• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Looking for constructive criticism on my first design

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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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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.
 
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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.
 
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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.
 
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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.:D
Thanks for all the great input!keep it coming.
 
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.
 
euro21:
thanks for the circuit.I see now.So the 5k resistor can help with the ringing at frequency extremes?

Osvaldo de
Banfield:
thanks for the link.I will have to study those circuits.

Plenty of good info here,glad I posted.This circuit seemed unique to me,so I thought I better get some opinions from those with more experience than myself before doing a final build..
Any other opinions on this circuit?
 
MelB
Thanks for the encouragement.I hope the thread continues to provide some scrutiny,I have much to learn too.
As far as sound quallity goes:In it's current breadboard state: rats nest wiring,jumper cables,and with average OPTs,it is sounding very good.
I think with a little fine tuning,and actual solder joints instead of alligator clips,I can get it to an even higher level.
 
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Don't waste those beautiful and very useful transformers like this.

First, you can clean up the Output section by an order of magnitude,
by simply adding only two more tubes, and a handful of caps and resistors.

What you get:

(1) 99% of PS hum and noise is rejected at the Anode of the output tubes (PSRR to max).

(2) Double the power output. The extra headroom is precious.

(3) Most of the non-linear distortion gone, through Push-Pull operation.

(4) True balanced output from the Interstage, without a splitter stage.

(5) No chance of DC flowing through the secondary of the interstage transformer, for 100% headroom and purity.

(6) NO DC or PS noise flowing through the Output transformer, meaning NO core magnetization at all, and 100% headroom out of Output tranny.

(7) Output tubes operating in a better range for maximum tube life.

(8) Jaw dropping uncanny clarity of the stereo staging; close your eyes and actually kiss Holly Cole.

Key points:

Split main PS voltage over the 5998 and the bottom tube, with the top 5998 getting 100 volts or more, and the bottom seeing hopefully about 250 if you've got it.

Set up the idle current in top and bottom tubes to achieve the DC idle at the bottom tube's plate to about 250 from ground.

You will never go back to SE again.
 

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Hi Nazaroo;

I don't like such bias of output tubes. I believe they do not like it as well. And I don't know why you need extra capacitors in top tubes of output stage.



Waveborne:

So I put another resistor in series with the 33k plate resistor of the gain stage,and connect this voltage divider to the cathode of the cathode follower stage via a capacitor?

You may split it, like 27k at the bottom, 6k2 at the top.

The result will be almost like CCS load.
 
Hi Nazaroo;

I don't like such bias of output tubes. I believe they do not like it as well. And I don't know why you need extra capacitors in top tubes of output stage.

Greetings Wavebourn:

do you refer to upper or lower tubes?
I left the bias of the tubes to the OP.
I have already assumed they (original circuit) are wrong, without looking closely.

As for the uppers, well, I have had great results with 5998s,
and much poorer results with 6AS7s.
I didn't bother to analyze why, since I already found a tube I liked.

The caps allow the top tube to function as a PS regulator.
This along with the xformer blocking cap keeps the audio signal
isolated and across the primary.

I also agree with your assessment of the input stage:
I think you can get better mileage out of a Mu-type CSS stage, or SRPP.
 
Hmmm... lets clean up the details for this circuit:

(1) If the Interstage transformer has no center-tap, (which would be connected to ground),
then you need a voltage-divider across the secondary to ground (see new diagram). The value of these resistors should be high enough to prevent excessive grid-current, but not so high as to induce instability in the (self)-bias D.C. circuit.


(2) You probably don't need the bypass-caps in my configuration, because the backward output-impedance seen by the speaker-load through the xformer is rather different now.

(3) With the lower voltage across the RS242, you'll have to recalculate the cathode-resistor, and perhaps its wattage.

(4) Depending upon available voltage at B+, you'll want to drop 10 to 50 volts across the load resistor between the upper and lower tubes. At 20-25 mA, that would be about 500 ohms to 2.5 k R.

(5) As mentioned, you need a 1k Grid Stopper for each triode in the 5998, and you should probably add one for the RS242 as well.

(6) The Resistor-values for the 5998 CSS circuit will be set by operating-point chosen. Start with 20-25 mA and your available voltage as a guide. Test it with the expectation of finding about +250-300 volts (to gnd) at the bottom of the 5998 cathode resistor.

(7) The BIAS for the 5998 can be referenced/checked against the specsheets for this tube. If you are substituting a 6AS7 or similar tube, check those datasheets.

(8) The blocking cap for the output primary doesn't need to be over-large; start with 10 uF polyprop and add a HF bypass cap.
 
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