Hi all,

Have a couple of EF183 Pentode amp valves and so far have managed to hook one up to the output of my regen radio, but the volume is very low due to only running 12volts on the anode. I wondered if anyone has a sugestion on how I can connect the other EF183 and therefore end up with a 2 pentode audio amp operating in space charge mode that might give enough otput to drive a pair of headphones with good volume via the output transformer .
I have a diagram of what I've done so far but not sure how to upload it to the forum.

Thank you,

Big Bill

 

Hello,

You can use the "manage attachments" button to upload images:

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Bertus

 

Umm - what HT supply voltage do you have available for the anodes?

 

If you only have 12v available, you need to build something like a flyback converter to give the anode a voltage within it's normal operating range.

Ronald Dekker made a great DIY'er page on boost and flyback converters, here:
http://www.dos4ever.com/flyback/flyback.html

The circuits shown certainly aren't the most efficient, but they're great as learning tools.

 

Hello,

There are special low voltage tubes.
These are called "space charged" tubes:
http://www.junkbox.com/electronics/lowvoltagetubes.shtml

Bertus

 

However the EF183 is not of that type, it's just a 'normal' type.

 

When operating pentodes at low voltages you need to abandon some of the traditional design methods.

You will need a large anode resistor (traditional starved designs used anything up to 5Meg!).

You also need to be aware of grid current, which will bias the valve very cold if you have a conventional grid leak resistor. A neat way to get around this is to connect you grid leak to the 12V supply, rather than to ground/cathode! Playing with the value, you should be able to get some useful gain.

Also, play with the screen voltage, as the gm of pentodes varies wildly at low screen voltages there will usually be a 'sweet spot'.

 

Hello,

On the page I posted there is this for using ordinary tubes on low voltages:

Low-Voltage Operation With Ordinary Tubes

Keep in mind that a lot of "ordinary" high-voltage tubes can be used very effectively with DC voltages as low as 3V on the plates.
Some of these tubes can be operated in "space-charge" mode with the first grid used as an electron accelerator, but that's not entirely necessary.
I cite some circuits below with ordinary triodes operating at very low voltages.
The problem with tubes generally is that taking power out of a tube circuit requires operating at a high voltage.
If you don't need significant output power (say, for a loudspeaker) and can settle for very low values of plate current, you can use plate voltage values under 20V, and often less than 10V.
When building radio receivers, this generally requires using high-impedence headphones for audio output, where "high-impedence" means 600 ohms or more at audio, the higher the better.
Such headsets are getting increasingly hard to find; Antique Electronic Supply used to sell them, but do not show them in the 2007-2008 catalog.
I do see them regularly on eBay.
Crystal earphones can be used, with one gotcha: They do not pass DC.
In many older circuits, the plate voltage passes through the headphones, which requires a dynamic headset (that is, one with wire-wound transducers) rather than a crystal transducer.
For an example, scroll down to the schematic for KB7NRN's 18V radio.
A crystal earphone will not work in this circuit, nor any circuit that passes B+ through the headset.

Many of the circuits you'll find in books and magazines specify 45V, 67.5V, or 90V batteries, which can be had but are expensive.
A lot of those circuits will work just fine on much lower voltage.
For example, I built the 3V4 BCB receiver from Harry Zarchy's kid-hobby book Using Electronics, and the circuit has a 100K resistor in series with the regen control pot.
The resistor was necessary because the specified 45V battery supply was too high. Without the resistor, the receiver oscillated at 45V irrespective of the setting of the regen pot—and worked perfectly at 9V.
I use a beat-up "hamfest special" adjustable lab supply for simple tube radio work, and I can spin a knob to vary the voltage between 0 and 50V.
When I have a circuit on the bench spec-ed for 45V B+, I set the supply at 45V, make sure the circuit works, and then start dialing down the voltage to see what happens.
Most of the time, increasing the amount of regeneration on the throttle control will compensate for the reduction in voltage.
Sometimes (as with Zarchy's circuit) you have to tweak component values a litttle to get down under 10V B+, but with decent headphones and a good antenna you will get signals.
The art lies in tweaking things to get the most signal at the least voltage.

Bertus

 

You may also like to read:
http://www.gtlab.net/gtlab2/filemanager/userpix/igor/low_anode_1.pdf
http://www.gtlab.net/gtlab2/filemanager/userpix/igor/low_anode_2.pdf
(You may get a danger message from your browser, but I have not found any malicious problems with these links)

Also by the same author:
http://b-kainka.de/roehrenexp.htm