a good coupling transformer for guitar signal

[Boogdish]

Rick's tube vibrato project has inspired me to try and design an all tube phaser.  I was planning on running it at high voltages and having it be mounted in a wood cabinet and sit on top of or next to the amp, sort of like the fender standalone reverb units.

Anyways, I want to build it with a 3-prong power cord for safety reasons, and I don't want a ground loop between this and the amp and I hate messing around with ground lift adaptors (I'm bad about losing small parts like that when I play shows).  So I was thinking about using a transformer on the output of the unit, to isolate the amp's signal ground from the phaser's. 

Could anyone tell me what I should be looking for in a transformer?  It's probably going to be fed by either a 12AX7 or 12AT7 and I'd like the output impedance to be in the range of what a single coil pickup's would be.  So far, the only transformer experience I have is output and power stages, I'm in the dark about figuring coupling stages.

Thanks guys.

[Auke Haarsma]

Check GEO for the humfree a/b/y. You can use idea's/parts of that circuit. RG also lists a transformer he recommends.

GL!

[terminalgs]

if your amp doesn't have a 3 prong plug, add one.   I don't think you need to worry about a using a transformer in the signal path.  it would have to have a wide range of operational frequencies, and I think those transformers get expensive.  I don't think it will buy you anything, really.  look at the Fender Reverb unit you mentioned, it does not make use of a transformer in this way.

on the three prong plug, your ground wire goes to chassis ground,  hot and neutral leads go to the power transformer ( through a DPDT switch and a fuse on one of the two, probably the hot.  neither hot nor neutral are ground before the transformer.  the  secondaries from transformer go to a rectifier circuit, and post rectification, neg. (-) is grounded.  your amp should be the same.  if its an old amp with a polarity switch,  you should change that to eliminate the polarity switch.

as for the output impedance,  if you have an extra triode, consider using it for a cathode follower to set a nice low output impedance that would easily feed any amp.

[Johan]

for guitar signal strength and frequency responce, you could get a groundloop transformer from radioshack..two tiny transformers inside, probably designed for around 10k Z and 0db signal..with guitarsignal you're about -20 to -10db. A guitars freq.resp. is pretty poor compared to hifi, and your impedances are much higher, so you shouldnt get any signal loss..
 

j