The Triad N-68X is popularly suggested as the fix (along a 3-prong cord and death cap removal) for AA4/5 radios and amps ("widowmakers"). In an old thread
https://www.diyaudio.com/community/threads/old-radio-quick-question-re-ps.85134/#post-988972
post #11, member Tubelab_com mentions some Hammond substitutes for the Triad:
Hammond 185D230
Hammond 185E230
Hammond 186E120
The Hammond 187 series is identical to the 186 except with a single primary. I'm interested in the 187 series as a substitute.
However Hammond lists the 169 or 171 series as substitutes for the Triad N series with no exact substitutes for the N-68X. Hammond identifies the Triad FD7 series as the match for the 186E120 and the F7 series for the 187E120.
I checked the specs of the N-68X and the 187E120 seem fairly close, especially the insulation rating and the hi-pot test. I'm inclined to think that using the Hammond 187 series would be safe. Also I can get it a little cheaper. So do you think it is safe to use the Hammond 186/187 series for isolation?
Assuming that the 187E120 is safe, would the 187D56 (56VCT,30VA) be safe for the build in the attached file (6N2P+50C5 amp).
Any and all comments welcome. Thanks.
https://www.diyaudio.com/community/threads/old-radio-quick-question-re-ps.85134/#post-988972
post #11, member Tubelab_com mentions some Hammond substitutes for the Triad:
Hammond 185D230
Hammond 185E230
Hammond 186E120
The Hammond 187 series is identical to the 186 except with a single primary. I'm interested in the 187 series as a substitute.
However Hammond lists the 169 or 171 series as substitutes for the Triad N series with no exact substitutes for the N-68X. Hammond identifies the Triad FD7 series as the match for the 186E120 and the F7 series for the 187E120.
I checked the specs of the N-68X and the 187E120 seem fairly close, especially the insulation rating and the hi-pot test. I'm inclined to think that using the Hammond 187 series would be safe. Also I can get it a little cheaper. So do you think it is safe to use the Hammond 186/187 series for isolation?
Assuming that the 187E120 is safe, would the 187D56 (56VCT,30VA) be safe for the build in the attached file (6N2P+50C5 amp).
Any and all comments welcome. Thanks.
The 187E120 will work fine as a replacement for the N-68X if you are in a 120 volt country. The 187D56 should work in the circuit posted if the 6N2P has a 150 mA heater. I'm not familiar with Russian tubes.
Thanks for pointing out the heater calculation. I was rushing and overlooked that. I was still relying on 12AX7 at 12V case. Upgrading to the 187E56 with 56VCT 56VA (1A max current vs .53A for the 187D56) should work .
No, just upgrading the transformer is not enough, you have a 150mA heater and a 300mA heater in series, that scheme will never work as such.
Thanks for the info. Guess I'll go back to a 12AX7 (heaters in series for 150mA) and use the 187E120. I would need to use a resistor to drop the voltage.
Btw would a 30VA transformer be enough for the circuit with a single 12AX7? The 187D120 is spec'd at 30VA and .25A.
Yes, should work. Your voltage requirement is actually 62.6V with the 50C5 and 12AX7, but 56V is close enough.
On paper the 30 VA transformer is good enough. In reality it isn't, especially working into a half wave voltage doubler which draws a bunch of current in pulses once every 16 mS. The transformer will get hot and possibly buzz. I have a little 4 tube guitar amp that uses 100 mA tubes, a pair of 32ET5's an 18FW6 and an 18FY6. B+ is from a full wave bridge. The Triad N-68X powers it fine. I tried a similar amp with 50B5's (same as a 50C5 except for pinout and price), a 12AU6 and a 12AV6. The Triad did not like it, got too hot to touch. The 187E120 would be my choice for a 150 mA tube string.
Also probably better to have a little too much and knock it down if need be. Out of curiosity, did you ever consider using 2 wall wart transformers for isolation/power? eg. (120AC -> 12VAC )-> (12VAC->120VAC)
The idea of using a pair of similar transformers back to back is pretty common and can be found in several DIY amp builds on these forums. If you are going to tap the first transformer for heater power it usually needs to be bigger (higher VA) than the second. Your overall losses will be nearly double what a single transformer will have and the overall size will usually be bigger, but for low powered projects the cost savings can be significant.
I had a pretty heavy 24 volt AC wall wart transformer that I used to power a tiny 4 tube guitar amp with 4 X 6 volt heaters (2 X 6AK6, a 6AK5 and a 6AQ6) wired in series and a 120 to 24 volt transformer wired backwards feeding a bridge and a cap for about 160 volts of B+. The main problem here was that any movement on the power cord would cause the heavy wall wart to fall out of the outlet.
That amp went through dozens of changes before meeting the trash can, but the final iteration held my ADHD powered attention for over a year. This all happened in the late 80's to early 90's before lithium batteries were common, so the cheap battery of choice was the common 7.2 volt NiCad pack for RC cars. The 7.2 volt battery powered the 6 volt heaters wired in parallel directly. B+ came from a DIY DC to DC converter that made about 160 volts at idle, and 140 when the amp was cranked to 11. Line power was a 6.3 volt Radio Shack transformer feeding a bridge made with Schottky diodes and a big fat cap. Maximum power output was about 2 watts when plugged in.
After that one got trashed I built a super tiny version that used a pair of subminiature tubes (don't remember the numbers) for the preamp and the same pair of 6AK6's for the output stage. A similar power system was used except that my DIY boost converter put out more voltage. Power was about 2 watts, and the whole thing went into a DIY guitar body. The tubes were visible under the Plexiglass pick guard on the front, and the 7.2 volt battery went into the pocket on the back where the bridge springs go. I used a Telecaster bridge which has no springs and put a 4 inch speaker in the body. I thought it was a neat idea, based on an old solid state version that was sold at the Lafayette Radio Electronics store where I grew up. The manager there used to let me play all the guitars since I was pretty good at it and it brought attention to them. As usual I grew tired of it and cannibalized it for parts.
I had a pretty heavy 24 volt AC wall wart transformer that I used to power a tiny 4 tube guitar amp with 4 X 6 volt heaters (2 X 6AK6, a 6AK5 and a 6AQ6) wired in series and a 120 to 24 volt transformer wired backwards feeding a bridge and a cap for about 160 volts of B+. The main problem here was that any movement on the power cord would cause the heavy wall wart to fall out of the outlet.
That amp went through dozens of changes before meeting the trash can, but the final iteration held my ADHD powered attention for over a year. This all happened in the late 80's to early 90's before lithium batteries were common, so the cheap battery of choice was the common 7.2 volt NiCad pack for RC cars. The 7.2 volt battery powered the 6 volt heaters wired in parallel directly. B+ came from a DIY DC to DC converter that made about 160 volts at idle, and 140 when the amp was cranked to 11. Line power was a 6.3 volt Radio Shack transformer feeding a bridge made with Schottky diodes and a big fat cap. Maximum power output was about 2 watts when plugged in.
After that one got trashed I built a super tiny version that used a pair of subminiature tubes (don't remember the numbers) for the preamp and the same pair of 6AK6's for the output stage. A similar power system was used except that my DIY boost converter put out more voltage. Power was about 2 watts, and the whole thing went into a DIY guitar body. The tubes were visible under the Plexiglass pick guard on the front, and the 7.2 volt battery went into the pocket on the back where the bridge springs go. I used a Telecaster bridge which has no springs and put a 4 inch speaker in the body. I thought it was a neat idea, based on an old solid state version that was sold at the Lafayette Radio Electronics store where I grew up. The manager there used to let me play all the guitars since I was pretty good at it and it brought attention to them. As usual I grew tired of it and cannibalized it for parts.
@Tubelab_com: Thanks for the detailed response. There are many useful ideas here that I can follow up on.
For now, the first idea that comes to mind is how I can return to using the 6N2P tube. I would stay with the 187D56 transformer and the original schematic but simply omit the 6N2P heater in the series filament string. Then I get a 120VAC to 6VAC wall wart rated for approx .5A and use that just for the 6N2P heater. Would this work? It'd be a cheap workaround.
For now, the first idea that comes to mind is how I can return to using the 6N2P tube. I would stay with the 187D56 transformer and the original schematic but simply omit the 6N2P heater in the series filament string. Then I get a 120VAC to 6VAC wall wart rated for approx .5A and use that just for the 6N2P heater. Would this work? It'd be a cheap workaround.
That should work. You might want to put a resistor in series with the 50C5's heater. Leave room for it, then measure your actual heater voltage after everything is built. Most Hammond transformers tend toward more voltage than spec. 56 volts isn't too much, but 60 is. I would aim for about 48 volts if you need a resistor. If you plan on using this in several places where the line voltage varies, adjust accordingly. My line voltage goes as high as 127 volts, so I aim for the nominal voltage. If your line is say 117 volts aim low so that the amp won't cook on a 127 volt outlet. The 50C5 is speced for 135 volts max on the plate and 117 volts on the screen. If you keep the screen in the 100 to 105 volt range the plate will eat several hundred volt as long as the average dissipation is kept below 7 to 10 watts. Class A is worst case so I don't push these little guys much past 7 watts, but I have a pair of 50B5's running AB2 in push pull on 340 volts making 20 watts. I set it at 20 watts and left it there overnight. The next day it was sitting at 20.02 watts with no issues.
The 50C5 runs HOT even by data sheet specs. The tube has a 7.5 watt heater and the "typical operation" data in GE's data shows it running at 98% of maximum rating which is 5.5 watts of plate dissipation. Anyone who ever fixed old radios knows that even with this heat load the 35W4 rectifier tube will die before the 50C5 does. Any reduction in unwanted heat will make the tube last longer.
The 50C5 runs HOT even by data sheet specs. The tube has a 7.5 watt heater and the "typical operation" data in GE's data shows it running at 98% of maximum rating which is 5.5 watts of plate dissipation. Anyone who ever fixed old radios knows that even with this heat load the 35W4 rectifier tube will die before the 50C5 does. Any reduction in unwanted heat will make the tube last longer.
Thanks for the tips. Now I can begin to get things together for this build. Won't be able to start till the end of May though. I'll get back here then. I appreciate the help.
I finally got around to building it this week. Big typo alert for anyone using the schematic (it was carried over from the one I based mine on): the first electrolytic capacitor (220uf 75V) in the power supply (before the "doubler" diodes in the Greinacher circuit) has the polarity backwards -- the positive side should point to the diodes.
It works (makes sound). I need to tweak B+ voltages and chase down some hum issues. It's being build into an old film projector case (RCA 400 jr) which has a 7 in speaker. For fun, I might add a VVR for the ultimate bedroom amp.
Thanks for the help. The Hammond 187D56 power transformer and a 6.5V wall wart definitely works well enough.
It works (makes sound). I need to tweak B+ voltages and chase down some hum issues. It's being build into an old film projector case (RCA 400 jr) which has a 7 in speaker. For fun, I might add a VVR for the ultimate bedroom amp.
Thanks for the help. The Hammond 187D56 power transformer and a 6.5V wall wart definitely works well enough.