Locating a noisy valve in Fender HRD iii

Philtre

Since I bought my s/h HRD iii last November there's been a bit of intermittent noise from one or more of the valves in the clean channel (I don't use the dirty channel). Sounds like a quiet scurrying windy, hissy, rumble. Nothing serious but I thought I'd locate it and swap out a valve or two.

Plugging the guitar into the return socket ("power amp in") eliminates the noise, so I concluded it's not an output valve.

Swapping out the 12AX7 in v3 and the noise is still there. Swapping out v2 and, touch wood, the noise is gone. But how can this be? I thought v2 was for the dirty channel?

Thoughts?

ICBM

Simple - V2A is used on both channels. It's only V2B that's exclusively on the dirty channel.

Philtre

ICBM said:

Simple - V2A is used on both channels. It's only V2B that's exclusively on the dirty channel.

Bingo! Thanks, that's it.

ICBM

Philtre said:

Bingo! Thanks, that's it.

If it ever matters, there's a catch on these amps too, that I've come across twice - a faulty power valve can produce noise in the preamp that doesn't appear when using the power amp in jack - it gets there via the filament wiring. The first time I came across this it took me ages to find it because I did what you did, and assumed it couldn't be a power valve.

Philtre

Right-o, I'll bear that in mind. That's a tricksy one. In my own case, replacing v2 has done the trick.

Philtre

For anyone else who might find this thread, I found this handy FAQ:

http://hotroddiy.com/original-faq.php

The section "Which preamp tubes are V1, V2, and V3? What do they do?" explains it.

Thanks to @ICBM for pointing out the function of V2A.

Out of interest, what physically is going on when a valve starts to produce odd noises like I experienced?

ICBM

Philtre said:

Out of interest, what physically is going on when a valve starts to produce odd noises like I experienced?

I'm fairly sure it's caused by either a tiny amount of air leakage into the valve, and/or contamination of the cathode coating.

The other main problems a preamp valve can have are microphonics - usually whistling or 'ringing', but sometimes lower frequencies - caused by the internal parts having come loose and effectively amplifying physically vibration - and hum, which is usually caused by breakdown of the insulation between the filament and the cathode.