Moderator: longfxukxnhair
That can be the case on some amps but NOT this bad boy. Like slow said, best to just get a matched quad and rebias them. If you are in the US there are many online tube sellers that will overnight them to you.Someone told me that if you remove the two outer tubes or the two inner tubes you'll be fine.
Just curious Steve, is the dangerous voltage rise using two tubes the actual plate voltage, or the screen voltage on the power tubes?SteveD wrote:You may well get away with it but I could never recommend it because it increases the HT on the plates of the remaining two valves thereby pushing them further from the KT66s comfort zone.
Like the guys said, if you can stretch to a new matched set and keep the others as spares, that's always a good idea.
Understood, thanks for the info.SteveD wrote:Both the screens and the plates will rise due to less loading on the HT. It's not actually a dangerous voltage rise as such, but the original mains input tolerance range the amp was designed for was +6% to -10% of a territory's nominal domestic mains voltage. The amps were officially safety approved as per their stock configuration when shipped from the factory. Hence, I can not recommend anyone to effect changes that vary from that in the position I am in.
If anybody wants to pull output valves then it is at their own risk.
Good point, I wouldn't be at all surprised. I'm not really sure why Steve Grindrod couldn't carry that over to the 2000 range. Maybe a test house issue.ClubAndCountry wrote:Might this have anything to do with the dropping of the valve failure fuses between JCM900 and JCM2000? Since it was possible to deliberately pull one of the fuseholders on the 100s to run the amp on two valves...
Can't comment on having to change the impedance and can honestly say I've never heard of that - doesn't mean to say it's not true mind. However, the VM transformer has the option of running at 4 ohms; you just need to open her up and swap either the 8 or 16 ohm plug over onto the 4 ohm option on the tranny.frank9310 wrote:I heard that if you're going to pull 2 tubes then you need to change your cabinet impedance to half of what it was. For example, if you have a 16 Ohm cabinet, then you need to change the impedance switch to 8 Ohms for it to be compatible. Otherwise, you risk damaging the amp.
The problem I see with the VM is my 2x12 cab uses 2 Marshall Celestion G12 V30s which are 16 Ohms each. So running those together in parallel, I get and 8 Ohm cabinet. So to pull 2 tubes, I would also need to set the selector switch to 4 Ohms.
You can't run the 2 x 16 Ohms speakers in series or you wind up with 32 Ohms. Since these are rated at 70 watts each and the 2466 is really supposed to be around 68 watts RMS or 110 watts peak, the 2 speakers should be adequate I would think.
The problem though is that the VM only has a 8/16 Ohm selector. Why was 4 Ohms eliminated or can the 16 Ohm selection be used? Reason I ask is on some Marshalls I've seen it where it says 16 Ohms and in parenthesis (4 Ohms). Does this mean you can use either one there?
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