I was just directed to this thread by a friend on another message forum. Several days back I won an auction on eBay for a '69 SVT blueline ($860 including flight case!!), and just two days ago received the amp. The auction description said that the amp was blowing fuses and had been taken to a tech who estimated something like $700 for a complete retube and conversion to 6550s. Lucky for me I do my own tech work so I wasn't scared off by that; I think that's the reason the auction ended so low, well that and the economy.
First thing I did was pull the amp apart and have a look at everything. Two of the rectifier diodes were cracked apart, one of them leaving a nice big char mark on the PCB. Also, every one of the paper electrolytics had a nice little glob of that battery-acid-looking stuff that leaks out of old caps when they go south. To me, these were great signs, as they might explain the blowing of fuses and lead me to believe the transformers are alright.
I tested out all of the tubes on both my Eico667 and Hickok6000A, and to my surprise, every single one tested stronger than new, with one exception-- one of the 6146Bs shows a short (on both testers) from the grid to cathode. So it's sort of funny to me that the seller's tech was calling for a complete retube when it needs nothing of the sort. The output tubes are all Westinghouse 6146B Made in USA with 88-12 stamped above the Westinghouse logo.
My plan moving forward is to order the new cap kit from Fliptops and pick up a bunch of 6146B's-- found a seller on eBay who sells matched sets (matched at lower than SVT voltages, but hey, who actually sells 6146Bs matched at SVT voltages?) and is willing to hook me up with a large quantity of tubes that are "matched" by his standards. I'm thinking if I get 12 or 14 that should be enough that I'll not have to think about output tubes for this amp ever again. Just need to save up the cash to pull it off.
In doing my research on the 'net, I found out that this amp appears to be from that original production run. First off, it has an unpainted chassis, which from what I read was a characteristic of that first run. Second, it has the relay socket, though like the one in JimmyM's amp, this amp has a plugin piece that hard-wires it for operation, and it's got a standby switch on the back plate. The naming on the back plate reads "The Ampeg Company, Inc -- Linden, N.J. USA".
All of the amp components and solder joints appear original, with a couple exceptions. First, the power cord... the amp has what appears to be a receptacle where a power cord could have plugged in, but it's been filled in with expoxy. I haven't tried to pull the epoxy out and doubt that I will. I'll put a grounded power cord on, but other than that will leave that aspect as-is. The second thing that I doubt to be original is the speaker wire, which isn't the four-conductor like you'd expect (I actually have a Neutrik 4-pin-to-1/4" adaptor that someone gave me with a B25B I bought a few years back, which I apparently still don't need) but rather has a standard 1/4" termination. The cable is awfully short, and it's quite likely I'll switch it to a standard 1/4" output jack so I can use my own speaker cable - the EV 2x15" cab I usually use for bass has the receptacle down near the bottom, so the speaker cable needs to be fairly long.
The tolex is in non-mint but very nice shape, and the faceplate is frighteningly perfect. From the photos in the eBay auction I was worried it was a repro, but it's not, it's just in great shape. I won't do anything to it as far as cosmetics go, since I think it looks great as-is and there's no need to de-vintage it.
This is only going to be the second SVT I've worked on. The first is a '73 that I picked up on eBay a couple years ago and recapped and retubed. It will be really nice to have both of them at the practice space at the same time and A/B them to see if the 6146 version performs any differently to my ears.
One thing I'm intending to do and would love to hear feedback on is to switch the screen grid resistors to 1K 5W ceramics. My '73 came stock with diodes paralleled across the screen resistors which from what I understand was a bad solution for an annoying problem. Anyone see any issue with using 1K 5W resistors as screen grid stops?
I'll take some before and after pics when the cap kit arrives. She's a beauty.
And in case anyone is interested, I don't even remember who directed me to it, but I have a copy of the mother of all SVT schematics, it's 15 pages and has every preamp and power amp variation I've ever seen, including both of the 6146B versions.
http://toomanyhelicopters.com/schems/svt.pdf
There are a whole bunch of other schems in that directory, feel free to poke around. The only one that's there that you couldn't find anywhere else (as far as I know) is a scan of a Traynor YBA-3A with some hand-written power supply modifications that I scanned from one of my 3A's. Other than that, they're all schems that you could find in other places if you do enough searching- that's where I got them!
