White Noise issue - Hiwatt T20 Combo

[66SuperTremMKIV]

Picked up a used ( As New Condition) Hiwatt Tube Series T20 Combo.

Plugging in and then turning on the power in standby mode - Silent.

Flick off the Standby and with no guitar plugged in , there is a very audible amount of white nose through the speaker.

Turn the switch to Half Power mode, the level of the white noise decreases slightly, but still overly audible.

Very well made amp but not sure of the quality of tubes fitted - Cheap Chinese I'd imagine.

Wondering what could be the cause - Should run quiet, my old Plexi's ('60s) are dead quiet in this mode & with guitar plugged in.

 

[springhead]

Some amps are noisier than others - circuit topology, gain structure, layout, component types etc.

Same on both channels? Goes quieter with the channel gain(s) down, or master volume? - that might point you to a noisy tube you could swap out.

 

[66SuperTremMKIV]

Tap tested all tubes, no microphonic's. Replaced V1 with a NOS Brimar, no difference. Replaced V3 with another NOS Brimar, white noise gone.
Only trannie hum remains.

BUT...... now when you turn the MV knob you get static/scratchiness occurring ? All other pots quiet.

Replaced with the Orig RUBY in V1, same static on MV......... pulled the Brimar V3 replaced it with a Hammond labelled Mullard UK circa 1960, all
static gone now. Just the trannie hum, cheaper grade trannies know doubt.

Very good qlty amp overall, retails (Street) $650 AUST scored it in As New condition $253 so for the price of a good effects pedal can't complain.

 

[Kristine]

 

[66SuperTremMKIV]

Try spraying some contact cleaner in the pots and valve sockets, and make sure the valve sockets are tight.

Thanks. Prior to swapping around the pre amp tubes all pots were silent in operation. Amp would be no more than 1 y.o. It seems to have been a pre amp tube issue. Hopefully that will fix it. No matter how well built the amps, they always use cheap tubes unfortunately.

Will pull the chassis at some stage, the spring reverb must be stuffed in there also :dang it is an accutronics apparently with springs !

 

[TM1]

Alot of these "budget" models don't use a choke in the power supply. A choke would probably remove 80% of your noise issues. Chokes cost a bit to make and fit to an amp. I'm sure they probably used a large wattage resistor in the 250 ohm range and the resistor would be a 5 or 10 watt type. A choke acts like a shock absorber for your amps power supply and has a great deal to do with touch response in the amp.
I would get a good, U.S. made choke about 10H and replace the resistor with the leads of the choke. I do this alot with new Vox and Marshall amps. Fender seems to be content to use really crappy Chinese made chokes.
Get a choke from Mercury Magnetics. those are about the best ones you can buy.

 

[callaway_1]

I would open it up and look at the guts.
Those are very poorly built. You would be very lucky if tubes solve the problem.
I'd shift it for something better.

 

[TM1]

I would open it up and look at the guts.
Those are very poorly built. You would be very lucky if tubes solve the problem.
I'd shift it for something better.

Wise choice I'd think.