With todays higher home voltages, are we better off using a 5U4 in place of the 5AR4 in our Dynacos? I had to keep adding resistance on the power cap to lower the B+ to my VTA board, and what about the rest of the amp as far as the higher voltage?
4 posters
Tube Rectifier
Bob Latino- Admin
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Location : Massachusetts
On an ST-70 you want to have about 425 - 450 VDC B+ voltage off pin 8 of the rectifier tube. If your line voltage is high then - yes - it might be best to use a 5U4 in the ST-70 amp in place of a 5AR4. What I would do is first check the B+ DC voltage off pin 8. If its over 450 VDC with the 5AR4 then I would use a 5U4. The nice thing about 5U4's is that it is still easy to get a quality USA made (RCA, GE, Sylvania) tube at a reasonable price. Many of the early '60's tube color TV's used a 5U4 rectifier so there are many good 5U4's still in circulation.
Bob
Bob
GP49- Posts : 792
Join date : 2009-04-30
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Beware, though: the 5U4 filament current is 3.0A; the 5AR4, 1.9A.
Since the power transformer in the stock Stereo 70 is marginal to begin with, you probably do
not want it to have to continuously supply an additional 1.1A.
Since the power transformer in the stock Stereo 70 is marginal to begin with, you probably do
not want it to have to continuously supply an additional 1.1A.
rudes- Posts : 63
Join date : 2012-03-07
Sorry...forgot to mention this is for a MK3 amp with VTA board. B+ needs to be 400-440v. With a 10K resistor instead of the 6.8K I'm at 440v. I also find I need to turn the bias pot all the way clockwise on one tube to get 40ma. If I reduce the B+ should take care of this. I just didn't know if I was better off just reducing the voltage to the driver board or the whole amp? Has anyone used a 5U4 with the MK3?
Bob Latino- Admin
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Location : Massachusetts
rudes wrote:Sorry...forgot to mention this is for a MK3 amp with VTA board. B+ needs to be 400-440v. With a 10K resistor instead of the 6.8K I'm at 440v. I also find I need to turn the bias pot all the way clockwise on one tube to get 40ma. If I reduce the B+ should take care of this. I just didn't know if I was better off just reducing the voltage to the driver board or the whole amp? Has anyone used a 5U4 with the MK3?
If you are still getting 440 VDC to the driver board with a 10K in place of the 6.8K resistor, then your line voltage is probably high? Have you checked your line voltage?
Bob
rudes- Posts : 63
Join date : 2012-03-07
Around 120v. Also using SDS board.
Last edited by rudes on Wed Dec 19, 2012 8:57 am; edited 1 time in total
rudes- Posts : 63
Join date : 2012-03-07
Also, is there a 5AR4 out there that puts out slightly less voltage than the others?
rudes- Posts : 63
Join date : 2012-03-07
After further testing I got the B+ on the VTA @ 410v with a 5U4GB and 420v with 5AR4. The output trans center tap voltage is 522v & 550v. The original amp calls for 480v. Is this increase OK?
Bob Latino- Admin
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I think you will be OK with the 5U4 in there since you have a Mark III. As GP49 has mentioned, the 5U4 does draw more current than a GZ34. Since the Mark III is a single channel amp, I don't feel the extra current draw of the 5U4 will cause any problems in your Mark III. Your tubes will last longer with the lowered B+ ...
Bob
Bob
rudes- Posts : 63
Join date : 2012-03-07
Thanks Bob. Do you feel I should use a surgistor on the line cord since the 5U4 has a harder turn on than a 5AR4 or will I be OK?
Bob Latino- Admin
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rudes wrote:Thanks Bob. Do you feel I should use a surgistor on the line cord since the 5U4 has a harder turn on than a 5AR4 or will I be OK?
The 5AR4 has an indirectly heated filament while the 5U4's filament is directly heated. As such, a 5U4 does warm up a little faster and does draw more current. IMHO, though, a surgistor isn't really needed with any tube rectifier. I have used 5U4 rectifiers in ST-70's and Mark III's quite a few times over the years without incident.
Bob
GP49- Posts : 792
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rudes wrote:Sorry...forgot to mention this is for a MK3 amp with VTA board.
Okay, then the 5U4 is safe. The Mark II with the identical power transformer was SPECIFIED
with the 5U4.
rudes- Posts : 63
Join date : 2012-03-07
OK. Was worried about the power trans. and additional current with the 5U4GB tube and also the fact that I am now heating 2 tubes on the driver board instead of one.
rmyauck- Posts : 9
Join date : 2012-02-18
Any negs. to using ICL's when using a directly heated rect. tubes or otherwise? Preferred position prim. or secondary leads? Thought it might help the tube filaments.
Could the higher B+ be compensated for in reduced tube life by lowering the dissipation by lowering the bias to where it still sounds good?
Randy
Could the higher B+ be compensated for in reduced tube life by lowering the dissipation by lowering the bias to where it still sounds good?
Randy