Bottlehead Forum
General Category => All things MagneQuest => Topic started by: 4krow on June 03, 2012, 12:58:25 PM
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I am going to go out on a limb here and see what the possibility/worth of upgrading the trannys on my Audio Nirvana amp would cost. I would be doing the swap myself, but what info is needed to get started, i.e. how would i go about choosing the right part for the job?
What I can tell you so far is that the AN amp is a 6V6 design, putting out about 12 watts per channel. I realize that there is a lot more info that you will need here, but thats what I know as of right now.
I was very impressed by what I have read about the MQ trannys.
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What are you wanting to achieve in terms of sonic improvements? It may be better to start upgrading/modding with simple steps. All commercial units are built to a price point. One can achieve positive improvements systematically by replacing a few relatively inexpensive passive components. Replacing the trannies would be, IMO, way down the list "high payoff" changes. To actually hear the improvement from the OPT's, you have to clean up everything in front of them.
Were it my integrated amp, I would start with the power supply and signal coupling capacitors. You have to get clean power to get clean music.
Then, look at the quality and type of resistors in the signal path. Are they metal film, carbon film, carbon comp or metal oxide? Replace any metal oxides with metal or carbon film. (I do not like the sound of metal oxides, even in the PS!)
Now that you will get likely better imaging, timbre and loundstage....Swap tubes for quality OS. Replace the volume pot with a stepped attenuator.
The unit is wired P2P. Make one change at a time and listen. Be sure to give caps sufficient burn-in to hear through the break-in period.
By the time you do all of this you will either love the amp, or be ready to sell it and move up. Either way, you have likely brought it too 95% of it potential and well into the land of diminishing returns.
Of course,
YMMV,
Geary
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You have to have a good understanding of the Audio Nirvanas circuit before you change output transformers. Is the circuit push-pull or single ended? Is the output parafeed or not? Are the 6V6's operated in triode or pentode mode? What's the anticipated output impedance of the circuit? As Magnequest tells its customers which transformers and chokes will work well in Bottlehead equipment its easy for bottlehead customers to swap in magequest transformers without knowing all the particulars.
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First, I would like to say that there is always a quick response at this forum, and second, the responses are usually the kind that I read seriously. After having this, I am equally grateful for the comprehensive and well written response here. Thank you. I'll end this thought with the fact that often times I read in forums responses or such that are poorly written, vague and just as 'tuff to get through, mispelled. Good lord, I think, is this what we have come to? Ok, so I am off my li'l box...for now.
In response to your suggestions, I have to agree when I look at the big picture. You are right even when it comes to the handful of components involved with making music.
As I have previously noted elsewhere , the volume control on this unit is an Alps 'blue velvet', so I am on the fence as to whether to change it out. Preferably, I do like the idea of a ladder type attenuator.
I will have to look again at the caps and resistors in this piece. I don't remember them being objectionable, but they were not top notch either. The power that feed to this unit is balanced and that in itself is a plus, but power is of a great concern to me, seeing as to what we have to work with when you are customer of the power company.
I did replace the Chinese tubes(because one failed quickly) with tubes of a more reliable nature(read JJ's and Harmonex).
As you stated, a look at the internal components would be cheaper and a more logical approach. I will keep you guys posted(I feel another 'adventure' coming on).
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Again, another good response to my question. I believe the circuit to be push pull, but as for the rest of the information, I can't yet say. Unfortunately, I don't believe that there is a schematic for this unit, which might help in some ways. Having said that I can only ask the designer and see what he has to say. In the meantime, it looks like I will be going after the fore mentioned mods.
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If you're getting 12 watts from 6V6s, it's probably push-pull. Are there four 6V6's?
Without a circuit, you need to talk to the designer - or else reverse-engineer the circuit (i.e. draw the diagram by studying the amplifier, tracing connections, reading parts IDs, measuring parts you can't read, recording voltages, etc. - it takes a LOT of time and effort!). Only the designer knows for sure what primary impedance is best for the amp - if it's push pull it's probably between 7K and 12K ohms, but you really want to know closer than that. If it's ultralinear then you need a transformer with the right UL taps. It probably takes feedback from a particular secondary, so you have to have the right secondary impedance available. If there's a LOT of feedback, there may be phase-adjusting components in the feedback network which are specific to the transformer used (they work with the stray capacitance and/or leakage inductance of the particular transformer).
We have worked with Mike to come up with transformers made specifically for use with our amps; those parts are not in the regular catalog. It's the only way we - and Mike - can be sure the parts will work well.
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Paul,
Come to think of it, I remember that it is of an ultra-linear design, and yes, push-pull. A few moments ago, I took off the bottom plate, and to get right to the point, it is one of the most beautiful works that I have ever seen inside. Makes me think to leave well enough alone. I did notice Rubycon 330 mfd, 400 v caps inside,as well as Rubycon caps used throughout except where teflon caps were needed. Those caps had a signature on them that I did recognize. Now I will be the first to say that just because it looks beautiful may not tell the whole story, and frankly, most every aspect of this amp I like. So, this may be a lesson in deciding just where to draw the line. If I were to pick on anything about this amp, it is that the 100k pot 'lets in to much sound too quickly', i.e. I have reached my max volume before 11 o'clock position. That is due to my speakers being 94 db sensitive I suppose. On the swich box that feeds the amp(for now), I have added a 20k stepped attenuator to compensate. A bandaid of an idea for now, soon to be changed with maybe a 250k Goldpoint? More to come, I'm sure...
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From the website:
"'Ultralinear' operation combines the best features of Single Ended and Push-Pull amplifiers."
This makes no sense. Both push pull and single ended pentode and tetrode amplifiers can be wired for ultralinear operation by using an output transformer with a tapped primary. Perhaps what the writer meant (or simply doesn't understand) is that ultralinear operation is commonly touted as combining some of the more desirable characteristics of triode and pentode operation.
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If your 100k pot gets to max volume right away without anything in between. Its probably a 100k linear tapered pot. A 100k audio A tapered pot should give you a more gradual increase in volume when you turn the volume knob. The reason for this is the taper of an audio A pot is desiged for the way sound increases while a linear pot is not. A linear tapered pot is not designed to deal with the fact that to double volume requires a 10 fold increase in power, while an audio A tapered pot is designed based on this fact.
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Just looked up Alps blue velvet pot. It is an audio taper so disregard what I wrote in the previous post.
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Going in over my head here, but I wondered if there would be a possibility of being able to wire in a switch in order to run the amp either in triode or pentode. I've seen it before, and yet I am sure that it is more than meerely placing a switch in circuit.
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BTW, it has become obvious to me that in order for me to have a more intelligent discussion about these matters, a better base of information would do me good. Any easy reading on the topics at hand that you could recommend? And yes, I would agree that this forum is of excellent value, but I don't wan't to wear on your patience while you educate me. Having said that, I will say it every time I think it, this is an incredible place to learn and get good ideas. And I'm not entering into a private little club attitude that I have seen at other sites. wow.
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If you feel you can get by with less power, It is a very simple mater to convert to triode mode. Just one resistor per 6V6...John
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Well, with my speakers being 94db efficient, I am more than convinced that say 3 watts or so would be adequate. This amp sports 12 watts per channel now, so there is plenty of leg room, i bet. Tell me more
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Greg, what you have is an entry level 12w push-pull amplifier and 94db/w/m speakers. It's pretty reasonable combination. What are you trying to accoplish ? Why you think you need to upgrade transformers and what you in your opinion is wrong with the existing transformers ?
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What I am always looking for is an increase in micro detail, and from my experience, that requires equipment that is well designed, and of course, quality parts. I would have to think that better quality trannys can help bring on the micro detail I like. Having said that, it does make sense to look at the amp as a whole. The tranny swap may be down the road at this point. It seems that there are some valuable suggestions by the members here that I must take into consideration first.
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Hi Greg,
To convert to triode mode, on each 6V6 there will be a wire from the output transformer connected to pin 4. Disconnect this wire and tape or heat shrink the end to insulate it and tuck it out of the way.
Now take a resistor 150-200 ohm or so 1/2 W, this is not at all critical. Connect one end to pin 4, the other to pin 3. There will be another wire from the transformer on pin 3, that stays.
All done
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Hi Greg,
To convert to triode mode, on each 6V6 there will be a wire from the output transformer connected to pin 4. Disconnect this wire and tape or heat shrink the end to insulate it and tuck it out of the way.
Now take a resistor 150-200 ohm or so 1/2 W, this is not at all critical. Connect one end to pin 4, the other to pin 3. There will be another wire from the transformer on pin 3, that stays.
All done
I wouldn't recommend doing this mode without rebiasing your amp. If you don't know how to rebias your amp for triode operation don't do this mode. Also triodes operate cleaner then pentodes, they don't need ultra-linear transformer taps or negitive feedback. If your amp has these things it may sound sterile in triode mode.
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Points well taken gents. I will be looking into these things, and then determine whats next. As a matter of fact this amp is run in ultralinear.
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I just have to ask; Does it matter if the amp is self biasing? I'm betting not.
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I respectfully disagree with Clifford. Unless the tubes are run to there absolute max, triode mode should be safe. Easy enough to check. What is the value of the cathode resistor and what is the voltage across it?
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When you get a self biased amp from a manufacturer you assume competence and don't worry about how the amp is biased as long as it sounds good. However when you do a major modification to a amp like changing from pentode to triode mode you should make sure the amp is biased optimally for the new mode of operation. If you look at the tube data sheets for the load line of the 6v6 in both triode and pentode mode they are not the same. For a given plate voltage and current draw a 6v6 in triode mode will generate a larger voltage drop across the cathode then in pentode mode, therefore a larger value cathode resister is needed in order to zero in on a spacific current draw and a chosen bias setting. Why hope for a good result when the tools are available to plan for one.
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Ok, there is 24VDC at the cathode pin, and the resistor is 680 ohms, +-5%. I think I should take photo of the circuit itself. There is something I would like you to see. I can try and send through here, but if that fails, email always works for me.
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What's your plate voltage its pin 3?
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PLATE VOLTAGE= 354 VDC
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NOTE! CORRECTION OF VOLTAGE READINGS! CATHODE VOLTAGE = 22.7 PLATE VOLTAGE= 339 THE REASON FOR THIS CORRECTION IS THAT I ONLY HAD ONE CHANNEL OF 6V6 TUBES PLUGGED IN AT THE TIME OF MEASUREMENT. SORRY ABOUT THAT
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A couple of photos of the 6V6 circuit.
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With the new numbers, I get a plate V of ~316 at 33 ma. Getting to the upper end but doable. 6V6 are tough. try it. I think you will like the sound. measure the voltages that way and if it looks like you are killing the tubes we can adjust it. not hard...John
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Based on the information you supplied your amp is biased at about 87%, 339vdc minus the cathode voltage drop of 22.7 = 316vdc times your current of 33 miliamps equals ( 22.7 divided by 680 ohms) equals 10.428 watts divided by max dispensation or 12 watts = 87%. Its probably biased a little lower because screen current wasn't factored in. Based on the rather high cathode voltage drop of 22.7 and the fact that your amp delivers 12 watts per channel it can be conclued that your amp is operating in ultra-linear mode which is a mode that includes the best features of both triode mode and pentode mode. Your amp has the clean low distortion advantage of triode mode without sacrificing the greater power available with the pentode mode. Your output transformer is designed to operate in this mode. Try the mode John suggested if you like it great if not you can go back to stock.
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I'm wondering if you have asked about these mods on the Audio Nirvana forum.
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After looking over the data it looks like your cathode resister is ok for triode mode. The amp may sound better in triode mode but will you be satisfied with significantly less power. Only one way to find out.
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I am going to try this mod and see what it amounts to. While I'm at it, I may replace the output caps. Sorry if this is redundant, I sent a message earlier this morning, but it didn't seem to take. Probably should be mentioned that because I live out in the sticks, all supplies will have to be ordered. I've got a list already, so this is a good time to do it.
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Greg,
I didn't see if you have said that you have a schematic. Do you? If so, scan it and send it to me. I can make some suggestions.
Posting a schematic of a current production product might be in bad taste.
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Grainger, I think that probably no one will ever have a schematic of this amp. David Dicks is not the kind of guy that gives things up easily. His idea when designing this amp was to use 'ordinary but decent' parts to achieve great sound. All in all, I think he achieved much. But of course, I can see room for improvement. For instance, the original tubes were Chinese, and not that they sound poorly, but one of them failed within a week. So I changed out the 6V6 tubes to JJ's and the 12ax7 to Electro Harmonex. The 6NS7 tubes are still Chinese. I will try this triode mode, but I think even after that there are a few improvements to be had.
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UPDATE:
I have carefully considered the pros and cons to the idea of revamping an already good amp, and for the cost, I have decided that simply upgrading the caps in the signal path, and bypassing others will be a better bet. By doing this, I will have affected(for the good) the current design, but not pushed a lot of money towards it. The other decision was to upgrade the 6Sn7 tubes as well. I stopped there to see that leaves me, and so far, I cannot make a judgement given the break in time of the new tubes/caps. As always, it was of great value to me to have the help and advice given here. I will report back when there has been some time with the ears..