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Thanks to Dimu..
Upgraded D7 to 2A...
Ordered E80CC philips and 6L6GC GE. Stunning beautiful..
I wrote to Elekit representative in Japan regarding the overall application of the E80CC preamp tubes with this amplifier and got the response on the following day - see attached.
Yes, do not use E80CC in stock amp to avoid any trouble. Upgrading heater rectifier and watching 6V transformer secondary budget makes it as safe to use as 12AU7. Upgrading power transformer to increase 6V secondary current rating allows combining E80CC with any power tube supported by the amp (which again is not a simple question as you have to watch out for multiple parameters including importantly grid voltage).
As for oscillation- my amp does not have any as per oscilloscope and spectrum analyzer tests I did earlier. Since oscillations depend on many factors including capacitances throughout the circuit your results may vary in theory. You can stick a basic analog oscilloscope at the amp output to see if you have oscillations (but do it in parallel to the usual load, do not replace speakers with oscilloscope as this is not good for the amp). I mostly tested with 1KHz signal. I am not too worried about oscillation with reasonable tube substitution as the amp has global NFB. I also use 12AT7 instead of 12AU7 in my TU-8500 preamp and thanks to heavy NFB there is no difference there. 12AT7 has much higher gain than E80CC which in turn has higher gain than 12AU7.
Once you have have proper heater power budget for the tubes the next important factor is heat dissipation. The amp gets very hot inside with bigger tubes. This is not good for all of the components inside, caps in particular. There were many posts on both passive and active cooling in this thread. I experimented with moving mosfets to the back of the amp case but mains noise pickup was difficult to deal with (but not impossible). I even tested configurations with paralleled mosfets for the power tubes where one mosfet remained on the board and another one was located at the back wall of the amp. After all that I ended up sticking small radiators on all four mosfets and opening up passive airflow by drilling small holes at the bottom and in the top cover over those mosfet radiators. Combined with EL12 spez my amp runs very cool now for many hours. External temperatures are at least 20C lower than if I used something like KT88 and internally the difference is even more significant.
That indeed is beautiful, enjoy! As far as look GE 6550 is one of the more stunning looking tubes in that amp.
I've been trying to find NOS 211 tubes after seeing Sunvalley kit at Victor's. The pictures don't really do it justice, those tubes are very impressive in person. See Sunvalley amp on http://www.vkmusic.ca/.
Allegroz - could you tell us which source and part # you used for 2A replacement for D7 diode (DB107)? I may upgrade it too in the future if time permits. Even if the D7 fails because of the extra current draw (which I still think unlikely) it is a 10c replacement part anyways. Yes, it is rated for 1A but has a peak current of 50A per datasheet. Been running E80CC tubes with EL34 and now GE 6L6GC 8 hours per day for more than a week already without a hiccup. No changes in sound, volume, temperature or a funny smell emanating from the amp as suggested in the other post. I really like how the Philips E80CC pre-amp tubes sound combined with NOS GE6L6GC power tubes. They produce a pretty mellifluous non-fatiguing tone with lots of transparency and details yet smooth and sweet, I can listen to this setup for hours without a break.
Would definitely like to try RCA with them some day I can find a good deal. I almost snatched a pair of interesting RCA 6L6GC blackplates on eBay last night for a good price. Likely a NOS/ANOS but someone grabbed them when my browser session expired on the phone because we lost our internet during the hurricane in Tampa last night. Too sad.
Another thing I am trying to understand in the schematic on Page 19 - the secondary winding from the stock transformer that feeds the power for heater tubes has 6V and 4A output that goes right into D7 rated at 1A. Why not use at least 4A-rated rectifiers instead of 1A here without having to rely on a particular tube type, etc.?
The default for D7 is db107 1A rated... I upgraded it to DB207 2A rated...
Transformers rated 6v n 4a...
It divided for two, first preamp tube via diode because it feed dc volt.. second powertube without diode, ac volt... total current powertube + preamptube must be 4a..
It will be wasting if we used 4a diode in D7.. because preamp tube only used 300+300 12au7 : total 600ma or 600+600 e80cc : total 1200ma..
Makes sense, thanks Allegroz. Just noticed that you also opted for the GE 6L6GC plus E80CC. Those GE NOS power tubes are great ones, I like them a lot too! I compared them to the modern TAD 6L6GC today and the GE are much cleaner and more transparent sounding with great dynamics.
I ordered a min batch of 20 DB207 bridge rectifier replacement for D7 from China through eBay for $4 shipping incl.
Unfortunately I could not find it anywhere to buy online within the US. It may take a couple of weeks or more to get from China. Once received, I will swap the 1A D7 for the 2A which will be like a 10 min job. No removal of the top PCB even needed, only a desoldering gun to remove the part.
If someone has a spare DB207 or two to sell from the US, please PM me asap.
There are many distributors of electronic parts such as Mouser and DigiKey for example. They have huge warehouses with many parts on hand and can ship them overnight. When I order stuff from those distributors I usually look at what else I would need. You can get better bypass caps for example from the same source.
There is also no reason to use specifically DB207, any rectifier with suitable physical footprint will do. Mine is 3A rated but I had to mount it creatively because pins are arranged differently. Voltage is not important in this case as voltage is nominally about 8.5V in this case. Temperature ratings are more important for this application.
When desoldering I use regular braid and lift iron temperature a bit, still being careful not to burn the tracks off the PCB. Combined with lots of good flux this sucks all solder clean from the vias, assuming you used proper solder when assembling the amp initially. I use heat gun for SMD soldering only and even then to basically reflow the components after initial soldering with micro iron.
Here is TU-8600 review from wall of sound.
I believe that the most important thing to conclude from the review is that this is the best sounding 300B amp that Noam had the pleasure of listening to (for long enough time).
I can't wait to hear it for myself, unfortunately Amtrans still hasn't got anything up on their website, so I'm assuming it's not available yet. As for the Elekit web shop, it should start selling on the 19th I believe.
All this talk about input tubes got me wondering.
Can anyone tell me whether these large E80CC input tubes will be safe to use with TU-8600?
Thanks all, the amp has been up and running fine, I wound up with kt88 tubes.
It's more than loud enough to make my ears hurt with speakers so plenty of power for my use.
I'm not worried about the heat as I have other tube gear (amateur radio tranceivers)
That run much hotter than the Elekit does..
The package with DB207 bridge rectifiers should arrive today from China, it only took around a week which is not bad at all. I may postpone replacing the stock rectifier until later and do more more upgrades to my amp altogether.
I am now considering replacing the AMTRANS AMCO C7, C8, C9, C10 caps with Mundorf Supreme Evo ones ordered from Victor based on the positive reviews on this great forum.
Pretty happy with my AMTRANS capacitors already, they sound very pleasing and musically with plenty of details. Just wondering if the Mundorf Supreme Evo are really worth the extra expense and provide tangible improvement to the sound. Or they are similar to some interconnects upgrade - after a specific point all the expensive audio cables fall into the common moot point area. For instance, honestly, I am having hard time telling the difference between my $2k Nordost Moon Glo Quattro Fil audio interconnects and the DIY solid-silver wires, both sound the same to me.
I would not mind trying the Mundorf caps if they help increase the resolution and lift some veil from the audio to make it sound a tiny bit cleaner maybe. Please advise.
I did some considerate research on the coupling caps for the last couple of days and decided to order the Mundorf MCap SUPREME EVO Silver/Gold caps to upgrade the existing AMTRANS ones. The Mundorf seem to be great capacitors and lots of people noticed subtle yet tangible improvements in the detail and overall sound. They however have a little bit of coloring to the high notes and are not neutral but I would not mind that.
Could someone please confirm if this is the right installation regarding the inner and outer foil (ground) polarity for those caps as posted by Dimu earlier in this forum? It appears to be correct to me and the ground (outer foil) must be soldered to the lower impedance or input of the signal and the inner foil goes to the output. The bottom left (C4) is upside down to mirror the C3 on the right, because the PCB layout is symmetrical which makes sense to solder them that way.