Hi All,
Someone gave me a Univox U-50 Lead amp that was not working. I took it home and found the power transformer had a shorted secondary.
Can't find the exact schematic for it but its virtually identical to the U-50 PA amplifier except for the front end. The U-50 PA schematic is easily found online.
I now have a replacement power tranny temporarily installed and things seem to be working "ok" except the output volume is much lower than expected. Amp sounds "normal" yet with volume at 12 noon the output at the speaker is very quiet and when cranked all the way is no where near what it should be.
In the process of going through "the usual checks" I'm checking the output transformer and find it has an "extra winding ?" on the secondary. The winding in question is isolated from the usual output winding and is then wired in parallel with it.
Transformer appears to have both 4 and 8 Ohm taps
4 Ohm tap is connected to the (single) speaker jack on the chassis.
8 Ohm tap is connected to the feedback loop.
The "extra winding" is in parallel with the 8 Ohm tap and has a much higher DC resistance than the "regular" secondary winding.
Anyone seen anything like this before? I would like to know what it is before I do any further testing.
Also, do the DCR numbers on the secondary seem ok?
Thanks!
Greg
Univox U-50 Lead Output Transformer
Moderators: pompeiisneaks, Colossal
Univox U-50 Lead Output Transformer
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Re: Univox U-50 Lead Output Transformer
Do you have a signal generator you can use to measure the turns ratios of the various secondary configurations? That would be a more useful metric than dc resistance imo.
Re: Univox U-50 Lead Output Transformer
I went ahead and measured the turns ratios and found the following:
4-Ohm tap (28.39)^2 * 4 = 3.2K
8-Ohm tap (20.55)^2 * 8 = 3.4K
"Mystery winding" (7.25) - not sure what to make of this...
Since there were no shorts primary to secondary or between the "mystery winding" or the normal secondaries I hooked it back up the same way I found it and tried to measure a 1 kHz sine wave and follow the voltage gain through the stages:
Input voltage = 0.060 VAC
Voltage out of 1st preamp stage = 0.258 VAC (measured at point "1" on the schematic - also note this version of the amp has no master volume)
1st stage voltage gain = 4.3
Voltage out of driver stage (the two BC169's in the Darlington configuration) = 4.33 VAC (measured at phase inverter input)
Driver voltage gain = ~17
Voltage on power tube grids = 105 VAC (measured at pin 5 of each power tube)
Voltage on power tube plates = 195 VAC
Voltage on 4 Ohm speaker tap = 4.65 VAC
Not sure what to make of the AC voltage measurements as I have never tried to measure the AC voltage gains through an amp before...
4-Ohm tap (28.39)^2 * 4 = 3.2K
8-Ohm tap (20.55)^2 * 8 = 3.4K
"Mystery winding" (7.25) - not sure what to make of this...
Since there were no shorts primary to secondary or between the "mystery winding" or the normal secondaries I hooked it back up the same way I found it and tried to measure a 1 kHz sine wave and follow the voltage gain through the stages:
Input voltage = 0.060 VAC
Voltage out of 1st preamp stage = 0.258 VAC (measured at point "1" on the schematic - also note this version of the amp has no master volume)
1st stage voltage gain = 4.3
Voltage out of driver stage (the two BC169's in the Darlington configuration) = 4.33 VAC (measured at phase inverter input)
Driver voltage gain = ~17
Voltage on power tube grids = 105 VAC (measured at pin 5 of each power tube)
Voltage on power tube plates = 195 VAC
Voltage on 4 Ohm speaker tap = 4.65 VAC
Not sure what to make of the AC voltage measurements as I have never tried to measure the AC voltage gains through an amp before...
Re: Univox U-50 Lead Output Transformer
My first thought is that the "mystery winding" is either inverted relative to the other secondaries and cancels some of the output to the speaker or non-inverted and adds to the output of the other secondaries.
The inverted case would be:
(28.39 - 7.25) * 4.65vac = 98.3vac on the primary
The non-inverted case would be:
(28.39 + 7.25) * 4.65vac = 165.7vac on the primary
Edit: And of course immediately after I posted and looked at your transformer drawing again I realized that the "mystery winding" is in series with both the 8 and 4 ohm taps and pretty much guarantees the math I just deleted is wrong. When you did the turns ratio measurements was the "mystery winding" connected as in the diagram?
The inverted case would be:
(28.39 - 7.25) * 4.65vac = 98.3vac on the primary
The non-inverted case would be:
(28.39 + 7.25) * 4.65vac = 165.7vac on the primary
Edit: And of course immediately after I posted and looked at your transformer drawing again I realized that the "mystery winding" is in series with both the 8 and 4 ohm taps and pretty much guarantees the math I just deleted is wrong. When you did the turns ratio measurements was the "mystery winding" connected as in the diagram?
Re: Univox U-50 Lead Output Transformer
I measured the windings individually for the earlier numbers.
Now that you mention it, I guess it was pretty short-sighted of me not to measure it connected to the "normal" winding as it is in the circuit.
Here are the readings with it connected:
4 Ohm tap: (27.82)^2 * 4 = 3.1K
8 Ohm tap: (19.99)^2 *8 = 3.2K
Not sure why it would be any benefit to have this additional winding. Maybe I'm measuring something incorrectly?
Now that you mention it, I guess it was pretty short-sighted of me not to measure it connected to the "normal" winding as it is in the circuit.
Here are the readings with it connected:
4 Ohm tap: (27.82)^2 * 4 = 3.1K
8 Ohm tap: (19.99)^2 *8 = 3.2K
Not sure why it would be any benefit to have this additional winding. Maybe I'm measuring something incorrectly?
Re: Univox U-50 Lead Output Transformer
It looks like you're putting 1vac on the secondary and measuring voltage (aka turns ratio) across the primary, right?
The 3k ballpark primary impedance numbers you're consistently measuring seems pretty low for a pair of 6L6, probably low enough to explain the lack of volume you're investigating. (See the example numbers at the bottom of the second page of the attached datasheet.)
You said there's only one speaker jack, is there an empty hole for an impedance selector switch or a second jack? Is the unused 4 or 8 ohm tap supposed to be grounded when the other is being used? To be honest the wiring of the pair of output jacks on that OT-50 schematic looks vague at best.
The 3k ballpark primary impedance numbers you're consistently measuring seems pretty low for a pair of 6L6, probably low enough to explain the lack of volume you're investigating. (See the example numbers at the bottom of the second page of the attached datasheet.)
You said there's only one speaker jack, is there an empty hole for an impedance selector switch or a second jack? Is the unused 4 or 8 ohm tap supposed to be grounded when the other is being used? To be honest the wiring of the pair of output jacks on that OT-50 schematic looks vague at best.
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Re: Univox U-50 Lead Output Transformer
Yes, I'm measuring by putting (roughly) 1V on the secondary, measuring the primary and then dividing the numbers.
3.2 K does seem low for a pair of 6L6. The only comparison I know is Fender output transformers were around 4K for a pair of 6L6 and if you mis-match the load on one of those the volume does not drop so dramatically.
I need to re-check my voltages and bias again. I thought I was in the ballpark but will double check to make sure.
Yes, there is only one speaker jack on the U-50 Lead. The 8-Ohm tap is tied to a lug on a terminal strip and drives only the feedback loop. No extra hole for another jack or selector. I'm pretty confident that's the way it was when it was made.
3.2 K does seem low for a pair of 6L6. The only comparison I know is Fender output transformers were around 4K for a pair of 6L6 and if you mis-match the load on one of those the volume does not drop so dramatically.
I need to re-check my voltages and bias again. I thought I was in the ballpark but will double check to make sure.
Yes, there is only one speaker jack on the U-50 Lead. The 8-Ohm tap is tied to a lug on a terminal strip and drives only the feedback loop. No extra hole for another jack or selector. I'm pretty confident that's the way it was when it was made.
Re: Univox U-50 Lead Output Transformer
Something still isn't adding up with the mystery winding.
The very small difference in the turns ratio numbers between mystery winding connected and not connected was unexpected. The first measurement of the MW (7.25) looked like it had a very significant number of turns.
The plate voltage to 4ohm load ratio doesn't match the low voltage tests. 195 / 4.65 = 42 Was that measured plate to plate, or plate to ground?
I personally would put 10v or 20v on the primary with a sig gen and measure the secondary windings on a scope so I could see phase and/or inversion issues as well as voltage.
I would also look at the nfb and maybe even disconnect it if anything is way out of tolerance.
The very small difference in the turns ratio numbers between mystery winding connected and not connected was unexpected. The first measurement of the MW (7.25) looked like it had a very significant number of turns.
The plate voltage to 4ohm load ratio doesn't match the low voltage tests. 195 / 4.65 = 42 Was that measured plate to plate, or plate to ground?
I personally would put 10v or 20v on the primary with a sig gen and measure the secondary windings on a scope so I could see phase and/or inversion issues as well as voltage.
I would also look at the nfb and maybe even disconnect it if anything is way out of tolerance.
Re: Univox U-50 Lead Output Transformer
Btw - Thanks for the suggestions - it is greatly appreciated!
Yes, I agree something is not adding up yet I have never seen a winding in parallel like this before so I'm not sure what to expect.
Measuring with a scope would be good as my voltmeter is probably not the last word in accuracy at such low AC voltage. Maybe I can grab/borrow a scope at some point as I currently do not own one. I will also double check the NFB loop and maybe just try disconnecting it and/or that winding. I did check the resistor values and they seemed to match the schematic.
Your question: "195 / 4.65 = 42 Was that measured plate to plate, or plate to ground?" I measured the input to the power tubes "plate to plate" (each probe on pin 3 of the 6l6) when driving the amp with a 1 kHz signal. For the speaker output I measured 4-Ohm tap to ground.
I checked the bias and it was very cold (way too cold) at 22 mA per tube. My new transformer gives a plate voltage of around 490V so I looked up on the "Jim Jones" bias tables and brought it up to around 36 mA. Testing after the re-bias the amp is noticeably louder yet still not what I would expect it to be. With a Les Paul plugged in and volume at 100% I'm almost at "small club gig" volume.
I'll keep going on it but might not have time to work on it for a couple of days.
Yes, I agree something is not adding up yet I have never seen a winding in parallel like this before so I'm not sure what to expect.
Measuring with a scope would be good as my voltmeter is probably not the last word in accuracy at such low AC voltage. Maybe I can grab/borrow a scope at some point as I currently do not own one. I will also double check the NFB loop and maybe just try disconnecting it and/or that winding. I did check the resistor values and they seemed to match the schematic.
Your question: "195 / 4.65 = 42 Was that measured plate to plate, or plate to ground?" I measured the input to the power tubes "plate to plate" (each probe on pin 3 of the 6l6) when driving the amp with a 1 kHz signal. For the speaker output I measured 4-Ohm tap to ground.
I checked the bias and it was very cold (way too cold) at 22 mA per tube. My new transformer gives a plate voltage of around 490V so I looked up on the "Jim Jones" bias tables and brought it up to around 36 mA. Testing after the re-bias the amp is noticeably louder yet still not what I would expect it to be. With a Les Paul plugged in and volume at 100% I'm almost at "small club gig" volume.
I'll keep going on it but might not have time to work on it for a couple of days.
-
Stevem
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- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Univox U-50 Lead Output Transformer
Grab or make up some 1 ohm 1% 2 watt resistors and place them in series to ground on the output tubes cathode .
Place a meter set for D.C. Volts across one resistor and measure the idle current, then drive the amp with a signal or a test tone, as you ramp up the drive signal you will see the current / D.C. Voltage go up if the rest of the amp is providing enough drive signal.
Also if you see the current going up the amp should get louder.
If your reading the current going up into the .070 amp range then you should also be seeing the amps B+ voltage start to sag down, and if you see these two things taking place and the amp is still not outputting over 8 watts then the OT is bad!
Seeing the output tube going up means they are doing there thing and trying to drive the OT , but can't due to a short.
Place a meter set for D.C. Volts across one resistor and measure the idle current, then drive the amp with a signal or a test tone, as you ramp up the drive signal you will see the current / D.C. Voltage go up if the rest of the amp is providing enough drive signal.
Also if you see the current going up the amp should get louder.
If your reading the current going up into the .070 amp range then you should also be seeing the amps B+ voltage start to sag down, and if you see these two things taking place and the amp is still not outputting over 8 watts then the OT is bad!
Seeing the output tube going up means they are doing there thing and trying to drive the OT , but can't due to a short.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Univox U-50 Lead Output Transformer
I finally had some time to do some more troubleshooting.
First I disconnected the preamp ahead of the PI and drove the output with another preamp and got the same result (low output).
Next I disconnected the NFB and the mystery winding - also no improvement.
An engineer friend of mine suggested an alternate test which honestly I do not fully understand: Drive the secondary the same way as I had (1-2 V) but with a series resistor 6-8 Ohms in line. With the amp off, the OT when driven should have a relatively high impedance so I should see little voltage drop across the series resistor. When tested I had a 4:1 voltage drop across the resistor.
Finally I received a replacement OT and swapped it in and bingo! Amp is rockin!
I'll have to do a post mortem on the old OT.
Greg...
First I disconnected the preamp ahead of the PI and drove the output with another preamp and got the same result (low output).
Next I disconnected the NFB and the mystery winding - also no improvement.
An engineer friend of mine suggested an alternate test which honestly I do not fully understand: Drive the secondary the same way as I had (1-2 V) but with a series resistor 6-8 Ohms in line. With the amp off, the OT when driven should have a relatively high impedance so I should see little voltage drop across the series resistor. When tested I had a 4:1 voltage drop across the resistor.
Finally I received a replacement OT and swapped it in and bingo! Amp is rockin!
I'll have to do a post mortem on the old OT.
Greg...
-
Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Univox U-50 Lead Output Transformer
Here's a handy way to test when a OT intended for 6L6 tubes is used.
Apply 10 vac to the OT in question primary and look for these output voltages.
2 ohm tap = .22
4 ohm tap= .31
8 ohm tap = .44
16 ohm tap = .62
Note that each tap is about a 40 Percent change, if your seeing a 35 or a 45 percent change there's a short on the primary or secondary winding.
If a OT for EL34 tubes where used the the numbers change to .24' .35, .49, and .69.
Apply 10 vac to the OT in question primary and look for these output voltages.
2 ohm tap = .22
4 ohm tap= .31
8 ohm tap = .44
16 ohm tap = .62
Note that each tap is about a 40 Percent change, if your seeing a 35 or a 45 percent change there's a short on the primary or secondary winding.
If a OT for EL34 tubes where used the the numbers change to .24' .35, .49, and .69.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!