LOW Output from 50's GA20

[Yeatzee]

Getting Deja Vu with my own personal GA40 I had a similar issue with and never got around to solving it. Have an early GA20 in, matches this schematic:

https://www.prowessamplifiers.com/schematics/Gibson/ga20.pdf

The amp was serviced somewhat already before it came to me, with replaced electrolytics and a 3 prong. The rest was largely untouched. Dimed the amp was barely bedroom volume. I measured the output wattage and it was putting out ~0.6w (2ishvac into 8ohm load) clean. Yikes! Here's what I've tried and done so far:

New rectifier, no change.
New power tubes, no change.

Replaced all of the waxy coupling caps with Mallory 150's, lowered the bias resistor to 150R from 220R, dropped the screen resistor from 10k to 2.2k. This got me up to a whopping 2.7vac clean into an 8ohm load for .9w. Really rocking! Same for both channels.

Here are some reference voltages with all of the tubes in the amp:

Plate to Cathode - 262vdc
Plate Current - 36mA
6v6 Plate Voltage - 275vdc
Screen Voltage - 255vdc
5Y3 Output B+ - 285vdc

Bias is hovering around 90% plate dissipation. Screens were crazy low (210vdc) before I reduced the resistor from a 10k to a 2.2k. I've not run into an amp that ran the screens so low compared to the plates so I decided to just experiment and get it closer, still a big drop with the 2.2k. Obviously that wasn't the fix either way, more of an aside than anything.

So where to next? I've been at it for hours so taking a step away, and hoping you guys can inject some new ideas as to what I'm missing! Amp otherwise sounds great, just low output. Worst case OT. Also don't have a spare 6SL7 phase inverter so may need to order one to test if that could be it.

[Yeatzee]

Adding some more voltage readings for reference. Seems quite low, but I have nothing to go off of. 1meg resistors dropping the voltage a ton for the preamps, but as designed?

[Yeatzee]

Welp, decided to test the OT primaries and secondaries. Primaries seemed normal about 212ohm each side to CT, plate to plate 423ohm. Secondaries, injecting 3.4 volts in via my variac to the 8ohm tap I got a voltage drop of about 1 volt so it's pulling current. DANGIT.

[passaloutre]

What's the AC voltage on the primary when you inject 3VAC on the secondary?

[BrianS]

Altering the design of the amp by changing component values is not a troubleshooting technique.

[Yeatzee]

Altering the design of the amp by changing component values is not a troubleshooting technique.

I've not run into an amp that ran the screens so low compared to the plates so I decided to just experiment and get it closer, still a big drop with the 2.2k. Obviously that wasn't the fix either way, more of an aside than anything

[Yeatzee]

What's the AC voltage on the primary when you inject 3VAC on the secondary?

100v on the primary = 2.6v on the secondary. I've been told a decent drop on the secondary from input voltage indicates a misbehaving OT secondary. Is that not the case?

[Rontone]

http://www.geofex.com/ampdbug/outtrans.htm

Here's a guide that explains a few more test methods for output transformer secondaries

[Yeatzee]

Almost 9w clean with a 5e3 OT subbed in.

[rafe]

Almost 9w clean with a 5e3 OT subbed in.
[/quote


I had to replace the OT in my 52 ga-40. It had been replaced in the distant past , so it was not an issue. I found a Hammond that spec'd out the same as the original and mounted in the original spot like it had been there forever....Still happy.....i don't know if they used the same ot in the ga-20 but the probably have one for it....

[HotBluePlates]

Welp, decided to test the OT primaries and secondaries. Primaries seemed normal about 212ohm each side to CT, plate to plate 423ohm. Secondaries, injecting 3.4 volts in via my variac to the 8ohm tap I got a voltage drop of about 1 volt so it's pulling current. DANGIT.

Almost 9w clean with a 5e3 OT subbed in.

Look at old school "Signal Tracers" like those made by Heath, Stark, Eico, etc.

They tend to have an output tube, an output transformer and a speaker to let the tech hear the noise/signal in the circuit under test.

But they also usually have binding posts for the OT primary (and maybe secondary) that allow the tech to tack the OT into the circuit in place of the amp's OT (for very low-volume testing).  Even though the primary impedance may be "wrong" for the tested circuit, it will be immediately obvious if the known-good OT lets the amp function properly, even with such low-volume testing.

Your experience with trying a new OT confirms this approach.  Other techs might keep a 4kΩ 40w OT on the bench as a "do-it-all Test OT" to tack in-circuit when trying to figure out if an amp's OT is smoked/failing.

[Yeatzee]

Welp, decided to test the OT primaries and secondaries. Primaries seemed normal about 212ohm each side to CT, plate to plate 423ohm. Secondaries, injecting 3.4 volts in via my variac to the 8ohm tap I got a voltage drop of about 1 volt so it's pulling current. DANGIT.

Almost 9w clean with a 5e3 OT subbed in.

Look at old school "Signal Tracers" like those made by Heath, Stark, Eico, etc.

They tend to have an output tube, an output transformer and a speaker to let the tech hear the noise/signal in the circuit under test.

But they also usually have binding posts for the OT primary (and maybe secondary) that allow the tech to tack the OT into the circuit in place of the amp's OT (for very low-volume testing).  Even though the primary impedance may be "wrong" for the tested circuit, it will be immediately obvious if the known-good OT lets the amp function properly, even with such low-volume testing.

Your experience with trying a new OT confirms this approach.  Other techs might keep a 4kΩ 40w OT on the bench as a "do-it-all Test OT" to tack in-circuit when trying to figure out if an amp's OT is smoked/failing.

I've been wanting to find / snag one of those for a while now, seems like such a handy piece of kit! And just cool to own.

The owner decided to buy a Mercury Magnetics OT for it, which worked great and the amp is significantly louder now (though still not a real loud amp!) and was doing closer to 7w or 8w clean iirc. Much smaller looking OT vs the 5e3.

I am getting a couple replacement OT's from AP to have on hand for this, it's the second time I've had a bad OT and no spare around to toss in to test making the repairs take way longer. Worth the investment!