subbing 7581A for 6V6GT in a Princeton

[Randall]

A customer wants me to put in a set of NOS Philips 7581A in place of the 6V6GT that are currently in his Princeton Reverb Reissue. My first thought is we will need to upgrade to a bigger OT. But what other considerations are there? I think there will be more current draw from the heaters, will the PT in there keep up? And since this amp's trem circuit modulates the bias, which I've never liked, how will that effect the bias resistor adjustment?

http://www.amparchives.com/album/Fender ... _orig.html

[ToneMerc]

Honestly, that would be an absolute waste of a perfectly good vintage pair of 7581A's. Unless you are going to change the topology of the circuit, it still will be an underpowered PR.

It would require a higher filament and secondary current rated PT, of course an OT and a type of this output tube would surely benefit from a LTPI instead of the stock cathodyne PI.

If this was an eyelet board, I would say first try the "Stokes" and "Paul C mod" and then change the OT to a larger 6V6 type.

TM

[Leo_Gnardo]

A customer wants me to put in a set of NOS Philips 7581A in place of the 6V6GT that are currently in his Princeton Reverb Reissue. My first thought is we will need to upgrade to a bigger OT. But what other considerations are there? I think there will be more current draw from the heaters, will the PT in there keep up? And since this amp's trem circuit modulates the bias, which I've never liked, how will that effect the bias resistor adjustment?

I've put plenty of 6L6 into Princetons and Deluxes, 7581's being a 6L6 variant should be no major problem. I've yet to see a PT melt down due to this. Only once a DRRI developed an intermittent open primary in the OT, very rare but considering it's a reissue, this could have happened just as easily with 6V6's. Yes you will have to adjust the bias. I just put a trimpot in series with the resistor loading the bias supply. It's also a good idea to have current limiting 1k5"stoppers" on the control grid and your choice of 470 to 1k on screen grids. Yes "shaking the bias" vibrato may be blamed for wearing output tubes prematurely. OTOH 6L6's/7591's are loafing in that circuit, and should give extended life & a dab more power at clip.

FWIW I've occasionally replaced the output drive tube with 12AT7, and that just about eliminates the premature collapse of one side of a sine wave when power testing & playing loud. OTOH with a 12AT7 the vibrato will just about disappear. To overcome this, a 220K-330K resistor bridging the 1M that's in the LFO signal chain to the depth control will restore vib. depth.

If you're worried about the PT, there's a good replacement that will easily support 6L6 filaments from Allen Amplification - TP24. However this transformer lacks a 5V winding for the rectifier tube. If your customer is after all the power he can get, going solid state for hi voltage rectifier will be a good move. And if he has deep pockets & feeling spendy get the Allen OT that goes with their PT for their Accomplice "double power Princeton."

[Randall]

Thanks guys. Just about what I thought, bad idea. I am going to try to convince him of it. I never really like doing work I don't believe in.

[Leo_Gnardo]

Maybe you could convince him to let you build an Accomplice kit for him. Depending of course on $$$ available.