Suggestion for DIY Tube Amplifier: ~$500 excludng enclosure/tubes

[Nanoha]

I'm looking to build a DIY tube amplifier (for low impedance cans) for about $500 (excluding enclosure and tubes) that can match or beat the performance of my PPX3 SLAM 6SN7. Any ideas?

[Nanoha]

$500 to $700.

[aerius]

I'll have to draw up the latest revisions of the schematic, but yes, it will easily beat the performance of a PPX3 Slam. You'll have to do the whole thing point to point as there is no PCB for my amp design. Not hard though as the circuit is quite simple.

[aerius]

Schematics, transformer hookup diagram, and PSU.

The 50uF cap near the transformer is a 240VAC motor run capacitor. Bypass with a fancy cap if it makes you feel better.

The 4pF caps are 500V silver mica caps. These are the neutralization caps and should be soldered right to the tube socket tabs. You want them to hang off the underside of the socket with the shortest leads possible.

0.22uF caps are any decent polypropylene film & foil cap. Best if rated for 250V or more. Wima FKP or Sprague Orange Drops work nicely enough, but feel free to go nuts if you feel like it.

If you're using the 6BL7 output tube, then use the 820R resistor.

If using the BX7, use the 1k resistor.

Finding rimlock sockets for the ECC40 could be a bit of an adventure. There's usually some on Ebay and European tube vendors usually have them.

ECC40 tubes are also pretty common on Ebay.

Try to keep your wiring short & neat or the ECC40's might decide to oscillate on you.

Power supply voltage isn't that critical, anything between 260V & 300V works fine.

You probably want to run the tube heaters on DC, some tubes hum, others don't, but you might as well play it safe.

How to hook up the Amveco 62080 transformer. This is a toroidal power transformer available for about $20 each on Digikey. Yes, it works really well as an output transformer.

Power supply is a 240V transformer followed by a 8uF film cap, a choke from 20-50H, a 220uF cap, 100R resistor, then 2 75uF motor run caps in parallel. Run a wire from here to the 6BL7's. Then a 500R resistor and another 75uF motor run cap, then run a wire to the ECC40's.

I probably missed a few things, so if you have any questions just ask.

[Nanoha]

Now I've got a new project over the summer. Thanks Aerius.

(I've seen your photo of your amp before. I've always wondered what it was. )

[aerius]

I just had another crackpot idea. Take the Espressivo Headphone Amp as a starting point, building it with the 6C45Pi tubes. Instead of the costly multi-tapped autoformer, substitute one of the cheap Amveco donuts, the 62081 or 62082 looks about right. For further cheapness, find an LED that can handle 30-40mA (or parallel a couple 20mA ones together) and biases at the required ~2.2V, this allows you to get rid of the bypass cap altogether. The Gary Pimm Current Source can be found here. If you use the MOSFET only one, I think it only drops around 20V so the B+ can be lowered accordingly and you can omit the cap & resistors to the left of the CCS. From what I've read, the 6C45Pi is a rather fiddly tube so you'll need short neat wiring and good grounding to keep it from oscillating and doing stupid stuff.

[Nanoha]

Oh just wondering, do you know of any DIY designs (preferably cheap) that use a 6SN7 tube (yeah yeah, I know it's lame, but I have a few of these tubes lying around) as the driver tube?

[JBLoudG20]

VanTOTL

[aerius]

Oh just wondering, do you know of any DIY designs (preferably cheap) that use a 6SN7 tube (yeah yeah, I know it's lame, but I have a few of these tubes lying around) as the driver tube?

There's probably at least a dozen of them floating around on Headwize, but I can't vouch for the goodness of any of them since I haven't built them.

Anyways, quick correction to the schematic. Use a red LED instead of the blue one. I used a blue one at first siince it dropped about the same amount of voltage as the cathode resistor I had in there before. However this runs the tube too close to cut-off and the sound suffers a bit compared to a red LED. My particular LED runs at a bit over 1.8V, and is rated for 20mA and 1400mcd. From my research on DIYaudio.com, brighter is bad because of higher impedance, so look for a lower mcd number. If possible, try to get an LED where the bias current of the circuit is in the upper half of the LED's current rating, apparently the LED is more linear when run that way.

[cotdt]

Nice work, aerius, but why are the tubes glowing purple? I bet the amp sounds nice. Seems like overkill for a headphone amp, though.

Nanoha, here is a design using 6SN7:

[Nanoha]

There is no such thing as overkill for headphones.

[cotdt]

what software is that, aerius?

[aerius]

Nice work, aerius, but why are the tubes glowing purple? I bet the amp sounds nice. Seems like overkill for a headphone amp, though.

Automatic colour balance on my digital camera was fooled by the glowing tube filaments. Soundwise it's up there, it's probably limited by the output tubes and transformers but it's still handily beaten every commercially available design I've heard so far. It'll squash a PPX3 Slam like a bug, the proof of concept build which used crappier transformers and a far less optimized design & layout with crappier output tubes was still dead even with that Singlepower.

Nanoha, here is a design using 6SN7:

Why? Why??!! It's a complete waste of a 45 or 2A3. All you'll hear is the sound of a 6SN7 straining to drive the output tube along with the sound of the output transformer. For the $500-700 budget you're not going to be able to afford good output transformers either seeing how there's also a bunch of chokes and likely a custom power transformer which also needs to be bought. At the very least use a 5687 family tube which has better drive capabilities. The 6SN7 has a plate impedance of nearly 8k Ohms, it's simply not going to be a good driver tube for any power triode. Use the 6SN7 for what it does best, either as an input tube or a driver for Pentodes and other easy to drive tubes.

[n_maher]

Oh just wondering, do you know of any DIY designs (preferably cheap) that use a 6SN7 tube (yeah yeah, I know it's lame, but I have a few of these tubes lying around) as the driver tube?

I believe that if you look hard enough you can find the schematic of the Wheatfield HA-2 which uses the 6SN7. With a little modification it would seem possible to drive both low and high impedance phones with it. I'm in the process of modifying/repairing mine right now so if you'd like Pete's list of suggested changes (for high ohm phones) just drop me a PM. I'm 99% sure the amp can be built for under $500 in parts per your criteria.

[philodox]

It'll squash a PPX3 Slam like a bug.

Yes and no. I am sure that your design is superior, but at my listening levels I prefer the PPX3 Slam. I would imagine it is possible to tweak your circuit [maybe even just different tubes] to have more gain though, right?

[deepak]

Automatic colour balance on my digital camera was fooled by the glowing tube filaments. Soundwise it's up there, it's probably limited by the output tubes and transformers but it's still handily beaten every commercially available design I've heard so far. It'll squash a PPX3 Slam like a bug, the proof of concept build which used crappier transformers and a far less optimized design & layout with crappier output tubes was still dead even with that Singlepower.

Why? Why??!! It's a complete waste of a 45 or 2A3. All you'll hear is the sound of a 6SN7 straining to drive the output tube along with the sound of the output transformer. For the $500-700 budget you're not going to be able to afford good output transformers either seeing how there's also a bunch of chokes and likely a custom power transformer which also needs to be bought. At the very least use a 5687 family tube which has better drive capabilities. The 6SN7 has a plate impedance of nearly 8k Ohms, it's simply not going to be a good driver tube for any power triode. Use the 6SN7 for what it does best, either as an input tube or a driver for Pentodes and other easy to drive tubes.

Are there any schematics for 300B/2A3 amps with the 5687 as the driver?

[aerius]

Yes and no. I am sure that your design is superior, but at my listening levels I prefer the PPX3 Slam. I would imagine it is possible to tweak your circuit [maybe even just different tubes] to have more gain though, right?

The last time you might've heard my amp was last summer. There have been many many changes since then, the most important ones being the improved biasing & new operating points on the ECC40 along with the grid neutralization for output tubes. Slightly more gain, but a lot cleaner and it won't distort or do anything funny at high volume like it did before. Everything else is also a lot better.

[Nanoha]

Heh, I just realized my PPX3 SLAM blows with low impedance cans. Should this be expected?

[aerius]

Heh, I just realized my PPX3 SLAM blows with low impedance cans. Should this be expected?

Yes. It's an OTL with 5687's, which means relatively high output impedance and low output current capabilities. This is not a good thing with low impedance 'phones like Grados. I believe KG measured the output impedance at something like 30 Ohms with a 32 Ohm load.

[aerius]

Are there any schematics for 300B/2A3 amps with the 5687 as the driver?

Speaker amps, yes, headphone amps, can't think of one. For speaker amps, see Amity and Karna.

[Nanoha]

Yes. It's an OTL with 5687's, which means relatively high output impedance and low output current capabilities. This is not a good thing with low impedance 'phones like Grados. I believe KG measured the output impedance at something like 30 Ohms with a 32 Ohm load.

Do you know what a headphone would sound like if the impedances don't match up? (Output impedance too high for the low impedance cans?)

I want to see if it's the effect I'm hearing.

[Nanoha]

VanTOTL

Oh no you don't! Not for low-impedance cans!

[philodox]

The last time you might've heard my amp was last summer. There have been many many changes since then, the most important ones being the improved biasing & new operating points on the ECC40 along with the grid neutralization for output tubes. Slightly more gain, but a lot cleaner and it won't distort or do anything funny at high volume like it did before. Everything else is also a lot better.

Cool. You coming to the TO meet in July? We might have to have another mini meet before then though... Maybe just invite some of the 'cool kids'?

My amp is apparently not a great match for Grado's with its output impedance, but pixelpushers PS1's sure did sound nice on it.

[JBLoudG20]

I'll be using it with a 701.

[Nanoha]

mine doesn't.

Eeeeeee.