I have just built a pre-amp based on the Velleman pre-amp/tone control unit (Philips TDA 1524 IC). The unit seems to work fine and audio output is circa 1.5v (to the power amp). Plugged into the input of my existing Arcam stereo amp it sounds fine (I realise I’m ‘doubling up’ pre-amps doing this but it proves that the Velleman pre-amp unit works and sounds good). Upon plugging into the Velleman 4040 valve amp and even with the pre-amp volume at full the sound is extremely quiet, ‘tinny’ and distorted (on both channels). Based on this I suspect it must be a power problem as it affects both channels. I have checked all of the test voltages on the 4040 and set the valve bias (all OK). The speakers are B&W 600s 8 ohm.
If we understand correctly the valve amp has never worked correctly?
If this is the case, then you will need to troubleshoot it step by step.
Start by measuring and comparing all voltages without tubes, next with tubes.
CAREFUL: VOLTAGES IN THE AMP CAN KILL. DON’T PERFORM ANY MANIPULATIONS UNLESS YOU KNOW WHAT YOU ARE DOING!
Thanks for your help - I’ve found two problems and the amplifier now works fine.:-
The heater 6.3v to V1 was not reaching the valve i.e. no heat! Due to the fact that V1 is not visible when the chassis is built-up I’d overlooked this (but owing to the fact that this would have been one of the few faults that would affect both channels it was the first thing I checked. The actual fault was a dry joint on one of the ‘J’ jumpers.
After fixing the heater issue only the right channel worked. Checking the ‘test’ voltages I found +V3L was not present at the valves V1 and V2, this was found to due to a lifted/broken PCB track. Once repaired the amp now works on both channels and sounds great.
One question - I’ve found that the chrome top surface of the amp gets very hot (particularly at the front centre). I suspect this is due to the decorative collars around the base of each EL34 transferring the valve heat to the chrome top. Would it be acceptable to put some thin fibre washers below the collars to reduce the heat transfer? I cannot see a problem with this.