VHT help needed !

[troublehead]

My 50/CL came with Mullard EL34's. The manual says A/B mode (where I keep it at) is fixed bias. Does that mean I can safely swap the Mullards for a matched duet of Ruby EL34B-STR's without internal bias ?

 

[Loudness250]

Well, if you have a bias rite or if you know how to bias with a multimeter. If you have some way of reading the bias. This is not recommended to do but for this special occasion, you could swap the tubes and just fire up your amp one time without using the standby, this will allow you to watch your bias rise as the tubes warm up. If the bias gets higher than 45 ma/mv Then you switch the amp off right away realizing that the fixed bias is too hot for these tubes. The reason for doing it like this is that if you did use the standby and just flick the switch, then all the current will rush in at once and your tubes could just get smoked. Ive done the method where I did not use the standby before and it never ruined or significantly shortened the life of the tubes but dont do it regularly just this once. So if the Bias is too high, you need to replace the bias resistor for one of higher value. Like say if the bias is 12 ma/mv too high, then maybe put a 56k in place of a 47k. All this is also assuming that you know how to check your plate voltage and figure out your bias in the first place. Hope that helps.

 

[Loudness250]

I would also highly recommend that you contact VHT if you dont know where everything is located on the circuit board of your amp. They can help you locate the resistor that needs adjusting and let you know what's what. But if you measure the bias and all's well then, your golden.

 

[guitarslinger]

My 50/CL came with Mullard EL34's. The manual says A/B mode (where I keep it at) is fixed bias. Does that mean I can safely swap the Mullards for a matched duet of Ruby EL34B-STR's without internal bias ?

When it says "Fixed bias" I don't think they mean "non-adjustable" (like mesa boogies).

You still need to get a 1 ohm resistor and a multi meter and open that sucker up.

If you don't know what you're doing, do a LOT of research before you touch your amp. Filter caps can kill you many years even after the amp has been unplugged. seriously. It's no small matter.

 

[Capulin Overdrive]

the 100CL i had, had adjustible bias?


probably should just make a quick call to VHT.

 

[guitarslinger]

My 2150 certainly has a pair of trim pots inside for bias. Actually, I bias it every now and then.