Hi all,
I have a 3-year old JTM45 Metropoulous/Valvestorm kit that had a failure a week ago. When I turned the power on there was a loud buzz (60Hz?) that faded away after about 20 secs. I thought I had a pedal or cable issue but it was the amp.
The first thing i found - no HT voltage, so the rectifier failed. This was a Sovtek 5AR4.
I replaced that, checked it, and then put in the preamp tubes - no problems. I inserted the KT66's and as soon as I turned off the standby there was again the loud 60Hz oscillation so I immediately turned the amp off. SO I have at least one bad output tube. I replaced the KT66's with a pair of spare EL34s that I had, biased it and it seems to be fine.
This is the 2nd time I've had a rectifier fail. The first time, 2 years ago, it just farted and blew the HT fuse.
The only thing that I can say is "different" about this Amp is that the PS voltage is pretty high at 500VDC (measured at the choke) compared to the expected voltages of 400 - 425. I assume that this is due to the transformer, since we don't have particularly high line voltage in the house. The tubes were biased for 35mA.
My question - is this high HT voltage an issue (for the rectifier)? Is it worth doing something to lower the HT voltage? And yes, the transformer is wired correctly. Sorry but I do not remember the brand but it is a "quality" brand.
FWIW the EL34's sound great - they saturate nicely...
TIA/Tim
I have a 3-year old JTM45 Metropoulous/Valvestorm kit that had a failure a week ago. When I turned the power on there was a loud buzz (60Hz?) that faded away after about 20 secs. I thought I had a pedal or cable issue but it was the amp.
The first thing i found - no HT voltage, so the rectifier failed. This was a Sovtek 5AR4.
I replaced that, checked it, and then put in the preamp tubes - no problems. I inserted the KT66's and as soon as I turned off the standby there was again the loud 60Hz oscillation so I immediately turned the amp off. SO I have at least one bad output tube. I replaced the KT66's with a pair of spare EL34s that I had, biased it and it seems to be fine.
This is the 2nd time I've had a rectifier fail. The first time, 2 years ago, it just farted and blew the HT fuse.
The only thing that I can say is "different" about this Amp is that the PS voltage is pretty high at 500VDC (measured at the choke) compared to the expected voltages of 400 - 425. I assume that this is due to the transformer, since we don't have particularly high line voltage in the house. The tubes were biased for 35mA.
My question - is this high HT voltage an issue (for the rectifier)? Is it worth doing something to lower the HT voltage? And yes, the transformer is wired correctly. Sorry but I do not remember the brand but it is a "quality" brand.
FWIW the EL34's sound great - they saturate nicely...
TIA/Tim
One thing that can cause rectifier failures is if the electrolytic caps used to filter the DC are stupidly oversized and connected right to the cathode of the 5AR4. If the amp was modified by the addition of capacitance to the power supply in this way, I'd remove the excess C. You don't want more than about 40uF on the 5AR4's cathode. What kills the tube is excessive peak currents into an oversized cap. After a choke or resistor in series with the 5AR4's output, then you can add more C.
Welcome to the Tubes / Valves Threads portion of diyAudio.
Perhaps the people on Instruments & Amps Threads portion of diyAudio have experienced the same problem with that Guitar Amplifier.
My first guess is two-fold, most of which have been mentioned many times:
1. Too Much capacitance that the 5AR4 has to drive, combined with too little series resistance (primary DCR, secondary DCR, and fixed series resistors before the first cap)
2. A 5AR4 that does not have a perfectly smooth and equal depth cathode coating.
Eventually, a 5AR4 can get tired of abuse and fail.
Then, you replace it with a new 5AR4 that is not a quality tube, like most old ones were.
Bingo, same problem even though the new tube hardly had the chance to warm up once.
Thermistors are temperature sensors, and do not fix inrush problems.
There are devices that Are made special, to fix inrush problems.
Take a good look at all of your amplifier characteristics, primary DCR, secondary DCR, Secondary rms volts, the uF value of the First filter capacitor, and the voltage rating of the first cap, versus 1.4 x the secondary voltage.
Then, check all the 5AR4 data sheets you can find at "Franks tube data".
Compare whether your amplifier meets or exceeds the 5AR4 spec. (many New 5AR4 tubes are Wimps).
Hopefully, if you used EL34 tubes, you tied pin 1 to pin 8 (do not let the EL34 suppressor grid float).
Perhaps the people on Instruments & Amps Threads portion of diyAudio have experienced the same problem with that Guitar Amplifier.
My first guess is two-fold, most of which have been mentioned many times:
1. Too Much capacitance that the 5AR4 has to drive, combined with too little series resistance (primary DCR, secondary DCR, and fixed series resistors before the first cap)
2. A 5AR4 that does not have a perfectly smooth and equal depth cathode coating.
Eventually, a 5AR4 can get tired of abuse and fail.
Then, you replace it with a new 5AR4 that is not a quality tube, like most old ones were.
Bingo, same problem even though the new tube hardly had the chance to warm up once.
Thermistors are temperature sensors, and do not fix inrush problems.
There are devices that Are made special, to fix inrush problems.
Take a good look at all of your amplifier characteristics, primary DCR, secondary DCR, Secondary rms volts, the uF value of the First filter capacitor, and the voltage rating of the first cap, versus 1.4 x the secondary voltage.
Then, check all the 5AR4 data sheets you can find at "Franks tube data".
Compare whether your amplifier meets or exceeds the 5AR4 spec. (many New 5AR4 tubes are Wimps).
Hopefully, if you used EL34 tubes, you tied pin 1 to pin 8 (do not let the EL34 suppressor grid float).
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Another factor is that most recent production 5AR4/GZ34 is horrible crap. The very latest Reflector products *may* be okay, but it remains to be proven. They've got a lot of 'splainin to do. You'd think a rectifier would be easy to make, but apparently not. Or maybe somebody picked a much lesser device from available stock and called it a GZ34 for sale to "toob guys".
All good fortune,
Chris
All good fortune,
Chris
This is a known problem with any JTM45 or late generation Bassman. It is caused by hot-switching the rectifier, because the standby switch is between the recto and the reservoir. When you throw the standby, the recto gets absolutely hammered by the inrush current. Early Bassman amps 'got away with it' because they used two rectos in parallel, with a much smaller reservoir. The later 5F6 pushed everything to 11, and the JTM45 copied it. Modern production rectos exacerbate the problem.
Best solution is to throw the standby switch in the trash.
Best solution is to throw the standby switch in the trash.