Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dickmandude on October 14, 2011, 12:46:51 pm
-
I got this amp at a rummage sale- A complete no-namer from China. Quad EL84, a couple of 7025's in the pre and 1 for phase inversion, all on a perfectly chinese PCB. The power transformer was for 220 V, exclusively (no 115 V taps). Secondaries were 260 V, 6.3 V (Both untapped), and 30V centertapped to run the reverb tank and channel switching sections of the preamp (presumably, since I have no schematic). I bought a Hammond transformer with 250 V CT and 6.3 no CT and I had a 30 V CT laying around and installed them. It seems to work all right, I still need to adjust a few voltages here and there (Bias and whatnot). My question is- Should the center tap on the 250 V secondary lead be grounded? I just taped it up, thinking that it would reflect the pre-modified version. Right where the HT and the filament secondaries solder into the board, there are bridge rectifiers. And on the filament circuit there is a little trimmer for hum balance. Like I said, it preliminarily seems to work fine, but the power transformer gets a little warm for my liking. Not sure if I'm just taxing it with all of those heaters, or if the cathode resistors need to be changed or if I need to ground that centertap. Just trying to eliminate all the suspects. Anybody have some thoughts on this?
Thanx, R.T. Bennett
-
Sounds to me like you did exactly the right and intelligent thing as you sit. re: Grounding vs leaving "alone" the CT. I'm not going to call it your CT wire "floating"...I would rather call it "unused" or "unconnected". From where you are now, grounding the CT of the power transformer will fundamentally change the nature of the rectifier circuit and, if it doesn't blow anything up, all other things being equal, will roughly cut your B+ in half. Obviously you don't want that. A CT in tube-level-voltage power transformers is to accomodate a 2-diode full wave rectifier circuit (a 5Y3 or a GZ34 or a 5U4 or many others but those are the most common) which depends upon a grounded center tap for the B+ circuit return. No such CT is needed with a 4-diode bridge. Grounding the CT, from where you are now, would have the tendency to suck down (more negative) the present value of your B+ and suck UP the present value of the ground side of the B+ circuit. Not good, certainly not what you'd want. The circuit as it is takes its "ground" (more properly, "zero volts") reference from the lower junction of the 4-diode bridge. Leave it alone! To make a 4 diode bridge using tube rectifiers would require 2 qty (or even 4!) recto tubes and good sized filament current to light them all up. And two or more sockets. And more chassis room. And dual tube rectos with two internal sections are generally set up with split anodes but common cathodes, so while it's trivial to make the "right" side of the diode bridge (common cathodes) I am not aware at this instant of a dual recto tube with split cathodes that would let you make the "left" side, of the bridge.
Silicon diodes are so cheap and compact and efficient, we do not really care what they cost, plus the ones you have are already there. I see no reason to mess with them.
Another way to look at this is "if it ain't broke, don't fix it". IMO you came up with an elegant solution to rescuing the amp. Good work! If this a hi-fi amp or a guitar amp? Post a pix if you can.
Transformers get hot! It's not at all uncommon for power trannies to get hot enough so that you really can't leave your hand resting on them.
-
I think that if there is a bridge rectifier, one "leg" of the bridge is grounded so you do not want to ground the PT center tap. Especially if it's working now.
This info from Merlin (a.k.a. "Valve Wizard") should help:
http://www.freewebs.com/valvewizard/bridge.html (http://www.freewebs.com/valvewizard/bridge.html)
Cheers,
Chip
-
Thanks guys! I'll post some pics of the beast once I get home and perform the required corporeal decompression.