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I summarized this for another group and thought I'd share here. Sorry for the repeat info from the previous post.

I gave myself a little light show. I was biasing the new to me Orange Rockerverb. I was using the transformer shunt method to measure current because I don't have a bias probe at home. To bias an amp, one needs to know how much power (in watts) the power tubes are using when at idle; doing nothing. To get this number, one needs to measure the voltage at the plate of the tube and the current running through the tube. Then, using the classic P=IV Ohm's law equation, you multiply the voltage and current together and get the result in watts of what is called plate dissipation.

Measuring the voltage is easy. The multimeter gets set to voltage mode, the meter's ground is connected to the amp ground, and the meter's positive terminal is connected to the power tube plate. The current is more of a challenge. The safest way is to have a 1 ohm resistor between the power tube cathode and ground. Then the voltage is measured across the 1 ohm resistor. Then the other Ohm's law is used; V=IR. Since R=1ohm, that means that V=I in the above case. So the voltage measure is also the current in amps. The dangerous way is to do the transformer shunt method. In this case, the multimeter is set to the mode to read current. Then the probes are placed on the output transformer center tap and the plate of the power tube. One half of the power transformer is shorted out and redirected through the multimeter. The internal resistance of the multimeter is essentially zero, so it gets almost all the current.

So I set my meter to voltage measurement and I took the plate voltage reading. Cool. Then I reconfigured my meter for a current measurement. I carefully took the current measurement. I did the math and realized that the bias was cold. So, I increased the current. Now, when one increases current, the plate voltage usually drops a bit. So, I needed to double check the plate voltage.

However, I forgot to put my meter back into voltage measurement when going back to measure the power tube plates. Remember, it was in current measurement, which is essentially a shorted circuit; no resistance. I got a large flash and the amp went silent. I basically short circuited the high voltage power supply through my multimeter, and inches from my fingers.

There are two 500mA fuses on the PT secondary. Killed them both. D'oh! Also, the fuses in the amp were 1A, but I guess in case of castastrophic f-up that 500mA and 1A fast blow fuses ended up performing the same.

So I got some 500mA fast blow fuses. I put them in last night. I turned the amp on and they blew almost instantly. What the fuck? I thought maybe my biasing mistake had broken something else in the amp. I was hanging with my nerdy friend Greg, and we went over all the possible points of failure. Nothing else was wrong. After a little more digging we realized that the amp needed slow-blow fuses, but for some reason, fast blow fuses of the wrong value were in the amp. Because they were the wrong value, they didn't fail with the inrush current at start up. So now I have the correct fuses on order. Fun times.

Replacing the pots on a David Smith curtis filter.

Traynor YBA-3A:

I was getting weird distortion when I played notes hard, and occasional popping and fizzing when sitting at idle. Took it apart and starting probing with a chopstick around and found a bad solder joint on one of the giant film decoupling caps between the PI and the output tubes. Reflowed some joints in that area, then secured the two caps to each other with silicone. Bench tested while probing and it seems to be solved for now.
Also fixed the non-functional bass boost switch. Turns out it was just oxidized. Contact cleaner and lube fixed that. I cleaned all the switches for good measure while I was in there.

Sunn Concert Slave

I know most folks will say it's sacrilegious to modify any gear that's entered that "antique" range...however, this one needed a mod that I'd argue is at least 35 years overdue.

purists need not worry, it's board tape & should come off easily. Cosmetically, adhesive residue would be least of its worries anyway

Hah!

Retrofitted my oddball 1-P90 walnut solidbody with an end-of-the-neck Kent Armstrong mini humbucker, put the P90 in the parts box, added a switched version of the G&L treble/bass-cut controls, put in an hour with my multimeter to figure out where i'd got the wiring mixed up. Sounds good to me - a gentle bass-cut on a neck pickup feels like a very useful control to have.

First time I've had a functional guitar in a few months. My instruments all look like Sid's toys and I love them for it, but it'll be really nice to be able to play again.

Audio Technica AT4060 Tube Microphone:



Bought the mic on a whim without any of the necessary accessories, like the power supply and 6-pin cable or shockmount.
Found a Sterling/Groove Tubes PSM-1 tube mic power supply with a broken 7-pin connector, so I swapped it for a 6 pin. Converted one of my star-quad mic cables from 3-pin to 6-pin, which worked because the mic only uses 5 of the 6 pins (6.3 V DC heater, 120 V DC B+, Audio +, Audio -, GND).
I had to tweak the power supply slightly, as it uses an LM317 in a Constant Current configuration to generate the heater voltage. The LM317 used in this way is less noisy than when it's used in the more common Constant Voltage mode. The PSM-1 was originally used with a mic that had a 6205 miniature tube, which only draws 150 mA, so the LM317 circuit was scaled for that. The tube in the AT4060 is a 6922, which requires 365 mA. I initially powered it up and could only get about 2 V on the heater, even after adjusting the trimpot (VR11). It just didn't have enough range.

PSM-1 schematic:


I added a 12 ohm resistor in parallel with R11 to scale the current sense input by about 40%. This allowed me to dial in the voltage such that I got 6.3 V under the tube's nominal current draw.

The mic itself worked fine once I got all that settled. Opened it up and found that somebody had upgraded it with a vintage Russian Reflektor 6H23π-EB, which was a nice surprise.

So I'm into this thing for about $300 total, which is less than half what these normally go for.

Nate Dort wrote: Sat Mar 23, 2024 9:03 pm Audio Technica AT4060 Tube Microphone:

Bought the mic on a whim without any of the necessary accessories, like the power supply and 6-pin cable or shockmount.
Found a Sterling/Groove Tubes PSM-1 tube mic power supply with a broken 7-pin connector, so I swapped it for a 6 pin. Converted one of my star-quad mic cables from 3-pin to 6-pin, which worked because the mic only uses 5 of the 6 pins (6.3 V DC heater, 120 V DC B+, Audio +, Audio -, GND).
I had to tweak the power supply slightly, as it uses an LM317 in a Constant Current configuration to generate the heater voltage. The LM317 used in this way is less noisy than when it's used in the more common Constant Voltage mode. The PSM-1 was originally used with a mic that had a 6205 miniature tube, which only draws 150 mA, so the LM317 circuit was scaled for that. The tube in the AT4060 is a 6922, which requires 365 mA. I initially powered it up and could only get about 2 V on the heater, even after adjusting the trimpot (VR11). It just didn't have enough range.

PSM-1 schematic:

I added a 12 ohm resistor in parallel with R11 to scale the current sense input by about 40%. This allowed me to dial in the voltage such that I got 6.3 V under the tube's nominal current draw.

The mic itself worked fine once I got all that settled. Opened it up and found that somebody had upgraded it with a vintage Russian Reflektor 6H23π-EB, which was a nice surprise.

So I'm into this thing for about $300 total, which is less than half what these normally go for.

Man, I got one of these w/ its power supply for about $600 but then sank another few hundred into it when I sent it to Audio Technica for the ocean sounds it was making at times. It took a couple trips back and forth to them but now the mic and power supply are almost new inside.

Still worth it, even if I wish I would have grabbed a different one. Curious how this one works out for you! It’s a really great mic.

A YBA3 ate shit on its way down from the Acadian homeland and now I’ve got a small mess. No thanks to that guy’s packing, unfortunately. The amp sounds great, even if the little fan inside of it sounds like Dying.

Anybody got a couple of Traynor knobs to sell?

What do I check (pot shaft and amp) to make sure there isn’t more damage?

I fired it up with a variac just in case and it sounds exactly like it’s supposed to. Pots seem to work… the one with the knob nub feels a little loose but I have taken the broken part off yet to check the shaft.