April 14, 2011 at 2:11 am #5187
First amp, first post; uhh, be gentle.
The TAN schematic seems to show a PT with 275-0-275, yet the link in the Parts checklist goes to the Mojo 330-0-330 part.
Has anyone built one with the 275-0-275 part, say from here?
Building this amp is too much fun.April 14, 2011 at 4:16 am #5409
I think the schematic I derived the TAN schematic from said 275V but you are right, both the Weber and the Mojotone transformers are 330V. That should be the correct voltage. I will fix that, nice catch, thanks.
A smaller transformer will change quite a bit in the amp. All of the voltages to the tubes are calculated based on the 330. Going lower in this amp probably isn’t too big of an issue. But the tubes will have less gain, the onset of distortion will change and there will be less overall power. Probably would do some bias calculations to figure out what it would do exactly though.April 14, 2011 at 9:23 am #5410
Thanks for quick answer, AJ.
I didn’t expect the 275 would have enough juice to reach some of those voltage ranges either. Can I follow up with a noob PT wiring question?
I’m stumped/terrified about wiring the PT, specifically whether to tie off or ground the extra leads. Here’s my PT:
(not sure that uploaded – I’ll post it to the gallery if not.)
DH writes, and the schematic shows, grounding the CT’s for the HighVoltage pair and the 6.3v heater pair. I think I can follow that correctly. But I’ve also got an unused wire on the primary side (blue in the .gif, for 115vac supply). In the secondary, there’s a gray 55v
lead (presumably for bias supply) and the spare 6.3v (green/yellow).
Which of these get taped/shrink-wrapped and which go to ground?April 18, 2011 at 10:22 pm #5427
You should be able to ground the blue and the gray. Hunter says to cut them off and that should be ok too but grounding it, from my understanding is a better practice. It helps in the case of a short, then there is a place for the power to go.
But I am not fully studied on this aspect yet.April 19, 2011 at 4:55 am #5428
Hang on a sec. I’m pretty sure you should not ground any of the leads you are not using, including any center-taps that might be included with a specific transformer that are not called for in the circuit. Best to cut off or cap-off (or shrink tube) the end of each unused transformer lead.
The yellow leads are the 6.3v taps for the heater circuit (this transformer does not include a center-tap for the 6.3 taps so you must add an “artificial center-tap with 2 resistors).
The Green (#10) & Yellow/Green (#11) leads create the 5v supply for the rectifier tube. Cap off the 6.3 tap (#12)April 19, 2011 at 5:07 am #5429
I’m sure Robin is right. I’m going to consult my guru
I’ve read where you need to reference it to ground and also where you want to tape them off. So the question is a great one.
Robin, do you ground the filament center tap, if you have one? The green/yellow?April 19, 2011 at 5:13 am #5430
As I read the transformer leads schematic, the Yellow/Green lead is a 5v
tap when connected to the Green lead and not a center tap.
But the heater circuit will, for sure, need an artificial center tap (for noise reduction), created with a 100r/1/2w resistor going from each yellow lead to ground. Here is a link that explains heater circuits and creating an artificial center tap: http://www.freewebs.com/valvewizard/heater.htmlApril 19, 2011 at 2:42 pm #5435beelzebumParticipant
just a quick note, if you are using the green, and yellow with a green stripe, for the 5v rectifier fillament. then make sure you cut and heat shrink the end of the 6.3v green/yellow lead. once the rectifier is on, that extra lead will have your full DC voltage riding on it. so make sure it is not touching anything. there are some transformers that do have a center tapped 5v lead, but that is never a ground lead. it would be your HV lead. i have seen many hammond transformers like this. also do not make a artificial center tap for your rectifiers 5v, only for the 6.3v going to the rest of your tubes.April 20, 2011 at 8:18 am #5436
Wow, Thanks Robin, AJ, & Beezlebum for your thoughtful input! I hadn’t recognized that the two-stroke schematic was calling for a 6.3v center-tapped heater circuit. I’ve read valvewizard’s pieces on so many topics- I hadn’t seen the one you linked to, so thanks again for that.
I’ve seen someone’s pictures of the process. Maybe I can find that again.
So much to learn, so little room for error.April 20, 2011 at 11:12 am #5437
Just to be clear, the heater circuit for the audio tubes requires 6.3v, the rectifier tube needs 5v for it’s heater circuit. No center tap is required for the 5v rectifier circuit but the 6.3v circuit needs a center tap, an artificial one if the PT does not include a center tap. Just as you use a twisted pair of wires to control hum, the center tap also works to control hum in the heater circuit.September 28, 2011 at 11:28 pm #5497GlocKbymnParticipant
Excuse, topic has mixed. It is removed
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