Ampeg SVT-6 Pro No Output

[ca7922303]

You want to have the Bias pots on both the Upper and Lower Power amp stages set to CCW. Forget where they were. Before applying AC Mains, lets double-check the Bias Pots and the do a diode-test of the Base-Emitter junction of Q2 and Q22 (bias xstrs of the two output stages) with the bias pots set to CCW. That should set the 10k bias pot so you're placing the 10k pot in parallel with the Base-Emitter junction. If it reads as a short, then the pot is set for Max Bias (Fully CW). That would have Q2/Q22 turned off, which lets the Gate Busses go to max voltage and turn the MosFET's on hard....which will kill the lot of them. I'm just going by what I see on the Power Amp Schematic to help get you oriented with this.

I have NO Idea why one of your IRFP240's is reading 0.4VDC across it's Source resistor, when all the others are reading 0mV. I see Vgs readings written on the Drain slab of the back of each MosFET. Are you re-using the Bergquist Blue-grey insulator strips that you had to pry the blown MosFET's off of, or do you have fresh insulator strips in place on the heat sink....or instead Greased Mica washers to insulate each of the MosFET when you mount the amp assemblies to the heat sink?

I believe I had advised about the potential damage that occurs when you have to pry the MosFET's off of the heat sink, which happens from the Thermoset of the non-mica insulator, and what usually happens is part of the embedded insulation breaks away from the sheet insulator. When that happens, you no longer have viable insulation. Reusing that can lead to shorting the Drain slab thru that damaged portion of the insulator to Chassis Gnd on the heat sink. Any chance we're dealing this this?

That's the only place I can see where you might have an issue. Unless you have an open Gate resistor. Those are the 47 ohm Flame Proof 1/2W resistors (stock Ampeg parts). I've been using small-body 1W Metal Oxide parts in their place, also flame proof. I think you had mentioned having some or all of those damaged.

What else would cause just one of these to be giving you what sounds like this one IRFP240 is turning on hard, reading 0.40VDC across it's Source Resistor while the rest are reading 0V? Open traces? PCB damage that wasn't found in the rebuild?

Q2/Q22 base to emitter diode test are both (0.6v). Both bias pots are at full CCW and have fresh insulator strips.

[nevetslab]

So, from this state, if you apply power, are you getting this one IRFP240 conducting, while all the others are turned off? That's 850mA of current you're reading thru this MosFET, while the rest are not conducting. What is the gate voltage at this MosFET, and what is the Gate Buss Reading?

[ca7922303]

So, from this state, if you apply power, are you getting this one IRFP240 conducting, while all the others are turned off? That's 850mA of current you're reading thru this MosFET, while the rest are not conducting. What is the gate voltage at this MosFET, and what is the Gate Buss Reading?

Lower board gate resistors(-258vdc). Have one 240 reading(-180vdc). Upper board gate Resistors (-320vdc) on all 10. Can you point me in the right direction of the gate buss? Thanks.

[nevetslab]

Lower board gate resistors(-258vdc). Have one 240 reading(-180vdc). Upper board gate Resistors (-320vdc) on all 10. Can you point me in the right direction of the gate buss? Thanks.

The Upper Half Output Stage Gate buss is common to: R28, R29, R30, R31 & R136 Gate Resistors....also common to D28 Anode, D4 cathode, R16, R10 & Collector of Q3. lower Half output stage Gate buss is common to: R40, R41, R42, R43 & R137 gate Resistors, also common to D29 Cathode, D7 cathode, R17, Bottom of AP1 Bias pot and Collector of Q4. Look at the schematic for the (+) and (-) halves of the Power Amp Schematic. These parts are on the (+) half of the Bridged-Pair Output stage You can read the schematic to see the (-) Half to get those designation numbers.

In the physical layout, each MosFET has a 47 ohm resistor (Gate Resistor) and a 2.2k resistor (current sense resistor). One side of the Gate resistors connect to the Gate. The other side is the Gate Buss. The Layout Drawing is PWA 1000W Bass Amplifier PCB Layout. You'll see the two Gate busses, as well as the two Current Sense busses.

Where on earth are you getting -258VDC, -180VDC, -320VDC readings? Your GND probe is NOT at Ground, and the highest voltage on these Power Amps is +/- 65VDC. I do see there is a +300VDC Power Supply to run the Front End Tube circuit ahead of the MosFET Output Stages, that work as a Bridge-Pair Power Amp.

[ca7922303]

The Upper Half Output Stage Gate buss is common to: R28, R29, R30, R31 & R136 Gate Resistors....also common to D28 Anode, D4 cathode, R16, R10 & Collector of Q3. lower Half output stage Gate buss is common to: R40, R41, R42, R43 & R137 gate Resistors, also common to D29 Cathode, D7 cathode, R17, Bottom of AP1 Bias pot and Collector of Q4. Look at the schematic for the (+) and (-) halves of the Power Amp Schematic. These parts are on the (+) half of the Bridged-Pair Output stage You can read the schematic to see the (-) Half to get those designation numbers.

In the physical layout, each MosFET has a 47 ohm resistor (Gate Resistor) and a 2.2k resistor (current sense resistor). One side of the Gate resistors connect to the Gate. The other side is the Gate Buss. The Layout Drawing is PWA 1000W Bass Amplifier PCB Layout. You'll see the two Gate busses, as well as the two Current Sense busses.

Where on earth are you getting -258VDC, -180VDC, -320VDC readings? Your GND probe is NOT at Ground, and the highest voltage on these Power Amps is +/- 65VDC. I do see there is a +300VDC Power Supply to run the Front End Tube circuit ahead of the MosFET Output Stages, that work as a Bridge-Pair Power Amp.

I assumed when you asked for voltages of the gate resistors that I was going black probe to ground. My mistake. So am I to measure voltage across the gate resistor itself. Red probe to one end of gate resistor, black probe to other end of gate resistor?

[nevetslab]

I assumed when you asked for voltages of the gate resistors that I was going black probe to ground. My mistake. So am I to measure voltage across the gate resistor itself. Red probe to one end of gate resistor, black probe to other end of gate resistor?

I'm looking to know WHAT the voltage of the Gate Buss is on the two Output stages...each one having a Positive side and Negative Side Gate Buss. Then, what the gate voltage is at each of the MosFET's. This is more for your checking so if there's one or more of the MosFET's having the gate voltage vastly different than the others, such as at this one MosFET that was turned on hard (where you measured 400mV across it's 0.47 ohm Source resistor. I would expect to see very similar gate voltages. The Voltage seen on the Source resistors, that should be done relative to the Output Buss. All of the Source Resistors are Common to each amplifier stage's Output Buss, so looking from there (treating that output as your '0V reference), it will tell us what the current balance is on each of the MosFETs.

[ca7922303]

I'm looking to know WHAT the voltage of the Gate Buss is on the two Output stages...each one having a Positive side and Negative Side Gate Buss. Then, what the gate voltage is at each of the MosFET's. This is more for your checking so if there's one or more of the MosFET's having the gate voltage vastly different than the others, such as at this one MosFET that was turned on hard (where you measured 400mV across it's 0.47 ohm Source resistor. I would expect to see very similar gate voltages. The Voltage seen on the Source resistors, that should be done relative to the Output Buss. All of the Source Resistors are Common to each amplifier stage's Output Buss, so looking from there (treating that output as your '0V reference), it will tell us what the current balance is on each of the MosFETs.

Upper board all gate resistors (0.00mv)
240 source resistors (334mv)
9240 source resistors (0.00mv)

Lower Board all gate resistors (0.00 mv)
240 source resistors(5vdc) 1@(3vdc)
9240 source resistors (0.00mv)

[ca7922303]

I'm looking to know WHAT the voltage of the Gate Buss is on the two Output stages...each one having a Positive side and Negative Side Gate Buss. Then, what the gate voltage is at each of the MosFET's. This is more for your checking so if there's one or more of the MosFET's having the gate voltage vastly different than the others, such as at this one MosFET that was turned on hard (where you measured 400mV across it's 0.47 ohm Source resistor. I would expect to see very similar gate voltages. The Voltage seen on the Source resistors, that should be done relative to the Output Buss. All of the Source Resistors are Common to each amplifier stage's Output Buss, so looking from there (treating that output as your '0V reference), it will tell us what the current balance is on each of the MosFETs.

Lower board busses(260vdc)
Upper board busses(325vdc)

[nevetslab]

Upper board all gate resistors (0.00mv)
240 source resistors (334mv)
9240 source resistors (0.00mv)

Lower Board all gate resistors (0.00 mv)
240 source resistors(5vdc) 1@(3vdc)
9240 source resistors (0.00mv)

You either have a major problem on the N-Ch side of your Upper and Lower Boards.....334mV across all of your Source resistors have these IRFP MosFET's turned on hard....711mA! The lower board....with 5V across the Source resistors...these are turned on VERY Hard...10.6Amps! Sounds like these devices are shorted.

Lower Board busses at 260VDC.....and Upper Board Busses 325VDC. Where in the hell are you measuring? Are you saying relative to GROUND, the output Buss on each of these amps is at those potentials? I find that very hard to believe. The +/- DC Supplies are nominally +/- 65VDC. and, you claim to have 4 and 5 times that positive buss voltage from +/- 65VDC. I don't believe you. You're doing something wrong.


Relative to GROUND, WHAT is the Gate Buss on the two amplifier's? Have you figured out HOW to identify the Gate Busses from the PCB Layout drawing?

[ca7922303]

You either have a major problem on the N-Ch side of your Upper and Lower Boards.....334mV across all of your Source resistors have these IRFP MosFET's turned on hard....711mA! The lower board....with 5V across the Source resistors...these are turned on VERY Hard...10.6Amps! Sounds like these devices are shorted.

Lower Board busses at 260VDC.....and Upper Board Busses 325VDC. Where in the hell are you measuring? Are you saying relative to GROUND, the output Buss on each of these amps is at those potentials? I find that very hard to believe. The +/- DC Supplies are nominally +/- 65VDC. and, you claim to have 4 and 5 times that positive buss voltage from +/- 65VDC. I don't believe you. You're doing something wrong.


Relative to GROUND, WHAT is the Gate Buss on the two amplifier's? Have you figured out HOW to identify the Gate Busses from the PCB Layout drawing?

Each MOSFET is connected to gate resistor. The other end connects to a common trace(buss), I am checking that point with one lead of dmm and other lead of dmm to ground. If this is not correct, please advise. Thanks.

[nevetslab]

Each MOSFET is connected to gate resistor. The other end connects to a common trace(buss), I am checking that point with one lead of dmm and other lead of dmm to ground. If this is not correct, please advise. Thanks.

That IS correct. Each output stage has two Gate Busses. N-Ch will be positive, P-Ch will be negative.

[ca7922303]

That IS correct. Each output stage has two Gate Busses. N-Ch will be positive, P-Ch will be negative.

So with one dmm lead on buss and one dmm lead on ground, why am I getting these high voltages do you think?

[nevetslab]

Probably because something else is wrong. You need to take this apart and find what mistakes you've made to be causing all this.

Also....these MosFET's were 'highly matched', from what I recall reading. What needs to be matched is the transconductance, which relates to how much current is passing thru the Drain to Source. If 'Matched' referred to the Gate Voltages, well....those typically ARE very close to one another, and you see those numbers written on the back of the metal slab of the parts. THAT's NOT what you need to be Matched. Those WILL BE that way

From what you're going thru on this project, I think you're way in over your head regarding your skill level and understanding. I'm trying to lead you to solving this, and we keep being hung up with basics that show your lack of understanding. The evidence is revealing you have major problems, and now you've introduced DC Voltages that you can't get unless you've somehow connected the Tube Power Supply over to what feeds the MosFET output stage.

Did you take photos of this amp, removed from the chassis, as well as before you disassembled it, so you have visual aids to guide you in being able to reassemble it? These are basic things when starting from scratch with an unknown. All of us here, when crossing into uncharted territories inside a chassis, go thru that photo-taking or sketching what is what, where things connect, placing tape labels on crucial wires so they go back properly, etc. Fail to do that, and things start to look the same to an uneducated eye.

I'll have to read all thru this thread to see just where you're at, as we should NOT be having this sort of conversation this far in. Sorry.

[ca7922303]

Probably because something else is wrong. You need to take this apart and find what mistakes you've made to be causing all this.

Also....these MosFET's were 'highly matched', from what I recall reading. What needs to be matched is the transconductance, which relates to how much current is passing thru the Drain to Source. If 'Matched' referred to the Gate Voltages, well....those typically ARE very close to one another, and you see those numbers written on the back of the metal slab of the parts. THAT's NOT what you need to be Matched. Those WILL BE that way

From what you're going thru on this project, I think you're way in over your head regarding your skill level and understanding. I'm trying to lead you to solving this, and we keep being hung up with basics that show your lack of understanding. The evidence is revealing you have major problems, and now you've introduced DC Voltages that you can't get unless you've somehow connected the Tube Power Supply over to what feeds the MosFET output stage.

Did you take photos of this amp, removed from the chassis, as well as before you disassembled it, so you have visual aids to guide you in being able to reassemble it? These are basic things when starting from scratch with an unknown. All of us here, when crossing into uncharted territories inside a chassis, go thru that photo-taking or sketching what is what, where things connect, placing tape labels on crucial wires so they go back properly, etc. Fail to do that, and things start to look the same to an uneducated eye.

I'll have to read all thru this thread to see just where you're at, as we should NOT be having this sort of conversation this far in. Sorry.

I took photos before disassembly and reinstalled with photos and schematics. Will check over it again. Would a diode test on mosfets be possible in circuit to show whether there are any shorts ?Thanks for your help.

[nevetslab]

I looked at the website where you bought the MosFET's. The only thing I found there, which had to be toggled to get additional information on what they had to offer....they made NO MENTION of the parts being Matched for Transconduction...which translates to yielding equal Current sharing which is needed on these Ampeg SVT-series MosFET Output Stage amps. So, we have NO idea what to expect. That's why I wanted to have you slowly turn up the Bias so we could start seeing them conduct, and get readings without any going way high in current (voltage across the 0.47 ohm Source resistors) while waiting for those not yet turning on to see where we are. Not dissimilar to setting bias on power tubes, and matching them.

What we have instead is some of these that are turning on hard (334mV across the 0.47 ohms) or 5V across them). I have NO IDEA how you have this occurring. 5V across is 10A thru the MosFET, which may be dead/shorted now. With them mounted, best you can do is to unsolder/lift the 0.47 ohm Source Resistors up, unsolder/lift the Gate Resistors up, and also the 2.7k Current Sense Resistors up. That now has the MosFET only connected to the +/- 65VDC power supply busses. That has them isolated enough (with NO POWER APPLIED....disconnected from AC Mains) to THEN check each and ever one of them for shorted Drain/Source, Gate/Source, Gate/Drain.

Have you learned how to turn one on and off with your DMM in Diode Test mode? With your meter in this mode, we'll do a IRFP9240 P-Ch MosFET. Place your BLK probe on the gate, the RED probe on the Drain Then move the Blk probe over to the Source, leaving the RED probe on the Drain. It should measure something like 0.005V in either Drain-Source or Source-Drain. Then bias it the opposite way....RED probe to Gate, BLK probe to Drain. Move the RED probe to Drain, and BLK probe to Source. Should read around 0.51, and reverse....BLK to Drain, RED to Source, open circuit. For IRFP240 (N-Ch MosFET, reverse the probes and do the same thing. My Fluke has an 8V potential behind it's 1mA Current Source, so there's enough voltage behind the current source to turn on a MosFET like this. Can't say about yours.

You'll have to fully isolate your MosFETS (forget the fact that the Drain is soldered into the PCB....there's no circuit path to the other two leads having lifted the gate and source resistors, as well as the Current Sense resistors.

I hope you don't have shrapnel now installed instead of working MosFET's.