figuring out rectified DC volts; reversing a transformer

[vaughandy]

hey folks,

So I'm needing a transformer with a 175V secondary for the power supply in my old Ace Tone 601. I've ordered one but it's a "made to order" sorta company so it's taking a while and I'm impatient.

I don't have a Variac, but I do have access to a variable DC power source. I hooked this up in the correct polarity to the circuit that indicates wanting a 175V AC power supply. This works for me, but I've found that I have to push it up to at least 225V DC to get any decent sound from it. I'm going to enclose the schematic of the power supply circuit, but I'm wondering how I determine how much DC i should feed that circuit to produce approximately the same voltage as if it had the 175V AC source and rectified it.

My second question is in regards to the safety of inverting a transformer, albeit temporarily. While I wait for my custom transformer, I do have a 220V primary 150V transformer to work with. If I connect 120V AC source to the 150V secondary winding, i believe the ratio would give me 175V out of that primary. Does this make sense? Is it safe to do, for maybe a maximum of 30 minutes at a time? Or am I talking about series / dangerous overheating issues?

Thanks!

[52 Bill]

Your amp uses a voltage doubler circuit to increase the voltage output to what I would guess to be around 400 vdc. The portion of the schematic that you posted has no voltages listed for the high tension supply, but the first filter cap is rated at 450 volts.

You are experimenting and trying things to avoid a proper repair of this amp. The more you do, the greater the chance of damage to the amp and or yourself. Either take the amp in for service or take the time to learn enough to experiment without risking life and limb.

[vaughandy]

Your amp uses a voltage doubler circuit to increase the voltage output to what I would guess to be around 400 vdc. The portion of the schematic that you posted has no voltages listed for the high tension supply, but the first filter cap is rated at 450 volts.

You are experimenting and trying things to avoid a proper repair of this amp. The more you do, the greater the chance of damage to the amp and or yourself. Either take the amp in for service or take the time to learn enough to experiment without risking life and limb.

Thanks I appreciate the info. That makes sense for sure.

As far as your warning, I do appreciate the concern for potential harm. I'll preface this by saying I'm not trying to sound like an ungrateful jerk, but I will admit to being a bit frustrated by people constantly saying I should just take this in for "proper repair." This is all a new hobby for me, but I'm not an idiot. I know very well how to treat high voltages, and I don't do so without undo caution. On top of the usual cautionary meaures, whenever I do any of this stuff I also double-glove with nitrile gloves.

What exactly is so wrong with experimentation? The ultimate goal of me working on this amp is primarily to LEARN, not to re-build the greatest amplifier in the world. I'm not so concerned with whether I'm doing something in the canonical fashion, I'm concerned with understanding how & why something will or will not work. I'm probably going to rise a lot of ire by saying that, but it's just getting a bit old.

[52 Bill]

Since I posted my response to this thread, I went back and read up on the 4 other threads that you have started regarding this amp.

In one thread you asked about how to hook up the transformer to run on 120 vac. Another next thread was about how the capacitors were getting hot, and then blowing up. Then another thread about the rectifiers diodes. In the other threads you were told that this amp has a voltage doubler circuit in the high tension circuitry, and that you should expect 350 vdc.

Do you understand what a voltage doubler circuit does? How it operates? If you had LEARNED from the earlier posts, you wouldn't be asking why you had to raise the voltage from your external power supply to at least 225v to get any sound out of the amp.

Learning is good, I learn things here all the time.

[vaughandy]

I do understand what a voltage doubler circuit does. I totally see now how this is a voltage doubling rectifier circuit, but I'd had conflicted information from one of the other threads that it wasn't. In fact it's the one you mentioned. If you scroll down, you'll see that one poster claims that he initially thought it was a voltage doubling circuit, but was mistaken. I took that at face value, as sometimes happens when one is getting into areas one isn't completely familiar with. See this post:
http://music-electronics-forum.com/t23729/

The capacitors getting hot issues is still a mystery to me. I've figured out how to avoid it, but it seems to be completely dependent on which polarity I plug the AC 220V power supply in. No one has been able to provide a solid explanation for this, but it is what it is. The rectifier diodes were an attempt to fix this issue, as I thought perhaps they were faulty and allowing AC power through to those caps.

[kg]

man, that's a funky power supply. looks like they just used whatever they had on hand and tried to make it work!

kinda like what you're doing.

seriously, if that were my amp, i'd rip out the PS section and redo it with a proper full wave rec. less hum, less likely to kill you if a cap shorts, etc.

if you like the sag of the half wave doubler, add a series R.

but that's just me.

[Jazz P Bass]

About the "have to plug it in one way".
One question comes to mind.
Is there a coupling cap to chassis ground on either L1 Or L2? (Mains Input)
If there is, toss it.
You may have a hot chassis.

[vaughandy]

About the "have to plug it in one way".
One question comes to mind.
Is there a coupling cap to chassis ground on either L1 Or L2? (Mains Input)
If there is, toss it.
You may have a hot chassis.

hmmm, interesting. i've checked the chassis for voltage about a million times, and always do so after changing anything in the power supply. haven't seen any voltage there yet. BUT, the cap in question that wants to blow if I reverse the 220V orientation IS connected to ground, so that sounds suspiciously possible.

[Prairie Dawg]

Wouldn't it be a little simpler to get a more robust mains transformer that delivers the kind of secondary voltage you need without all this voltage doubler business?

[Jazz P Bass]

hmmm, interesting. i've checked the chassis for voltage about a million times, and always do so after changing anything in the power supply. haven't seen any voltage there yet. BUT, the cap in question that wants to blow if I reverse the 220V orientation IS connected to ground, so that sounds suspiciously possible.

There is "Ground Polarity Switch" with a .005 cap on it. Remove that cap.

And why oh why is this not a 3 wire grounded chassis configuration?
2 wire power cords are basically unsafe on an amplifier.
If not illegal.

[vaughandy]

There is "Ground Polarity Switch" with a .005 cap on it. Remove that cap.

And why oh why is this not a 3 wire grounded chassis configuration?
2 wire power cords are basically unsafe on an amplifier.
If not illegal.

oh good call! regarding the 2 vs 3 wire power cord, I've actually got it properly wired up with a 3 prong cord as per Install - 3 Prong Power Cord on a Vintage Fender Amp | Guitar Files(TM). I attached the "extra" bottom ground to the chassis underneath the transformer as recommended in that link. Does that all seem okay?

[Bruce / Mission Amps]

I don't want to get into a pissing contest but I really think anyone (novice amp techs, like yourself) who thinks wearing any pair of gloves,
CHRIST, especially "doubled up nitrile gloves", while working on high voltage amps,... is a safe approach ... well, I'm sorry but 52 Bill's concerns about your knowledge and well being are VERY well founded!
If you don't know why , ask.
A little mentoring is a good thing but letting yourself be insulted by it is really a negative attitude to take at this stage in your hobby.

[vaughandy]

I don't want to get into a pissing contest but I really think anyone (novice amp techs, like yourself) who thinks wearing any pair of gloves,
CHRIST, especially "doubled up nitrile gloves", while working on high voltage amps,... is a safe approach ... well, I'm sorry but 52 Bill's concerns about your knowledge and well being are VERY well founded!
If you don't know why , ask.
A little mentoring is a good thing but letting yourself be insulted by it is really a negative attitude to take at this stage in your hobby.

i'm not insulted. I understand that the concerns are well founded. It's cool. I'm glad you more experienced / trained folks are looking out for novices' safety.

for the record, i don't think wearing nitrile gloves somehow makes me immune to shock and allows me to be sloppy / brash around high voltages. it's just one of the extra precautions i'm taking. anyway.

[defaced]

An extra precaution is only an extra precaution if it actually does something. The only word I could find to describe Nitrile's dielectric properties were "poor". Most only mentioned it's properties against oils/fuels and its mechanical properties.

ESI Compound Properties
ElastoTech SouthWest Inc. - Compound Profile: Nitrile Rubber

The funny thing about high voltages is that they can make things that don't usually conduct, conduct. So yea, typically at say 50v (used purely as an example) nitrile is an insulator, but at 400v (again pulled out of thin air), it will be forced to conduct.

Here's some mentoring for you. In a field that involves circumstances that could kill you, when you start something that you have zero experience in and no one to teach you (like stand there and smack your hands when you're reading B+ voltage and you go to use two hands to hold the two probe leads), it should be 100% reading, 0% doing. After you get a good lay of the land and things start to make sense and you know what can kill you/general safety practices, maybe shift it to 90% reading and 10% doing. Of course this trend continues and at some point you're going to become proficient and you'll settle around 50% reading, 50% doing - assuming you still want to get better and not reinvent the wheel. Jumping in feet first is great, until you figure out - after you've jumped in and can't do anything about it - there's a big ass shark there who wants to kill you. Then you wish you would have read the sign on the shore line that said "No Swimming, Sharks in Water".

[vaughandy]

An extra precaution is only an extra precaution if it actually does something. The only word I could find to describe Nitrile's dielectric properties were "poor". Most only mentioned it's properties against oils/fuels and its mechanical properties.

ESI Compound Properties
ElastoTech SouthWest Inc. - Compound Profile: Nitrile Rubber

The funny thing about high voltages is that they can make things that don't usually conduct, conduct. So yea, typically at say 50v (used purely as an example) nitrile is an insulator, but at 400v (again pulled out of thin air), it will be forced to conduct.

Here's some mentoring for you. In a field that involves circumstances that could kill you, when you start something that you have zero experience in and no one to teach you (like stand there and smack your hands when you're reading B+ voltage and you go to use two hands to hold the two probe leads), it should be 100% reading, 0% doing. After you get a good lay of the land and things start to make sense and you know what can kill you/general safety practices, maybe shift it to 90% reading and 10% doing. Of course this trend continues and at some point you're going to become proficient and you'll settle around 50% reading, 50% doing - assuming you still want to get better and not reinvent the wheel. Jumping in feet first is great, until you figure out - after you've jumped in and can't do anything about it - there's a big ass shark there who wants to kill you. Then you wish you would have read the sign on the shore line that said "No Swimming, Sharks in Water".

Swallowing my pride. I'll do some serious thinking about getting a bit of training.