There have been three users who had their FRED diodes zapped when flipping the standby switch. One user also had their 5AR4 fatally arc over from flipping the standby switch.
I have found that the voltage spike comes from the collapsing magnetic field in the power transformer. There is current flowing through an inductor (the transformer secondary) when the switch is opened the current has no path to flow, so the voltage can build up until it finds a path. This is the same principle used in an automobile ignition coil. If the switch just happens to be opened at the current peak of the AC cycle the voltage can rise to very high levels. I have seen 2500 volt spikes which can zap the FRED diodes, and possibly the rectifier tube or even the power transformer. The magnitude of the spike seems to be related to the power transformer itself. The surplus transformers that I have used to make amplifiers create a small spike, and all of the transformers are the same, while the Allieds (Hammond made) vary considerably from unit to unit. Adding a resistor and capacitor across the switch does seem to dampen the spike considerably, but doesn't completely eliminate it. There may be some combination of transformer and diodes that could still be vunerable.
It should also be noted that the diodes are not needed if you never intend to use solid state rectification. Just leave them out of the PC board.
For soft start add an "inrush current limiter" in series with the AC line input. There are special thermistor type devices. When cold they have a high resistance (say 50 ohms) which is in series with the AC input causing a low current to flow into the amplifier and causing the current limiter to heat up. As the limiter heats up its resistance drops until it is about .5 ohms. This process takes about 5 seconds depending on the ambient temperature. I use GE CL-80 devices for small tube amps like the SimpleSE (Digikey KC008L). CL-90 is suitable for smaller stuff while the CL-60 goes in the big boys.
The "proper" method for switching from triode to UL is to shut power off. I have flipped the switch live many times in my amps with only a small pop in the speakers. Again, there may be some combination of parts that could respond unfavorable to this action, so I can't recommend it. It would be especially unrecommended with expensive highly efficient speakers where the transient could be disastrous.
I have found that the voltage spike comes from the collapsing magnetic field in the power transformer. There is current flowing through an inductor (the transformer secondary) when the switch is opened the current has no path to flow, so the voltage can build up until it finds a path. This is the same principle used in an automobile ignition coil. If the switch just happens to be opened at the current peak of the AC cycle the voltage can rise to very high levels. I have seen 2500 volt spikes which can zap the FRED diodes, and possibly the rectifier tube or even the power transformer. The magnitude of the spike seems to be related to the power transformer itself. The surplus transformers that I have used to make amplifiers create a small spike, and all of the transformers are the same, while the Allieds (Hammond made) vary considerably from unit to unit. Adding a resistor and capacitor across the switch does seem to dampen the spike considerably, but doesn't completely eliminate it. There may be some combination of transformer and diodes that could still be vunerable.
It should also be noted that the diodes are not needed if you never intend to use solid state rectification. Just leave them out of the PC board.
For soft start add an "inrush current limiter" in series with the AC line input. There are special thermistor type devices. When cold they have a high resistance (say 50 ohms) which is in series with the AC input causing a low current to flow into the amplifier and causing the current limiter to heat up. As the limiter heats up its resistance drops until it is about .5 ohms. This process takes about 5 seconds depending on the ambient temperature. I use GE CL-80 devices for small tube amps like the SimpleSE (Digikey KC008L). CL-90 is suitable for smaller stuff while the CL-60 goes in the big boys.
The "proper" method for switching from triode to UL is to shut power off. I have flipped the switch live many times in my amps with only a small pop in the speakers. Again, there may be some combination of parts that could respond unfavorable to this action, so I can't recommend it. It would be especially unrecommended with expensive highly efficient speakers where the transient could be disastrous.
Hi Jimazz,
Yes, I did find a way, thanks to some help from a friend. I could describe all the connections if you'd like. Are you building an amp and you need multiple output taps as well? Do your transformers have all the necessary output taps?
Mike
Yes, I did find a way, thanks to some help from a friend. I could describe all the connections if you'd like. Are you building an amp and you need multiple output taps as well? Do your transformers have all the necessary output taps?
Mike
yes, I'm building the amp (still gathering the parts tho), and the opt (transcendar 5K with UL tap and 4 and 8 ohm ), so
it would be great if you could describe the connections , or a pic could work fine ..... thanks
it would be great if you could describe the connections , or a pic could work fine ..... thanks
Jimazz,
Okay, here are the connections that I made:
1) 0 ohm tap is connected to the negative (black) binding post AND to cathode feedback (as shown in the schematic)
2) 4 ohm tap is connected ONLY to the 4 ohm positive (red) binding post
3) 8 ohm tap is connected to the 8 ohm positive (red) binding post AND to ground on the board (!)
4) 16 ohm tap is connected ONLY to the 16 ohm positive (red) binding post
Does that cover all the connections you needed to know about? If not, just let me know. I'll attach a photo of the wiring, too.
By the way, I really like the sound of this amp. I ended up spending a fortune on good parts ($1200 total!!) but it seems to be worth it. A friend of mine here in Boulder built one as well (using the same high-end parts), and he feels that it's one of the best amps he's ever heard. I wouldn't disagree. Some audiophiles have a problem with the electrolytic caps in the circuit, but it sounds great, regardless.
Good luck with it! It's fun building your own amp that sounds great.
By the way, I'd be glad to email you my bill of materials if you're interested.
Mike
Okay, here are the connections that I made:
1) 0 ohm tap is connected to the negative (black) binding post AND to cathode feedback (as shown in the schematic)
2) 4 ohm tap is connected ONLY to the 4 ohm positive (red) binding post
3) 8 ohm tap is connected to the 8 ohm positive (red) binding post AND to ground on the board (!)
4) 16 ohm tap is connected ONLY to the 16 ohm positive (red) binding post
Does that cover all the connections you needed to know about? If not, just let me know. I'll attach a photo of the wiring, too.
By the way, I really like the sound of this amp. I ended up spending a fortune on good parts ($1200 total!!) but it seems to be worth it. A friend of mine here in Boulder built one as well (using the same high-end parts), and he feels that it's one of the best amps he's ever heard. I wouldn't disagree. Some audiophiles have a problem with the electrolytic caps in the circuit, but it sounds great, regardless.
Good luck with it! It's fun building your own amp that sounds great.
By the way, I'd be glad to email you my bill of materials if you're interested.
Mike
Attachments
thanks mluckow,
yeap, that covers all the connections I need to know
feel free to mail me, also can you tell me what part upgrade do you think was more critical??
thaks again
yeap, that covers all the connections I need to know
feel free to mail me, also can you tell me what part upgrade do you think was more critical??
thaks again
Hi Jimazz,
I tried to send you an email but the system wouldn't let me. What's your email address? If you prefer, you can email it to me at: mluckow ( a t ) comcast dot n e t. (Can you understand my email address? I altered it to prevent spam software from figuring it out.)
I'm not an expert, but I was told that the most important components of the amp are the output transformers (OPTs), the signal wires, the volume potentiometer, and the two coupling capacitors (C11 and C21). Some audiophiles say that the rectifier tube is also important, and the best is a Mullard 5AR4 or GZ34, which you can buy on eBay. (However, I'm using a cheap Sovtek rectifier tube now and it seems to sound fine to me. But many people say that Mullard is better, so I'll probably buy one, too.) Also, if you have any switches, it's important that they're good so they make good connections.
Mike
I tried to send you an email but the system wouldn't let me. What's your email address? If you prefer, you can email it to me at: mluckow ( a t ) comcast dot n e t. (Can you understand my email address? I altered it to prevent spam software from figuring it out.)
I'm not an expert, but I was told that the most important components of the amp are the output transformers (OPTs), the signal wires, the volume potentiometer, and the two coupling capacitors (C11 and C21). Some audiophiles say that the rectifier tube is also important, and the best is a Mullard 5AR4 or GZ34, which you can buy on eBay. (However, I'm using a cheap Sovtek rectifier tube now and it seems to sound fine to me. But many people say that Mullard is better, so I'll probably buy one, too.) Also, if you have any switches, it's important that they're good so they make good connections.
Mike
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