Mains Transformer Question

[MlinarS]

Hello all,
If you read my prevous posting "What did I do wrong with LEGACY" posted a few days ago, you probably remember that I have problem with the Mains Trafo (to low voltage). After conversation with KevinKR (thank you Kevin), decission was made to replace transformer.
In corespondence with trafo maker I specified tranfo parameters for secondaries as follows:

1) 460-0-460V @ 0.500A
2) 6.6V @ 3A
3) 6.6V @ 3A
4) 6.6V @ 3A
5) 6.6V @ 3A
6) 20V @ 0.250A
7) 7.5V @ 3A

Due to dificulty in making trafo with so many windings he sugested changing specs as:

1) Lowering the HT current rating
2) Combaining 6.6V windings into 2X6.6V @ 6A each

Question about point 1:

If output stage and driver stage draws combined current of 130mA. What would be the safe margin to specify HT current rating? 2x... 3x... or something else?

Question about point 2:

-As you can see from the schematic two diferent windings are feeding heaters of the two GZ34 conected in paralel. If we combine this two windings into one winding of 6.6V @ 6A and feed both heaters in paralel will the rectifiers work properly ?

-The other two windings are feeding the heaters of two 6AS7 with the current @ 2.5A each. In addition one of the windings also feeds heater of EF86 @ 0.200A. If we combine this two windings than the current drawn from all tree heaters would be 5.2A. Is the 6A safe enough margin for the aplication or should I specify higher current rating ?



http://www.fortunecity.com/rivendell/xental/1179/projects/legacy/legacy.html

TIA for help.

Bob

 

[Tom Bavis]

Transformers are normally specified for LOADED voltages, so you will have higher voltage if you use less current. Specify the windings for the EXACT current and voltage you want. Instead of asking for a higher current rating, ask for a lower temperature rise. The GZ34s are 5.0V, 1.8A - use a single 5V/4A winding for both (no series resistors). The 6AS7s MUST have aseparate filament winding! The heater-to- cathode voltage is very high. I would not put the EF86 on the same winding - I'd put it on a winding with the SV83s and bias it to +50V or so. Reduces H-K voltage on the tubes, maybe reduces hum too.

Also - rather than overspecifying the current ratings, I would prefer to use more turns to reduce the flux in the core. Less mechanical hum, less stray magnetic field, less core loss, less magnetizing current, less inrush current... it WILL cost you though.

 

[MlinarS]

Hello Tom,

Thank you for the post.

The GZ34s are 5.0V, 1.8A - use a single 5V/4A windings for both (no series resistors).

Point taken.

The 6AS7s MUST have a separate filament windings!

Sory but I'm nos sure I understand what you seing. Please let me ask you different way. Can I run heaters from both 6AS7s in paralel and feed them from single winding specified as 6.6V/6A or I must have separate winding for each heater ?

Thank you.

Bob

 

[Tom Bavis]

The 6AS7s can share a filament winding, but the cathodes are 425V above ground. The filament winding for the 6AS7s must be separate from all other tubes to avoid heater-cathode breakdown.

The 7.5V winding will carry about 4A with two 300Bs plus capacitor ripple current.

So you need:
460-0-460 @ 260 mA
6.3V @ 5A for 6AS7s
6.3V @ 2A for drivers and EF86
5.0V @ 4A for rectifiers
7.5V @ 4A for 300B filaments
20V @ 250 mA for whatever

I don't think the last winding belongs on this transformer - I'd use a separate 6 VA transformer, with its own input fuse.

If you specify a higher current than you need, you will have margin, but your voltages will be high at the lighter load. If you specify the current you need, but ask for a lower-than-normal temperature rise, you will have the margin, but your voltages will be correct.

 

[MlinarS]

Hello Tom,

Now I understand what you ment (sory, english is not my mother tong).

I don't think the last winding belongs on this transformer - I'd use a separate 6VA transformer, with its own input fuse.

This winding will be used for the driver heater as I changed driver from SV83 to C3M wich requires 20V/125mA.

Since you mentioned separate transformers, should I have a separate winding for the heater of EF86 and use separate trafo for the C3M ? Will this be a better solution ?

Thanks.

Bob

 

[Tom Bavis]

I would never specify a transformer with any winding less than 5% of the total VA. Reason is safety - if that winding gets shorted, it won't draw enough extra current to blow the main fuse. Believe me, I've seen it happen!

As for the EF86, a small transformer is the best choice if it's the only remaining 6V tube. 6.3V/0.6A and 20V/0.25A transformers are available inexpensively from many manufacturers. Make the 6.3V transformer 2A if you think there's any chance you'll ever use 6V driver tubes...

 

[MlinarS]

Hi Tom,

Thank you for your valuable input.

Ask for a lower-than-normal temparature rise

Would you kindly elaborate a bit on this ? What is the normal temparature rise ? Is there any treshold regarding this parameter ?

Thanks,

Bob

 

[Tom Bavis]

Transformers are usually designed for minimum cost while meeting requirements for current ratings (total VA) and temperature rise. A 50 deg C rise might be the design value - 40 is better. The transformer will be larger than the "optimum" they would otherwise choose, and a bit more expensive.

 

[MlinarS]

So if transformers temparature rise is rated 35*C would that be considered good ?

Bob

 

[Tom Bavis]

Yes 35C at rated load is a conservative design.

 

[MlinarS]

Thank you very much for your help.

Bob