Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: billcreller on September 22, 2015, 07:52:41 pm
-
The Sunn amp I fixed up a while back, is on my bench, with ZERO audio output. I first checked the high voltage from the rectifier, which is like none....
The 5AR4 is lit, shows like 800 + volts across the plates, but no DC ......
I didn't think it was possible for a rectifier tube to quit working that way. Maybe something else is going on......(?) I don't have another tube to try. This tube was new when I did the re-cap on the amp. It's a Sovtek brand...
Any words of wisdom ??
It's a 200S model by the way....
-
No real words of wisdom other than getting another rectifier and do a non-loaded test to try to get your DC back. I would be looking for a short in your power supply while waiting on a new rectifier to come in.
-
You could briefly run a 5U4 (3 amps vs 2 amps filament) on the theory that everyone has a 5U4 or two lying around and it's generally speaking a cheap tube to blow up. Use a lamp limiter in case the filter caps are shot!
I have a feeling that Russian/Chinese GZ34/5AR4 are subject to odd failure modes. That is just from reading several posts here, not from personal experience.
-
I have a few 5U4s, and I can re-wire the socket to try one easy enough.....Sure seems like a weird situation.......
Thanks for the replies :icon_biggrin:
-
No rewiring needed, just plug it in and take your measurements reasonably quickly. HVAC to pins 4,6
5 v fil to pins 2,8
Don't worry about which of 2,8 the amp pulls B+ from. Doesn't matter, in this instance.
-
I put a 5U4GC in there, and it works normal. I'll have to get a new 5AR4, but not a Sovtek brand.....
-
The best GZ34/5AR4 being made today is the one from Ruby Tubes. Here is a quote I found from Stan (km6xz on Ampage) from Russia and he had praise for this tube.
Greg
The Ruby 5AR4/GZ-34 is made in Tom's own factory as a unique part. We modeled it after one example of a new old Mullard tube he had and tried various versions of materials and spacing until it came very close, closer than any other makes of 5AR4 by the other major brands in the 50s and 60s. That Mullard tube really performed differently than others with the same designation, with lower forward drop, higher current etc, the more it was tested the more impressed I was with the original. If you have an amp that uses one, or are designing one, that is the only tube rectifier I would recommend for a guitar amp.. Smaller loose sounding harp or blues amps could use the 5U4 for a looser compression effect that would be nice. The higher impedance of that 5U4 means less voltage regulation so the difference in response and note recovery.
-
Yes, I read that as well. I use 5AR4's almost exclusively and have found none that have decent reliability, including the Ruby. There just is no substitute for the real thing.
-
Clyde, by "the real thing" I assume you mean an RCA/GE/Sylvania 5AR4/GZ34.
Do you have a similar opinion on 5V4s? (eg; domestic NOS or good used only)
Actually, I am not aware of new Russian/Chinese built 5V4, but I assume they exist.
-
Actually, I am not aware of new Russian/Chinese built 5V4, but I assume they exist.
Doug sells them. I bought 1 and the vdc was spot on with a couple of NOS 5V4's I have. :icon_biggrin:
Here's the link, scroll down;
http://hoffmanamps.com/MyStore/catalog/parts11.htm (http://hoffmanamps.com/MyStore/catalog/parts11.htm)
-
Clyde, by "the real thing" I assume you mean an RCA/GE/Sylvania 5AR4/GZ34
Do you have a similar opinion on 5V4s? (eg; domestic NOS or good used only)
Actually, I am not aware of new Russian/Chinese built 5V4, but I assume they exist.
Mullard variant GZ34's as well.
I use only NOS 5V4's when required, I like this tube. I have new production Sovtek 5V4's from New Sensor but they're not like the NOS-voltages are different and they ramp up rather quickly.
-
I see NOS RCA/Sylvania 5V4s on ebay for very reasonable ($10) now and then. GZ34's, they want $75 for in Amperex, Mullard, GE, any known brand. And old NOS, sometimes $500!!