High gain output valve

[G8HQP Dave]

For a pentode, gm is the best measure of valve gain; gm times anode load resistance gives stage gain. Mu becomes relevant for triodes, or triode-connected pentodes.

[Neil Purling]

As a difference between the EL84 and a 6CH6 is that the EL84 has a gm of 6.7 and the 6CH6 has a gm of 11. If I put a 3 volt p-p signal into a EL84 and 6CH6 what signal level might I encounter on the anodes of each?
There are minor pin differences, the cathode and g3 are not internally connected in the 6CH6 and g2 is on pin 8, because g3 is on pin 9.

[Herald1360]

As already noted, the voltage will depend on the anode load resistance. For the same load, the 6CH6 will give about 4.5dB more output- a bit over twice the power.

Distortion may well be worse. The recommended operating conditions for audio output valves are just the manufacturer's idea of the best compromise in terms of power and distortion. It will depend on the relative shape of the characteristic curves of the two valves under similar operating conditions.

Looking at datasheets, it's interesting that the 6BQ5, oft quoted as an equivalent to EL84, actually has similar gm to 6CH6

[dominicbeesley]

I can't remember the exact details but I do remember using a TV output valve (PCL82 I think - I have a lot of them) with 16V for the HT and LT - it wasn't exactly hifi or loud but worked ok as a signal tracer.

You don't need much power, I have always thought this and, for the first time, I have measured the voltage on my amplifier 'speaker terminals just now. I had to turn up the volume to make the meter read anything useful. It came out at (very) about 250mV (peak ish) into 8 ohms (one channel) for a loud sound, not cinema effects loud but much more than is needed to listen to the radio. That is 8mW per channel, 16mW total. Conclusion, you don't need a high gain valve, this is confirmed by the observation that most vintage volume controls have the first 1/8th of their travel worn out, just where they where used.

[Bazz4CQJ]

I noticed some time ago that a number of "guitar amp" circuits were showing up using ECC8x in push-pull output. Does such a stage offer anything over a single-ended circuit in this context?

B

[Herald1360]

Different distortion characteristics. P-P tends to cancel even harmonics so distortion becomes predominantly odd order. Otherwise unless you bias the P-P pair towards class B operation and drive them harder, you won't get any more grunt out than from paralleling in class A single ended.

[Bazz4CQJ]

This lengthy old thread on ECL80's https://www.vintage-radio.net/forum/...L80+amp&page=2
may be worth a visit

B

[bikerhifinut]

Originally Posted by merlinmaxwell

You don't need much power, I have always thought this and, for the first time, I have measured the voltage on my amplifier 'speaker terminals just now. I had to turn up the volume to make the meter read anything useful. It came out at (very) about 250mV (peak ish) into 8 ohms (one channel) for a loud sound, not cinema effects loud but much more than is needed to listen to the radio. That is 8mW per channel, 16mW total. Conclusion, you don't need a high gain valve, this is confirmed by the observation that most vintage volume controls have the first 1/8th of their travel worn out, just where they where used.

I concur,
An awful lot of portable radios in the 60's had outputs around 250mW to 500mW and with a decently efficient loudspeaker they seemed to fill a room with sound.
I wondered if using an output stage circuit from a known valve radio wouldn't be the answer? Plenty of single valve outputs out there? Didn't some even have the detector diode as part of the output pentode?

And finally, if you put a known 1 watt of power through your domestic Hi Fi you'll be well able to hear it in the next room. Power isnt everything.

I have an old Practical Wireless with an article on the EF50 valve and circuits for everything from RF through IF amps to detector stages and audio pre and output circuits.
I don't know for sure what the modern version is? But if its an EF80 they more or less get given away.
If you'd like a scan of the article pm me.

Andy.

[Neil Purling]

I have a stack of old editions of P.W., I don't know if I have the EF50 article.
However, it was presenting something to do with a valve which was exceedingly common at the time & therefore very cheap.
It was done in Dec 1962 for the EF91. I made that with a Repanco DRR2. It was not a nice TRF. Juggling tuning, regeneration & hand capacity effects was not fun, though the audio o/p would be enough for the bedroom given a good speaker.
There must be some ex TV valves that'd do the whole job with the one envelope, like a ECF80 or even a PFL200. I don't know where i'd get a 10 pin base or a 16V heater transformer though

[FERNSEH]

PFL200: a 6-3 volt heater version was made and it actually made it to the States as the 6Y9.
http://www.radiomuseum.org/tubes/tube_efl200.html
The slope of the "L" section is 20mA/V. Doesn't get much better than that.

The 6Y9/EFL200 was used in a low cost mono TV set. In fact I believe the set used two 6Y9s, one as the intercarrier amplifier and audio output. The other was used in the function the valve designed for, video amplifier and sync separator.

DFWB.

[Herald1360]

16V for heater on one valve is the sort of odd supply that an old wall wart is handy for. In fact ISTR 16Vac being the supply for something like an old BT modem and quite powerful dc output units abound as laptop supplies etc. Losing a few volts in a resistor or some series diodes widens the range too.

http://www.ebay.co.uk/itm/Universal-...QAAOSwQoFWPO23

Just as an example........

[grampy2]

In 1950 I built a two valve regenerative set using an old RF 25 chassis and SP 61's detector was strapped as a triode,second SP61 served as an output to a 3inch speaker. As I remember it the volume was perfectly adequate.the whole thing lasted several years until the smoothers gave up and I threw it away.

[FERNSEH]

If you want to try out the EFL200/6Y9 you can still buy the valve from here:

http://www.thetubestore.com/Tubes/6T...pes/6Y9-EFL200

I'm sure they ship to the UK.

DFWB.

[glowinganode]

Try finding a 10 pin socket to suit though....
Rob.

[FERNSEH]

B10B sockets: No problem, you can still buy them.

http://www.ebay.com/itm/2-B10B-10Pin...-/250718062463

DFWB.

[Neil Purling]

I haven't bothered to buy anything from the US for some considerable time, due to the postage costs. I did not know that there was a EFL200. The first telly I bashed back to life used the 'P' version, in the frame osc/sync sep stage?

I would have thought it'd be easy to get a PFL200. However finding a heater supply...
I would not have thought of using a PSU of a laptop. Are they not producers of appalling amounts of RFI? Just what you do NOT want near a low' level RF circuit, unles you have it remote from the chassis. Then there is the problem of getting a 10 pin base, whatever version you procure.

Is the SP61 anywhere near the characteristics of a EF39 or a 6K7, apart from the Mazda base?

[FERNSEH]

The EFL200 might be available from European vendors.

Worth searching .de and .fr auction sites.

DFWB.

[FERNSEH]

Originally Posted by Neil Purling

Is the SP61 anywhere near the characteristics of a EF39 or a 6K7, apart from the Mazda base?

The EF39 and 6K7 are vari-mu pentodes, slope about 1 -2 mA/V.
The sharp cutoff pentode MO base SP61 has a slope of 6mA/V. The rare(ish) Z66 is similar on the international octal base.

DFWB.

[Neil Purling]

I have a EF39 or 6K7 enough to use a different one for each day of the week!
I hadn't thought of using another as an output stage.