Class A/B whats the difference?

[John]

So how would my ears detect the difference, my amp has a knob on the back , fully one way is is class A and fully the other is A/B and any where in between is........well just that! any where in between.

[JimTidmarsh]

I'll bet you're really glad you asked now, John!!

[RayL]

Having looked at the diagram, there is no negative feedback. The lower power rating is because the EL34 valves are cathode biased, rather than dc biased.

Ray L

[Didier]

Fender Pro Junior is definately a Class A...
Jimbo

The Fender Pro Junior delivers 15W with a pair of EL84 in push-pull. If it was pure class A, it would be 10W at best, so it's class A/B.

Didier

[Graylion]

Sorry for the delay guys! Been away then VERY busy running a charity! Yes, Roger, that's the diagram of my amp. I've mislaid the original AND the photocopies but that post is just about clear enough for an expert (i.e. NOT me!) to read. As Didier says, 25 Watts (in reality it might have actually been less if measured) isn't very much for 2 x EL34s. It was fairly common though with a few amps at that time - early 1960s. When I bought mine in 1962 I don't remember there being any amps with 50 watts output in the UK. Selmers were up to 30 watts until 1963 - then they brought out the 50 watt and later 100 watt.

In their October 1961 catalogue the highest output amp was the "Twin Selector-Tone", which had 2 x KT88s for 30W output. The "Selector-Tone Automatic" had 2 x EL34s running at 25W, like mine (but with less valves than the Bird!).

By October 1962 Selmer had the "Bassmaster" with 2 x EL34s at 30W and the "Twin Selectortone" with 2 x KT88s at 30W. It also had the "Automatic Selectortone" which had 2 x EL84s at 15W. By the June 1963 catalogue they had uprated this with 2 x EL34s at 25W. In Sept 1963 the Selectortone series had gone and were replaced by the "crocskin" covered "Zodiac Twin 30" (2 x EL34 at 30W) and "Thunderbird Twin 30" (2 x KT88 at 30W). This was joined by the "Bassmaster 50" with 2 x EL34s now running at 50W! In those days we didn't have The Trades Descriptions Act so the outputs were "Salesmen's watts"! Viz - the Watkins Dominator MkI probably wasn't 17W from 2 x EL84s! Makers tended to use the valve maker's data sheets and quote the maximum possible - then didn't design them to give the quoted output!

All the dates quoted are the catalogue publishing dates from the originals, which I collected as a schoolboy at the time! Only the Selmer amps had their valve complement listed - not the Futuramas, Gibsons or Fenders. If anyone is interested in details from these, which range from October 1961 to September 1963, please let me know. I also have the Selmer guitar catalogues from Oct 1961 to Aug 1963 (the last one is Fender & Gibson)

[Didier]

the Watkins Dominator MkI probably wasn't 17W from 2 x EL84s! Makers tended to use the valve maker's data sheets and quote the maximum possible - then didn't design them to give the quoted output!

The Bugera V22 which also uses 2 x EL84 is quoted at 22 W. It seems rather optimistic !
http://www.bugera-amps.com/EN/products/V22.aspx
Now if it's real, the EL84s will not last long...

Didier

[Graylion]

Thanks for the link Didier - some nice looking amps at reasonable but you're probably right about that V22 - it must run really hot and burn out the valves within a short while! Just add to that my UK mains voltage of 245V and I'll be cooking breakfast on it!
Cheers, Lionel

[Graylion]

Thanks for the info Ray. Does that mean the bias on my Bird (amp!! ) is fixed so I wouldn't have to worry about re-biasing if I changed valves? I have to say that it is the quietest valve amp I have ever known, even compared with my more modern (by 30 - 40 years!) amps! My Carlton Camel 100W (4xEL34) is a "hummer"! I intend to try swapping out pre-amp valves and disconnecting the reverb tank (a simple phono plug) to see if it makes any difference. If not I'll swap the EL34s around - AFTER marking where they came from, of course. Failing that I'll delve inside and look at the possibility of star-earthing and check for any obvious poor design - armed with a guide on how to do it!
Cheers, Lionel

[RayL]

Thanks for the info Ray. Does that mean the bias on my Bird (amp!! ) is fixed so I wouldn't have to worry about re-biasing if I changed valves?
My Carlton Camel 100W (4xEL34) is a "hummer"!
Cheers, Lionel

That's correct, Lionel, no bias to set up.

Although not a fix for your Carlton Camel, one hum-reduction technique for the perfectionist is to rectify and smooth the 6.3v heater supply. This means that a.c. stops at the power supply and there are no 50Hz alternating currents anywhere in the audio side of the amp. It also means that the heater supply can be made variable. For longer valve life, the heaters can be run at a lower voltage. For more audio power (and shorter valve life!) use a higher heater voltage.

Ray

[Graylion]

Thanks Ray - yes, I've read that the possible AC heater supply problem can be checked by disconnecting it and temporarily using a 6V battery. If the hum goes, then that's the cause eh? It wouldn't be rocket scince to have an off-the-shelf stabilised DC transformer inside.