HUUUUUMMY new build

[dehughes]

So, get this....

I've done a lot of tinkering (amp is much better/quieter/easier on the ears now...) but I've been having some crazy noise/buzz issues which just started yesterday, and in doing some troubleshooting I noticed about a 200v disparity between plates on the 5AR4. One reads about 340vDC (which is correct for the PT/tube/circuit) and the other reads about 130vDC. Swapped out all tubes with known good ones...no change. Swapped out tube rectos with known good ones of same and different types...no change. Only once did the noise go away, and that was when the voltage on the "low" recto plate mysteriously and inexplicably rose to about 290vDC....but when it dropped back off after about 10 seconds, the noise came back. I then measured the plate to ground resistance of both recto plates, and one measured 127.7 ohms, where as the "low" plate measured out at infinite resistance.

Sound like a bad PT? I'm going to take it out and send it back to Mercury Magnetics tomorrow, so we'll see... I've never experienced a bad PT before, so your suggestions are appreciated...

[tubeswell]

So you swapped the rectifier?

[dehughes]

So you swapped the rectifier?

Yup. Three different (known good) ones. Same result, same secondary measurements.

[tubeswell]

Are the secondaries (HT and rectifier) VAC the same at both ends without any rectifier tube plugged in? (you will need to disconnect the line to the reservoir cap to properly measure the 5V winding)

[dehughes]

Are the secondaries (HT and rectifier) VAC the same at both ends without any rectifier tube plugged in? (you will need to disconnect the line to the reservoir cap to properly measure the 5V winding)

The B+ line secondary voltages are different (340v/130v), the 5V voltages are the same on each end.

The tranny is on its way back to Mercury Magnetics...we'll see what they say.

[VTron21]

Sorry for the noobish question, but I'm at a loss as to what "HT" stands for when refering to Tube amps.

I'm actually having a similar issue as dehughes with a Tube amp i'm working on. All of the 12AX7 tubes have a 60hz bleed through. Not sure if the Power Tubes are because I don't have them installed at the moment. and from reading this thread I'm looking at either going to a DC Voltage for the heaters or raising the DC voltage on the Heater center tap. (it's currently tied to chasis ground)

[pontiacpete]

[QUOTE=VTron21;143715]Sorry for the noobish question, but I'm at a loss as to what "HT" stands for when refering to Tube amps.

Isn't that the British way of referring to B+, and means 'high tension'?

[tedmich]

So, get this....

I've done a lot of tinkering (amp is much better/quieter/easier on the ears now...) but I've been having some crazy noise/buzz issues which just started yesterday, and in doing some troubleshooting I noticed about a 200v disparity between plates on the 5AR4. One reads about 340vDC (which is correct for the PT/tube/circuit) and the other reads about 130vDC. Swapped out all tubes with known good ones...no change. Swapped out tube rectos with known good ones of same and different types...no change. Only once did the noise go away, and that was when the voltage on the "low" recto plate mysteriously and inexplicably rose to about 290vDC....but when it dropped back off after about 10 seconds, the noise came back. I then measured the plate to ground resistance of both recto plates, and one measured 127.7 ohms, where as the "low" plate measured out at infinite resistance.

Sound like a bad PT? I'm going to take it out and send it back to Mercury Magnetics tomorrow, so we'll see... I've never experienced a bad PT before, so your suggestions are appreciated...

so the disparity is measured at the socket without the 5AR4 present? Whats the voltage coming off the PT under a resistive load?

British identifier sentence: I was working on my Austin's valve stereo when the bonnet fell on my head and I got a shock from the HT lead and had to go to hospital.

[tubeswell]

[quote=pontiacpete;143718]

Sorry for the noobish question, but I'm at a loss as to what "HT" stands for when refering to Tube amps.

Isn't that the British way of referring to B+, and means 'high tension'?

Yup

[VTron21]

I'm actually having a similar issue as dehughes with a Tube amp i'm working on. All of the 12AX7 tubes have a 60hz bleed through. Not sure if the Power Tubes are because I don't have them installed at the moment. and from reading this thread I'm looking at either going to a DC Voltage for the heaters or raising the DC voltage on the Heater center tap. (it's currently tied to chasis ground)

So I tried both a DC voltage to my heaters on the 12AX7's and a DC Voltage to the center tap of the Heater side of the power transformer and still have a 60hz wave on the output of the tubes.

Turns out the guy who did the initial wireing before I came in to trouble shoot why the amp wasn't working put the -Bias power supply on the farthest end of the amp from the power transformer. So it's supply line from the Transformers main out put went right through the entire preamp section causeing a bleed through from to the various stages. So I moved the -Bias supply next to the power Transformer and bingo, no hum.

now I just have to figure out why the trem circuit isn't working properly >.>