[Frihed89]

No speaker taps, output transformer coupled.
 
How many input/gain stages?  What circuits? Coupling between stages?  What OPTs?
 
Heaters- AC or DC?  hum pots?  Other bright ideas to keep hum down.
 
Anything else you want to add?
 
 
 
 

 

[FrankCooter]

Good project. I've built a number of similar amps. 
 
1) Two stage, direct coupled, works well in this situation. I like the DRD topology favored by Jack Eliano of Electra- Print.  After trying most everything available, I like the German postal tubes such as the C3G and the D3A for drivers, You might want to consider the 46 in place of the 45 as an output tube, it's cheaper, has more gain, and many people (including myself) think it sounds better.
 
2) Filaments will have to be DC, or perhaps ultra-sonic AC. I used Tentlabs supply modules.
 
3) Power supply will need to be a separate chassis in order to prevent interaction of the power and output transformers. I used an over-scale "brute-force"  B+ supply with mercury rectifiers, multiple chokes, and lots of PIO motor-run capacitors, If I did it again, I'd probably use Schotkey rectifiers and some sort of tube based shunt regulator.
 
4) DHT outputs put any amp at a whole new level. Good luck with your project!

 

[Frihed89]

Thanks very much.
 
On 1.  I talked with Don Garber about his design and he said it wouldn't work at all for headphones.  I translated that to mean that all DC amps were trouble for headphones.  I was wrong?  I think I've seen and saved the electraprint schematic.  A question I have is what works best bewteen the OPT and the headphone jack?
 
How about a 26 into a 45?
 
How about OPTs?  MQ or Lundahl?
 
I have never seen or heard a 46, but I can feel "cheaper"!
 
On 2.  OK.  How about a battery supply?  I have a custom source with a lot of experience.
 
On 3. Full SS rectification?  The same guy on 2. has built a number of  SS power supplies for SS and tubes with tons of filtering and they work well, but none is hooked up to a DHT.  They have one case with the rechargeable batteries and power supply and caps or another case stuffed with more caps, a lot more caps.
 
 

 

[FrankCooter]

Direct coupling (or any other type of amplifier stage coupling) has nothing to do with the intended output load of the amp. Speaker and headphone amps differ in gain, power, and output impedance, but the design principals are the the same.
 
A 26 DHT driving a 45/46 DHT is possible, but your net gain will probably be under 6dB and your output power will be very low. This is OK for something like a Grado, but probably not enough for any ortho. The 26 is a very difficult tube to use in this application. Most of them are noisy, microphonic, and hum.  The 112A, basically a 26 with a 5V filament, is much more stable. I've built a 112A/71A all DHT direct coupled amp that I use with Grado's and JVC DX-1000.  Sounds great, but I went through a dozen 112A to get a reasonably quiet pair. Same with the 71A. Gave up on the 26 after trying 20 or so.
 
I love batteries as filament supplies, but not very practical. The 26 has a 1.5V@1.25A filament, the 45/46 has a 2.5V@2.0A filament ( I may have these numbers slightly off as I'm just speaking from memory here). You're  going to need BIG batteries and you're going to burn up a lot of your battery capacity in waste heat. The 112A and the 71A both have 5V filaments, so they're a little more practical for battery operation.
 
Sowter and Luhndal both make excellent transformers and are located in Europe. You need something like a 5K:32@35mA. Shouldn't be a problem. The transformer secondary connects straight to the headphones.
 
 
 

 

[Frihed89]

Thanks again.