First build, only getting faint volume

[knavewave]

Just finished my first build (its a two stroke from the guitar amp handbook), and while I am getting a very nice sound out of the amp, its only audible if your head is a few inches from the driver.

While my electronics skills are supremely lacking, the rough output I'm observing on my scope / multimeter looks reasonable from input jack up to the output tubes (6v6). In particular, I can observe the amplification / phase changes / coupling effects acråoss both passes through the 12ax7, and appear to be getting a 200V to 220V peek-to-peek swing arriving at each 6v6 while playing. I have also hooked the scope up to the speaker jack wires, and have observed suspiciously low voltage swing... Unfortunately I don't have the numbers at the moment, but I think its in the 1 - 10mV ballpark, and with only minor changes to the waveform occurring when playing.

Here's the readings I'm getting to each 6V6 pin...

Code: Select all

                          6V6#1    6V6#2
Pin 2 - Heater          - 3.1V   - 3.1V
Pin 3 - Plate (Anode)   - 464V   - 464V
Pin 4 - Screen Grid     - 444V   - 444V
Pin 5 - Control Grid    - 177V   - 178V  - 220 peek-2-peek at full tilt
Pin 7 - Heater          - 3.1V   - 3.1V
Pin 8 - Cathode         - 444V   - 444V  - 220 peek-2-peek at full tilt

When using the scope I also noticed that if merely touch my scope's 1x probe to any point between 6v6 grid and the final 12ax7 coupling cap I get a boost in volume, though it remains very very low. My initial thought was that there might be a bad / loose joint, but after prodding each output stage component without any effect I'm convinced that it has something to do with the scope impacting the circuit.

Any ideas?

After doing some Googling 440V seems pretty high. Is this too high? Could it produce very low volume levels?

How can I troubleshoot between the 6v6 and speaker jack? What voltages should I see?

[JazzGuitarGimp]

Does your build have both "speaker" and "external speaker" jacks, where the main speaker output jack shorts the OT secondary to GND when there is no speaker plugged in? If so, are you sure you're plugging the speaker into the correct jack?

[sunnydaze]

The 440 volts is high for 6v6's. But the big problem is see is the cathode voltage is very high as are the control grids. The cathode voltage is almost equal to the plate. You didn't say in your post, but I'm assuming this amp is cathode biased. If it is the control grid should be zero with no signal and the cathode should be much, much lower. If the amp is supposed to be fixed bias, the cathode should be zero and the control grid should be negative.

Can you post the schematic and some pics?

Mike

[Tillydog]

...the big problem is see is the cathode voltage is very high...

+1

It looks like you have a missing connection to the cathodes (or the ground connection to the big cathode resistors is missing).

The high voltage on the control grids is only because the cathode is (almost certainly) open circuit.

The 6V6s won't be drawing any bias current at the moment, so the supply voltage will also be high. It should come down some when you get the cathode connection sorted.

Be careful - it will be loud when you fix it !

[M Fowler]

Just in case you need voltages.

Mark

[Structo]

How are you measuring voltages?

You should ground your black probe to a chassis ground when measuring DC volts.

Does pin 8 of the 6V6 go to the 500 ohm 5w and 25uF pair, then to ground?

[knavewave]

Wow. You guys are awesome! Thanks for all the great replies.

The problem was the high cathode voltage... I was completely missing the ground on the cathode resistors as tillydog suggested. I now have a very loud amp, and am looking forward to all the tinkering / getting my electronics knowledge up to par

Many, many thanks!

[JazzGuitarGimp]

That's what I get for skipping over the voltages! All I can say in my defense is that I had had just returned home from a long gig.... Sheesh!

[Tillydog]

I now have a very loud amp, and am looking forward to all the tinkering / getting my electronics knowledge up to par

Congratulations!

Enjoy