Two quick KT77/EL34 questions

[Julesman64]

1. What is the max safe bias for an EL34 or E34L? I've read as high as 40ma.
2. What is the max safe bias for a KT77?? I've read as high as 45ma.

Just scored a free pair of Gold Lion KT77s (yes free Gold Lions) and was thinking of swapping them into my JCM 800, but I wanted to make sure I set them up correctly before putting any juice through em. Also, I just checked the bias on my JJ E34L's and they were at 37/35ma, which sounds a little low to me.

From what I have read you can bias your 77's hotter than 34's. Fact or Fiction?

[SteveD]

The maximum permissable bias current on any valve is strongly dependant on the HT voltage at the anode (plate). It is the maximum anode power dissipation that matters and EL34s are nominally rated at a 'design max' of 25W per valve as are the KT77s.

[Julesman64]

I suppose a better question to ask would be how much dissipation should I be biasing for, 60, 65, or 70%???? I see people going to 70% but that sounds a little high to me. Dont want to see any red flashes coming from my amp.

[SteveD]

You can go as high as you want as long as you don't exceed the max dissipation. However, if you go very hot it will be at the expense of valve life. Brian Robertson of Thin Lizzy used to like his bias pretty hot.

[ClubAndCountry]

I find that running them *much* cooler than is currently fashionable sounds better, but that's just my opinion. I like around 13.5W for EL34s in a Marshall circuit, or only about 55% of rated dissipation at idle - which at typical plate voltages is only about 30mA. I know this is *far* cooler than the numbers we're used to seeing nowadays. To me, the amps sound more open, dynamic, powerful and respond better to changes in the EQ and the guitar like this. You will also find that in an amp as powerful as a JCM800, they have much less of a tendency to go into screen overload and fail! Remember that the bias setting is the *idle* current, and the valves will draw a lot more when the amp is cranked up - if you start off too high at idle, they will be way over the rating at full power.

I would also bias GEC KT77s the same as EL34s, but like all old-production valves they probably exceed the design spec by a fair margin, and don't seem to suffer unduly from running hotter than modern ones. Many vintage amps ran standard Mullard EL34s at or above the 25W even at idle, some even over 30W, and it didn't seem to do them much harm.

At the end of the day it's down to what sounds good to you though.

[SteveD]

Remember that the bias setting is the *idle* current, and the valves will draw a lot more when the amp is cranked up - if you start off too high at idle, they will be way over the rating at full power.

The amp will pretty much draw the same max current it always does at full power. The only thing that changes with the bias setting is the 'Class A' proportion of the Class AB biasing.

[ClubAndCountry]

The *maximum* current draw is the same, but the proportion of the cycle above cutoff is greater with hotter bias (even beyond the point of forward clipping) and so the total power developed in the valve is higher. This is effectively what you're saying too! :-) But it certainly changes the overall power dissipation. You only have to watch the valve screens at full power to see it...

[Hellatone 60]

I have a question for you Steve... Is it possible to put EL34's in the VM?

[slowpokerhino]

I have a question for you Steve... Is it possible to put EL34's in the VM?

http://www.marshallvintagemodern.com/vi ... hilit=el34