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Old 13th July 2012, 10:06 PM   #1
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Default Is my power transformer bad?

I am hoping you guys with more tech knowlege than me can shed some light on this problem. Amp is the Fisher 30A mono. 1 pair of EL84 output tubes. Voltage coming out of the Multi Filter cap is way high. It is orig. cap and the amp has played fine daily for a more than a decade after restoration by the Fisher Doctor. Starting to eat output tubes, unless that is just a coincidence since the Russian tubes don't seem to have the life of the NOS stuff. I figured it was the old multicap and was ready to replace it/them. But, I tested the voltage from the Transformer itself and it is also way high. Remember now, the transformer is still in the circuit, so would that make it test out of spec because of a fault elsewhere in the amp? If it matters, here are the differences in voltage from the cap(s) compared to schematic:
Cap1-40mfd: 418v, should be 385v.
Cap2-40mfd: 374v, should be 315v.
Cap3-40mfd: 318v, should be 270v.
This amp does sound distorted. A staticy sound riding along with the music that I have determined not to be a tube noise. It also had trouble with low pitched conga like drums...I thought the speaker was rattling, but turned out to be the amp.
I have other 30A's with all orig. parts that test spot on. And another that tests slightly high, but nowwhere near this much.
So, no question that the Filter caps should be changed at this age, but do you think that is actually the root of this problem?
Sorry if my query is dumb or the reason academic, I am not as tech savy as most of you DIYers. Any comments or even questions are gratefully appreciated, Thanks, Chris
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Old 13th July 2012, 10:17 PM   #2
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AC line voltages in the USA are quite a bit higher now than the 115 that was nominal back in the Golden Age. The numbers you're measuring are actually much less than some other folks would be seeing.

A "bucking transformer" in a separate box, 122 volts (or whatever) in and 115 volts out, is a common recommendation.

All good fortune,
Chris
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Old 13th July 2012, 10:18 PM   #3
asmith is offline asmith  United States
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I'll bet your amp was built at a time when the standard line voltage was lower, somewhere in the range of 110-117. If your like most of us your line voltage is somewhere around 125 or so. 10 volts difference times the ratio of the transformer could account for the higher then normal reading your getting.
There could also be a dropping resistor in the early stages of the filtering that has shorted.

Short awnser your transformer is still good.
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Old 13th July 2012, 11:29 PM   #4
kevinkr is offline kevinkr  United States
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The problem isn't with your power transformer, more likely bad output tube or tubes or some other component failure.

Note that if the electrolytics are original they are well overdue for replacement. Failure modes including shorting, venting, and just turning into high value resistors or low value caps or both.

Can you post a schematic of the amplifier and we can advise what to measure to trouble shoot the amplifier.

Note that the original voltages listed in the schematic were probably done with a 10Kohm/V analog multimeter and the measurement range was probably specified with a wide limit like 10 - 20%.

Since the voltages are on the high side it seems likely that the output tubes are not drawing the expected amount of plate current. (Or as mentioned modern line voltage, later Fisher gear was all specified at 117VAC IIRC, but this is not universal.)

If you are not acquainted with safe measurement techniques at these voltages please read the safety and newbie threads at the head of this forum..
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Old 16th July 2012, 03:47 PM   #5
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Thanks guys for your comments so far. I would like to post a schematic as requested, but don't have one here in the computer right now and scanner is dead. Also, I am dumber with digital than I am with Vacuum tubes. As I remember, the line voltage when testing was only 117v. Though I can certainly see the possible problem with ancient analog multimeters used to prepare the original specs... didn't think of that. Remember, I do have another pair of amps that test spot on, so what gives? I will try again with other output tubes and set a Variac for 115v. I have been reluctant to do this because of recent tube failure in this amp. Don't have a known good pair I'm willing to sacrifice. Maybe those Sovtek "M's" with a bunch of hours on them from the good testing amp. They don't test perfect, but are running audibly well in the other amp.
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Old 16th July 2012, 04:51 PM   #6
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Quote:
Originally Posted by HiFiHarv View Post
I am hoping you guys with more tech knowlege than me can shed some light on this problem. Amp is the Fisher 30A mono. 1 pair of EL84 output tubes. Voltage coming out of the Multi Filter cap is way high. It is orig. cap and the amp has played fine daily for a more than a decade after restoration by the Fisher Doctor. Starting to eat output tubes, unless that is just a coincidence since the Russian tubes don't seem to have the life of the NOS stuff. I figured it was the old multicap and was ready to replace it/them. But, I tested the voltage from the Transformer itself and it is also way high. Remember now, the transformer is still in the circuit, so would that make it test out of spec because of a fault elsewhere in the amp? If it matters, here are the differences in voltage from the cap(s) compared to schematic:
Cap1-40mfd: 418v, should be 385v.
Cap2-40mfd: 374v, should be 315v.
Cap3-40mfd: 318v, should be 270v.
This amp does sound distorted. A staticy sound riding along with the music that I have determined not to be a tube noise. It also had trouble with low pitched conga like drums...I thought the speaker was rattling, but turned out to be the amp.
I have other 30A's with all orig. parts that test spot on. And another that tests slightly high, but nowwhere near this much.
So, no question that the Filter caps should be changed at this age, but do you think that is actually the root of this problem?
Sorry if my query is dumb or the reason academic, I am not as tech savy as most of you DIYers. Any comments or even questions are gratefully appreciated, Thanks, Chris
The first thing you need to consider is that the voltages "should be" X, Y, or Z volts is NOT correct. That number is NOT a firm spec. You need to look at the tolerance - it's likely + or minus 15% if it's an older Fisher.

So at a nominal 117 volts the voltages could be as high as 442, 430, and 362 volts respectively - and still be in tolerance.

Second, all transformers have a spec called "% regulation" which in simplest terms tells you the % difference between full load and no load voltages on a trafo secondary. If the trafo was not in circuit the voltages will be way high. As well, no one can wind a trafo EXACTLY on the "spec", the winding has some tolerance of its own too.

Third, if the line voltage was above 117 VAC then the secondary voltages will be high.

Fourth, if the circuit is drawing less current than it should the voltages will be higher.

Fifth, as Kevin said, the higher impedance of today's digital meters will cause the readings to be higher than with an older analog VOM.

The LAST thing I would even consider would be a problem with the transformer - there are very few scenarios (if any) that can cause a transformer itself to deliver more voltage than it did.

So here's what I'd do:

1. If the can cap is original replace it - it's time.

2. Check ALL the tubes (there's only 4), especially the power tubes. Replace any that are weak.

3. CAREFULLY measure the voltages on the trafo primary under load. If they are higher than "spec" you can count on the secondary being higher.

4. You can bring the voltage down at C1A (which will reduce the other voltages as well by increasing the value of R16. The schematic shows a 5 volt drop across the 100 Ohm R16, but that is not correct - the output tubes alone draw a nominal 70 ma. So add the 12AX7 and a bit of leakage, etc., and I suspect the draw is about 75 ma. with an average tube set.

Therefore each additional Ohm of resistance at R16 will drop the voltage about .75 volts. So I suspect replacing the 100 with somewhere in the 180 Ohm to 270 Ohm range will do. 5 watts is plenty large enough too unless you go larger than 270.

You can bring the voltage down other ways too, but that might be the simplest. Just be sure you have a healthy filter cap and tubes before you jump into anything else. If you can't find the parts or tubes you need you can drop me a note, but most are common.

The schematic is below.
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Old 16th July 2012, 09:16 PM   #7
MelB is offline MelB  Canada
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My best guess as first thing to check, noting the symptoms and voltages is a poor connection to pin 9 on the EL84 sockets (g2). If you look at the manufactures specs R14 should be dropping 45 volts and it is. R15 should be dropping 70 volts it is not. It is only dropping 44 volts. Tells me one of the grids is not drawing current? Something to check.
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Old 17th July 2012, 04:51 PM   #8
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Thanks Jim and Mel for the good advice. I will delve deeper into the amp today and see whats up. You have both given me good ideas as to where to focus my efforts. And thanks Jim for the suggestion on R16. I suspected I could do something like that if needed, But wasn't sure it would be technically OK. I know Jim, there are large tolerances in play here, but it worried me that all my readings were so high rather than a mix of high and/or low. By the way, I checked the amp yesterday at 110vAC with known output tubes and the readings were much improved: Trans. out=322vAC (right on spec) Cap1=385v, Cap2=344v, Cap3=294v. Mel, the first thing I will do is check the output pin connections as you suggested. I cleaned and re-straightened these a while back, but it does get hot in there, who knows. Thanks again, Chris H
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Old 17th July 2012, 05:11 PM   #9
kevinkr is offline kevinkr  United States
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Might want to see how it is behaving with those "known" output tubes, it could be that the ones in there were bad, at least you will know that it was not the tubes if you give it a quick listen before diving deeper.
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