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Old 16th June 2013, 11:00 PM   #11
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The output transformer is 2000R.

I thought I was using fixed bias with 120R connected to two cathodes ?
Unless you mean fixed current ?

My power transformer supplies 200VA so about 900mA.

The output transformer only has a 4R tap so I cant do much there.

The basis of the circuit is from a 100watt Simms-Watts PA I used to have but pentode connected instead of ultra linear. But I don't use a negative voltage bias I just rely on a 130R double cathode resistor to get the bias.

The Simms-Watts PA was extremely loud for 100 watts. I couldn't stand anywhere near the speaker without hurting my ears when it was on full power.
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Old 16th June 2013, 11:14 PM   #12
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It shows ultra linear on diagram but the connections are just pins on a pcb.
It can be wired for triode mode, ultra linear or pentode mode.

Click the image to open in full size.
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Old 17th June 2013, 09:17 AM   #13
Enzo is offline Enzo  United States
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Quote:
I thought I was using fixed bias with 120R connected to two cathodes ?
Fixed bias means a negative supply voltage is applied to the grids. The voltage can be adjustable, but it is still considered fixed. it is fixed because it does not vary with the action of the circuit.

What you have is cathode biased tubes. It is not fixed, because the voltage developed across the resistor depends upon the tube current.

In other words in a fixed bias amp, I could remove the B+ from the tube entirely and the bias voltage would remain. If I removed the B+ from the cathode biased tube, the bias voltage would disappear.
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Old 17th June 2013, 01:23 PM   #14
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The Simms-Watts PA amp uses fixed bias from a negative supply.
It means you can lose the cathode resistors that would be quite bulky and probably get hot.
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Old 25th June 2013, 05:06 PM   #15
Struth is offline Struth  Canada
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Hi Guys

If you want 100W why are using cathode bias? That is the limiting factor here. Maybe you should read about how tube power amps work.

Wire up a bias supply and assure that the bias pot provides a sweep from around 10% of absolute B+ to 15%. If B+ is 470V, then -47V to about -60V or so. This assures that you can turn any tube sample 'off' and have complete control over how 'on' it is. Ideally, individual bias pots and current-sense resistors for safe ground-referenced bias measurment and adjustment.

Another detail is that you can wire the tubes as triodes and this is a good tone for some players. Make sure the screen resistor is ALWAYS in series with pin-4. basically, treat the other end of the screen-stop as the "flexible" connection point.

Despite what some readers believe, there is NO specific OT primary impedance that is "ideal" for each tube type. The common big-bottle tube types used in MI all work into a very widely overlapped range of loads quite happily. This means that if there is heater current to support EL-34s, then you can readily plug in 6L6, 6550, KT-66/77/88 and all the equivalents, and in most cases 6V6 also. Just bias the new tube properly for the voltage environment.

Our FAQ and our TUT-series books explain all of this in detail.

Have fun
Kevin O'Connor
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Old 25th June 2013, 05:12 PM   #16
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I looked at a Simms-Watts amplifier circuit and that uses fixed bias with pots.
I didn't have -50 volts available so went for cathode bias.
I am only losing 3 watts in the cathode resistor so it isn't a disaster.

As it stands the amplifier is very loud.

Last edited by nigelwright7557; 25th June 2013 at 05:15 PM.
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Old 25th June 2013, 05:20 PM   #17
Struth is offline Struth  Canada
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Hi Guys

You seemed to miss the point.

You can definitely cathode bias the tubes if you wish, or if this provides the tone you want. You will just not get 100W this way.

Also, you need more than -50V to assure the tubes will be under your control.

With respect to cathode biasing: at the very least you should use two Rk's for the four tubes. Each Rk should be shared by a push-pull pair to promote dynamic balancing. You can take this a step further and have individual Rk's, losing dynamic balance but allowing tubes to be pulled safely for power reductions and/or asymmetric tones.

Without Ck the output will be cleaner but lower in power. With Ck added to any of the above, the amp will transition to class-AB at some power level.

How much power is spent in Rk is of little concern to output power. Rather, it is relevant to placement of Rk and Ck, and in sizing Rk power-wise.

Have fun
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Old 25th June 2013, 05:27 PM   #18
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The cathode voltage is 20 volts with zero signal so why do I need more than -50 volts ?

The bias current is 40mA.

I am using the amplifier in pentode mode to get decent power out.
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Old 25th June 2013, 07:05 PM   #19
Struth is offline Struth  Canada
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Hi Guys

I thought since you had a Fender OT and wanted 100W that you would be using more typical supply voltages. Looking back at your previous post I see there is only 350Va. That would put the fixed-bias grid voltage around -35V or so. With cathode bias, more like -19V. Your 20V reading across Rk lines up with this.

Note these voltages are with respect to the cathode, which is the tube's reference point.

You might get about 60Wrms with this arrangement, mostly limited by the supply voltage. You lose the cathode bias voltage right off the top as far as supply utility goes. Then there is the internal resistance of the tube that further restricts how close the plate voltage can be pulled towards the cathode voltage.

60W through normal guitar speakers in large cabinets will be very loud. If you need it louder, add more speakers and arrange them to focus the sound. SPKR shows cabinet layouts for best sound on stage or at home.

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