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Old 22nd May 2012, 12:14 PM   #1
pebmrb is offline pebmrb  United States
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Default Looking for constructive criticism on my first design

Enclosed is a schematic for an amp that I currently have in the breadboard stage.Given that it is my first build that wasn't a direct clone of someone else's design,I was hoping to get some constructive criticism before doing a final build.I ended up with this circuit due to the fact that this power tube requires class A2 operation,which took me into unfamiliar territorry,and after experimenting with a number of driver circuits,I ended up with this.I have learned a lot,and this circuit seems solid,and sounds good to me.
Thanks in advance
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Last edited by pebmrb; 22nd May 2012 at 12:33 PM.
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Old 22nd May 2012, 01:17 PM   #2
TheGimp is offline TheGimp  United States
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If I were going to go to the trouble and expense of an inter-stage transformer, I would choose a better output transformer. The one you have will limit your frequency response.

Although the data sheet I've seen (Telefunken) does not specify anode resistance, since the tube is an output triode I'd be inclined to drop the transformer down to at least 5K primary, probably 3500R for better load matching.

Your anode voltage is a bit high per the data sheet.

I've never run a triode like that, but from the one graph on the data sheet it looks like the tube is meant to run positive grid current to get linear operation.

Hopefully someone with more DHT experience will chime in.

Last edited by TheGimp; 22nd May 2012 at 01:31 PM.
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Old 22nd May 2012, 02:33 PM   #3
euro21 is offline euro21  Hungary
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RS241 (same as 242) datasheet:
Ra: 3100 Ohm, thus 10K OPT is perfect.

IMHO 330V not too much, and -9..10V, 25mA, 330-340V operating point also OK.

Maybe interstage is ringing above 100kHz (and below 10Hz), use secunder loading (5K with output tube capacitance enough).

At this operating point A2 region above 20Vpp Ug (0.6Veff input), with 0.3% THD.

Last edited by euro21; 22nd May 2012 at 02:39 PM.
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Old 22nd May 2012, 02:40 PM   #4
pebmrb is offline pebmrb  United States
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TheGimp:
Thanks for the good advice,I will definately consider a better OPT in the final construction.In regards to the Loading,The RS242 is supposed to be the same as the RS241 which has an anode resistance of >3k if I recall.I can quite easily try different loads with the multi tapped Hammond.

Last edited by pebmrb; 22nd May 2012 at 02:45 PM.
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Old 22nd May 2012, 02:43 PM   #5
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In place of the interstage transformer, take into account the possibility of direct drive from a WCF or a SRPP, reducing costs and weight. A suggestion only.
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Old 22nd May 2012, 02:58 PM   #6
pebmrb is offline pebmrb  United States
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euro21:
Thanks for getting those specs.
I am not entirely clear on what you are describing about the interstage transformer,though.Can you elaborate?

Osvaldo de
Banfield:
I did consider those circuits in my earlier designs,but the ITs have already been snuck into the house,past the wife,so if possible,I will try to go with em.
Thanks for all the great input!keep it coming.
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Old 22nd May 2012, 03:20 PM   #7
TheGimp is offline TheGimp  United States
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Looking at the plate curves for the 241, I agree that 10K is probably the best load-line.

The 242 is slightly different as the anode voltage rating (300V) is lower than the 241 (400V). Grid curves are also different. The Ua 300 line for the 241 shows a 5mA lower anode current compared to the 242 (8-10% difference).

Also, it is interesting that both are called 15W Sender Tubes, but in the data sheet the 241 shows 12W max plate dissipation and the 242 shows 15W max plate dissipation.
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Old 22nd May 2012, 03:24 PM   #8
pebmrb is offline pebmrb  United States
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So,I guess I better back off on the B+ a bit if I want these things to last,eh?
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Old 22nd May 2012, 03:35 PM   #9
euro21 is offline euro21  Hungary
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Schematic.
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Old 22nd May 2012, 04:34 PM   #10
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Perhaps it must be adapted, but I give you another idea:

http://jrossmacdonald.com/jrmpubs/04...deFollower.pdf

Using less voltages, and other tubes, you can drive the A2 amplifier directly.
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