Ampeg SVT Classic

[Valvehead]

I just got this one and it had a blown main fuse. Still shows Fault light (red led , front panel) with no power tubes in it. I read on here about some issues with the protection circuit / relay and i have this so far =

relay clicks on power up

Q1 tests E= -3.5v B = - 2.1v C= +2.5v

pin 1 of J36 to cathode of D1 is 19 ohms

R2 measures 10 ohms


From Mr Enzo---

"And if the resistor reads 10 ohms. Then measure the circuit. Measure resistance from pin 1 of J36 to the cathode of D1. That measures R2 THROUGH the circuit.

If you want to test the relay circuit without having the drive signal through R3, just short Q1 E-C. That SHOULD energize the relay. "

I tried that and no change .

I dont see anything burnt or corroded connections --cant find any bad diodes , none of the big power resistors are bad , corrected all solder joints on main pcb

i havent looked at the preamp / control section because i cant seem to get it out of the head shell...!!! any tips ?

[nevetslab]

From your voltage measurements on Q1 of the Relay driver, Q1 isn't in saturation, so the relay doesn't look to be pulled in. You are still getting RED falult light status, which with the protection circuitry, inhibits powering of the HT transformer.

As you already have the amp and preamp out of the case, remove the power tubes, marking each as to their position (use the parts placement diagram on the Power Tubes PCB for your guide. What we're going to do next is power up the chassis with the power tubes removed, and see if you are out of the fault condition, where you should then have GRN status indicator on the front panel.

Then, one pair of tubes at a time (V1 & V4, V2 & V5, V3 & V6 are up/down pairs)....we want to find out which power tubes have a fault. @hat will usually happen in this test is one or more tubes may 'flash' from an internal short, and will fire the protect circuit. I always have my finger on the Standby switch if I see a flash, and also have the amp powered thru a variac/AC Power Analyzer for accurate monitoring. You may want to do one thing prior to this power tube checking test. That is to check to see if you've lost any of the screen resistors. You could use your ohmmeter and measure pin 4 from one tube to all the other tube sockets' Pin 4 position. You should measure 440 ohms (or 44 ohms if the screen resistors are still 22 ohms and not 220 ohms). If you get open circuit in one of the readings, then you'll have to use a different tube socket to determine which screen resistor is open, since we're using two tube's screen resistors in series for testing. Often a screen resistor will burn open from the tube failure.

Anyway, a failed power tube is usually the cause for the Fault Light to be on. If that is the case, and all you have is the one SVT CL, you're now down to less than 6 power tubes. I get thru this problem having an accumulation of pulled KT77's and 6550's from servicing so many SVT's, and usually can come up with a match to the other's plate current at a given bias setting.

If, on the other hand, the fault light still remains lit with no power tubes installed, that often indicates a shorted rectifier diode in the power supply circuits on the HT or Screen supply/bias supply.. We can go forward from there with the troubleshooting.

[Valvehead]

The red fault light is with no power tubes in.

i checked everything on the power tube board--all is good. It has been converted to the 220 ohm res / diodes removed.

All the larger resistors check ok

I think ive checked all the diodes on the main pcb --all check ok

I havent checked anything on the preamp/ pot section because i cant get the damn thing out of the head shell ! does it have to be completely un-wired and slid out the front ??

[nevetslab]

Yes, the preamp has to be disconnected to come out, then for bench testing, re-connected. Depending on the vintage of the amp, the older ones have two black wires on the power switch, and two colored wires on the SB switch. Recent amps have altered that, and began using insulated fast-on's that refuse to unplug. So, make sure to mark or record the two pairs of wires for those switches. Latest versions have change the middle connector to a single-in-line crimped cable, which usually has seating problems . The older ones use a dual-row IDC ribbon cable connector.I also mark the pair of 2-cond connectors at the far end, which carry the 6VAC input and the +/-3VDC cable that feeds the Relay PCB at the other end.

So, you'll have to troubleshoot the protect circuit, which is split up between the main PCB and the preamp PCB.

I recently helped out one of our members on a similar Protection Circuit problem...I'll have to look for that post and add that to this reply, as it does walk you thru the circuits.

Easiest way to find these two posts (and many more) is select Advanced Search, type in Ampeg SVT Protection and hit the Search button. It will come back with two pages of related posts.

Take a look at SVT2 Pro Fault Troubleshooting on pg 2 of the posts listings, dated 12/10/17 started by Zachdude and
Ampeg SVT CL Protect Relay 1/12/18 started by shzmm These two posts cover a lot on the protection circuit and control of the relay.

I did recently have one SVT-CL that was caused by two shorted diodes in the Screen Supply.

Also, I've had some SVT's go into protect from false triggering of IC2B (comparator ckt in the protection circuit). The fix on that is adding a 0.1uF cap between pin 6 & 7.

[Valvehead]

ok i have the amp apart finally..its the version with the long gray ribbon cable. I will read those posts mentioned .

edit--i just went back through the entire amp and cant find the problem--tested every diode in there for shorts....no corroded connectors ..

which tube are you adding the .1 uf cap ?

[Valvehead]

Can you advise where to start looking next?

[nevetslab]

Not a tube...t's IC2B on the main PCB. Now, there may be a 0.1uF cap already in place, as I see in one of my schematics Ampeg had added that part. it's the IC that takes the summed voltages from all the power tube cathode resistors on the (+) input forming a comparator. The 0.1uF cap goes from output Pin 7 to (-) input Pin 6 to suppress false triggering of the fault circuit.

[nevetslab]

You're still in Fault mode, with no power tubes installed, and the relay is NOT pulled in, (normal condition for Fault mode). I would start in the Preamp. Pull the PCB out, and look closely at the pair of 2-pin headers, one that receives the 6.3VAC Heater voltage, and the other that sends back +/- 3VDC. I find those 0.062" square pins develop solder fractures...pin plating & flow-soldering over time, tiny radial fracture rings often appear. I also find them on the 5-pin Power header at the other end of the Preamp PCB, where the HT voltage comes in. Also look closely at the tube socket pins, the input jack connections, and the solder connections on all the pots. Solder fractures on the SVT-CL develop over time if you're gigging with the amp, and it's massive weight in transit helps shake solder joints apart I inspect all connections on that main PCB, particularly supply caps. I rarely find fractures on them, but have in the past. Also, the three M3.5 standoffs that support the main PCB at the front side of the chassis...the one on the output xfmr side is the Grounding path to the chassis from the main PCB. that standoff must be tight, or the amp will hum.

After you've verified solid connections everwhere on this PCB, put it all back together, and then look really close at the main PCB of the power amp chassis. Those Molex square pin headers invariably develop solder fractures...one is heater supply, the other is HT & signal...the other end of that 5-pin/4 wire harness to the preamp. Then, look closely at the two bias pots and all of the other rear panel connectors. The PCB is being supported by those parts, and they take a beating from the weight of the amp during transit. Likewise on the driver tube and input tube sockets. Likewise on the feedback 2-pin header on the edge of the PCB that connects to the Output PCB.

While you're in there, assuming you lift up the main PCB, time to tighten the Output Xfmr mtg screws...two are hidden under the main PCB, one under the output PCB and only one is readily available without having to move anything. Same with teh mtg hardware on the Power Xfmr. One under the AC Mains PCB.

When you remove the AC mains PCB and the output PCB, look for solder fractures on both of those boards. I constantly find the AC mains connector having fractures, often the AC Mains fuse holder on that board. Rarely elsewhere on that AC Mains board. On the Output PCB, the Phone jacks, the impedance switch and the multi-pin transformer secondary connector often have solder fractures.

As you would have the Power tube PCB lifted out during servicing the main PCB in the chassis, look closely at the power tube sockets. Not always, but sometimes I find a board loaded with radial solder fractures on those tube socket pins.

You're ready to re-assemble the chassis and power it up, without power tubes installed. If it's still in Fault mode, it could be a bad IC2 IC3 thru IC6 are the Window Comparator IC's that form the RED/GRN Bias Indicator LED circuit, which ranges from 147mV to 280mV for GRN Below 147mV, NO LED, and above 280mV, RED.

IC2B, if in Fault mode, sends it's signal up to the preamp to drive the LED Status indicator, as well as the inhibit line to the Relay PCB to turn off the HT Xfmr, or prevent it from turning on.

I've read some folks really dislike this Protection circuit, but, I've found it saves a lot of expense that would otherwise occur by letting a power tube do more damage until a mains fuse blows. It also is really handy when starting back up during testing the power tubes. I can got more into detail on that procedure, though it's in my post on the SVT2 Pro Troubleshooting mentioned earlier

[Valvehead]

Thanks for all the info !

Ive pulled every pcb out of this now and ALL had a TON of cracked solder joints. The pre amp pcb had at least 20 . All that has been re-flowed now.

Both the big transformers had loose mount screws ..... the .1 cap on IC2B is in place .

wired it back up and powered on.....no power tubes or the 3 12ax7 in the main pcb ...

going into Run mode....we have a green LED !!

I will start installing the power tubes a pair at a time--with my current limiter on---arent these arranged in some sort of weird pattern ?


Ive never seen an amp with more bad joints than a Marshall jcm 2000 .....wow!

[nevetslab]

Ah....welcome to the effects of stresses from mass and weight of the SVT CL amps. What you've just found everywhere is what go thru keeping a fleet of these amps tamed when facing the riggors of tour life. Loose power transformers with the amp in transit behave like a large shaker table, and vibrate the dickens out of everything else in the amp. And, the mistaken belief by the equipment designers that solder terminals is all that's needed to mount PCB's to a chassis with. Who needs such cost increases like support brackets and additional support hardware anyhow?

The power tubes are arranged in two two triangles. When viewed from the rear, the back row, reading left to right is V5, V6 and V1, and the front row is V4, V2 and V3, with the upper triad of tubes being V1, V2 & V3, and the bottom triad being V4, V5 & V6.

Did I understand you correctly, having 3 12AX7's in the output stage? The two driver tubes V8 & V9 are supposed to be 12AU7's and the input tube is a 12AX7. 12AX7's wont work as drivers in this amp.

When I"m finally at the step of checking my power tubes, I do them in pairs, and have the power amp chassis standing on it's power Transformer edge, with a small block under the pwr xfmr to keep it stable, as shown below:



I will normally set the bias voltage equal on the two bias pots. You could pre-set them to -47VDC (found on Pin 8 of the two driver tubes) and look at the first pair of tubes. It may be too much, and you'd have to increase the bias to another level to get into the nominal window of 230mV across the 10 ohm cathode resistor, which is where I'll normally begin. Each tube will be different. I clip the black test lead to the Ground lead on one of the cathode resistors (assuming they're not gooped with RTV preventing that)

I record each tube's cathode current in this process. and I normally use the same tube sockets in the test. Go back to S/B, wait for the Status light to go to RED then pull the power tubes, which should have been marked as to which tube they are. Insert the next pair and let them warm up. I normally leave the amp on S/B, and switch them out with the heater voltage on. After a couple minutes of warm-up. switch out of S/B and watch the tubes for any signs of arcing. I have a Vaiac and a power analyzer at my bench and I'm always looking at the the analyzer and tubes when I power them up, as often I will find the one tube that caused the amp to go into Protect during this process.

When you're thru all six tubes, you'll have a list of their plate currents. I may at this re-arrange the six tubes for best current matching. I'll install the six tubes at this point, assuming you have six similar tubes. Now, having two bias pots helps, as you can compensate for some that ready much higher currents than the others, and just turn the bias down to be equal to the opposite half.

Disregard the scope probe and soldered-on test points. I did that for a different test procedure, allowing me to look at the AC voltage across the cathodes. But, the image also shows the locations of each power tube's 10 hm cathode resistors, which I used to measure each pair of tues during the initial testing of tubes..

Your amp still has the thick clear vinyl 'garden hoses' as protective housings for the two harness bundles between the power amp and preaamp. On all the amps here at CenterStaging, and others I service regularly, I've replaced those hoses with Tech Flex, which can still handle the heat in the vicinity of the power tubes, while making it so much easier to manage when runing the amp outside of the cabinet. The preamp stands on it's side as seen in teh photo. The stiff garden hoses normally won't allow that..

[Valvehead]

Did I understand you correctly, having 3 12AX7's in the output stage? The two driver tubes V8 & V9 are supposed to be 12AU7's and the input tube is a 12AX7. 12AX7's wont work as drivers in this amp."

looking at the rear of the amp---does the AX7 go in the first/ left hole of the 3 ?

the amp is up and running and im ready to bias--what should i set it at , in mV , the points you show in that 3rd picture ?

1 / 3 / 6 seem to be one side and 2/4/5 seem to be the other of the ot

its got a low freq hum but adjusting the bias pots really helps that.. 128mv / 98 mv the amp is quietest.

[nevetslab]

The 12AX7 goes into V7, wich is closest to the Output transformer. The 12AU7's go in next into V8 & V9.

With the rear end of the amp facing you, and looking down on the power tubes, the front-side row of tubes, left to right are V5, V6 & V1, and the rear side of the tubes, left to right are V4, V2 & V3. There are two triangles of tube grouping. V1, V2 & V3 on the right, and the second triangle is upside down, being V4, V5 & V6.

I'd pre-set the vias boltage at -47V and see what you get when you install V4 and V3. If the RED bias LED's light up, then you'll have to turn the bias pots a bit CCW, to maybe -48VDC and try again.. I want to get the first pair at 23mA, which is 230mV across 10 ohms. Then, I swap out V3 7 V4, install V2 & V5 and record their plate current numbers, then swap them out for V1 & V6 and recor their plate curren values. If the tubes are still well matched, then you can keep the original orientation. If not, then group them for best plate current matching. With all tubes installed, you can fine-tune the bias. I usually have to juggle the power tubes untilI get a best-fit matching. If one tube is a rouge, being 10mA off from the others, I'm replacing that tube, going thru my inventory of pulls, staying with teh same brand of tubes.

More on this when I get home.

[Valvehead]

Thank you sir--

but where exactly are you measuring this on the pcb---when you say across the 10 ohms....is it each of the 6 points in that picture or ? Just want to make sure im in the right spot.

i noticed that when adjusting the 2 bias pots, i can get both red and green led's to light up together...but not separate..or just green. When they are both lit up i have a LOUD low freq hum. I know i need to go with my meter readings ...ive just never worked on one of these Ampegs

[Enzo]

I know it was already discussed, but when I see the lights acting that way, my first reaction is a bad 12AU7.

[Valvehead]

correct ??