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Old 19th September 2005, 11:12 AM   #1
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Default Dynaco rectifier replacement?

Just wondering if the Dynaco would take a 5R4 or 5U4 rectifier in place of the stock 5AR4. I have plenty of the 5R4's and 5U4's but no extra 5AR4's. I thought about SS rectification but then my B+ exceeds the rating of the power supply caps. It would also push the B+ above 300V on the plates of my 12AT7's in my non Dynaco front end and driver board.

Any thoughts?
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Old 19th September 2005, 11:23 AM   #2
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I think the data sheets would make it clear if a) they fit physically in the same socket and b) if they can support the same or larger voltage and current. Or is there some other magic factor I missed??

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Old 19th September 2005, 11:25 AM   #3
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I would assume the current draw might be higher on the 5R4 and 5U4. I don't know if the Dynaco transformer could handle it.
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Old 19th September 2005, 12:57 PM   #4
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Quote:
Originally posted by burnedfingers
I would assume the current draw might be higher on the 5R4 and 5U4. I don't know if the Dynaco transformer could handle it.


Ahh, you mean the heater current? How much more is it?

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Old 19th September 2005, 01:29 PM   #5
Sherman is offline Sherman  United States
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Quote:
Originally posted by janneman




Ahh, you mean the heater current? How much more is it?

Jan Didden

5R4 heater is 2A, the 5AR7 is 1.9A and the 5U4 is 3A. I have no idea what a Dynaco transformer can supply.

I believe the 5U4 voltage drop is 44V and the voltage drop for the 5R4 is 60+V so your B+ may go up a bit with a 5U4. The voltage drop for a 5AR7 is about the same as the 5R4 (within a couple volts I think).

It appears the 5AR7 is a better replacement if matching original specs is the goal.
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Old 19th September 2005, 01:53 PM   #6
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I wouldn't worry about a 10% delta in heater current, but I wouldn't make the jump from 2 to 3 amps without checking the actual heater voltage with the higher load. If it stays up it is probably OK.

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Old 19th September 2005, 02:49 PM   #7
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5R4 will work. B+ voltage will be lower, but since it's directly heated, voltage will be high until output tubes warm up. Cathode stripping is not a problem with fixed bias, so filter cap is only worry... they're designed to take SHORT period overvoltage.

Rebiasing required, of course. Output power (especially bass) will be reduced somewhat, not a problem with efficient speakers. In return expect better life from your EL34s..
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Old 19th September 2005, 03:53 PM   #8
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The manual I have for my ST-70 says that you can use the 5U4 as a replacment,but i'm not sure they recommend it indefinatly,more like as a "back up" rectifier untill you can get a new 5AR4/GZ34.

I've been using a NOS RCA 5U4 in mine since my Sovtek 5AR4's kept arcing over. Seems to work just fine,but the Power X-fmr might run a tad warmer.
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Old 19th September 2005, 04:50 PM   #9
amperex is offline amperex  United States
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Some audiophiles prefer the lower voltage output of the 5R4 with the higher line voltages now-a-days.
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Old 19th September 2005, 05:33 PM   #10
Sherman is offline Sherman  United States
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Quote:
Originally posted by amperex
Some audiophiles prefer the lower voltage output of the 5R4 with the higher line voltages now-a-days.

Good point. If I simulate something using PSUD for instance I always get higher voltages when I build it since my mains run 120 rather than 115.
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