Need serious help

One would think that I would have learned by now that ignoring it won't make it go away.

For some time my "red" M-125 has been making a soft, intermittent, crackling, hiss. Think initial hit of needle to vinyl on a 10 to 40 second frequency.
I swapped tubes, one at a time from offending amp to the other until all tubes swapped. The noise stayed with the "red' amp. At some point I felt that by slightly wiggling the output tube in the V-2 position I was able to make the noise go away. Hard to tell with the intermittence. Cleaned and tightened socket.

So....... Ignore it........

Now I have lost the tube in the V-2 position, no fanfare, just quit. Running amps on two output tubes now while waiting on replacement with readjusted bias. Next the sound quality dropped off on that amp. Check bias, and it will not hold steady. Replace GZ34 with the Webber WZ68. Bias comes back everything is cool, until the Webber goes TU and the bias is all over the board again.

Can't really ignore this ..........

Any help will greatly appreciated as my degree is mechanical, not electrical. Please be specific, this is all new to me.

Thanks

I would seriously look at your power transformer outputs!
Transformers usually do not tend to go south, but it has and can happen.
Also, perhaps go through a typical 'initial startup' checking procedure.
Below is a direct copy of the M125 assembly manual.

Initial startup
1. Make sure that the 5 amp SLO-BLO fuse is in the fuse holder.
2. Place the two 12BH7 (or 12AU7) driver tubes in the sockets on the top of the driver board. DO NOT
PLUG IN THE RECTIFIER OR THE KT88 OUTPUT TUBES at this time. Install the IEC power cord
into the IEC connector on the rear of the amp. Plug in the amp to an AC outlet and turn on the amp
with the front switch. Wait about 10 seconds to see if the two 12BH7 tubes light up. If they do and
they stay on for a minute then turn off the amp and proceed to step 3. If the 12BH7 tubes don’t
light up or if the fuse blows check the wiring below and measure the AC voltage where both pair
of twisted wires connect to the driver board. Check to see that you get about 6.1 - 6.5 volts AC. If
not, check the wiring back to the power transformer for an error. Turn the amp OFF.
3. Get your multitester and set the range for 0 to 2 volts DC. CONNECT A SPEAKER. (NOTE - the
amp will produce a slight hum through the speaker without the RCA input jack terminated with an
interconnect plugged in and terminated on the other end into your preamp) Place the WZ68 solid
state or GZ34 tube rectifier into V5 and just the ONE LEFT SIDE and ONE RIGHT SIDE 6550/KT88
output tubes in their socket. Initially turn the two bias adjusters on the driver board fully
CLOCKWISE (to the right). This will start your bias out at its lowest point. Turn on the amp. Allow
the amp to warm up for a minute and check to see that the two 6550/KT88 tubes light up properly.
Measure the bias on the output tubes by placing the BLACK NEGATIVE probe into the lower
center (negative) test pin and the RED POSITIVE probe into either the left or right side test pin.
The LEFT bias potentiometer biases the LEFT tube (or BOTH LEFT TUBES in the four output tube
mode) with the RED test probe in the LEFT test pin. The RIGHT bias potentiometer adjusts the
bias on the RIGHT tube (or BOTH RIGHT TUBES in the four output tube mode) with the RED probe
in the RIGHT test pin. With a small screwdriver, adjust both bias adjusters on the driver board to
read .600 VDC (600 millivolts or .6 volts DC). Turn the adjusters COUNTERCLOCKWISE TO
INCREASE BIAS one tube at a time. Turn the bias up to .600 volts on both tubes. .600 VDC gives a
bias current of 60 milliamps for each of the two output tubes.
4. For TWO OUTPUT TUBE operation you must use ONE LEFT and ONE RIGHT side tube. Although
you may use either LEFT or either RIGHT tube socket, it is probably better to use the two FRONT
left and right sockets. The amp will run a little cooler with the output tubes further away from the
power transformer.
5. For FOUR TUBE output tube operation place a KT88, 6550, KT90, KT100 or KT120 in each one of
the FOUR output tube positions. Turn the amp on. The LEFT BIAS adjuster now biases BOTH left
output tubes and the RIGHT BIAS adjuster now biases both RIGHT SIDE tubes. Set the bias
voltage for each bias adjuster for FOUR output tube operation to 1.20 VDC. 1.20 VDC will also give
each of the four output tubes a bias current of 60 milliamps.
6. If at any time a fuse blows IMMEDIATELY when you turn the amp ON, check carefully all the power
transformer wiring for an error. Check all wiring against the pictorial. If a fuse blows AFTER A
FEW SECONDS and/or the rectifier tube shows arcing after a few seconds check carefully all of
the QUAD CAP, SCM and ESL capacitor connections against

One thing to look at (well two) generally - what is your line voltage where you are at, and are you using tube era outputs or modern re-issues?

Line voltages can run pretty high and, just as importantly varies quite a bit. This makes setting bias kind of a moving target. Many people find that modern re-issues are not nearly as tough as the old school valves. There may not be anything wrong other than being a little on the high side of where the amp and tubes will settle in.

I had never checked line voltage, I had always ASSumed. At least on this Sunday, my Fluke tells me that, from the wall I am getting 119.8 to 120.3 VAC.
Almost all my tubes are reissue.

Also, for anyone who grew tired waiting for a response about my wood surrounds or dust covers, I answered those questions in the photo section today.

Transformers usually do not tend to go south, but it has and can happen.
Also, perhaps go through a typical 'initial startup' checking procedure.
Below is a direct copy of the M125 assembly manual.


I knew you would be there Montana,

I have gone through Initial start up a couple of times now with nothing of note. I will do it again however.
Can you tell me how to check tranny with parameters....... like I said......Newbie......

Triple check soldering on all points of multiple connections, especially pin 5 on output tubes....

Don't ask me how I know.....

audiobill wrote:Triple check soldering on all points of multiple connections, especially pin 5 on output tubes....

Don't ask me how I know.....

Yeah, what Audiobill said about pin 5 under the hood, also atop it on the pin-holders. Tighten them up and Deoxit them. Turns out not all tube pins are identical in diameter or circumference, you'll find this out in a hurry if you're tube-rolling even amongst the same brands. Bad contact on the 5-pin and everything goes very quickly pear-shaped. It's pretty to watch though.

I looked at the bottom side of this thing for an hour before I sat down and asked your help.

How about the 10 ohm 5w resistor at pin 8 not being soldered to the ground lug? I bet soldering it will help.

This forum is anonymous isn't it?

5/35 wrote:I looked at the bottom side of this thing for an hour before I sat down and asked your help.

How about the 10 ohm 5w resistor at pin 8 not being soldered to the ground lug? I bet soldering it will help.

This forum is anonymous isn't it?

No, we'll find ya.

5/35 wrote:I looked at the bottom side of this thing for an hour before I sat down and asked your help.

How about the 10 ohm 5w resistor at pin 8 not being soldered to the ground lug? I bet soldering it will help.

This forum is anonymous isn't it?

hehehehe...know how ya feel...I've done some doozies....and am sure I'll do some more in the future.
So we assume that fixed your problem?....that is the main thing...don't really matter why and how....its fixed and running well....that IS THE most important thing!!

I just finish the amp I thought I had finished months ago. Just for kicks, I went through initial setup on both amps. Dead silent, Webbers holding all four outputs at 1.20 VDC.
Still don't quite believe that I looked over the top of that again and again and again.

All is well and the sun came out. Time for a beer and Rock and Roll!

Thanks to all that helped !!! Best spot on the internet!!!

Great - wise to tug at EVERY connection with needle nose , bright light and magnifying glass before powering up!