Hammond Transformers and EZ80/EZ81

[oodi]

Hi,

I am looking at building my first tube amp based on the ECL82. Just selecting the various transformers. For PT I selected the Hammond 270FX 138VA (same price as 270EX 118VA, I understand the 270DX 83VA is marginal for this application). For a choke I am looking at 156M (3H, 100ma, 86 ohms) or 158Q (5H, 150ma, 105 ohms, 400VDC). OTs are 125CSE just because they're less expensive then 125ASE or 125BSE.

I realize that a SS rectifier will perform just fine. However, I like the glow and I am willing to spend a few extra $$$ just to have a rectifier tube on the chassis. The Hammond transforemers have two secondary filament windings, on at 5V and one at 6.3V. Will the EZ80 or EZ81 function with its heater conencted to the 5V secondary?

As far as transformer choice, should I look at compensating for the additional loss of the rectifier tube via a 300-0-300 sec PT, like the 272BX (300-0-300 @ 100mA) or 272DX (300-0-300 @ 125mA)?

Thanks,
Adi

[poobah]

EZ80 & 81 require 6.3 Volts... not good at 5 volts. Barring the temp coefficient of the heater you're looking at a ~40% reduction in heater power... not good.

[kevinkr]

EZ80/81 need 6.3v on their filament to function correctly. Note that the cathode insulation is sufficient on both types for operation on a grounded 6.3V winding at these voltages provided that the overall current rating of the winding is sufficient for the filaments connected to it.

5AR4/GZ34 are cheaply and readily attained, clearly more than needed here - still a good choice and will run fine off of the 5v winding.

Kevin

edit: fix minor typo

[oodi]

I thought about the GZ34 but doesn't it use an octal socket? It therefore wouldn't match the ECL82s from a visual perspective.

Are there any disadvantages to running the EZ80/EZ81 on a grounded 6.3V winding? All the transformers I am considering have ample filament current to support the two ECL82s and a EZ80/81.

One other thought, if I were to run the ECL82 filaments on DC power the 5V secondary should give me more then 6.3V DC if I use a shottky bridge. Not sure if that makes all that much sense, though.

Adi

[poobah]

Oodi,

Actually if you "run the numbers" you will find it very difficult To derive 6.3 DC from 5 VAC...even with Schottky... you will need huge capacitance. The same for a 6.3 from 6.3.

As kevinkr pointed out you have very good rating to run a grounded 6.3 V winding.

[oodi]

Great! A single 6.3V heater winding for all tubes is fine with me.

Just need to think about transformer data now. The EZ80/81 require a certain resistance from the transformer. I have not been able to find this information anywhere regarding Hammond transformers. Short of buying the transformer first, where can I find that information? Perhaps the numbers are high enough in practise that there's no need to worry about it?

Just need to decide on the transformer now, which secondary voltage. With a SS rectifier 275V should be ample, but with the EZ80/81 perhaps I should pick a 300V secondary.

Thanks,
Adi

[poobah]

The numbers from a Hammond trans will be very "normal". I would not "worry" about it. Hammond makes "good" transformers; not "great" transformers. You could surely email Hammond and wait for one day.

Do you have the the power supply design software, FREE, from Duncan's Amp Pages?

[oodi]

I've been playing with PSUDII. I have no high voltage transformer so I have no clue as to what resistances to expect. I plugged 350 Ohm source resistance into it since this is minimum requirement for EZ80. I have the circuit configured per datasheet with 50uF cap (followed by 3H/86Ohm and 47uF) and 80mA. PSUDII complains about rectifier IRFM exceeded. I cannot get rid of that message even if I reduce C1 to 2uF...

Adi

[kevinkr]

Try the EZ81 (6CA4) instead, it is rated for higher currents and voltages than the EZ80 (6V4) - it should also provide a somewhat stiffer supply due to lower internal impedances.

It is acceptable practice to add series resistance between the plates and the transformer winding or if you prefer (and I do) in series with the grounded center tap.

[oodi]

What are the experiences with Hammond power transformers? Are they conservatively rated, i.e. is it acceptable to to run the transformer very close or at the rated loads?

I expect my ciruit to be drawing about 80mA from the tranformer high voltage secondary. Current draw from the 6.3V filament will be <2.2A. If there's a second filament it won't be used. Since I don't have a transformer yet I am looking at this short list:

• 270DAX 75VA, sec. 260-0-260 @ DC ma 90, 6.3V @ 3.5A
• 270DX 83VA, sec. 275-0-275, DC ma 90, Fil.#1(rct) 5.0v @ 2a, Fil.#2(htr) 6.3v @ 3.0a

Then there are these additional choices:

• 370DAX 76VA, 260-0-260 @ DC ma 90, 6.3VCT @3.5A
• 270EX 118VA, sec. 275-0-275, DC ma 125, Fil.#1(rct) 5.0v @ 3a, Fil.#2(htr) 6.3v @ 4.0a
• 270FX 138VA, sec. 275-0-275, DC ma 150, Fil.#1(rct) 5.0v @ 3a, Fil.#2(htr) 6.3v @ 5.0a

My question, are the 270DX or 270DAX acceptable or should I look at the next higher rated transfomers? The 370DAX is much more expensive then all the other choices, is it worth the difference?

Thanks,
Adi