I’ve done about 10 gigs with my new Genz Benz STM-900 after buying it on impulse several weeks ago. When I first tried out the amp it was an awesome experience, but over time something had changed. At first I thought it was my imagination, but after about 40 hours of playing-time it had reached the point where it was undeniable.
Play-testing fresh out of the box, I did notice an occasional “crackling” sound when I wasn’t playing – very faint but definitely there, as if a resistor or a tube was noisy. I tried not to worry about it since the Streamliner had oceans of tone and ultra-smooth tube compression, and virtually unlimited output power, and basically it sounded just gorgeous. I sort of figured either the crackling would either go away by itself, or else eventually it would degenerate into more of a full-blown problem.
Over a several-week period, almost imperceptibly at first, I noticed I had to run the volume control higher and higher to get a decent amount of sound out of it. This was going through a 15 inch neodymium speaker in the bottom section of a Carvin BX515N combo amp. Eventually I got to running it with the volume control turned all the way up, and the master volume control at about 80% of full-blast, and this was just to get it to a reasonable level for a tavern-sized gig (and we don’t play all that loud, on the scale of things).
I finally realized something funny was going on at a recent gig where we were the “guest” band, i.e., the house band had us come on and play a set. The house bassist was using a Streamliner STM-600 through an Eden 2×10 cabinet – I believe it was a D210XLT – on top of some kind 15″ speaker, or maybe it was 2×12. I’m not sure. Interestingly, the tone controls were set just about the way I’ve been setting them, with just a small amount of bass boost, with the control set at about 1 – 2 o’clock, and with the mid control set to about 10:30 for a moderate amount of cut, at 600Hz. The treble control was set straight up.
The gain control was set to about 11 o’clock, and the master was at about 12:00, and it sound was out of this world, to my ear it was amazingly powerful and deep, filling a very large room with high rafters. And it seemed vastly more powerful than my setup, notwithstanding my single 15″ speaker versus the stack.
The thing that cliched it was the volume control. The house bassist had it set at about 9 o’clock. But I’ve had to turn mine all the way up and it wasn’t anywhere near that loud. And my amp is supposed to have 50% more power.
I called up Genz Benz and spoke with Jeff Genzler himself, who implied that one of the tubes might be bad. He thought it would be the “first” one, which I guess would be the one on the left. I also asked him if he could share the schematic but he declined, which was somewhat of a disappointment. I told him I might have to reverse-engineer it.
So began the tortuous (not to mention torturous) process of testing the tubes, just as I used to do back in the 70s when I was in high school, and worked in the TV repair shop in the afternoons. First I had to locate an old Superior Instruments Model 85 tube tester which I happened to have in storage, which involved about a day of unpacking boxes stacked to the ceiling in the downstairs closet – to no avail – until finally my wife discovered it in the storage shed out by the carport, along with a 3-ring binder with quite a few pages, with the settings for each tube type. Both items were a bit mottled from quite a few years in the dank environment. I cleaned up the tube tester exterior with a soapy sponge and dish towel, took off the lid, blew off the dust, plugged it in, and let it warm up. The binder was fairly well delaminated so I didn’t try to clean it up, but was just very careful with the pages.
Next I had to remove the STM from the top of the Carvin, which first involved locating a number 2 Phillips screwdriver (everything else was in the tool kit, except that) , and then removing the rack-mount front panel / amp assembly, and then unscrewing the amp from the panel.
After more frustration locating a suitable Allen wrench in yet another box of junk, which turned out to be 5/64″ I believe, and unscrewing all the screws, I popped the top cover and exposed the three 12Ax7A / ECC83 / 7025 tubes (these part numbers are all synonymous).
I was surprised to find a mix of two different tube manufacturers, namely JJ and Ruby. The tube on the left, the one Jeff thought would be bad, was the JJ. It was barely on the passable side of the red/green line, reading of just under 19 on the scale (this was an arbitrary tube-tester expression of transconductance, expressed in Mhos, the inverse of Ohms), on both sides of the tube.
The other two tubes were both Rubys. Ruby Tube doesn’t actually make tubes, but they relabel them somehow. Some Ruby Tubes are known to be JJ, which is synonymous with Tesla, but others are likely made in China. The second tube (the one in the middle of the row) turned out to be the culprit . One of the sections gave a reading of about 8, which was definitely into the red zone, and the other side of the tube showed as completely dead. The third tube, also a Ruby, gave a reading of about 18.7 or so, which like the original JJ tube was barely into the good zone.
As I was digging for the Superior tube tester I also uncovered a fairly large box of of guitar-amp tubes left over from an old guitar-amp project I had done back in the early 90s. It was like a pot of gold – a box full of Sovtek and Tesla 12aX7As with a bunch of Ei 12AT7s from Yugoslavia and also a selection Sovtek 6L6 / 5881 tubes with a handful of EL34s from the Czech republic.
I tested some of the Sovtek and Tesla 12Ax7As and they all gave readings that were higher than the stock tubes in the Streamliner, typically around 22 transconductance units. Somewhere on the web I saw some test results of 12Ax7s, and the Teslas rated better as far as tone and low noise, so I installed a full set of 3 of them in the amp.
Put it all back together for a bit of play-testing, and the results are astonishing. Suddenly the amp has all it’s incredible character back. I’ll try and post a picture of the Genz – Carvin combo when I get a chance.