Ampeg Gemini II repair

[jtn191]

I bought a Ampeg G15 chassis off ebay which said the main problem was missing the 7199 tube...I rewired it for 6u8a and powered it on today...only to hear no sound but a hum. The hum is present with/without the output tubes (however there is a fizzing sound, sounds like the tubes are warming up?) and doesn't change when the volume knobs are turned. I've ordered a can cap (40-40-40) to replace the one there. The can cap in it now looks original and quickly drops to 40v when powered off. Should I replace any other caps--and do you think this could be the problem?

I've built a 5e3 and a bunch of pedals but would appreciate the help. Full voltages coming soon...

Is the big brown cylinder a cap? it has two connections on one side, one on the other, no markings. Should probably replace the red cap (doesn't seem to hold much voltage)

[01redcobra]

The brown one is also a cap. Once it is out of the clamp, you should be able to find the values somewhere on the cardboard shell. Do you have the schematic? That should help you out.

[52 Bill]

Judging cap quality by how long it takes to discharge is iffy at best. Please try and fix the amp before you randomly recap it.

If it powers up without blowing a fuse, you've got a good first step there. Get the schematic and start noting the voltage readings and compare to the schematic values. Double check the work that you did to make the tube swap.

Once the amp is passing a signal you can start looking to make it better.

[jtn191]

Here's the schem



Ah found it! just a bad connection on pin 7!

[Jazz P Bass]

Man, those voltage readings just gave me a mean headache.

What's with all of the Vac readings?

The important static (ie: no signal) readings are the preamp plate, grid & cathode measured in Vdc.

The heaters are measured across pin 4/5 & pin 9. (this has been omitted)
Vac in this case.

The signal readings are taken in Vac.

A Vac reading is usefull when checking filter & decoupling caps, also.

Looking again (ouch) something is up with V5 pin 3. (zero volts?)

[jtn191]

hehe thanks guys, turned out it was a poor connection on pin 7 of the 6u8a...
wish I hadn't spent 2 hours measuring voltages...