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Post by leonski on Oct 20, 2021 20:06:44 GMT -5
Nutty but DOABLE. I'd read the meter every Sunday Morning and settle up with the Jar. 8x KT88s would be maybe 500$ or so, current prices. NO IOUs ALLOWED.
As for 'averaging out'......I've worked with an OPTICAL Flat, a monochrome light source and Quartz Blanks ground on various 'tables' than polished. We would adjust the
tables to produce a certian curvature.....I think 2 'rings' of interferrence was the goal.....I don't remember if pushing down on the center of the flat should make the rings go 'in' or 'out'.
Whtn the rings went in, the table was 'high'.....and I THINK that was the desired conditions.
We were grinding quartz wafers to be used in the mass production of tiny Tuning Forks......
The standard for a telescope mirror might be considered 1/8th wave......a measure of extreme flatness.....
Will OUTPUT tubes last 2000 hours? That's about 5.5 hours per day......Or about 38.5 hours per week.......might be easier to manage......
In any event? 500$? 2000Hours? 0.25$ per hour....
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Post by DavidR on Oct 20, 2021 21:54:47 GMT -5
Or you could check the tube with a tube meter
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Post by leonski on Oct 21, 2021 0:05:26 GMT -5
Or you could check the tube with a tube meter 2 reasons why not..... First? I don't OWN a tube checker. Finding a good one? Hasn't been one at the local drug store in maybe 30 years! I generally lack ROOM to store one and it'd be used maybe every couple years..... It'd be easier to find a fellow-hifi guy within driving range and make a house call......Bring a 6-er of some good beers? Second? Don't want to remove / install tubes unnecessarily. At some point I'd end up having to retension the socket for a 'firm' grip on the tube. Last time I used a tube checker would be mid-70s. We had a machine at work which had 2 shopping bags FULL of 'em....... We had a problem and the boss sent me to the drug store with ALL the tubes to check..... But the point you DON'T make is the one that is maybe most telling. How many tube amps come WITH an hour meter? I'll bet not many...... And I also suspect the bias of the tube may nosedive at some point. The ST70/120 have ONE test point per tube which makes that easy.... |
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Post by jbrunwa on Oct 21, 2021 0:13:18 GMT -5
Or you could check the tube with a tube meter 2 reasons why not..... First? I don't OWN a tube checker. Finding a good one? Hasn't been one at the local drug store in maybe 30 years! I generally lack ROOM to store one and it'd be used maybe every couple years..... It'd be easier to find a fellow-hifi guy within driving range and make a house call......Bring a 6-er of some good beers? Second? Don't want to remove / install tubes unnecessarily. At some point I'd end up having to retension the socket for a 'firm' grip on the tube. Last time I used a tube checker would be mid-70s. We had a machine at work which had 2 shopping bags FULL of 'em....... We had a problem and the boss sent me to the drug store with ALL the tubes to check..... But the point you DON'T make is the one that is maybe most telling. How many tube amps come WITH an hour meter? I'll bet not many...... And I also suspect the bias of the tube may nosedive at some point. The ST70/120 have ONE test point per tube which makes that easy.... I sold my mint Eico 667 dynamic tube tester a couple years ago, As I recall it tested tubes under a load to give a more accurate reading. Looks like ones in similar condition are $600 or soon ebay. |
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Post by leonski on Oct 21, 2021 1:00:04 GMT -5
For 600$ I could get MOST of the way to a new lens for my camera!
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Post by leonski on Oct 22, 2021 17:31:11 GMT -5
I just looked on EPray. LOTS of tube checkers from maybe 100$ on UP to over 2000$......
And some real brands from the past, like KNIGHT or even HEATHKIT......for the DIY types.
Sure....Eico is popular
I would think that a CLUB could take up a collection and BUY one which is available to ALL club
members. New member kicks in while if the 'owner' of the meter leaves the club, it is given to another member for safekeeping.
At least that way, the meter would get some use and be properly cared for.....While an individual wouldn't have to go the full expense.....
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Post by leonski on Oct 29, 2021 22:27:00 GMT -5
I've looked thru a bunch of stuff.....about tube bias.
I see some places referring to MV and see the test points and the potentiometer, for which I have that nice Long plastic screw driver.
But I also see references to current......MA.
Can any tube guys translate this for me?
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Post by audiobill on Oct 30, 2021 2:13:00 GMT -5
Bias is measured by voltage at the test points, but is affecting the steady state current through the tube.
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Post by leonski on Oct 30, 2021 16:16:16 GMT -5
Yes.....I talked with a buddy and got the short course in 'bias resistor' and proper choice makes voltage and current equal....so you can measure voltage and have the correct current......
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Post by leonski on Nov 12, 2021 16:01:52 GMT -5
I see 'sold out' until Late November or some such on the VTA website.
Might still be able to get a kit before Christmas!
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Post by audiobill on Nov 12, 2021 16:20:38 GMT -5
I told you!! 🤡
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Post by leonski on Nov 12, 2021 18:29:50 GMT -5
 Yes.....but I'm being pulled 5 ways at once. And I STILL need to get to LaLa Land and listen to those KLIPSCH. Though I wouldn't be upset to find the M125 ran my panels just fine, either. I completed demo of my old patio cover. About 1000lb of wood or more. And I fit it in a 'bagster' which was taken away last week. Next up was pulling the 3 posts / bases. They were over 200lb EACH and a lot of digging. I still need to find a truck to borrow for an hour....and some labor. 8 foot Max length and some of my boards were 16 feet. LOTS of cutting and I ended up buying one of those SawzAll things. I could cut your CAR in half with that brute. Next up? Remove about 250sq/ft of 4" thick concrete and replace with PAVERS while at the same time installing post bases for the NEW patio cover. Wife wants a solid roof for use during RAIN. And I have to at that time, build a table for my Big Green Egg which goes over on the END where I left a 'slab' which won't be roofed over. I think a TREE needs to come out, too. Fun Times...... Image is a SketchUp Rendering of my new table....proposed. The light brown is a granite slab I had cut. 32mm and cut to fit the Egg, using the 'table stand'..... Wanna Drop By for some fun? I'll provide LUNCH and BEER but you bring your own Linament...... |
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Post by leonski on Nov 21, 2021 16:55:13 GMT -5
I've been doing some 'digging'. Found a time delay relay AND auto bias which seem superior to the VTA offerings....and pretty much a 'wash' as to $$$.
Time delay board? small and NO relay. Uses an SCR and CAP for timing and has only one connection to the B+. Auto Bias? A little more complicated BUT
will allow tubes to be used to their fullest. I don't know WHAT tubes can be matched for, but I suspect BIAS isn't on the list.....
At the same time as installing the TD relay? Go to a SS rectifier. Several plusses from that move and no real downside. Not the least of which is NEVER
having to change that particular tube ever again.
Amp also comes with a synthetic tube socket.....Celanex, by name. Real Ceramic is available and might be preferred....Unfortunately? Expensive. But you can
make worthwhile changes at this time....making certain that, for example, the KT120 tube goes ALL the way into the socket and doesn't interfere with the socket
mounting screws....You can leave the rectifier socket Alone and change only the 4 output sockets....X2 amps....
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Post by audiobill on Nov 21, 2021 20:40:49 GMT -5
Having built over 100 VTA amps, my advice is not to do TDR or autobias. This is Bob’s advice as well.
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Post by leonski on Nov 21, 2021 22:46:22 GMT -5
Having built over 100 VTA amps, my advice is not to do TDR or autobias. This is Bob’s advice as well. Good advice to build 'stock'........I may change to ceramic tube sockets AND improve on the SCM with higher values. this would be a 'low cost' improvement. Better speaker posts and input jack? That might be possible, as well, without altering any instructions or intent. And not a chage in instructions, but rather just 'parts' for better..... But that being said? Going SS for the rectifier IS quite tempting. And takes the load off the 5v winding on the power transformer....though it might benefit from a 'dummy load' of some sort. rubli.net/HT_delay/ This board is maybe a little larger than 2" x 1" wide.....and uses an SCR for switching. No relay, solid state or 'other'.... I suspect it best to SAVE those changes for later. Instructions for such mods for ME, anyway, must be written for the non-professional amp builder. Some of the abbreviations and conventions are NOT familiar to me, so there is room to 'slip up'...... The auto bias board, while a really neat idea, looks TOO much for a novice like me to install. |
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KeithL
Administrator 
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Post by KeithL on Nov 22, 2021 10:17:15 GMT -5
Switching from a vacuum tube rectifier to a solid state one changes several things:
It will RAISE the B+ voltage - sometimes more than you may expect. Solid state rectifiers have a lower source impedance so you may end up with a significantly B+ voltage - especially under load. This can raise the power output - but can also risk exceeding the limits on some parts like high voltage filter capacitors.
It will cause the B+ voltage to spike when you first turn the amplifier on. The power supply will reach full output before the output tubes load it... so the voltage will probably jump up quite a bit when you first turn it on.
This can cause the voltage to exceed the ratings on the filter capacitors for a short time (see above).
There is also the risk of something called "cathode stripping". Basically it is claimed that, because the output tubes receive full PLATE voltage before their cathodes have a chance to heat up, this "stresses" the coating on the cathodes of the tubes. (The idea is that a cold cathode, with sufficient voltage applied to it, emits electrons differently... and this damages the coating.)
In theory this may cause the tubes to wear out prematurely. Most early premium amplifier designs had a delay which prevented B+ from being applied to the tubes until after the cathodes had time to reach operating temperature...
(However, the debate over this has been going on since tubes were new technology, and there doesn't seem to be any evidence to suggest that it actually happens.)
The point is that you should NOT "just drop in a solid state rectifier in place of a tube" unless you're sure it's OK in the particular design involved...
Having built over 100 VTA amps, my advice is not to do TDR or autobias. This is Bob’s advice as well. Good advice to build 'stock'........I may change to ceramic tube sockets AND improve on the SCM with higher values. this would be a 'low cost' improvement. Better speaker posts and input jack? That might be possible, as well, without altering any instructions or intent. And not a chage in instructions, but rather just 'parts' for better..... But that being said? Going SS for the rectifier IS quite tempting. And takes the load off the 5v winding on the power transformer....though it might benefit from a 'dummy load' of some sort. rubli.net/HT_delay/ This board is maybe a little larger than 2" x 1" wide.....and uses an SCR for switching. No relay, solid state or 'other'.... I suspect it best to SAVE those changes for later. Instructions for such mods for ME, anyway, must be written for the non-professional amp builder. Some of the abbreviations and conventions are NOT familiar to me, so there is room to 'slip up'...... The auto bias board, while a really neat idea, looks TOO much for a novice like me to install. |
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KeithL
Administrator
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Post by KeithL on Nov 22, 2021 10:23:11 GMT -5
I'm not quite sure about that part about "making the voltage and current equal".
Current THROUGH a resistor is ALWAYS proportional to the voltage ACROSS that resistor. Yes, you can pick a resistor that's "a nice round number" to make the calculations easier - for example, with a 1k resistor, that would be 1 mA / 1 V measured. However, the math isn't exactly complicated, and the best value for that resistor is often chosen for other reasons.
(And, if your amplifier uses cathode bias, you DO NOT want to arbitrarily change that value.)
Yes.....I talked with a buddy and got the short course in 'bias resistor' and proper choice makes voltage and current equal....so you can measure voltage and have the correct current...... |
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Post by leonski on Nov 22, 2021 15:22:07 GMT -5
The idea is to use VOLTS to end up with the proper current....thus the choice of the dropping resistor...
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Post by audiobill on Nov 22, 2021 16:43:54 GMT -5
BTW these amps use a “constant current source” semiconductor to maintain bias current.
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Post by leonski on Nov 22, 2021 16:46:46 GMT -5
Switching from a vacuum tube rectifier to a solid state one changes several things:
It will RAISE the B+ voltage - sometimes more than you may expect. Solid state rectifiers have a lower source impedance so you may end up with a significantly B+ voltage - especially under load. This can raise the power output - but can also risk exceeding the limits on some parts like high voltage filter capacitors.
It will cause the B+ voltage to spike when you first turn the amplifier on. The power supply will reach full output before the output tubes load it... so the voltage will probably jump up quite a bit when you first turn it on.
This can cause the voltage to exceed the ratings on the filter capacitors for a short time (see above).
There is also the risk of something called "cathode stripping". Basically it is claimed that, because the output tubes receive full PLATE voltage before their cathodes have a chance to heat up, this "stresses" the coating on the cathodes of the tubes. (The idea is that a cold cathode, with sufficient voltage applied to it, emits electrons differently... and this damages the coating.)
In theory this may cause the tubes to wear out prematurely. Most early premium amplifier designs had a delay which prevented B+ from being applied to the tubes until after the cathodes had time to reach operating temperature...
(However, the debate over this has been going on since tubes were new technology, and there doesn't seem to be any evidence to suggest that it actually happens.)
The point is that you should NOT "just drop in a solid state rectifier in place of a tube" unless you're sure it's OK in the particular design involved...
Good advice to build 'stock'........I may change to ceramic tube sockets AND improve on the SCM with higher values. this would be a 'low cost' improvement. Better speaker posts and input jack? That might be possible, as well, without altering any instructions or intent. And not a chage in instructions, but rather just 'parts' for better..... But that being said? Going SS for the rectifier IS quite tempting. And takes the load off the 5v winding on the power transformer....though it might benefit from a 'dummy load' of some sort. rubli.net/HT_delay/ This board is maybe a little larger than 2" x 1" wide.....and uses an SCR for switching. No relay, solid state or 'other'.... I suspect it best to SAVE those changes for later. Instructions for such mods for ME, anyway, must be written for the non-professional amp builder. Some of the abbreviations and conventions are NOT familiar to me, so there is room to 'slip up'...... The auto bias board, while a really neat idea, looks TOO much for a novice like me to install. That is the point of the TD relay setup. Keep the B+ OFF until heaters have a chance. I've seen up to 40 seconds bandied about.....Which only SOUNDS like a long time, but really isn't. Board I've seen is 31mmX63mm and maybe 25mm tall.....Easily placed. In the case of the M125 amp? It could go either between the chokes OR replace the Triode / Pentode switch....which I consider a near-useless feature..... You do NOT need but to break into the B+ line and that's IT. Board has a B+ IN and OUT. Once the SCR is ON? microscopic voltage drop. One large (isn) cap on board which I presume sets the time.... Also? the new kits feature a 120 or 117volt primary. This gives a LITTLE wiggle room on B+ output voltage. And for that matter? You don't want the 6.3v of the heaters running 'too high', either..... The mod I will NOT ATTEMPT for at least a year or more? The AutoBias board. A little more difficult to place. Needs a good ONE TIME adjust. But may extend tube life by providing EACH output ..... |
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