Simple EL84

[HDTVman]

The one you refer to (dynaco Low Powered) has a 100R cathode R which is in common for both triodes and is decoupled with a 100µF cap.>>

I see 100R between the cathode 1 and cathode 2, with cathode 2 taken to ground through the 100uF cap, so where's the bias resistor for the second section, and in any case wouldn't we expect round 1K?

The DC bias path is via the 1K feedback resistor and the secondary winding of the output transformer to ground.

Can you see it now?

Later BZ

 

[andyjevans]

The DC bias path is via the 1K feedback resistor and the secondary winding of the output transformer to ground.

Can you see it now? Later BZ

Hmmm. I've never seen this arrangement - I just assumed that the 1K was the feedback resistor. Haven't seen this anywhere else.

Apropos the concertina in Dyna-based EL84 amp above, why 27k and 33k? I've only seen equal resistors in concertinas? What's the intention? Cheers, Andy

 

[andyjevans]

The one you refer to (dynaco Low Powered) has a 100R cathode R which is in common for both triodes and is decoupled with a 100µF cap.>>

I see 100R between the cathode 1 and cathode 2, with cathode 2 taken to ground through the 100uF cap, so where's the bias resistor for the second section, and in any case wouldn't we expect round 1K?

While we're on 'puzzled of UK' why does the concertina in the previous schematic have unequal resistors - first time I've seen that. What's the idea?

 

[HDTVman]

I see 100R between the cathode 1 and cathode 2, with cathode 2 taken to ground through the 100uF cap, so where's the bias resistor for the second section, and in any case wouldn't we expect round 1K?

The DC bias path is via the 1K feedback resistor and the secondary winding of the output transformer to ground.

Can you see it now?

[/B]

The DC path for both triode sections is common to the 1K resistor and the transformer. The 100 ohm resistor is just in series with it to provide some AC decoupling for the cathode of triode #2 with the cap to ground. This also give a different amount of feedback to the two triode sections.

This type of feedback circuit is very common with equipment that is as old as I am. Back in the old days designers made an effort to get the most out of the circuit without adding more parts but by making what was there multitask a bit.


Later BZ

 

[pengboon]

Hi guys

Just wondering have you ever make this Amp....

http://www.diyparadise.com/simpleel84.html

I just wondering how it sounds...??

Just looking at some old threads

By not using a seperate phase spliiter like the above example, will the power be lower that with? I seem to recall reading somewhere that it does, though I can't remember where or how...

 

[gingertube]

Yes the power will be less.
The EL84s are being run as a diff amp and so are their own phase splitter... err.!! that should be phase inverter. I have never tried this BUT I expect it to have a fair amount of 2nd harmonic distortion due to imbalance in the output stage.

The reduced power is due to it being strictly Class A - Not sure what its clipping behaviouir would be like but don't expect it to be great.

Cheers,
Ian