I did a repair on a Strauss Amp this weekend past and noted that only 2 of the quad of EL84 had screen resistors. I note that the Peavey Classic 30 schematic shows the same thing. Also, in both cases the screen resistors which are fitted are only 100R.
This seems "dodgy" to me. Surely all 4 tubes need screen resistors and 100R is way too small.
On the Strauss the 4 EL84 are all in a line and it is the centre 2 which have the screen resistors.
Is this typical?
Is there any reason for it?
Since it is the center 2 tubes which have them I thought maybe it might be a heat management thing but the 100R value would argue against that.
B+ was 420V which also seems abusive (to the tubes).
I ask because I looked on my old amp chassis "donor" shelf last night for a new project and noted I have an old Mono PA Amp which had 4 x 6BQ5 (EL84) so I have power and output trannies to suit a quad of EL84 Amp.
Tempted to build a Vintage AC30 copy.
Cheers,
Ian
This seems "dodgy" to me. Surely all 4 tubes need screen resistors and 100R is way too small.
On the Strauss the 4 EL84 are all in a line and it is the centre 2 which have the screen resistors.
Is this typical?
Is there any reason for it?
Since it is the center 2 tubes which have them I thought maybe it might be a heat management thing but the 100R value would argue against that.
B+ was 420V which also seems abusive (to the tubes).
I ask because I looked on my old amp chassis "donor" shelf last night for a new project and noted I have an old Mono PA Amp which had 4 x 6BQ5 (EL84) so I have power and output trannies to suit a quad of EL84 Amp.
Tempted to build a Vintage AC30 copy.
Cheers,
Ian
I wonder if those 100R screen stoppers are just there for HF stability? I would think that damping one EL84 of a parallel pair would hold the parallel EL84 stable as well.
I read somewhere on the internet (sorry not a great reference!) that parallel pentodes can sometimes oscillate mutually between the two.
In guitar amps bigger screen stoppers are usually a good idea to limit screen current during continuous clipping of the output stage, and hopefully prevent failure due to over-dissipation in the screens.
It seems to be a recent trend in guitar amps to increase the plate voltage on EL84s well beyond anything envisaged in the data sheets. I suspect that probably correlates with complaints about limited tube life of EL84s!
If there is 420V on B+, what is there on the screens?
I read somewhere on the internet (sorry not a great reference!) that parallel pentodes can sometimes oscillate mutually between the two.
In guitar amps bigger screen stoppers are usually a good idea to limit screen current during continuous clipping of the output stage, and hopefully prevent failure due to over-dissipation in the screens.
It seems to be a recent trend in guitar amps to increase the plate voltage on EL84s well beyond anything envisaged in the data sheets. I suspect that probably correlates with complaints about limited tube life of EL84s!
If there is 420V on B+, what is there on the screens?
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