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Millett "Starving Student" hybrid amp - Page 414

post #6196 of 6806
Quote:
Originally Posted by vixr View Post

Kim,

    what phones do you use? I have some HD-600 and the gain is a bit low on my last build. I think I could benefit from a 12AX7...


I did very limited experiments back when I built the 12A_7 but I do remember that HD-600s work well with the 12AX7; Grados, however, already have problems taming them.

post #6197 of 6806

Ah, yeah. I forgot about this point. I mainly use the Q701 with my MSSH. It has plenty of gain to drive them. I can't use my SRH440 with the MSSH because these cans are too sensitive, the noise level is too high to enjoy the rest of the audio. For some reason, the Q701 is silent on the MSSH, and at normal listening level I can't even hear the noise.

 

Another factor is the output level of your DAC. The NFB-12 has a very high line out volume level. I guess this helps, but it has more to do with the input resistors of the amp than the gain.

 

Vixr, are there input resistors in the build you use with the HD600? If the gain is too low, you could simply bypass them, or lower their value. They are optional, after all. I did some test with and without them, and the difference is very significant. Without them, the volume knob was useless with both my headphones. I'm curious how this scales with the HD600.

post #6198 of 6806

yes...A few years ago there was a version of the SOHA that we used SIP sockets under the input resistors...they could be jumpered or used to tweak the resistor value...thanks for reminding me of this option. I will give it a try. I actually have a few boards left from the GB on that amp. I may have to build one again just for grins, I sold my original.


Edited by vixr - 9/13/12 at 5:32am
post #6199 of 6806

about to grab some tubes off ebay, couple questions first, how important is it to have matched tubes? and what is the differance between the 12au7a and the 12au7?

post #6200 of 6806
Quote:
Originally Posted by shamazo View Post

about to grab some tubes off ebay, couple questions first, how important is it to have matched tubes? and what is the differance between the 12au7a and the 12au7?


You don't need matched tubes for the SSMH. Matched tubes are only "needed" in push-pull amplifiers tu minimize distortion (and even there I've read a couple of articles that say there are no real benefits to matched pairs). Unless your couple tubes vary WILDLY (as in one is dying and the other is brand new) it's not likely you'll hear any difference between the channels.

 

You don't need matched triodes either (each 12A_7 tube contains a couple triodes and some companies will sell tubes with matched triodes) as the parallel connection of the two triodes in the SSMH averages the differences between them.

 

The 'A' suffix indicates controlled heater startup time. Some say it also indicates a higher amplification factor variant of the tube but my old RCA tube manual lists the same amplification factor (17) for the 12AU7 and the 12AU7A

 

cheers!

post #6201 of 6806

It's been a while since I last worked on this little amp, but motivated by a friend of mine who saw my build gathering dust in my closet and asked to try it out I finally decided to implement a modification that's been on my mind for over a year.

Inspired by the design of the Millett Hybrid, Millett [Mini]Max, SOHA II, a re-reading of Morgan Jones' "Valve amplifiers"  and some posts by Dsavitsk where he mentioned that the best mod you could do to the SSMH was substituting the plate loads for constant current sources, I thought I might try my hand at it.

The benefits of using a CCS as plate load are many and described in detail in many pages (Google is your friend). I'll just summarize a quick list here:
Improved distortion figures
Fantastic power supply ripple rejection (PSRR)
Reduced crosstalk
More gain from the tube


I decided to use the ring of two transistors design, just as in the MiniMAX, since the necessary parts (a couple PNP transistor and a couple resistors) are easily available most anywhere in the world. I settled for the BC327 PNP transistor since that's what was easily available to me where I live.

Based on some current measurements I did in the 12A_7 version, I went for a plate current of ~0.7 mA per tube which based on the details posted here (http://www.diyforums.org/MiniMAX/MiniMAXccs.php), and after rounding to common resistor values, resulted in one 680 ohm resistor and one 10K resistor for the CCS.

I decided to also substitute the cathode bias resistors (R5, R11) for 2K trimpots, so as to be able to set the plate voltage; again, just like in the MiniMAX. I did not use bypass caps around the trimpots.

I put the components in a little perfboard and, after removing the plate load resistors (R1, R7), I ran wires from the junction of R13 and C6 (V+), each tube plate and ground to my CCS "module".

 I must say that it didn't work at first, which was terribly frustrating. After much testing, debugging, rewiring, even swapping out one of the output MOSFET's, I found out that one or both transistors in one of the CCSs were blown. After substituting them... Ah!  voilá! Sweet music!
 
 I proceeded to measure the voltage at the junction of R13 and C6 (V+) and found 42.6 volts. Then I hooked up the meter to each tube plate and, using the trimpot, I set each plate voltage to 21.3 volts. I measured the cathode voltage to ground and found out it was 0.9 volts. Finally, and just to make sure everything was well before pluggin in my beloved Grado 'phones, I measured DC at the output: 0.0 volts. :) So all looks good.
 
 Finally, I got set for a listening session using my collection of 320Kpbs MP3 files, Foobar200 and Bantam DAC. What I'm going to say is of course highly subjective without any detailed measurements to back it up, but, given that disclaimer I'll go ahead:  I was absolutely enthralled by the sound coming out from the amplifier. It seemed to have more clarity and definition. It also seemed to me that it had a 'blacker background'. Drums were a delight: listening to Tony Williams' drumming in the second great Miles Davis quintet recordings was a real joy: the transient response, the "air" around the drum sound. Piano was a treat too, whether it was jazz, classical, blues. I just kept listening to album after album and falling in love with my little amp as the added clarity, definition, quieter background and transient response all built up to enhance the 'you are there' sensation.

Again, I'm perfeclty aware that all the above is highly subjective, but for the price of 4 transistors, 4 resistors, a couple trimpots and 2 or 3 hours of your time (just for the build, don't come to me complaining that you spent your whole weekend sitting by the amp with your headphones on!) it seems to me that the bang for the buck factor of this mod is stratosferical.

A couple final things. In the first place, the amp is harder to control now with the additional gain; not a  problem with 300 ohm but certainly hard with 32 ohm Grados. It's probably time for me to add those input resistors to attenuate the input signals.

Second, taking into consideration the great PSRR that the CCS adds to the tube stage, I think those who built your own linear power supply should give it a try.


I'll post some pictures later today and will try to add the CCS to the 12A_7 schematic.

cheers!
 


Edited by the_equalizer - 9/24/12 at 8:44pm
post #6202 of 6806

Ok, so here are a couple pictures from my amp with the CCS mod. Since I built it just as a test it's not very neat and I haven't removed the unused parts from the regular build (cathode bias resistor and bypass electrolytic capacitor). Now that it's working I'll split the CCS board in two and fix each little module between the tube socket and the terminal strip. The location will also help with lead length and I could also use a couple connectors for the wiring so the modules are easy to replace or service.

 

 

 

In the following descriptions I'll refer to the 12A_7 / 17EW8 pins and to right channel parts. The left channel CCS should connect to the homologous points in the left channel; and naturally other tubes (like the 19J6) will have different pinouts.

 

Black wire is ground, brown wire comes from the tube cathodes (pins 3 and 8), red is power (from the junction of R13 and C6) and orange goes to the tube plates (pins 1 and 6).

 

The blue 2K ohm trimpots replace the cathode bias resistors and are simply wired as rheostats. You could maybe do without them but it's definitely nicer being able to dial in the plate voltage and hence get the best sound from the tubes.

 

 

 

Here's the schematic for the CCS. It's just the typical ring of two transistors. The circuit simply replaces the plate load resistor R1.

 

 

I've been listening to the amplifier constantly and I just keep playing album after album. I'm really enjoying the sound from it with all kinds of music I've tried. I'm really glad I decided to build the CCS mod.

 

I hope someone else also tries it. It's fun and it works great!

 

cheers!

post #6203 of 6806

After building 4 kits and 2 P2Ps I thought there wasn't anything left to try on this amazing little design and then Bang, Equalizer saves the day again with a great idea!!!

 

Any chance we can get a pic of the flip side of your new board?

 

I recently finished a 17ew8 version and must say I am very pleased with the way it sounds! With a Mu factor of 50 it's a little more unforgiving

of generic mp3s but the mids are right in your face. i found myself adjusting the equalization on my source player a wee bit and everything was just fine!
 

post #6204 of 6806
Quote:
Originally Posted by Legis31 View Post

After building 4 kits and 2 P2Ps I thought there wasn't anything left to try on this amazing little design and then Bang, Equalizer saves the day again with a great idea!!!

 

Any chance we can get a pic of the flip side of your new board?

 

I recently finished a 17ew8 version and must say I am very pleased with the way it sounds! With a Mu factor of 50 it's a little more unforgiving

of generic mp3s but the mids are right in your face. i found myself adjusting the equalization on my source player a wee bit and everything was just fine!
 

 

You certainly have experience building this amp! I've only built two of them, and wired the 12AE6 variant on protoboard.

 

Here's a pic of the flip side of the board, but there's just nothing special to it, nothing particular about the layout.

 

 

Good to hear you're enjoying the sound from the 17EW8. One thing I like about that variant is that the MOSFET's don't get as hot as they do with the 12A_7 build.

 

About the CCS setup, just like TomB mentions in the MiniMAX CCS page, there's nothing special at the current I chose, certainly do try with other currents. I was thinking of setting it to 2mA so each triode gets 1mA.

 

BUT here's something important you need to keep in mind. If you set the CCS to work at more that 1mA each, you'd do better to hook them up directly to the power line (C1) just like the MOSFETs are. The reason is that the extra current pulled by the CCSs will increase the voltage drop across R13, and since R13 forms part of the voltage divider (R2/R4) that biases the MOSFET gates, that extra voltage drop across R13 can mess up that biasing.

 

If I get the chance this sunday I'll experiment with that setup. In the meantime I'm going back to my listening session (netbook + Linux Mint + Wine + Foobar2000 + Bauer Stereo-to-binaural -> Bantam DAC -> 12AU7 CCS SSMH -> AKG K701)  :D

 

cheers!

post #6205 of 6806

Thanks big time for the pic and especially the labels! it'll help more than you know! Your 12au7 pics got me through my 9 pin conversion without a hitch.

 

Granted I'm using 1 1/2" sinks on my 17ew8 and probably should have used 2", but the temp runs at 130-135F whereas I can hold the 19j6 sinks all day at approx 85F.

 

Keep up the great work....this thread might have died out a long time ago if not for you! Thanks again!

post #6206 of 6806
Quote:
Originally Posted by Legis31 View Post



Granted I'm using 1 1/2" sinks on my 17ew8 and probably should have used 2", but the temp runs at 130-135F whereas I can hold the 19j6 sinks all day at approx 85F.


Hmmm...did you use R2 and R8 = 220K like in the 19j6 version? Otherwise, if you're using 390K, you're feeding the 17 volt heater in the 17EW8 with 12 volts, which would explain why your MOSFETs are running as hot as in the 12A_7 variant.

I'm glad that the pic(s) helped and thank you for you kind words but it's all of us here, all interested in this rocking little amp, who help keep the thread alive.

If you plan to add the CCSs to your amp, I'm certainly interested in reading your opinion, impressions.

Cheers!
post #6207 of 6806

Equalizer,

as per your question about which resister i used, I ran a 220K and 33k in series which gave me 17.2V on both heaters.Darn close to what the tube manual says at 17.5v. I did use IRF 610s if that could be the cause?

I do find it odd that my sinks get that hot just dropping 2 additional volts from the 19j6 version, though. My other measurements are as follws.

 

Power supply     47.5V

tube plates        32/32

tube grids           0/0

tube cathodes  61.2mv/370.7mv

mosfet gates     21v/21v

dc offset            1mv/1mv

 

Pretty well balanced 'cept for the cathodes.

 

it may be a while before i can try the CCS rig.... wifey says its fall clean up time around the house but I'll definitely keep you posted!

post #6208 of 6806

Interesting measurments, thanks. I guess 2 volts is enough to heat up the MOSFETs that much.  Good luck with the clean-up!

 

cheers!

post #6209 of 6806

very interesting! i might have to give the ccs mod a try... my starving student is one of the original 19j6 versions, should still work right?

post #6210 of 6806
Quote:
Originally Posted by kchapdaily View Post

very interesting! i might have to give the ccs mod a try... my starving student is one of the original 19j6 versions, should still work right?

 

Certainly. It's been a looong time since I had a 19J6 build but I seem to remember the current going through each 19J6 is ~0.5 mA; so I'd set the CCS to that value. The default resistor values used in the MiniMAX ( 1.13K and 11.3 K )  will do for that.

 

cheers!

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