12AH8 - battery valve substitute?

[Al (astral highway)]

Brewing away in the background I have a plan to either build or mod something with a triode-heptode frequency changer.

This one I know was very popular in its time, rarer but not silly priced now. I know ECH81 will do the same job with no heater centre-tap. But... anyone know of a close equivalent with a 1V4 filament?

It doesn't have to be a pin-for pin equivalent, even if such a thing existed.

My knowledge of battery/B7G/ valves is highly limited although I have a DF91 from I don't remember where.

Cheers folks.

[G6Tanuki]

I don't know of any sane battery-valve triode-heptodes; the trend with directly-heated battery valves was always to have a single electron-stream frequency-changer to conserve battery-power rather than having a separate heater-power-wasting oscillator triode. The first "pentagrid converters" had issues - the oscillator part of the 1A6 was hopeless and would stop oscillating - to the point where it was not unusual for manufacturers wanting sensible shortwave performance to wire a separate triode across the oscillator-part of the 1A6!

As "Battery" valves go, you could always look at the 12-volt-HT ranges (with 6.3V heater) like the ECH83; I used one of these with a 12V-HT pentode (EF97/EF98, can't remember which) RF-amp in a crystal-controlled mobile shortwave-to-MW converter in the 1970s.

[Al (astral highway)]

Originally Posted by G6Tanuki

The first "pentagrid converters" had issues - the oscillator part of the 1A6 was hopeless and would stop oscillating - to the point where it was not unusual for manufacturers wanting sensible shortwave performance to wire a separate triode across the oscillator-part of the 1A6!

Originally Posted by G6Tanuki

....as "Battery" valves go, you could always look at the 12-volt-HT ranges (with 6.3V heater) like the ECH83; I used one of these with a 12V-HT pentode (EF97/EF98, can't remember which) RF-amp in a crystal-controlled mobile shortwave-to-MW converter in the 1970s.

Ahah, that's a good plan. I haven't done anything with 12V HT and it has its attractions and is intriguing to me.

Cheers for the recommendation.

[G6Tanuki]

The 12FX8 was a US 12V-HT triode-heptode.

Also 6DS8.

There were also a range of US-designed 12V-HT single-electron-stream frequency-changers, such as:

12GA6
12FA6
12AD6
12AG6
12CS6

that were used in 'hybrid' car-radios [valves for the RF/FC/IF-stages, transistors for the audio output] in the 1950s and early-1960s.

There were even 12V-HT triodes designed for use as mixer/oscillator at VHF for FM car radios.

12AJ6, 12AE7, ECC86.

It's a fascinating area to investigate, seeing what can be achieved with pitiful levels of HT.

[VT FUSE]

Hi Al

You say that your prospective project could be built using a Triode Heptode like ECH81,would be delighted to send you a NIB ECH83 12V version by Mullard and if a Heptode DK91 1.4V is of interest then you are most welcome to a new Mullard one as well.
Let me know if interested and if you would like sockets as well ( I also have some prototyping FRG4 pcb's with sockets mounted and positions for components which may be handy)
Not after anything for them,you really would be very welcome.


Best regards

Mike

[Heatercathodeshort]

What a generous offer there.

The ECH83 were selected from the ECH81. Most ECH81 will oscillate with 12V 'HT'

This is great fun. Practical Radio published an article around 1948, the '6K7 Pocket Receiver' that used a 4.5V battery for the single 6K7 heater and anode supply. The 6K7 has a 6.3V heater but according to readers letters it worked remarkably well! John.

[G6Tanuki]

If you're interested in low-HT operation then this article:

http://www.r-type.org/articles/art-094.htm

could be worth a read: there were various 'bigrid' tetrodes in the 1920s/early-1930s which used low HT and a sort-of space-charge-neutralising approach for the second grid. The idea never really caught on though.

[VT FUSE]

I am curious as to what Al is interested in,I do agree John, that to every intent and purpose the ECH81/3 were exactly the same valve,possibly characterised in early production but proven the same by etch codes.I have read up on the UK & US valves and have most examples but many more UK/European versions.

The 12AE7 is a dual dissimilar Triode for AF duty on section 1 and drive duty on section 2 for the OP Transistor in late US (1958) hybrid radios and is very different to the ECC86 which is rumored to be a selected ECC88.

As a periodic distraction I will quite often assemble a receiver with criteria of 12V max for anodes.

Another valve that lends itself well to space charge type circuits or Bi-Grille applications is the former USSR military 2HS27L Pentode with g3 brought out to its own pin with 2.2V fil but excels at low HT esp if the fil Volts are run at 1.5-2V,this is often called starved Cathode operation as alluded to in the case of 6K7 by John,at 6.3V on the heater it is abysmal at low Anode Volts but with heater underrun quite significantly at 4.5V it really can perform.

[Al (astral highway)]

Originally Posted by VT FUSE

...would be delighted to send you a NIB ECH83 12V version by Mullard and if a Heptode DK91 1.4V is of interest then you are most welcome to a new Mullard one as well.
Let me know if interested and if you would like sockets as well ( I also have some prototyping FRG4 pcb's with sockets mounted and positions for components which may be handy)

Hello Mike, this is really generous of you and I’d love to take you up on all that you’re offering there, including the sockets. Thank you so much !

I’ll PM you details ...

Thanks again, that’s made my day!

I’m out at the moment but will post more about my plans later or tomorrow

[Argus25]

Originally Posted by astral highway

Ahah, that's a good plan. I haven't done anything with 12V HT and it has its attractions and is intriguing to me.

Al,

12V HT valves are fun.

Usually for normal high voltage HT valves, a triode-hexode or triode-heptode as a converter is superior to a pentagrid converter. For example the ECH35 has a better conversion transconductance than a 6A8 (pentagrid) or even the 6k8 which is a variation of the triode-hexode concept but with the osc mixed in at G1, rather than G3 where the received signal is injected, but for the ECH35 the received signal is injected at G1 and osc mixed in at G3.

In any case, on the face of it, the ECH83 should make a great converter.

When I tried it it did, but due to the capacitances of the triode section it is not much use over 12 to 14MHz. So when I made a shortwave/MW radio with it, I had to abandon the triode section for the 5.7 to 18 MHz short wave band, and replace the L/O with an MPF102 circuit attached. But if you were just making an MW band low HT voltage radio with the ECH83, it would be perfect.

[kalee20]

Agree with Argus! Yes, pentagrid converters are usually reckoned to not perform as well as triode-hexode types. But they do use the electron flow from the single cathode or filament effectively, important when this is limited!

The 6K8 is unusual in that it has inner-grid injection of oscillator. Most use the outer grid, with the inner grid having the RF signal. The ECH81 could be used either way, as there's no internal connection between triode and hexode (apart from a common cathode connection), but I don't know of any circuit where the roles of the grids are swapped.

As to original question - have you considered a KCF30 (or FC2A) triode-pentode frequency changer? Not quite 1.4V filament - it's 2V - but it is a decent valve and certainly performs on SW. Pye used it in the 67A battery table radio.

[Al (astral highway)]

Hello all,

I don't yet have a specific design in mind, but I'm interested in at least exploring a VHF front end + , edging toward a whole radio. The AF stage will likely be with transistors.


If that's too ambitious for the valves I end up working with, then I'll happily start at medium wave instead. There's not a huge hurry; I'd like the result to be sensitive and selective, as well as good-looking and distinctive. This is very much a harvesting components stage so that I don't end up with a design that needs something hard to find!

Hugo, thanks for those technical notes, that's very useful to know.

Peter (Kalee20),, thank you also for your technical insights.

About your Q in the last line of your post: no, I hadn't considered it but only because I'm not familiar with it. It sounds like it has a good pedigree.

[Heatercathodeshort]

Originally Posted by kalee20

As to original question - have you considered a KCF30 (or FC2A) triode-pentode frequency changer? Not quite 1.4V filament - it's 2V - but it is a decent valve and certainly performs on SW. Pye used it in the 67A battery table radio.

Yes the KCF30 by it's appearance is almost certainly a Mazda valve made for Mullard. It's not seen very often these days but would be a good candidate. The huge FC2A is an Octode and may not like to oscillate with a very low anode supply but you never know. J.

[G6Tanuki]

If you're after going VHF then I'd suggest you use the triodes, since multigrid mixers are significantly noisier, and with low HT you have less freedom to play with optimising the electrode-voltages.

The 12V triodes - you could use a pair on a cascode RF amp, one as a triode mixer and another as the oscillator. Once you've got down to 10MHz-or-so you could use a single pentode IF amp followed by a crystal-controlled ECH83 frequency-changer to get down to 450-470KHz for the second-IF. There's a 12V-HT double-diode-triode available for detection/AGC purposes.

I vaguely recall a 1950s VHF-airband transceiver that used 12V valves for the receive-strip to reduce power-dissipation (and eliminate the need for a vibrator-pack). Though that might have used 28V as the "HT" since that's what plenty of aircraft had available.

Might be worth looking in some old ARRL Handbooks (aren't these available on the americanradiohistory site?) for ideas.

[VT FUSE]

A frequent consideration of mine has been to obtain the whole front end tuner from a valve AM/FM receiver of the usual sort, ie , ECC85 VHF double Triode (often bereft of emission) and rework it with an ECC86 12V double Triode to operate from the lower voltage.

I have previously had some success with Super regenerative VHF FM receivers but those purpose built VHF FM front ends have much more functionality.

Any thoughts out there?

I agree that a solid state hybrid approach to the OP stage would make sense.

It occurs to me that the ECH83 is often used as RF amplifier in hybrid designs by strapping as a Pentode,thus a Triode Pentode should result-a versatile valve.
Mike

[Synchrodyne]

Regarding the use of the ECH83 at HF, Mullard made the following comment in respect of its initial MF-HF car radio circuit based upon 12 V-HT valves: “To simplify the changeover from a conventional car radio receiver to the hybrid design, it is considered that the oscillator valve, when operating on 12V, should have sufficient slope to permit the use of existing car radio coils. The ECH83 triode section is unsuitable in this respect for a short wave receiver, therefore a separate oscillator valve has been introduced. The EF98 connected as a triode was found to be suitable for this application.”

Also, the MF-HF receiver used an EF97 RF amplifier rather than the heptode section of the ECH83, pentode-strapped as in the LF-MF receiver. The rationale was given as: “The EF97 r.f. amplifier provides some advantages over the ECH83 heptode section which was used in the previous design. Cross modulation is considerably improved, and the receiver will handle larger inputs without overloading. The maximum value of input signal for no modulation distortion should be 1.0V r.m.s. on the EF97 control grid, at 1Mc/s.”

The EF97 and EF98 were respectively the remote-cutoff and sharp-cutoff RF pentodes in the Mullard 12 V-HT series. The EF98 was also used, tetrode-strapped, as an AF driver.

The MF-HF receiver was described in Mullard Technical Communication #29 of 1958 March, with the LF-MF version described in #25 of 1957 June.

I think that there has been some debate as to whether the ECH83 was a selected ECH81 or simply a rebranded ECH81. The argument in favour of the former is supported by Philip’s own use in (European) TV circuits. Initially it used the ECH81 as a noise-gated sync separator, changing to the ECH83 when that became available. As the it was in a low-HT circuit, presumably the ECH83 was more dependable. Eventually it developed the ECH84 specifically for the purpose, with a short-grid base, sharp cutoff heptode section. I’d guess that the proportion of ECH81 production that met the ECH83 specifications was quite high, and only part of that was required for actual ECH83 labelling.

The 12FX8(A) was unusual valve in two ways. Firstly, as best I can determine, it was the only American miniature triode heptode. Secondly, it did not follow the European norm for such valves. Rather the heptode section was intended to be used as a self-oscillating mixer of the customary American pentagrid type, with the signal on grid 3. The triode was intended for use as an RF amplifier. I guess that it was a way of providing an RF amplifier for car radios without adding another valve envelope. Probably the 12FX8(A) could be used in the conventional way, with the triode as oscillator and the heptode as mixer. Many pentagrids were also specified for use with a separate oscillator as well as for self-excitation.

The 12AH8 appears to have been a “necessity” product from Brimar, developed to meet the British and European setmaker preference for triode-heptode frequency changers, its standard frequency changer being the 6BE6/12BE6 pentagrid. The use of a split heater allowed a single valve to address both the 6.3-V (AC-only) and 150 mA (AC-DC) heater requirements. For reasons unknown Brimar chose a moderate conversion conductance of around 0.5 mA/V at a time when the competition was offering around 0.75 mA/V.

The ECH81 was something of a “do everything” valve, its multiplicity of functions aided by not having the triode grid internally connected to the heptode grid 3. As well as AM frequency changer, its triode was specified for use as a self-oscillating FM mixer and as an AF amplifier, and its heptode, when pentode-strapped, for use as an IF amplifier.


Cheers,

[VT FUSE]

In the past I have tried to find the online text which is referred to,namely Mullard Technical Communication 29 of march 1958 and any associated with the hybrid range of valves,

Thanks for posting this info,I have seen it in disparate forms but not presented as seen in this last post as far as I recall,do you have links?

The US range of 12V valves is rarely seen in the UK,however I have examples of most,the 12FX8(A) is very rare,introduced,I think,just prior to Transistors becoming available economically for most stages.I only have one 12FX8A.

[Argus25]

Originally Posted by kalee20

The 6K8 is unusual in that it has inner-grid injection of oscillator. Most use the outer grid, with the inner grid having the RF signal.

I once asked my late friend John Stokes about the ECH35 and 6k8.

(he wrote this famous book:

https://www.amazon.com/Years-Radio-T.../dp/0911572279 )

The reason was both were specified in the Eddystone 640 radio and I wanted to know which was the better one to use. He seemed unimpressed by RCA's 6k8 and said it wasn't a true triode-hexode converter and advised the ECH35 and explained to me the conversion transconductance issue.

Also, it is interesting , that the specs of the ECH35 are not dissimilar to a much earlier valve, the X41. The X41C (ceramic base version) is used in the HMV 904/5 TV set and the oscillator in this case runs around 37MHz, below the incoming signals at 41.5 and 45 MHz.There is a circuit on page 2 of this pdf showing "ultra-short wave" reception:

http://www.r-type.org/pdfs/x41.pdf

When I restored my 904, I could only find two NOS Marconi X41C's in the whole of the UK, but X41's were plentiful. Though there are probably some more hiding somewhere.

[Synchrodyne]

Originally Posted by VT FUSE

In the past I have tried to find the online text which is referred to,namely Mullard Technical Communication 29 of march 1958 and any associated with the hybrid range of valves,

Thanks for posting this info,I have seen it in disparate forms but not presented as seen in this last post as far as I recall,do you have links?

This article is not on-line, as far as I know. I have a hard copy in a bound volume. I’ll scan it, and post it here. It might take a few days, though.

There is a good article on the theory and practice behind 12-V HT car radio valves in RCA Engineer for 1958 August-September, available here: https://www.americanradiohistory.com..._Issue_Key.htm.

And there is an extensive treatment of frequency changers in Langford-Smith, p.962ff in the Classic Edition. The second section reviews some of the specific valves involved, mostly American, but the Osram X79 gets a mention.


Cheers,

[VT FUSE]

Did wonder at my lack of leads regarding the Mullard tech docs.
Would be very interested in a scan here if you find the time,I know the Mullard documents were pretty much trade exclusive circulation.

There is quite a lot on the web regarding the American valves including those specifically designed for relay driving in auto tune AM car radios.

Must admit to a sneaking admiration for this tech ever since being given a discarded PYE TR1000 almost 50 years ago and marveling at the lack of vibrator or high Voltages.

Mike