Two-stroke amp not making any sound

[raits77]

Hello.
This is my first post. I just finished my two-stroke amp yesterday (exactly from the book), but it's not making any sound, not even a hum.

The pilot light is on and all the tubes light up, thus the 6.3V circuit should be fine. I double checked all the connections and the voltages from the book. The voltages between the output tubes (pin 3) and ground were 340VDC (compared to 340-360VDC in the book), the voltages between the gird (pin 4) and ground was 298VDC (300VDC in the book) but the voltage between the the 12AX7 (pin1 and 6) and ground was 107VDC (140-160VDC in the book).

Is the low voltage the problem and how could I fix this? The only difference from the schematic in the book was that I used a .0047uF mallory cap (as in the parts list) for the tone control instead of a .047uF. And a random .47uF cap

(Mouser Electronics, Inc.)

for the bypass cap (instead of a mallory series 150 one). But the parts list says that you should use a 47uF 400V axial cap. Can that cause the problem? I also noticed that the 68k resistor at the middle of the board (between the 1.5k resistor and the speaker jack) reads a resistance of 1.5k instead of 68k.

Considering how much time I spent on the amp I would really appreciate some help here.

Thanks

[wagdog]

I've built that amp. I ended up modding mine to be more blackface champ like. Anyway...

As far as your cathode bypass cap, you should use the correct value. It won't prevent the amp from working though. You should also use the correct value for your tone control cap.

The 68k resistor is your negative feedback resistor. It should read 68k. Are you reading that value w/the speaker plugged in?

Can you possibly take a close-up pic of your board and post it?

[raits77]

Hello wagdog,

I appreciate your help.

Well, the amp doesn't make any sound at all, not even a hum. I read the 68k value when the speakers are plugged in or not, same value. The resistance shouldn't be affected by that. Which one is the correct value for those caps? The parts list and the diagram are showing different things :S All the tubes are lighting up. It's refusing to upload multiple pictures, will try to add more.

[52 Bill]

I don't know the book or the project, but have you gone back and double checked all of your wiring? No odd wiring errors?, no wrong pin number on a tube socket?, etc.

[Tom Phillips]

Have you tried a different speaker yet? What's the DC resistance reading between the tip and the body of the speaker plug when it is not plugged in to the amp?

Caution: It would be best if you don't try to play through the amp until at least hear some hum at the speaker.

[wagdog]

Hello wagdog,

I appreciate your help.

Well, the amp doesn't make any sound at all, not even a hum. I read the 68k value when the speakers are plugged in or not, same value. The resistance shouldn't be affected by that. Which one is the correct value for those caps? The parts list and the diagram are showing different things :S All the tubes are lighting up. It's refusing to upload multiple pictures, will try to add more.
[ATTACH=CONFIG]18889[/ATTACH]

Does the 68k from your input jack connect to v1a pin2? I can't see that in your pic. Maybe it's underneath?

Your tone control cap should be a .0047 (4.7n).

Is there a ground rail under the board, that the power-supply caps are hooked to? I'm assuming there is. Is everything that is supposed to be grounded, grounded?

After the .022uf coupling cap from v1b, there is a 220k grid leak to ground - how the signal getting to the power-tubes after that? Is that under the board as well?

Just checking 'cause I can't see under the board in those pics!

Hope that helps.

[Tom Phillips]

A couple of more things to check:
1) Have you got good connections from your power tube cathode pin (#8) all the way thru the cathode resistor to ground? You should measure ~500 Ohms from pin 8 to ground and you can probe from the top of the socket with the tube removed to insure that you are measuring the complete path. I ask this because some of the solder joints look suspicious [Edit] and I can't see the connection at the ground end of the cathode resistor which I assume is under the board.
2) Have you go good continuity from your speaker output jacks through the impedance selector toggle switch to the OT secondary?

Nice high resolution photo you posted. Please post a photo of the top of the chassis.
Regards,
Tom

[Tom Phillips]

I just noticed that you have two power tubes in your amp wired in parallel and each one has its own cathode resistor. Therefore, its unlikely that my suggestion about checking the cathode circuit will find the cause of you completely dead amp. (Unless both circuits have a problem)
Please check the speaker and switch first.

[Bruce / Mission Amps]

If the power tubes are in conduction there will be a strong DC voltage on lugs 8 of their sockets... if not, it isn't wired right.
Also, what is the toggle switch doing over there by the preamp tube and is the yellow and white wires part of the secondary? Like so you can switch the impedance ratio around and use one power tube or both? I'd suspect that too.
I find my builders frequently wire an input jack wrong and ground the signal out before it even gets to the grid of the first preamp tube.
The other scenario is that, from the volume pot, they have shorted the center conductor in the shielded cable to the shield on one end or the other.
But if you hear NO HUM... I am still suspecting the power tube(s) is-are not "on" for some reason or the speaker jack and or switch is bogus.

[loudthud]

1.5K is what you will read across the 68K. One end has 1.5K to ground and the other has the OT secondary (very low resistance) to ground. Actual reading should be 1.468K which is 1.5K in parallel with 68K. YMMV

[Sticky Sam]

I have just built the Two Stroke amp and have the same problem. Valves ( tubes ) glow, 3 including pre amp valve. I have a non contact voltage tester. Obviously there is power from the wall socket along the power cable to where the black wire connects. Also voltage in the two red cables coming from the power transformer. I will read through this thread ( do I need to start a new one ) and buy a multimeter tomorrow.
ta for any help.