Ralph,
It may be possible to reduce the amount of hum injected by a slight re-balance. Your circuit works by assuming that equal amounts of ani-phase 50Hz are injected into each side of the filament (heater).
There are 2 compromises here:
1 Assumption that the centre tap is exactly central.
2 Assumption that the filament is perfectly symetrical.
To compensate for both these imperfections, it is common to fit a hum balance pot.
Unfortunately, it is not easy to incorporate a hum pot into a centre tapped transformer scheme. But there are other ways.
Swapping the wires on the filament pins may make the hum better or worse. Both 300B's might be different.
While I think about the best way, perhaps you could measure the hum AC voltage, (across the speaker terminals). I would expect a value of 5mV or so to be typical.
If it is this value, but unacceptable to you, you may have to change to DC heaters.
Cheers,
It may be possible to reduce the amount of hum injected by a slight re-balance. Your circuit works by assuming that equal amounts of ani-phase 50Hz are injected into each side of the filament (heater).
There are 2 compromises here:
1 Assumption that the centre tap is exactly central.
2 Assumption that the filament is perfectly symetrical.
To compensate for both these imperfections, it is common to fit a hum balance pot.
Unfortunately, it is not easy to incorporate a hum pot into a centre tapped transformer scheme. But there are other ways.
Swapping the wires on the filament pins may make the hum better or worse. Both 300B's might be different.
While I think about the best way, perhaps you could measure the hum AC voltage, (across the speaker terminals). I would expect a value of 5mV or so to be typical.
If it is this value, but unacceptable to you, you may have to change to DC heaters.
Cheers,
John,
Don't ask me how, but I got my tweeter back on both speakers. Very very strange indeed. As for the hum, I think I'd have to make some progress first on the correct voltage and current settings for the 300B's and to choose an appropriate biassettting. After that, building it into a nice chassic and see what the hum is after all that. Now I measure 0,06V (left) and 0,07V (right) on the AC scale. This is not acceptable for me, but we'll fix this later. I've anticipated for DC on the 300B's so I got me a extra 0-14V secundary on the trafo's. So I can make an extra DC supply like the one I use for the driver tube.
Cheers, Ralph
Don't ask me how, but I got my tweeter back on both speakers. Very very strange indeed. As for the hum, I think I'd have to make some progress first on the correct voltage and current settings for the 300B's and to choose an appropriate biassettting. After that, building it into a nice chassic and see what the hum is after all that. Now I measure 0,06V (left) and 0,07V (right) on the AC scale. This is not acceptable for me, but we'll fix this later. I've anticipated for DC on the 300B's so I got me a extra 0-14V secundary on the trafo's. So I can make an extra DC supply like the one I use for the driver tube.
Cheers, Ralph
Hi Ralph,
With hum balancing you might not get down to more than half that value.
It seems that DC is probably the way to go on this one.
When you lay out your chassis, make sure to allow for heatsinking of the heater supply regulators, and possibly the rectifiers as well. There's going to be a LOT of heat.
What VA rating are the heater transformers?
Also, consider magnetic field interaction between the transformers and chokes.
Cheers,
John
With hum balancing you might not get down to more than half that value.
It seems that DC is probably the way to go on this one.
When you lay out your chassis, make sure to allow for heatsinking of the heater supply regulators, and possibly the rectifiers as well. There's going to be a LOT of heat.
What VA rating are the heater transformers?
Also, consider magnetic field interaction between the transformers and chokes.
Cheers,
John
Heat,n,stuff
Ralph,
The transformers seem Ok. You'll be dissapating about 10 watts in the regulator, although there are ways to reduce this.
I believe you will need to use Schottky diode rectifiers to avoid the ripple drooping below the regulator's "drop-out" voltage.
Factors to take into account are the the heatsink's efficiency,(thermal resistance in w/degC), the devices thermal resistance, and the maximum ambient temperature.
Alternatively, copy someone else.
It seems to me that you are following a fairly common path trodden by Dutch DIY'ers on this project. Has Dick van de Merwe had any part?
Cheers,
Ralph,
The transformers seem Ok. You'll be dissapating about 10 watts in the regulator, although there are ways to reduce this.
I believe you will need to use Schottky diode rectifiers to avoid the ripple drooping below the regulator's "drop-out" voltage.
Factors to take into account are the the heatsink's efficiency,(thermal resistance in w/degC), the devices thermal resistance, and the maximum ambient temperature.
Alternatively, copy someone else.
It seems to me that you are following a fairly common path trodden by Dutch DIY'ers on this project. Has Dick van de Merwe had any part?
Cheers,
Hi John,
Aha, nice to know that my amplifier is built typical Dutch style? Well it's a small world ofcouse, and I know Triode Dick's site well but the design is not by him but by another Dutch DIY'er. I think the schema is very simple and straightforward and by using good parts I hope to achieve a good sounding result (in the end).
In fact it's a bit of a guess for me this project. I never had any SE tube poweramp in my system and I don't even have (your typical SE) sensitive speakers. I use a 2-way homemade speakersystem with ScanSpeak units, also a Dutch design. But a friend of mine built a 300B SE amp and it sounded lovely in my system, so I thought I'd have a go at it!
Ralph
Aha, nice to know that my amplifier is built typical Dutch style? Well it's a small world ofcouse, and I know Triode Dick's site well but the design is not by him but by another Dutch DIY'er. I think the schema is very simple and straightforward and by using good parts I hope to achieve a good sounding result (in the end).
In fact it's a bit of a guess for me this project. I never had any SE tube poweramp in my system and I don't even have (your typical SE) sensitive speakers. I use a 2-way homemade speakersystem with ScanSpeak units, also a Dutch design. But a friend of mine built a 300B SE amp and it sounded lovely in my system, so I thought I'd have a go at it!
Ralph
Hi Ralph,
I'm sure it will sound at least as good as your friend's, maybe even better if you have better transformers.
What rectifier diodes did you have in mind?
BTW
I had a look at your preamp thread, from a long time ago. Did you ever get it sorted out?
It occurs to me that you might have reduced the hum pick-up by re-orientating the output transformers. And yes, I do expect hum pickup with it switched off too, though the sound of the hum might change with power.
Cheers,
John
I'm sure it will sound at least as good as your friend's, maybe even better if you have better transformers.
What rectifier diodes did you have in mind?
BTW
I had a look at your preamp thread, from a long time ago. Did you ever get it sorted out?
It occurs to me that you might have reduced the hum pick-up by re-orientating the output transformers. And yes, I do expect hum pickup with it switched off too, though the sound of the hum might change with power.
Cheers,
John
Hi John,
My schema and my friends differ quiet a bit, I use interstages and very different tubes. I'm sure it must sound better!
I never solved the hum problem with my preamp, but used a workaround. The optx are very sensitive to hum pickup but there was also another problem. My poweramp has an input sensitivity of 685mV. This was _way_ to sensitive for my preamp, so I heard the hum very loud. I use a voltage devider now that drops the output of the preamp by some 12dB. No hum anymore
Back to my new amp, I use 1N5408 diodes for the 5687 heater and bias circuit. For the 300B I use JAN-5R4-WGB tubes and for the driver I use 5Y3GT's. The diodes for the 5687 heater are paralled with 4 small 100pF caps.
My schema and my friends differ quiet a bit, I use interstages and very different tubes. I'm sure it must sound better!
I never solved the hum problem with my preamp, but used a workaround. The optx are very sensitive to hum pickup but there was also another problem. My poweramp has an input sensitivity of 685mV. This was _way_ to sensitive for my preamp, so I heard the hum very loud. I use a voltage devider now that drops the output of the preamp by some 12dB. No hum anymore
Back to my new amp, I use 1N5408 diodes for the 5687 heater and bias circuit. For the 300B I use JAN-5R4-WGB tubes and for the driver I use 5Y3GT's. The diodes for the 5687 heater are paralled with 4 small 100pF caps.
Ralph,
The path trodden by mant Dutch DIY'ers, is I believe, a good one.
If you haven't ordered the diodes for the KR300B rectifiers, I recommend:
STPS745S Available from Farnell part # 527-210.
The reason is, they are Schottky, and have a plastic coated mounting tab. It makes fitting to the heatsink simple and reliable.
The 25 off price is very good, at least in the UK, and you could share some with a friend.
Mmm, preamps. I don't really like them myself. And as for amplifying the signal, just to attenuate it, well your S/N will suffer.
Maybe you can use that gain with your new amp.
Cheers,
The path trodden by mant Dutch DIY'ers, is I believe, a good one.
If you haven't ordered the diodes for the KR300B rectifiers, I recommend:
STPS745S Available from Farnell part # 527-210.
The reason is, they are Schottky, and have a plastic coated mounting tab. It makes fitting to the heatsink simple and reliable.
The 25 off price is very good, at least in the UK, and you could share some with a friend.
Mmm, preamps. I don't really like them myself. And as for amplifying the signal, just to attenuate it, well your S/N will suffer.
Maybe you can use that gain with your new amp.
Cheers,
PREAMPS
Hi,
Sorry for jumping in...
If you don't need them,junk them and go passive.
Now that is very sensitive indeed.Was this a commercial thing?
John,
The Dutch have it from the French who have it from the Japanese who have it from god knows where??
Thing is,with audio less is often more.
Ciao,
Hi,
Sorry for jumping in...
If you don't need them,junk them and go passive.
Now that is very sensitive indeed.Was this a commercial thing?
John,
The Dutch have it from the French who have it from the Japanese who have it from god knows where??
Thing is,with audio less is often more.
Ciao,
SCHOTTKY
Hi,
You mean for the bias supply?
If yes....I would certainly recommend them.
And while you're putting together an order why not the diodes in the heater supply for the input tube as well.
You may have to readjust the output on the LM317 but that's no problem.
You'll put those 25 pcs to good use in no time.
Cheers,
Hi,
You mean for the bias supply?
If yes....I would certainly recommend them.
And while you're putting together an order why not the diodes in the heater supply for the input tube as well.
You may have to readjust the output on the LM317 but that's no problem.
You'll put those 25 pcs to good use in no time.
Cheers,
Actually, I thought not. Then I simulated it:
At the moment, if your transformer gives out exactly 17volts, your LM317 is going into "dropout". (This means that in the troughs of the ripple, there is insufficient volatage acroos the regulator for it to function properly). This will lead to ripple getting through.
There are 2 easy cures. You can do either one:
1) Change to Schottkey diodes.
or,
2) Increase C1 to 2000uF.
So it might need slight adjustment.
The LM317 will dissapate a little more though.
Happy Schottky shopping....
Cheers,
SE
Hi,
Well,the Belgians are the multilinguists that acted as a go between for the anglosaxons who took 15 years to read the encrypted message?
Actually the French speaking part of Belgium picked it up from the French since the late seventies.
Science evolves slowly ,n'est-ce pas?There is always that occasional *resistor*.
Cheers,
Hi,
Well,the Belgians are the multilinguists that acted as a go between for the anglosaxons who took 15 years to read the encrypted message?
Actually the French speaking part of Belgium picked it up from the French since the late seventies.
Science evolves slowly ,n'est-ce pas?There is always that occasional *resistor*.
Cheers,
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