Ameg SVT6 PRO help

[KD8PIQ]

So my beloved SVT 6 PRO messed up the other day.

I have been running the dog piss out of this amp for 5 years or so, and I love it.

After jamming for some time, the sound just stopped.

I shut it down, and the next day checked it out..

plugging a 8 ohm speaker into the amp, then powering it up..
as always it takes about 30 sec or so for the final section's relays to engage.. But now when they do engage, it is as the amp is fully loading the speaker. With all the pot's at min, and nothing plugged into the input, the speaker is jumping full hard out of its cage.

I am a competent technician, but have never worked on an bass amp such as this.

I am guessing a fet, or few died on me.. But even after removing the screws holding them to the heatsink I cant get them to budge.

I found replacements at mouser for cheep, but before I go messing anything up I though I would try and research for help on this project.
I have obtained schematics through Ampeg.
Any good info on this would be greatly appreciated.

Thank you for your time,
-Scott

[two fingers]

There's a guy here named Gerrold (forgive me if I misspelled it) who used to work for Ampeg. If I'm not mistaken, he may have even been involved in the SVT6 project somehow. Anyway, he is very helpful ans usually comes up with the answer after asking a few questions. If he doesn't come by, somebody else will jump all over it. Love this place!

[KD8PIQ]

Originally Posted by two fingers

There's a guy here named Gerrold (forgive me if I misspelled it) who used to work for Ampeg. If I'm not mistaken, he may have even been involved in the SVT6 project somehow. Anyway, he is very helpful ans usually comes up with the answer after asking a few questions. If he doesn't come by, somebody else will jump all over it. Love this place!

Thanks for the input. I hope to get it fixed soon.

[B-string]

Jerrold Tiers is the man. If the speakers jump out and stay there the amp is outputting DC and will fry your speakers. Jerrold may know if this amp has DC protection that has failed as well.

[Jerrold Tiers]

hah..... yes, you have DC.....

The 3PRO and 6PRO for some reason which I cannot explain, have no DC protection...... Every other amp does, and the odd part is that those two amps were designed years apart, by different people.....

Now, as to WHY you have DC..... it is, of course, possible that you have a shorted output mosfet or two..... But if you did, the speakers should be about smoking as well as pushing out.... it would have connected 65 volts DC to the speaker.

Maybe that did happen...... but I think you would be more emphatic about the speakers if you had all 65V behind the problem..

The other possibilities include .....

a leaky cap from the tube driving stage, that is creating some DC offset on the output

A failed DC correction circuit (IC2A, or IC2B along with the connected parts). That would allow the DC to drift off of zero, but maybe not so far.

Something failed in the bias circuit (Q3, Q4, Q18, Q19 and surrounding parts), so that the bias circuit itself has a DC offset which the correction circuit cannot fully correct.

There are lots more possibilities, but those stand out.

tech time..... Since you seem to be a tech type, presumably you intend to fix it yourself.....

The bias circuit provides a small current to operate the bias, small enough that the tube output can drive it to produce output. The DC correction operates to adjust the two current sources to balance the voltage on output to zero...... there is an integrator which passes only DC and very sub-audio frequencies which senses the output DC and makes the adjustment through the 220k resistors.

[Kmonk]

I have a US built 6PRO and have had great luck with it. Everyone I talk to says that it is the head to have. Best combination of power and vintage tube sound. Makes me wonder why Ampeg discontinued it.

[two fingers]

See? Love this place. (Sorry I misspelled your name big guy.)

[KD8PIQ]

GREAT, I don't have time right now to look into it much at the moment; but I will get back to you soon on this. I will note that I have recently stared running a rather strange speaker configuration that may have led to the problem. With my Fluke I measure 2.6 ohms as my load from each feed line. I bet this caused my problem running the master about 3/4.

[Primakurtz]

A terrific, wonderful amp - but not below 4 ohms.

[two fingers]

Originally Posted by KD8PIQ

GREAT, I don't have time right now to look into it much at the moment; but I will get back to you soon on this. I will note that I have recently stared running a rather strange speaker configuration that may have led to the problem. With my Fluke I measure 2.6 ohms as my load from each feed line. I bet this caused my problem running the master about 3/4.

So, you're running an 8 ohm cab and a 4 ohm cab in parallel?

[KD8PIQ]

Originally Posted by two fingers

So, you're running an 8 ohm cab and a 4 ohm cab in parallel?

I recently "started running" 8 ohm and 4 ohm in series. (1) 4 ohm Alpine type R 10", and (1) Kustom KPC15 15"/horn cab ratted at 8 ohms.
(X2)

I use to run 4 of the 15" cabs, but took 2 of the speakers out of line for guitar support.


Bad idea, but I think of mistakes as learning experiences. I was hoping there was a protection circuit in the amp to prevent any problems.

[Jerrold Tiers]

Originally Posted by KD8PIQ

I recently "started running" 8 ohm and 4 ohm in series. (1) 4 ohm Alpine type R 10", and (1) Kustom KPC15 15"/horn cab ratted at 8 ohms.
(X2)

I use to run 4 of the 15" cabs, but took 2 of the speakers out of line for guitar support.


Bad idea, but I think of mistakes as learning experiences. I was hoping there was a protection circuit in the amp to prevent any problems.

Series should not be any issue other than lower power output.....

There are protection circuits, for protection against short circuits, over-temperature, etc. The failure may be just the random type that can occur, and anyhow you don't know what failed yet.

[KD8PIQ]

Originally Posted by Jerrold Tiers

Series should not be any issue other than lower power output.....

There are protection circuits, for protection against short circuits, over-temperature, etc. The failure may be just the random type that can occur, and anyhow you don't know what failed yet.

You are reassuring Mr. Tiers.

I just tested the impedance of the 15" cabs, and with the Fluke I'm reading 8 Ohms. These cab's are rated 125 RMS 250 PEP. So they are not blown. Nor are the 10's. So that is good.

I did use a 12" Celestion G12H-100 single cab for bench test after checking tubes. I found 3 out of 6 dead. I replaced all tubes with NOS 12ax7's. As for all tube where 12ax7's. I did note in the schematics I think the final drivers are suppose to be 12au7's. But they are not.

All pot set to 0 I burnt the speaker to hell. it is dead welded now.

This is the information I have at this point.

Thank you for your time.

-Scott

[Jerrold Tiers]

Originally Posted by KD8PIQ

As for all tube where 12ax7's. I did note in the schematics I think the final drivers are suppose to be 12au7's. But they are not.

All pot set to 0 I burnt the speaker to hell. it is dead welded now.

Yes, final driver 12AU7, previous tube 12AX7.

If you fried it that bad, probably it did get full DC. That still doesn't mean the problem can't be something other than output devices though...... if something else fails, even among the things I mentioned, it is possible to get pretty much full 65 V DC on the outputs.

The protection will allow a LOT of current at full voltage. The amount allowed is reduced as output voltage goes down.

[KD8PIQ]

I have taken HD photos and uploaded them for people to view.
My first approach is to remove the mosfet PBC board.

Then individually test each fet. I have removed the mounting fasteners and even did some de-soldering on the fet's and cannot get the board to budge, nor the fets.

Are these glued to the heat sink? HAHA

Any ideas what I'm missing here?



The only signs of heat are a few wire-wound resistors. I'm guessing they are not a problem.








Here is an over all top view.

[Jerrold Tiers]

Actually, that is not the best first move.

I suggest first, that you turn it on without disconnecting anything (except of course no speakers). Put back the wires and fasteners first, so that you cause no MORE damage than what has already occurred. It seems that the unit is not so shorted that it cannot be on, since you had time to notice the speakers, and it didn't open the fuses or breaker.

Then, measure all the important places for voltage.

Check of course output, also drive point voltage (at bias) , output of the DC correction amplifier, voltages in the current source circuits that supply bias, etc. Check the bias voltage at the bias circuit.... it will be fairly high, because of the mosfets, somewhere in the several volt range.... 4 to 8.

While you are at it, check for all the power supply voltages, including the +- 100V supplies.

Then turn off and let it sit a while. Then check to see if any of the 0.47 ohm resistors in the output are open.... open ones mean dead mosfets. Those (if any) will probably have open 47 ohm gate resistors, and may have open protection resistors as well (2.2k)

The results will probably tell you many things.

You can't remove the mosfets easily because the insulator material tends to stick to them... it has a one-side adhesive as well, but both sides stick after time and heat. Unless you have lots more of that particular material, do not remove anything that you are not forced to in order to replace bad parts.

Checking regular function is always better than a slog through trying to check parts...... if trying to check parts, you often miss leaky parts and bad things that only happen with full voltage present.

[RickenBoogie]

Earlier, the OP mentioned running cabs in "series". I question this, as so many players assume daisy-chaining cabs as series, when in fact, this is parallel. So, to the OP, how exactly, were the 2 cabs wired up? One cable from amp to cab A, and another cable from cab A to cab B?, or did you actually have a specially wired spkr cable made to run the 2 cabs in series?

[Jerrold Tiers]

Good point..... if paralleled, could be bad.

he suggested he was a reasonable tech............. oughtta know that......

[KD8PIQ]

Originally Posted by RickenBoogie

Earlier, the OP mentioned running cabs in "series". I question this, as so many players assume daisy-chaining cabs as series, when in fact, this is parallel. So, to the OP, how exactly, were the 2 cabs wired up? One cable from amp to cab A, and another cable from cab A to cab B?, or did you actually have a specially wired spkr cable made to run the 2 cabs in series?

I'm sorry, I should have been more clear.
The "speaker CABS are in series", not there speakers.

The wiring is PARALLEL, just like jumping a car battery. positive to positive, negative to negative.

As I have stated the Impedance total load on each of the 2 speaker feed lines measured 2.6 OHMs with my fluke.


For those not following.

In the most basic sense series circuits can be defined as any given number of electrical or electronic components, having resistive, capacitive, inductive or conductive properties, connected in an end to end fashion where the current through each is equal. Assuming the total opposition to current flow (be it resistance, reactance, or so on) of each
component is equal, the voltage across each component will also be equal.
In parallel circuits all components are connected to common positive and negative terminals. Assuming the power supply is well regulated, the current through one component has no affect on the current through other components
and the voltage across each component is equal to the supply voltage. This is not true, however, if the power supply is not regulated. A good example of this is the dimming of the lights in a house or other building caused by the starting of an electric motor or large appliance.

[Jerrold Tiers]

Well, that is "PARALLEL" and not the "series" you wrote originally.......That makes a big difference in what you were doing to the amp, which appears to have been overloading it, if you had an 8 and 4 in parallel..... While that shouldn't be an issue current-wise, it does get everything hotter, and heat is the enemy of electronics.

I must have missed the DCR, or ignored it, because DCR can be all over.

Originally Posted by KD8PIQ

I recently "started running" 8 ohm and 4 ohm in series. (1) 4 ohm Alpine type R 10", and (1) Kustom KPC15 15"/horn cab ratted at 8 ohms.
(X2)