Hi,
Anyone out there running a 26 tube preamp or heard one. I have several 226's burning a hole in my pocket and am tempted to make a very simple preamp. I have read that it can be hard to make them quiet. I have previously built a 71 amp and really like the sound of these old DHTs. I started with AC filaments but ended up with DC since I was using headphones and had a hard time taming the humm. AC sounded better to me. Has anyone tried an SMPS for heaters? I have already seen the VT52 web page projects and would probably use that for a start.
The alternative that I am considering is an Aikido but I love the globe tubes...
Any opinions out there?
Thanks a lot,
Paul
Anyone out there running a 26 tube preamp or heard one. I have several 226's burning a hole in my pocket and am tempted to make a very simple preamp. I have read that it can be hard to make them quiet. I have previously built a 71 amp and really like the sound of these old DHTs. I started with AC filaments but ended up with DC since I was using headphones and had a hard time taming the humm. AC sounded better to me. Has anyone tried an SMPS for heaters? I have already seen the VT52 web page projects and would probably use that for a start.
The alternative that I am considering is an Aikido but I love the globe tubes...
Any opinions out there?
Thanks a lot,
Paul
My friend hoarded 26 tube in various forms (gave me a pair which I haven't collected) and built a preamp. It hums loud, and he has tried various tweaks but I can never convince him to use battery for the filaments.
It sounds glorious on his system (he's using a type 50 amp) and he's tolerating the hum. Me, I can't. So if I finally collected my pair from him, I would do battery filament, or at least DC.
I'd use diyAudio's CCS to load it on the anode or sink on the cathode, I don't know yet. Just thinking out loud.
It sounds glorious on his system (he's using a type 50 amp) and he's tolerating the hum. Me, I can't. So if I finally collected my pair from him, I would do battery filament, or at least DC.
I'd use diyAudio's CCS to load it on the anode or sink on the cathode, I don't know yet. Just thinking out loud.
arnoldc said:
Thanks for the input. I am in the collecting stage for ideas and yours are very welcome. I went to your web page and saw the 417a/45 amp you made. Very nice. I made a very similar amp with a 417a and a 171 tube. I started with a Lundahl interstage but converted it to choke load the input driver instead. I made this one just as a headphone amp. I wish that I never sold it.
I love DHT's.
Best,
Paul
arnoldc said:
A bit OT but I have vested interestI was thinking of going that way, using LL1660. Can you share your experience why you liked choke loaded over the IT instead?
Boy, this is a little tough, mainly because I only remember that I prefered it. I tried a lot of things with that amp. I loved AC filaments but could not kill the humm. I ended up with DC filaments but put some small chokes in the filter. It sounded somewhere inbetween AC and DC. I also used battery bias on the driver tube. If I remember correctly I used the LL1660s also. I used an 80 for the rectifier. The rectifier tubes made a pretty big difference in the sound. Here are a couple of photos. If anyone out there has this amp let me know how you are liking it...
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
mourip said:
Give a look here,
a phono preamp (Loesch like) with 26 linestage.
http://www.teresaudio.com/haven/preamp/csb_riaa.gif
I hope this can help you
ciao cicero32
I use variable DC bench power supplies for the filaments of my balanced line stage, which has four UX4 sockets which can take any DHTs that fit the socket. I like having the variable DC for filaments so I can try out different tubes, and there are plenty that fit besides the 26. Just dial in the voltage you want and off you go. Simple. I don't have hum problems with my speakers, which are about 87db efficient. I tried batteries but in my case they were no improvement. My PSUs are Thurlby Thandar, which is nice, but there are plenty of others cheap on ebay all the time. Useful things to have for filaments.
Other tubes that come to mind that will work well in this design include the UX112A/12A, and the 301/201/01. This design will also work with the 27 IDHT as well and its variants. Hum obviously is less of an issue with one of these..
It is really important to minimize magnetic fields around the tubes and transformers, and a quiet filament supply is a must.
I strongly recommend a separate chassis for the psu, that said it is possible to build one on a single chassis with good results.. I mounted the power transformer at one end of the chassis on top and the output transformers internally mounted on back and side panels about as far away as possible.
I recommend electrostatic shields around the tubes as unattractive as that may be..
The speakers I use are extremely efficient >103dBspl at 1W and there is no audible hum from this pre-amplifier with a power amplifier having in excess of 20dB of voltage gain.
Of the above types the 26/226 is by far the most common and easily obtained.
I noted the link to my schematic, and I am delighted that people are sharing it here, but please (!!) read the associated article which you can find through a link on my site. There is a lot of material about this design and what is needed in general to build one successfully. A number of my local friends have also built this design without serious difficulty.
edit: fix omissions
It is really important to minimize magnetic fields around the tubes and transformers, and a quiet filament supply is a must.
I strongly recommend a separate chassis for the psu, that said it is possible to build one on a single chassis with good results.. I mounted the power transformer at one end of the chassis on top and the output transformers internally mounted on back and side panels about as far away as possible.
I recommend electrostatic shields around the tubes as unattractive as that may be..
The speakers I use are extremely efficient >103dBspl at 1W and there is no audible hum from this pre-amplifier with a power amplifier having in excess of 20dB of voltage gain.
Of the above types the 26/226 is by far the most common and easily obtained.
I noted the link to my schematic, and I am delighted that people are sharing it here, but please (!!) read the associated article which you can find through a link on my site. There is a lot of material about this design and what is needed in general to build one successfully. A number of my local friends have also built this design without serious difficulty.
edit: fix omissions
If you have hum issues with that speaker, I'd say you're in trouble
Our speakers are in the 98-101dB sensitivity and that's why hum is an issue. If we put my friend's 26 preamp in your system, it would be dead quiet.
Hi Kevin,
glad you brought up that 01 type. I dug my 01 pair from my pile and since they're spec'd for DC filaments, i thought of starting with them first before the 26.
I have a 226 in the my system with a 103 db.
Things to do
1) have a subchasis for the 26.
2) the subchasis must be suspended by something plastic or rubber. Use pcb standoff but plastic. Brass will transmit vibration and very much more microphonic.
3) avoid suspending via spring methods , the springs soak up vibration and retain the vibration and still will effect the 26
4) Build a separate chasis ps Keep the emi at bay. Better than trying to cure the problem. Prevention is best
5) Filament pay close attention here. Battery is good but well executed fillament is best. I use CLC -current source . Smps works but inject noise into the tube. What ever noise at the fillament will show up at the plate. Avoide SMPS at all. Give the brighter and more dynamic sound due to the noise.
6) Use transformer or choke to load the 26. From test this give lowest noise figure for the tube better than the standard resistor coupling.
7) Cheat by using a step down IT to reduce the gain as while as HUm
8) Use battery bias sound better.
I hope this helps i did 4-5 months trying to solve this stupid tube. Thankfully it paid off. For kicks if ccs voltage is high i pluck in a vt-62 or type 10 to replace the 26. Not the right bias but stil fine. Make sure fillament voltage is very much higher.
Cheers
Nicholas
Things to do
1) have a subchasis for the 26.
2) the subchasis must be suspended by something plastic or rubber. Use pcb standoff but plastic. Brass will transmit vibration and very much more microphonic.
3) avoid suspending via spring methods , the springs soak up vibration and retain the vibration and still will effect the 26
4) Build a separate chasis ps Keep the emi at bay. Better than trying to cure the problem. Prevention is best
5) Filament pay close attention here. Battery is good but well executed fillament is best. I use CLC -current source . Smps works but inject noise into the tube. What ever noise at the fillament will show up at the plate. Avoide SMPS at all. Give the brighter and more dynamic sound due to the noise.
6) Use transformer or choke to load the 26. From test this give lowest noise figure for the tube better than the standard resistor coupling.
7) Cheat by using a step down IT to reduce the gain as while as HUm
8) Use battery bias sound better.
I hope this helps i did 4-5 months trying to solve this stupid tube. Thankfully it paid off. For kicks if ccs voltage is high i pluck in a vt-62 or type 10 to replace the 26. Not the right bias but stil fine. Make sure fillament voltage is very much higher.
Cheers
Nicholas
Thanks
Thanks for all of the replies. This is far better than I'd hoped for. Lots of good ideas.
Any preferences between a transformer or a cap on the output?
I was going to use a cap since I had some nice Jensens in my treasure box but now I am thinking transformer input, loading, and output. You know, two chassis and 40 pounds for a two tube preamp!
Also how about parafeed? Perhaps some nice Magnequest OPTs...
I am way over budget on paper but now really into it!
Best,
Paul
Thanks for all of the replies. This is far better than I'd hoped for. Lots of good ideas.
Any preferences between a transformer or a cap on the output?
I was going to use a cap since I had some nice Jensens in my treasure box but now I am thinking transformer input, loading, and output. You know, two chassis and 40 pounds for a two tube preamp!
Also how about parafeed? Perhaps some nice Magnequest OPTs...
I am way over budget on paper but now really into it!
Best,
Paul
Rick
Been so busy did not have the time to draw it out.
BUt i will try to describe it here
Input transformer fitted with a -9volts 9v ppa batery
226 grounded cathode.
Plate is a CCS using gary pimm stuff with a 4.7uf MKC cap to a 1:1 interstage c-core
battery fillament is a ronan reg style with fillament choke
Been so busy did not have the time to draw it out.
BUt i will try to describe it here
Input transformer fitted with a -9volts 9v ppa batery
226 grounded cathode.
Plate is a CCS using gary pimm stuff with a 4.7uf MKC cap to a 1:1 interstage c-core
battery fillament is a ronan reg style with fillament choke
Hi ,
When I tried the 26 , even with CCS fils , it hummed . Could only get rid of the hum with shielding around the valve . Then came the microphony , some were MUCH worse than others . I ended up suspending the valves on rubber springs on a subchassis . Chassis was damped and the whole thing sat on sorbothane feet , problems solved ! Eventually , after rebuilding with all batteries and an LS8A DHT , the original problems came back so I do not think the 26 is an isolated case of being a noisy valve . I think that with DHT's special precautions should be taken during the design ie; if the chassis rings when tapped so will the valves so damp it ! make the chassis heavy enough , isolate and shield the valves etc .
cheers
316a
When I tried the 26 , even with CCS fils , it hummed . Could only get rid of the hum with shielding around the valve . Then came the microphony , some were MUCH worse than others . I ended up suspending the valves on rubber springs on a subchassis . Chassis was damped and the whole thing sat on sorbothane feet , problems solved ! Eventually , after rebuilding with all batteries and an LS8A DHT , the original problems came back so I do not think the 26 is an isolated case of being a noisy valve . I think that with DHT's special precautions should be taken during the design ie; if the chassis rings when tapped so will the valves so damp it ! make the chassis heavy enough , isolate and shield the valves etc .
cheers
316a
316a makes a lot of valid points which agree closely with my experience using the 26 and other small signal DHT's. The shielding he mentions (and I mentioned in a previous post) is very important to getting the hum out.
The plate area of the 01/12A/26 is quite heroic for a tube with moderately high rp, this makes it very vulnerable to electrostatic pickup which is why shielding is so effective. (If magnetic pick up were the primary issue only mu metal or steel would make a significant difference.)
I much prefer transformer coupling with these tubes because it provides you with a line stage with very low source impedance, unfortunately that does come with a price, both high cost for a good transformer, and a big reduction in available gain.
I have used 9V battery bias applied to the grid and this works exceedingly well.
I am less keen on battery bias in the cathode circuit, although I was a strong advocate of this approach for a while. Some batteries exhibit far worse colorations than a decent electrolytic with cathode bias or a high quality film cap with grid bias.
The old NiCD battery to my ear has less coloration than the more modern NiMh cells, but they seem to vary a lot from brand to brand.
NiCD in general seem to have lower and more uniform (with frequency) source impedances than comparable NiMh and I assume that at least part of the difference lies here. These of course were used in the cathode circuit.
Don't use rechargeables in grid bias applications.
I have also used lithium batteries for grid bias - these should NEVER
be used in the cathode circuit due to possible explosion hazard.
The plate area of the 01/12A/26 is quite heroic for a tube with moderately high rp, this makes it very vulnerable to electrostatic pickup which is why shielding is so effective. (If magnetic pick up were the primary issue only mu metal or steel would make a significant difference.)
I much prefer transformer coupling with these tubes because it provides you with a line stage with very low source impedance, unfortunately that does come with a price, both high cost for a good transformer, and a big reduction in available gain.
I have used 9V battery bias applied to the grid and this works exceedingly well.
I am less keen on battery bias in the cathode circuit, although I was a strong advocate of this approach for a while. Some batteries exhibit far worse colorations than a decent electrolytic with cathode bias or a high quality film cap with grid bias.
The old NiCD battery to my ear has less coloration than the more modern NiMh cells, but they seem to vary a lot from brand to brand.
NiCD in general seem to have lower and more uniform (with frequency) source impedances than comparable NiMh and I assume that at least part of the difference lies here. These of course were used in the cathode circuit.
Don't use rechargeables in grid bias applications.
I have also used lithium batteries for grid bias - these should NEVER
be used in the cathode circuit due to possible explosion hazard.
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