dragonworks
Banned
an output transformer? I have an old masco amp I have been working on and I have a large amplified signal right up to the output transformer.
How to test
- Thread starter dragonworks
- Start date
Steenamaroo
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I'm a proper noob here, but surely you can disconnect the transformer and check resistance across primaries, and then across secondaries.
At least that'd tell you if either side was open, right?
Even just disconnecting the output side and checking for output/voltage would be helpful in eliminating a short after the transformer, or whatever.
At least that'd tell you if either side was open, right?
Even just disconnecting the output side and checking for output/voltage would be helpful in eliminating a short after the transformer, or whatever.
dragonworks
Banned
the transformer works somewhat. I did the tests you already mentioned and it seemed okay. I can see a 160 step up in amplification on my scope, until it reaches the output tranny, then a large drop, but still a clean low signal. I just found another NOS transformer and will replace this one, that will tell me for sure. When I first got it and turned it on both can filter caps melted. I replaced them, then the coupling caps for the 6l6s, then a bunch of other caps and some very large resistors. New rectifier tubes and a new phase inverter tube and two new diode tubes, two new 6L6s .Each time getting a better larger signal. Now everything seems to be working correctly with the correct voltages etc until I reach the OPT.
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Steenamaroo
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The output transformer has gnd and 3 taps for 4, 8 and 16ohm.
Do you get the same result on each tap? Is the black grounded OK?
Although if you've ordered the new part anyway I suppose it doesn't matter.
Do you get the same result on each tap? Is the black grounded OK?
Although if you've ordered the new part anyway I suppose it doesn't matter.
dragonworks
Banned
The output transformer has gnd and 3 taps for 4, 8 and 16ohm.
Do you get the same result on each tap? Is the black grounded OK?
Although if you've ordered the new part anyway I suppose it doesn't matter.
no I do not get the same result on each tap. I am not sure what I should be seeing but I get .5, 1, + 1.5 ohms when I test. The ground is solid as far as I can tell. I have to take the "board" that holds most of the components off to get the transformer out. I figure I will just cut the wires so I have the color code and I won't have to label anything. I figure if I am going to go that far I might just as well replace it.
Steenamaroo
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Those readings look close enough to spec, give or take.
What about the primary? (disconnected it from everything) It should be, but I guess won't be, 320 ohms.
Best way to test an OPT is "backwards" using the 6.3V heater supply*.
Establish which is the 16 Ohm winding and apply heater AC to it.
For (what look like) 6V6 valves the anode to anode load should be around 8k and so the stepup ratio about 1:22 giving some 138volts across the anode pins (this should not kill but it CAN bite!) Then check that you have half that centre tap to each anode.
Best to check the other impedance ratios on the low Z side, 4 Ohm tap should be 3.18Vish.
Note: If the transformers are for guitar amp use don't sweat it if they are off the paper spec for the valve. Gitamp designers tend to go for power and "grit" rather than fidelity. Naturally if you want hi fi the traff needs to be pretty spot on.
*I used a quite expensive and nice Neutrik analyser!
Ah yes! One side of the secondary MUST be solidly bonded to chassis.
Dave.
Establish which is the 16 Ohm winding and apply heater AC to it.
For (what look like) 6V6 valves the anode to anode load should be around 8k and so the stepup ratio about 1:22 giving some 138volts across the anode pins (this should not kill but it CAN bite!) Then check that you have half that centre tap to each anode.
Best to check the other impedance ratios on the low Z side, 4 Ohm tap should be 3.18Vish.
Note: If the transformers are for guitar amp use don't sweat it if they are off the paper spec for the valve. Gitamp designers tend to go for power and "grit" rather than fidelity. Naturally if you want hi fi the traff needs to be pretty spot on.
*I used a quite expensive and nice Neutrik analyser!
Ah yes! One side of the secondary MUST be solidly bonded to chassis.
Dave.
dragonworks
Banned
If anyone would care to look at the sams photofact and the schematic, it is here. I do not have the preamp. When you get to the link you have to click on where it says photofact folder masco.
And this is a great site for other manuals and schematics.
http://bama.edebris.com/manuals/masco/cm20
And this is a great site for other manuals and schematics.
http://bama.edebris.com/manuals/masco/cm20
dragonworks
Banned
maybe a stupid question, probably is. I have the new transformer, does it matter which of the two plate wires go to which plate of the push pull section? I replaced the old one and I have a huge signal coming out now, totally distorted and howling while there is no input. The only thing I might have hooked up wrong is the plate wires. I am going to switch them but I highly doubt that is the problem. Before I put the new one in, I had a great big beautiful signal coming off the plates of the 6L6s. Now, if I pull the 6L6s out I have a very nice signal coming out of the inverter, as soon as I plug one in, everything goes to hell. I checked all the voltages again, to the plates, the grids, the bias, and they all seem correct. Trying to learn this stuff on my own is not easy. It is possible that this transformer is no good? It is NOS from the 1950s I think, at least I was told that when I bought it and it sure looks like it was never mounted and the leads showed no signs of ever being soldered.
Yes it matters!
But only if, as is surely the case here, the amplifier has "global" feed back from speaker out to PI cathode.
Flip the wires and you should be ok. I say "should" because a replacement transformer might not have the same characteristics as the old one and while I am pretty certain that the major screaming will stop there might be some twitchiness under drive and bursts of oscillation.
If this is the case we really need some detail of the circuitry around the OPT and PI. Push comes to shove, you don't NEED feedback. Many guitar amps do well without, most famous, the AC30.
Dave.
But only if, as is surely the case here, the amplifier has "global" feed back from speaker out to PI cathode.
Flip the wires and you should be ok. I say "should" because a replacement transformer might not have the same characteristics as the old one and while I am pretty certain that the major screaming will stop there might be some twitchiness under drive and bursts of oscillation.
If this is the case we really need some detail of the circuitry around the OPT and PI. Push comes to shove, you don't NEED feedback. Many guitar amps do well without, most famous, the AC30.
Dave.
dragonworks
Banned
Finally got the pdf file converted so i could upload it.
Thank you Dave, some of this is over my head at this point. You can go to the link I provided earlier in the thread and you will find the schematic there. In the parts list, it listed the transformer I bought as a direct replacement for the unit. On the 16 ohm tap there is a wire going back to the first tube in the series through an RC. If you would like I could send you the PDF. I have been working on this thing forever and have learned quite a bit while doing so, one being able to relate what I see to the schematic, etc. I think I stated before that when I was given this and plugged it in the filter caps just melted. Also, they were can caps, 10, 10, 20, 20. I could not find that particular can cap and replaced them with 20, 20 20, 20 caps. I was told by a EG at work that this would cause problems, I was told by just about everyone else that I know that has worked on an amp that this will not cause problems? This is not a guitar amp but a High Fidelity amp from the 50,s
P.S. what does PI stand for?
Thank you Dave, some of this is over my head at this point. You can go to the link I provided earlier in the thread and you will find the schematic there. In the parts list, it listed the transformer I bought as a direct replacement for the unit. On the 16 ohm tap there is a wire going back to the first tube in the series through an RC. If you would like I could send you the PDF. I have been working on this thing forever and have learned quite a bit while doing so, one being able to relate what I see to the schematic, etc. I think I stated before that when I was given this and plugged it in the filter caps just melted. Also, they were can caps, 10, 10, 20, 20. I could not find that particular can cap and replaced them with 20, 20 20, 20 caps. I was told by a EG at work that this would cause problems, I was told by just about everyone else that I know that has worked on an amp that this will not cause problems? This is not a guitar amp but a High Fidelity amp from the 50,s
P.S. what does PI stand for?
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So sorry!
I just assumed it was a rough arsed guitar amp! Right!
Yes, just swapping the anode leads should cure the instability and assuming the transformer is an exact replacement and of equal quality, all should be fine.
The replacement capacitors will not have any detrimental effect, quite the opposite I would say!
I have the schematic now. Forgive me but I do not trust sites that do not give a clear indication of a download. Spent FAR too much time in the past clearing out dodgy speederuppers and toobars!
If the amplifier still does not perform to your expectations make a list of all the voltages*, especially around the first two valves and the cathodes of the output valves. Note, the mains line voltage will not be exactly 117 and this will throw all the readings out but no matter. Check the voltage at C29 and then relate all the others to this, i.e if VC29 is up by 5% then so will all the others and that is fine.
*And some will read wrongly on a digital meter...But THAT is whole other can of worms!
Dave.
I just assumed it was a rough arsed guitar amp! Right!
Yes, just swapping the anode leads should cure the instability and assuming the transformer is an exact replacement and of equal quality, all should be fine.
The replacement capacitors will not have any detrimental effect, quite the opposite I would say!
I have the schematic now. Forgive me but I do not trust sites that do not give a clear indication of a download. Spent FAR too much time in the past clearing out dodgy speederuppers and toobars!
If the amplifier still does not perform to your expectations make a list of all the voltages*, especially around the first two valves and the cathodes of the output valves. Note, the mains line voltage will not be exactly 117 and this will throw all the readings out but no matter. Check the voltage at C29 and then relate all the others to this, i.e if VC29 is up by 5% then so will all the others and that is fine.
*And some will read wrongly on a digital meter...But THAT is whole other can of worms!
Dave.
Steenamaroo
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Steenamaroo
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+1. I built this little amp a while back.
Transistor amp, sure, but similar layout.
Initially I had the transformer primaries the wrong way around and all hell broke loose, so I'd look at that first.
Transistor amp, sure, but similar layout.
Initially I had the transformer primaries the wrong way around and all hell broke loose, so I'd look at that first.
dragonworks
Banned
going downstairs to switch out the leads soon. That site is pretty trustworthy, I have downloaded a few schematics from them and I am the one who uploaded this photofact to that site.
Another great site, I have downloaded quite a few books from here
Technical books online
I have checked all the voltages and they are with 10% of what is listed on the schematic.
thanx all and lets hope for the best.
P.S., these are both .org sites not .coms and they are not looking to sell you anything.
Another great site, I have downloaded quite a few books from here
Technical books online
I have checked all the voltages and they are with 10% of what is listed on the schematic.
thanx all and lets hope for the best.
P.S., these are both .org sites not .coms and they are not looking to sell you anything.
Hah! I guess you are too young to know this Steen' but the earlier valve amplifiers used interstage transformers as well.
The one in the (Deacey?) amp shown is doing duty as a phase inverter but in valve amps they also gave a decent voltage gain and were used because valve performance was poor and valves very expensive (not to mention the amount of support power, HT, heater and grid bias)
Note to OP. The 5V4G rectifier can be replaced with the far more rugged GZ34 if needs be.
Dave.
Steenamaroo
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I'm pretty sure it was the final output transformer I had wired wrong as, possibly, dragon does.
Wouldn't fight you on it though.
dragonworks
Banned
okay thats done. Seems like I am closer to my goal except, now I have a ton of motorboating, and distortion, but the signal is coming through loud and there is no squealing etc. When I hit the guitar hard I get lots of distortion. I replaced coupling caps to the 6l6s in the past. Will have to take all voltage measurements again. I have replaced most of the caps in the unit. One strange thing I have found is I have replaced the gain pot 4 different times now and none of them seem to work properly. Tried audio and linear taper. I bought one of the most expensive ones I could find feeling the cheapies I bought from mouser were no good. I have to bypass it. Another question, why does this amp have a preamp tube, V4 if it is supposed to be just an amp? Questions, Questions, sorry to bother you guys. P.S.,Am using a joe meeks for a preamp. The amp motorboats with no input.
Thanx All
Thanx All
Steenamaroo
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Glad to hear it.
Yup. I'd do that first.
Perhaps focus on this once you've tested all the voltages.
I'm thinking the problem might not be the pot.....?
If I don't reply again it's because I'm out of my depth.
