Metropoulos Forum

I've built a Marshall Silver Jubilee 25/50W clone according to the SDM layout (2550x) and carefully made it through all of the critical first-time power up steps. I used the steps from Paul Ruby and a lightbulb current limiter to get to the final stage which is where I am stuck.

I removed the limiter and my B+ voltage is fine (458V) and I was even able to use the bias test points that I added to the plan to get the power tubes to 32-33mA.

Things don't line up when I test the voltages and try to see that "In general, the triode gain stages should have ~1V on the cathode, 0V on the grids and ~1/2 B+ on the plates."

Instead, of the 1/2 B+, I am getting almost full B+ voltage (430-425V) on the plates (pins 1 and 6) of the 12AX7 preamp tubes and 0V on the grids (pins 2 and 7).

There is a slight hum (in a good way, it seems) from the 8ohm speaker connected during the final stages of testing, but no response from switching between the clean/gain stages or raising the volume pot. No LED either.

I saw an old post by SDM with voltages (jpg) uploaded, but the link wasn't working so I used the Ceriatone 2550 voltage chart to ballpark initial values which confirm the above. Here are my readings for the preamp tubes:
HT - 350
B+ (OT CT) - 458

Expected/Actual
Pin1 - 212/430
Pin2 - 0/0
Pin3 - 2.2/0
Pin4 - 0/0
Pin5 - 0/0
Pin6 - 183/425
Pin7 - 0/0
Pin8 - 1.7/0
Pin9 - 0/0

If anyone has the info showing add'l expected voltages for the schematic that would be great as well.

I did post this issue on AX84.com and received some guidance to check into the cathode resistors and grounding connections so my apologies if anyone is reading twice. I was hoping to get input from those most familiar with this project since it may be obvious what the issue is.

Thanks in advance!

I would agree that the high preamp voltages make it seem as if the tubes are not conducting. Open cathode resistors or bad grounds and solder joints would cause this. Are voltages really high across all preamp tubes or just one in particular? Do you have proper heater voltage at all tubes? Should be AC unless you modified the filament supply.

Thanks for the quick reply!

Are voltages really high across all preamp tubes or just one in particular?

I've found the high B+ voltage on the plates of all three preamp tubes.

Do you have proper heater voltage at all tubes? Should be AC unless you modified the filament supply.

There is AC voltage on the heaters, I believe the reading was around 4.3V or so... I only had a few minutes to poke around the amp this evening but plan to dig around tomorrow evening to check for bad grounds / solder joints with DMM. I verified that the cathode resistors were the correct values and not accidentally grounded. I will also verify the AC voltage on the heaters for the three preamp tubes.

Thank you so much for your guidance and quick reply! Your suggestion helped me narrow down the issues and I was able to focus on where the problem may be. After some time with the schematic and DMM (and some head scratching) I realized that I hadn't connected pins 4 and 5 on two of the three preamp sockets... Doh!

Now it sou s like it's working fine, time to rock out. Thanks again so much!

Glad you found it! It's always the simple things that give us problems. Rock on!!!