Do you already have a power xfmer for this project? If so what are it's ratings and available outputs? If you do not yet have the power xfmer is it your plan to scrap the P-P amp later when you build the SET amp for the power supply parts, or will you leave the P-P amp intact and start the SET from scratch?
I'll probably start from stracth, but I'm not sure if i will even be able to make an SE amp this summer, one might be enough for someone going into College (tube amp+dorm=VERY BAD!).
I'm looking at a quite simple HK amp that I want to make because I love the brand, and there are fairly few caps in the audio path. The only problem that I can't quite figure out is that the power transformer has an output of 150v AC before the power supply circut
This amp Packs a punch though, 25wpc
http://manuals.harman.com/HK/Owner's Manual/HK250-om.pdf
any help on tackling this would be wonderful
I'm looking at a quite simple HK amp that I want to make because I love the brand, and there are fairly few caps in the audio path. The only problem that I can't quite figure out is that the power transformer has an output of 150v AC before the power supply circut
This amp Packs a punch though, 25wpc
http://manuals.harman.com/HK/Owner's Manual/HK250-om.pdf
any help on tackling this would be wonderful
That Harmon Kardon amp uses a solid state diode voltage doubler circuit. You don't have to use that.
If it were my amp I'd get a 400-0-400 volt @200 mA xfmer (Hammond 278X) and use a choke input filter with a two diode full wave rectifier. You can use a 5AR4 tube rectifier or a pair of 1N5408's. The SS diodes will give you more output power and punch. Choke input filter allows you to use much more filter capacitance as the first B+ filter which will give more current available for power transients in the output tubes. It gives better B+ regulation as well. Choke could be 5 to 10 Henries at 200 mA rating. If you use SS rectifiers you should add a time delay or you will have too much B+ immediately at turn on until the tubes warm up. A slick way to do that is to put a 5AU4 damper diode in series with the B+ out of the two diodes before it hits the choke. The 5AU4 won't pass the DC until it warms up at the same rate as the 6L6's.
If it were me, I'd start simple with a 5AR4 rectifier. If you need more power then the same socket will support a 5AU4 and the SS diodes hang under the chassis no problem.
If it were my amp I'd get a 400-0-400 volt @200 mA xfmer (Hammond 278X) and use a choke input filter with a two diode full wave rectifier. You can use a 5AR4 tube rectifier or a pair of 1N5408's. The SS diodes will give you more output power and punch. Choke input filter allows you to use much more filter capacitance as the first B+ filter which will give more current available for power transients in the output tubes. It gives better B+ regulation as well. Choke could be 5 to 10 Henries at 200 mA rating. If you use SS rectifiers you should add a time delay or you will have too much B+ immediately at turn on until the tubes warm up. A slick way to do that is to put a 5AU4 damper diode in series with the B+ out of the two diodes before it hits the choke. The 5AU4 won't pass the DC until it warms up at the same rate as the 6L6's.
If it were me, I'd start simple with a 5AR4 rectifier. If you need more power then the same socket will support a 5AU4 and the SS diodes hang under the chassis no problem.
rcavictim said:
Wow! okay thats alot to take in all at once, however most of it makes sense, however what should I use for the filtering caps, and If i use an SS rectifier (two diodes) in order to prevent premature B+ voltage, could I just include a standby switch?
Chokes scare the crap out of me, I think they are the last passive component I don't really understand, I know their genral purpose, and how they work (Woo! go AP physics) but i'm just not comfortable with them yet
alexmoose said:
Yes you could simply place a SPST toggle switch between the CT of the HV winding and ground to turn the B+ on and off.
If you do not use a choke you will have higher inrush currents which makes capacitor selection more critical and will have to go to a lower voltage power xfmer. A CRC supply filter will be needed.
Chokes aren't THAT scary. The only thing to watch is if you disconnect them while they are passing DC current they will develop a high voltage across their terminals as the mag field in their core colapses. As hard wired into the amp power supply that will not be an issue. I highly recommend using a choke for the great benefits that it will provide, like taking the beating and heating out of the power supply electrolytics, and smoother, better regulated DC with less ripple (hum). The screen grids need really good voltage regulation in this amp. Any change in voltage will equate to distortion. With a choke input filter and larger main filter you have the ability of reducing the screen grid supply fluctuation based distortion.
You're right, and you know alot more about this stuff than me, the choke you described earlier is only 24.95 from hammond, however these parts added together are probably going to breach my $300ish budget. Where do you guys buy your Iron, and other components? Most every part people have mentioned on this forum, I have looked up on tubesandmore.com, is there a better place?
I heard someone mention that microwave oven transformers can be used as plate chokes. Use the HV winding. This is capable of passing at least 200mA without significant heat issues. These can be had as salvage at any recycling place.
Shoog
Shoog
Shoog said:
Actually that was me. Yes that would probably work fine but one would best install such a choke in the negative supply end so that the core of the MOT which is tied to one side of the HV winding, can be grounded to the chassis.
Good idea Shoog!
Okay, This concept sounds good to me, however now is the tricky part, envisioning exactly how the circuit is going to be laid out. I know some of how powersupply circuits need to be laid out however this seems a little big for my modest knowladge of HV circuits, so does anyone have any suggestions and ideas as to how to bring the concept to life?
Hi Alex,
Most nuker transformers are rather heavy. You may be further ahead using a real choke. You may not need one at all with a 6BQ5 P-P amp. Many commercial units did not use a choke. That will save some money and weight.
-Chris
Most nuker transformers are rather heavy. You may be further ahead using a real choke. You may not need one at all with a 6BQ5 P-P amp. Many commercial units did not use a choke. That will save some money and weight.
-Chris
I agree that Nuker transformers are a little too heavey, espicially when the Hammond one I need is only 25 dollars, and alot smaller. I have 4 6BQ5s already, which would reduce the cost of the amplifier as whole, however I kinda want to try 6l6s, as I did repair an SE 6BQ5 Hifi earlier this year. I also truely love Harmon Kardon, so I really want to make their deisgn
Hi Alex,
The 6BQ5 type will be a little more forgiving, and the outputs are much less expensive.
If you are fixed on using 6L6's then up your budget and do it right. I personally like them for sound as well. Understand that your costs just went up a lot. Gain is higher and lead dress more critical.
I would start with a 6BQ5 project and listen to it as you build the 6L6 version later. You will learn some things along the way.
-Chris
The 6BQ5 type will be a little more forgiving, and the outputs are much less expensive.
If you are fixed on using 6L6's then up your budget and do it right. I personally like them for sound as well. Understand that your costs just went up a lot. Gain is higher and lead dress more critical.
I would start with a 6BQ5 project and listen to it as you build the 6L6 version later. You will learn some things along the way.
-Chris
Best to decide on what output tubes you wish to use before designing the power supply, or anything else.
I have changed my mind about this I think. I'd like to now suggest that you just buy a ready made Dynaco ST-70. It is on budget.
I have changed my mind about this I think. I'd like to now suggest that you just buy a ready made Dynaco ST-70. It is on budget.
"Actually that was me. Yes that would probably work fine but one would best install such a choke in the negative supply end so that the core of the MOT which is tied to one side of the HV winding, can be grounded to the chassis."
How does that work with their use as plate chokes. Do you have to isolate them from the chassis, or can you disconnect the winding from the core ??
Shoog
How does that work with their use as plate chokes. Do you have to isolate them from the chassis, or can you disconnect the winding from the core ??
Shoog
Shoog said:
Give us a detailed explanation of your avatar, and I'll give you a detailed explanation of using a MOT as a audio plate choke. 😀
Just our neighbours dog(border collie) who stuck her nose into my digital cam one day - I liked it so I use it.
Shoog
Shoog
anatech said:
I could go with 6BQ5S if you guys think its easier, I have of 4 of them two vintage 1962, two brand new, however I had trouble finding a good p-p schematic for EL84/6BQ5s.
I did find "Baby Heuy" http://www.diyparadise.com/simpleel84.html
However I was driven away by the massive cost of the Transformers. I think that price for the Iron 280 dollars, is a little more in reach if i get some quick cash on the freebie sites. does seem like a good porject? or is it too much cost for too little output?
http://www.diyparadise.com/buildel84c.html This is the more current, better version
Hi Alex,
Price out Hammonds at some of the suppliers. There are other suitable brands as well.
You can then clone an HK design if you wish. Other good sounding amps are Eico and Heathkit. I believe SY has a very good sounding 6BQ5 type amplifier project on the go. That would be very worthwhile.
Actually. Build SY's amp. I'll try and find a link for you.
-Chris
Price out Hammonds at some of the suppliers. There are other suitable brands as well.
You can then clone an HK design if you wish. Other good sounding amps are Eico and Heathkit. I believe SY has a very good sounding 6BQ5 type amplifier project on the go. That would be very worthwhile.
Actually. Build SY's amp. I'll try and find a link for you.
-Chris
Shoog said:
I have been looking at that thing for a long time and wondering what the heck it was! I thought it was some kind of fish but now I can see the dog. 😀
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