Hello good people
I have just purchased a pair of Welborne DRD 45 power amps, and as a newcomer to this amplifier I have a few questions, that don't seem to be covered elsewhere.
1. On first appearance it seems to have a valve rectifier, but when I look inside, it's only the heater that's connected to the rectifier valve for effect. Is this normal?
2. Can anyone direct me to a link for the schematic and build manual?
3. It plays beautifully, but the levels of hum from my open baffles is annoying. I have minimised it using the hum pots, but what else can I do?
4. Is there a forum where debate on these amps is centred?
Thanks Chris
I have just purchased a pair of Welborne DRD 45 power amps, and as a newcomer to this amplifier I have a few questions, that don't seem to be covered elsewhere.
1. On first appearance it seems to have a valve rectifier, but when I look inside, it's only the heater that's connected to the rectifier valve for effect. Is this normal?
2. Can anyone direct me to a link for the schematic and build manual?
3. It plays beautifully, but the levels of hum from my open baffles is annoying. I have minimised it using the hum pots, but what else can I do?
4. Is there a forum where debate on these amps is centred?
Thanks Chris
That build schematic clearly shows a fully connected vacuum rectifier.
If yours is not assembled this way, that may have some to do with your hum.
If yours is not assembled this way, that may have some to do with your hum.
AC heating of directly heated triodes are a common source of hum. Using DC on the filament will eliminate this. That's one of the reasons I designed the Universal Filament Regulator (and built it into my DG300B amp).
Another source of hum is the B+. A voltage regulator such as my 21st Century Maida Regulator is a way to address this. You can also apply LC filtering, but that tends to get expensive due to the cost of the inductor.
Tom
Another source of hum is the B+. A voltage regulator such as my 21st Century Maida Regulator is a way to address this. You can also apply LC filtering, but that tends to get expensive due to the cost of the inductor.
Tom
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A good quality/condition 45 triode can be very quiet in a properly designed and built circuit. My 45 SET amps exceed 80dB S/N referenced to 1-watt output with AC filaments and a common CLC power supply filter. I have a few pairs of 45s that approach 90dB S/N with AC heating.
You should likely measure the output noise and get a reference. Also check to ensure the filter caps are all good, you can also look at measuring the ripple on the plate supply. Finally, the hum adjustment pot is not going to give you a sharp null due to it's overall series resistance. You can pad it with a pair of 12-15 ohm resistors, one from each end to the wiper. This will reduce the series resistance from about 25 ohms down to about 5 ohms.
If you still have trouble getting lower noise, you could have either a circuit problem, layout issue or a set of 45 triodes that are in poor overall condition.
Regards, KM
You should likely measure the output noise and get a reference. Also check to ensure the filter caps are all good, you can also look at measuring the ripple on the plate supply. Finally, the hum adjustment pot is not going to give you a sharp null due to it's overall series resistance. You can pad it with a pair of 12-15 ohm resistors, one from each end to the wiper. This will reduce the series resistance from about 25 ohms down to about 5 ohms.
If you still have trouble getting lower noise, you could have either a circuit problem, layout issue or a set of 45 triodes that are in poor overall condition.
Regards, KM
Thanks guys, some good info there.
I am still looking to discount any simple things that could cause the hum, such as system ground loops.
Here is a little experiment I did last evening:
1. Discernible hum when system all powered up. The hum does not change if the volume control of the TVC is turned right up
2. All other components of the system then turned off, except the two power amps. The hum remains the same.
3. Input phono cables to the 'live' power amps unplugged and the hum gets considerably louder.
Not sure if that means anything to you experts 🙂
I am still looking to discount any simple things that could cause the hum, such as system ground loops.
Here is a little experiment I did last evening:
1. Discernible hum when system all powered up. The hum does not change if the volume control of the TVC is turned right up
2. All other components of the system then turned off, except the two power amps. The hum remains the same.
3. Input phono cables to the 'live' power amps unplugged and the hum gets considerably louder.
Not sure if that means anything to you experts 🙂
There is one glaring problem with the amp: all the current for the output tube goes through the wiper of the hum pot, P1. I have a Welbourne DRD amp and when I paralleled 10 ohm resistors with the wiper the hum was dramatically reduced and the amp sounded much better. The two 10 or 15 ohm resistors go from each end of the pot to the wiper so the majority of the current goes through the resistors not the wiper. It was a dramatic improvement.
That sounds a neat idea. What affect does it have on the pot. Does it change its null point position or its sensitivity. At the moment it seems very crude adjustment and also crackles. Would it help to clean it with DeOxit?
Oddly, I already addressed this in my previous post, but apparently everyone missed it. Padding the balance pot has been a common concept for decades. It doesn't change the null point, but makes the adjustment less sensitive, i.e., a bit wider. The reason it reduces the hum is due to the series resistance (with the bypass cap) going from 25 ohms down to about 5 ohms.
Regards, KM
Regards, KM
Thanks.
I am not clever enough at electronics to understand all this detail, but I have a friend who is, and will pass on all contributions.
I very much appreciate people helping.
I am not clever enough at electronics to understand all this detail, but I have a friend who is, and will pass on all contributions.
I very much appreciate people helping.
I have a further question now 🙂
It seems that Ron Welborne switched from valve to SS rectifiers many years ago, claiming a better sound.
My amp seems to be equipped to take either and is at present SS.
If I was to play devils advocate I would argue that the SS rectifier is cheaper to produce and easier to assemble, so it would obviously get a recommendation from Ron on the commercial side. Also companies like Audionote on their very expensive 45 Paladin amps still use valve rectifiers.
Anyone have any strong opinions on this?
It seems that Ron Welborne switched from valve to SS rectifiers many years ago, claiming a better sound.
My amp seems to be equipped to take either and is at present SS.
If I was to play devils advocate I would argue that the SS rectifier is cheaper to produce and easier to assemble, so it would obviously get a recommendation from Ron on the commercial side. Also companies like Audionote on their very expensive 45 Paladin amps still use valve rectifiers.
Anyone have any strong opinions on this?
Most will have opinions on this. In short, you can design a good power supply with either type of rectification. Also, it should be obvious that a pair of diodes are going to be less expensive than a single tube socket, much less the tube itself. This pretty much states that cost can be a high factor, as vacuum tube rectifiers can command a high price tag, especially the 5AR4 and it's European counterpart GZ34.
Now, getting back to the circuit at hand, note that the 45 triode operates best at around 34ma of current. Also note that it takes a little bit of warm-up time before conduction evens out across the filament/cathode wire. The input/driver tube will take much longer before it starts to conduct, so with a direct-coupled design, the 45 will wind up running hotter until the 6N1P warms up.
This is due to the fact that solid-state rectification will provide a plate supply almost instantly. A vacuum tube rectifier will ramp up voltage more slowly and allow time for the other tubes to warm up and stabilize. In my view, this is a better design and if done correctly will easily provide a high quality power supply for the amplifier circuit. My smaller SET amps (45 and 2A3 outputs) use a 5V4G rectifier tube which can easily handle the current requirements.
Of course the above counts as my opinion... and I'll stick with it, as it's based on my involvement in this stuff for half a century.
Regards, KM
Now, getting back to the circuit at hand, note that the 45 triode operates best at around 34ma of current. Also note that it takes a little bit of warm-up time before conduction evens out across the filament/cathode wire. The input/driver tube will take much longer before it starts to conduct, so with a direct-coupled design, the 45 will wind up running hotter until the 6N1P warms up.
This is due to the fact that solid-state rectification will provide a plate supply almost instantly. A vacuum tube rectifier will ramp up voltage more slowly and allow time for the other tubes to warm up and stabilize. In my view, this is a better design and if done correctly will easily provide a high quality power supply for the amplifier circuit. My smaller SET amps (45 and 2A3 outputs) use a 5V4G rectifier tube which can easily handle the current requirements.
Of course the above counts as my opinion... and I'll stick with it, as it's based on my involvement in this stuff for half a century.
Regards, KM
Thanks KM
My friend is also advising a change to valve rectification. This is how he describes it "It is kind of like having wings but still being on the ground instead of flying."
My friend is also advising a change to valve rectification. This is how he describes it "It is kind of like having wings but still being on the ground instead of flying."
Did you get this sorted out?
There was an upgrade package that included the SS power supply that also added a common mode choke to the 45 filament supplies.
The common mode choke is just like a ferrite core with the filament supply wires wrapped round it. This apparently was an upgrade to DRDs running in America but caused hum in DRDs running on 230V - 240V AC.
If your amp has this common mode choke it's best to remove it.
There was an upgrade package that included the SS power supply that also added a common mode choke to the 45 filament supplies.
The common mode choke is just like a ferrite core with the filament supply wires wrapped round it. This apparently was an upgrade to DRDs running in America but caused hum in DRDs running on 230V - 240V AC.
If your amp has this common mode choke it's best to remove it.
Yes thank you. In the UK we have a start up business called SW1X run by Slawa Roschkow who is a genius at valve gear. He junked the SS power supply, which was full of cheap components, added extra capacitance and changed the rectifier valves for 5U3C military grade.
The amps are nicely quiet now and sound beautiful
The amps are nicely quiet now and sound beautiful
My hum pots are still very scratchy but the hum can be much better adjusted out when they have been cleaned with Deoxit. So I have decided to replace them and also do this mod with the parallel resistors.
I have two questions to optimise SQ
1. Is the quality of the pot important and what would people recommend.
2. Are the quality of the parallel resistors important and what would people recommend
Thanks
Trying to get my head around the internals of the hum pot.
Assume its a 100R pot and that the minimum hum point is exactly mid position. Does each leg now see 100 ohms, 50 ohms or zero ohms?
Assume its a 100R pot and that the minimum hum point is exactly mid position. Does each leg now see 100 ohms, 50 ohms or zero ohms?
Quality of the pot is reasonably important, but only so far as needing to be something of good quality, and being able to dissipate the heat it will produce at the desired voltage (with some headroom for derating so it doesn't run too hot)
Resistor quality is about the same. Basic carbon or metal film will be fine, wirewound being less preferable but it would still work. Derate the wattage on these as well so they don't run too hot.
In such a position/application going for super fancy audiphool types would be a fool's errand. Good quality parts of adequate wattage will do perfectly fine, sourced from a reputable retailer.
At mid position the pot will have 50R on either side of the wiper, basic resistor divider theorem. With AC from the filaments across the pot, and the pot adjusted for balance, you should see equal voltage on either side of the wiper if adjusted for proper hum cancelling, but in the real world this may not be the case, depending on how well the filaments of the tubes were assembled.
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