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Old 11th November 2010, 12:32 AM   #1
ron944 is offline ron944  United States
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Default 5aR4 Rectifier Problem, Help! Little Long

I recently purchased a Mullard 5aR4 00 Getter Rectifier for my Dynaco ST70. I was told by the seller that I know that it tested as new on both sides. When I received the tube it tested 0 on both sides. I figured mabe there was a problem with my tube tester and after talking with him I installed it alone in the Dynaco. When I turned it on it flashed blue, I turned it off and the back on, and again it flashed blue then settled down. Pins 2 & 8 read 536vdc (manual shows 435vdc) pins 4 & 6 read 460vdc (manual show 360). My ac is 123v show I expect readings to be a little high. Is this tube bad and if so would I get these kinds of readings or none at all. Thanks in advance.
Ron
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Old 11th November 2010, 12:57 AM   #2
kevinkr is offline kevinkr  United States
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I have worked on a lot of ST-70 and have never seen a properly functioning unit with less than 490V (~120V line) at the first filter cap in the pi-filter. I have no idea where they got the 435V value so often quoted.

The voltage at the output of your rectifier is exceeding the rated working voltage of the stock or standard replacement electrolytics if fitted. Are the output tubes installed and properly biased?

I've had problems with arcing even in Teles and Mullard 5AR4 in this amplifier and recommend placing a UF4007 rectifier diode in series with each plate, this seems to prevent the arcing at turn on and the resulting destruction of the rectifier.

I can't vouch for the condition of your rectifier, but significant blue glow may be an indication of excessive gas or just fluorescence in the envelope glass. Where is the blue appearing? You might consider returning this tube if you have any question at all.
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Last edited by kevinkr; 11th November 2010 at 01:00 AM.
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Old 11th November 2010, 03:38 AM   #3
ron944 is offline ron944  United States
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When I tested the rectifier I left all the other tubes out, just in case. Without a true load will the rectifier B+ be higher. When I uprgraded the amp about four years ago I replaced everything other than the three transformers and the choke. I also installed an SDS Labs Capacitor Board and individual Bias pots for each tube. Do you think I should install all the tubes and check the voltages again?
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Old 11th November 2010, 05:16 AM   #4
kevinkr is offline kevinkr  United States
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If you are reasonably confident that the rectifier is OK I would install the rest of the tubes - only way to get valid voltage measurements. I believe the SDS board is rated well above the voltage you measured. Truthfully I haven't experienced a rectifier damaging tubes, only supply caps and very rarely a power transformer if the rectifier develops an internal short. The reverse is not true however.. I do recommend installing the diodes I mentioned in a previous post.
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Old 11th November 2010, 06:03 AM   #5
cerrem is offline cerrem  United States
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You need to have all the tubes in the amp..since you need a load current to pull the B+ down to a sensible operating level..The B+ will be initially high until the amp warms up and pulls current...
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Old 11th November 2010, 10:41 AM   #6
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Quote:
Originally Posted by kevinkr View Post
If you are reasonably confident that the rectifier is OK I would install the rest of the tubes - only way to get valid voltage measurements. I believe the SDS board is rated well above the voltage you measured. Truthfully I haven't experienced a rectifier damaging tubes, only supply caps and very rarely a power transformer if the rectifier develops an internal short. The reverse is not true however.. I do recommend installing the diodes I mentioned in a previous post.
Hi Kevin,

The Triode Electronics (SDS) cap board's first section is 800 volt rated but the other 3 sections are only 500 volt rated. This is too low IMHO to use on the Dynaco ST-70 which undoubtedly has a surge above 500 VDC at turn on. It is interesting that Triode sells the CE 80, 40, 30, 20 cap and mentions that this cap, which is 475 volt rated, "is NOT high enough to be used in the ST-70 or MkIII amplifiers." See link below ..

80-40-30-20 uF at 475V Twistlock Capacitor at Triode Electronics

They are saying the 475 volts is not enough for the ST-70 but their 500 VDC rated cap board is high enough.

People tend to knock quad caps because older electrolytics do tend to go bad after years of not being used. The original Dynaco quad cap was a real weak spot on that amp. The ST-70 dates back to 1959 and some of these original quad caps are now 40 - 50 years old. You can't expect the quad cap to work properly after this many years.

Instead of the Triode SDS board, I favor the F&T quad cap which is made in Germany. This cap is rated at 550 volts continuous with a 600 volt surge rating on the first three sections and 525 volt rated on the 4th section. There is usually a dropping resistor between the 3rd and 4th sections anyhow that drops the voltage down to near 400 volts on the last section. I have used over 100 of these F&T quad caps in the last couple of years. Not one has failed. The F&T caps don't have the total capacitance of the Triode (SDS) board but having 500 uF on an ST-70 is a fine example of overkill. The ST-70 amp IMHO doesn't benefit from anything over 175 - 200 uF. I once put an extra 165 uF of capacitance off the second section of the quad cap raising the total capacitance to well over 300 uF. Even at high volume levels this ST-70 amp didn't play any differently with the added capacitance. You don't need 500 uF of DC power storage on an ST-70. Link below to the F&T caps ...

F&T quad caps

Dynakitparts stocks the F&T 550 volt KTL25 quad cap at the link below.

Dynakitparts 550 volt rated quad caps

Bob Latino
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Old 11th November 2010, 01:02 PM   #7
ron944 is offline ron944  United States
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Thanks for the replys, I can install the rest of the tubes and check the voltages. I am not sure the rectifier is okay due to the "0" readings on my tube tester. Should I check pins 2 and 8 for ac voltage also. If a rectifier is bad will it pass ac rather than converting it to dc? I guess I need to study up on the internal working of tubes.
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Old 11th November 2010, 01:16 PM   #8
20to20 is offline 20to20  United States
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When I turned it on it flashed blue, I turned it off and the back on, and again it flashed blue then settled down.
Hi Ron,

I'm curious about the arc you observed. Did it happen at the instant you threw the switch, or did it happen with a few seconds delay as the heater was coming up to operating temp? Were you cautious with the first power on because you didn't trust the tube and turned it off immediately and then energized it again when nothing exploded, and then saw the arcing?
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Old 11th November 2010, 01:16 PM   #9
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Quote:
Originally Posted by ron944 View Post
I turned it off and then back on...
It's generally not a good idea to "short cycle" a tube amp. After you turn the amp off, always wait for the tubes to cool before turning it back on again.

Do you have a known good 5AR4 to check in your tester? Does it give the expected readings? Maybe your tester is bad.
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Old 11th November 2010, 01:25 PM   #10
20to20 is offline 20to20  United States
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Quote:
Originally Posted by Ty_Bower View Post
It's generally not a good idea to "short cycle" a tube amp. After you turn the amp off, always wait for the tubes to cool before turning it back on again.

Do you have a known good 5AR4 to check in your tester? Does it give the expected readings? Maybe your tester is bad.
Sounds like a loose heater contact on the tester socket. Maybe heater didn't come up. I hate when that happens! Wiggle, wiggle, wiggle....
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