Maybe someone here has experience with these transformers.
The price seems reasonable, but I would definitely order the version with 4, 8, and 16 Ohm taps.
Perhaps the unequal winding resistances that you measured indicate an internal short.
Does the bad transformer have the lower reading?
The price seems reasonable, but I would definitely order the version with 4, 8, and 16 Ohm taps.
Perhaps the unequal winding resistances that you measured indicate an internal short.
Does the bad transformer have the lower reading?
One more thing to check:
You have Common, 4, and 8 Ohm taps.
If one of the secondaries is incorrectly wired, that might explain the difference in output voltage, when it drives the same load resistor.
Otherwise, I suspect the primary impedance of the 2 primaries may not be the same.
You have Common, 4, and 8 Ohm taps.
If one of the secondaries is incorrectly wired, that might explain the difference in output voltage, when it drives the same load resistor.
Otherwise, I suspect the primary impedance of the 2 primaries may not be the same.
They appear to be correctly wired - believe it or not I did think of that, but at least going by secondary wiring colour, they both appear to be wired correctly. I also checked DC resistance and 4 and 8 ohm taps match (although both are a very low DCR which really you would want kelvin leads to be sure of I think). Primary DC resistance does not match however.
Fran, guitar amps often wreck the OT; partial short. Some of them were expensive, so I worked out how to test them.
Find a 6.3V heater transformer. Feed the 6.3v into the 8 Ohm secondary; then measure the AC RMS voltage that appears on the primary.
Compare the reading with the calculated value (or the known good one). If it's a partial short, the primary output voltage will read low.
Find a 6.3V heater transformer. Feed the 6.3v into the 8 Ohm secondary; then measure the AC RMS voltage that appears on the primary.
Compare the reading with the calculated value (or the known good one). If it's a partial short, the primary output voltage will read low.
Rod, that is really helpful and a great tip to have in the toolbox.
So on the bad side I get 76VAC, and on the good side I get 110V. It just removes any doubt that the OPT is not at fault. My buddy has ordered a set from that primary windings company in the UK, so lets see how this one goes.
So on the bad side I get 76VAC, and on the good side I get 110V. It just removes any doubt that the OPT is not at fault. My buddy has ordered a set from that primary windings company in the UK, so lets see how this one goes.
One other thing I would mention is that 14K is an extremely low load resistance for a 6SN7, something like 22K would be near the minimum I would consider acceptable.
I place a fairly high resistance (say 4 - 8X the rated tap impedance) across the secondary winding and this may prevent a flyback situation in the output transformer primary if a speaker lead becomes disconnected during use. The other thing that I have seen is filament to grid shorts in certain Russian made 300Bs, that will fry the OPT unless you are very lucky. Fuses if used should be appropriate high voltage types placed in the lead from PSU to transformer primary. (Fuses in the cathode circuit are not always effective.)
Edit: Forgot to mention that the outcomes may be different for cathode vs fixed bias - in the fixed bias case the winding dcr is the only thing effectively limiting the primary current, in the case of cathode bias the results depend heavily on the bypass cap used and what happens to it when it sees near plate voltage.
I place a fairly high resistance (say 4 - 8X the rated tap impedance) across the secondary winding and this may prevent a flyback situation in the output transformer primary if a speaker lead becomes disconnected during use. The other thing that I have seen is filament to grid shorts in certain Russian made 300Bs, that will fry the OPT unless you are very lucky. Fuses if used should be appropriate high voltage types placed in the lead from PSU to transformer primary. (Fuses in the cathode circuit are not always effective.)
Edit: Forgot to mention that the outcomes may be different for cathode vs fixed bias - in the fixed bias case the winding dcr is the only thing effectively limiting the primary current, in the case of cathode bias the results depend heavily on the bypass cap used and what happens to it when it sees near plate voltage.
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As long as the bad transformer (76V on primary), gives about 1.4 times the voltage on the 8 Ohm tap as it gives on the 4 Ohm tap.
1 volt on 8 Ohm tap, and 0.7V on the 4 Ohm tap.
That verifies the secondary taps.
1 volt on 8 Ohm tap, and 0.7V on the 4 Ohm tap.
That verifies the secondary taps.
So say a 68R-100R or so - 1 or 2W maybe? This is easy to do.....
An easy change as well - the first stage has a 24k plate resistor, and the second stage is 14k at the anode. Anything else I need to change when doing that?
So would something like this be appropriate - 500mA, 1kVAC rating, fast blow (given that the OPT is 100mA and B+ to the OPT is 410VDC or so). Would a lowish value fusible resistor be an appropriate alternative?
I didn't specifically measure both taps using Rod's 6.3VAC method - but I did check that output from 4 ohm was less into 8 ohm - that it wasn't that the winding was made with the wrong colour secondary for example.
I want to say again - thank you all very much, it really is an education here.
Yes, something in the range of 68 - 100 ohms should be fine. (I am using 100 ohms across the 8 ohm tap on my GM70 amps)
Checking both taps is a sensible precaution to make sure the secondary windings are not swapped.
For this use I would recommend about 2X the maximum rated plate current for the tube type - something like 200mA - 250mA should be OK, to be effective it has to open quickly. (So fast blow) There may be a small risk of nuisance fuse blowing so include spares. You can also put a 0.047uF - 0.1uF film capacitor across the fuse terminals with a voltage rating sigificantly higher than the supply voltage.
A small fusible resistor might be an option. I have not tried that, be sure to read the manufacturer's application notes to size it correctly for the condition where you want it to blow. Resistance value should be a few ohms or less?
Checking both taps is a sensible precaution to make sure the secondary windings are not swapped.
For this use I would recommend about 2X the maximum rated plate current for the tube type - something like 200mA - 250mA should be OK, to be effective it has to open quickly. (So fast blow) There may be a small risk of nuisance fuse blowing so include spares. You can also put a 0.047uF - 0.1uF film capacitor across the fuse terminals with a voltage rating sigificantly higher than the supply voltage.
A small fusible resistor might be an option. I have not tried that, be sure to read the manufacturer's application notes to size it correctly for the condition where you want it to blow. Resistance value should be a few ohms or less?
Its probably just easier to use a fuse - and definitely more understandable for the next person to fix it. 250mA fuses are pretty cheap and even 480V capable fuse holders are a small enough investment given the OPT cost.
Just to confirm, I repeated Rod's test again, feeding 6.3VAC into the 4 and 8 ohm secondaries, and reading the voltage on the primary.
| feeding 6.3VAC into 4 ohm secondary | feeding 6.3VAC into 8 ohm secondary | |
| Bad OPT primary VAC reading | 94 | 75 |
| Good OPT primary VAC reading | 151 | 106 |
Well, a little update.
My buddy ordered a pair of transformers from the company linked back a bit - primarywindings in the UK. Took a few weeks between one thing and another, but they look top notch, and are a lot bigger than what was in the amp originally. I'd say they are maybe 1.3-1.4 times bigger which is pretty substantial. Anyway, managed to shoehorn them into the chassis and all seems back on track now. All voltages line up now, and seem to make sense with typical values for a 6SN7/300b amp. I'm getting almost 8W into 8r at clipping. I see a little roll off starting gently above 15khz or so and the square wave isn't exactly solid-state-like as the frequencies rise over 10 or 12khz - but again this seems to be the norm.
A soak test for about 5 hours and all seems good.
I took many of the great suggestions that you told me about - thank you all - most notably the fuse on the OPT feed, and the power resistor on the secondary. After that I also did a general tidy up - there were several half baked resistors and caps and all of these have now been replaced with good quality/high temp/long life types. There were many long and bare leads from point to point wiring and those carrying voltage are now all sleeved. So hopefully this amp has a long second life ahead of it. The weak point now is the PCB for the power supply which is aged and doesn't take kindly to desoldering etc.
Thanks for all the help everyone.
My buddy ordered a pair of transformers from the company linked back a bit - primarywindings in the UK. Took a few weeks between one thing and another, but they look top notch, and are a lot bigger than what was in the amp originally. I'd say they are maybe 1.3-1.4 times bigger which is pretty substantial. Anyway, managed to shoehorn them into the chassis and all seems back on track now. All voltages line up now, and seem to make sense with typical values for a 6SN7/300b amp. I'm getting almost 8W into 8r at clipping. I see a little roll off starting gently above 15khz or so and the square wave isn't exactly solid-state-like as the frequencies rise over 10 or 12khz - but again this seems to be the norm.
A soak test for about 5 hours and all seems good.
I took many of the great suggestions that you told me about - thank you all - most notably the fuse on the OPT feed, and the power resistor on the secondary. After that I also did a general tidy up - there were several half baked resistors and caps and all of these have now been replaced with good quality/high temp/long life types. There were many long and bare leads from point to point wiring and those carrying voltage are now all sleeved. So hopefully this amp has a long second life ahead of it. The weak point now is the PCB for the power supply which is aged and doesn't take kindly to desoldering etc.
Thanks for all the help everyone.