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On/Off switching for pairs of power tubes
Hi guys,

to cut along story short, I have a PA-10 with two boards, each  with a pair of tubes. A PA-66 with two EL34s and a PX-66 (actually a cut PA-66) that carries two 6L6s. I regularly use either EL34s or 6L6s. Since there is a lot of heat under the hood, I would like to turn off the pair I don’t use while playing through the other one. I'm not talking about muting, rather about powering on/off. 

In TUT, p. 6-57 there is a paragraph on Cathode switching as a stand-by for power tubes. Is this the best way to reduce heat? I am thinking about a solution that completely turns off either the EL34 or the 6L6 pair if technically possible.  

Is there any mechanical switching that would turn off the B+ plate supply and the heater feed at a time between PSU-PA and PA-66 respectively PX-66? Maybe anything better?

Unfortunately, most DPDT switches can only handle DC voltage up to 250V. B+ plate supply usually is much higher. The heater feed can easily be broken. It is 12 V from the power supply.

Can anybody help me with this issue?

Best wishes 
Hi Kai

See your thread about preamp tube switching for the part numbers for the switches.

Yes, you can disable both heaters and plate + screen supplies for the tubes in question. If we assume the two sets of tubes share an OT, then it might take three DPDTs to select the tube pair. Ine DPDT can select the plates, with the switch wipers tied to the ends of the OT. The second DPDT can select the screens to be powered with the switch wipers connected to any T-U-P switching in place. The third DPDT can select which heater pair is powered. The switches noted have a 3A rating, so one pole per exclusive tube pair.

The use of multiple switches can be ergonomically problematic inasmuch as it is easy to not switch all the switches. Adding relays to perform the actual mode switching allows a single SPDT to select the tube set. You could mechanically link the three toggles or just be diligent about switching the three together - place them in close proximity so one hand motion gets all three easily.

Have fun
Hi Kevin,

thanx for your reply.

Do you have any relay in your product line that would do the job?

The only obstacle: There isn't much space left in the 4U chassis power amp unit. Thus, it should be pretty small.

Best wishes
Hi Guys

As in the preamp tube switching thread, look at the insulation ratings of the relay you wish to use and that will tell you if it is okay for selecting power tubes. Usual ratings are 1kVrms up to 2k5Vrm. Think of the maximum signal on a tube plate as being twice the B+.

You will be switching these things with the power off, so no worries about pops or thumps. Yes, the power must be off since you want to switch everything including heaters. Were you only switching the plate and screen then you could do so hot - even with signal present, which actually makes the switching quieter.

Have fun

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