Build Your Own Clone Message Board

Hello Everyone!

My main issue is that when plug the I amp in and turn it on I get a very low amount of noise out of my speaker (a little hum and a little hiss) but nothing changes when I add a guitar and guitar cable to the inputs. The signal hiss does not increase in volume with the volume pot either. This is my first amp build so I expected a few hiccups in the road haha. So far I used my multimeter to check continuity on the cables underneath the board, and all the wire connections coming from the turret board, and they all seem to have continuity to their respective tube/pot lugs.

I'm getting 120VAC into the power transformer from the power cable, through the volume pot, and into the fuse; It comes out strange though. The Green Leads are sending 3.5VAC instead of 6.5VAC, the Red Leads are 330VAC instead of 360VAC and the Yellow Leads (Heater Wires?) seems to be the biggest issue; I'm measuring 900VAC instead of 5VAC. I feel this may be causing issues down the line but I don't know how to correct it if that is the issue. Comparing the voltage notes on the schematic:

1. Pins 1&6 of 12AX7 = 200VDC; they are 214VDC on pin 1, 308VDC on pin 6
2. Pins 3&8 of 12AX7 = 1.8VDC; They are 1.8VDC on pin 3, 158VDC on pin 8
3. Pin 8 of 6v6 = 22VDC; I'm getting 200VDC

On the Board Side it looks like in terms of resistors my (R1) 1M resistor is showing 0 resistance. (R10) 22k resistor is showing 9k of resistance (and looks burnt). (R3+R4) 68k resistors are showing 32k of resistance each. The filter caps are all showing the correct voltages on the positive leads (C5 16uf at 411VDC, C6 8uf at 375VDC, and C7 8uf at 344VDC; This is all with Fully Loaded Tubes).

That's about all I know how to diagnose at the moment as I don't know what voltages I should be seeing through each component or how to test that yet. I've included some photos that hopefully will help but if anything more is needed please let me know!

Thank you very much,

Brendon

To measure the AC voltages coming out of the transformer secondary, you want to measure between the taps, not to ground. So for example, you should get 6.3VAC when you measure from one green wire to the other. If you measure from one green wire to ground, you'll only get half, so I'm assuming that's what you're doing.

Keep in mind that one of the yellow rectifier heater wires is also connected to the rectified DC output, so your meter cannot read AC voltage there. Also, keep in mind that the voltages on the schematic are approximate. It's normal to see a difference of 50V on the DC side.

So for right now, are you getting approximately correct voltages at test points A, B, and C? Is your pilot light coming on. Are all the tubes glowing like you would expect?

_________________
*patience is a virtue*

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

Yes as of now I am getting correct voltages at test points A, B, and C. My Pilot light is on and tubes are all glowing.

Please take voltages of all solder terminals for the 6V6 socket. Take one set of voltages with the tube in and one with the tube out.

_________________
*patience is a virtue*

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

So here are the voltages for the 6V6 tube socket.

Soldered Tube Terminals
2 - 6.5 VAC when measured against 7 with and without tube
3 - 458 VDC no tube, 395 VDC with tube
4 - 437 VDC no tube, 370 VDC with tube
5 - 2.94 VDC no tube, 6.3 VDC with tube
7 - 6.5 VAC when measured against 7 with and without tube
8 - 3.5 VDC no tube, 203VDC with tube

I also noticed the faint hum stop when I probed lead 5, and it made a louder crackling as I probed lead 3.

Based on those comparison of tube to no tube I would assume your problem is at pin 8 as you suspected. Most likely a bad 6V6 or a bad 25uF/25v bypass cap. If your measurements are correct, having 200VDC at that cap has probably damaged it since it's only rated for 25V. If you have a spare 6V6 that you know to be good (or can borrow one from another amp), I'd try that first. If you're still getting 200VDC at pin 8, remove the bypass cap. You'll want to replace it asap, but it is non-essential to the basic function of the amp, so you should be able to test without it.

_________________
*patience is a virtue*

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

Eureka! you are correct it was still around 200vDC when I changed tubes. taking out the cap did indeed make the amp work. what does that capacitor do and why does it work without it? So I should replace it with the same capacitor and it should limit the current to what it is suppose to be?

Thank you so much!!

I'm really sorry about that. We can send you a replacement. Please contact sales@buildyourownclone.com and let them know you need a new 25uF/25V for the Champ.

The 470ohm resistor is the part that is absolutely necessary for it to work. That capacitor's purpose is to filter out residual AC noise and to achieve the maximum gain from the 6V6 (or whatever device you're using) within a specific frequency range. So without the cap there, you are getting more of a "full range", which might sound like a good thing, but guitar is all about midrange. Without a bypass cap, you'd get a more "loose" sound. The bypass cap helps the 6V6 to focus more on the midrange for a "tighter" sound. If you used a 100uF instead of a 25uF, you'd have a more bottom heavy sound. If you used a smaller cap, like a 1uF, you'd have a thinner sound.

_________________
*patience is a virtue*

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

No worries! I'll send an email to them right away. Thank you for the quick diagnosis of the problem!