Update :73 Silverface Twin

[gtrplayr1976]

Yes, any of the section with sufficient slack between the line cord input and the PT primary as it makes its way through the fuse and On/OFF switch.

OK. Got everything revised as stated. Grid ,and shielded cables removed from ties. Nothing changed while moving wires around. I checked both channels ,and the oscillation appears on both. I started checking through the signal to see where the oscillation may stop/start. (not sure if this was the correct way to go). I proceeded to adjust the volume as I had done before as I started with V1b pin 6,and worked my way to the MV. The wave was smooth as I adjusted the volume to the point from before. 3-4. The voltage at the DVM on the load had gotten as high as 23. All waves were smooth until I got to the 250pf cap at the MV. One side was smooth ,and on the other side I started to get oscillation again. Again, I'm not sure I was going at this the right way ,but I thought I would poke around with the probe to get some ideas. The pictures show where I moved the grid wire ,and the meter, and scope readings at the 250pf cap on both sides. The last two pictures are with the probe at the other side of the cap.

Note: I performed similar tests as I had done before on the load, but I had moved the scope probes to the output jack inside the amp. Even though I am still using 2.25 amp fuses I never blew one after doing this. All those results were the same as before.

[gtrplayr1976]

This just get's more confusing. Video seems to be making things worse, much easier to see the pictures.
It appears that you are getting about 30V p-p on the scope when clipping starts? What is the AC voltage reading on your meter at this time?
When you are measuring the resistance of the load resistor, it must be disconnected from everything, the amp, other test equipment etc. What is it measuring when cold? Hot?
You said something about the loud tone heard having to do with the meter being set on AC, that should not happen, was the meter set to ohms?
Also, can you link to the probes you were having trouble with so we can all avoid them? Or is there any chance they are 100X probes?

They are 100mhz probes from N&K Trading on Ebay. Yallstock is the seller. Some info on the probes.
Products Number

TE-MT299
Products Name

Oscilloscope Scope Clip Probe 100MHz Tektronix HP
Bandwidth

DC-6MHz
DC-100MHz
Attenuation Ration

1:10
Input Resistance

1Mω:10Mω
Input Capacitance

1X:85pF-115pF
10X:14.5pF-17.5pF
Compensation Range

15pF-35pF
Rise Time

<3.5ns
Working Voltage

10X:<200vDC+ACPeak
Cable length

120cm
Temperature opertating
nonopertating

-10°C ~ +50°C
-20 °C ~ +75°C
Humidity


≤85% (relative humidity)
Item Net Weight

70g / 2.5oz

[gtrplayr1976]

OK. Got everything revised as stated. Grid ,and shielded cables removed from ties. Nothing changed while moving wires around. I checked both channels ,and the oscillation appears on both. I started checking through the signal to see where the oscillation may stop/start. (not sure if this was the correct way to go). I proceeded to adjust the volume as I had done before as I started with V1b pin 6,and worked my way to the MV. The wave was smooth as I adjusted the volume to the point from before. 3-4. The voltage at the DVM on the load had gotten as high as 23. All waves were smooth until I got to the 250pf cap at the MV. One side was smooth ,and on the other side I started to get oscillation again. Again, I'm not sure I was going at this the right way ,but I thought I would poke around with the probe to get some ideas. The pictures show where I moved the grid wire ,and the meter, and scope readings at the 250pf cap on both sides. The last two pictures are with the probe at the other side of the cap.

Note: I performed similar tests as I had done before on the load, but I had moved the scope probes to the output jack inside the amp. Even though I am still using 2.25 amp fuses I never blew one after doing this. All those results were the same as before.

Something else I noticed when I had the scope on the 250pf cap at the MV. When I increased the volume to a point above the 23 volts at the load resistor the sound of the transformer singing stopped ,or was at a frequency I couldn't hear.

[Tom Phillips]

...When I increased the volume to a point above the 23 volts at the load resistor the sound of the transformer singing stopped ,or was at a frequency I couldn't hear.

This is strange. If your test frequency (400Hz, 1kHz or whatever) is still present I don't know why the singing from the OT would stop.


Sorry to hear that the parasitic oscillation issue hasn't been solved. I'm still thinking about what to suggest next but I'm fully occupied with home plumbing and electrical system repairs &upgrades that is taking up all my available time.
Later,
Tom

[gtrplayr1976]

This is strange. If your test frequency (400Hz, 1kHz or whatever) is still present I don't know why the singing from the OT would stop.


Sorry to hear that the parasitic oscillation issue hasn't been solved. I'm still thinking about what to suggest next but I'm fully occupied with home plumbing and electrical system repairs &upgrades that is taking up all my available time.
Later,
Tom

No worries Tom. I'm in no big hurry . I understand all the home repair stuff. I've been there too. I'm going to reroute a few wires in the amp to see if that helps. I read somewhere that if the input wires were next to the chassis it could cause issues. When I wired this one I thought it would look less cluttered if I ran them under the board. Changing those may help ?

[gtrplayr1976]

New development over the weekend. After moving some wires trying to find the parasitic oscillation I fired it up. No changes in the oscillation. After a few minutes set at 20.9 volts on the output I pulled the MV control to the out position ,and noticed the voltage dropped about 1 volt. Shortly after the voltage dropped to nearly half 11.2 maybe. I'm not sure what's going on with that.