I have scored some on Ebay for $3. The word is that these are factory rejects, or at best untested back door tube sales. Test them in something non critical at more stringent conditions than you plan to use them in. I got 8 for $30 including shipping about 2 years ago. They all worked and are all still alive, but the alignment isn't as good as the Ruby Tubes selected versions of the same tube (the pair that have been seen glowing in many of my pictures). The cheap ones will develop hot spots on the plates when cranked, while the good ones glow evenly and accept more power without redness.
I tested them in the Eico and they all tested well. As far as uneven alignment...I have yet to see a Chinese 6L6 that had good alignment at all. I've bought a few from AES and others and they all had lots of mechanical inconsistencies. The ones that had uneven glow in the SSE went into my power supply. 🙂
These were all labelled "Mesa", for what it's worth.
These were all labelled "Mesa", for what it's worth.
George and Michael thanks for info!
I understand in a SE amp changing the OT trans. to a higher primary impedence improves sound quality at the loss of some power output.
Does this apply to PP amps?
George also has advanced PP boards he is working on. Maybe out this year?
Randy
I understand in a SE amp changing the OT trans. to a higher primary impedence improves sound quality at the loss of some power output.
Does this apply to PP amps?
George also has advanced PP boards he is working on. Maybe out this year?
Randy
My experience is that's generally true for the case of class A no-NFB triode amplifiers, SE or PP. Improved sound quality in this case would be due to better loudspeaker damping and/or lower THD.
For pentodes and class AB amplifiers the situation is not so simple and sometimes lower primary impedance will result in lower THD. Damping factor vs. negative feedback is another consideration.
In a local "Schade" feedback topology like the DCPP amp, the damping factor will be higher with higher Zpri, but the DCPP is partial local feedback + light global feedback that can be adjusted to suit (within some range).
With local feedback only, the damping factor in class AB would be limited to approximately Zpri/4 * gm where gm is the output tube dynamic transconductance.
I'm reviewing schematics now, but are there any EL34 designs you'd collectively recommend? There are a ton out there, but I'm looking for some customer satisfaction.
More to come...
Edit: It also should be noted that this will be driven from a CDP / iPod and I'd like to keep it passive pre (read: volume pot).
Kofi
More to come...
Edit: It also should be noted that this will be driven from a CDP / iPod and I'd like to keep it passive pre (read: volume pot).
Kofi
I don't know how you feel about PCBs, but you can save some time by using one of the plethora of Dynaco ST70 driver boards. They come in just about every topology you can think of using both noval or octal tubes or even (horror!) some solid state!
I've heard from quite a few people that this is a very good sounding design (pp1c).
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I have searched and only come up with PCBs for the driver board, so I'm assuming this means the power stage is off board?
Looks pretty perfect to me... but a PP with no NFB seems strange. How do we get around notch distortion?
Also-- could this be run ultralinear? And would the OPTs need to be about 120mA to allow for headroom?
Kofi
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Just because it's PP doesn't mean it can't be Class "A". No notching or cross over problems occur, when both tubes of the PP pair have a 100% duty cycle. Allen Wright has advocated forced symmetry, where a CCS is used to set the "idle" current in a Class "A1" PP pair. He loves balanced circuitry and that's as balanced as they come.
The concepts being discussed can be extrapolated to Class "A2" triode wired 6L6GCs (no CCS). Tubelab's PowerDrive would be used between the voltage amplifier and the "finals". Some serious power O/P would be available. As long as the speakers used don't require a "balls to the wall" damping factor, no NFB triode mode circuitry will be OK.
I have a completly balanced amp that was designed in another thread:
http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp.html
That thread covers the entire design from wild idea to finished amp. It is a fully balanced design using two cascaded LTP's. The thread is now at over 400 posts, and Chris if now building his amp. Mine is still in the breadboard stage due to lack of time.
I tried the CCS in the cathode of the optput tube experiment The CCS will force A1 operation even with PowerDrive. You will be rewarded with about 10 of the best sounding watts you ever heard (using trioded 6L6GC's). I plan to revisit this topology with bigger tubes when I have time.
Pure A2 might get you some more power, but why limit things. I have tested AB2 triode mode without any feedback. The sound quality was excellent, and yes "serious power" flowed freely. I believe that this setup would work with any speakers capable of being deiven with a SE amp. My implementation ran enough idle current to keep the tubes in class A for most of the normal listening range. What is serious power, about 40 watts with 6L6GC's and 75 watts with KT88's.
No, this isn't a "simple" amp. In fact the power supply is more complicated than the amplifier is.
A triode strapped EL34 will put out about as much as a UL EL84. Triode sounds sweeter. Where I buy them they (EL34 and EL84) cost about the same (give or take a few pennies).
Does not SE require about 'twice the iron' of a PP?
Mr. Annan's budget is $500 or thereabouts.
Now if you can find an old ST70 (in a garage sale) like the one linked below and use that as your base you'd save a lot of time and money.
Dynakit Stereo 70 – metaruss
http://vacuumntubes.com/red-monster-kt88el34-6sn7-tube-amp-power-amplifier
http://www.flickr.com/photos/goa2008/2735072489/
BTW SY has a EL34 schematic with 6SN7 and 6SL7 tubes somewhere on this forum or maybe this amp is something you can build (maybe replace the 6SJ7 with a 6SL7)
http://www.one-electron.com/amps.html
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That seems to be the sticking point at this time. I like the Allen Wright EL34 job, but I think the iron would be about $300 alone. That would leave $200 for the rest of the project including tubes and chassis, which would likely be tough to do.
Let me research the cost a little further. I'll probably give Jack Eliano a call (Electraprint) and see if he can work some magic on the iron. Believe it or not, his prices have been great and his iron is exceptional to boot.
Kofi
Maybe an all wood diy chassis from recycled wood like I've seen on diyaudio's Bass Horsmen's site might save you $. They have that antique look too. I've seen a few others too so not unpopular.
If you run into a vintage ST-70 amp for low $:
The latest boards for ST-70 are Gregg or Geek on diyaudio from classicvalvedesign.ca. It adapted in SS CCS from Fred Nachbaur's RA-100 direct coupled amp which used tubes only except PS You could also build Fred's. Gregg has as separate PS board. He has a form too & see diytube form.
K & K Audio has an interesting transformer coupled board with CCS. Looks like might be fairly expensive ST-70 board, but no PS board needed.
For non CCS boards diyaudio Elli D. liked the Triode Electronics EF86, ECC99 board as there are two. There is a separate PS board. I think both boards with optional parts kits come to under $175 less tubes.
Another non CCS board is Bob Latino's which looks to be popular. See tubes4hifi.com. He has ST-70 & ST-120 amp kits which use this board and uprated transformers over ST-70 for ST-120. See diytube form.
Randy
If you run into a vintage ST-70 amp for low $:
The latest boards for ST-70 are Gregg or Geek on diyaudio from classicvalvedesign.ca. It adapted in SS CCS from Fred Nachbaur's RA-100 direct coupled amp which used tubes only except PS You could also build Fred's. Gregg has as separate PS board. He has a form too & see diytube form.
K & K Audio has an interesting transformer coupled board with CCS. Looks like might be fairly expensive ST-70 board, but no PS board needed.
For non CCS boards diyaudio Elli D. liked the Triode Electronics EF86, ECC99 board as there are two. There is a separate PS board. I think both boards with optional parts kits come to under $175 less tubes.
Another non CCS board is Bob Latino's which looks to be popular. See tubes4hifi.com. He has ST-70 & ST-120 amp kits which use this board and uprated transformers over ST-70 for ST-120. See diytube form.
Randy
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That's right. Besides the octal sockets, the only things off board audio-wise are the grid stoppers and the cathode resistors used to measure the bias current. Needs a power supply, of course.
I have Bob's board in my ST70 now. It's much better than stock, but I think it would be even better with a CCS in the LTP tail. It also has quite a bit more gain than I'd prefer. It can be built as a low gain version with 12BH7s rather than 12AT7s, but I think I'll end up going to a different board entirely when the time comes.
Minimum cost for new iron for your project as spec'ec seems to be a little over $200 assuming Edcor 50W OPTs ($70 each) and an Edcor PT (about $70). You could reduce the output power below 30W and get into the $50-60 range transformers. Then you might consider EL84s...
You might find a deal on used transformers that could save a little more money.
Yeah, the Dynamutt driver is the one I was probably thinking of:
Classic Valve Design - Dynaco Clone and Original Design Boards
Classic Valve Design - Dynaco Clone and Original Design Boards
Yes DynaMutt! That was the name. Just out in Dec.
He mentions you don't need to find a Pl tube that has matched sections because of CMFB in his design. Can use 9 pin or octal input tubes and has individual bias & balance controls. He mentions a resistor value change in his PS notes for 6L6 family of tubes also. Detailed info in this form and his.
Randy
He mentions you don't need to find a Pl tube that has matched sections because of CMFB in his design. Can use 9 pin or octal input tubes and has individual bias & balance controls. He mentions a resistor value change in his PS notes for 6L6 family of tubes also. Detailed info in this form and his.
Randy
I paid S$200 (about US$150) for 2 Thailand made OPTs in Singapore. (Rs 3000) US$60 for a Power torrroidal transformer made to Wayne's (cogncogs) specs made in India. US$100 for 4 EL34 Svetlana , 2 6SN7 and 1 6SL7 EH tubes and sockets in Singapore and about $110 for the rest of the parts including a SS chassis.
$150+$60+$100+$110 = $420. I think a EL34 PP should fit under $500.
The cats were away yesterday evening. This thread inspired me to hook the ST70 up to the main system again. What fun...it's been a while. Is there a difference between a 15WPC and 35WPC? Yes there is when you want to play it loud. It has JJ KT77s in there.
Back to your original post...is the amp going to sit on a book shelf?
Back to your original post...is the amp going to sit on a book shelf?
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