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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: 01 Oct 2010, 16:53 
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Joined: 24 Feb 2009, 03:24
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One schematic 2A3 /it is suitable for 300B/ without interstage cap and with fixed bias /without cathode cap!!
From Russian forum:
http://audioportal.su/attachment.php?at ... 1158128485
http://audioportal.su/attachment.php?at ... 1158128500
/I thing - I'll check - I have used some tubes 6G2....I can give free/.

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Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/


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PostPosted: 02 Oct 2010, 00:14 
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m.saunby wrote:
What's wrong with a high quality wire-wound pot for setting cathode bias? Then you can have whatever value is best for the tube.

As long as it's well bypassed to make the wirewound inductance moot, nothing is wrong with that.

If controls like that were good enough for the military of the day, it's certainly good enough for HiFi.

Cheers!

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PostPosted: 02 Oct 2010, 03:14 
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Hi everybody,

a couple words on opts. I had the privilege to compare Edcor and Hammond transformers with identical specs in the same amp. I would love to use Hammond transformers, they are cheap and easy to get for me, but they do not sound the same, not at all, at least not in the OddBlock amp, so I cannot put a Hammond transformer on a OddBlock for a customer by keeping a clear conscience. Now I don't dare to generalize that, but this is what my own experience taught me. Would be very interesting to compare those opts in different amplifiers, this way I could form a final opinion, which I do not have at this point, just that I am very careful using Hammond opts now.

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- 832 / GU32 tube push-pull amplifier project


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PostPosted: 02 Oct 2010, 03:52 
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Hi Ben,

Regarding your PP OddBlack, here is a page that may interest you and explain things going on with Hammond vs. Edcor in that configuration:
http://www.angelfire.com/electronic/fun ... ck_UL.html

Cheers!

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* Ratings are for transistors - tubes have guidelines*
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PostPosted: 02 Oct 2010, 11:56 
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Joined: 05 Jan 2009, 05:18
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Geek wrote:
m.saunby wrote:
What's wrong with a high quality wire-wound pot for setting cathode bias? Then you can have whatever value is best for the tube.

As long as it's well bypassed to make the wirewound inductance moot, nothing is wrong with that.

If controls like that were good enough for the military of the day, it's certainly good enough for HiFi.

What do you calculate would be required to effectively bypass the inductance? My estimate would be, at audio frequencies, zero/zilch/nothing. Am I wrong? If so why?


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PostPosted: 02 Oct 2010, 16:52 
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Hi,
m.saunby wrote:
What do you calculate would be required to effectively bypass the inductance? My estimate would be, at audio frequencies, zero/zilch/nothing. Am I wrong? If so why?

Oh, nothing very scientific... depends on the control, but 47nF is enough for any wirewound I come across.

Basically you want to cure any RF that gets caught up.

Cheers!

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PostPosted: 03 Oct 2010, 18:31 
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Joined: 06 Apr 2009, 10:08
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Location: US Pacific Northwest
In answer to the question of why not use a wire wound pot and manually set the bias, I'll touch on a little point of design philosophy. When designing for myself, I would be very tempted to do just as you describe. I tend to do a of of things like putting in manual adjustments and hardwiring monitoring points when I build an amp for myself. However, when building an amp for a client it is important to remember that human nature says if there's an adjustment available, someone will adjust it. Now if you have painstakingly tuned the amp before you send it to the client, the last thing you want to have happen is for the client or someone else to start adjusting things. In general they are not going to make performance any better and they could actually make it much worse, or even damage the amp or tubes in the process. In any event, there is a very real probability that the net result will be a very unhappy client. :censored: I'm not saying that someone who has an amp built for them doesn't know what their doing. In fact in some case they are very knowledgable. It's just that they are paying you to do the design. So my philosophy is just do the engineering up front so that adjustments aren't really needed. :D

Now, on to other matters. It's time to talk about the driver stage. Now according to the requirements, we only get one driver stage (i.e. it's to be a two stage amp), we do get to specify the use of a preamp, and the driver is based on a 6SN7. Now actually these three requirements work well together. The being limited by the lower gain of the 6SN7 is not a problem in our two stage amp because we can specify a preamp. Now just how much of a preamp we need we'll see a little farther on.

I am actually rather surprised that no one mentioned the two things I neglected to address in the design of the power stage. One is the absence of the grid circuit design and the second is no mention of the value of the cathode bypass capacitor. The reasons for these two omissions are that the grid circuit is integrally tied to the performance of the driver and as such is better addressed during the driver stage design and the bypass capacitor values should all be addressed at once after both stages are designed and we are setting the overall frequency response of the amplifier.

Lets begin by addressing the limitations placed on the design by the 6SN7. The first limit is B+. The 6SN7 has a max B+ of 300v. As the power stage will require a power supply in the ~450v range we are going to need to include a dropping resistor in the power supply to get the lower voltage required. The next limit is the gain. The limited µ of the tube is going to limit effective amplification factors to around 15. Since we need 71v peak to get max power out of the driver stage, this means we need approximately 5 volts peak input. This puts the bias point for the 6SN7 up around 6 volts (allowing for headroom).

Now we need to address the grid circuit of the power stage. This is important because the DC operating point and DC load line for the driver stage may be set by the plate resistor, but the AC load is set by both the plate resistor and the grid circuit of the following stage. Now the max grid circuit resistance for the 300B in a self biased operation is 250kΩ. This resistance will show up in parallel to the driver plate resistor in AC operation of the driver stage. This means that the driver stage AC load line will be rotated clockwise from the DC load line. We need to insure that the effect will not prevent the driver stage from reaching the required voltage swing necessary to drive the power stage. Personally I like to limit the driver stage plate resistor to not more then 20% of the following stage grid circuit resistance. This suggests a driver stage late resistance of 50kΩ and an AC load on the driver stage of 41.66kΩ. The following figure shows the driver stage operational design using the aforementioned limits and assumptions.

Attachment:
Driver Stage Design 1.jpg


The blue lines represent the DC design parameters of the driver stage, and the red lines show the AC operation. As I indicated earlier, by limiting the plate resistor to no more then 20% of the following stage grid circuit resistance, the AC load line is close enough to the DC load line that things generally work out ok. One of the problems with using high µ tubes (like the 6SL7 or 12AX7) to drive the power stage is that those tubes require much higher plate resistances and as such, the rotation of the AC load line would put the driver tube into severe compression on the negative going input voltage swing. This causes excess harmonic distortion and would destroy the sound of the amplifier.

This design actually works out fairly well with almost no tweaking required. Rk is 2.07kΩ which is close enough to 2kΩ as to not be a problem, the driver current is only 2.9mA (not too bad), and the amplifications and distortions look acceptable. The required input voltage to fully drive the power stage is 3.47v-rms or 4.91v peak. This is a very modest output voltage for most preamps so the client shouldn't have any trouble driving this power amp. The design has about 1.5v of headroom before driver stage grid conduction sets in which means that the limiting factor on amplifier power output will be the power stage. Now I calculated the cathode impedance at this operating point and assumed a 25µf capacitor for a 5.2Hz half power point. We will revisit this decision when we set the overall frequency response of the amplifier.

Using the driver stage 1.43% distortion at full scale and the power stage distortion of 3.5% (since we decided to go with the Edcor output transformers) means that the total distortion for the amp is 3.8% with a Ds value of 0.47%/W. This amp should sound excellent! :up:

Next time we'll talk about bringing it all together and setting the overall frequency response of the amp.


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PostPosted: 04 Oct 2010, 00:02 
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Hi Matt,

I am kind of reluctant to post here, because you know so much more than I do, so if I write something not entirely true, it is because I simply don't know any better, but what I say comes with the best intentions. :)

Did you ever consider of trying a SRPP pre stage? I do that pretty standard, because of it's low distortion and the relative high voltage swing output, about a third of supplied B+. I could always drive any tube into clipping with my (well Bruce's) SRPP pre stages. Even the PL509, and this is a difficult to drive tube. Each time I find a schematic I like, I am usually only interested in the output stage and I use SRPP as pre stage, until now, always with good success. I construct my pre stages always almost complete onto the 9 pin tube socket, very short component leads and no chance to collect noise or hum from anywhere. I have a lot of time but only little money, so I learned to stick to things I know they work, this works for me.

Kind regards from Holland
Benny

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My DIY Audio Projects:
- Single-Ended 12B4A tube amp with ECC802S driver
- DIY Push-Pull KT88 tube amp (OddWatt amp from scratch)
- 832 / GU32 tube push-pull amplifier project


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PostPosted: 04 Oct 2010, 07:44 
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Joined: 18 Jun 2010, 12:46
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Location: Northern VA
Looking at your loadline, your negative swing is in the knees quite a bit. Looks to me that if you upped the current to 8 ma you'd be in much more linear teritory. CCS loading the tube would get you a Mu of about 20, this would require less drive from the preamp, and allow for a lower cathode voltage and more current to pass through the tube. Also your AC load line would be much flatter using a CCS and your PSRR would be excellent.
Have you considered using a couple series LED that give you the desired cathode voltage while eliminating the bypass cap?

I've run my NOS and new production 6SN7 at 400 volts (plate to cathode) with no problems, and all the datasheets I've seen shows a Max DC voltage of 450V. (not surprising given the vertical deflection duties it was designed for) Where did you find 300V?


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PostPosted: 04 Oct 2010, 09:10 
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As for plate voltage, the original 6SN7 specifications defined 300v as the maximum DC plate voltage and 1200v peak in the vertical deflection amplifier role. These were the specs for the 6SN7GT and can be found in these data sheets. When the 6SN7GTB was specified, the max DC plate voltage was increased to 450v. Since I cannot be assured to which set of specifications a particular tube was built, I am being conservative and going with the 6SN7GT specifications.

As for your other comments, you're correct that I might get a little more amplification by using a CCS but it would still not be enough to alleviate the need for a preamp and the distortion would increase (especially in the higher order harmonics where the effect on sound quality can be large). When I get some time, I will post over on the SET thread a discussion of distortion order and its effects on sound quality.

If I had problems with the initial design of this driver stage I might have contemplated some of the approaches you describe (especially given that the 6SN7 is a requirement). However, as the simplest design also results in only 1.43% distortion at full swing, I am quite happy with these results.

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