What a great thread! Personally I love the EF86 on the front end and the 12AT7 LTP. I'm interested to see where this all goes. I know everyone is aiming for 50 to 60w. I will point out that Dynakitparts has a nice package deal on the transformers for the Dynaco Mk Iv amp (40W) - $165 for the PT (740vct, 165ma) and a remake of the 4.3k ST-70 transformer. I actually ordered 2 pairs today with this project in mind.
There's no reason you can't build with EL34 class tubes. Three good variants out of Russia are available: SED (=C=) EL34s, EH 6CA7s, and "reissue" GEC KT77s. FWIW, I'd SS rectify the B+ and use combination bias, which allows a single bias trim pot. in each channel to work very well.
I'm using those 6CA7's in my first build. They sound great, and don't look half bad either!
An externally hosted image should be here but it was not working when we last tested it.
The advantages of combination bias are taking advantage of the extra Volts SS B+ rectification yields, stabilizing the operating point (decent sized grid to ground resistors cease to be problematic), and reducing trim pot. count.
I follow the KISS principle. So, a shared 100 Ω/470 μF. network of appropriate ratings goes between each PP pair's cathodes and ground. Ohm's Law makes setting the "idle" current a complete no-brainer. I'd omit the bypass cap. in a no loop NFB SE DHT design, especially if a 300B is the "final". IMO, that type has excessive 2nd order HD. Remember, that Harvey Rosenberg, who loved the 300B, was careful to say that the type puts on a show. OTOH, I will not argue about the linearity of the 2A3.
Hi guys, I'm back! 
Let's have some tube talk, I have been looking on eBay from time to time and also found some bigger dealers selling new/reissue brands, mostly Russian tubes like Electro Harmonix, Sovtek but also Tungsol, Mullard, Gold Lion (was the last correct..Genalex) and all seem to be made in Russia, anyway I would have liked to use only new/reissue brands from New Sensor Corp. who owns most of the aforementioned brands as the NOS will disappear one day and their prices are soaring all the time.
Well what I wonder are these eBay shops trustworthy? I noticed one shop by name BNB tubes do have for instance Platinum quad matched KT88 for $US105 (they were $99 or so for some week ago but gone a bit up, on the other hand Genalex Gold Lion could be had for $190, gone down in price) and their prices are mostly lower than even New Sensor' prices on their home page, can anyone tell are these real things and how come New Sensor take more?
Also I wonder about the Gm and Ip values, I asked BNB tubes if EH KT88 could be had at Gm = ~8000 but was according to them very high and normally 5-6000 so I told that they had pictures on Gold Lions with Gm 8300 Ip = ~70 mA something, it's said these two brands (Eh & Genalex GL) in particular are the same tubes with some minor difference in material, so what values should they be then as BNB tube can per customers wish pick a higher or lower value. They do sell also 12AT7 and a matched tube was $2 in addition.
Tubemack, nice picture of your amp!
I noticed in your picture as elsewhere, that your left tube pair have the black print without the EH face while the tube pair on the right have the gold coloured print with the EH face, do you or anyone else now why is that, is the black print the older version?
Cheers Michael
Let's have some tube talk, I have been looking on eBay from time to time and also found some bigger dealers selling new/reissue brands, mostly Russian tubes like Electro Harmonix, Sovtek but also Tungsol, Mullard, Gold Lion (was the last correct..Genalex) and all seem to be made in Russia, anyway I would have liked to use only new/reissue brands from New Sensor Corp. who owns most of the aforementioned brands as the NOS will disappear one day and their prices are soaring all the time.
Well what I wonder are these eBay shops trustworthy? I noticed one shop by name BNB tubes do have for instance Platinum quad matched KT88 for $US105 (they were $99 or so for some week ago but gone a bit up, on the other hand Genalex Gold Lion could be had for $190, gone down in price) and their prices are mostly lower than even New Sensor' prices on their home page, can anyone tell are these real things and how come New Sensor take more?
Also I wonder about the Gm and Ip values, I asked BNB tubes if EH KT88 could be had at Gm = ~8000 but was according to them very high and normally 5-6000 so I told that they had pictures on Gold Lions with Gm 8300 Ip = ~70 mA something, it's said these two brands (Eh & Genalex GL) in particular are the same tubes with some minor difference in material, so what values should they be then as BNB tube can per customers wish pick a higher or lower value. They do sell also 12AT7 and a matched tube was $2 in addition.
Tubemack, nice picture of your amp!
I noticed in your picture as elsewhere, that your left tube pair have the black print without the EH face while the tube pair on the right have the gold coloured print with the EH face, do you or anyone else now why is that, is the black print the older version?
Cheers Michael
IMO, the risk is too great. Unfortunately, Flea Pay is more lawless than Dodge City, KS, was before Wyatt Earp rode into town.
Also, I suspect that BNB is selling factory "matched" goods. New Sensor's Saratov, Russia, plant makes good tubes, but the factory matching SUCKS.Buy from a reputable dealer who takes the time to burn tubes in, cull out duds, and match tubes properly under real world conditions. There are several individuals that meet the description. Jim McShane is 1 of the good guys and he ships world wide at reasonable cost.
Oh yeah, there are substantial differences between the EH and GL KT88s. You have to take 'em apart to see the differences. The 2 varieties don't sound the same. If money is a problem, buy EH. They are quite decent and are a good value. The GL is simply the best big "beamie" in production and are priced accordingly.
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That's a nicely done project. Is that an ST-70 variation? Where did you get the chassis from?
..Todd
Are we all here? I last thought (post # 363) that we are also up against practical available components. If this is to be a monoblock, power transformer problems should fall away (at the expense of an extra transformer). Also I encouraged the higher output, not because of a dramatic difference, but so that we do not fall back at wasting KT88s. The 400W version could just as well use 6L6GCs, with quite a saving.
But important now: What is the consensus regarding an obtainable (affordable?) power transformer? Unless that is known (good for 460 - 480V I hope), design will hang in the air.
I have done some sleuthing regarding 12AT7, ECC99, E182CC, 6N6P1 and 5687 for phase inverter. Not being the proud possessor of a good spectrum analyser at present, or all those tubes, I had to resort to cumbersome analysis per Crowhurst's graphical method (as outlined in the reference 'Steve's ..."). A few surprises emerged .... Hope to come up with these shortly; time has been busy with collecting and suitably enlarging good and poor Va-Ia graphs, apart from personal blah blah blahs.
Meanwhile an informative discussion on availability of components - can be at least as important as the design.
Previously, reproduction Dyna MK3 and Edcor power trafos have been mentioned.
AFAIK, the best deals in power trafos available to North American builders come from AnTek. AnTek's stuff is toroidal. So, you mount the "iron" on the underside of the top plate, with only an "acorn" nut showing, or you obtain a cover. Antek's model AN-2T350 seems appropriate for a PP KT88 monoblock. Its filament windings are quite substantial and only 1 of them provides enough current to energize a pair of KT88s, a 12AT7, and either an EF86 or a 6GK5. Voltage multiply the 2nd filament winding to obtain any and all negative voltages needed.
The current PS was designed around the Edcor XPWR002. Eli likes the Antek, and with the nice cover they offer, I like that option better than I thought I would. I have no idea about the availability of these in other countries?
Eli - Which model antek would be the best choice for 465V to 480 B+? The AN-2T350 seems low on voltage for a higher B+?
Johan, I think most of us are all set on the higher B+.
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You are overlooking the nominally 115 V. primary and regulation effects. Examine the linked data sheet.
A possibility for a yet "taller" B+ rail is to parallel the 350 V. windings and wire 1 of the filament windings in series with them, using the boost configuration. Bridge rectify the conglomeration. A small additional trafo would be needed for negative rail purposes.
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Is there something unsuitable about the Edcors we were previously 'using'? Or are we just compiling a list of alternatives? The MKIII transformers I looked at had substantially higher output voltage; over 400VAC (well over 550V DC), and their price was double the Edcor model.
Johan, let me clarify one thing. My tangent about a stereo unit, using a stereo transformer, was just my own musings about how I might alter this project to suit myself. I did not intend to supplant the original objectives. I shouldn't have mentioned it, it derailed the thread more than I expected.
..Todd
Johan, let me clarify one thing. My tangent about a stereo unit, using a stereo transformer, was just my own musings about how I might alter this project to suit myself. I did not intend to supplant the original objectives. I shouldn't have mentioned it, it derailed the thread more than I expected.
..Todd
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If anyone is interested, I'll run a PSUD model with the Antek and SS bridge rectification. I get the basic idea for Eli's filament voltage multiplier but would probably need some hand-holding.......
I'm also assuming this would be a "lower cost" alternative to the spec'd Edcor.................
I'm also assuming this would be a "lower cost" alternative to the spec'd Edcor.................