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Trout 7th December 2005 12:49 AM

Transformer selection question
 
Hi Guys,
Im looking at a catalog from a well known componet source and am very curious about selecting a transformer for a single ended amp using 6V6GT, 5Y3,& 12AX7, Basically fender champish.

So, My question is more about the MA requirements. My OPT is rated for 40ma (Transformer is rated 100 Hz to 20 KHz -2 dB at 5 watts with 40 ma (maximum recommended) primary current)

The power transformer I am wonder about has the following specs.
650CT 40ma (after rectification)
5V 2A
6.3CT 2A

Is the PT 40ma rating to low?
Also With The 5V-2A rateing, The 5Y3GT requires 2A heater current, Is this also to close?

Or does anyone know a sutible source for a PT that meets the tube requirements listed abouve for $25.00us or under?
Gene

Eli Duttman 7th December 2005 01:20 AM

Gene,

Assuming that 40 mA. of DC is with a cap. I/P filter, a pretty conservative estimate for the AC RMS current capability of the rectifier winding is 60 mA. You can get at all 60 mA. if you use a choke I/P filter. In order to get a decent B+ rail voltage, you'll have to SS rectify. UF4007s are pretty quiet to begin with and they can be snubbed, if necessary. A small cap. (< 1 muF.) in front of the inductor allows you to tweak the O/P voltage of the PSU. It's quite safe to use the fudge factor cap. to make the O/P voltage = AC RMS voltage. A 325 V. B+ rail is not unreasonable for use with a 6V6. Tieing off the 5 V. winding puts 10 VA back into the "pool" that just might be useful. Your PSU filter is LCLC (ignoring the fudge factor cap.).

For a 325 V. PSU using a 10 H. 1st inductor, Icrit is approx. 33 mA. The 40 mA. idle current of the 6V6 easily takes care of that requirement.

Be sure to use a standby switch in the line from the rectifier winding's CT to ground. Let the tubes get good and hot before throwing the standby switch.

Trout 7th December 2005 02:13 AM

So Tube rectified with 3 X 20uf caps would be out of the question?
Guess I best keep looking for a low cost alternative. That one was 23 bucks + SH

These things are alot harder to find in an affordable price range, or at least I can't seem to beat the $35.00 barrier.
Gene

Eli Duttman 7th December 2005 03:03 AM

Gene,

Are you willing to try something a little different? How about a tube rectified voltage doubler supply somewhat in excess of 300 V.?

Mouser stock # 553-N51X is a 35 VA isolation trafo that costs $12 each. Use that trafo to feed a "full wave" doubler made from 2X 6AU4 damper diodes. The doubler stack consists of 2X 250 WVDC 68 muF. 'lytics. Follow the doubler with a LC section.

"Rat Shack" cat. # 273-1511 is a 3 A./12.6 VAC CT filament trafo. that costs $10.49 each. 6AU4 heater draw is 1.8 A. Wire the 6AU4 heaters in series and connect the junction to the trafo's CT. Use a 12V6 for the "final". Same guts as a 6V6, but costs less. 12V6 heater draw is 225 mA. 12AX7 heater draw is 150 mA. The total heater draw is well within the filament trafo's rating. $22.50 plus shipping buys the power "iron". As this is a "12" V. heater setup, you can use an $8 GE 7058 instead of an 'X7.

Trout 7th December 2005 03:13 AM

Honestly,
I think I'll just end up biting the bullet and getting a couple from Triode or Hammond. I was hoping to pop a couple of cheap champs together for some buddies. I would really prefer to stay as close to the normal schematic as possible given my 70/30 success rate LOL

Maybe I can dig something up on evilbay.

Gene

tubelab.com 7th December 2005 02:48 PM

I assume you are looking in the Allied Electronics catalog, since the 6K3VG transformer meets the specs that you list. I think that this one is a bit undersized. The DC voltage will drop a bit when the amp is pushed into clipping, which gives a unique sound, but this is might be too close. I have tested the Triode OPT and it will work up to 60mA so you may want more headroom. At $23 it might be worth a try if you are looking to build low cost amps.

I have been using Allied transformers in my amps for about 4 years. The only one that failed is the one that got wet during a hurricane last year, and it lasted 3 months after that. Autopsy revealed all sorts of green growth due to water. I routinely run these at or slightly above the maximum ratings. They do get hot, but don't fry.

In my SE (45, 2A3 etc) audio amps I use the 6K56VG transformer with a 5AR4 rectifier tube. This gives 320 volts with 90 mA load. You will get less voltage with a 5Y3 and more if you use SS diodes. This one costs $38 though.

For more voltage I use the 6K7VG. It gives 350 volts with a choke input filter at 130 Ma load (300B amp), and over 400 volts with a cap input. This might be too much voltage for your application. It's $43 also. I used these when I built my "turbo champs" a few years ago. Dual rectifier, switchable filter, capable of melting a 6V6 if turned up full. User education required. The schematic will be on my web site with the next update, I found a paper copy and scanned it.

If using a Hammond, try T272X. Around $40, you will get about 350 to 375 volts.

Trout 7th December 2005 08:20 PM

Quote:

Originally posted by tubelab.com
I assume you are looking in the Allied Electronics catalog, since the 6K3VG transformer meets the specs that you list. I think that this one is a bit undersized. The DC voltage will drop a bit when the amp is pushed into clipping, which gives a unique sound, but this is might be too close. I have tested the Triode OPT and it will work up to 60mA so you may want more headroom. At $23 it might be worth a try if you are looking to build low cost amps.
Yep,, Thats the unit. I have a discontinued unit from them in a Marshall clone that is very impressive but I was a $5.00 special at a estate sale 6K6 model. I sure wish they still offered that model!

On the SAME topic, But Different project. I have 3 matching PT's in perfect working order from some salvaged consoles. Looking at the schematic and the tube compliment, I am thinking a couple 18W clone projects.

Transformer Specs on Schematic

I am guessing that it is safe to assume I can run 2-EL84's and 4-5 12AX7's on the filament winding, And 210MA on the HV seems pretty close to safe, or borderline to much?
Once again, Triodes OPT rated at 80ma per side in PP 25 watts,Primary 6600 ohm CT .

I am already going winter crazy here LOL it was the big "ZERO" degrees this morning and not expected to get above the single digits for overnites till maybe saturday BRRrrrRRRrrrrrr
Good time to tinker by the glow of a few warm tubes :D

Gene

tubelab.com 7th December 2005 09:03 PM

The power supply schematic that you provided shows that it provides 360 Volts with a 210 mA load. 210 Ma is about right for 4 6BQ5's and 2 12ax7's. If you use this to power an amp that uses 2 6BQ5's and 3 12AX7's (about 110 mA) you will get 375 to 400 volts of B+ voltage. This is on the edge of meltdown for garden variety 6BQ5's and EL-84's. If you have 7189's or 7189A's this is not a problem. The other possibility is to use a tube that can live with the higher voltages like the 6L6 and it's relatives.

Since you have a transformer that was made to power 4 6BQ5's and 2 12AX7's, you could build an amp that uses this tube line-up. An AC-30 clone maybe. You will need a larger output transformer, like the Triode 22848ULT which costs more.

An extra 12AX7 would not likely be a problem if you needed it. it could be connected to the extra 6.3 volt winding. By the way a 12AX7 draws .3 amps when run from 6.3 volts.

amperex 7th December 2005 09:11 PM

Filament is fine. 40ma is on the edge, but if rated into a capacitor input load such as 40uF, it should be ok. Weber speaker is a cheap source for a Fender Champ type transformer.

Trout 7th December 2005 09:40 PM

Lets not forget the transformer also powered the tuner filaments so figure at least 5 more 6.3V tubes. The Pilot windings are basically not needed.

Looks like everything I have is either to big( PT's) or to small (OPTS):bawling:

Maybe I should just build a stereo guitar amp haha, Hey that might sound pretty nice with the stereo chorus unit I have.

I so hate looking in the garage for more stuff to salvage, But It looks like thats this evenings entertainment.
I have more junk out there than a resale shop.
Gene


Hmm thinking maybe I have a PT & OPT for 6L6's in an old pacemaker pa amp.
I know I have a OPT & PT that drove an old wurlitzer organ with 6L6's also, But no specs on the transformers.
But hey, I haven't melted anything yet this week:D
gene


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