Tweet
Hi,
A friend of mine gave me this Fender Hot Rod Deluxe to be repaired.
Here's the schematic and layout :
http://support.fender.com/schematics..._schematic.pdf
The problem is : when in more drive mode, there are some oscillations happening.
Here are the tests and reasonings I've done so far :
- No oscillations happen in clean or drive mode, so I guess those are caused by the very high gain of the more drive mode (gain increased by bypassing both V2 cathode resistor)
- I switched the preamp tubes, oscillations still happen with erevy tube I put into the amp
- The oscillations are not affected by the drive gain pot or the master pot. The only control that affect the oscillations is the treble control. When this is at a very low value, the oscillations are reduced. I guess this is because less "high" frequencies come to V2 and excite the oscillating frequency.
- I ran the amp without the power tubes to get rid of any "acoustic" coupling : no results. I even removed the phase splitting tube (12AX7) without any result.
- I shorted R23 and R24 to get rid of the jfet switches, nothing changed.
- When I touch the middle of ribbon cable of V2 (it connects G1-C1-A1 and G2-C2-A2 to the main board), I get some squealing. The wires in the middle of the ribbon cable connect A2 and C1. Seems to me that the circuit is instable and that the "link" made when I touch the ribbon cable is enouth to get the circuit into frank oscillations.
I guess I could get rid of those oscillations reducing the gain of V2B by lowering the plate resistor, or adding a resistor in series with the cathode bypass capacitor (the one that are "activated" in "more drive" position). I could also put some low value capacitor (a few pF) in parallel with V2's plate resistor to lower the HF gain, but those mods would affect the amp's tone for sure.
I also doubt that Fender let an oscilating amp go through their test procedures, so some part must have gone wrong !
Here is a sound file illustrating the problem :
https://drive.google.com/file/d/0B6K...ew?usp=sharing (please let me know if the link doesn't work)
This has been recorded through the "preamp out", without the output tubes, so no sound at all on the speaker. There is no speaker simulator or LPF, so the distortion sounds harsh as expected
At the beginning, the guitar's volume pot is turned all the way down. We can already hear some "phantom" oscillation behind the hiss. Then I played some notes where you can hear the oscillation ringing, especially during the sustain of the notes. Then I touched the V2 ribbon cable, and you can hear those frank squealings.
Has anyone any idea of what could have gone wrong in this amp to cause such instability ?
Thanx in advance.
Best regards.
Eric
A friend of mine gave me this Fender Hot Rod Deluxe to be repaired.
Here's the schematic and layout :
http://support.fender.com/schematics..._schematic.pdf
The problem is : when in more drive mode, there are some oscillations happening.
Here are the tests and reasonings I've done so far :
- No oscillations happen in clean or drive mode, so I guess those are caused by the very high gain of the more drive mode (gain increased by bypassing both V2 cathode resistor)
- I switched the preamp tubes, oscillations still happen with erevy tube I put into the amp
- The oscillations are not affected by the drive gain pot or the master pot. The only control that affect the oscillations is the treble control. When this is at a very low value, the oscillations are reduced. I guess this is because less "high" frequencies come to V2 and excite the oscillating frequency.
- I ran the amp without the power tubes to get rid of any "acoustic" coupling : no results. I even removed the phase splitting tube (12AX7) without any result.
- I shorted R23 and R24 to get rid of the jfet switches, nothing changed.
- When I touch the middle of ribbon cable of V2 (it connects G1-C1-A1 and G2-C2-A2 to the main board), I get some squealing. The wires in the middle of the ribbon cable connect A2 and C1. Seems to me that the circuit is instable and that the "link" made when I touch the ribbon cable is enouth to get the circuit into frank oscillations.
I guess I could get rid of those oscillations reducing the gain of V2B by lowering the plate resistor, or adding a resistor in series with the cathode bypass capacitor (the one that are "activated" in "more drive" position). I could also put some low value capacitor (a few pF) in parallel with V2's plate resistor to lower the HF gain, but those mods would affect the amp's tone for sure.
I also doubt that Fender let an oscilating amp go through their test procedures, so some part must have gone wrong !
Here is a sound file illustrating the problem :
https://drive.google.com/file/d/0B6K...ew?usp=sharing (please let me know if the link doesn't work)
This has been recorded through the "preamp out", without the output tubes, so no sound at all on the speaker. There is no speaker simulator or LPF, so the distortion sounds harsh as expected
At the beginning, the guitar's volume pot is turned all the way down. We can already hear some "phantom" oscillation behind the hiss. Then I played some notes where you can hear the oscillation ringing, especially during the sustain of the notes. Then I touched the V2 ribbon cable, and you can hear those frank squealings.
Has anyone any idea of what could have gone wrong in this amp to cause such instability ?
Thanx in advance.
Best regards.
Eric
Comment