Will this work on a AB763 Twin

[Gone_Fishing]

Hi guys,,,,I have a 2475-65 Music Man chassis only,,,,I want to make a Twin clone,,,,why?, I don't know, maybe I think my back is going to get better or I might take up playing the Steel Guitar,,,,I want to use the "Low,Off,High" power switch on the chassis front,,,I knew about pulling two tubes,,,,I searched the Net and found this gentleman's input on a Forum when someone asked about getting half power in his Twin without pulling the tubes,,,,can this work?,,,is it safe?,,,is the way he is pairing the OP tubes below correct?,,,,is it not V7&V10 and V8&V9? (inside or outside pair)

http://www.tdpri.com/forum/archive/index.php/t-77015.html

The Radium King  May 31st, 2007, 12:29 PM
switching the cathodes is a lot easier. a cathode lifted from ground kills the signal from that tube. in a twin, each tube pair has a common cathode to ground (v7 and 8 on one side of the ot, v9 and 10 on the other). rearrange the cathodes such that v7 and 9 share a common cathode to ground, and v8 and 10 are a pair. now put one pair on a switch and you can lift it from ground as required, leaving you with only two tubes running.

some tricks include using different tube types in each pair, so that tone changes as well when you lift one pair of cathodes. ensure that you can bias everything properly if you do this. another trick (which is good if you don't want to drill a hole) is to use the cathode lift as standby. replace your standby switch with a dpdt 'centre off' switch (put a jumper where the original was). wire the switch such that, when up, no pairs are lifted, down one pair is lifted, and middle both are lifted - standby, no sound.

as stated above, power tubes are like parallel resistors; lift a cathode from ground and the resistor is no longer in the circuit and overall resistance of the circuit will increase. that means that the impedance that the ot wants to see will increase. in a twin that wants to see a 4 ohm speaker load, remove two tubes from the circuit and now it wants to see 8. you can achieve this by disconnecting one speaker (fenders run two 8 ohm speakers in parallel for 4 ohms, and with only 2 tubes running you are only putting out about 50 watts, so a single speaker shold handle it) putting in a multi-tap ot or ignoring it (tube ot's can handle a mismatch of impedances; in a twin you are good for a 2 to 8 ohm speaker load).

someone can correct me here, but disconnecting the plates may cause 'cathode poisoning'? without a plate voltage there will be current flow from the negative grid to the cathode, poisoning the cathode?

Thanks in advance,,,,Eual

[HotBluePlates]

... I want to use the "Low,Off,High" power switch on the chassis front,,,I knew about pulling two tubes,,,,I searched the Net and found this gentleman's input on a Forum when someone asked about getting half power in his Twin without pulling the tubes,,,,can this work?,,,is it safe? ...

Yes, it will work because electrically it's the same thing as pulling the tubes out of the socket.

If you want to maintain optimum loading of the remaining 2 tubes to get the maximum output power from the ones left, you would also switch the speaker load to reflect double the impedance on the primary.

What does that mean? Normal Twin OT is 2kΩ:4Ω with no other taps on the secondary. You yank two tubes (however you accomplish that); now you want to apply an 8Ω load to the existing 4Ω tap to reflect a primary impedance of 4kΩ, perhaps by disconnectioning one of the two parallel speakers.

Or, you have an OT with multiple taps, and apply your existing 4Ω speaker load to the 2Ω tap. This will also reflect 4kΩ to the primary.

Or, if you are unable to do either, you keep your 4Ω speaker load and run the 2 output tubes with the existing 2kΩ primary. Power output will be reduced somewhat, and distortion may increase somewhat.

... is the way he is pairing the OP tubes below correct?,,,,is it not V7&V10 and V8&V9? (inside or outside pair) ...

The Twin's output stage has 4 tubes. The 2 on the left are connected to one side of the OT primary, the 2 on the right are connected to the other side.

People telling you to pull tubes want you to remove one tube from each side of the OT primary. V7, V8 are on one side and V9, V10 are on the other. You can remove one tube from each side with any of the following combinations of tube left in the amp:
V7, V9
V7, V10 (outer pair)
V8, V9 (inner pair)
V8, V10

It's easier to tell people who don't know what they're doing how to pull correct tubes by giving an easy visual reference (outside 2 tubes, middle 2 tubes), than to say, "pull tubes 2 and 4".

I don't know why Radium King wrote it the way he did (he used to hang out on this forum too around that same time). Maybe he was thinking arranging the tubes that way might yield some easier way to wire something or maybe it was just how he though of pairing the tubes which act on either side of the OT.

[Gone_Fishing]

I sure appreciate it Blue Plates,,,,the rear of the Music Man chassis has a switch for 4-8 omh,,,,I plan on getting a 4-8 ohm OP transf. to ease in the use of the half power setup,,,I understand now what you're saying about the OP tube pairing,,,it finally "sunk in"

The Music Man chassis also has a Master Volume on the second (Reverb/Tremolo) channel, so I want to use it as well and put a PPIMV on this amp as well,,,,I'll look on the Forum for how to wire one if one's there.

Again, thanks a lot.

Eual

[Gone_Fishing]

Oh yeah, I'm going to call it my "Twin Music Men" amp.