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Old 9th Dec 2016, 4:33 pm   #21
paulsherwin
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

You decrease the grid bias by reducing the cathode resistor. Increasing the resistor makes the bias more -ve.

Watch the current through the output transformer though - there's no knowing what it's rated for. 29mA is low for an EL33 so it may be set at that level to protect the transformer, though that's an odd thing to do unless Roberts were trying to use up excess stocks of battery valve transformers.
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Old 9th Dec 2016, 4:42 pm   #22
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Grid bias should be negative and should in theory be measured between cathode and grid, but where "auto bias" is used, measuring the voltage across the cathode resistor amounts to the same thing.

When the voltage is measured between ground and the grid, this shows whether the audio coupling capacitor (THAT Capacitor) is leaking. If the capacitor is OK the voltage will be close to zero. My experience has been that if the capacitor is leaking the voltage will be positive and in double figures.
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Old 9th Dec 2016, 4:46 pm   #23
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Thank you both, I'll get out the Avometers when I get home and have a play around, time permitting.

N.
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Old 9th Dec 2016, 5:14 pm   #24
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

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Originally Posted by Nickthedentist View Post
Assuming the control grid is held at 0v, to reduce the amount of -ve grid bias I presume I'd need to increase the voltage on the cathode by increasing the value of the cathode resistor (R17) still further. This would also reduce the anode current below the 29mA where it stands currently.
The grid bias voltage sets the operating point of the valve, the operating point will determine the standing anode current, the valve needs -ve grid bias, that is to say the control grid (g1) needs to -ve with respect to cathode, the grid is connected to the cathode (via the chassis) by the grid resistor and the cathode resistor, therefore when the cathode is +ve the grid is -ve with respect to the cathode, changing the value of the cathode resistor changes the grid bias and thus the valves operating point and therefore the standing anode current, it's the combination of screen grid current and anode current that determines the grid bias voltage developed across the cathode resistor.

The 560 ohm cathode resistor you fitted would develop across it approx. 16 volts with the anode current of 29 m/A that you measured, add to that the screen current and there will be more than 16 volts developed across the cathode resistor, with the HT voltage being what it is in your receiver 16 odd volts -ve bias is going to put the valves operating point towards the lower part of the valves Ia/Vg curve, this can lead to distortion of the one half of the signal.

Reducing the value of the cathode resistor will reduce the grid bias voltage and shift the operating point to the straighter part of the valves Ia/Vg curve and will reduce distortion.

But reducing the grid bias will increase the anode current, that current flows through the output transformer and the rectifier hence me saying "if the rectifier/transformer(s) can take it" The rectifier should be no problem given the voltage it's fed with, the output transformer might be ok also.

This is assuming that the valve you have is ok.

Apologies for any waffle.

EDIT: Post crossed.

Lawrence.

Last edited by ms660; 9th Dec 2016 at 5:25 pm. Reason: correction, just noticed there's no secondary HT winding
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Old 10th Dec 2016, 4:56 am   #25
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Recently I used the EL33 in an Argus TV project, instead of the 6V6. I found that a suitable value of cathode resistor was between 390 and 470 ohms. I was able to get some EL33M's at a reasonable price (these have metalization on the glass near the base connected by a wire link to pin 1 on the octal socket). If you use one of these, double check that pin 1 is connected to ground in your set, sometimes, in odd cases, this socket pin could be used a tie point for other components, then its possible to get a shock from the metalization.

PS: with EL33's it is not a bad idea to put something like a 33 ohm or 47 ohm resistor in series with the anode or the control grid, it helps prevent parasitic RF oscillations.
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Old 13th Dec 2016, 12:55 pm   #26
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Thanks for all the replies. I did some measurements with different values of cathode resistor, and two different valves from my junk box, see the attached table.

Datasheets that I've seen state the grid bias as -12.5V for the 6V6G, and -8.5V for the 6P25.

Bearing in mind that the OP transformer is titchy (this is a portable set, after all, and only has a 5" speaker), I'm a bit reluctant to increase the anode current beyond the 29mA stated in the Trader Sheet.

What do you people think I should do next? Should I scope the output to see what it looks like when driven with a sinewave?

Thanks,

N.
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Old 13th Dec 2016, 1:21 pm   #27
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Something not quite right there Nick.

Apparently increasing the cathode resistor of a 6V6 from 220R to 330R changes the bias, but makes no difference to the anode current.
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Old 13th Dec 2016, 1:33 pm   #28
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Yes, I was thinking the same. To be fair, I was very tired last night. Will try again...
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Old 13th Dec 2016, 1:52 pm   #29
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Unless something else is limiting the current availability to 42mA?

David
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Old 13th Dec 2016, 2:07 pm   #30
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

I think the 6V6 is on the way out.

Does the output sound distorted with the 6P25? If not, then just use the 6P25 with 330R. The 6P25 has closer characteristics to an EL33 anyway so is a better choice.
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Old 13th Dec 2016, 5:46 pm   #31
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

As a substitute for an EL33:
A KT61 will show a lower anode current.
A 6V6G will show a much higher anode current.
A KT61 will give slightly higher gain
A 6V6G will give significantly less gain.

Your 6V6G is showing healthy DC conditions so is unlikely to be low emissions.
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Old 13th Dec 2016, 10:02 pm   #32
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Thanks for your thoughts, David, Paul and PJL.

Paul: The output sounds fairly good with either valve, though perhaps not entirely "clean". Your 330R cathode resistor choice seems sensible.

PJL: Those comparisons with an EL33 are very useful, I suspect you may be right about the 6V6G being OK, but either way, it's not going to be chucked out, but will go back into storage if it's not used in this set.

Sticking with the 6P25, I've done some more measurements, see attached table. All comments welcome.

Nick.
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Old 18th Jan 2017, 9:06 pm   #33
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Hi Nick,
I have a used EL33 in my bits and pieces, the only thing that I can measure is that the heater is intact I cannot measure it for emissions. It is from an old Tandberg Radio from the late 40s.

Please drop me a pm if you want me to send it to you, I live in Norway, alternative I will be over in the Brighton East Sussex area in early March, the 10th until 13th if you want me to post it from there.

Tony
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Old 18th Jan 2017, 9:13 pm   #34
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Hi Nick do you still want a KT61 if so I have a used one you can have for the postage, I will test it tomorrow. Mick.
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Old 18th Jan 2017, 10:35 pm   #35
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Right, Nick, let's go back several steps and DESIGN a basic output stage from first principles.

Audiophiles might want to look away, now.

Lessee... A nice table radio might do 4 watts, I suppose, and the HT might be around 250v running on load.

These two guesses do rather a lot for us. Let's assume that the resistance of copper wire forming the output transformer primary is fairly low, low enough to neglect the ohmic losses at the fairly low currents involved. This is a reasonable assumption. So the average voltage on the anode of the valve must be 250 volts! Running with a sinewave signal, the anode voltage will swing symmetrically to either side of 250v.

Time for another assumption. A real world valve isn't going to be turned on hard enough to pull its anode right the way down to zero volts, and if you got anywhere near zero, you'd have hellish distortion (other adjectives are available. A guitarist might even like it)

So let's make another assumption, let's say the anode only gets pulled as low as 50v above ground.

So the max voltage across the primary is going to be 250-50 = 200v
But as the waveform has to be symmetrical, when the valve is almost turned off, the primary voltage on the transformer must swing as far the other way... -200v across the primary, so the anode must go up to 450v.

But the power supply is only 250v, so where does 450v come from? Well the half of the wave where the valve is pulling its anode voltage down below the 250v HT passes current through the primary winding and charges up the field in the inductance that winding presents to the world. This field is stored energy, and when the valve is driven to lower than quiescent currents, the transformer gives up its energy and drives the anode voltage ABOVE the HT voltage. (This is why single ended power output stage transformers have a gap in their cores to stabilise the inductance value and to prevent saturation)

So our anode voltage is swinging sinusoidally between extremes of 50v and 450v, with an average value of 250v, so there is no dc component of voltage across the primary winding.

So we've made a 400v peak-to peak sinewave. That's 220v peak or 200 * 0.707 = 141.4 volts RMS across the primary.

Now say the speaker is 3 Ohms, and we want 4 Watts. Power = Vsquared over R. R is 3, power is 4 so voltage squared is 12, so the RMS voltage across the speaker is root(12) which is 3.5 as near as makes no odds. Now we have the turns ratio of our transformer because it's the same as the voltage ratio. 141.4/3.5 so we need a 40.4:1 ratio.

Easy?

Now what about that transformer? We want to have some nice bass. We don't want the transformer limiting whatever the speaker and cabinet can do. Let's decide to make the low frequency roll-off be 3dB at 30Hz which would make for a good quality table radio.

Now that 3 Ohm speaker seen through a 40.4 to one ratio transformer looks like the turns ratio squared times the speaker impedance if viewed from the primary.

so 40.4 squared, times 3 is 4896 Ohms. For that 3dB roll off we need the reactance value of the inductance of the transformer primary to equal this at 30Hz.

Reactance is 2 * Pi * F * L =4896 we know F is 30 so we can rearrange and calculate L
So L needs to be 26H which is quite an amount of inductance. If we relaxed that 30Hz choice, and allowed the bass to roll off higher, the inductance required would scale down pro-rata.

Hey, but here we are, we've just designed the requirements for the output transformer and no-one's even mentioned what type of valve we might be using

This is why output valve substitutions work so well without people changing output turns ratios. The turns ratio is set mostly by the choice of HT voltage, by the wanted amount of power and by the speaker impedance. Different valves might change that assumption of taking the anode voltage down to 50v, but that has a smaller effect on the transformer than the other parameters.

Now, what about anode current and biasing?

Power = Isquared * R

4W into a 3 Ohm speaker gives us 1.15 Amps RMS. now that is plus or minus 1.633 A on the peaks..... into the speaker.

At the transformer primary, this is reduced by the turns ratio so 1.633/40.4 gives us plus or minus 40mA on the peaks. Remember, this is the signal current component and adds with the quiescent current.

We don't want the valve current to cut right off on the peaks, or it flat-tops and we get nasty distortion again, but this calculation is with it going right to the 50v minimum anode voltage, so we can play just as close at the other end, but let's leave it 10% of the signal current, 4mA so to get 4ma left when the signal is -40mA, then the quiescent current needs to be 44mA!

And we still haven't involved any specific valve yet.

So we want one which can run at an average 250v and survive 450v peaks
We want one we can bias at 44mA and will handle an 84mA peak

The 250v and 44mA gives an average anode dissipation of 11 Watts

So how about that 6V6, then?

DC anode volage long term 315v that'll do!
Pek anode voltage 1200v (even if triode connected) that'll do!
Anode dissipation 12W yup, that's in.
Peak cathode current 105mA yup, that's plenty to allow some screen current.

Now we've got these, what about the transconductance? That essentially sets the gain you get, which tells you how much oomph you need in whatever drives the chosen valve.

The next step is to look at the curves and pick a cathode resistor to give us 44mA quiescent current.

The output-side work can be done by laying a load line across a set of characteristic curves, and it'll make the onset of distortion more visible, but these wet-finger calculations will do the job.

David
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Old 20th Jan 2017, 10:43 am   #36
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

It's all gone quiet. Was it something I said?

David
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Old 20th Jan 2017, 11:53 am   #37
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

An enlightening theoretical analysis David, but I wouldn't expect anyone to respond directly. Meanwhile Nick seems to have resolved the immediate problem with his junk box 6P25.
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Old 20th Jan 2017, 11:57 am   #38
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Put in valve, adjust stuff for best sound with minimum smoke.

Lawrence.
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Old 20th Jan 2017, 1:07 pm   #39
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Quote:
Originally Posted by paulsherwin View Post
Meanwhile Nick seems to have resolved the immediate problem with his junk box 6P25.
I've just stumbled upon this thread today, THANKS for the replies and very kind offers, will be back soon when I've got a few mins spare...
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