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?
Any thoughts?
janneman said:
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.
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..
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..
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.
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.
How old is that Russian 5AR4 that arced over? There's plenty of feedback that suggests more recent Russian and Chinese 5AR4s have improved significantly. FWIW, Gordon Rankin likes recently made Russian 5AR4s. JJ (Slovakia) has also entered the 5AR4 "arena" and has gotten decent reviews.
In NOS, Matsushita may be the best deal. Mullard set up the Jap. production line and the product is a good copy of the Mullard original. Durability might not be as good, but the sonics ARE there.
BTW, forward drop in 5AR4s is unusually low for a vacuum rectifier. Some damper diodes might do a little better, but the 5AR4 is champ among the 5 V. filament types, AFAIK. 5Y3s and 5R4s have the largest forward drops.
In NOS, Matsushita may be the best deal. Mullard set up the Jap. production line and the product is a good copy of the Mullard original. Durability might not be as good, but the sonics ARE there.
BTW, forward drop in 5AR4s is unusually low for a vacuum rectifier. Some damper diodes might do a little better, but the 5AR4 is champ among the 5 V. filament types, AFAIK. 5Y3s and 5R4s have the largest forward drops.
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