heater schem question

[J Rindt]

This Traynor YCV-20.
The three preamp tubes.

1. Are they running one AX7 in series at +15 VDC.
And the other two in Series/Parallel at -15 VDC .?

2. Would that mean each heater is running at 7.5 VDC.?
But they are dropping That Down to closer to 6.5 Volts via R1 R86 and R87.?
Thank You

On page #7
 
http://traynoramps.com/downloads/servman/sm_ycv20.pdf

(Is there a way to add this to the forum schem archives.?)

[pompeiisneaks]

They're using only 15 volts through that dropping 18 ohm resistor do lower hte voltage closer to the 12 volts needed.  I am thinking, though, if I'm doing the math right, then it is a bit under powered, V=IR, and for a 12AX7, 300mA for heater current is .3 * 18 = 5.4 so it should drop it to sub 10V but maybe I'm not doing the math right?  At any rate, I'm pretty sure that's why they're using the 15V side for the tubes.

As for the schematic, I think that one of the mods can put it up, or Doug can if he doesn't already have it.  (you can also post it to the schematics sub forum if you want)

[sluckey]

Those tubes can be operated from 6.3VAC/.3A or 12.6VAC/.15A, depending on how the filaments are wired. I'm gonna just say the tubes operate at 6VDC or 12VDV because that makes the math simple.

Each tube is wired for 12V operation. That means the two filaments inside each tube are in series. Each tube is powered from a 15V regulated supply. That means you need a series resistor to drop the voltage at the tube to 12V. Each 18Ω resistor must drop 3V. 3V/18Ω=0.167A. That's the current flowing through the resistor and also through the tube. and that's close enough to the required filament current to properly heat the tube.  (These numbers will change slightly if you assume there will be 12.6V rather than 12V at the tube. You can do that math if you want to.)

One thing bothers me about this circuit... The 18Ω resistor will dissipate .5W with 3V across it. (3V²/18Ω=0.5W). Yet the resistor is only rated for .5W! It would be wise to double the power rating of that resistor to prevent failure due to heat.

Send Doug a pm if you want this schematic added to the library.

[J Rindt]

Yeah...the round numbers make it easier to Ask/Explain.
I will send Mr. Hoffman an email.
Thanks

[shooter]

Yet the resistor is only rated for .5W

this amp followed your recommendation


Is there "sound" engineering to opt for all the extra parts?

[sluckey]

Of course! 

[PRR]

> a 12AX7, 300mA

Let's use the right numbers. 12AX7 can eat:
6.3V 0.300A
12.6V 0.150A

Ass-uming they aimed at 0.150A target, 0.150A * 18r is a 2.7V drop.

15V - 2.7V is 12.3V at the "12.6V" heater.

I believe the tube is utterly happy.

However it will suck less at 12.3V of nominal 12.6V. A naive resistance assumption suggests 0.146,4A. Heaters tend to more constant-current and I would dart-board 0.148A. This is really splitting frog-hairs, but 15V-(0.148*18) makes 12.336V.

Using the round numbers: The resistor wastes 2.7V*0.150A. This is 0.405 Watts. In a 1/2W part!! As Sluckey says, this is far too close for long-term comfort. I would call it a "30 day special". If you warrant your stuff more than 30 days, you should probably up-size the resistor 2X, a 1W part. (I have seen a 4X suggestion-- if the difference in price is small, the extra 10 cents today may avoid a $5-$50 hassle or lost-gig down the decades.)

But wait! Are you ever going to cold-start? A cold heater has resistance 1/5 to 1/10 of hot resistance. Taking 12V 0.15A that is 80 Ohms hot, so 8 to 16 Ohms cold. Taking 9 Ohm guess, combined with 18 Ohms, and 15V, at cold-start the 18 Ohms sees 10 Volts! And 10^2/18 is 5.55 Watts! This is only for a second, though current will not drop to nominal for several seconds. A 1/2W part will go red-hot at 5.5W. The material expand suddenly. The element develops micro-cracks, and again at every turn-on, until it fails. A 5W resistor would not be overly conservative, though I could see using a 3W part.

Main-cap sag and regulator limits may modify these peak stresses, though a thumb-count does not suggest a major dip.

[pompeiisneaks]

PRR thanks!  that helps me understand why my math was 'right' with the wrong data ;)

~Phil

[shooter]

also thanks, i'll stick to the old n tried methods

[J Rindt]

FWIW.....this amp is now about 12 years old, those resistors have never failed, and show no sign of abuse.

[pompeiisneaks]

age doesn't matter as much as how much they are used.  If you mean it was used heavily for 12 years then maybe it's fine, but Sluckey generally knows what he's talking about :)  Your call, though, your amp.

[J Rindt]

It belongs to a friend. If i still had it, I would go ahead and just replace them.
But yes.....it has been played A Lot.