does anyone have a schematic or some advice about rigging up a standby switch in an st70 to avoid cathode stripping when using a SS rectifier?
thanks
thanks
Chop, nice to hear from you again. Since you asked, I'd pull that SS stuff and turn it back into a real '70.
There isn't much of a problem with indirectly heated tubes...
But you could simply put a low Vdrop tube rectifier in series with the rectified B+, that makes for a slow turn on.
Or you can use a thermistor arrangement too...
_-_-bear
But you could simply put a low Vdrop tube rectifier in series with the rectified B+, that makes for a slow turn on.
Or you can use a thermistor arrangement too...
_-_-bear
you're right but i just want to have it for back up and it's another tinkertown project
(1) In the '70 the bias voltage is applied immediately.
(2) the cathodes will just sit there until the heaters come up anyway.
apparently, the B+ voltage comes up so fast in SS rectification that the cathodes havent had time to build up a space charge and so cathode damage results. this significantly reduces tube life i am told. makes sense
i think the cathodes get damaged when there is no space charge
but there is a full B+ voltage.
bias voltage comes up immediately this means that the cathodes are protected from the B+ voltage by the negative bias voltage on the grid?
googling this query there are people who have modded the st70 to accomodate SS rectification with a voltage stabilizer tube or something or a standby switch
thanks very much
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The cathods are almost grounded and there will be no conduction until they are hot. There can be no conduction without a space charge.
The negative bias is what keeps the tube from conducting at full max current... It blocks current flow. It is there to regulate immediately. But there will be no conduction until the cathodes are hot anyway. If the cathodes were somehow "blasted" by plate voltage, that would constitute conduction, for any flow to occur.
yeah that makes sense.
joe curcio (manufacturer) says
If you search for research on cathode stripping you will find scientific studies (relatively) that find evidence both that it exists and that it does not and consequently I have concluded that empiric evidence needs to prevail.
Several points:
1. There are several commercial companies with very sharp experienced engineers producing hundreds (if not thousands) of amplifiers that apply B+ to the tubes (using ss rectification) year after year. I'm thinking that their database should support any evidence for the need to delay the HV for so called cathode stripping if needed.
2. I've designed, built and delivered my fair of tube amplifiers and never have I seen any evidence of cathode stripping.
3. I frequently design SMPS power supplies and from that experience it is clear that that current surges stress components severely (as does excessive voltage and excessive power dissipation) and the suggestion to apply B+ after the filaments have reached temperature makes the cure more hazardous than the perceived illness. These pulse currents are very traumatic to all of the components in the chain (including the power supply, regulators if used and the tubes themselves). To that point however, the inrush current limiter (into the cap bank) has real merit.
Yep, that addresses the stress on the input filter cap going above its voltage rating, caused by the limited resistance of the SS rectifier and lower voltage drop than with the tube rectifier.
There was some discussion on this recently, and at length.
http://www.diyaudio.com/forums/tubes-valves/168981-lets-settle-b-cold-tubes-issue.html
In essence, no empirical evidence currently exists to suggest that tube life is reduced by not letting them warm up before applying the HT, although there is evidence which suggests the opposite (e.g., cathode poisoning).
(If there were evidence for it you can be sure it would be plastered over every tube forum on the internet, yet none is forthcoming).
http://www.diyaudio.com/forums/tubes-valves/168981-lets-settle-b-cold-tubes-issue.html
In essence, no empirical evidence currently exists to suggest that tube life is reduced by not letting them warm up before applying the HT, although there is evidence which suggests the opposite (e.g., cathode poisoning).
(If there were evidence for it you can be sure it would be plastered over every tube forum on the internet, yet none is forthcoming).
No such an evidences, except when powerful RF output tubes work in class C mode and get negative bias from rectification of exciting signal by control grid current.
It is the result of following of common belief that does not apply to each particular case. There are lot of such cases, both in science and engineering. The most obvious is, Sir Isaac Newton himself clearly stated that the square law in case of gravitation does exist; that he does not know what causes it, and he can't explain it from the point of view of the model of physics he developed; but so called "mainstream" scientists since then tend to deny all evidences that can't be explained using Newtonian model of physics. That means, "Mainstream" scientists and engineers don't think outside of the box where schoolbooks are kept.
thanks! i guess there is nothing to worry about. SS i go then. less heat less hassle
then im going with SS rectification because it creates less heat and just makes me feel less nervous about that tube which i have replaced so many times already
Are you blowing rectifier tubes? I have heard of this happening in the ST70. A simple mod may be to add a UF4007 diode in series with each plate of the rectifier tube. I just did this yesterday when wiring up my 6L6 PP monoblock. The plates of the diode are pins 4 and 6, so I just place the diodes on the socket between pins 3&4 and 5&6, then wire the HT to pins 3 and 5 rather than 4 and 6. An inrush current limiter on the centre tap to ground of the transformer completes the job. You end up with less stress on the rectifier tube, less inrush current to the caps, and you still get a slow B+ rise well after the bias voltage has come up. All this for three cheap parts and no chassis modification!
Cheers,
Chris
(1) For the '70 higher amounts of input capacitor than designed are not advised.
(2) You may have a problem with your choke not doing its job anymore.
(3) Newer cheap GZ-34's have a bad reputation for arc resistance. Pair that with any one of the above and you will blow rectifiers.
Are you past the point of no return on any of these?
Until the tube is hot all you have are two pieces of metal separated by an a gap. The inside of a cold tube is no different from the contacts of an open switch.
The real purpose of the standby switch in the old days was to protect the capacitors. With no current draw from the tubes the B+ would go above the cap's rating.
I think the cathode striping argument is a hold over from ham and commercial radio transmitting tubes that run in the kilovolt range. These voltages create a strong enough electric field to ionize gas in the tube. But with our (about) 400V tubes I don't think the effect is big enough to mater in the one minute it take to warm up.
Now with SS diodes in an otherwise stock ST70 I wonder if the caps are stress before the tube conduct? Adding a standby switch is not hard, You might consider putting it on the AC side of the diodes. Switches are typically rated higher for AC.
Chopchip,
The ST70 has a SS rectified bias supply. That means even a slight delay in B+ rise is sufficient to dodge any cathode stripping that might occur. Put a CL150 inrush current limiting thermistor between the diodes and the 1st filter cap. and you have those few seconds of delay. The "instant" on bias supply, which makes the O/P tubes control grids negative with respect to ground, provides electrostatic shielding for the cold cathodes.
Have you taken any actions to deal with the extra Volts SS B+ rectification yields?
The ST70 has a SS rectified bias supply. That means even a slight delay in B+ rise is sufficient to dodge any cathode stripping that might occur. Put a CL150 inrush current limiting thermistor between the diodes and the 1st filter cap. and you have those few seconds of delay. The "instant" on bias supply, which makes the O/P tubes control grids negative with respect to ground, provides electrostatic shielding for the cold cathodes.
Have you taken any actions to deal with the extra Volts SS B+ rectification yields?
But is there a thermistor that works well at the very low currents in tube amps? Easier to find thermistors that will go in the power transformer's primary. Putting one on the primary also limits inrush to the caps. On the primary side there is enough current to heat the thermistor so that it can fall to 2 ohms or so in steady state. It will also give the heaters a slow start too and I think show starting the heaters might help tube life.
Many of the transformers we use were designed the 117V mains so now with 120 or 125V we have "voltage to burn", so why not?
Caps don't go over their voltage rating during in-rush/ As they charge up to working voltage the current goes down. The over voltage happens later after they are fully charged but there is no current being pulled from the power supply. Today anyone would buy over ratted caps so it's a non-issue.
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