I just snagged my first Stereo 70. I was going to modify it using Joe Curcio's boards...then I had an idea but first I need further information....
My idea is basically to buy a second power transformer and create two monoblocks. Here's my question:
How much power will A470's be able to handle? Would it be possible to run 4 EL34's into each transformer?
My understanding is I can run the St70 in mono mode and simply parallel the outputs. My thinking here is rather than have two transformers back feeding into each other, it might be better to use just one. Any opinions about this? Does it make any difference?
Lastly, can the ST70 use any other output tubes besides the EL34's? Any recommendations?
I just thought that I'd toss this out there as an idea...
Thanks,
My idea is basically to buy a second power transformer and create two monoblocks. Here's my question:
How much power will A470's be able to handle? Would it be possible to run 4 EL34's into each transformer?
My understanding is I can run the St70 in mono mode and simply parallel the outputs. My thinking here is rather than have two transformers back feeding into each other, it might be better to use just one. Any opinions about this? Does it make any difference?
Lastly, can the ST70 use any other output tubes besides the EL34's? Any recommendations?
I just thought that I'd toss this out there as an idea...
Thanks,
The Curcio circuit works great.
You can double up the EL34s, but you won't get much more power, since you're still dealing with a 4300 ohm p-p impedance. It will allow you to run closer to Class A, if that's important to you.
The power handling of those transformers will depend on how low you ask them to go. At midband, 100 watts is not unreasonable to expect, but don't ask the poor things to do that at 20 Hz. Other tubes that will work include 2A3, 7027, 7591, and 6550.
You can double up the EL34s, but you won't get much more power, since you're still dealing with a 4300 ohm p-p impedance. It will allow you to run closer to Class A, if that's important to you.
The power handling of those transformers will depend on how low you ask them to go. At midband, 100 watts is not unreasonable to expect, but don't ask the poor things to do that at 20 Hz. Other tubes that will work include 2A3, 7027, 7591, and 6550.
Primary impedance of A470 should be 6.6K...
You can use 6L6GC/7027 series, like the first ARC tube power amp (modified from a ST-70, I do have the schematic if needed)
Don't try to parallel the power tubes, it will draw double amount of current through the output transformers, will probably kill you outputs...
You can use 6L6GC/7027 series, like the first ARC tube power amp (modified from a ST-70, I do have the schematic if needed)
Don't try to parallel the power tubes, it will draw double amount of current through the output transformers, will probably kill you outputs...
Really... I didn't check with the Dyna catalogue, but I remember only the output from Dynaco Mark III is 4.3K primary 100W which can handle parallel tubes. I did mod over 30 pcs of ST-70 in the past, the spec of the outputs are pretty nice (better than MKIII, but not the oldest potted one since it's actually Acrosound) but can't handle too much current (fried couple in experiments, 70ma per side is the max... total don't over 150ma)
janneman said:
Mike LeFevre of Magnequest, the holder of the designs and rights for a lot of the classic iron now reckons were about the same counting for inflation. I'll see if I can find the post.
His handmade MQ470's are only about 6x the price of the mass produced Dyna's several decades later.
But it would still be nice if they were the same actual dollar amounts today. I'd really like a pair of the A410.
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