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Old 24th March 2015, 02:22 PM   #1
rg2 is offline rg2
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Default Aikido Problem - B+ dropping too much

Hi guys!

This is my first post and first tube build (long time lurker).

I bought an Aikido Noval board and PS-14 (PS-14)

I'm trying to run 4 12FQ7's at 300V but the B+ voltage seems to drop more than should be expected (I think).

I'm using an Antek 1T230 (AS-1T230 - 100VA 230V Transformer - AnTek Products Corp). I have the 230V secondaries wired in parallel. And the 6.3V secondaries wired in series to give me 12.6V. On the PS-14, I'm using the full wave bridge for both.

Using the chart at the back of the Aikido manual, I've set V1 & V3 to operate at 7.3mA (Rk=470ohm) and V2/V4 at 10 mA (Rk=243ohm)

Unloaded, the PS-14 gives me 341V B+ and 12.6V at the heater pad. That seems right.

Now, as soon as I attach the heater pads to the Aikido board, the B+ drops to 294V-303V (seems to vary every time I turn it on). Is it normal for the B+ to drop this much when only the heaters are connected? This is with only 10ohm as the dropping resistor (R7 on the PS-14 schematic).

When I connect the B+ to the Aikido board, the B+ drops to 265V. This is with the lowest R12a/R12b combo provide, which is a combined 889ohm. I get 252V at the tubes.

So, I'm wondering...is it normal for the voltage to drop this much with such low value dropping resistors? I know the circuit can run at 250V but I can't help but feel that something is wrong here.

Any thoughts would be really helpful.

Thanks!
RJ
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Old 24th March 2015, 02:53 PM   #2
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4 x 12FQ7 @ 300mA = 14.4W

4 x 10mA @ 350V = 14W

So--Theoretical power-cons from the whole board is around 30W.

Looking at this, I would say there's something incorrect, I wouldn't have expected such a voltage-drop. Are you sure there's no short/leaks anywhere,--maybe on the heater-supply, voltage drop when connecting only heater seems quite high...

Maybe that mains Tx just has awful regulation I don't know.......

--Why such a 'large' dropper-resistor? 900 ohm seems a lot. Whats the layout of the PSU stage? R-C-L-C....?

I would probably use summit like a 220-330 ohm depending on what voltage I had decided to set the stage to run at....
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Old 24th March 2015, 03:12 PM   #3
rg2 is offline rg2
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Thanks Alastair! I had checked the PS for shorts before but maybe I missed something. I'll double check again.

I think when you combine the two boards, it's a CRCRC.

I can't find the exact manuals online but the PS-14 is basically the same as the PS-3 from what I understand (http://glass-ware.com/User_Guides/PS-3.pdf). The schematic is on pages 14 and 15. I've got 10 ohms at R7 there.

This is the All in One manual...I believe the aikido section is the same as my Noval board (http://www.tubecad.com/2009/03/13/Ai...20in%20One.pdf).

For the dropping resistors (R12A/B on page 8), I was just using the values from the chart at the end of the manual (page 13). So, you're thinking that's too much?

Thanks!
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Old 25th March 2015, 02:35 AM   #4
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Can you pinpoint where the voltage drop occurs? Is it just after the rectifiers or after the dropping resistor or even at the primary? Are you using any voltage stabilizer for your mains?

Measure the voltage between the dropping resistor legs and divide by the resistor value. This gives you the actual current flowing, see if it makes sense.
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Old 25th March 2015, 10:22 AM   #5
DF96 is offline DF96  England
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I suspect what is happening is that the large heater supply caps (C5 and C6 in the PS-3 circuit) are loading the 6.3V secondaries too much, so lopping off the peak of the AC waveform. The PS-3 manual suggests using two separate transformers.

1.2A DC is about as much as a 3A AC winding can supply, so you should expect some voltage drop. Some of this will arise from the secondary itself, but some from the primary and this will affect the high voltage output too.

Something to try:
- wire the heater supply as a full-wave CT instead of bridge; this means only one diode drop instead of two
- then reduce the value of C5,C6 (or use just one of them if the ripple current rating will allow)
The idea is to reduce the peak charging current by allowing the reservoir cap to droop a bit more, but making up for this by dropping less voltage in the diodes.
Alternatively, add a low value resistor in series with the heater secondary - but be careful to maintain enough voltage for the regulator.
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Old 25th March 2015, 01:05 PM   #6
20to20 is offline 20to20  United States
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Why would you expect the 230v. secondaries to provide much more than 230v. once the tranny begins to be loaded? Self regulation should start being a real factor at a 10% load and be close to spec by 25% load. Just ballpark numbers. Are you trying to run the PS in the unloaded range for higher voltage?

Edit: And so with a full cap input PS you could expect about 15% above the 230v. for B+ when the tranny is loaded. Maybe @ 260v.

Last edited by 20to20; 25th March 2015 at 01:14 PM.
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Old 25th March 2015, 03:13 PM   #7
rg2 is offline rg2
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Quote:
Originally Posted by ballpencil View Post
Can you pinpoint where the voltage drop occurs? Is it just after the rectifiers or after the dropping resistor or even at the primary? Are you using any voltage stabilizer for your mains?

Measure the voltage between the dropping resistor legs and divide by the resistor value. This gives you the actual current flowing, see if it makes sense.
Thanks! It was happening both after the rectifiers and after the dropping resistor. But I didn't think to check the Primaries..that's interesting. I'm out of the studio until tomorrow, so I won't be able to check that today.

I'm not using a voltage stabilizer on my mains. Do you think that would help?
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Old 25th March 2015, 03:21 PM   #8
rg2 is offline rg2
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Quote:
Originally Posted by DF96 View Post
I suspect what is happening is that the large heater supply caps (C5 and C6 in the PS-3 circuit) are loading the 6.3V secondaries too much, so lopping off the peak of the AC waveform. The PS-3 manual suggests using two separate transformers.

1.2A DC is about as much as a 3A AC winding can supply, so you should expect some voltage drop. Some of this will arise from the secondary itself, but some from the primary and this will affect the high voltage output too.

Something to try:
- wire the heater supply as a full-wave CT instead of bridge; this means only one diode drop instead of two
- then reduce the value of C5,C6 (or use just one of them if the ripple current rating will allow)
The idea is to reduce the peak charging current by allowing the reservoir cap to droop a bit more, but making up for this by dropping less voltage in the diodes.
Alternatively, add a low value resistor in series with the heater secondary - but be careful to maintain enough voltage for the regulator.
Ah! This is starting to make sense.

I did notice that he suggests two transformers. But when I did my research on this, it seems like most people were building it with a single transformer.

Based on his manuals, it seemed like this transformer would supply enough current. 4*12fq7's should require 1.2A. His calculation in the manual for a full wave bridge is Iout = Iac/1.8. So, that would be 1.67A...so, I thought I would be ok. Out of curiosity, how did you get 1.2A DC as the limit from the heater windings? Should I be using a different calculation?

I'll try the center tapped configuration with your suggestions...I'll be able to give that a shot tomorrow and I'll report back.

Also, I'm not opposed to using a separate transformer for the heater if that would help. Might that be the best way to approach this?

Thanks!
RJ
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Old 25th March 2015, 03:23 PM   #9
rg2 is offline rg2
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Quote:
Originally Posted by 20to20 View Post
Why would you expect the 230v. secondaries to provide much more than 230v. once the tranny begins to be loaded? Self regulation should start being a real factor at a 10% load and be close to spec by 25% load. Just ballpark numbers. Are you trying to run the PS in the unloaded range for higher voltage?

Edit: And so with a full cap input PS you could expect about 15% above the 230v. for B+ when the tranny is loaded. Maybe @ 260v.
So, I thought the unloaded DC voltage I would get from a full wave bridge would be Vdc = Vac *1.4. That's actually pretty close to what I got...341Vdc. Am I mistaken?
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Old 25th March 2015, 03:49 PM   #10
20to20 is offline 20to20  United States
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Quote:
Originally Posted by rg2 View Post
So, I thought the unloaded DC voltage I would get from a full wave bridge would be Vdc = Vac *1.4. That's actually pretty close to what I got...341Vdc. Am I mistaken?
That is a calculation for the peak voltage. It only exists with no load and a fully charged input cap. But once you start to load the circuit the B+ will gradually come down to the full regulation point of the tranny which is the RMS voltage plus about 15% with a cap input PS. If you used a choke input PS you would be even lower and closer to the 230v. speced for the output.
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