Jump to content

captainbeefheart

Regulars
  • Posts

    933
  • Joined

  • Last visited

Recent Profile Visitors

The recent visitors block is disabled and is not being shown to other users.

captainbeefheart's Achievements

Forum Veteran

Forum Veteran (4/9)

592

Reputation

  1. I agree they wanted to keep the same connections but I also think there could have been a happy middle ground where you keep the same ins/outs yet arrange the components with a little more care instead of how they do it. The way they do it has a clean look with the resistors all lined up in rows but it's not optimal. As for the new design with all triodes and the LTP, the results is why I said it has worse performance yet they claim better performance, you may be correct in that there was something wrong with the amp Amir tested because for what they have to work with they should easily get much better numbers without much effort. I like the addition of the CCS on the tail of the LTP but they chose a pretty poor component as I showed with the graph because their impedance above 10kHz drops like rock, you want a constant impedance over the entire audible frequency range at the very least. With the same amount of tubes, 3 12AU7's they could have used one bottle for a cascode which will behave just like a pentode but with less noise and then DC couple to a shared bottle where one triode per channel is the split load phase inverter. This would the closest to the original and still improve upon it with less noise. The VTA is essentially a Mullard circuit with a triode first stage instead of a pentode but with worse performance. If we are trying to keep the original sonics then the cascode - split load would be the best option. If you want to change it completely then either do the Mullard circuit right or better, just use two dual triodes per channel and do a Williamson circuit which IMHO is a really great circuit. Does anyone have a VTA 70 version in the New England area? I can take some measurements to see if they align with Amirs then we know the one Amir tested wasn't broken. It sort of makes sense something was wrong because you would have to put forth some effort to get that high distortion at only 10 watts with this topology, something is way off balance here.
  2. Thanks for your reply I really enjoyed it and I agree with you so I'll clarify my position so it accurately depicts my thoughts on "better" for this specific instance. I do not get hung up on .5% THD with a tube power amp especially if it's lower order harmonic distortion. The transducer will always add far more distortion especially at higher power levels so to me it's moot if the amplifier has .5%THD at 35 watts because the speaker will most likely be something like 8% THD especially at low frequencies, literally 30% THD isn't uncommon at lower frequencies with some speakers at higher output levels. We have UL push pull EL34 output stage, when I look at the original vs the newer front end I can see the logic in that they feel a long tailed pair would be a better phase inverter, this can be true when the loads of the two phases are not equal; e.g. driven near 0v where the grid circuit acts like a reversed biased diode and starts to conduct clamping the voltage. But the amp shouldn't be driven into clipping like a guitar amplifier. The split load phase inverter used in the original is actually a very good phase inverter with great balance. The better option would be to just add one more bottle per channel, keep the split load but use the extra bottle as common cathode drivers to drive the power tubes, ala the Williamson circuit. Although the split is under unity gain, with the addition of the extra common cathode stages after the phase inverter you end up with more gain than with just one stage driving a long tailed pair. The original design is very elegant, the pentode/triode single bottle has a ton of gain in the pentode stage and then the triode section is a very accurate phase inverter. The load they use at the tail has frequency dependent loading problems, at high frequencies the tail is at a much lower impedance. It's just odd the new design has such high distortion at such a low power output, 1% at 10 watts, that's pretty terrible for a push pull EL34 UL amplifier as far as anyone is concerned. The newer design is a clone of the famous mullard circuit, even back then with no active load for the LTP tail they were getting .05% THD at 20 watts!!! Now as I mentioned I really don't get too hung up on vanishingly low distortion but being an engineer for a very long time you learn with little effort where certain designs should end up at in regard to distortion. So seeing a Mullard clone with poor specs tells me the circuit wasn't really designed very well because for what it is naturally the distortion should be much lower. Mind you I'm not saying all good amps have very low distortion, I have single ended pentode amps that have 1% THD at 12 watts so that's pretty bad a push pull amp performs worse when it already has a huge advantage of second harmonic cancellation in the output stage and the fact my single ended amp is second harmonic dominant. So my SEP amp with 1% THD at 12 watts will sound much cleaner because it's second harmonic dominant where the new design of the ST70 is third harmonic dominant, but second harmonic is quite high for a push pull amp telling me the front end is not very linear. Overall I just don't think they did a very good job with what they had to work with.
  3. The original used one triode and one pentode for the front end per channel. The newer design uses three triodes in the front end per channel, I should have stated it this way in that they added another active device in the front end per channel and it didn't improve the performance. I don't think I know him, is he a regular on here?
  4. Screw it, my wife triggered the inner voice of my conscience to kick in and so I just changed the price to $250 including shipping. That covers what I paid for the transformers,chassis, and connectors. The wiring inside is all shielded Mogami and not just standard wire giving this extremely quiet operation. I remember when I made it I waved a transformer under load around the outside of the unit and the shielding works very well, nothing bled into the signal. New price: $250 shipped. When I get finished making the two custom amps I am working on now I'll build another unit with my spare transformers and sell that one also. Or if any diy'ers are interested in the transformers themselves just send me a PM. I'd hate to see these transformers go to waste as they are really amazing sounding and I know for a fact you cannot touch the quality of them for the price.
  5. Here is the datasheet for the transformers
  6. I honestly have no idea how to price these things so don't be afraid to throw an offer at me, I am very reasonable and just hate to see fantastic gear sitting in boxes in a closet not getting used. The older I get the more I worry the kids will just throw all in a dumpster when I croak and that would have me rolling over in my grave. I also don't want to get completely taken advantage of and sell of something far lower than it's worth but I understand there needs to be a happy middle ground. My wife gave me a crazy look and said "someone will pay $400 for that thingy? What's it do?"😆 She doesn't get how expensive high quality audio transformers are especially rare vintage ones. So if anyone thinks I am being unreasonable don't be afraid to PM me about the SUT's, I don't want to price gauge anyone here and only want to be fair. As I said I am only pricing these things from poking around the web for a few minutes. I'll edit the first post to add in "OBO"
  7. I went to look at current production 6SN7 tubes and made this look I am dropping the price to half what a current production one is, $15 each plus $10 for shipping. That's a total of $70/quad or $40/pair Someone will get some great tubes for cheap😁
  8. Here is the style I was contemplating, I can actually make a completely new unit with my extra transformers if anyone is interested that way there I don't have to disassemble the black one in the extruded enclosure. I can also add a switch for different loading options which people may like to experiment with, say 50, 100 and 250 ohms which should cover a wide range of loading options. I have other paint options besides black and can make wooden chassis as well.
  9. The ebay listing transformers are super crusty looking if you ask me, the transformers inside mine are in mint condition and were new in boxes when I got them. Here is some of my extra transformers, you can see how clean they are.
  10. This unit uses very rare vintage RCA 12399 transformers with a gain of 14x (+23db) and works excellent for MC carts with an output voltage of .3mV and up. I no longer use MC carts and if I do in the future I have more of these transformers so I can always just make another unit. The unit is configured for a load of 250 ohms to your cart but I can change this to whatever spec you want. I used 250 ohms because with my Dynavector this gave me no losses and also no ringing or any unwanted nastiness. I have seen some carts that prefer a lower load so again the load isn't set in stone just let me know and I'll change it before shipping out. I was going to make a more fancy chassis with the transformers sticking out the top plugged into sockets before selling but I figured I'd post it as is and then if someone wants a custom chassis we can discuss it. I wanted a small footprint which is why I chose this type of chassis but I have seen other SUT units with this type of transformers and kinda like the look of the transformers outside the chassis, so it's an option for anyone interested. Nothing compared to these transformers, I have used Altec and many modern types and these by far are just unbelievable and can understand why the price of these have gone through the roof. Here is the only pair I found for sale and they want $650 just for the transformers!!! I was looking at other devices for sale using these specific transformers and they are selling for $800. https://www.ebay.com/itm/163967138650?chn=ps&norover=1&mkevt=1&mkrid=711-213727-13078-0&mkcid=2&itemid=163967138650&targetid=4580496732614411&device=c&mktype=&googleloc=&poi=&campaignid=418233788&mkgroupid=1230353745471221&rlsatarget=pla-4580496732614411&abcId=9300542&merchantid=51291&msclkid=d97149dba07611356b9f2bbd4f185aef Asking $250 EDIT: I have no idea how to price selling this and don't want to come across as greedy, so I changed the price to include offers and We'll see what happens. There just isn't a large enough online presence to know what it should sell for or the value of the transformers are. I do think $100 per transformer is a very good deal because the price of high quality audio transformers are VERY expensive.
  11. I went and matched up another quad of these because I had more requests for them after the other 3 quads were sold. Just PM me but there is another for sale thread with the other quad or two pairs.
×
×
  • Create New...