I don't mean B+ comes after; I mean inrush current in primary that is reflected sum of inrush currents in secondaries is limited until DC voltage on filaments is enough to click the relay.
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Zenith, I saw your Piedmont and wondered if it was the town of or the area of.. It's cool to know someone close by, for sure...
Back to the problem...
It's easy to fix the problems created by mods of the ST-70. The original wasn't broken, so it didn't need fixed. Don't monkey with success, put it all back like the original and just listen to the sweet, sweet, music.
Perhaps the best advice on this thread regarding the original problem!
In fact, I'm listening to an ST-70 as I write this. It's a Bob Latino kit I purchased and built just a little over three weeks ago. The only differences in the power supply between mine and an original are a larger transformer and greater capacitance. That's it. There's no fancy "soft-start" other than the recifier tube itself.
I do like Wavebourn's switch-shunted series resistor, though. I'm building an RH84 (I just happened to have the parts, but mainly it's a last-ditch effort to try to like SE amps), and will probably incorporate this feature.
Just enjoy putting 'er together, and everything else will fall into place. Besides, if you're like me, you'll screw up some other small detail and create a real problem! 😀
And if you do, there are lots of us on this forum ready to help...
Ze...
If I recall you have the 80uf/40uf etc., etc., cap, with the 40uf section used for the input cap. That 40uf is probably the ragged edge limit where you can sleep without wondering if the rectifier is going to arc as long as you use a quality tube. Little discussed on this or any forum I've read is the formula for understanding the inherent current limiting DCR of the power tranny and the limits of the GZ34 with respect to the input capacitance. The data sheet on the GZ34 specifies a 100R DCR total needed for reliable operation when 40uf input cap is used. The stock ST-70 has a total of about 40R, but, you are allowed to have less than 100R DCR if some choke is also used and less than 40uf. The stock ST-70 uses 30uf input capacitance so it is reliable. Any added capacitance requires membership in the Experimental Aircraft Association in my opinion. With today's higher line voltage and "cheap" import rectifiers, the slippery slope starts at the end of our toes.
I've got a little Magnavox 6BQ5 SE on my bench hooked up to some 8 ohm Technics and I think it sounds on par with my Sansui G-6000 if not better.
Maybe we could put a BBQ together this fall...?
When I did a project with a SS power supply, I separated the heater supply from the HV DC supply. That way, I can power the heaters, let the cathodes warm up before turning on the HV. The main consideration here was to avoid any over volting since the design used DC coupling (first voltage amp to the LTP splitter/second voltage amp, and cathode follower grid drivers to the 807 finals). With all cathodes good to go, there won't be any over volting of small signal control grids, or exceeding Vhk ratings.
Cathode stripping really isn't a consideration unless you're dealing with the extreme voltages of max legal ham rigs, or something like an 845 SET that operates at ~1.0KV. Otherwise, it would be a problem when using types like the 5U4 or 5Y3 for the HV DC: the plates get the full AC voltage from the PS secondary before the filaments warm up. I haven't heard of any failures due to possible cathode stripping. With hollow state diodes, the real killer seems to be excessive reservoir capacitance in CLC or CRC power supplies or worn out HS diodes that've lost emission. Haven't had a problem with 5U4GBs and 34uF (68uF / 300V capacitors in series with 100K equalization resistors) up front with a 220uF / 450Vdc filter capacitor behind a 7H ripple choke.
Cathode stripping really isn't a consideration unless you're dealing with the extreme voltages of max legal ham rigs, or something like an 845 SET that operates at ~1.0KV. Otherwise, it would be a problem when using types like the 5U4 or 5Y3 for the HV DC: the plates get the full AC voltage from the PS secondary before the filaments warm up. I haven't heard of any failures due to possible cathode stripping. With hollow state diodes, the real killer seems to be excessive reservoir capacitance in CLC or CRC power supplies or worn out HS diodes that've lost emission. Haven't had a problem with 5U4GBs and 34uF (68uF / 300V capacitors in series with 100K equalization resistors) up front with a 220uF / 450Vdc filter capacitor behind a 7H ripple choke.
That and inductor input filters where the initial voltage can surge way up before the filaments/heaters are hot enough to promote emission. The capacitors can see really high voltages according to the simulations I've run.
20to20:
You know, I've made the same observations about that cap arrangement. The kit shipped with a Ruby 5AR4 that has worked flawlessly - so far. It's even survived a couple of "hot-switching" events with no apparent arcing. Thankfully the power transformer that ships with the kit is wound for a 120V primary, as Progress Energy keeps our mains at a rather brisk 121 V or so.
Take a look at this amp if you want to see a rectobulb pushed to its limits (look for the 60W 6550 Williamson Schematic). A look at the specs for a 5V4 will tell you that the rectifier would live a short and brutal life in this circuit. Yet, I've had correspondence with at least one person on this forum who says his 5V4's have only been replaced two or three times since he built a pair of monoblocks to this circuit in the late '50's. He says they're still operational today. Go figure!
I "liberated" one of those Magnavox amps from a console awaiting curbside trash pickup. The Leatherman tool paid for itself that day. 😎
It's also funny you should mention the EAA. As a young teen in the early '80's I absolutely lusted for a Mitchell U2. The plans are still available, and I may buy them yet!
I'll be willing to wager my wife and I have crossed paths with you at some of the local hamfests and antique radio swap meets. Send me a PM...
Miles:Take a look at this amp if you want to see a rectobulb pushed to its limits (look for the 60W 6550 Williamson Schematic). A look at the specs for a 5V4 will tell you that the rectifier would live a short and brutal life in this circuit. Yet, I've had correspondence with at least one person on this forum who says his 5V4's have only been replaced two or three times since he built a pair of monoblocks to this circuit in the late '50's. He says they're still operational today. Go figure!
I "liberated" one of those Magnavox amps from a console awaiting curbside trash pickup. The Leatherman tool paid for itself that day. 😎
It's also funny you should mention the EAA. As a young teen in the early '80's I absolutely lusted for a Mitchell U2. The plans are still available, and I may buy them yet!
I'll be willing to wager my wife and I have crossed paths with you at some of the local hamfests and antique radio swap meets. Send me a PM...
I've separated heater and HV supplies like that, too. Works great if you have the chassis space for the extra transformer. Enjoyed your website, BTW. I've got some 807's floatin' 'round somewhere just waiting to be lit.
Gimp:
You're absolutely right; those high voltages show up in real life, too - not just in simulations. I breadboarded a 6550 Williamson, then hooked it up "Tubelab Style" to about five of those EL-Cheep-O Harbor Freight DVM's before the initial power-up (I hesitate to use the term "fire-up" in this context). I was astounded to see the voltage on that first filter cap, a 10 µf 500V electrolytic, rise to nearly 700 volts before settling down to around 465! 😱
I promptly replaced that one with two series-connected 22 µf 450 V units, with 220 k equalizing resistors...
I promptly replaced that one with two series-connected 22 µf 450 V units, with 220 k equalizing resistors...
I have often worried about cathode stripping with solid state rectifiers, but I have come to the view that this is not a real issue with small audio valves.
FWIW, the cathodes on the GZ34 rectifiers fitted to my Quad valve amps heat up much more quickly than the cathodes in the KT66 output valves. This means that the HT voltage peaks at around 435 volts for a few seconds before the KT66's start to draw any appreciable current.
These amplifiers are now well over 50 years old, and are used several times a week, so I can only presume that Peter Walker knew what he was doing! 😉
Admittedly the valves don't last forever, (the last set of KT66's were fitted around 15 years ago), but this seems to suggest that cathode stripping is not a problem with this set up at least.
As a point of interest, I got a stock of GEC KT66's from a factory that I worked at in the 1970's. They had a big PA system which used 24 KT66's, and were changed annually. The amplifiers were part of the fire alarm system, and so were never ever switched off. I just happened to be in the right place when this equipment was serviced one year!
Gravissima
FWIW, the cathodes on the GZ34 rectifiers fitted to my Quad valve amps heat up much more quickly than the cathodes in the KT66 output valves. This means that the HT voltage peaks at around 435 volts for a few seconds before the KT66's start to draw any appreciable current.
These amplifiers are now well over 50 years old, and are used several times a week, so I can only presume that Peter Walker knew what he was doing! 😉
Admittedly the valves don't last forever, (the last set of KT66's were fitted around 15 years ago), but this seems to suggest that cathode stripping is not a problem with this set up at least.
As a point of interest, I got a stock of GEC KT66's from a factory that I worked at in the 1970's. They had a big PA system which used 24 KT66's, and were changed annually. The amplifiers were part of the fire alarm system, and so were never ever switched off. I just happened to be in the right place when this equipment was serviced one year!
Gravissima
I have often worried about cathode stripping with solid state rectifiers, but I have come to the view that this is not a real issue with small audio valves.🙄
Just a thought!
If cathode stripping is not a problem: Why is it a problem to run heaters at low voltage as long as the tube works..Also % allowed variation (over voltage I can see), under voltage? O.K so the tube may not work as well as it should (Low emission). Would this cause damage?
I have never seen cathode stripping cause a problem, however I always think that the "Old techs must have known more than we do"? I have seen a tube get destroyed with high back E.M.F from coils!
Regards
M. Gregg
Just a thought!
If cathode stripping is not a problem: Why is it a problem to run heaters at low voltage as long as the tube works..Also % allowed variation (over voltage I can see), under voltage? O.K so the tube may not work as well as it should (Low emission). Would this cause damage?
I have never seen cathode stripping cause a problem, however I always think that the "Old techs must have known more than we do"? I have seen a tube get destroyed with high back E.M.F from coils!
Regards
M. Gregg
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You do do that to a point, but at reduced temperatures the cathode is more susceptible to surface poisoning by combination with stray gas.
(Although the effect is reversible by 'reactivating' the cathode at a high temperature for a short period)
A true story!
I asked an amp tech many years ago about stripping he said!
Do I understand how sacrificial electrodes work in electroplating.
Why do I think a valve gets reduced emission after long period of use!
Then he said your B+ rail will rise to high levels with HT on and no heaters.
Why was I surprised to see Hi HT when my meter measures RMS AC volts and peak DC and the caps will climb to the highest transient on the AC waveform!
Then he said can you electro plate in a vacuum with high voltage!
Electrons are negative and flow from cathode to anode. Then he said if the tube gets low heater volts and high voltage possibly exceeding the working voltage of the tube what will happen!
Then he said wait for the cathode resistor on the O/P tubes to blow as you apply ht with no magnetizing current in the Output TX and flash over through the output tubes due to the high HT. Then the power caps blow due to the exceeded voltage which also happens when the heater supply fails!
Also tube rectifiers loose more volts than semiconductors even higher HT.
I am still interested to see!
Regards
M. Gregg
I asked an amp tech many years ago about stripping he said!
Do I understand how sacrificial electrodes work in electroplating.
Why do I think a valve gets reduced emission after long period of use!
Then he said your B+ rail will rise to high levels with HT on and no heaters.
Why was I surprised to see Hi HT when my meter measures RMS AC volts and peak DC and the caps will climb to the highest transient on the AC waveform!
Then he said can you electro plate in a vacuum with high voltage!
Electrons are negative and flow from cathode to anode. Then he said if the tube gets low heater volts and high voltage possibly exceeding the working voltage of the tube what will happen!
Then he said wait for the cathode resistor on the O/P tubes to blow as you apply ht with no magnetizing current in the Output TX and flash over through the output tubes due to the high HT. Then the power caps blow due to the exceeded voltage which also happens when the heater supply fails!
Also tube rectifiers loose more volts than semiconductors even higher HT.
I am still interested to see!
Regards
M. Gregg
All I read here is your "amp tech" asking you a lot of questions, leaving it up to your imagination to fill in blanks that he couldn't answer with definitive recognized facts.
Hear hear!
I think we need to remember that in the heyday of valve equipment, there were a lot of 'valve swappers' working in repair workshops all over the place. In many cases their knowledge extended little further than simply swapping the most likely valves until the equipment worked again. If the problem was more 'technical' (i.e. an open circuit resistor), the equipment would be given to somebody more experienced, who knew how to use an Avometer properly!
I fondly remember the TV repair man coming around to our house in the early 1960's to fix our black and white television. He always wore a white coat, and brought a big suitcase full of valves, which he would swap until the set worked. He was clearly an 'expert', and nobody would dare question his wisdom.
By the late 1960's I was taking a keen interest in electronics, and I remember telling the man that I thought the line output transformer had failed, (symptoms of no picture, a high frequency rasping sound and a strong smell of ozone). He gave me a blank look, and said I was not old enough to understand, but had to take the set away for repair. Oddly enough, the old part looked remarkably like a line output transformer to me. 😉
Much the same thing happens today with 'module swappers'; except that it is rarely economic to repair modern equipment. 🙄
Nigel.
I agree,
However I have seen all the things he said except cathode stripping. This guy worked on very complex equipment including Xray! Maybe the voltage has not been high enough. However I have never seen it!
We are talking 20+ years ago and he is not with us now!
Regards
M. Gregg
However I have seen all the things he said except cathode stripping. This guy worked on very complex equipment including Xray! Maybe the voltage has not been high enough. However I have never seen it!
We are talking 20+ years ago and he is not with us now!
Regards
M. Gregg
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