Build Your Own Clone Message Board

Hi, thanks in advance for any response.

I got through my Brit45 build and ran into a snag. Step 9 of the testing procedure tells me to turn the amp on in standby and measure the voltage on the red terminal of the 32uF cap. I read 0V between the chassis and this lug when the amp is in standby. I went ahead and proceeded with testing and here are my results when standby is off - installed all tubes and the plates are glowing. Note, when I put the amp back in standby the voltage on the 32uF cap's red terminal drops back down.

A: Expected 450VDC @ Red 32u - Measured 0VDC on Standby, 448V Standby Off
B: Expected 440VDC @ Yellow 32u - Measured 460VDC
C: Expected 380VDC @ Red 16u - Measured 461VDC
D: Expected 310VDC @ 16uF on board - Measured 465VDC

Note that C and D are reading high. When I get to step 18 I read 0V between pin 1/8 from either power tube and the chassis and no sound from the speakers (not even a pop coming out of standby).

I've triple checked continuity and verified that the components are in order as specified by the instructions. Thoughts? Do these symptoms point to anything obvious that I'm missing?


Thanks!
-Tyler.

Hi Tyler - welcome to the forum!


This is actually a mistake in the instructions. The red terminal of the 32uf cap is located after the standby switch, so if the the amp is in standby mode, there will not be any voltage present at that location.


This doesn't seem possible. The highest DC voltage in the amp should be at pin 8 of the rectifier tube, which connects to point A when the amp is not in standby. You should not be able to get any higher voltage after that point. Pin 8 feeds the B+, then the voltage should drop a little after the choke at point B, then lower after the 8.2k resistor at point C, and then lower after the 10k resistor at point D.

Is this with the power tubes installed? Are you sure you measured DC and not AC voltage? Are you measuring at the correct point? Is the black lead of your probe grounded?


Is your meter reading the right voltage range (DC millivolts)?

Photos can often help with troubleshooting. Please post clean, well lit photos of your build.

_________________
MasterDelayer/Reverbrador/Ampaholic/TopJacker

Thanks for the quick response!

I believe the power tubes were installed when I took the measurements posted above. I'm definitely measuring DC voltage. I've been clipping the black probe to the chassis and testing the appropriate points with the red probe. I have the meter set to 200mV when testing pins 1/8 on the power tubes - reading 0V.

Here are a few photos from before I started testing (notice the fuse is not installed yet, but was installed during the test):
https://flic.kr/p/YMa3X3
https://flic.kr/p/XLCuff
https://flic.kr/p/Z4WZuT


-tyler.

I should note - the photos above have a few cold looking joints. I went over the board again when I was testing for continuity and made sure all of the joints were solid.

Yes. The fact that your voltages are getting higher as you go further down the power rail seems impossible. Perhaps you need to change the battery in your meter.

Aside from the voltages being off, does the amp seem to be working OK?

_________________
*patience is a virtue*

Please do not PM me. email is prefered. keith@buildyourownclone.com

If there were a high frequency oscillation, that could make a voltmeter read incorrectly. Just a thought.

The 220µf/25v V1 cathode bypass cap - is it grounded? I can't see a ground wire there.

_________________
MasterDelayer/Reverbrador/Ampaholic/TopJacker

Made it back to the amp last weekend for more testing and I was especially confused that my B+ voltage was extremely low coming out of the rectifier - ~97V. After some head scratching I figured out that my 500mA fuse had blown. Replaced the fuse and voltage was back up to what I'd expect to see, but switching off standby blew the fuse again. I found an issue with continuity on one of the inputs - re-soldered and gave the turrets another once over, checking for cold joints. The amp fired up as expected, adjusted the bias and everything sounds fantastic!

Thanks for your help.


-Tyler.