Ampeg SVT Classic

[Enzo]

J8, J9 are the 12AU7 sockets. There are three small tube sockets on that chassis, the two right behind the two bias controls are the 12AU7 sockets. They are the two closest to the corner. The 12AX7 is in J7, which is near the centerline of the board, next to the slave out jack.

I think your drawing has the 12AX7 on the wrong end of the row.

[nevetslab]

The power tube positions are NOT correct. The right triangle will be V1 at top, V2 and V3 at the bottom, with V3 next to the output Xfmr, and the upside-down triangle is V5, V6 at the top, and V4 at the bottom, with V5 &^ V4 next to the Pwr Xfmr. And, as Enzo pointed out, V9 and V8 are the two 12AU7's, V9 next to the Pwr Xfmr, and V7 12AX7 closer to the Output Xfmr.

Further notes on set-up for measuring the power tubes. I had been (in the past) picked a pair of tubes, usually V3 and V4, and used those front-row tube positions (re: rear view of chassis) and adjusted the two bias pots to get 220mV (22mA) across their respective cathode resistors. Then, left the bias pots set that way, and got current readings on the next two pair of tubes. I still ended up with a list of the 6 tubes' cathode current readings relative to that bias voltage setting, and re-sorted the tubes for a best match to go into the two triads.

I more recently began using a set bias voltage setting, read on Pin 8 of the two driver tubes, and adjusted the bias voltage level equally until I was at or near 220mV across their cathode resistors. While this might be more realistic to show how all six tubes behave at a set bias voltage setting, you'd still find six unique current readings, and then group the tubes for two best-match sets. From there, if one set is higher than the other set, you can still adjust up or down so the end result averages out to 22-23mA per tube. Either method works. You still usually have to juggle the tubes for a best match, re-bias, then let them burn in / idle at that setting for some time, 1/2 hr to an hour, and see how stable they are. During that burin-in process, I have a fan box in place to keep them cooled down.



The two bias pots on the rear panel.....the left-most one (rear view) adjusts the bottom set of tubes V4-V5-V6, while the right side bias pot adjusts the upper set of tubes V1-V2-V3. I always have a set of marked-up schematics on my bench during this procedure, just to remind me the location of the six cathode resistors, as you're looking at the foil side of the PCB during the set-up measurements.

On the chassis, I have the cover plate mounted with only six tube socket screws installed, and no hold-down clamps in place, so I can rapidly install and remove the power tubes. I'm also wearing photographer's print-handling gloves to keep my finger oil off the tube glass, as well as to keep from burning my fingers (have to work fast, the gloves not really much thermal insulation).

When I've made my best-fit match, I remark the tube bases as to their tube position, remove them to a cushioned box to hold them, then go back and install the hold-down clamps.

As seen in some of my instructions, I've also been installing hi temp silicon rubber sleeves on the hold-down spring/top-hat joint to prevent cracking the glass during equipment transit, and also fashioned top-had grommets to fit inside the 1" ID hole, so the tubes are well cushioned. That cut down the failure rate of tubes from getting cracked from the spring/clamp joints digging into the tubes.



Where am I measuring the cathode current at? Look again at the image below (disregard teh scope probe where I had added test points on this set-up):

[Valvehead]

ok---thanks--- the amp seems to be nice and stable at the moment . I will crank it up tomorrow before i put it back together .........man these are tough to go through !

[Enzo]

Like anything unfamiliar, the first one is the hardest. The next time, you will already know how to take it apart, what NOT to take apart, where the wires need to go, etc.

[Valvehead]

yea...im not so sure about another Ampeg svt-Cl...........


do you guys charge extra labor $$ for these ?

[Enzo]

I charge the same rates for most everything. Something that is more difficult takes longer, so my bill will be larger because of that. I don't charge more per hour.

I can check out and do maintenance on a basic Fender amp fairly quickly. If I have a 48 channel mixer with 10 aux feeds, it can take two or three hours just to check all the functions, let alone go inside and clean and lube stuff. Customer pays for my time by the hour.

[nevetslab]

When I find one of these with loose hardware, solder joint fractures everywhere, and needing one or two power tubes to establish a balanced set, it's about a 7 hr job by the time it's done, but then is solid as a rock. They get more exciting when the M3.5 truss head screws on the power tube plate strip out, and the mating standoffs below on the tube PCB un-thread, so you can't get the bloody screws removed. new ones use really cheap cage nuts that fall out of their chassis holes, and lave the chassis screws on stage, so now loose hardware is moving about in the 650V chassis to add excitement. And the PCBs are full of RTV booped everywhere. Still, great amps once set up.

[Valvehead]

Well , everything was fine so I unhooked all the wiring and re-assembled the whole thing back into the head shell. Of course...now it doesnt work....cant wait to take this apart again........

I have output signal for sure..i can hear the amp humming (it was pretty quiet before i put it back in)
no input / preamp signal....... no fault light-still green .

maybe i have one of the 2 wire cables / plugs 180 degrees out ??

or maybe that long gray ribbon cable came unplugged in the preamp case while wrestling this 100 pound bundle of joy

[nevetslab]

I've mis-installed that dual row grey ribbon cable into the preamp before, and the amp lived to tell the tale. It gave me no signal, and I forget what weirdness it gave me,. I was one row off. The pair of 2-pin headers....I have missed one of the pins there, and that didn't allow the relay to close, henc no sound, I've also mis-connected the 5-pin plug and gotten away with that. Fortunately there's a blank pin next to the 345VDC suply wire on that end.

With the cables re-installed into the preamp, you can removed the four screws holding it to the roof, pull it forward to discover where the mistake was made. Assuming it's in the preamp.

Now, getting it mis-installed on the motherboard...that's harder to deal with. And, of course, the preamp has to be disconnected to allow the chassis to be extracted fo find what went wrong there. When I work on these amps, I've never fully unplugged EVERYTHING from the PCB's..Only what's essential to lift up the main PCB, lift out and rotate the AC mains board. The Output board, I do take that out completely. And, of course the preamp board. I hope you don't have to go all the way down to extract the power tube PCB, that's SO MUCH WORK to get it out! I doubt if it's that, since you DID have all working with it out of the cabinet.

BTW, where did you end up with the 6 power tubes, after you checked, grouped and finished biasing up the tubes?

I normally select the tubes for the best / closest current match per upper and lower sets. Form there, you will find if you adjust the bias pot for one sicde enough in either direction , you will start hearing hum. When the two sets are evenly matched and balanced, the hum nulls out.

If you didn't re-install the Ground screw into the AC mains PCB assy, you'll get hum. And, if the ground reference standoff on the Output Xfmr side of the mIn PCB is loose where it attaches to the chassis screw, you will definitely get hum. But you had it working 'open-0face' without that, so I think you're looking in the preamp.

I did finally cobble a tube position drawing and posted it a few threads back...#17.

[Valvehead]

I found it------i was 1 pin off on the gray ribbon to main pcb. Amp is working great now !!! Thanks so much for your knowledge of these horrid beasts sir !! I hope to never cross paths with one again.

Side note--i see you work at Center Staging. A good friend of mine worked there around 20? years ago , drum dept. , name is Paul Thompson.....know him ?

[nevetslab]

Paul Thompson.....I've not had the pleasure of meeting him. I've been there since March 2009.

There have been times I wish I never had to again take the plunge into those amps. I do for sure have to be of a certain mind set to do it. They are an interesting case study of how bad self-inflicting 'wounds' get from sheer mass and weight moving about if never taken care of. You saw it first hand. I often think 95% of my work is digging into products to find solder joint fractures. Anyway, it's been a pleasure.

[Valvehead]

Much appreciated !