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Hello all!I want to build a PP amp using some of my HW but I can't find a good schematic I like.
The HW I am going to use is 700VCT@500mA power transformer with plenty of 6,3V and 5V heater windings and I have a pair of Hammond 1650R OPT(100W, 5k).
Can you recommend some good circuit to use as a base for my project?
I prefer octal preamp tubes and would like to use EL34 in the output stage.
The main goal is good sound ... power in the range of 20 - 40W ... and to have some fun.
Open to all suggestions ...
Cheers!
speleo
Follow Ups:
find an amp with the 6sj7as input :6267 values are also close.Float the 6BL7 heater if you need to: it is high 200v so ok.
6SJ7 dc into grid of 6BL7 for general values see vinnie 845 driver.
just add resistor under cathode to bring it up= (anode voltage of 6sj7+ 15volts bias of 6bl7) divided by currrent of 6bl7 (probably 22ma is good)
then put same resistor on top of cathode of 6bl7. this creates your phase inverter a la' dynaco/hafler. then cathode & anode film/foil cap coupled to grids of kt77. then you need an overall feedback path which is the real downside of the hafler path.
williamson is worse: remember this.One solution for no overall feedback path. Use phase inverter first, say 6bl7: great tube for this.
run into 6SL7 dc into KT77's grid. To do this you must float the cathodes of the KT77's up to the 6SL7 cathode ouput voltage.
Then run a single resistor unbypassed this means straight into the cathodes of the 6SL7's so that you get degenerative feedback from the anodes of the kt77's to the grids of the KT77's. you probably will get enough feedback that you will not have to use overall loop feedback, assuming you are running the kt77's UL.
So, have fun: also I assume now that you want already drawn tested schematic. I am being hard on you: I am sorry. It is just that this is a genuinely insightful answer to your question (and will probably build a better amp from your parts.)
Big Peace,
-3db
A Williamson, with 6SN7's and 6L6GC outputs. Can't go wrong. Are they monoblocks? I'd suggest EL34's but at 1.5A of current versus .9A for the 6L6's, a stereo chassis is tougher. 5K should be fine for either, which is nice.
I like the Williamson too, but being a cathode biased triode
design, it should need closer to a 10K OPT, right? Triode connection
is quite forgiving as compared to pentode or ultralinear mode, but
going from 10k to 5k would seem like a distortion introducing stretch,
UNLESS he paralelled a pair of EL34 tubes --then he'd be spot-on
with about 24-26 watts of tride output power, COOL! His stated OPT
should easilly handle the current, and the williamson driver should
drive the additional input capacitance of a second pair of EL34's
no prob. I would, however delete the feedback line, and bypass the
first stage cathode resistors with about 16V 470uF caps too. For that
matter, once the feedback loop is cut, the first 6SN7 stage becomes
superfluous (loop gain now higher) and I'd delete it and just use
the uneeded half of the 6SN7 as the other channels input stage --
still bypassing the second (now 1st stage after mod) stage's
cathode resistor with the 470uf cap. Should have plenty of gain to
be driven to full output with normal input levels. REMEMBER to
triode connect the EL34s.This is definitley a strong possibility.
-Tom
I forgot the second stage of williamson was the split-load inverter!
What was I thinking???Anyhow, keep the driver circuit unchanged (don't delete the first
stage) except for the before mentioned elimination of the feedback
line connection, and the addition of a 470uf cap to the first stage
cathode resistor *Which you will now have to recalculate for a
single resistor OR simply parallel a 1.2K with a 470 Ohm and this
would be your cathode resistor. (Sorry, too lazy to calculate at
the moment for single R!)-T.M.
..you'll have a ridiculous amount of gain - no?Regards, Allen
And one would have to build it carefully to avoid hum and excess
noise due to the gain as well --all doable. On the flip side:
You'd never need a separate preamp to drive it.-T.M.
Actually, if your OPT's are rated to 100W, no reason not to push them a bit. KT-88's or 6550's, no question. 6L6's would be a waste IMO
fixed bias ( because of your 5000ohm OPT impedance).
Then use the Van der Veen direct coupled 12AU7 driver: 1/2 direct-
coupled voltage amp to 1/2 split-load inverter.IMPORTANT: feed the above mentioned Menno Van der Veen drive
circuit (both halves of each channel's 12AU7) from a regulated
300v supply comprised of a stack of two 150V VR tubes which are
fed through a single power dropping resistor from the same +400
volt poer supply supply point as the EL34 output tubes. This dropping
resistor feeding the driver stages should be calculated so that
the VR tube stack draws about a static 10-15mA current to ground
after the currebt draw to the driver stages is factored in.
each of the four 12AU7 sections (remember, both channels here)
should have it's own INDIVIDUAL 22uf filter cap (450V rating)
charged to +280 volts through INDIVIDUAL dropping resistors which
tie together directly at the regulated +300V supply. DO NOT apply
ANY capacitance DIRECTLY to the +300 VR tube supply rail --this
will case instability and oscillation. The four little droppping
resistors which reduce the voltage from +300 to +285 to each one
of the drive stages 22uf filter caps will provide such isolation.This will give you a drive circuit with AWESOME filtration and
isolation form the output stage transients -even at less than 1Hz!.
This will result in truly excellent bass and transient performance
--FAST dynamics.Try it --you will thank me.
Again, you can use Menno Van der Veens exact fixed bias P-P EL34
amp design schematics and just change the b+ supply line
feeding the driver stages to +300V VR tube regulation --that's it.Oh yeah, and I strongly suggest you do use TRIODE connection config
(this nets around 12-15 watts out) and do NOT use ANY loop inverse
feedback.See: 'Tubes and Transformers in Power Amplifiers' by Menno van der
Veen or his newer 'Modern Valve Amplifiers' book for the schematics.Good luck.
-T.M.
.
Have you considered the Super Compact 150. This is a Push Pull design that squeezes 150 watts out of a pair of EL-509 Pentodes. The tube lineup per mono-block is (1) 12AX7, (1) 12AU7, (1) 6BM8 and (2) EL-509's. The power supply uses three power transformers and the OPT suggested is a Hammond 1650T.I plan to build me a pair later this year but would love to hear from anyone that has built one of these. The design is from Transcendent Audio and has been around for some years now.
The schematic is available on PDF from Transcendent Audio. With 150 Watts the choice of speakers can be quite broad.
BR/
Rich
here this is simple and very good..I also have a version of this with 12bh7s and no feedback that I can post.This circuit was flat from 10hz to 100khz
I have breadboarded the PP1 amp and tried el34, 6550 and KT66 in pentode mode with a regulated screen supply below 300 volt. I also tried S11E12 (=12E1) tubes. I didnot like the somewhat heavy bass on my speakers (damping factor) so I tried local feedback with a 100K resistor from plate el34 to plate of top ecc88 (i used 6N23). The sound is better now but I have far less power. Can I lower the 470 ohm resistor in the tail of the cascodes to get more gain ? Any other suggestions ?
... to get more gain ?Yes. I tried local feedback as you describe for many years on and of - but always came back to no feedback with triodes. But if you (insist) on using pentode connection then it will be a big help - and certaainly will sound far better than using loop feedback!
Be aware that with the 100k F/B r's in place - your anode voltage will go up and you may need to change the running current to ensure the driver anode V is still at the optimum level for maximum driver swing and minimum distortion.
Regards, Allen
"Yes. I tried local feedback as you describe for many years on and of - but always came back to no feedback with triodes. But if you (insist) on using pentode connection then it will be a big help - and certaainly will sound far better than using loop feedback!"I agree here - but I have no idea why. Does keeping the output transformer out of the feedback loop have something to do with it?
Probably more to do with the # of stages involved than the OPT per se.
The frequency response limitations and hence the consequent phase shifts are vastly larger in any OPT than in a direct tube to tube local loop, or even a couple of tubes in a not so local loop.The other big importance of taking the NFB off the anodes of the OP tubes rather than the seconadary of the OPT is that it reduces the driving impedance as seen by the OPT - hence reducing the transformer's own inherent distortions.
The RDH states: A transformer's distortion is zero if the driving impedance is also zero...
Regards, Allen
A surprising result. But probably true. With a zero driving impedance the driving voltage is maintained no matter what the secondary load is. And when the driving impedance is zero, you can remove the core entirely and the performance will be the same.The primary inductance doesn't have to be much at all to prevent a complete short at low frequencies and so the core is not doing anything useful to a zero driving impedance. All the core ever does is support low frequencies in transferring the energy to the secondary by raising the inductance for the low frequencies for the case when the driving impedance is higher than zero.
Since the core has no effect, it is acting like an air core transformer all the way. And there is no distortion in air core mutual inductance.
Kurt
As well as the tube amp hobby I design servo amps for stabilsed platforms in aircraft and laser scanners in my day job.In any feedback system there is a very usefull "Rule of Thumb" for best performance.
For stability (without special compensating techniques) the loop gain can only be as high as the ratio of the two most dominant poles. In a tube amp with global feedback one (well 2 actually) poles are set by the Output Transformer leakage inductance and interwinding capacitances. You then want to introduce a dominant pole = to that pole divided by the loop gain (say 20 for a typical example). That means for common garden variety Output Transformers you would be wanting to introduce a pole at 1.5 to 2 KHz. Imagine what that would sound like !!!!.So unless using a Toroidal Output Tranny with bandwidth to 250kHz, whenever you apply global feedback, you Will always be fiddling with special compensating techniques. These techniques (from a first principals point of view) seek to modify (reduce)the high frequency gain (amplitude) whilst attempting to leave the phase response intact. This basic point is largely missed or not understood and 98% of "stabilising circuits" I've seen are done poorly. It is also true that to a certain degree (not completely) reducing high frequency gain whilst leaving phase response intact are mutually exclusive requirements.
Do the math - that zobel network across the anode load resistor in the first stage you see as a typical stabilising technique should have a resistor value of 1/10th the anode load - how many times have I seen a value of 1/2 or 1/3 used - a sure sign that someone doesn't know what they are doing and are proceeding by trial and error - mostly the latter.
Aside: HF response will be nearly 6dB down when Zobel Xc + R = the load resistor BUT the 45 degrees phase shift (usually associated with the -3dB point in a simple low pass) will be when Zobel Xc = the Zobel R
Thats why you want the Zobel R = to 1/10th the load resistor (certainly never more than 1/5th)- to maximise the difference in frequency between the amplitude -3dB point and the 45 degrees phase shift point.Having said all the (sorry - I pushed my own button)I'm stongly with Allen on this - any way to avoid global feedback and hence avoid inherently compromised stabilsing techniques should be embraced. As he says reducing the effective rp of the output stage itself helps at both ends of the frequency spectrum - it lowers the low frequency roll off (6dB/octave) associated with rp and Lp (OT Primary Inductance) and raises the High Frequency roll offs associated with rp and leakage inductance and interwinding capacitances (12dB/octave total) Hence I use triode mode or ultralinear with balanced shunt feedback from the output tube anodes. If you still have to introduce global feedback to control Zout then the rp reduction will have made this simpler by extending the frequencies at which problem phase shifts occur in the output tranny.
I know he is not an Ultralinear fan. From the above you can see why that might be. Some OT phase shifts are involved in the feedback to the screens. The same arguement would apply to cathode feedback schemes.
Cheers,
Ian
From the above, in a global feedback system, there are a couple of other "facts" that fall out.
In the forward path all other poles MUST be above the OT introduced pole by at least a factor of 3, preferably 5 to 10. MANY driver stages fall down at this point. The theory says that it doesn't matter where in the forward path the dominant pole is introduced and many an amp has been designed an built with "accidental" stability due to inadequate driver stage. Always check your amp open loop to find out where the most dominany pole is located and if required slug that stage harder whilst make sure ever other stage is significantly faster - if that turns out to be the driver stage, that is perfectly acceptable but do it by design rather than by accident.The feedback path MUST be faster than the forward path and here large values of feedback resistor interacting with Miller capacitance at tube grid can be a problem so most feedback is to a low impedance point - usually a cathode so that a low value of feedback resitor can be used and the capacitance at the feedback point will not cause a low pass filter at the frequencies of interest.
You can see why avoiding global feedback simplifies things enormously and reduces the possible number of problems.
Remember Kiss.
Simple circuits have simple problems
Simple problems have simple solutionsAs one of my software lecturers used to point out:
Its 3 times a difficult to debug your software as it is to write it.
If you are as clever as you possibly can be when you write it - when do you reckon you'll have it running by - answer NEVER.Cheers,
Ian
> The RDH states: A transformer's distortion is zero if the driving impedance is also zero...I assume the context of this statememt discounts non-linearities of the core material (?)...
...but it was pretty much an ultimate statement.Regards, Allen
http://www.tubecad.com/2006/07/blog0073.htm
Dude,Other posters have skipped over your Octal small signal tube request for a reason. The triode(s) used in a phase splitter should be high gm/low Rp. AFAIK, nothing Octal really fits that pattern, but the 6SN7 comes close. You are going to get very little gain from a 'SN7 wired up as a differential phase splitter. So, you will need an effective voltage gain block in front of the splitter. A choke loaded 6SL7 in SRPP will do the job.
The 'SL7 SRPP is linear enough as is. You will need loop NFB only around the splitter and "finals". You can "clone" the basic topology used in "El Cheapo" for splitter and "finals".
Use a pair of 6AU4 damper diodes as the B+ rectifier. Voltage multiply the 5 VAC winding to get the B- rail.
Eli D.
"The triode(s) used in a phase splitter should be high gm/low Rp."
That's true for a cathodyne/concertina/split load phase splitter, but the opposite is true for a cathode coupled/long tail pair/differential splitter (who thinks of all these silly names?)The 6SL7 makes a good LTP splitter but it must be followed by a low Rp driver. If you want a concertina splitter, a 6SN7 is better.
Ray,One of the bugbears in circuits with loop NFB is slew limiting due to a significant HF error correction signal. High gm types are LESS likely to slew limit. The Mullard circuit is BETTER using a 12AT7, instead of a 12AX7, as the splitter.
I can see a 6SN7 buffered 6SL7 LTP driving PP 2A3s sounding GOOD. Loop NFB would not be used; so, slew limiting is not a problem.
Eli D.
Using Allen Wrights schematic as a starting point. El34's, 6550/KT88/KT90, and 300B are all possible options for output tubes depending on desired power/sound (even 2A3 choke input supply for lower B+). There are other tubes that can work for the front end if you dislike the 6922.
Russ
I've used Allen's circuit w/300Bs (cathode biased) and works great.
...as long as you organise a higher B+ of up to 500V. Follow Russ's point of a choke input filter if at all possible, as it makes a BIG difference to the sound.If you go for two pairs of output tubes, then I'd suggest using 6H30's as the driver tubes with 12k anode loads and lots of current - in fact this is the exact tube line up you can expect to see very soonas a kit from Brian Cherry's DIYHiFi Company (Hong Kong), and also from (my) Vacuum State in Europe.
Triode connected - two pairs of 6550 will easily deliver 50 watts of extremely good pure Class A1 sound with this circuit.
Regards, Allen
Have some nice pair of 8K PP OT's without UL taps and am looking for an ultimate EL-84 PP amp schematic.
__________________________________________________
Boo!
This design is the best amp in my collection. Of course we all think our own design sounds the best BUT it is truely stunning. I called it the "Baby Huey".Bas Horneman has kindly hosted the schematics on his site.
The Amp Schematic:
http://basaudio.net/schematics/pp/ppel84_baby_huey.gifThe power supply Schematic
http://basaudio.net/schematics/pp/p...by_huey_psu.gifRecent Mods:
1)The EL84 CCS Cathode bypasses are now 1000uF/25V Blackgate NX in place of the 4700uF/470uF/1uF networks shown.
2)10 Ohm + 220nF zobel added across the 4 Ohm secondary tap (the tap I use)
3) 47 Ohm series resitors in the the HV feeds to Left and Right Channels dropped to 4R7
4) Note that there is a very small amout of global feedback and local balanced shunt feedback set by the 15K which connects across the 2 47K resistors off the EL84 Anodes.IF - 2 Ohms Output Impedance is acceptable to you THEN ditch the global feedback altogether by removing that 12K global fedback resistor and increase that 15K to 27K - THIS is how I run my amp (I tried 33K but thought it a bit much)
IF 2 Ohms Output Impedance is not acceptable then leave the global feedback as is but increase the 15K to 20K. This was the best option with global feedback intact for an Output Impedance of just over 1 Ohm.
B.T.W. This whole design grew aout of an ECL86 (6GW8) circuit by Yves Monmagnon.
here
http://www.diyaudio.com/forums/attachment.php?s=&postid=552739&stamp=1105777406The ECL86 version I built and subsequently sold was also seriously stunning.
Cheers,
Ian
http://www.artimusica.nl/wim/dc_coupled_amp.pdfa nice direct coupled amp.
Do a search for stc amps. Those Japs know their tube stuff
...and change the cathode R's to get the running current you need.Nothing else needs to change at all.
Except if you want to do it correctly - you must use a choke input PS - big sonic difference!
Regards, Allen
It might be called a case of overkill as the el84 is so easy to drive. Take a look at Poinz's "music machine". It is along the same line but more for the 6v6/el84.
Russ
...my cascode circuit is not there to drive difficult loads - it's there because it came out tops sonically after 20 years of trying most every circuit, simple and complex!Regards, Allen
Hey, I love your circuit Allen. That is why I posted it first off (which I hope you don't mind).All I was trying to say is it almost seemed a waste to have a driver that could swing...what....four, five?? times what an el84 needs. But hey, this is DIY land and overhead is always nice. Besides, he would have lots of room to grow (say hy69 later?? and it would darn sure be in a very very linear area driving the el84).
But Poinz's circuit is adequate for the el84 and might be easier/cheaper for some to build. That's all:)
Russ
... and "adequate" didn't sound like what he was after.But yes - thanks - I'm happy for that circuit to be known, talked about, and used. Thanks for mentioning it.
Regards, Allen
Allen,I remember to have seen in an old Glass Audio magazin a version of the hedges cascode with on top of the cascode top tube a cathode follower. Did you try that too ? The author claimed that it bettered the regular cascode.
The key to the cascode in audio IS it's high output impedance. If you then load it with a lowish value non inductive resistor, you'll get a very well defined gain and consistant output impedance across a very wide frequency range, with often very low distortion as well!Adding a CF would, IMO, only bring in the typical CF problems - and perhaps mess up the positives of the cascode.
I'll keep them seperate, thanks.
Regards, Allen
speleo
The 1650r is a killer transformer and they dont cost that much being as large and well made as they are..However its impeadance is as such that its made for 6550/kt88 so you want to make the circuit to run kt88s and then you can run el34s at times if you wish or 6L6 type tubes.
I would use the vacuum tube audio circuit that is used in the VTA 70..You can get them as the mk3 boards or the st70 or you can just build it from the schematic..I have that.The reason recommend it is,you can run 12at7s or 12bh7s or a combo of both and you have enough drive in that circuit to drive 3 pairs of outputs per channel.
nt
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