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Old 27th December 2003, 11:52 AM   #1
icebear is offline icebear  Norway
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Default Resistors in power supply

There is too high voltage on my Mullard 5-20. Without the tubes inserted, I measure 690V after the rectifier. It shold not bee that high. More like 400. On the original schematics there is two resistors between the trans. and the rectifier. The tranny is 410-0-410. What values should these resistors be? It's R26 and R27.

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Old 27th December 2003, 04:22 PM   #2
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Yes........without the tubes inserted you are asking for trouble with that high voltage........YOu shouldn't test the HT unloaded...without the output tubes in sockets......If you are using the indirectly heated GZ34 rect, the warm up should be the same as for other tubes...so the HT doesn't rip off. If you are using the same rated components as per Mullard 5-20 design; C12-13 on power supply (post rect+choke) are rated for 450V.....don't stretch this with 690V, or they will <go-for-launch>.

The original quote <440V DC on centre tap of o/p tranny>;
Mains tranny should be capable of delivering 2x410V AC @180mA if the unit is to be used with FM/AM tuner. If not 2x150mA is sufficient.
<R26-R27 will depend on resistance of the transformer which for the GZ34 should be at least 110 ohms>

rich.....
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Old 27th December 2003, 04:48 PM   #3
icebear is offline icebear  Norway
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Default More testing

Hi

I'v tested more with the tubes in place.

The tranny's are rated to 300mA. It's a stereo chassi, so it has to supply both channels. But I'm not using any tuners, so it shoul be ok.
Measured the resistanse of the tranny (with a voltmeter), and got 63 ohm's.

I tried a couple of values for the R26&27. First with 50 kohm, but the voltage only got up to 25V on the first cap. Then I tried 220 ohm. I measuerd while I switched it on. It takes a couple of seconds before the voltage raises. When it vent over 450V I switched it off. So someplace between these values should do it. 5k maybe? Any suggestions?
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Old 27th December 2003, 08:45 PM   #4
SHiFTY is offline SHiFTY  New Zealand
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What rectifier are you using? If it is a "directly heated" one, like a 5U4, the voltage will surge quite high on turn on, as the rectifier warms up much faster than the output tubes. When no current is being drawn from a power supply, its voltage tends to rise.

What will happen is that the output tubes warm up and then your voltage will drop to normal levels. However, this could take 15-20 seconds, longer with EL34s. If your caps aren't rated for the high "surge" voltage, they might go pop after a while.

Solutions:

Best solution: Use a GZ34 or similar slow-warm up tube. These take 30sec to start working, by which time the output tubes are working, and there is no high voltage surge.

Another solution is to use a "standby" switch, in the secondary centre tap of the mains transformer. This lets you control when the B+ is applied, and you can warm your tubes up before you apply the B+. I use this with diode rectifiers.

Or, if you have the caps available, put two 450V caps in series with a 220K 1W resistor across each. This (sort-of) gives you a 900V cap of half the capacitance. However, you need twice the caps.

The resistors in the schematic are probably meant to be adjusted to get your voltages spot on. Probably in the range of 100-300 ohms, 10W would be recommended. These will not fix the surge in any way.

Also be careful! Those voltages are quite nasty, make sure all caps are discharged before you work on the unit!
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Old 27th December 2003, 08:52 PM   #5
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Before we go further, some questions:

A couple of seconds and over 450V suggests a directly heated rectifier. What type rectifier tube are you using ??

What voltage rating and Cap value are the power supply smoothing caps ?

rich
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Old 27th December 2003, 08:55 PM   #6
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Hi,

Quote:
Best solution: Use a GZ34 or similar slow-warm up tube. These take 30sec to start working, by which time the output tubes are working, and there is no high voltage surge.
The 5-20 uses a GZ34/5AR4.

Cheers,

EDIT:

Original diagram is here:

MULLARD 5-20
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Old 27th December 2003, 10:00 PM   #7
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Frank......got variants of two sim versions of 20W Mullard circuits both with GZ34. 1st(1960) shows 50uF 450V on DC ex rectifier and 2nd (undated) uses 8uF; all other components are same.

There is also a Philips version which has uprated 30W Pout.

I presume the version with 8uF+ 10H choke+8uF is the original Mullard as 50uF's electyt's weren't around at that time. Correct ??

However it appears the original 5-20 circuit parts list C12_C15 uses 8uF paper types then these would be rated at 500V.
Resistors R26_R27 in each rectifier anode leg are shown as 6W min rating.

rich
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Old 27th December 2003, 11:54 PM   #8
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Hi,

Quote:
I presume the version with 8uF+ 10H choke+8uF is the original Mullard as 50uF's electyt's weren't around at that time. Correct ??
I think so too.

I have another one using the GZ32 as a rectifier. Just after it, it states a B+ of 465 V so these 8F caps should have a 500V rating at least.

So I assume the resistors need to be adjusted so we have a raw B+ of 465V.

The reason they weren't specified was that they need to be adjusted according to the xformer specs.

Not having a tuner and/or preamp taking their juices from the same supply would also make the B+ go higher by a few volts too.

Cheers,
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Old 28th December 2003, 12:29 AM   #9
icebear is offline icebear  Norway
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Default Rectifier

Hi

I'm using the 5AR4.

After the choke I'm using two caps at 220uF/400V in series. Then there is 15k resistor. Then a 500v/47uF, a new resistor (270k) and the last cap same as the other.
Does these "unoriginal" cap values make any difference?

bjrn
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Old 28th December 2003, 12:51 AM   #10
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Hi,

Quote:
Does these "unoriginal" cap values make any difference?
They very well could as your PS becomes more "efficient" the voltage will tend to rise.

There's also the risk that the choke will go into saturation at switch on.

What's the value of the first cap just after the rectifier?

Cheers,
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