At any rate try to get it going OK with one tube first, so you are sure there are not the other factors throwing things off.
Jan
Jan
;
Excellent!
By now there should be output.
What is the characteristic of it?
You are right — if the TP 9 is running only 4.2 volts, it elicits a question: what is the present value of R15?
If 96 Ω, then the 12AQ5's are really being starved for current flow. You're only working with 44 mA for quiescent (no signal) current, for the 3 valves combined. As jan.didden observed, when hooked up this way, without individual bias-setting resistors inline with each cathode, the quiescent cathode-anode current flow can be quite different, tube to tube.
Tubes (like semiconductor devices) can be quite “piggish” and will “hog” the current when hooked exactly in parallel. Although you don't presently have 3 ea, 48 Ω resistors, you do have 2 of them. This would allow you to remove one 12AQ5, and use a separate cathode resistor (and of course, bypass capacitor) for each of the two.
If 48 Ω, then I'm beginning to suspect a bad valve, or something similar, involved.
So…
R15?
-= GoatGuy ✓ =-
Hello again!
R15 was a 48Ohm.
I also suspect that the current is too low. I'm a bit tired tonight and it's getting late (1:50 AM).
Will try them tomorrow and let you all know.
I have to note 2 things.
1. I actually have one 12AQ5 and 2 6AQ5s but the 6AQ5s' filaments are in series.
2. All 3 of my tubes have different shapes of plates. The two 6AQ5s have shorter plates and one has like 70% plate area of the 12AQ5.
BUT! When I removed the two 6AQ5s, the volume decreased significantly (could say 1/3-1/4).
I know that the 12AQ5 is from RFT because I saw an identical one with the 12AQ5 writing and the plate structure of an RFT and it was exactly the same. Not sure about the two 6AQ5s.
Thank you for the update.
GO TO BED.
We'll get to this tomorrow.
Now, if it is the morning…
Since you have 2 ea., 48 Ω resistors, I recommend putting them in parallel. With a single 48 Ω resistor for R15, at 4.2 volts
E = IR … rearranges to I = E/R
I = 4.2 ÷ 48 ( × 1000 to get mA)
I = 88 mA
Since you're working with 3 completely different tubes, I think jan.didden's advice is now more useful than ever. Plug in just the 12AQ5, and neither of the 6AQ5's. Use a single 48 Ω resistor. I = 4.2 ÷ 48 ( × 1000 to get mA)
I = 88 mA
Let's get measurements at the TPs (that are directly related).
I'm interested especially in TP–5, TP–9, TP–8 and a more exact TP–2 vs. TP–3. (you ought to be able to measure the voltage difference between them, supposing that your volt-meter can 'float' with the black wire to one, the red to the other).
Also, confirmation of resistor values. I've checked-and-rechecked the “legend”; any confirming information would be quite helpful.
-= GoatGuy ✓ =-
Hi!
I have to tell you something about R7. I didn't have a high wattage 27K, so I used a 5W 33K.
The test points with only the 12AQ5 connected (there is a lot of error because of the 1M-100K voltage divider):
TP2: 67V
TP3: 54V
TP4-5: 150V
TP8: 152V
TP9: 0.40V (with a 48Ohm)
TP9: 0.25V (with a 24Ohm)
You should be getting a lot more current through that 12AQ5.
Did you measure the voltages right at the socket?
The heater is at 12,6V and glowing?
If so, the valve is defective.
Try the other ones.
About the output transformer: you say it has an 18:1 ratio.
What load do you have at the secondary?
An 8 ohm speaker would reflect as 2k6 on the primary side. Should be fine for 2 xAQ5's.
What's the DC current rating? This depends on the wire size and width of the airgap. It does have an airgap, right?
Did you measure the voltages right at the socket?
The heater is at 12,6V and glowing?
If so, the valve is defective.
Try the other ones.
About the output transformer: you say it has an 18:1 ratio.
What load do you have at the secondary?
An 8 ohm speaker would reflect as 2k6 on the primary side. Should be fine for 2 xAQ5's.
What's the DC current rating? This depends on the wire size and width of the airgap. It does have an airgap, right?
Thank you, AbaddonD. Substituting 33 kΩ 5 W for the 27 kΩ is just fine. Good choice.
Parafeed813 quickly saw that the 12AQ5 is almost certainly functionally dead, as a valve goes. With 48 Ω, it is only conducting 8.3 mA, with a grid bias of 0.40 V. It should be conducting about 25 mA, with 150 VK-A (TP8 minus TP9); it should be conducting even more than that with a bias of only 0.40 V.
Poor tube.
Give it a decent burial.
By the way, what is the actual B+ voltage? Again, with your 'imprecise' voltage divider, you can measure this easily enough.
Again, Parafeed813's advice to pull the 12AQ5 and substitute in the pair of 6AQ5's is very good.
Do that, and let's see what's up.
Thanks for playing along!
Just Saying,
-= GoatGuy ✓ =-
https://frank.pocnet.net/sheets/049/1/12AQ5.pdf is not great, but has key numbers for AF amplification.
__________
PS: do you have a 10 or 11 kΩ resistor? Put this in series with the 100 kΩ resistor on your voltage divider (digital voltmeter), in order to be much closer to 10.0 to 1 ratio. However, do not do this yet. While we're "talking values", it'd be a shame to introduce a whole new set of them with the adjustment. For now, whatever values you are reading are about 9.1% of their full-value. The 'back corrector' is exactly 11x. Have you been multiplying by 10x or 11x to get the adjusted values listed for the TP readings? Thanks!
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All the values have been measured right at the sockets.
The heater voltage is 12.2V and the cathode does glow red.
It could be? The tubes are in very good shape cosmetically (IK you can't say a tube is good because it's clean, I mean they don't seem to have been used a lot).
The transformer is an 18:1 power transformer which was rated for above 3A. It is handmade (made about 20 years ago).
I have two 6Ohm speakers hooked up in parallel, so 3Ohm. They are rated for 125W RMS.
They were listed as used and the seller which I had also bought the PCL805s from him and they were functioning as intended, told me that they work correctly and they are functional.
The actual B+ is 170V-173V.
Yes. Well, I have multiplied the readings by 10 to account for the "10:1" voltage divider.
Excellent, AbaddonD. Could you do a 'spot measurement' of the B+ with your voltage-divider? … just for reference.
It'll help show the difference between B+ and TP{3,4,5,6,7,8}; this difference of course then is used to estimate current by Ohm's Law E=IR.
Yours,
-= GoatGuy ✓ =-
Does the power transformer thats being used for output have an airgap? If not I bet it's saturated...
I don't think so.
About the saturation, I had mentioned before that with the PCL805's pentode as the amplifier, it had an output power of about 4W. While this does less than 1W with a lot of distortion.
You mentioned before, that you had 20W!! output too. Driving that transformer with ~ 150mA (three tubes parallel) will saturate it for sure without a gap. Distortion comes from saturation.
Thank you, AbaddonD. I was hoping it would be close to that.
Working with your last set of test-point values:
TP2: 67V … ΔV = 13 V compared to TP3
TP3: 54 V … see above — ΔV a bit higher than expected, but not completely whacko
TP4–5: 150 V … is ΔV = 2.0 V compared to B+
TP8: 152 V … is ΔV = 0 V compared to B+
TP9: 0.40 V (with a 48 Ω)
TP9: 0.25 V (with a 24 Ω)
TP8 being ΔV=0 is confirmation of just how low the current thru 12AQ5 is. VERY low.TP3: 54 V … see above — ΔV a bit higher than expected, but not completely whacko
TP4–5: 150 V … is ΔV = 2.0 V compared to B+
TP8: 152 V … is ΔV = 0 V compared to B+
TP9: 0.40 V (with a 48 Ω)
TP9: 0.25 V (with a 24 Ω)
Importantly, the TP4, TP5 ΔV of 2.0 thru the 5.1 kΩ resistors shows 0.4 mA approx of screen grid current. Entirely within expectations.
Thanks for the measurements.
Now to try your pair of 6AQ5's.
… and TP2 being 67 V also indicates that
E = IR …
I = E/R
I = 67 V ÷ 33,000 Ω
I ≈ 2.0 mA
-= GoatGuy ✓ =-I = E/R
I = 67 V ÷ 33,000 Ω
I ≈ 2.0 mA
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The TP values with two 6AQ5s in circuit.
B+:145V
TP2: 63.5V
TP3: 52V
TP4: 143V
TP5: 140V
TP8: 140V
TP9: 0.46V (with a 48Ohm)
TP9: 0.28V (with a 24Ohm)
(all values have been measured with the inaccuracies of the voltage divider)
The problem of 'copy-and-paste' is that it replicates measurements either made-but-not-transcribed-accurately, or … just-not-made-at-all. TP9 is suspect. — especially considering that TP8 is now at 140 V. Quite a voltage drop on your output transformer primary. Don't know its 'R' coefficient, but the 5 volt drop implies that TP9 should be substantially different than “no change from before”. If you know what I mean.
TP2 and TP3 showing a bit of droop from before. This also implies that the current-draw of the pair of 6AQ5's is dropping the supply somewhat. Which is to be expected. Maybe good!
Whoops, I see the B+ is now 145.
Well, there you are.
It makes sense.
And TP4 at 143 remains consistent.
But TP5 and TP6 are showing (Δ = 5V), so 1 mA of screen current.
Again, very probably just fine.
TP9 again?
GoatGuy
TP2 and TP3 showing a bit of droop from before. This also implies that the current-draw of the pair of 6AQ5's is dropping the supply somewhat. Which is to be expected. Maybe good!
Whoops, I see the B+ is now 145.
Well, there you are.
It makes sense.
And TP4 at 143 remains consistent.
But TP5 and TP6 are showing (Δ = 5V), so 1 mA of screen current.
Again, very probably just fine.
TP9 again?
GoatGuy
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