care and feeding of series heaters

[Andy Le Blanc]

I've been noodling with a rig that uses all octal tubes in the pre-amp.
And my solution for hum disturbances is a 12v DC fil. supply for the heaters.
Its much quieter, and flexible for various tube types is you use series/parallel wiring.

The issue that arises with series heaters is the different current requirements
of tube heaters when you wish to mix it up with different tubes in your application.

so... my application uses a 12.6v supply and I want to use a mixed bag of 2 6sn7, a 6sj7... and a 6v6 rc stage for giggles.

At first you might think... 6 and 6 make 12... no problem

but the tube types all have different current ratings and won't see the right voltage for the heaters... they'll light, but it'll be over or under
and you definitely want to baby your old bench tubes and have happy clients with long lasting and reliable amps

the 6sn7 have .6 amp heater... 6sj7 have .3... and the 6v6 has a .45 heater

So... take the 6sn7 R = V/I 6.3/.6 = 10.5r..... thats the resistance of the heater

you want the other tubes to equal this so that the voltages will be the same across the series heaters string... is the largest current draw

the 6v6 = 6.3/.45 = 14r.... So you use the difference between the two heater ratings

.6 - .45 = .15 then 6.3/.15 = 42r you want to parallel the 14r 6v6 heater with a 42r resistor to match the 10.5r 6sn7 heater

double check 42 x 14 / 42 + 14 = 588 / 56 = 10.5

thats your basic resistors in parallel equation R1 x R2 / R1 + R2

the 6sj7 works out to .6 - .3 = .3 ... so ... 6.3/.3 = 21r

so the application works with 2 12.6v series heater strings
one has a 6sn7 series with a 6sj7 paralleled with 21r resistor

the other is a 6sn7 series with a 6v6 paralleled with 42r resistor

and since the two strings are 21 r thats 21x21/21+21 = 441/42 = 10.5r

I=E/R 12.6/10.5 = 1.2

The heaters will require 1.2 amps of the heater supply current

[darryl_h]

How about parallel 6SJ7's connected in series with the 6SN7. That leg then has a current flow of 0.6A.

If you then used a 15 ohm resistor ( the closest preferred value ) of at least 5W rating in series with the 6V6 heater, the current flowing in that leg is .45A.

Total heater current 1.05A.

Connecting heaters in series or series/parallel can cause voltage peaks in excess of the heater's ratings unless they have controlled warmup times. A heater in series with a dropping resistor will also take considerably longer than usual to reach full temperature. This is because the cold resistance of the heater is much less than its hot resistance.

[Meat&Beer]

Interesting, and quite timely! Now I have to ask, can a pair of identical 6V tubes be run in series in an amp that has 12V tubes everywhere else on a 12V heater tap?

[darryl_h]

Interesting, and quite timely! Now I have to ask, can a pair of identical 6V tubes be run in series in an amp that has 12V tubes everywhere else on a 12V heater tap?

Most certainly!

Many of my builds use 12 volt heater supplies. The 6 volt output valve heaters are connected in series pairs, whilst the 12A*7 valves are connected across the 12 volt supply, using pins 4 and 5, leaving pin 9 floating.

If there is an odd 6 volt heater ( for example an EF86 ) you can use a dropping resistor in the heater supply to that valve alone.

[Meat&Beer]

Thanks darryl, I appreciate the confirmation. What had me worried was this:

Connecting heaters in series or series/parallel can cause voltage peaks in excess of the heater's ratings unless they have controlled warmup times. A heater in series with a dropping resistor will also take considerably longer than usual to reach full temperature. This is because the cold resistance of the heater is much less than its hot resistance.

Is that just in reference to working with different tube types on top of differing voltages while having to accommodate with dropping R's?

I'm not trying to hijack this thread, just looking to wrap the ol noggin around what's being discussed. It also happens to line up directly with a build that I'm drawing the layout for at this very moment!

(Who hid my 12CG7's?!?!?!!!!)

[darryl_h]

If you have two identical valves with their heaters in series, there will not be any voltage surges, as the heaters will warm up at equal rates.

There is some historical information about series heater strings in television sets here: http://www.clarisonus.com/Archives/Tube ... String.pdf

(Who hid my 12CG7's?!?!?!!!!)

I did . . .

[Andy Le Blanc]

I put a 6sn7/12sn7 on each leg exactly because the tube type has a controlled heater warm up period.

and balancing the current over the heater string to avoid a hot flash is the point

using identical tubes or identical current demands balances the voltages over the string

its all very old solutions to using say... 12.6 v for a car radio

[Stevem]

I use a Extron power supply model 28-071 that can be had most times for under 15 bucks on ebay, in fact if you know anyone who has been in the AV line of work fro more than ten years they likely have a stack of these that they would be thrilled to get even 5 bucks for!
This is a regulated 12 volts at one amp so powering 2 12ax7s is a breeze and in all my testing Ii have found that there is really no need for anything thing more than the first two gain stages of any amp section to be on dc for low noise!

[Meat&Beer]

Another question to add... When using 12v AC filament with two identical 6v tube types in series, does it matter (for hum/noise) how you wire them, and if so, why?

For instance, F-4-5-4-5-F, vs F-4-5-5-4-F...

[Andy Le Blanc]

Yes, if the 12v string is referenced to ground, then the tube filament that is
closest to the ground potential will be the quietest.

so the order of the tubes can help with hum, the highest gain tube or the first
tube in the pre amp.

[Meat&Beer]

...so the order of the tubes can help with hum, the highest gain tube or the first
tube in the pre amp.

Thanks for taking the time to answer, I appreciate it! Maybe I should explain a couple more details for this example.

The 12v filament supply being spoke of has a center tap if that matters or factors in to this situation. The amp in question is actually a stereo hifi build, and the two identical 6V tubes (6CG7) I'm asking about are both in the same position, in each channel. They are the phase inverters, one in the left channel, and one in the right. So, they're technically both in the same position under the same conditions you could say...

In this illustration, is there an optimum order of pins to follow? I'll whip up a quick sketch showing how I'm presently picturing it in the amp. Shown is f-4-5-4-5-f, or would f-4-5-5-4-f be better?

Again, thank you!

[matt h]

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[Meat&Beer]

Thanks, Matt!

It's a paraphase, plucked directly from a Conn organ schematic. This amp is being built via the "building blocks" approach, with values adjusted per recommendations from someone so kind as to be helping and guiding me along this build with his tips, tricks, and trade secrets. I've agreed not to share them, just so that's been said.

Here's the PI schem as it appears in the original Conn schematic. (C10 is omitted for this amp, and the 12AX7 is actually going to be a 6CG7. NO NFB!) Correct you are, Mr. Matt, this amp is essentially all parallel with one series exception in the heater arrangement.

I wasn't planning on elevating the heaters. When the inevitable trouble shooting stage comes, however, who knows what may need to be implemented. (Implement. Haha! Inside joke. )

Andy's comments about the "one being closest to ground" is a very helpful tip that's very worthy of being kept in mind. This situation is just a different one, however. It'd be easy enough just to swap the series arrangement after the fact, but I'd really like to solder things once is all. Just seeing if there's a general consensus for this approach.

[matt h]

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[Andy Le Blanc]

just be consistent... in your wire dress, so that if you have an issue it can be resolved, dress both the tube sockets the same.
you can also over design the DC heater supply, if you have a scope you can observe the supply ripple and choose a design with the least % at the Vout.