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Old 8th Jan 2024, 12:37 pm   #1
Malcolm T
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Default Low voltage Valve audio amplifiers

Has anyone had any success with low voltage "around 12 volt" stereo valve amps. I am looking for circuits and articles on this topic.
Is there any groups for low voltage valve projects?.
I have quite a few valves and wot not around the place.

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Old 8th Jan 2024, 3:09 pm   #2
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Default Re: Low voltage Valve audio amplifiers

You won't really get speaker-driving levels of power from the 12V-HT valves; in their intended car-radio applications the audio versions of these valves operated as a voltage-gain device and invariably drove a transformer-coupled power transistor or two for delivering the actual power to the speaker.

There are some 'odd' low-voltage audio valves around though; the 26A7 has a 26.5V heater and with 26.5V on the anodes [it is a dual pentode] delivers an eardrum-bursting 0.18 Watts of output!

(It was designed for use in aircraft radios).

See https://www.radiomuseum.org/tubes/tube_26a7gt.html
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Old 8th Jan 2024, 3:20 pm   #3
Malcolm T
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Default Re: Low voltage Valve audio amplifiers

Well i,m not looking for ear shattering Db levels just to experiment with and see whats possible with low voltage from there i consider improvements . if i get minimal watts it will still be a success .
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Old 8th Jan 2024, 4:50 pm   #4
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Default Re: Low voltage Valve audio amplifiers

Not that I have a huge amount of experience in this, but the valve car radios I've seen have had a vibrator invertor to create decent HT from the low voltage car electrics.

As I recall, Valradio made a number of small mechanical choppers that either fitted inside the car radio, or in the back of the dashboard, driving what was effectively a mains transformer in the radio.

S.
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Old 8th Jan 2024, 4:58 pm   #5
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Default Re: Low voltage Valve audio amplifiers

Quote:
Originally Posted by Sergeauckland View Post
Not that I have a huge amount of experience in this, but the valve car radios I've seen have had a vibrator invertor to create decent HT from the low voltage car electrics.

As I recall, Valradio made a number of small mechanical choppers that either fitted inside the car radio, or in the back of the dashboard, driving what was effectively a mains transformer in the radio.

S.
On a similar note there are 12 Volt to HT inverters on Ebay that can provide HT at round 40 Watts, they run typically around 100khz so would be well out of the audio region.
But I am guessing the original poster wanted to use low voltage HT valves and I was wondering if some of the Russian Rod pentode valves would give some output at such low voltages.

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Old 8th Jan 2024, 5:45 pm   #6
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Default Re: Low voltage Valve audio amplifiers

Quote:
Originally Posted by Sergeauckland View Post
Not that I have a huge amount of experience in this, but the valve car radios I've seen have had a vibrator invertor to create decent HT from the low voltage car electrics. S.
That was obsoleted by the arrival of "12V HT" valves in the mid-50s, and the first generation of Germanium power-transistors [which wouldn't work sensibly above a few hundred KHz].

"Hybrid" car-radios were then a thing. ECH83, EF97/98 valves for the RF/IF stages, and something like an OC16 or OC28 as the single-ended output transistor.

There was even a range of 12-volt-HT Nuvistors intended for FM car-radios!

Mullard 1960 manual snippit attached, using an EF98 to drive a TO3 Germanium power transistor in a car-radio design.
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Old 8th Jan 2024, 8:46 pm   #7
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Default Re: Low voltage Valve audio amplifiers

Yes, most of the DL9* series valves were for a higher Ht , but would work at 45 volts. The 12V anode valves did not have an output valve in their range so the OC16 and transistors of that ilk were used.

It might be worth trying some of the "rod pentodes" but do not expect much power output

Ed
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Old 8th Jan 2024, 9:02 pm   #8
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Default Re: Low voltage Valve audio amplifiers

The basic problem is that valve technology is voltage-oriented, not current-oriented like transistors. This means that they don't amplify well at low HT voltages, and it's really impossible for them to deliver decent LS volumes at very low HT voltages.
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Old 8th Jan 2024, 9:35 pm   #9
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Default Re: Low voltage Valve audio amplifiers

John Hunter discusses what's possible with low voltage valve circuits, using standard valves in radio and audio, on his web site in this article:
https://www.cool386.com/12vsuperhet/12vsuperhet.html
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Old 9th Jan 2024, 9:51 am   #10
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Default Re: Low voltage Valve audio amplifiers

Quote:
The basic problem is that valve technology is voltage-oriented, not current-oriented like transistors. This means that they don't amplify well at low HT voltages, and it's really impossible for them to deliver decent LS volumes at very low HT voltages.
Paul's right a valve is a high voltage low current device, a transistor is a low voltage high current device.

To power a typical 8 ohm speaker to 3 watts you'll need about 600mA of current, there's no valve that can shift that much current with a HT of 12v.

Therefore you either need a speaker with a higher impedance say a few kilo ohms or parallel quite a few valves together, say 500, all off the cuff figures.The only practical way to build a low HT amp using valves is to use a valve front end and a mosfet/s to drive the speaker.

Andy.
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Old 9th Jan 2024, 10:09 am   #11
Malcolm T
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Default Re: Low voltage Valve audio amplifiers

Thanks for all the replies , The John Hunter circuit looks interesting and the low voltage valve types.
I know it wont be hifi but, and do appreciate the voltage difficulty and output .
However i will give anything a go
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Old 9th Jan 2024, 1:45 pm   #12
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Default Re: Low voltage Valve audio amplifiers

BTW, why the 12v DC provision? Is it that your off grid or just don't like working with high voltage?

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Old 9th Jan 2024, 3:29 pm   #13
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Default Re: Low voltage Valve audio amplifiers

A challenge really, but also i dont have any HT transformers and dont want to get into anything HT.
The idea is to keep it below 50V so 12V seemed like a good starting point as i have power supplies for 12V
I have mains here too.
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Old 9th Jan 2024, 8:02 pm   #14
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Default Re: Low voltage Valve audio amplifiers

If you want to keep it below 50V why not go for 49V? There'll be oodles more grunt from the output stage than at 12V.

I seem to remember an article in the BVWS Bulletin by someone who made a loudspeaker radio using 24V HT (and LT), but I can't remember the details! I can envisage the main problem being the driver stage, because the output stage will be running with so little grid bias that the signal peaks will take it into grid current.
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Old 9th Jan 2024, 8:29 pm   #15
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Default Re: Low voltage Valve audio amplifiers

Quote:
Originally Posted by kalee20 View Post
I can envisage the main problem being the driver stage, because the output stage will be running with so little grid bias that the signal peaks will take it into grid current.
Hence I would suggest the idea of push-pull zero-bias triodes, in class-B - http://www.r-type.org/articles/art-103.htm - like the dual-triode I mentioned upthread; yes, the driver stage needs to provide power rather than pure voltage - and from a low impedance so grid-current doesn't confuse the biasing.

"Quiescent Push Pull" was the 1930s equivalent. Seee http://www.r-type.org/addtext/add019.htm
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Old 10th Jan 2024, 11:37 am   #16
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Default Re: Low voltage Valve audio amplifiers

EL/PL504 is a possible solution.
Unconventional low voltage amp ...
Good article here ...

There is a PCL82 suggestion here ...
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Old 10th Jan 2024, 11:41 am   #17
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Default Re: Low voltage Valve audio amplifiers

This is what I was looking for ...
Space charge and other low voltage tubes ...

If you google 'space charge' and 'diyaudio' then there are also some quite serious uses of these tubes, as current sinks or in headphone/line amps
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Old 10th Jan 2024, 8:25 pm   #18
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Default Re: Low voltage Valve audio amplifiers

With 40/50 Volts you might get adequate power from 6080's.
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Old 11th Jan 2024, 6:36 pm   #19
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Default Re: Low voltage Valve audio amplifiers

This is a different approach to this discussion. Whether it is of interest depends upon your aspirations.

In the January 2020 issue of Silicon Chip magazine, which is published in Australia, there was a stereo valve pre-amp project using the 6P1 valve. The project was reprinted in the January 2021 issue of Practical Electronics in the UK.

You can find more information about the 6P1 tube at https://diyaudiostore.com/products/korg-nutube You will see that at low voltages the tube has a very low gain. You can find some discussion at https://www.diyaudio.com/community/t...nutube.329270/

I have no connection to Pete Millett.

HTH and 73 John
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Old 11th Jan 2024, 6:37 pm   #20
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Default Re: Low voltage Valve audio amplifiers

A modest increase in HT voltage will give a significant increase in audio output
Common solutions in days gone by included a small HT dry battery connected in series with the 12 volt supply. A purpose made 45 volt dry battery, or several 6 volt handlamp batteries in series.

For isolated premises with a 12 volt lighting system (often either a vehicle battery, regularly swapped for one in a vehicle, or a larger battery charged by a wind turbine.)

A home made HT lead acid battery, charged in sections from the 12 volt supply was another possibility. Purpose made HT lead acid batteries were manufactured, often in ten volt units.

With all the LT and part of the HT supplied from an existing 12 volt system, battery costs were much lower than a portable dry battery set.

Acceptable audio power could be obtained from 32 volts HT, and this was a reason to use 32 volt lighting plants rather than 12 volts.

These days I would suggest an external 12 volt supply and few 9 volt batteries inside the set connected in series with the 12 volt supply for HT.
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