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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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PostPosted: 29 Aug 2019, 05:01 
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You can always use any op amp other than INA828 as a diffrential op amp, just that standard ones don't have DC level shifting option


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PostPosted: 18 Feb 2020, 17:09 
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poty wrote:
There is also a simple mod that almost eliminating the need of the 25Ohm pot (use its own CCS for each tube and connect the power tube cathodes together by a small resistor of 10-15Ohm value). The mod also helps with heat problems and allows the usage of lower current devices.



Regarding the use of separate CCS for each tube, will the attached schematics work that way? Is there something to modify?


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PostPosted: 18 Feb 2020, 20:45 
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It will not work


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PostPosted: 19 Feb 2020, 01:33 
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Yes, all is OK on the schematic. In practice the 10R resistor may be bigger in value, but it works even that way.

ILoveHiFi wrote:
It will not work
Why do you think so?


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PostPosted: 19 Feb 2020, 12:33 
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poty wrote:
ILoveHiFi wrote:
It will not work

Why do you think so?
Because by separating the current sources, you are forcing the same amount of current through both halves of the transformer primary even at AC conditions. It is the imbalance in drive between tubes, while maintaining the same TOTAL DC current that allows this topology to function. Once you split the current sinks between the two halves of the primary, there is no AC coupling and the amplifier will not function.

You need to solve for both DC and AC conditions when attempting to modify the topology.

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PostPosted: 20 Feb 2020, 00:41 
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While analysing circuits you should take into account not only an idea jist, but implementation too. There is a common element (namely 10R resistor) which made your assumption wrong.


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PostPosted: 10 Apr 2020, 08:36 
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Hey Guys
Im having trouble getting the tubes to stay balanced. They hover up to 1mA. When I first built them they stayed puy. I have been rolling tubes such as nos phillips and mullard el34 but they wouldnt settle. I put back the kt88's (jj blue) and they even seem to go up and down about 1ma . this is a new phenomenon so im not sure what to look for here
Any advice would be appreciated
Regards and stay safe
NIck


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PostPosted: 12 Apr 2020, 21:54 
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Hi, The tubes will often "wander" a milli amp or two as they heat up. However it doesn't matter until the go more than about 10ma off. At that level there is a slight loss of power 1-2 watts. You will probably not notice it. If they get really off say 25% or so then the tube consuming the most current will get close to the maximum dissipation rating. Remember this is a class A design and the tubes conduct maximum power at idle. The design is such that there is a fudge factor designed in that allows for up to 15-20% out of balance condition before reaching max dissipation. The amp will still sound fine even then though. I find that over a period of six months mine drift only a milli ampere or two. Some brands are worse than others. In the early days of the design (about 8-10 years ago) the then current crop of EH KT88s would not balance at all. The issue was likely a thermal one as they changed how they functioned as they got hotter. Unfortunately the hotter one got the more current it consumed and it would tend to run away. It is not necessary to use matched pairs of tubes, but they should be fairly close. If it won't balance at all then they are too far different. BTW If you use Tungsol KT120s in the amps set for the lower power (about 70ma / tube) the sound is exceptional. The loss of power is not as great as it might seem and full power is about 20WRMS. Unless your speakers are really inefficient you will not notice the difference (preamp volume setting will be a little higher though). That is how I use mine now. The "big" bottles sound great.

Good listening
Bruce

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PostPosted: 13 Apr 2020, 08:32 
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HI Bruce
I was puzzled by this for days and noted once the lm317's got over 35 degrees celcius then it would happen. I have a square section fan cooled heat sink (50mm x 50) and it has internal fins.
Im thinking i will use 2 lm317's in parallel.. that way i can 1/2 the dissipation rate . I have mistakenly placed the single 317 at one end of the heat sink and i think i will reposition them to be more centralised. I have sent photos for your reference.
Currently (excuse the pun) the LM317 's are running at 45 degrees on the black plastic part of the 317 and 52 degrees c at the metal backing that you mount it on.
They sound nice and punchy Bruce, I commend your design and that you freely shared this.I will take your advice and get some kt120's. I did try a quad of shuguan kt66's i had, one wouldnt balance but the other pair did and I liked its sound.
Take care bruce and I hope we are all still here after this covid thing settles


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PostPosted: 13 Apr 2020, 10:04 
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Hi, Heating doesn't really affect the 317s from doing the CCS duty. 45C is not all that hot and they are good for a lot more. Exactly how much depends on the heat sinks. You can parallel two if you want and they behave fine in pairs if the ICs are from the same lot. Matching is not 100% but pretty close. I use them in pairs in the higher power KT120 amps with as much as 300 ma running through a pair. The 317s can easily handle 20 watts dissipation and in the KT88 versions of the amps are only handling around 7 watts. Well within ratings on a heat sink. I would have to check it, but I believe they are rated at over 100C max.

On some tubes that won't balance...try swapping the positions. It seems some don't like the signal being derived from the cathode of the other. Swapping seems to effectively make about a 1 volt difference in the requirements of the tubes to balance. The 25 ohm pot only has a range of 4.5 volts swing from one side to the other. (don't use larger values as it will alter the sound) I figured if the tubes were less equal than that then they would be non-linear in the audio range (was true IAW my distortion analyzer) as well and unsuitable for use.

BTW. In virtually all the tube amps I have encountered the current flow drifts a bit over time. It doesn't seem to matter what type of bias they use. Ones with sophisticated bias adjustment circuitry are better...but at much more complexity and cost. I didn't figure it was suitable for a diy design and it really didn't add any improvement in the sound. IMO (YMMV) it is a marketing gimmick.

Good listening
Bruce

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