GA-20 "bias" problem

egdon · Post by **egdon** » Thu Apr 28, 2011 4:46 am

Hi, I have an older [octal] Gibson GA-20 amp with an odd problem. I hope one of you can help me with some ideas. The amp was in terrible shape when I got it, and a lot has been replaced and I have cleaned it up pretty thoroughly.

I can hear the amp has a good sound in it, but layered over the top it has an ugly distortion on pretty much any volume. The voltages look at least reasonable to me, and it is cathode biased. I noticed using a dual bias probe that one tube was measuring about 6-9mV [depending on the tube], and the other was at 88-95mV!

The OT is new, and measures 162 ohms between ct and brown, 155 ohms between ct and blue. Speaker is good. Tried swapping the 6V6s in their sockets as well as 2 other matched 6V6 pairs. Very little difference between any set of tubes, voltages are about the same.

With the output tubes removed:
Rectifier B+ 408 VDC - 408 VDC
6V6 plates 379 VDC [V4, OT brown] - 378 VDC [V5, OT blue]

Both 6V6s see the ~200 ohm cathode resistor value the same. The resistor and its bypass cap are new.

With output tubes in:
Rect B+ ~330VDC on both sides
6V6 plates 292 VDC [V4] - 282 VDC [V5]

Again, using the socket type dual bias probe:
V4 6-9 mV
V5 88-95 mV [!]

Voltages between OT leads:
ct-to-brown 1.0-1.3 VDC
ct-to-blue 11-13 VDC

So one is about 10X the other, with any set of 6V6s, but only with the tubes in the amp.

Any have an idea what this could be?

Thanks for the help,

egdon · Post by **egdon** » Thu Apr 28, 2011 4:50 am

meant to add the schematic as an attachment

egdon · Post by **egdon** » Thu Apr 28, 2011 7:23 am

try try again

martin manning · Post by **martin manning** » Thu Apr 28, 2011 10:58 am

What is the voltage on the grids of the 6V6's? I'm thinking leaky coupling cap on the "blue" side...

Andy Le Blanc · Post by **Andy Le Blanc** » Thu Apr 28, 2011 1:50 pm

As you were going through the beastie, did you check for leaky coupling caps?
could be the OPT... but could also be a grid return

check the ..05 caps for leaky dc and the 220k/4.7k/220k grid return
its also the fixed paraphase inverter tube source

DonMoose · Post by **DonMoose** » Thu Apr 28, 2011 3:48 pm

martin manning wrote:What is the voltage on the grids of the 6V6's? I'm thinking leaky coupling cap on the "blue" side...

Suggest you measure this both WRT GND and WRT cathodes (they'll be above GND a ways). We'd like to know how negative the grids are WRT cathodes.

(WRT = With Respect To)

If that's not it, what are your screen resistors up to?

Hope this helps!

martin manning · Post by **martin manning** » Thu Apr 28, 2011 4:27 pm

The cathode should be positive WRT ground, but the grid should read zero. If the grid is above ground, that indicates a leaking coupling cap.

egdon · Post by **egdon** » Thu Apr 28, 2011 8:36 pm

Thanks much for all of these replies!

I tried to measure the voltages as you have suggested, but I'm not sure these readings look right

On the cathodes, I can't really measure any voltage WRT GND...0.00 - 0.02 volts. But the grids are another story. One V4 [brown] I get a very small DC voltage WRT GND. On the other grid [blue] it is up around 18VDC

So measuring these against the cathodes, I get a slight negative at V4 grid WRT the cathode, and I get about +18V on the other.

The 220Ks and the 4.7K that feeds the inverted signal back measure very close to their rated ohm values

On to the coupling caps...I disconnected the grid end of both .05 caps. After they 'settle down' they come to pretty much zero. Even with them disconnected, the grid on V5 still has the 18VDC on it

I also rechecked the wiring around all these components as well, and sweated a few solder connections that looked a little suspect on the 6V6 sockets

martin manning · Post by **martin manning** » Fri Apr 29, 2011 1:15 am

You're right it's not making sense. The cathodes (pin 8's) should be identical and around 20VDC to ground, and the grids (pin 5's) should be ~zero. Are you sure you are measuring at the right pins and that you don't have a wiring error?

egdon · Post by **egdon** » Fri Apr 29, 2011 6:47 am

Martin,

Yes, I was measuring the right pins, and yes, there was a wiring error. I am embarrassed that I didn't find it, especially since I checked over the wiring before my last post. The funny thing is, the error was made by the monkey that came before me, and not only didn't I find it, I assumed it was good and changed things around it to make it worse. Two monkeys, half a brain. Luckily I got the rest of the amp right...

Thank you for pointing me out of the fog, and thanks to all of you who posted replies, they were all helpful and on the mark.

I am just hearing this amp now working properly for the first time. It's really gratifying to finally get it right, and I definitely learned a lot!

martin manning · Post by **martin manning** » Fri Apr 29, 2011 10:59 am

You're welcome on behalf of the group and ourselves and I hope we've passed the audition ;^) Good to hear you got it sorted out.

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GA-20 "bias" problem

GA-20 "bias" problem

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