Restore or Mod A Maggie 6V6 Console Pull

[thebes]

Oops I am a  dull fellow.  Forgot that a 6V6 is a Pentode, like the El34 etc . Got the term confused with a "quad" of power tubes used for the push/pull. .  It's funny but I've never strapped my Marantz8B into triode.  Should do that one of these days. 

 

I'm also a little leery of total Class A operation, but I'd be interested in learning more., because to me one of the great benefits of  A/B to my limited understanding anyway, is that they tend to produce more robust bottom end, than many Class A circuits.  Like anything I'm sure that's open to debate. Finally tested the tubes and they match up well, plus I have at least 6 or 8 more 6V6's handy. 

 

I do have a scope on hand but it's seen very little use since I'm not really up to speed on them and any chance to rectify that lack of experience would be of interest to me.Carry on lads, I'm too tied tonight to type much more, but I look forward to this project.  Hopefully other will also follow along and learn some things.

[xxJPMxx]

I'm also a little leery of total Class A operation, but I'd be interested in learning more., because to me one of the great benefits of A/B to my limited understanding anyway, is that they tend to produce more robust bottom end, than many Class A circuits. Like anything I'm sure that's open to debate. Finally tested the tubes and they match up well, plus I have at least 6 or 8 more 6V6's handy.

 

 

Hello thebes, I wouldn't let that myth about Class A amps hold you back. The reasons for people most likely associating robust bass with A/B amps is most A/B amps are higher powered amps with a lot of global feedback, the better damping factor gives them "tighter" bass. I will try and explain class of operation.

 

Picture a sine wave.

 

 

 

 

 

Picture an amp with one power tube. To faithfully recreate that signal it would have to conduct current the full 360 degrees of the waveform, so pretty much all single ended amplifiers are Class A. Now when you have two devices operated in 180 degree opposition the signal in the output transformer is the difference of two signals. At lower signal levels the A/B amplifier operates in Class A meaning each device conducts current the full 360 degrees. How an A/B amp affords more power is this: Since the two tubes are in opposition one tubes grid sees a positive swing and that valve starts to conduct more while the other tube's grid sees a negative swing and starts to conduct less, it gets to a point where the tube on the negative swing stops conducting and this is called cut off. When this device cuts off it's portion of the OT winding is now out of the circuit and the opposite tubes plate now sees half the load (from 4k to 2k). For a brief period the tube is being operated beyond it's max ratings but it's okay because on the next cycle it will be "off" for a brief period of time so the plate has time to cool. So A/B amps operate in Class A for the majority of time unless you are cranking it. The issue with A/B amps is when operated too close to Class B one tube stays off too long of the cycle and there will be a notch at the 180 degree crossover point, hence why they call it crossover distortion. These rules apply to all frequencies so as you can see it should have zero effect on bass response. My thinking is that whith higher power A/B amps you just feel the bass more because there is more power, or a lot of times Class A power amps will be triodes and no global feedback giveing them a low damping factor and wooly bass.

 

 

Maynard has another option on biasing the power tubes that will allow for manual adjustment of the valves current in order to mitigate standing DC current in the output transformers. I will let him explain his idea, but in the meantime I will look at the cost difference for the parts. I order parts all the time and don't mind making you the constant current sources so all you have to do is solder them in place, one solder connection to pin 8 and the other connection to ground. I make them all the time and they cost me $5 each, you will need 4 so that's $20. I will look up the parts for Maynards method. With Maynards method (yuck! ) if you decide to swap tubes you will have to manually readjust where with mine you just plug and play, they will always pull whatever current you set! But the downside to my method is you will be limited on power to about 10 watts due to the tubes staying in Class A.

 

I know Mike has chimed in with some good ideas, lets here some from you other tube dudes out there. I have been crunching numbers over here on a couple possibilities, Mike's idea on a CCS in the tail of a LTP differential amplifier. This method doesn't allow a ton of open loop gain so the amount of feedback possible isn't a lot, I can get distortion low enough but output Z is still around 3, which we all know with some of these speakers the low end impedance is around there so the DF will be 1, not that tight but maybe good enough. The other one I was playing around with was one that Maynard had suggested in an email to me this morning, it's funny because I had the best results with this method going over some simulations yesterday. This method is a simple common cathode voltage amp (1/2 12AX7) into a cathodyne phase inverter. This gave me more open loop gain to use more feedback, dampening factor improved slightly and distortion is way down. I have also been playing around with self inverting push pull stage which frees up a triode, I used the triode in a half mu setup, the benefit of this is the front end has a lower ouput impedance and also is actively loaded, meaning the gain won't be effected by tube age I should have proposed schematics for after the holiday. I haven't played around with the other paraphase setup Mike posted, I will get to that too.

 

 

Gobble Gobble,

-JP

Edited November 26, 2015 by xxJPMxx

[mike stehr]

I think the cathodyne phase inverter is used on the old Eico HF-81 stereo integrated, the HF-12 mono integrated, and the HF-14 power amplifier.

[tube fanatic]

JP is certainly correct about class A push-pull amps being capable of producing a robust bottom end.  No concerns about that in my view.  I've attached  schematics of 3 proposed output stage arrangements (you will have to reduce the size when viewed- there are some issues with a recent software update).  The first allows the dc balance to be set.  The second is the same as the first, except a desired amount of current feedback is employed which can allow a global loop to be eliminated.  As JP said, when tubes are replaced the bias will have to be reset.  To me, that's not a big deal (in fact, with brand new output tubes if it's set initially, and then rechecked at around 50-100 hours, you will probably not have to check it again for years).  As is obvious, there are many ways of doing this.  The idea of a self-inverting push pull output stage is also a neat method (the 6V6s are very well suited to that type of operation).  The third schematic is a possible self-inverting differential output stage.  It is driven the same way as a single ended amp.  The cathode resistor cannot be bypassed in this application so drive requirements will be on the higher side.  Regarding the cathodyne phase inverter, it is self balancing so concerns about the AC balance of the amp are eliminated.  JP, if you have time, can you model all 3?  I haven't thought about p-p amp design in so many years my memory may be a bit foggy as to some of the methods I had success with (old age you know!).  And, yes, I hate p-p amps!!!!!   

Maynard

marty-p-p-3-1.pdf

[thebes]

Sorry for the delay in getting back. 

 

First, thanks very much JP for you kind explanation of class of operation.  It was most informative.

 

I've been thinking about this nascent project a bit, and doing a bit more research. Since we are going to be modifying the circuit, I'm thinking maybe a full mod would be in order , with a new circuit from the power supply forward, but not ruling out changes to the power supply to meet the requirements of the new circuit.  I'm thinking that even with new OPT's the cost would still be very modest.  I've been looking at the Edcor push/pull amps which are about half the price of similar Hammonds. I've used them in the past and have no trouble with their sound and build quality.

 

Maybe these two would do with the later, and more powerful alternative, preferred  should the circuit design support it.  Also maybe a new circuit would entail using different iron altogether.

 

https://www.edcorusa.com/gxpp10-8-8k

 

https://www.edcorusa.com/cxpp25-600-8k <https://www.edcorusa.com/cxpp25-600-8k>
 

 

Sorry to change direction on this, but a simple new circuit might keep sand in my shoes, but not in my  amp.

 

What say ye?

[xxJPMxx]

Hello Marty, The first transformer could work but the second one is for 600ohm load.  The first one is only for 10 watts, I would suggest getting this one. https://www.edcorusa.com/gxpp15-8-8k

 

Or for an extra $20 each these are better, if they will fit. https://www.edcorusa.com/cxpp25-ms-8k

 

I used the latter in my last push pull build that came out great, you will get extended low end using these.  I am trying to get my buddy whom I built the amps for to join the forum, he uses Heresy's but we hooked them up to Cornwalls and let me tell you, devestating!!! 

 

The best part about getting new output transformers is the sound will be much better! The Edcors come with ultra linear taps so now that is another option. Again my last push pull build was ultra linear mode and the results were great, very low distortion and high output power. I am still working on my data for your amps, I have some ideas from Maynard I need to look over and run some simulations etc.......   I am glad you are taking the plunge with these and getting new iron and a more updated circuit, you will be most pleased.

[thebes]

Hey JP, thanks for your Edcor suggestions. My bad on the 600ohm OPT. I tried to correct the links last night but the thread wouldn't open.  It appears that the tech folks have fixed the issue.

 

I'm leaning towards the Cxpp25-ms-8k if the circuit you guys come up with will be suitable.  Haven't taken the measurement but I will tonight to make sure they fit.

 

Yeah there is cheap and there is cheap.  After all this is my main winter project, although I do want to freshen up a Dyna FM3 tuner, and figure out why my newly acquired Ampex RTR makes a loud buzzing noise instead of music. 

 

But I'll leave those two for later.

[JL Sargent]

I was able to rebuild a nice similar amp with a lot of help in this thread.

 

https://community.klipsch.com/index.php?/topic/127925-old-tube-amp-revived/?hl=%2Bstromberg+%2Band+%2Bcarlson#entry1417951

[mike stehr]

I do have a scope on hand but it's seen very little use since I'm not really up to speed on them and any chance to rectify that lack of experience would be of interest to me.

 

Does the oscilloscope work? Do you have a probe of some sort?

 

If you are bored with some time to kill, do a internet search on how to operate a oscilloscope. The Tektronics pdf file for scope operation is piled up everywhere on the 'net. There are also other primers online that are more simple to understand if the Tektronics primer is a bit daunting.

 

A scope is kind of self-intuitive once you get the hang of things...but it can get confusing...

 

Invest in a function generator if you don't have one already. They can be found on eBay for reasonable prices. Some BNC cables, (one with clips on one end) and some load resistors.

 

Something to ponder...

[xxJPMxx]

I am still cooking something up, I have a real simple schematic good for about 10 wpc with really low distortion and decent damping that should work perfect with the $30 Edcors found here; https://www.edcorusa.com/gxpp10-8-8k

 

Stay tuned

[xxJPMxx]

Okay I had some more time this morning to look over some of my ideas. I am sure there are mistakes so please I need other tech eyes to look them over.

 

First one is common cathode stage driving a cathodyne phase inverter w/ fixed bias so there is no bias resistor in the cathode messing with the balance. Output stage is ultra linear class A/B with cathode bias. R15 and R16 is a 50 ohm pot for DC balance, since the models are perfect it is set in the middle for simulation. 10db of global feedback.  Full output is around 10 watts @ .5% THD

 

Schematic;

 

 

FFT @ 1 watt,  .1% THD mainly second harmonic from input stage.

 

 

 

 

 

The second schematic is a bit more fun if you don't need a lot of power and want to try something a bit more exotic. First stage is an SRPP driving a self inverting Class A push pull output stage. I like the SRPP because you will not get degraded performance with older valves that have lower transconductance, basically tube age will not effect performance. Since we need to force Class A operation there is half the power. Elegant and simple if you ask me, no phase inverter to worry about AC balance.  R15 and R16 serve the same purpose and will be replace with one of these: http://www.mouser.com/ProductDetail/BI-Technologies-TT-Electronics/93PR50LF/?qs=sGAEpiMZZMvygUB3GLcD7jZSyzxDqivkwIQj05pIUWQ%3d

 

Or any similar pot will work. I want to mention that it might be a good idea to place 1 ohm resistors in series with the power tubes cathodes as current sense resistors, these are easy to add and I recommed you do.

 

6db of global feedback. Output power is 5 watts per channel @ 1% THD.

 

Schematic;

 

 

 

 

FFT @ 1 watt, .2% THD

 

Edited December 5, 2015 by xxJPMxx

[thebes]

 

I do have a scope on hand but it's seen very little use since I'm not really up to speed on them and any chance to rectify that lack of experience would be of interest to me.

 

Does the oscilloscope work? Do you have a probe of some sort?

 

If you are bored with some time to kill, do a internet search on how to operate a oscilloscope. The Tektronics pdf file for scope operation is piled up everywhere on the 'net. There are also other primers online that are more simple to understand if the Tektronics primer is a bit daunting.

 

A scope is kind of self-intuitive once you get the hang of things...but it can get confusing...

 

Invest in a function generator if you don't have one already. They can be found on eBay for reasonable prices. Some BNC cables, (one with clips on one end) and some load resistors.

 

Something to ponder...

 

Yes it does. It's a BK Model 1471B 10mhz scope.  I've even had a person show me how to use it but the lesson didn't stick.  I do not have and need some clips but I do have a couple of probes.  I also have a Heathkit Oscilloscope Calibrator but no Function Generator.

 

I do need to learn how to sue this thing. Maybe I can get a simple lesson from you guys as the end part of the build process.

[thebes]

Wow JP, you have been busy. Thank you for all your work on modelling these. Given my general duncehood when it comes to understanding tube circuits, not to mention schematics, I think I'd better await comments from your fellow experts.    Also again excuse my ignorance, but what are V1 and V2 in these schematics, the inputs?

 

Oh, I have done some physical measuring and I think the 25 watt Edcors are too tight a fit.  I can probably shoehorn them in but I don't like them sitting that close together, so these lower wattage approaches are fine by me.

[tube fanatic]

Marty, are you planning on removing the factory output xfmrs, or leaving them in place with the new ones behind them?  If the latter, I have a couple of interesting and novel circuit alternatives which I'll review with JP.  Let us know!!!

 

Maynard

[xxJPMxx]

Also again excuse my ignorance, but what are V1 and V2 in these schematics, the inputs?

 

 

Hello Marty No need to apologize for great questions, if they don't get asked then nobody learns. In the schematics V1 and V2 are voltages, in my simulation software they are in the sense perfect voltage sources (for now anyway). V2 is the input signal in AC volts (you were right!) and V2 is the high voltage B+ created by the power supply. You can see for V2 when I do an AC analysis it uses .1 volts peak amplitude and sweeps through whatever frequencies I choose, say 1Hz to 1MHz. Under that you see sine 0 .6 1k, this is for transient analysis, it is a sine wave with 0 volts DC offset, .6 volts peak amplitude, @ 1kHz. You may have noticed I don't have schematics for the power supply yet,  we will work on those after we know what kind of circuit you plan to run. After we design the power supply I could add in their parameters like 120Hz ripple from rectifier, output imedance, etc...... to the simulation.

 

I ask you then, what are your requirements? We can start with simple goals like power out, bandwidth, distortion, and damping factor. So you could say something like; "I am looking to build a push pull  amp that has at least 5 watts of power, I want full power down to 34Hz for my Cornwalls with distortion <5%, and a damping factor of at least 2. Push pull designs cancel the second harmonic that is created in that stage. In the second schematic I presented the input SRPP stage has low distortion when optimised so looking at the FFT chart I plotted you can see that the second harmonic is low, this circuit has much less distortion in open loop than the first I presented. The reason the first schematic has less distortion is because I had more open loop gain to add more feedback which cancels distortion. The first stage in the first schematic is a common cathode gain stage and is rich in second harmonic, if this stage was more linear the FFT would look like the second amp with mainly a dominant third harmonic. What I am getting at is you can taylor your distortion to how you like, you said you wanted a push pull amp and in my experience push pull amps tend to have more third harmonic. Why? Well most of the distortion will be created in amps is in the output stage and like I said earlier the second harmonic distortion in a push pull amp gets canceled so the third dominates. The first schematic I presented is a push pull amp but with a distortion spetrum similar to single ended. I didn't want to present two schematics that had the same distortion spectrum. You can even name amps you have heard before and really liked and we can not copy the schematic, but copy it's characteristics and using your parts come up with a similar sound!! Our own Carver challenge, sort of.    Pick your flavor

 

 

I hope others join in on the discussion. I know Maynard has been following.

[thebes]

Maynard, you've got me intrigued for sure.  I could shoehorn the 15 watt OPT's into that space behind the existing ones at right angles to the power transformer.  They'd be pretty close to the existing transformers, although they should also be at right angles to the originals.  With enough separation to prevent cross talk? I'll leave that to you.

 

Hey JP, I kinda suspected it V1 and V@! were part of the modelling software, but , as you know, most schematics show (usually a,b,c etc) points where the power supply meets the amplifier circuit, so I wasn't sure if I was looking at that.

 

Oh, I do like the sound of my Marantz 8b, but I suspect doing  this project will cost me roe than the $50 I paid for mine Sorry but I never get tired of working that little goodie into any audio conversation.

Edited December 8, 2015 by thebes

[thebes]

Well just a bump up to say that I haven't forgotten this build project.  Everything's been delayed a bit  though.  I had a guest for six weeks who was recovery from surgery, plus the holidays.  Then I bought a set of decorator Khorns and I've had to steal the 6V6 rebuild money to freshen the caps, enclose the tops, seal the back etc. 

 

 

 

Fear not, though. Your intrepid shockmeister has a couple of pieces of gear up for sale and those proceeds will be used on this project.

[Noremac82]

I am new to the forum and I know this is an older thread but the OP was in the same situation a year ago as I am right now.  I acquired this same amp at a really good price with the intent of restoring it to original.  After reading a little about the sound of the original design and hearing about some of its shortcomings, I am not sure it is what I am looking for in my build.  I am not trying to rattle my windows or anything but i do what some bass in my music.  I am also afraid that one of the OPT is NG so i will be replacing them and i really like what i am reading about UL OPT's.  Since this will be my first full build I also like that it has a low parts count and I dont mind that the output drops to 5W.  Based on what I have read to this point, it seems to be a good fit for what I am wanting to do and I think my 60 bucks wont be wasted because I have a good platform to start building. I also think it will be a good opportunity to learn a few things and hopefully end up with a really good sounding amp in the end.  

 

I dont want to hijack the OP's thread so would it be OK to continue discussion here or would it be better if I start a new thread?  I am new here and dont want to start off by stepping on peoples toes so i dont mind to continue this discussion in a new thread.  If its OK to revive this post, and the gentlemen are still willing to help, i can follow up this post with a couple questions along with some additional information about my goals for this build.

 

Take care everyone and thanks for putting this information out there for everyone to learn from.

[tube fanatic]

Many threads are resurrected when related questions arise, so I'm sure that Marty won't mind at all!  Most importantly, are you experienced in working with high voltages in a safe manner?  Under the chassis you will have possible exposures during testing which could be lethal.  This needs to be taken very seriously.  That being a matter aside, I see in your profile that you are using Elac speakers.  I recall reading a review on the B6 some time ago in which it was found to be a fairly easy load for a tube amp, although its low efficiency will limit how loudly it can be played.  You mentioned one opt not being good.  How did you draw that conclusion?  To me, since you're considering replacing the opts and rebuilding the amp, and also want to have some bass potency, it may make more sense to build a nice 4-5 wpc SEP.  If that appeals to you I can post a design which is likely to work quite well.  As to ultralinear, some say that it combines the best qualities of triode and pentode operation, and others the say it's the worst qualities.  Taking an amp designed for pentode operation and pulling the screen supply from the UL tap doesn't necessarily work out since juggling the circuit parameters is often necessary.  So, fire away with other questions.  Unfortunately JP, who posted extensively in this thread last year, is no longer involved with the forum.  He was the resident push-pull guy!  

 

Maynard    

[Noremac82]

Maynard, thanks for the response.  I have done some basic work (tube changes, transformer replacement, cap replacement) for friends in the past but never done a full build of an amp.  I have wired homes with 110/220 and had some training through work that was meant to prepare me for high voltage DC work involving LiPo systems for vehicles.  As we all know the whole electric vehicle thing didnt really stick like expected so i never got moved to that program and in turn never really used what i learned in the training.  So long story short,  no i have never specifically worked with the higher AC voltages contained in an amplifier but I do however feel like i have a grasp of what i will be working with and i am aware of the safe working practices needed to do this without melting my fillings.  Plus i am not afraid to ask questions so if i come across something i am not familiar with or dont feel comfortable with, i wont hesitate for a second to ask someone with more knowledge than me for help or guidance.  I also have to equipment, multimeters, oscilliscopes, function generators, etc. that should help me safely troubleshoot any issues that may come up.  Overall my confidence level on this is about a 6 or 6.5/10.  I think that will go up with some additional knowledge and experience but since i am completely new to this i am replacing some confidence with caution for the time being.  

 

I am glad to hear that those speakers should pair well with a tube amp.  I do not need a whole lot of volume since my listening room/man cave isnt exactly a sprawling room of luxury.  Its pretty much a tiny spare bedroom with a television, stereo and a chair.  Might upgrade to a mini fridge in the near future but for now i am making due walking back and forth to the kitchen.  

 

I suspect the OPT is NG due to its cosmetic appearance and comparing the Resistance measurement of the output of one to the other.  Since i wrote that i did a little more research and am thinking the difference i am seeing may be due to the difference in feedback resistor value of CH1 vs CH2 since i measured at the connector instead of the output of the transformer.  So i am back to where i started and I dont really know if the one is OK or NG.  It does look pretty rough though.  Actually they both have seen better days but one is in a lot worse shape than the other.  The gentleman i got it from said it was working when it was removed from the cabinet but he didnt specify if both channels were working or just one.  Even if this one is good i would like to replace them at some point with something that gives me a little better frequency response and is cosmetically a little more appealing.  If they are both good then i can make due with them for now and put some of that money towards improving other aspects of the amp.  

 

As to SE vs PP, i am not partial to one vs the other.  My main concern is getting the best sound i can get and have enough power left over to upgrade my speakers down the road.  I do not know enough about the differences between the two but i would like to keep the original look somewhat  intact.  I would also like to keep the parts count down for simplicity sake which is why that second schematic really appealed to me.  I am leaning more towards a PP amp just because i feel like i owe it to the amp to leave it in some resemblance of its original form.  Now if that is going to compromise SQ at all then its off the table.  If not and its going to be a matter of preference, i think i would like to do PP.  Nothing against SE because honestly i couldnt tell the difference between the two at this point in time.  i am basing that decision on trying to retain some of the original form of the amp.  If there is a schematic that allows me to use the current tube layout, 2 12AX7/46v6, or some variation thereof, i would not be opposed to using it so long as it is a good sounding amp.  I know this is very subjective but i think we can all agree that there is a baseline as to what is above average SQ or good SQ and that anything above that is mainly preference.  I would also like to have something that has the ability to be improved upon at a latter date if i choose to go that route.  For example, one thing i would like to do, and i dont even know if it is possible, but i would like to put an headphone out on this amp at some point.  I plan on getting a pair of HE-6's at some point in the future and they require quite a bit of power.  For now i will stick with my HD650's but i would love to be able to plug them into this amp for late night listening.  I have looked at a couple of things to try and match impedance while preserving frequency response but have not came up with anything definitive yet.  Maybe someone here has tried that in the past and could point me in the right direction.  

 

Sorry for such a long winded response but right now i am somewhat overwhelmed with all of the information i have available to me.  Once i begin to understand it all a little more i believe my confidence will improve.  Again, thank you for taking the time to respond and hopefully we can get this thing up and running soon!  I have the money to throw at it right now so i guess its as good of a time as any since i have been wanting to do this for a while.  I dont want to frivolously spend on it but i dont have a problem buying quality components if they will truly improve the listening experience.  Right now i have the chassis, a working set of tubes (not matched or even the same brand but they are all there), original transformers and choke (one OPT is suspect but i hope to upgrade them after i get it up and running), i have ordered fuse and new power cord along with RCA terminals and speaker terminals.  I have a lot of odds and ends components laying around but i will be placing a component order once i have finalized the decision on schematic.  I am going to research the difference between PP and SE today and will report back once i have reached a conclusion.  Gut feeling is i want to stick with the PP design but i want to thoroughly weigh my options before making the final decision.