EF80 - anyone tried it as an audio amp?

[kalee20]

Yes!

At RF, a bit of hum coupled in would not be a problem. The tuned circuit or IFT in the anode would simply filter it out. So, even though the EF80 and EF86 would in many cases have been handling millivolt or even microvolt signals in their intended applications, only the EF86 needed the hum-cancelling heater.

[Sideband]

Originally Posted by Aerodyne

I'm surprised it was used as an output valve, but I shouldn't have been, knowing the 'any port in a storm' approach to TV design in the mid 20th century.

Quite a few 'non-output valves' have been used as output valves. The EF91, EF80, ECC82 (yes in push-pull as well as single-ended) and I believe in one TRF radio the EBC33/6Q7. Thorn used a PCF80 as an audio amp and output valve in some of their portables.

Conversely the ECL80 which IS an output valve has been used as a mixer/oscillator in one TV......

[G6Tanuki]

Yes, designers will often use-what-they-can-get-hold-of-cheaply, which leads to some interesting outcomes - like the little 1-V-1 "midget" radios offered by Premier etc. I've seen these with an ATP4 directly-heated power-pentode as the output-stage, and also one with a CV309/QV04-7 VHF beam-tetrode).

In the case of TV designers there may be significant business sense in using less-than-ideal valves if by doing-so you can increase the size of the order for that particular valve and so wring an extra sixpence per valve 'volume discount" out of your supplier. It also makes spares-stockholding easier for everybody.

[Robert Gribnau]

Philips and Revox did use the EF80 in audio amplifiers.

Greetings,
Robert

[Heatercathodeshort]

The EF91 was also used in some KB television receivers. It gave a very good account of itself.

EMI models: Marconi VT153. HMV 1821. Yes Tony, the 1840 series including large console models 1840, 1841, 1842, 1843, 1844, 1845, 1846, 1847 and 1848. HMV 1814 1816 [Highlight models] 1824 series comprising over 5 models including large consoles and probably more. I think the trick was in the quality of the output transformer.

Strangely the sound was about the only good thing witht these nasty models. They were generally hated in the trade and often let their owners down. John.

[Valvepower]

Hello,

Not an EF80, but I’ve used its newer replacement the EF184 in a hook-up amplifier driver/phase splitter based on the Jeffery circuit using two EF184’s.
The fist stage is pentode connected with a bootstrapped anode load and the second stage is a triode connected concertina phase splitter.

This is or was work in progress, and I’m just awaiting the time to build a complete amplifier. I envisage this driving a couple of EL84’s.

Regards
Terry

[rontech]

Originally Posted by Ed_Dinning

Hi Gents, there were many designs for complete amps, radios etc using EF80's and also EF50's in the magazines of the day.
It would be interesting to see the above amp using a set of nice red EF50's, possibly less micro-phonic as well !

Ed

Back in the day ( 1958 ) I used a red can EF50 as the head pre amp in an experimental tape recorder ( DIY type deck kit with full width tape head! ) : the results were quite good.

[Aerodyne]

Robert's REVOX circuit uses the EF80 without regard to hum problems other than fitting hum-bucking resistors across the AC heater supply, so it can't have been that troublesome.
I must say I'm surprised to see the extent of the use of this valve - but then I did say it was 'ubiquitous'.
Many thanks for all the replies, a fascinating read for me.
Yes, perhaps I should get out more often...

Tony

[Synchrodyne]

Regarding the use of high-slope pentodes for AF purposes, it may be noted that both AF voltage amplifier and AF output were listed applications for the pentode section of the ECF80, see Mullard “Valves, Tubes & Circuits” #26 in Wireless World (WW) 1955 February. Although the PCF80 pentode was not the same as the EF80, it was broadly similar, and it is reasonable to readacross the ECF80 applications to the EF80. With 2.5 W anode dissipation, the EF80 would be a more capable power amplifier than the 1.7 W ECF80 pentode.

Dynatron used the Z77 (EF91) for AF purposes in its LF612 and LF613 amplifiers, albeit triode-strapped. One was used as a driver, and another as phase splitter. Triode-strapped Z77s were also used in the AF section (“line” level, not gramophone preamplifier) sections of the T99 and T139 tuner/control unit combinations.

Notable was Baxandall’s use of the Mazda SP61 in his tone control circuit of 1952. The SP61 was the predecessor to the 6F13 and 6F1. A high-slope valve was chosen for best performance. (The benefits of high open-loop gain in this circuit came to the fore again in the discrete solid-state era, when obtaining such was not quite so easy.)

In valves that might be used for TV and FM frequency changer work, some attention was paid to minimizing microphony. (Perhaps this was because in part, the 6J6, once used for this purpose, was found to be rather microphonic.) The microphony resistance of the EF42 was discussed in Mullard “Valves and their Applications” #13 in WW 1948 January, also attached to this post: https://www.vintage-radio.net/forum/...0&postcount=22. I imagine that the EF80 would have been similarly endowed.

Another factor supporting wideband use is that the EF80 was specified for use as a video amplifier. The latter group seems to straddle the realms of both “EF” and “EL” valves.


Cheers,

[Hybrid tellies]

In the BRC980 small screen all valved tv chassis they use the EF80 in the IF stages and as the video output valve and a PCF80 as the audio amp and output valve.