Picked up a nice old Ampeg.

[sangandongo]

I found an ad on Craig's List for a lot of 260 NOS tubes and a tester for $150 bucks. I went down to pick them up and the guy had an Ampeg Gemini II G-15 in the back of his pickup with the tubes. He said Guitar Center would give him $225 for it. I ended up skipping on the tubes and buying the amp from him for $200.

I got it home, pulled off the baffle to figure out what year it is. The serial number says its a '67 but the pot codes say '68. The power tubes in the amp were bad, as were the 12AX7s, but the rectifier and other preamp tube tested very high. Unfortunately, I don't have my tube biasing tool anymore, so I have to buy another one (all my amps are cathode biased early '60s from Gibson or DanElectro).

I'm pretty sure the amp is biased wrong right now because those tubes burn like the sun. I don't want to fry the screens, so I'm going to let it sit until I get a new tool.

Anyhow, does anyone have experience with this amp? What are your thoughts? I'm debating selling it but will have to wait until I bias and retube it to decide.

I think mine is something of a misfit, because it has the Ampeg logo on the grill cloth rather than the frame of the cabinet. Also, the tolex isn't checker board and there is a gold lining running around the front edge of the cabinet. Even the knobs don't look like other G-15s, though they do match other Ampeg amps I've seen.

--
john

[woodyng]

that sounds like a killer deal- i just bought an ampeg sb-12 and was happy to get it for $600.00. mission accomplished,bushbucks blown!

[sangandongo]

Nice one! I'm actually looking for a bass amp from that era. I wonder if this G-15 can handle the lower registers without blowing the cone out. Perhaps i'll sell this puppy and put it towards finding something like an SB-12.

[woodyng]

i am not all that familiar with these amps, but i would think if it you have a 12 or 15 inch speaker in there,you should be able to play bass through it,at least at a low volume. if it is an open back type of cabinet,it might not work very well for the best bass tone. mine had the original speaker replaced with something that tended to distort a little quicker,probably to benefit the sound for guitar,but it sounds good to me for bass as well-my other amp is a markbass combo,so i was looking for some dirt with this one...unfortunately,i purchased it while on vacation,and it is in oregon,while my basses and i are still in memphis! (waiting to move....) i played several basses through it at the shop in eugene where i bought it,but they didn't have any rick basses laying around,so i am real anxious to hear how it's gonna sound with the 4000,the 4004,not to mention the danelectro hodad......

[soundmasterg]

With the tubes pulled, and the amp on, you should be able to get a voltage reading on the grids of the power tubes to see at least if the negative supply is working or not, and if the voltage is too low or not. The schematic should list a negative voltage at the grids, which would be approximately the negative voltage desired to bias the tubes correctly. The actual supply voltage should have the capability to go higher and lower than that nominal voltage listed on the schematic, though with the tubes out, it will read high most likely. Once you have verified that a negative voltage is being generated and is showing up on the grids of the power tubes, then you can try with your future bias tool and the tubes plugged in to see if you are able to bias the tubes correctly without smoking them. I do have a Bias King Pro that could sell if you were interested in it. It works just fine, although it doesn't check the plate voltage and the bias current like the Weber meters do. It only checks the bias current. If you are interested in it, then PM me.

Greg

[sangandongo]

Greg,

Its been a while since I've had to bias an amp and can't recall which points I need to hit with the voltimeter to find the current. The echo of a memory says pin 8 for positive and ground on negative.

I keep blowing fuses in the amp. Not sure what the cause is yet. I will probably convert it to 3 prong while I have the chassis open. I actually threw a pretty big arc the day I got it because I inadvertently mixed it on a strip with a 3 prong amp. Yes, I kicked myself, no need to reiterate it.

--
john

[soundmasterg]

To properly set the bias on a fixed bias amp, you need a way to measure the cathode current. This needs to be done in series instead of parallel. So you can solder a 1 ohm 1 watt resistor between each cathode (pin 8 usually) and ground, disconnecting the wire that connects the two sockets, and disconnecting any other wires that connect pin 8 to ground. You can also get one of the meters like the Weber Bias Rite, the Bias King Pro like the one I'm selling, or many others that are similar, and plug your tubes into the meter sockets, then put the meter sockets into the tube sockets. The meter will show you the cathode current, though when you multiply it out to get the dissipation as I explain below, you have to remember to convert that number to milliamps. So a reading of 25.6 would be 25.6 milliamps, or .0256.

If the amp is cathode biased, then you can measure across the cathode resistor like you mentioned and you would get a voltage reading that could be converted to the current using ohms law and the value of the cathode resistor.

With both, you also need to be able to measure the plate voltage (pin 3 usually) and then multiply that times the cathode current to find out the plate dissipation, and then compare to the specs for whatever tube you have in the amp. You should be aiming for 60% to 70% of the maximum dissipation of the tube in question. Strictly speaker, when you measure the cathode current, you're getting the plate current and screen current added together, but the screen current is usually small and it errs on the cold biased side of things, so you can usually ignore it.

Blowing fuses can be bad power tubes, a miswire somewhere, or a bad power transformer. Try the power tubes first. If the fuse still blows, then disconnect all the secondaries on the power transformer, making sure they don't touch anything, and leaving the primaries connected. Take the tubes out, and replace the fuse, then turn the power on. If the fuse blows, the transformer is bad. If it doesn't then it is good, and the problem could be somewhere else.

A 3 wire power cord is a very good idea for safety reasons. Please be sure to wire it correctly and safely. If done incorrectly it can be very dangerous!! The black wire should go to the fuse, and then the switch, and then the transformer. You may have to get some terminal strips to be able to accomplish this as the vintage way is to stick the fuse and switch on opposite sides of the transformer and it has to be changed. It will still function if you didn't do that, but it is not safe because in the event of a fault condition, the AC voltage could be interposed onto the chassis, and this is dangerous. The white wire should go direct to the transformer. The green wire should be on it's own solder lug and bolted to the chassis in it's own spot at least an inch from the plug where the wires come into the chassis.

Hope that helps?
Greg

[scott_s]

If I'm not mistaken, Peter Buck used a Gemini II for much of the "Murmur" sessions.

- Scott