair this amp has had before there's really no telling.

I don't believe he checked the phase shift, but I can't say that for sure. I'll try that myself and report back.
 
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#119
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.
 
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#120
What good guess, and when along the way did you make the guess?

Again, measurements would relatively quickly identify what's going on and where.
 

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