OT - amp question - squealing noise generated in pre-amp

[brett]

Hi.
Apologies for asking an amp question here, but I asked it over at 18watt.com and I've not got any response.

I have an 18 watt Lite IIb that has been working great for a year or so.
This is the schemo:http://www.18watt.com/modules.php?name=Downloads&d_op=getit&lid=141
Just the other day it started loud squealing as I plugged or unplugged the guitar lead. This very lous squeal happens even at 1 or 2 on the volume scale (the output from the first 12AX7 when this is happening is about 30V ac) .   I can also pick up radio stations, electric fences and other interference by placing my finger a few mm from the tip of the lead.

I've tried disconnecting the input sockets and 1M grid-grounding resistors and applying my finger direct to pin 2 (a grid), and I get the same result.

The B+ is 320V, the grids are 0V and the cathodes are 1.1V (which seem about right). However, the plates are only at 140V, which seems too low.  The 12AX7 load line for a 60k resistor (ie 120k shared between the 2 triodes) and at -1.1V grid should be about 200V according to the 12AX7 Ip-Ep graph).

I've swapped tubes, and this didn't improve anything.  The only thing that makes it go away is to earth the input.

This is frustrating the hell out of me.   I'd be grateful for any ideas..
thanks for any help

[brett]

Bump

[brett]

Hi.
It's been a long, frustrating time solving this.
After replacing many bits and following false leads, I tried a new rectifier tube, and the problem was partly resolved.  This made me very curious.  I wondered whether a new tube raised the B+ ?  Anyway, I left the amp just sitting there open for a few days, and one day as I walked past I thought I'd have another (1001st) look inside.

The problem was obvious.  I had joined 100K and 22K resistors in series by twisting the leads.  The far ends were massively soldered, but I had forgot to solder the join between the resistors.    

Maybe the leads were making and breaking electrical contact at high frequency?  That would explain the low (average) plate voltage.

I am lifted from my depression and I have promised my wife that I will take this as a sign to practice a lot more and really enjoy my amp.  An 18 watter is a beautiful thing (especially when driven by a Rangemaster!)