Thanks, I knew that I had seen this problem mentioned in the forum, and that the diode and inrush current limiter cured it.
how many watts is this resistor then.
so the charging of the capacitors at start-up calls for a moment of high current and this causes the tube to flash. when it flashes the fuse blows because the current is too high, i guess flashing is arcing. putting a resistor somewhere in this part of the circuit reduces the inrush current and slows this start up.
why do some tubes flash and others dont
Rectifier tubes all have a voltage drop because of the space between the cathode and the plate. In the GZ34 the voltage drop is extremely low for a tube because the electrodes are very close together. This makes it easy for an arc over in the GZ34. Manufacturers try to reduce this with coatings but if it's a cheap tube, you get what you pay for.
i get what i pay for? so if the distributor marks a tube up from $2.00 to $20.00 and the packaging advertises the tube to be of good quality and then it fails it is my fault? or if an allegedly good tube is 40 dollars do i get what i pay for?
i have payed very high prices for items and they were of very low quality.
this adage "you get what you pay for" is a convenient way to justify poor business practices and degenerate ethics and pass the responsibility from the principle-less profiteer onto the trusting and cooperative buyer. and ofcourse the audio industry is replete with this.
buy low and sell high, cut costs without remorse even if it means compromising the quality of the product. why? to make more money. why? so you can afford to buy ever lower quality and ever more expensive items. absurd?
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Ditto that. It #%$#%$ me off too. You'd think that all of the business we give Chinea that you could get for trading their low cost of production for a lower price, that you could get equal quality we expect. But it seems that they have learned to make things look nice, built cheaply, and reap a double benefit. I tried to build my house with screws from Chinea and I twisted the head off of 25% of the screws or stripped out the slots because of the cheap steel they dumped into america in the guise of product.
If you plan on using SS rectification in our version of the ST70, you can do one of two things.
1. Add an In-Rush Current Limiter like a CL-90 across the primary of the power transformer.
2. Add a stand-by switch. This can be accomplished by removing the choke wire that's connected to the (B+) pad on the capboard, connect that choke wire to one side of the stand-by switch, and connect the other side of that standby switch back to the (B+) pad on the capboard.
Best Regards,
-Chad at Triodeelectronics.com
anyone have an idea what the specs of the switch would have to be.
cl-90 is 2 amps cl-130 (available on ebay) is 1.6amps would this be enough you think?
1. Add an In-Rush Current Limiter like a CL-90 across the primary of the power transformer.
2. Add a stand-by switch. This can be accomplished by removing the choke wire that's connected to the (B+) pad on the capboard, connect that choke wire to one side of the stand-by switch, and connect the other side of that standby switch back to the (B+) pad on the capboard.
Best Regards,
-Chad at Triodeelectronics.com
anyone have an idea what the specs of the switch would have to be.
cl-90 is 2 amps cl-130 (available on ebay) is 1.6amps would this be enough you think?
I'd use a relay... the switch would have to be a 600vdc rated at minimum. I'd use a DPDT switch (two contacts in parallel).
Same with the relay, two or more contacts...
If you "bang" the choke with the second capacitor, there will be a large inrush current through the choke and the likelyhood that the voltage you are applying is higher than normal, watch for that. If you switch before the choke, then there is still the higher than normal voltage and current to charge the second cap AND the draw of the load...
I would opt for the SS rectifiers and a SINGLE diode in the form of a TUBE in series with the output of the SS rectifiers, or if you prefer after the filters (not quite as good). If you choose to use a standard rectifier tube, you can now parallel both sections for higher current rating.
This creates a time delayed conduction through the tube rectifier... there are various choices for tubes that do this job well.
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the CL-90 and CL130 are likely more than big enough - I don't recall if they need to be matched to the load for the resistance to drop fully or not... a spec sheet would reveal all. Larger current if the current through the device is largely irrelevant is probably better, but both are far in excess of the current required by the two channels at clipping...
_-_-bear
Same with the relay, two or more contacts...
If you "bang" the choke with the second capacitor, there will be a large inrush current through the choke and the likelyhood that the voltage you are applying is higher than normal, watch for that. If you switch before the choke, then there is still the higher than normal voltage and current to charge the second cap AND the draw of the load...
I would opt for the SS rectifiers and a SINGLE diode in the form of a TUBE in series with the output of the SS rectifiers, or if you prefer after the filters (not quite as good). If you choose to use a standard rectifier tube, you can now parallel both sections for higher current rating.
This creates a time delayed conduction through the tube rectifier... there are various choices for tubes that do this job well.
---------------
the CL-90 and CL130 are likely more than big enough - I don't recall if they need to be matched to the load for the resistance to drop fully or not... a spec sheet would reveal all. Larger current if the current through the device is largely irrelevant is probably better, but both are far in excess of the current required by the two channels at clipping...
_-_-bear
A resistor by itself is not a good option. If you are dissipating 120W on a line voltage of 120V then you have 1A of current flowing. A 18 Ohm reisistor will drop 18V dropping you effective voltage by 15%.
A relay is used with a time delay to short the resistor after a suitable period of time to allow the filaments/heaters to soft start, and limit the inrush current to the B+.
18 ohms is sufficient to limit the inrush to probably 2-3A in the primary which in turn limits the secondary filament/heater current as well as the B+ charging the caps. Since a relay is used to short the resistor it has no effect after the short power up period. The value has to be large enough to limit current, but small enoug to allow sufficient power flow for the heaters to operate. I suspect anything from 15 ohms to 30 ohms would work fine.
For comparison, the CL-90 has a cold resistance (25C) of 120 Ohms, which drops to 7.8 Ohms at .5A, 3.04 Ohm at 1A and 0.27 Ohm at 2A.
The same 1A of current flow thorugh the CL-90 would only result in a drop of 3.04V instead of 18v, and would reduce the waste to about 2.5% instead of 15%.
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The resistor idea works. I was introduced to it when I built my first amp (Rick Spencer's 12V6/12SL7 beginner's amp in a 2001 AudioXpress). He put a resistor in series with the heaters to tame the startup surge, then shorted it with a switch to apply the full 6.3 V after they had warmed up sufficiently.
20to20: It's good to see a fellow North Carolinian in here! I see you're in W-S. I work in GSO. 😀
20to20: It's good to see a fellow North Carolinian in here! I see you're in W-S. I work in GSO. 😀
Hang on.
Don't you mean to apply the full B+ after the tubes have warmed up?
I've seen this concept before, with a resistor and switch.
He did it for both, IIRC. For the heaters, the idea was to increase the number of possible on/off cycles by reducing heater current, thereby (hopefully) preventing them from "popping" like light bulb filaments. There were three switches: preheat (series resistor in the circuit), full heater (series resistor shunted), and the usual B+ switch.
And actually, I did goof 😱 - the heater voltage was 12.6 V.
That was a really good amp, BTW. I literally built it out of junk. The OPT's came from a Heathkit AA-151 I found almost folded in half and buried in mud at a junkyard. The ceramic tube sockets came from an old unrestorable military transmitter, and the chassis top and bottom plates started life as a DOT road sign.
I had maybe $100 in it, and it sounded great! 😎
And actually, I did goof 😱 - the heater voltage was 12.6 V.
That was a really good amp, BTW. I literally built it out of junk. The OPT's came from a Heathkit AA-151 I found almost folded in half and buried in mud at a junkyard. The ceramic tube sockets came from an old unrestorable military transmitter, and the chassis top and bottom plates started life as a DOT road sign.
I had maybe $100 in it, and it sounded great! 😎
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What size resistor would you recommend for the B+ 50mA/285V for example?
Maybe a 2 Watt? 285 * 0.005= 1.425
And how many ohms? 18 as was suggested before?
I assume this is on the secondary.
Which waste?
12V relay clicks as soon as voltage on a filament supply is enough to click the relay. It shorts that 18 Ohm resistor, no more waste of power, no waste of additional components for meaningless power delay relay.
Brit01: I know what you mean! That's one aspect of the WAF (Wife Acceptance Factor) that is frequently overlooked.
I've read of arrangements that overcome the multi-switch hand-jive by using digital circuitry, which is a 'bit' beyond me, if you'll pardon the pun.
The problem there is such efforts are usually met with an incredulous "Well geez - why couldn't you just buy an amp at (insert big box store here)." 🙄
Seriously, Wavebourn's point is sound. It can be easily and cheaply implemented without any weird parts. It can be used in portions of the power supply that aren't subject to extreme switching transients (i.e. no arc-welded switch or relay contacts), and it is quite versatile.
I've read of arrangements that overcome the multi-switch hand-jive by using digital circuitry, which is a 'bit' beyond me, if you'll pardon the pun.
The problem there is such efforts are usually met with an incredulous "Well geez - why couldn't you just buy an amp at (insert big box store here)." 🙄
Seriously, Wavebourn's point is sound. It can be easily and cheaply implemented without any weird parts. It can be used in portions of the power supply that aren't subject to extreme switching transients (i.e. no arc-welded switch or relay contacts), and it is quite versatile.
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Wavebourn: Would you be able to describe how I wire this so the B+ comes on after the heaters have warmed up?
Thanks
Even though this entire thread started with it being pointed out that a B+ delay is unnecessary?
On primary, 20W white ceramic wirewound resistor. 12V relay that shorts it is powered from 12V filament voltage regulator. That's it. It is for 2x100W push-pull tube amp; I set inrush current on 6A peak limit such a way. For lower power you may take higher value of resistor.
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