How can I find out the output resistance/ohms of my amp? Gibson GA77RET

[kman007]

I am replacing the pair of 10" speakers in a early/mid 60's Gibson GA77 RET tube amp and need to know what speakers to use and or wiring to match the impedance up with the amp The tubes compliment is 3x6EU7, 2x12AU7, 6CG7 (aka 6FQ7), 2x6L6, 0A2, about 25 watts. No indication of the ohms on either the main or extension speaker jack(s). I am a noob but can install some speakers (I thought). Any help would be greatly appreciated.

[The Dude]

According to the schematic the GA77RET uses two 8 ohm speakers in series (16 ohm total load).

[flyingdutchman]

If you still have the original speakers with their voice coils intact, you can use any decent multimeter to determine the speaker's impedance.
Put the multimeter in low ohms range (usually 200 ohms on a run-of-the-mill DMM) and firmly press the probes to the speaker terminals.

The reading you get on the meter is usually slightly lower than the speaker's nominal impedance. So if the meter reads 7 Ohms, you're probably dealing with an 8 Ohm speaker.

Before doing the actual measurement, it might be a good idea to touch the probes together and check the reading. In theory, it should read 0 Ohms, in practice, it won't. Resistance in the test leads and connections will conspire with offsets in the meter circuit such that a dead short may read around 1 Ohms. Keep this in mind whenever you use a (cheap) multimeter to measure low-ish resistance values.

[kman007]

First off a big "Thank You" to His Dudeness, schematics are more art and awe than info. That was my plan but at this point my reasoning was that I had a very limited selection, now I have the confidence to advance. Flyingdutchman, at least one of the speakers I was taking out was already a weirdo replacement and they were real shot...just the occasional "gurgle buzz" if you attacked the signal.

But thanks again, what results! I really appreciate the guidance.

[The Dude]

Given the age of that amp the "gurgle buzz" you are hearing could be bad electrolytic caps. Most of the old Gibbys I've worked on have had bad caps. You may want to hook up the speakers you have to another amp first and see how they sound. It may not be the speakers at all.

[kman007]

I was doubtful as I have blown some speakers in my day but never to the point where it didn't make any noise. But when I was told that our local reliable amp tech had performed a full service (filter caps, tubes, grounded cord, etc) I got the stroke of genius to try it with an extension cab and it sang strong. Do you have much experience with these amps? Some say it had the ole Tel Ray oil can reverb and others say a more traditional spring reverb setup. I see three RCA jacks on the chassis...the owner seems to remember our tech saying it needed a rare tube to get the reverb working but IMO the tubes are easy finds. I bought a strong tremelo tube for some added strength perhaps but it is fairly mild. Also saw something about the tremelo being based on a light sensor/bulb setup and thought maybe that is what the tech was describing to the owner. Regardless, any advice or info on those RCA jacks would be helpful, otherwise it still sounds nice without reverb. Thank you so much.

[The Dude]

The schematic is available HERE.
The tremolo is a traditional LDR setup (LDR-1 on the schematic). The schematic also shows something other than a typical tank reverb. I haven't seen one of these for a long time, and I don't remember for certain what the jacks are for and I hate to give you misinformation. Someone here will know for sure. The tubes are all available, albeit some are expensive compared to more common tubes.

[flyingdutchman]

The schematic also shows something other than a typical tank reverb.

Looking at the circuitry, I'm pretty sure that this something is indeed an "Oil Can" Echo/Reverb. The clues are the high impedance drive circuitry, and the presence of something that looks like a motor.

I haven't seen one of those in ages. The last one I encountered was in an old Farfisa amp, and was no longer functional.