Help diagnosing tube amp hum.

[brownmule]

Hello I've got a crate turbo valve 120 with an extremely loud hum. I've changed the filter caps and tubes. I've attached a picture of the schematic. I thought I'd found a leaking coupling cap c34,it had 50v after it and right before the phase invertor. I lifted one leg of the cap and still have 50v on pins 2 and 7 of pi. I'm stuck at this point point any help would be appreciated

[brownmule]

Also should note the him is worst on clean side and volume control doesn't change it. I can upload schematic of needed.

[Enzo]

It is normal to find such voltage on the grids of the phase inverter. The grids should be within a volt or two of the cathode, and would be if measured from the cathode instaed of to ground. Your meter impedance is fooling you.

Schematic:
http://bmamps.com/Schematics/crate/C...Schematics.pdf

Don't just change parts hoping to change the right one. Find out what you need then just change what is necessary. How much ripple is on your B+? How much ripple is on the bias supply? Did you check C60,61?

Plug the guitar into the FX return and zero its volume control. Still hum? if not, then the preamp has the hum, if it still hums, then the power amp is hummy. If the power amp, pull V3, hum stop? No? Pull V5, hum stop?

[brownmule]

Thanks, I'll do that when I get home and report back.

[brownmule]

Ok after some testing I found some interesting things. I had the amp on standby for about 30 minutes testing voltages, I had all the tubes in except v4 (reverb tube) and switched it off standby and it was dead quiet. I posted down and started Checking around v4. Once the app cooled down ipowered it up without v4 and the hum was back after it warmed up it is gone again. Any thoughts what would cause it to quiet down after warming? The hum on start up is very loud especially on clean channel.

[brownmule]

Most of the voltages are really close except pin 6 on v3. It's about 100v over

[loudthud]

Most of the voltages are really close except pin 6 on v3. It's about 100v over

130V is probably a typo, 230V looks about right.

Check the heater Voltage to ground. Each side should be about 3.2VAC. If not check R116 and R117.

[brownmule]

Thanks for the reply, there are 2 test points labeled tp6. Both should read around 140. One of them read right on and the other is reading 240, don't think it's a typo

[52 Bill]

Thanks for the reply, there are 2 test points labeled tp6. Both should read around 140. One of them read right on and the other is reading 240, don't think it's a typo

Why would you think that there would two test points at different parts of the circuit and both be labeled TP6 and that they should both read the same voltage? Maybe the second one should be TP8?

That would be a typo to me.

[loudthud]

1.5K is the most common cathode bias resistor used on 12AX7s in guitar amps. With that in place, the tube will conduct about 1mA. At 1mA a 100K plate load will drop 100V. For V3-B that will put the plate Voltage at 220V and the cathode at 16.5V. On the V3-A side you have a 220K plate load resistor. The tube will try to conduct 1mA because it has the same 1.5K cathode bias resistor, but the 220K will cause the plate Voltage to drop too low and current will be slightly less. 130V is reasonable for that tube, the plate current is 0.863mA. There is no way both sides of that tube will have the same plate Voltage.

Did you check the heater Voltages ?

Edit: I can't find TP8 anywhere. I suspect it should be V3 pin 6.

[brownmule]

I finally had some more time to test some things. After turning the amp on and letting it warm up a few minutes and flip the standby switch the hum is very loud (afraid the speakers will blow). Flip the standby back on and let it sit for maybe 10 minutes then Try again and the amp is dead quiet. This happens with any combination of tubes in it. I can turn it off then back on and is quiet as long as its warmed up. I can remove v2 and power it up and it's quiet at all times. Also all the voltages are about 20 or 30 volts high. My wall voltage is 125.

[brownmule]

Thanks for replying, yes all heaters are around 6.3 volts.

[brownmule]

Also if someone could help me understand the switching path between channel 1 and 2 I think it would help. The hum is far worst on clean channel.

[J M Fahey]

Did you remember to ground filament wiring center tap, either physical or 2 x 100 ohm resistors?

[cosnerguitars@gmail.com]

Hi, do you have a link for this schematic, or can you post or email me one? It would be much appreciated! :-)