OT - Firefly Lives! But I need a little help...

[Paul Marossy]

OK, I did the smoke test. At first I had a horrible hum when I turned it on, and the controls would make it squeal when I turned them too far in either direction. So, I checked my wiring one more time, looking at my connections to ground. I made a dumb mistake... I forgot to connect the filter caps and the 100 ohm resistors on the heater supply to ground.  :oops:

So, having fixed that, I fired it up and thought it sounded great. It's quiet and has a nice tone. I was using my Seymour Duncan Convertible's 12" speaker, which is a Celestion Sidewinder for my test. But, I noticed that it seemed very, very loud. At least a lot louder than I was expecting it to be. So, after playing it for about 15 minutes, I decided to check all of the B+ voltages just to see how close they were to the schematic. B+1 measured 585 volts! I'm surprised that my filter caps didn't blow up and other things didn't melt down, too.

I shut it off again, and scratched my head a little bit. I checked my connections to the rectifier bridge and it is correct. But, it seems as though it's acting like a voltage doubler somewhere on the secondary side, which measures about 220 volts before the bridge rectifier, IIRC...

I used the Hammond 269EX power transformer which has a center-tapped secondary. Should that center tap be connected to ground? It seems to me that it should be. If so, can it be connected directly to ground or does it need to go thru a resistor? And can I keep the 4-diode bridge?

Anyone?

[Nasse]

Aargh!!! Could it be wrong configuration of rectifier diodes and center tapped transformer???

Would these pics be any help, or just for something to reference further discuss?

a)


b)



found this link, basic power transformer and rectifier circuits and nice pics but unfortunately in finnish, but one picture tells more than thousand words.

http://koti.mbnet.fi/~huhtama/ele/ml11.htm

[Joep]

Hi Paul,

You are applying 380 volts to the bridge now instead of 190.

You should use picture B) in Nasse's post to wire it. Both of the red wires go to one of the diodes, the red/yellow wire to ground.

You can use A) but then you have to connect the centre tap (Red/yellow) and ONLY one of the other Red wires to the bridge. Isolate the other red wire. This is not the prefered solution, because you loose half of the power the transformer can deliver.

This will get you the right B+ voltage.

Great build btw. Paul!!

Good luck,

Joep

[Paul Marossy]

Thanks guys. I looked up how to connect a center tapped full-wave rectifier in a book of mine, and it's like the second picture. The detailed description on which color wire goes where is helpful. My only experience with center-tapped secondaries is for creating a bipolar power supply...
You learn something new every day.

I'll go fix it now.  :wink:

[idlefaction]

check out duncan's power supply calculator:
http://www.duncanamps.com/psud2/index.html

like Joe pointed out, if you're running the 269EX as a bridge, then you're feeding it 380V and should get 1.4 * 380 = 532V off the first cap.  if you have an inductor, that would explain the rest of the voltage.  yeah, definately change to a full wave recto!!!

i built a vox AC4 circuit on the weekend for my gf - EF86 + EL84 into a really old 'silver bell' alnico celestion, it sounds fantastic with a les paul!!!   hooray for amp building aye.    i might try practising with it cos it's a really LOUD  4 watts *grin*  i should take some photos actually, i covered it with blue fur and put googley teddy bear eyes on the front!!!!!!!!

congrats, are you hooked yet?  

[ncc]

Hello,
Thanks for posting the request/ solution.

I am about one day or two from applying power to
my firefly and I was about to make the same mistake.
I followed the instructions on the Doug's schematic but
he is not using a 269EX transformer.  I did assume
that the 269EX would work if I ignored the center tab red/yellow
wire but after reading this posting, it makes sense that the
190VAC rectified through 4 diodes would show up as 380V.
Yikes!

This is the best place to learn.
take care,

ncc
PS. Paul, I can't wait to read about your progress in your next post.

[Doug H]

Guys,

I apologize for all the confusion over this (and I'm glad you didn't blow anything up, Paul).

In amp circles, this pwr supply stuff is kind of 2nd nature and we tend to gloss over some details. I never expected this amp to catch on over here at Aron's pedal site so I need to explain things a little better for a different audience (and I don't mean that as a dis   ).

Nasse's diagrams are exactly right. Here's the deal: The 269 transformer is center-tapped. These types of transformers are typically rated at half their full secondary voltage. So a "190VAC" (or 190Vrms) center-tapped transformer is actually 190-0-190, which means it swings a total of 380Vrms from one end to the other. The reason they are rated at half the voltage is because they are traditionally used with a full-wave rectifier- one diode (sometimes 2 in higher power circuits) on each end and the center tap grounded. The filter caps will get charged to the peak value of the 190Vrms AC voltage- 190 * 1.414 = 268.66V. Now this will be affected by your actual line voltage and the amt of current being drawn by your circuit- but it gives you a general idea of the DC "B+" voltage you will get out of the transformer. (If you use a vacuum tube rectifier instead of diodes, there will be more loss and it depends on the tube as to what your B+ actually will be.)

So if you are going to use a center-tapped power transformer for this amp, don't even bother with the diode bridge. Just follow Nasse's "b)" diagram and put a diode from each outer leg of the transformer secondary to the first filter cap, and ground the center tap. Then it's a done deal.

The only reason I used a diode bridge is because I could get a 207->110VAC isolation transformer inexpensively. It doesn't have a center-tap and swings 207Vrms total from end to end.

I will add an update to the schematic to show the alternate pwr supply wiring for those who want to use a center-tapped xformer like the hammond 269.

Once again, I apologize for all the confusion over this.

Doug

[Paul Marossy]

Thanks, Doug. I checked out your new schematic. At least I know how these center tapped secondaries work now.  

Everything is just fine now. Thanks for posting that at your site, and for all the rest of the stuff you have contributed.

For anyone who wants to see more details of my project from finish to end, check out this page: http://www.diyguitarist.com/GuitarAmps/FireFly.htm

[MarkB]

Congrads, Paul!!
Looks great.

Is the layout on your site updated with the proper transformer wiring?
"-)

[Paul Marossy]

Well, the layout on that page shows how it should be wired with the Hammond 169EX transformer. So, yes, if you use that transformer, the layout is correct. If you use a different transformer, see Doug's schematic for how to wire it.  

[Alex C]

Hey, I'm having some trouble too.  Please read my post here:   http://diystompboxes.com/sboxforum/viewtopic.php?t=18704  

Thanks!


Alex

[Paul Marossy]

Alex,

If I understand your question, I put the standy switch between the filter caps and the rectifier diodes.

On the transformer, the center tap is connected to ground, each "end" of the secondary is connected to a diode. Those two diodes are connected together (like the 2nd diagram above), and from there you having your starting B+ voltage. Here is the actual measured B+ voltages in my Firefly:

B+1 - 299V
B+2 - 265V
B+3 - 254V

These are a little higher than what is shown on Doug's schematic, but it works fine.  :wink:

[Alex C]

Thanks for your help, I know I was a little unclear with my language.  I used Radio Shack switches because I had 'em left over from another project.  They're rated at 250v, and I feel that I shouldn't use this one for the standby if the voltages are going to be higher than 200 or so (but the one for the 115V mains to the transformer should be fine).  That's why I was inclined to switch the center tap to ground instead of switching the B+.  I was worried about the switch handling the voltage.  I'll get another switch and try it this weekend.  

Another question, though- assuming yours (Paul) is wired correctly, why are you getting 250V+ from the transformer that's "rated" (I know that's not the correct word, but you know what I mean) at 190V?


Alex

[Doug H]

Another question, though- assuming yours (Paul) is wired correctly, why are you getting 250V+ from the transformer that's "rated" (I know that's not the correct word, but you know what I mean) at 190V?


Alex

190V is the RMS voltage. It will provide DC at the peak voltage so you multiple the RMS value by 1.414 to get the DC voltage when using diodes for rectification.

Doug

[Alex C]

I saw you use that in an equation in an earlier post, but I didn't understand it.  Thanks for the explanation!   You've been very helpful to many people, thank you very much.  


Alex

[Paul Marossy]

Alex,

I think I have those same ratshack swiches, too. The ones that have the "on/off" plate, right? I personally think that the standby switch is probably fine if you look at the current that it is seeing, even though the voltage is higher than 200 volts.

In other words, if the switch is rated for 3 amps at 200 volts, then theoritically, it should be fine at 1 amp at ~250 volts. If it were 3 amps at 250 volts, well then, that's a different story.  
(someone correct me if I'm wrong)

The only problem with switching that secondary center tap to and from ground is that if you do it that way, then B+ current is always being supplied to your circuit. I don't think it would work like you would expect it to.... When you flip the power switch, B+ voltage would be supplied immediately to the circuit, not giving the tubes time to warm up. That kills tubes a lot faster.

[Alex C]

Hey Paul, thanks for your reply, and thanks for your help too.  

I think my switches are like yours, they say the same thing, but the lever part is black on mine.  I think I'll get a different type just to be safe.  

I had considered what you said about the B+ always being supplied, and I had thought about that, but I figured that if the AX84 amp did it, it couldn't be bad.  Also, here's my probably very-wrong idea about how this is working:  I thought that disconnecting the center tap (via the standby switch) would be comparable to disconnecting the negative end of a power supply, and therefore the circuit would not be complete, and the voltage would not actually be present at the B+ points.  I am probably wrong, but that was what I concluded while building the P1.   I don't even know the basics of transformers.  Could someone please explain this to me?


Alex

[Paul Marossy]

Well, all I know is that I didn't have the secondary center tap connected to ground, and it still worked...  
If you break the B+ supply, that works for sure!

I have a really good book on transformer design that was given to me, but I haven't taken the time to read it thoroughly.

[Alex C]

No sound comes out when I have it on standby the way it is now (switching center tap to ground), but when I am playing and then turn it on standby, it takes probably ten seconds before I can't hear anything.  This is much longer than my Carvin MTS tube amp and my P1 (which uses the same transformer configuration).  

I'll wire it with the B+ switched by the standby just to be sure that's not my problem.  Thanks for your help and suggestions, Paul.


Alex