[adamus]

Hi all,
 
I am going to build an amp for a mate shortly. He has a big(ish) budget and wants a valve based head amp to power his HD650's and possibly hd800s down the line.
 
Thoughts so far
 
Amp Section
6h30p
CCS active load, dn2450 cascode
LED bias
Sowter 8665 50% mu metal tranformers
probably mundorf or equivalent caps for parafeed
Pot will be either alps in shunt, or a dact if he wants.
 
Powersupply
As i will be using the cascoded CCS the powersupply is 'slightly' less critical.
Probably go simple, CLC, film caps used throughout.
Valve rectifier, something readily available and directly heated
 
I'll go for a two chassis build, use FPE for panels and Modoshop for the chassis.
 
 
I havent done the circuit diagram yet, but its a very simple design that will use very high quality components.
 
How would you better it - any ideas welcome.
 
Cheers
 
Adamus
 
 
 
 

 

[nikongod]

I'm not always such a fan of cathode LED's. Perhaps its the cheap LEDs I use, or a circuit issue the few times I have had them work poorly (and I have had them work well too... Im just superstitious now I guess), but I have had them sound flat out bad (harsh and gritty to be specific) where other biasing methods in the sounded GREAT. OTOH it will look a bit like a mini SY "red light district" once you get enough LEDs in series and parallel to make whatever voltage & current you need which is pretty neat.
 
Anyways, I would try a couple things out to bias the tube.
 
As a second bit:
I have always thought that 2-chassis amps were a bit of a PITA. Perhaps Im just weird like that. Why not 1 medium-large sized chassis? The amp wont be too heavy to move comfortably and you should be able to fit everything into a medium sized chassis. I understand the benefits to noise from keeping the power transformer further from the audio stuff, but you can design around that.
 
It should work out well either way. The 6h30 parafeed single tube is a nice fun amp.

 

[dsavitsk]

I'll disagree with Ari -- two chassis design is a big help with OPTs unless you have a really really big box.  This is especially the case with nickel core OPTs as they pick up a lot of magnetic signals.  Doesn't matter with speakers so much, but it does for phones.
 
Also, I am yet to see a case where LED bias did not sound and measure better than a resistor + cap. I have also found it to work much better than a WE connection. There are other fixed bias options you can try, but LEDs are a lot easier and work as well as anything.
 
Only other comments I'd make on the design is that (a) a regulator is a good thing -- not necessarilly to get rid of noise, but to feed the CCSes with a low Z supply, and (b) I like a solid block of low (50%) nickel better than half high (80%) nickel and half steel interleave, but that's just a preference.
Quote:

I'm not always such a fan of cathode LED's. Perhaps its the cheap LEDs I use, or a circuit issue the few times I have had them work poorly (and I have had them work well too... Im just superstitious now I guess), but I have had them sound flat out bad (harsh and gritty to be specific) where other biasing methods in the sounded GREAT. OTOH it will look a bit like a mini SY "red light district" once you get enough LEDs in series and parallel to make whatever voltage & current you need which is pretty neat.
 
Anyways, I would try a couple things out to bias the tube.
 
As a second bit:
I have always thought that 2-chassis amps were a bit of a PITA. Perhaps Im just weird like that. Why not 1 medium-large sized chassis? The amp wont be too heavy to move comfortably and you should be able to fit everything into a medium sized chassis. I understand the benefits to noise from keeping the power transformer further from the audio stuff, but you can design around that.
 
It should work out well either way. The 6h30 parafeed single tube is a nice fun amp.

 

[adamus]

Thanks for the advice.... i have looked at your pages and they are helpful. I do have a HV PCB knocking about somewhere (salas designed off DIYaudio). That would make a nice reg poweruspply.
 
Are we talking small gains though... the CSS on the single stage would reduce any noise significantly. I can always try it though, or do a simpler tip50 reg or something as the sals shunt may be overkill. 
 
I'll draw up a schematic over the next few days and post it up
 
Re LED bias - I too have only had positive experiences with led bias. Must suit my ears!

 

[adamus]

any comments on the tube choice - it looks good on paper, but could i do better?

 

[dsavitsk]

Quote:

any comments on the tube choice - it looks good on paper, but could i do better?

It's a great tube.  You'll need to do the math to figure out of you'll have enough gain.
 
I do basically this same amp here (http://www.avguide.com/blog/can-jam-chicago-2010-international-head-fi-meet-part-2?page=1 Can I post this? No idea what the rules are about this.)  But I use 6C45's which I like better and, due to higher mu allow for higher ratio transformer which seems to cut down on THD a bit. Downside is that they can be difficult to get stable.
 
There is a free downloadable model of the 6h30 that will make your life a lot easier, though. Worth playing with.
 
http://www.glass-ware.com/GlassWare_True_Curves.htm
 

 

[FrankCooter]

Quote:

any comments on the tube choice - it looks good on paper, but could i do better?

Looks like you're well on the way to a very nice amp. I've built several similar to what you're proposing.
 
A few minor suggestions:
 
1) Definitely keep the power supply on a seperate chassis. Hum based on transformer interaction is a real possibility with this type of amp. The extra real estate can be used for an LCLC filter with motor-run oil capacitors replacing any electrolytics. Regulation is a matter of choice, try it both ways. I like damper diodes for rectifiers. Look at the octal 6AU4. Plentiful, sounds great, and cheap.
 
2) Try a bunch of different output tubes besides the 6H30. Nothing wrong with the 6H30, but it's "supertube" status is simply BAT marketing hype. There's a lot of candidates here. My favorite is the Western Electric 418a, which is basically a 4 elemant version of the legendary, and nearly unobtanium 437a. Sounds great in triode mode. Frequently found on Ebay for about $20.00. I'm not a fan of the 6C45Pi, individuals are all over the map and there's a sort of solid state harshness to the sonics.
 
3) I agree with nikongod's concerns about bias topology. I sometimes use LED's, but my favorite is a lead/acid battery in the grid circuit. If you use a cathode resistor, use the best bypass capacitor available, hopefully you can still find a Blackgate.
 
4) If you can swing it, go for the Dact attenuator. In a simple circuit like this, every part will signifigantly effect the sonic signature of the amp.

 

[adamus]

Thanks for the advice.
 
With such a simple circuit design, i should easily be able to try different tubes and bias configurations.
 

 

[dsavitsk]

 
Quote:

I'm not a fan of the 6C45Pi, individuals are all over the map and there's a sort of solid state harshness to the sonics.

SMD stoppers touching the pins on the plate, both grids, and all 4 cathodes, and this goes away. As I say, a pain to work with.

 

[regal]

I agree on the separate chassis,  it adds a lot to the price but is almost essential with headphone OPT amps.  I've always had good luck with properly rated LED's,  but I hear horror stories and makes me paranoid.  Someone showed me that a simple grid bias with a tread adds negligible noise and it sounded better than an electrolytic bypass,  you may want to consider that if you can't find a battery in the voltage you need (you will have to add an input cap.)
 
I'm building a similar amp but single feed instead of parafeed.  You may also want to consider a 5842 tubes.  As with anything the chassis is the hardest part and has me procrastinating.

 

[FrankCooter]

If you go with the 5842, use the Western Electric version, which is the 417a. You'll definitely hear the difference over a generic Raytheon. Besides the 418a, my favorites of this type are triode connected 7788's and D3a's. If you want to go dirt cheap, try a 12gn7a. Lots of good stuff out there.

 

[adamus]

I will certainly try the d3a, by all accounts it seems a very godo tube indeed, if a little difficult stabilise.