Hot Rod Deluxe 6L6WGC too hot?

[MickeyB]

Its a 1996 with original 6L6GC's in it and I have a brand new matched pair of TAD 6L6WGC-STR short bottles that I put in and it will not bias under 95mv. The old pair bias from 35 to 80mv. Do I have to change screen resistors in order to use these? I didnt think these tubes were that different.

[Enzo]

Put the tubes in one at a time and see how one tube biases, then the other. Obviously if two tubes are to bias at 60ma, then one tube should sit at 30ma. If both tubes are just that hot, we could modify something. But if one tube is doing most of the current drawing, then you have a bad tube. You shouldn't have to change anything to use the tubes.

Also try one socket, then the other. to see if there is any difference in the amp between them. And if there is, then start looking at voltages around teh socket - particularly pins 4 and 5 - screen B+ and bias voltages.

[yankeerob]

From memory - the bias is fed from a winding on the PT that also supplies the switching circuit and a handful of op-amps that also serve as reverb drive and recovery, drive/more drive etc (which is why these amps should not be allowed to be called 'all-tube' - because they simply aren't but we digress)...

The point of mentioning this is that if there is a problem somewhere else down the line in that supply chain it might very well result in the bias supply voltage being too high... One suspect area in these amps (and I have now looked up the schematic) are the two big ceramic wirewound resistors (R78 and R79) that provide feeds for the +16V and -16V rails - which again - from memory are not too far away from the bias pot. They are known to fail (and/or cause heat damage to the PCB) and if one or both have in fact failed - and have gone high - it is completely possible that the bias supply voltage could rise as a result... please be aware that this is pure supposition and completely off the top of my head...

A lot of care (and I do mean a lot!) must be taken to protect the very fragile solder pads and tracks on these PCBs... you basically only get one shot at desoldering and resoldering and I would suggest that you use a good quality solder braid to desolder and be very aware that the slightest excess of heat will cause the copper to seemingly evaporate right before your eyes...

On the other hand - I have found the stock Sovtek 5881WXTs to be perfectly good for purpose... I have only used two pairs of TADs ever and they were not very durable... they started to drift apart after a couple of months and once they'd gotten 5mA apart they really started going south in a hurry... and they ran very hot physically... you pays yer money... you takes yer choice...

Good luck with it... and like I said... be careful with that PCB

[Mick Bailey]

I've had a few fender amps recently that have run out of bias adjustment with certain tubes. Check you're getting -50v or so on the wiper of the bias pot (or the - end of C38). If everything else checks out ok and the bias is insufficient I increase R77 to 120k or 150k. But ONLY if there isn't a fault elsewhere.

An essential check is to make sure R83 (51K) isn't fitted - should be an empty space in this particular model but sometimes a resistor is fitted by some owners to give a hotter bias.