Hello all-
First post to this forum. Got into tubes about 10 years back, and got out of it due to cost. Now, I'm back into tubes (not because they're less $, but I guess I can afford it now).
Purchased two ST-70's on ebay (I hate ebay). My goal was to get at least two good OPT's and a power trannie. Yes, I paid too much. Don't ask!
I will be building a very modded ST-70 with triode driver, regulated current (not voltage) filaments, blah, blah, blah. I'm an EE, so I have this bad habit of overbuilding.
Anyway, my question is this: out of the four A-470's (all cloth wire), I am getting some interesting ohmic values on the primary. Hopefully someone with experience in the
A-470 can help me.
I took resistance readings from the center tap to the two green and two blue leads. My results are as follows (in order of blue, green, blue/white, green/white).
OPT1: 90, 30, 103, 34
OPT2: 160, 30, 102, 34
OPT3: 90, 30, 105, 35
OPT4: 163, 30, 103, 34
My interpretation is that #2 and #4 are defective (possible loss of negative bias?).
What is interesting is that 1 and 2 were from the same amp, and 3 and 4 were from another amp. Typically, I would not expect two failures from two different amps to show identical test results.
Further, I took each OPT and placed a 60Hz signal on the primary, measured and looked at output waveform with an oscilloscope (4 ohm dummy load). All OPT's gave similar results.
Is anyone able to confirm the expected values with a know good A-470? If I had a working amp, I might be able to let my ears make the decision.
Thanks so much for your replies !!
First post to this forum. Got into tubes about 10 years back, and got out of it due to cost. Now, I'm back into tubes (not because they're less $, but I guess I can afford it now).
Purchased two ST-70's on ebay (I hate ebay). My goal was to get at least two good OPT's and a power trannie. Yes, I paid too much. Don't ask!
I will be building a very modded ST-70 with triode driver, regulated current (not voltage) filaments, blah, blah, blah. I'm an EE, so I have this bad habit of overbuilding.
Anyway, my question is this: out of the four A-470's (all cloth wire), I am getting some interesting ohmic values on the primary. Hopefully someone with experience in the
A-470 can help me.
I took resistance readings from the center tap to the two green and two blue leads. My results are as follows (in order of blue, green, blue/white, green/white).
OPT1: 90, 30, 103, 34
OPT2: 160, 30, 102, 34
OPT3: 90, 30, 105, 35
OPT4: 163, 30, 103, 34
My interpretation is that #2 and #4 are defective (possible loss of negative bias?).
What is interesting is that 1 and 2 were from the same amp, and 3 and 4 were from another amp. Typically, I would not expect two failures from two different amps to show identical test results.
Further, I took each OPT and placed a 60Hz signal on the primary, measured and looked at output waveform with an oscilloscope (4 ohm dummy load). All OPT's gave similar results.
Is anyone able to confirm the expected values with a know good A-470? If I had a working amp, I might be able to let my ears make the decision.
Thanks so much for your replies !!
It's possible that there are shorted turns, but I can't see the resistance going UP... maybe if it was seriously overheated.
You can easily measure the voltage ratio at 60 Hz (do you have a variac?). 4K to 4 Ohms is 1000:1, voltage ratio is square root of that, about 32:1, or about 16:1 for 16 Ohms. Primary on these is at least 300 Henries, from what I hear... so the full primary would be about 100K ohms at 60 Hz, around 1 mA at 100V, and you could compare the inductance of the segments accordingly. A shorted layer (even a few shorted turns), would reduce the inductance considerably. Use low voltage until you know it's OK (the primary sees over 700V at 35W, so 120V won't hurt it - if it isn't shorted).
You can easily measure the voltage ratio at 60 Hz (do you have a variac?). 4K to 4 Ohms is 1000:1, voltage ratio is square root of that, about 32:1, or about 16:1 for 16 Ohms. Primary on these is at least 300 Henries, from what I hear... so the full primary would be about 100K ohms at 60 Hz, around 1 mA at 100V, and you could compare the inductance of the segments accordingly. A shorted layer (even a few shorted turns), would reduce the inductance considerably. Use low voltage until you know it's OK (the primary sees over 700V at 35W, so 120V won't hurt it - if it isn't shorted).
Got it. And yes, I have a variac.
Basically, I'll keep the 4 ohm dummy load on, and energize the primary with 120VAC. Measure current, compare the values between the four transformers.
It would make some sense to impress enough voltage (120) in order to force any weak insulation to manifest itself.
Thx.
Basically, I'll keep the 4 ohm dummy load on, and energize the primary with 120VAC. Measure current, compare the values between the four transformers.
It would make some sense to impress enough voltage (120) in order to force any weak insulation to manifest itself.
Thx.
Got my results. Wish they were more conclusive.
Current from center tap to Blue and Blue/White were very equal at 95 mA on what I believe are the good OPT's.
On the 'bad' OPT's, I get 95 mA on center tap to Blue/White, and only 90 mA from center tap to Blue. From the standpoint of DC resistance, these numbers make sense, I guess. I was hoping to see current increase as a result of shorted ampere turns, which did not happen.
My last resort will be to put about 300-400V on each half winding, to attempt to make the problem rear its ugly head with a little more emphasis.
Thx again.
Current from center tap to Blue and Blue/White were very equal at 95 mA on what I believe are the good OPT's.
On the 'bad' OPT's, I get 95 mA on center tap to Blue/White, and only 90 mA from center tap to Blue. From the standpoint of DC resistance, these numbers make sense, I guess. I was hoping to see current increase as a result of shorted ampere turns, which did not happen.
My last resort will be to put about 300-400V on each half winding, to attempt to make the problem rear its ugly head with a little more emphasis.
Thx again.
Well, I suspect all four OPT's are bad.
Tests of excitation current at 360V were as follows:
CT to Blue: 6.5 mA
CT to Green: 13.2 mA at 100V (could not push much more)
CT to Blue/White: 6.5 mA
CT to Green/White: 13.2 mA at 100V
What are the odds that all four OPT's from two independent amps are bad? Unbelievable !!
Coupled with the fact that all the failures were identical !
Anyone know of a good source of cloth wire OPT's besides the evil ebay?
Tests of excitation current at 360V were as follows:
CT to Blue: 6.5 mA
CT to Green: 13.2 mA at 100V (could not push much more)
CT to Blue/White: 6.5 mA
CT to Green/White: 13.2 mA at 100V
What are the odds that all four OPT's from two independent amps are bad? Unbelievable !!
Coupled with the fact that all the failures were identical !
Anyone know of a good source of cloth wire OPT's besides the evil ebay?
Wait a minute, I must be on 'ludes or something.
The green and green/white leads are the screen taps, and much closer to B+ center tap than the blue and blue/white.
So I guess these numbers make some sense?
It still doesn't address the discrepancy in ohm readings.
ARRRRRGGGHHH the frustration !!
I love electrons and grain-oriented silicon steel
The green and green/white leads are the screen taps, and much closer to B+ center tap than the blue and blue/white.
So I guess these numbers make some sense?
It still doesn't address the discrepancy in ohm readings.
ARRRRRGGGHHH the frustration !!
I love electrons and grain-oriented silicon steel
The primaries on the Dyna trannies are not symmetrical.
That's due to the differences in the winding position on the bobbin.
So the DCR of each half is "wrong" - but the turns ratio is "right".
Different production runs of the transformers can also vary a little.
There are two main variants of power trannys - the "big stack" and the "small stack." I think, iirc, the taller stack was earlier.
Then there are the potted ones - who knows what's in them.
I had a few with potted trannies over the years...
The driver circuit sux, dump it - especially if ur an EE.
The power supply filtering is anemic.
The connects and wires blow.
The output iron is ok.
But run the tubes in triode, unless you crave that "pentode splash".
If ya got two, run em in some sort of mono... for best results.
Have fun!
_-_-bear
PS. next try some 300Bs in P-P... more better fun!
PPS. then some 845/211s... report back...
That's due to the differences in the winding position on the bobbin.
So the DCR of each half is "wrong" - but the turns ratio is "right".
Different production runs of the transformers can also vary a little.
There are two main variants of power trannys - the "big stack" and the "small stack." I think, iirc, the taller stack was earlier.
Then there are the potted ones - who knows what's in them.
I had a few with potted trannies over the years...
The driver circuit sux, dump it - especially if ur an EE.
The power supply filtering is anemic.
The connects and wires blow.
The output iron is ok.
But run the tubes in triode, unless you crave that "pentode splash".
If ya got two, run em in some sort of mono... for best results.
Have fun!
_-_-bear
PS. next try some 300Bs in P-P... more better fun!
PPS. then some 845/211s... report back...
Now I feel a little better, knowing the DCR is not always expected to match (although it still hurts the brain to be soooo different). In the end, I guess I'll have to wait for the finished product to see if these are any good. It's only money, right?
I test 3750 kVA, 25 kV transformers for a living, where if DCR does not match, you got serious problems. No doubt audio is a teenie bit different.
Good suggestions, everyone.
I will be going to cascode triode input.
Completely new PS, chassis, wiring, and bias.
OPT and PT are the only items remaining.
Stick with pentode P-P for now, I've heard varying thoughts on triode.
Got some interesting slow start timing and controls I might share with y'all in the future.
Maybe the next project (if this actually works) would be SE for tweeters.
It doesn't have to make sense, just be fun.
Thanks again everyone !!
I test 3750 kVA, 25 kV transformers for a living, where if DCR does not match, you got serious problems. No doubt audio is a teenie bit different.
Good suggestions, everyone.
I will be going to cascode triode input.
Completely new PS, chassis, wiring, and bias.
OPT and PT are the only items remaining.
Stick with pentode P-P for now, I've heard varying thoughts on triode.
Got some interesting slow start timing and controls I might share with y'all in the future.
Maybe the next project (if this actually works) would be SE for tweeters.
It doesn't have to make sense, just be fun.
Thanks again everyone !!
I wanted to mention that it might make sense to pair up the transformers with the closest DCR, regardless of which amp they came out of.
People over the years have replaced iron on these amps quite a bit, although it is rare that outputs fry on the ST-70, it is possible that they were swapped around at some point in time...
And even if not, pairing them up might be a good idea, although probably not necessary.
_-_-bear
People over the years have replaced iron on these amps quite a bit, although it is rare that outputs fry on the ST-70, it is possible that they were swapped around at some point in time...
And even if not, pairing them up might be a good idea, although probably not necessary.
_-_-bear
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