Having problems with my 50 watt build need help

[pleckZZ]

Ok I’m new to this first build I ironically work in the building that the cathode ray tube was invented but let me get to my problem. I followed the 50 watt marshall instructions on metro wiki to the tee. I purchased my own parts to save some money. The face plates,chasis and ptp board I cnc machined looks real good I used different pt ,ot and bridge rectifier with the same specs witch I bench tested and values looked good. Build went good solder joint were very good nice and clean, slight problem with
pot they were crime plated and needed to sand to attach buss but hers my problem First part of test went good values were good within 3 percent However 2nd part of test I’m getting strange values on valve 2 pin 1 ---232 v (should be 150) , pin 7 ---235 v(should be 150),pin 8--- 236 (should be 152) I went over the whole build several times. Could use some insight from
Someone with more experience please help


ps I have a dxf file of chasis if any one needs it

[soundguruman]

Are you testing it with the output power tubes in?
Have you adjusted the output tube bias?
What is your B+ voltage, what is your bias voltage on pin 5 of the output tubes?
Did you change, increase the values of the power supply capacitors?
Are you using low current output tubes, or high current?
After the output tubes have been biased, the output may draw more current, and the B+ voltage will be lower, making your preamp voltage lower.
But does the amp work?
It is not unusual for the preamp voltage to be higher than what it says on the schematic, but try biasing the output tubes first, and check your voltages again.

[drshewhart]

Something similar to me in my 50w plexi, but much higher on V3 pins 1 & 6.
Electrolytic caps are stock, trannys are Magnetic Corp.

These are the readings I got from every tube. Measured with bias pot at the lower position, all tubes installed and with no speaker conected:

pin1 pin2 pin3 pin4 pin5 pin6 pin7 pin8 pin9
V1 165 - 1,1 3,16ac 3,16ac 223 - 1,95 3,13ac
V2 175 - 1,2 3,15ac 3,15ac 319 175 177 3,15ac
V3 368 - 5,14 3,19ac 3,19ac 366 - 5,14 3,16ac
V4 - 3,18ac 469 462 -39 467 3,17ac -
V5 - 3,2ac 468 460 -39 467 3,19ac -

Mains voltage is 240ac, normal here in Spain.
The amp sounds but very low volume (almost noticeable) and very distorted.
Any ideas what can be wrong?

[soundguruman]

Something similar to me in my 50w plexi, but much higher on V3 pins 1 & 6.
Electrolytic caps are stock, trannys are Magnetic Corp.

These are the readings I got from every tube. Measured with bias pot at the lower position, all tubes installed and with no speaker conected:

pin1 pin2 pin3 pin4 pin5 pin6 pin7 pin8 pin9
V1 165 - 1,1 3,16ac 3,16ac 223 - 1,95 3,13ac
V2 175 - 1,2 3,15ac 3,15ac 319 175 177 3,15ac
V3 368 - 5,14 3,19ac 3,19ac 366 - 5,14 3,16ac
V4 - 3,18ac 469 462 -39 467 3,17ac -
V5 - 3,2ac 468 460 -39 467 3,19ac -

Mains voltage is 240ac, normal here in Spain.
The amp sounds but very low volume (almost noticeable) and very distorted.
Any ideas what can be wrong?

For one thing, you should never run the amp without a load connected to the output transformer

[Wesmont]

I wonder if you figured out what the problem was in your build. I am building a JCM 800, and have almost the exact same readings, and the same sound too, unfortunately.........

[toner]

The low volume and V3 voltage readings above indicate that the PI cathode resistor (connected to V3, pins 3 and 8 ) is wrong. It should be 470 ohm (yellow-violet-brown), not 470k (yellow-violet-yellow).