| arnoldc |
I am a lucky bastard :D
My friend asked me to pick a pair of 26 from this-
I was like a confused jackass :D Tung-sol, RCA, Silverline, National Union, Cunningham, Songbird... he only has six pairs globe 26 including this blue Arcturus...
So I exercised some delicadeza and settled for these, even though I was holding already a pair of 1930's globe Cunningham UX-326 :smash:
ST bottle Arcturus with red labels :D
Heck he even gave me the preamp where to plug them tubes!
The preamp uses LED bias (with current feed) and current regulated filament supply. Silent!
Talk about Christmas in July :D
I was able to hear the blue Arcturus, which has the best sonics, followed by the globe UX-226 and Arcturus ST bottle, then by the Tung-sol ST bottle.
Wow, what a DHT experience! |
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| andyjevans |
| Did you find the high treble slightly rolled off? That's been my only problem with the 26 - otherwise it has gorgeous mids. |
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| arnoldc |
@andyjevans, in the system we tested last night and in my system now, no.
in my system, the Arcturus ST has very good balance, while it's true that it has excellent mids (which the Arcturus blue globe betters) the highs and lows are amazing. The highs have plenty to offer, micro-details galore, without sounding tizzy, very smooth detail sound. The bass is very articulate and detailed, AND this tube has got speed and dynamics, both macro and micro... which I think the globes I heard (26, 226, 326) are noticeably slower and mellower. |
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| Gluca |
| Is it transformer coupled to the output? |
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| Zen Mod |
| quote: | Originally posted by arnoldc
I am a lucky bastard :D
....................... |
they're ugly!
:devilr: |
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| andyjevans |
| I have a 3 stage all DHT amp under construction for a friend, and we did some tube rolling in the driver stage (outputs are 2a3) . The 26 globes were particularly nice, better than the 26 ST and the 112A. Particularly noticeable on voice and piano. The globe 26 is a very fine tube if you like the warm sound - it is quite fat and euphonic, compared to say a 30. In fact, 30 works nice on the input to this amp to give some balance. |
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| arnoldc |
| quote: | Originally posted by Gluca
Is it transformer coupled to the output? |
No they're not. In fact, the preamp uses cheap Auricaps (because my friend is also the distributor here)
| quote: | Originally posted by Zen Mod
they're ugly!
:devilr: |
Very ugly indeed!
| quote: | Originally posted by andyjevans
I have a 3 stage all DHT amp under construction for a friend, and we did some tube rolling in the driver stage (outputs are 2a3) . The 26 globes were particularly nice, better than the 26 ST and the 112A. Particularly noticeable on voice and piano. The globe 26 is a very fine tube if you like the warm sound - it is quite fat and euphonic, compared to say a 30. In fact, 30 works nice on the input to this amp to give some balance. |
I want to make an all DHT amp too. With a preamp, I am hoping I will survive with just a single DHT input, and DHT power. Maybe a 10Y and 45.
Globe or ST is a matter of preference, and synergy with the rest of the gears. |
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| kevinkr |
I have both globe and st type 26 from the like of CECO, Sylvania, RCA/Cunningham and several others. These do duty in my transformer coupled 26 pre-amp. It's survived 7 yrs now with just an ongoing series of improvements, and it is still my reference.
I think I like the CECO globes the best overall for casual listening, warmest presentation, clean extended highs and good bass. The current tubes are nos/nib Sylvanias and I think these are the best overall compromise.
My transformers (UTC HA-113) have permalloy cores and excellent high frequency extension. The problem with choke and capacitor coupled pre-amps using this tube is the fairly high RP (>8K) means that low capacitance cables are a must along with an amplifier input stage with modest amounts of miller capacitance.
The one mistake I made was to include the power supply on the chassis, this results in unavoidable hum pickup in the transformers. I have got it down to quite a low level, but considering that all of my power amplifiers have noise floors that are lower still it is the dominant noise source.
12's, 26's, 27's (idht) and 30's are all quite prone to electrostatic pick up so I made little, and rather ugly copper foil shields to go around them, grounded to the chassis - depending on potentials on adjoining component chassis (and me) this can result in a reduction of hum and buzz of about 10dB or so. (Makes a BIG difference.) |
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| arnoldc |
Hi Kevin, honestly I was surprised how my friend achieved no noise in his preamp. I remember him enjoying his prototype even with hum. We used to joke around and say the hum is the soundstage :D He was also suffering from microphinics and had to wrap his tubes with aluminum foil. He was using AC then, and was really stubborn in sticking to AC even though I've been trying to convince him to go current regulation (DC).
Then he came up with these (he made 6 of these, the black one)
By arnoldc, shot with K750i at 2007-07-10
Filament supply is now current regulated with LT1085.
I was stumped! There's no noise exhibited like before, in fact it was so silent, no microphonics too, even if we tube rolled different brands. Maybe he got lucky, and so do we!
ps.
re: Reference. I have to admit that my 407A and 12B4 pre might be gathering dust soon... |
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| Audio_idiot |
Hi Arnoldc,
Could you share the filament circuit?
Thousand Thanks |
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| arnoldc |
I haven't really dissected it and don't have a schema ready but it's fed from a 5V AC supply -> diodes -> 10,000uF cap -> LT1085 (set for 1 something mA for the 26) -> filaments
It's like a simplified version of the Ronan Regulator (current regulator version). |
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| Audio_idiot |
Hi Arnoldc,
hope you will publish the circuit once you havce had it.
I have a pair of 26 waiting for me to find a good hum free filament treatment...
You are sooooooo.... Lucky, I wish I have more generous friend like you have...
Cheers |
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| andyjevans |
I guess most of you are using single ended preamps, but mine is balanced with differential pairs over a CCS. I've "economised" on filament supplies by using one per pair and running the filaments in series. Sometimes this works well, but sometimes I get hum, so this may be caused by imbalance in the filaments. So it seems this needs a second look. I think going back to one filament supply per tube may help here.
I have 26s in my driver stage, and here they seem OK with a series supply for the pair.
For those interested, you can construct a simple Mullard style direct coupled two stage input with DHTs. Sounds really good. With 2a3s in the output in PP you DO need a couple of tubes with mu of about 9. For example 30 into 26 gives you enough gain in the amp, but go down to a mu of 5 and use 31 or 46 (great choices) and you'll probably need extra gain in the preamp. Unless your speakers are very sensitive, that is - I'm working on the basis of about 90db sensitivity in the speakers.
Incidentally, if you use LT1085 or LM1084 or the like as a voltage reg, then a nice little common mode choke after it improves the sound. But if you use a current source for the filamants, then a CMC seems to make the sound worse. I prefer the sound of a current source myself. |
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| WALTER BURKHARD |
Any diagramm from ur balanced with differential pairs and CCS preamp??
cheers Walter |
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| JoshK |
The problem with PP's is finding matched quads of good old stock 26's and the like. I seem to see plenty of pairs still but quads are hard to come by.
I often wondered if using a 26 (et al) in the first position of a 3 triode aikido would be interesting. Broskie shows a 3 triode variation with all 12au7's in one of his earlier blogs. In this case, one could use the DHT for the gain and something like a 5687 for the output pair.
This wouldn't allow for all the tube swapping abilities of a standard aikido but I could get over that. You would also need seperate filament supplies. |
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| arnoldc |
Hi Josh, but why would you "color" the 26 DHT with something else?
My experience tells me that a single 26 really gives an amazing experience. The free-flowing character, the openness and width and depth gave the music a whole new direction made me a convert, and I'm not turning back. |
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| kevinkr |
| quote: | Originally posted by arnoldc
Hi Josh, but why would you "color" the 26 DHT with something else?
My experience tells me that a single 26 really gives an amazing experience. The free-flowing character, the openness and width and depth gave the music a whole new direction made me a convert, and I'm not turning back. |
Arnold you've summed up in a nutshell why the only component in my system that hasn't been replaced in the last seven years is my 26 dht transformer coupled pre-amp..
:D :D
(It's undergone a considerable amount of improvement, particularly in terms of noise floor, ground loop issues, minor power supply and bias supply design improvements, not to mention passive component quality, but the fundamental character remains unchanged.) |
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| JoshK |
It was just a thought...I don't have enough experience to know how it would sound. I just figured the gain portion would be the predominant influence on sonic character and that the 5687 follower would just lower the output Z and make it drive better.
I guess that is why there is breadboarding...
What operating points do you guys like for the 26's? I have a pair or two of them to try. I also have a pair of 112a's. |
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| kevinkr |
| I typically run them around 6mA or so at 135V |
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| arnoldc |
Hi Kevin, I guess I now belong to the 26 camp as well :D
Hi Josh, I don't really want to screw up the preamp, so I did not open it and just asked my friend who replied with "must be 6mA" so I guess that's the 26 sweet spot. He did not mention the B+ though but I know there's a 24K Riken as a plate load as I saw the bottom briefly before I took it home. |
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| burnedfingers |
The 26 is a sweet sounding tube but I could never get the hum out of it. I tried batterys and that didn't work and to be honest there ought to be a way to have it almost silent hum wise without going to an elaborate battery and charging system.
Didn't manufactures have 26's in the preamp sections of many of the old radio circuits?
I've seen Kevin Kennedy's schematic of a 26 showing the UTC HA-133 transformers but haven't been able to source any of them at a reasonable price.
Is there a substitute transformer that is more cost effective that will also work? |
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| Gluca |
Cinemag has a few items in catalog (mic out and output) that would work. I used a CM9589 in my 801A preamp (parafeed).
(very) Good value for the money.
Gianluca |
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| kevinkr |
| quote: | Originally posted by burnedfingers
The 26 is a sweet sounding tube but I could never get the hum out of it. I tried batterys and that didn't work and to be honest there ought to be a way to have it almost silent hum wise without going to an elaborate battery and charging system.
Didn't manufactures have 26's in the preamp sections of many of the old radio circuits?
I've seen Kevin Kennedy's schematic of a 26 showing the UTC HA-133 transformers but haven't been able to source any of them at a reasonable price.
Is there a substitute transformer that is more cost effective that will also work? |
Check with Jack over at Electra-Print, he made me some custom transformers for my headphone amplifier project and I was quite pleasantly surprised by both the quality (performance) and the cost.
Unfortunately for looks 26's benefit greatly from electrostatic shielding.. External fields are often a problem with this tube. I got a 10dB improvement in hum by making a simple set of grounded copper foil shields. Aluminum foil works well in a pinch..
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