Am I reading wrongly this schematic or heaters are really lift of 280V DC ?

[kagliostro]

I was giving a look to this schematic because it uses a tube "bridge" rectifier (2 x 6X5 and 1 x 5Y3)


and my attention was captured by the CT of the HV winding that is connected to the CT of the 6X5 heaters winding


are those winding really lifted of a so large amount of DC (280V) ??


K

[DummyLoad]

its's a seperate filament transformer. 6X5 is indirectly heated. so yes heaters for 6X5 are elevated. perhaps if you look at it as a capacitor input FW CT Bridge Doubler it may help you see what's going on.


--pete

[kagliostro]

Some time ago I asked about a similar circuit used in a Philips PA amp and I learned what was happening,


at the CT you have a positive voltage that is 1/2 of the rectified voltage

I was doubtful because 280V seems a very high voltage to be applied to an heater winding CT

Thanks Pete

Franco

[jojokeo]

I was doubtful because 280V seems a very high voltage to be applied to an heater winding CT

kag - all you need & want is 35-40 Vdc on the heaters.

[kagliostro]

I know that 30-60v are usually used


I was astonished seeing that the schematic used the 280v to lift the heaters


Franco

[HotBluePlates]

I was doubtful because 280V seems a very high voltage to be applied to an heater winding CT

kag - all you need & want is 35-40 Vdc on the heaters.

You may only want the heaters 20-40vdc higher than the cathode. But there are circuits out there where some tube cathodes sit several hundred volts above (or below) ground, so you would want to reference the heaters of those tubes a similar amount above or below ground.

That's said, the situation is rare in guitar amps. Cathode followers and long-tail inverters are the most likely place for it to arise, but typical guitar-amp practice is to rely on the near-100v heater-to-cthode voltage rating. Hi-fi practice generally spends more on extra heater supplies & protects the tubes more, but they also typically spend $$$$$ on tubes.

[kagliostro]

That's said, the situation is rare in guitar amps

Yes, sure, those in the schematic is a RF circuit (seems to be a CW transmitter)


Franco

[HotBluePlates]

Yep, for a morse code transmitter (That's what the "Key" is: a big momentary switch to tap out the code).

[PRR]

> its's a seperate filament transformer.

Yes, but it could have been on the main core. (Except the main core's 6V winding is in use for the 6146's grounded heater.)

We want the HEATER-CATHODE voltage to be not-high. Under 100V-200V(*) for most small tubes. 450V for 6X5, which often has 300V-400V on cathode but gets heater power with the small tubes which use a grounded or semi-grounded heater circuit.

In this very strange plan, the 6X5 cathodes are waving up and down at the ends of the HV winding. I believe the HV winding is 311VAC each side of the 280VDC point. If the heater winding were grounded, there would be 715 Volts peak across the 6X5 heater-cathode insulation. This exceeds the 450V rating, so it would short-out, smoke, stink. By tying the heater winding to the 280VDC tap, the heater-cathode voltage is 435V peak, and the rating is not exceeded (however it is very close).

It would be better to have *two* 6V windings, one for each 6X5 (plus one for the 6146). Then each 6X5's H-K voltage could be zero. But this is a very crude transmitter, with very common parts, and as few parts as possible.

Also note that 6146 heater is grounded, but in the key-up condition its cathode will float up to above 74 Volts. Since the H-K rating is 135V, this is OK. And in an RF stage, radio signal tends to follow all leads, even heater leads, so hard-grounding the heater circuit is probably best.

(*) Some (not all) tubes have heater-cathode leakage which will cause hum in low-level stages. Taking the heater somewhat positive reduces this. So 10V-50V of heater lift is often seen.

Some circuits have trouble with heater-cathode voltage. The 5F6a second stage has a cathode at 2V and a cathode at 200V. In modern thinking, we might lift the heaters to +100V so none of the H-K insulation feels more than 100V. However 12AX7 *was* often used in DC amplifiers with cathodes sitting at all voltages. It is rated 180V H-K. And H-K rating is worst-case.... 90+% of tubes will be much better. (The problem is small flaws in the insulation.) Apparently Fender had no trouble working the tubes he got at 184V H-K, or with H-K leakage hum, and did not use lifted heaters.

[kagliostro]

Many thanks for the explanation PRR

Please can you go a bit deeper about the use of two separated heater winding for the 6X5 tubes ?

It would be better to have *two* 6V windings, one for each 6X5 (plus one for the 6146). Then each 6X5's H-K voltage could be zero

Thanks

Franco

[sluckey]

Please can you go a bit deeper about the use of two separated heater winding for the 6X5 tubes ?

It would be better to have *two* 6V windings, one for each 6X5 (plus one for the 6146). Then each 6X5's H-K voltage could be zero

By using two totally separate filament windings for the 6X5s you could directly connect the cathode to one side of the heater for each tube, ie connect pin 7 and 8 together. Now there is zero voltage difference between heater and cathode.

[kagliostro]

Thanks Steve

now I've understand

Franco