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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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 Post subject: Re: 6EM7 power amp
PostPosted: 06 Nov 2011, 20:03 
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Thank for your comments as usual Matt. I may change them.

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 Post subject: Re: 6EM7 power amp
PostPosted: 07 Nov 2011, 05:18 
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Matt: Just checking that the inter-stage cap from the driver triode to the power triode in the 6EM7 is 0.022uf. Its just one of those valuegs I do not have two of. Please confirm. What range can I work with here?

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 Post subject: Re: 6EM7 power amp
PostPosted: 07 Nov 2011, 06:23 
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Try to find some replacement caps for Paris. Any good suggestions: 0.022uf and 0.47uf??

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 Post subject: Re: 6EM7 power amp
PostPosted: 07 Nov 2011, 10:26 
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mwhouston wrote:
What range can I work with here?
Just don't go any smaller than 0.02uf. You should be able to safely go to 0.047uf or 0.068uf without a problem. I would however not go above 0.1uf or bias excursions my become a problem.

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 Post subject: Re: 6EM7 power amp
PostPosted: 07 Nov 2011, 15:14 
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Matt: What cap brands and types do you favour?

While playing the amp for about 10 hours the other day I notices a slight resonance or distortion on some notes. It ocured mainly on low notes of the female voice. The easiest change for me to make was to change the preamp tube. At the time I think it may have been a 12AT or Y7 in the amp (markings are gone). I replaced it with a NOS Toshiba 12AU7. I found the 12AU7 sounded one of the better tubes in both the 4S MKIII and 4S MKIV Universal. Sure enough play the same tracks again resonance has gone. The tube I was using, though it tests 80% emissions, I think has come from an old TV and is well used. It may just have been a bad tube rather than some Tube/preamp interaction.

I have a few Gold Lion 12AU7 and 12AX7s. I must try these.

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 Post subject: Re: 6EM7 power amp
PostPosted: 07 Nov 2011, 21:50 
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mwhouston wrote:
Matt: What cap brands and types do you favour?
Actually, I tend to be rather pedestrian in my choice of most components; sticking with what has generally worked for me in the past without issues. I tend to focus on technology for the given application and then go with a reputable quality manufacturer.

Case in point, for signal pass elements at audio frequencies (like interstage coupling caps) I like quality polypropylene film capacitors. These are generally very stable, have good tolerances, good aging characteristics, high voltage tolerance, are in many cases self healing, and exhibit graceful degradation at end of life. So this is what I'd use.

As for manufactures, I really don't want to start a fight. Everyone has their favorite and prices are all over the map. In general, the wilder the marketing claims, the less likely I am to buy. I know there are people out there paying big bucks for special paper and aluminum foil capacitors rolled on the thighs of Japanese virgins by the light of the full moon... but I'm not one of them. I also try to stay away from commodity manufactures in central and east Asia because I can't be guaranteed of their performance. I've had very good luck sticking with companies like IC or Sprague (Vishay). (Like I said... pedestrian). A tremendous number of the supposedly "Audio" capacitors are either "manufacturer of the month" with good marketing, or rebranded major brands at 1000% to 100000% markup. Beware of anyone that want's more than a few dollars for a signal capacitor. You're likely paying for marketing rather than quality.

Here's a good tip. Any quality reputable parts manufacturer will have a technical support line you can call and actually talk to an Engineer. A parts Engineer will talk your ear off concerning the minute details of part construction, materials, processes, etc. If you get a "that's proprietary" response every time you ask a question, that's a red flag that they don't actually make their own parts. Buyer Beware.

I know I haven't really answered your question, but it's about the best I can do. I do have to say that I am always amazed that the same people who will extol the virtues of a $100 capacitor will, in general, pull a random schematic off the internet and begin changing things without a thorough analysis. I have always believed that proper design and analysis comes first and (sensible and prudent) component selection second.

Well, now I've done it. Let the firefight begin! :eek:

mwhouston wrote:
While playing the amp for about 10 hours the other day I notices a slight resonance or distortion on some notes. It ocured mainly on low notes of the female voice. The easiest change for me to make was to change the preamp tube. At the time I think it may have been a 12AT or Y7 in the amp (markings are gone). I replaced it with a NOS Toshiba 12AU7. I found the 12AU7 sounded one of the better tubes in both the 4S MKIII and 4S MKIV Universal. Sure enough play the same tracks again resonance has gone.
I was think about your topology and it may be that you are seeing mild bias excursions that tended to resonate at a particular frequency.

The basic amp circuit first stage gain is about 17.6 and can handle an input voltage of approximately 3.5 volts peak before going into grid conduction. The power stage is however biased at -34v so it will go into grid conduction any time the input voltage to the first stage goes beyond about 1.9v peak.

With a 12AU7 in the "universal" topology, the gain is about 20dBv (unbypassed). With the volume control at half volume it's abut a -20dBv loss so the result is unity gain. With a 12AT7 for example, the gain is about 26dBv so with the same volume setting you have a gain of ~6dBv (2.0 v/v). So dependent on your input voltage and dynamic range, you could be pushing the power stage into grid conduction and bias excursions. Of course, all of this assumes that the power stage is running at full power (~2.1W).

If you were not at near full power, then it could have also been due to the preamp stage going into grid conduction. With the 12AT7 this would occur at about 2.7v peak input, but the 12AU7 would require about 3.2v peak input. This may not seem like much of a difference, but if you were operating on the edge (with just some dynamic peaks popping over the limit), you could see it.

Or it could also just be that the 12AU7 has only about 30% of the distortion of the 12AT7 in the universal design.

Or it could be that, as you say, the 12AU7 was just a better tube.

This is what makes tube rolling so addictive. :up:

B.T.W. watch the gain on that 12AX7. With a gain of 29dBv (~28v/v) it will be really easy to overdrive the 6EM7 input stage. (~0.07v peak minus the volume setting. e.g. 0.7v peak with the volume control at 1/2 volume.)

In any event, it's good to hear that you're enjoying the amp. I'm very glad that the design worked out so well. Have fun.

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 Post subject: Re: 6EM7 power amp
PostPosted: 07 Nov 2011, 22:38 
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Matt: I must have pre-empted your "polite" comment about signal caps and ordered these early this morning: Sprague Series 716 Polypropylene; "Orange Drops. Ultra high quality, copper leads, +/-5% tolerance, 85C operation. Ideal for Hi-End Audio & Guitar Amps". To quote an Australian supplier. I did this many hours before reading your recent post. They range between $3 and $6 each. I have paid $200 for "well marketed" caps in the past. I may not any more. I should have the caps in a day or two.

I have used these caps a number of times on different projects and always thought they were OK. As you can see they are very moderately priced and of course come from a reputable manufacturer. I have devised a system which will allow me to keep the originally installed X2s and have the Orange drops in also. This will allow the user to switch between two sets of signal caps. There is just somehting about the current sound that has me captivated. I'm not sure if is good or bad but it is just nice. I don't get a strong focus in the center though and speakers are in-phase. A switch will switch one set of caps out and the other in. I have wanted to do this for many years. How many amps do you know where you can taylor the sound by switching in and out cap sets???

As you described the "resonance" I was getting sounds about spot-on. It was on louder notes and at a certain frequency. This is not happening with the 12AU7. So I think that explains what was happening. I will try the 12AX7 next.

And finally "yes". I am enjoying the map immensely. It glows well too. Lots of hot tubes.

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 Post subject: Re: 6EM7 power amp
PostPosted: 08 Nov 2011, 17:08 
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Is anyone aware of a way of testing signal caps. This would need to be done without expensive test gear but go beyond "that sounds good". And if we could test signal caps what would we be looking for beyond brand name and market hype.

If there is no scientfic procedure which alludes to how a cap should sound and we have to resort to subjective analysis, again what key points should we lookout for?

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 Post subject: Re: 6EM7 power amp
PostPosted: 08 Nov 2011, 19:53 
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Well I wish I could read an analogue meter. Where I thought the PS was delivering about 260V make that 340V!! Here are some voltage readings around the citcuit. Currently the 6EM7 is dissipating about 8.9W. Funny the bias figures are very close to Matt’s calcs:

HT prior to 820ohm limiting R= 340V
HT after limiting R which feeds the OPT = 300V
HT after the 2.2k limiting R which feeds the small triode in the 6EM7 and the preamp section = 280V
6EM7 small triode plate to cathode V=190V
6EM7 large triode plate to cathode V=245V
6EM7 small triode voltage drop across cathode R = 2V (Matt’s calcs = 1.9V)
6EM7 large triode voltage drop across cathode R = 35.5V (Matt’s calcs = 34V)
6EM7 voltage drop across small triode plate R = 85V
6EM7 voltage drop across large triode (tranni) = 5.9V

Now lets look at the 4S Universal stage:
12AU7 plate to cathode voltage = 70V
12AU7 drop across cathode R (1.2K) = 2.4V
12AU7 drop across plate R (100K) = 195V

So do I need to get the HT down to 260/250V?? Or is it OK here. Next some frequency and power testing.

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 Post subject: Re: 6EM7 power amp
PostPosted: 08 Nov 2011, 20:29 
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Mark;

I'm a little concerned by your numbers. I'm having trouble making the numbers work out. With the section 2 triode biased at 35.5 volts across an Rk of 750Ω means that the current is 47.3mA. With a plate to cathode voltage of 245v this means that the triode is dissipating 11.6W (opposed to 8.9). This is really too high for a 10W triode. My design was shooting for about 215v and 45.5mA for a total dissipation of 9.8W. 47.3 mA is also creeping up close to the 50mA average cathode current limit for that triode. Are you sure that the 245V you quote is not plate to ground voltage (i.e. including the bias drop across Rk)?

Here is my load line for the power section.
Attachment:
Sec 2 Load Line.jpg

Bottom line is that although the section 1 triode is still well within design limits, I think you're burning up life on the power stage. It's really a testament to the ruggedness of these little tubes that you can run at 16% over rated dissipation for as long as you have without any problems.

I think you need to tame that power supply a little. ;)


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