recommend me a diode

[daz]

I have 7.1 v on my heaters, and i know some will say thats not a problem. But every so often i see a post somewhere where someone with the same issue is told it's not a problem and others say it is. I went with the latter in the past and left it alone. But i now decided to err on the safe side and add some diodes to drop the voltage. Here are the PT specs, can someone recommend a NTE part# that would be rated well over what i need for this?

Hammond Power Transformer 272JX

"Classic" Enclosed Plate & Filament Power Transformers
Primary 115 VAC, 60 HZ. Enclosed, 4 hole chassis mount. Minimum 6" long leads. Class A insulation (105 degrees C.) Hi-Pot test of 2,000V RMS. Conservatively rated and CSA approved.
272JX 236VA, sec. 300-0-300, DC ma 250, Fil.#1(rct) 5.0v @ 4a ct, Fil.#2(htr) 6.3v @ 8.0a ct.

From valve wizard....

[Hardtailed]

How current are you actually pulling?

I think you have 3 12AX7 and 2 EL34? So that's 3.9A total if my brain is still functionning.

Did a quick search on Mouser, and 6A parts (to be on the safe side) seem to drop too much voltage, they have pretty slow recovery time too...

What about using a resistor?
3.9A and you want to drop 0.8V, that gives us a R of 0.2ohms... 0.22ohms is usually available. Power dissipation would be over 3W, so a 10W part would be safe. In a pinch you could parallel 5 1ohm 2W resistors. Just thinking out loud here...

Perhaps someone knows of a 6A diode that only drop 0.7V or so... I don't

The question though is: has tube life been a problem so far?

[daz]

The question though is: has tube life been a problem so far?

Not that i'm aware of, but thats not the point. The point is i hear both sides from equally smart guys on this and other forums, so whether or not it's an issue i'm going to do this to err on the safe side. The tubes are 3 ax7's and 2 34's, yes.

[ThermionicScott]

I like Hardtailed's idea. It doesn't take much resistance to bring down the heater voltage to a healthy level.

- Scott

[fyl]

can someone recommend a NTE part# that would be rated well over what i need for this?

Merlin suggests a 60S1, the equivalent NTE part would be the 5812.

http://www.nteinc.com/specs/5800to58...nte5812_17.pdf

[daz]

I like Hardtailed's idea. It doesn't take much resistance to bring down the heater voltage to a healthy level.

Yes, but i seem to recall there are issues doing it that way aren't there? They can get too hot and fry or something as i recall. Or at least when i asked way back when there was something like that said. What wattage would you say is necassary?

[fyl]

I have 7.1 v on my heaters

Is it under load or unloaded?

If unloaded, you'll have close to 6.3V under load.

If under load, 7.1V is on the high side, you should have 6.3V ± 10% => 5.67 to 6.93V, and preferably ±5% for maximum tube life.

[fyl]

Perhaps someone knows of a 6A diode that only drop 0.7V or so...

A standard 6A diode will drop app. 1V at 6A but only .85V or so at 4A. Using a higher rated part, say a 10A or 20A diode, will reduce voltage drop.

OTOH, you can pick a Schottky barrier diode, there are hundreds different models rated at 10A or more. A good example is the MBR735, a 10A/35V part, which will drop only .75V @ 7.5A and around .7V @ 4A.

http://www.fairchildsemi.com/ds/MB/MBR735.pdf

NTE equivalent:
http://www.nteinc.com/specs/6000to6099/pdf/nte6083.pdf

[stingray_65]

I had ran in to the very same issue building a Princeton ish amp.

I was using vintage Iron and the secondary came right where I wanted but the heaters were too high for my liking, in the 7.4 and 6.3 V range.

I used 2 sets of IN5408 diodes run back to back as you pictrured, one pair on my 5V and one on my 6V.

they dialed in REAL close to where they should be.

if you subscribe to the 20% tolerance 5v + 20% = 6v and 6.3 + 20% = 7.54v

where you are right now is acceptable in most aplications, just on the high side.

I use a lot of vintage tubes and often they are pulls from old equipment and I try to stress them as little as practical.

in all honesty the major reason I opted for the diodes is to keep from buring up #47 bulbs. at 7.3 v they look like lazers and don't last as long as a paycheck does these days.

[ThermionicScott]

Yes, but i seem to recall there are issues doing it that way aren't there? They can get too hot and fry or something as i recall. Or at least when i asked way back when there was something like that said. What wattage would you say is necassary?

If you do it wrong, sure. Here's an example: Say you have a 2x6V6 amp with three 12AX7s in the front end, all running off the same heater winding. That's 1.35 amps of heater current. To bring that 7.1V down to 6.3V, you'd need 0.59 ohms of resistance in series with all of the heaters. 0.8V * 1.35A makes 1.08W of heat to deal with. Use a 2W precision resistor, mount it so it can dissipate heat (like you would a cathode resistor), and you're done!

My dad and I built an amp while I was in high school using a bunch of old iron and we used this trick in several places to keep the heater voltages in check.

- Scott