2 6L6, 5 12AX7 Current

[ChrisM]

Trying to figure out if a 180mA power transformer will be able to handle 2 6L6s and 5 12AX7. Looking at the Vibrosonic amps I would say yes. From my calculations it would work.

180mA enough?

[tubeswell]

I say 180 ought to work as long as the heaters are up to it. (You need 4A minimum- preferably 5A or more). Put a 250mA fuse on each side of the High Tension winding just in case.

But I would prefer 200mA for that

[ChrisM]

The heater is rated for 4A so I am set for that. I only need 3.3A.

There is a PT that is the exact same as the one I am looking at but it is rated at 200mA. It is almost $20 more and a bit bigger. Size is a issue with this project I am working on.

How close I am I going to be with 180mA?

[tubeswell]

Re: your heater winding - did you take the lamp draw into account? My rationale is that if the heater winding is 'bigger' than needed, it'll help keep the PT cooler, which will help a bit with any strain on any other windings

but having said that, 180mA ought to be ok for the High Tension winding

[ChrisM]

No did not take into account the lamp. How much does that draw?

Hammond apparently over winds their heater winding by a bit (I heard 30%) so I think I will be ok.

[Phil_S]

What the lamp draws depends on the lamp used. If you use a standard #47 bulb, it needs .15A, so add that to 3.3 for 3.45. I'd say 3.5A is a reasonable minimum.

How much B+ current you need depends, in part, on your design. You did not provide enough information to judge if 180mA is enough. I'd guess it is adequate for many 6L6 designs, but not all. You reference the Vibrosonic, but your post suggests that is not what you are building. Do you have plate and screen voltages in mind?

[ChrisM]

Ya I am going for around 440V on the plates with about 5 - 10V less on the screens. From my reading of Valve Wizard it should be enough. I am not sure of the plate to screen ratio for 6L6 that Merlin mentions when calculating screen current.

[ChrisM]

I bought a transformer it is 325V-0V-325V rated at 180mA(DC).

I found this handy calculator a while back http://www.dreamtone.org/Calculate_Current_Form.htm
And when I typed in my info a while back I thought I was fine.

I went back today and just rechecked. Two 6L6GCs, 4 12AX7s, SS recto.

My current draw will be 180.9mA. Is this correct? This would be very hard on my PT wouldn't it?


The 6L6GCs would be biased cool, ala Duble.

[passfan]

Give it a shot. It will run at it's rated output with a little more heat than you would normally see at lower outputs but it should run within specs. You can save on heater current by using a 120 volt bulb on the primary side of your PT.

[ChrisM]

You think keeping the heater current down would help the overall "stress/heat" down?

[tubeswell]

That PT should be fine - provided the heater winding current rating is at least:

.9 (6L6) + .9 (6L6) + .3 (12AX7) + .3 (12AX7) + .3 (12AX7) + .3 (12AX7) + .3 (12AX7) + say .5 ( for the lamp) = 3.8A

[ChrisM]

The heater winding is rated at 4A. Should be good then.

[Andy Le Blanc]

You can use the first two 12ax7 to cathode bias the 6l6.

You'll get a regulated DC heater for the most critical stages.

you'll also regulate the plate current of the 6l6 to around 150ma.

you'll take the load off the heater winding.

you wont waste space or components on bias

and you can save more space by dropping the stand by.

the heaters are dressed series to ground, very simple.

[Tonegeek]

You can use the first two 12ax7 to cathode bias the 6l6.

You'll get a regulated DC heater for the most critical stages.

you'll also regulate the plate current of the 6l6 to around 150ma.

you'll take the load off the heater winding.

you wont waste space or components on bias

and you can save more space by dropping the stand by.

the heaters are dressed series to ground, very simple.

That is really interesting. But, won't the voltage vary to those heaters as the signal changes on the 6L6's and in effect be AC anyway as the signal on the power tubes turns the DC bias into AC? Do you wire the x7s for 6v or 12v operation?

[M Fowler]

Allen amps sells a 150mA replacement for Princeton and other chassis wanting to run 6L6 or EL34's the PT 25. So your 180mA should be good to go.


http://allenamps.com/parts.php#transformers

Upgrade Power Transformer for BF/SF Champ™, VibroChamp™, Bronco™, and Princeton™. 310V-0-310V @ 150ma, 6.3V at 4.5A, 5V at 2A. 50V bias tap, center-tapped heater winding and internal hum shield brought out to a lead for grounding. 2" by 2-1/2" mounting centers, 5.1lbs. Unit is approximately 1" taller than a stock transformer. Now supports the use of both 6L6 and EL34 power tubes (with proper socket wiring).