# Direct Heated Valve in Series - Channel Separation Question

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#### ballpencil

Single ended pentode amp using type 27 and type 47 tube per channel. 27 is indirectly heated while 47 is directly heated and both heaters consume 2.5V@1.75A. Let's say i have 12VDC supply and I put the heaters in series like so: +12V > 47 Left Ch > 47 Right Ch > 27 Left Ch > 27 Right Ch > Power Resistor > Ground.

Target Vgk bias is -15.6V for each type 47. So.. for the left channel, the 47's cathode is already elevated 9.5V above ground and the right one is 7V (think filament bias). I will make up for the remaining Vgk with fixed bias.

What happens to channel separation? Since the 47s' cathodes are at fixed voltage relative to ground (clamped by the 12V supply), i think there is no problem with channel separation when music is playing but i may overlook something. Opinion?

#### JonSnell Electronic

If the filaments are in series, what appears on one will interfere with the other. If wired in parallel, the same will apply.
Always use separate filament supplies that are isolated.

#### Tom Bavis

Since the total heater string is 6.8 Ohms (12/1.75), a cathode current swing of 40 mA could result in crosstalk of 270 mVp-p. Keep in mind that any channel-to-channel crosstalk will be out-of phase. Bypass the junction of the two 47 cathodes with at least 4700 uF.

#### ballpencil

Thank you. I will keep the 4700uF idea in mind. How about if i parallel each filament with LED? 2.5V LED.. i guess that's the blue one? I think then any cathode current will swing through the LED, instead of the resistive heater chain. The question that arises is how effective it is? SY in his Red Light District EL84 article mentioned a Red LED has about 4.5 ohm impedance.. while a single 47/27 heater has 2.5V/1.75A=1.42 ohm. I could of course parallel a few more LEDs to reduce it even further but is it necessary? I know that 4700uF has ESR but is it lower than 4.5 ohm? Which one is better: LED or bypass cap?

#### smoking-amp

If you were to use a 2.5VCT xfmr for the tubes all in parallel, then the signal current drops would be occurring across sub Ohm copper resistance (to the CT).

Another approach with the series filaments would be to use C bootstraps from the driven tube's filament CT back to the driver plate load (B+ end, with another series R to actual B+), so that the drive signal would be developed relative to the filament V. A small issue would still remain due to the low driver Mu of 9, which would be somewhat sensitive to the bootstrap varied plate V. (1/9)

#### BinaryMike

I know that 4700uF has ESR but is it lower than 4.5 ohm?

A quick scan of Mouser's 4700uF 25V offerings gives an ESR range of 17~106 milliohms.

#### ballpencil

I have this fetish with powering tube amps with 12-24VDC brick adapters so using 2.5VCT xfrmr is not an option It's either wire each channel in parallel with separate huge hot power resistor or put them in series as i mentioned for lower heat and better efficiency. That mouser 4700uF ESR is lower than i thought. I guess that is the cheapest best option. Thank you.

#### petertub

Paid Member
I have this fetish with powering tube amps with 12-24VDC brick adapters so using 2.5VCT xfrmr is not an option It's either wire each channel in parallel with separate huge hot power resistor or put them in series as i mentioned for lower heat and better efficiency. That mouser 4700uF ESR is lower than i thought. I guess that is the cheapest best option. Thank you.
Well, then it's simple. Just buy e brick adapters for the filaments, 1 each for
the direct heated, and 1 to share between the indirect heated ones ( + one for your B+ )

Life is sooo simple !

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