Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on July 30, 2011, 04:04:38 am

Title: Power Supply - one other question
Post by: kagliostro on July 30, 2011, 04:04:38 am: Is near impossible to find Power Transformers (for preamp circuits) that can fit in U1 rack units

So the use of small PCB transformers (10VA) back to back
220v – 9v / 9v – 220v that rectified give near 300v B+ ( 220v x 1.4 = 308v)

But there are circuits that need higher B+

Someone tried to obtain more B+ using different rating transformers back to back
220v – 12v / 9v – 220v (220v/9v = 24.4 than 24.4 x 12v = 293v) that rectified gives near 410v B+

But this way there is core saturation and the transformer must be oversized or became hot quickly (also oversize with U1 rack don’t match)

To use two back to back arrangements in series (220v – 9v / 9v – 220v + 220v – 9v / 9v – 220v) will give an AC voltage of 440v that rectified is 616v, too high for preamp purpose

So I’ve think to use a different back to back arrangement like this

220v – 6v / 9v – 220v + 220v – 6v / 9v – 220v that will give 146v + 146v = ~293v AC that rectified will give near 410v

The question is:
Giving to the second transformer a lower input voltage (6v instead of 9v) will be a safety way to obtain the needed B+ voltage avoiding core saturation & other problems ?

Thanks
Title: Re: Power Supply - one other question
Post by: jojokeo on July 30, 2011, 04:24:38 am: I think this could work for you, have you seen these before?
http://www.antekinc.com/details.php?p=655 (http://www.antekinc.com/details.php?p=655)
or this one
http://www.antekinc.com/details.php?p=639 (http://www.antekinc.com/details.php?p=639)
Title: Re: Power Supply - one other question
Post by: VMS on July 30, 2011, 05:36:42 am: Here's couple more:

http://www.tube-town.net/ttstore/product_info.php/info/p4786_Torodial-81VA-Power---Ultra-Flat.html (http://www.tube-town.net/ttstore/product_info.php/info/p4786_Torodial-81VA-Power---Ultra-Flat.html)

http://www.tube-town.net/ttstore/product_info.php/info/p782_Torodial-27VA-Preamp---Engl-520.html (http://www.tube-town.net/ttstore/product_info.php/info/p782_Torodial-27VA-Preamp---Engl-520.html)
Title: Re: Power Supply - one other question
Post by: Willabe on July 30, 2011, 08:20:44 am: How about these from Hammond?

http://www.hammondmfg.com/229.htm (http://www.hammondmfg.com/229.htm)

Brad :think1:
Title: Re: Power Supply - one other question
Post by: kagliostro on July 30, 2011, 12:13:42 pm: Many thanks friends

Jojokeo

the price is good and also transformers seems to fit U1 space and needed current and voltages

Unfortunately I live in Italy so I'm looking for Europe market suppliers

VMS

the first transformer is a bit expensive for a preamp need but may be nice for a little amp

the second one (Torodial 27VA Preamp / Engl 520) seems to be a good choice (240v x 1.4 = 337v DC) also if not perfect for High Voltage request of some preamp

Willabe

Hammond 229 series, I know that (and can buy it directly here in Italy) but the B+ voltage isn't so high (220v x 1.4 = ~308v)

Thanks

Kagliostro

p.s.: technical question

using the 220v - 6v / 9v - 220v + 220v - 6v / 9v - 220v to obtain the ~293v AC I think I loss current, isn't it ?
Title: Re: Power Supply - one other question
Post by: jojokeo on July 30, 2011, 12:54:45 pm: Quote from: kagliostro on July 30, 2011, 12:13:42 pm
the price is good and also transformers seems to fit U1 space and needed current and voltages

Unfortunately I live in Italy so I'm looking for Europe market suppliers

Just a suggestion. They will ship as many transformers in a box as possible for one shipping fee and could utilize flat rate int'l shipping which combined w/ the low pricing of the tranny's would make things more do-able & reasonable for you and justify the higher shipping cost? Medium flat rate is about $45 but you may get away w/ small flat rate for $13.95 US and they may still be able to fit in 7 or 8 easily? But if you only needed one or two then the shipping fee is still reasonable.
Title: Re: Power Supply - one other question
Post by: kagliostro on July 30, 2011, 01:24:07 pm: Jojokeo

there isn't only the shipping problem

you must (unfortunately) consider also TAX ( :sad2: don't want say more about :sad2:)

Kagliostro
Title: Re: Power Supply - one other question
Post by: jojokeo on July 30, 2011, 01:33:38 pm: How much would that be? Sorry, I have no idea what it would be for you, what would you have to pay in Italy?
Title: Re: Power Supply - one other question
Post by: kagliostro on July 30, 2011, 01:58:20 pm: I'll pay the tax applied in the USA (VAT) more the tax applied in Italy (I.V.A.) more the customs duty

the I.V.A. will be 20% and custom duty 4.2% (EDIT someone told me there is a more, not better specified 4% to be paid)

to consider also that sometime ago I ordered a book in the USA (Guitar Amplifier Power Amps - Richard Kuehnel) and I wait for it 3 months

So you can understand I am compelled to prefer european suppliers :dontknow:

Kagliostro
Title: Re: Power Supply - one other question
Post by: PRR on July 30, 2011, 11:36:22 pm: 6V applied to a "9V" winding is safe.

The current rating is unchanged. So the total VA or Watts-DC is 6/9 or 2/3 of what you paid for. This is a "tax"; but much less than the man in the uniform would take for importing luxury goods that compete with your home industry.

> two back to back arrangements in series

You don't need a lot of current. Caps are cheaper. Use the Voltage Doubler.
Title: Re: Power Supply - one other question
Post by: jojokeo on July 31, 2011, 12:03:27 am: I have to ask since I've been on a cap info mode recently: would balance resistors be used across the caps to even the current between the two like when used is series situations?
Title: Re: Power Supply - one other question
Post by: PRR on July 31, 2011, 12:12:16 am: In this type voltage doubler, no. It works as two half-wave power supplies. Each cap is *forced* to 205V, no matter what's happening on the other cap.

In conventional supplies that don't have a half-voltage connection: yes.
Title: Re: Power Supply - one other question
Post by: kagliostro on July 31, 2011, 03:03:36 am: Thanks PRR for your intervention

Quote
So the total VA or Watts-DC is 6/9 or 2/3 of what you paid for

the PCB transformers I have are rated at 10VA
(I got it on the cheap, € 2.00 each, initially thinking to use only for filament purpose, they fit near exactly the U1 available space)

so I can have near 0.03A (6/9 = ~0.7 -- 10VA x 0.7 = 7 -- 7/220 = 0.03A)

than I must consider losses of the transformer ........... may be 0.025A .......... is that correct ?

using a doubler I loss half of the current, so ~0.013A ........... I think is too poor, isn't it :w2:

can you give a rule of thumb as to estimate VA requested in relationship to the kind of tubes usually present in the preamps (such as 12ax7 - 12at7 - 12au7 - 12ay7 - 6sn7 - 6au6 - 12sn7 - ef86 - 5879 - 5751 .........) ?

don't know if that is possible but this will be of GREAT help

Thanks

Kagliostro

p.s.:

Quote
much less than the man in the uniform would take for importing luxury goods that compete with your home industry

Hope one day this will be only a bad memory, but I think we must wait much before that happens .................
Title: Re: Power Supply - one other question
Post by: HotBluePlates on July 31, 2011, 11:07:15 pm: Quote from: jojokeo on July 30, 2011, 01:33:38 pm
How much would that be? Sorry, I have no idea what it would be for you, what would you have to pay in Italy?

The biggest problem with shipping from the US to Italy isn't even the tax (although it is bad). The problem is customs.

I sold something on ebay one time to an italian buyer. I shipped it promptly. It sat in customs for 4 months(!) with the buyer unsure if I stole his money. He did eventually get his item, but I swore off shipping anything to Italy after that.