Fender Twin Reissue - phase inverter tube issue

[disorder]

Hey guys, I have a Fender Twin Amp reissue that gets a serious hiss when switched from the 25w setting to the 100w setting. At 25w it plays and sounds just fine. At 100w it also plays fine but you are playing over a serious hiss with the occasional crackle and very light pop. I pulled the preamp tubes one by one and the problem completely goes away when the phase inverter tube is pulled. It is V4 on this amp. Any other tube removed did not cure the issue.

Here is the schematic
http://support.fender.com/schematics..._schematic.pdf

At the 100W setting I am reading ~275VDC at the plates of V4 and about ~90VDC at the cathode. The schematic says that there should be 36VDC at the cathodes of V4. What could cause this? As you can see from the schematic it is the "Y" supply that is feeding the plates of V4 and my reading of the "Y" supply line matches the schematic pretty close at ~320VDC (schematic reads 315VDC).

[pdf64]

1. Check the power tube bias at both power settings (see note 9 on schematic)
2. Confirm that the overdrive channel hasn't kicked in somehow on 100W setting
3. Try a different tube in V4 slot, if you haven't got a known good spare then try swapping with V2 (overdrive channel) or V6 (trem oscillator).

[disorder]

A known good 12AX7 still causes the hiss. Power tube bias is proper for both sides at both 100w and 25w settings. Overdrive has no kicked on, in fact you can still kick it on and play the drive channel over the hissing noise.

While checking voltages at components around V4 I got some weird results. In 100w mode, if I probed one end of R103 (other end of DMM is at chassis ground) the hiss went away, and also I could not play guitar through it anymore, it went silent. The same thing happened when I probed EITHER END of C103 (coupling cap at beginning of PI stage). Both ends of C103 read about 59VDC.

If I went to 25w mode, probing R103 or C103 did nothing to affect the guitar signal, could still play guitar through it.

Could C103 be leaky?

[melvin]

90V at the cathode is most likely caused by improper current flowing through V4. Check the tube first, then the path from cathodes to ground. A leak at C103 would also cause this and a leaky coupling cap can cracke and pop. Once you're satisfied with V4, another common source for this kind of crackle/pop is the standby switch and the high/low power switch. Also, check for bad solder joints.

[disorder]

thanks for the tips... I replaced C103 and that did not fix it.

there is no path from cathode to ground... as the schematic shows the two cathodes of each triode are tied together and also to a resistor network. pretty common phase inverter setup I believe. the schematic shows that C103 follows the Preamp Out Power Amp In jacks. The jack tips have the same 59V on them that is entering (and for some reason leaving) C103. This doesn't sound normal to me, having DC on those jacks. Is that something to consider?

here is a cropped portion of schematic showing phase inverter
http://img696.imageshack.us/img696/7...10116at516.png

[g1]

Just ahead of the loop jacks is a muting circuit (J113 fet etc.). It sounds like when you kill the hiss and signal with your meter probe in 100W. mode, you are somehow triggering the mute circuit. What doesn't make sense is why it only happens in 100W mode. May be worth removing the mute fet to see if it has anything to do with the hiss or DC issue.

[pdf64]

Often with modern amps there's a component between circuit ground and chassis ground - diode / resistor / cap, some combination of these, intended to prevent ground loops etc. Can't locate it on the schematic, though different symbols are used for circuit ground and chassis ground.
Maybe something screwy is happening with that link?
No way should there be Vdc on the jacks.

[km6xz]

thanks for the tips... I replaced C103 and that did not fix it.

there is no path from cathode to ground... as the schematic shows the two cathodes of each triode are tied together and also to a resistor network. pretty common phase inverter setup I believe. the schematic shows that C103 follows the Preamp Out Power Amp In jacks. The jack tips have the same 59V on them that is entering (and for some reason leaving) C103. This doesn't sound normal to me, having DC on those jacks. Is that something to consider?

here is a cropped portion of schematic showing phase inverter
http://img696.imageshack.us/img696/7...10116at516.png

The ground reference is through R107. What voltages are you seeing at R107, and the junction of R104 and R105, and does it change when inserting a dummy plug into the AMP In jack?

[pdf64]

Just noticed that the circuit ground and chassis ground are bonded at the guitar input jack socket, p1 D8, top left of schematic. If you are getting any voltage between circuit ground and chassis ground when the high cathode voltage on V4 is evident, then it may be beneficial to check that link.