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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: 16 Dec 2011, 08:39 
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I was senior network manager for a large international accounting firm for 20 years up to a few years back and I know what it is like to earn a large income with benefits. But I would like to think on this forum we all add a bit and take a bit and it all balances out in the end. As you have suggested Matt, it is nice to have some of your designs built.

Thanks for your great efforts for the last design.

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PostPosted: 16 Dec 2011, 14:46 
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mwhouston wrote:
I would like to think on this forum we all add a bit and take a bit and it all balances out in the end.
Very true Mark. After all, I wouldn't do what I do here if I didn't enjoy it.

But I do have to poke fun from time to time. ;)

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PostPosted: 21 Dec 2011, 17:38 
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Just received another matched pair of TJ Full Music 300BSE carbon plate limited edition tubes from Mable Audio. Again I got them for around the $650.00 mark. I don't intend to build another amp with these tubes but more as a backup to the pair I am currently using. I have a pair of Shaguang 300B-Z s for another 300B build for which Matt has provided an updated schematic. There is a few projects ahead of this though.

A member of the audio club (for whom I built the tube phono preamp) came over to pickup the preamp and took a short listen to my system. His amp uses 6550 tubes. He claims the Silver Dragon sounded very good and had the signature sound of a 300B of which he has heard a few. Musical flow, great sound stage and excellent mid-range he claims are the signature markings of a good 300B. The tube phono preamp I built for him sound extremely good in my system. Should he decide to take the one I have built him I will have to replace it. Yes another build.

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PostPosted: 27 Jan 2012, 02:05 
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Matt's original calcs. suggested the Silver Dragon my produce about 7.5W. On test this was closer to 5W. Matt though the bias points may be wrong. I have just checked some of the resistors which should effect the gain or output of a amp. I did this with out removing the base but work outside of the amp. All appear OK except one. The input resistor (grid to ground on the 6SN7) should have been 200K. But when I measured from the RCA to ground I get 20K, an easy mistake to make. This may explain why I'm not getting a lot of gain from my single stage tube preamp when used with the Dragon. They are not design to work into low impedance loads. Tomorrow this resistor will be replaced by the correct one and when I get my tube preamp back from loan I will see if the gain increases.

I'm not sure that the grid to ground resistor on the 6SN7 would effect overall gain and power of the amp. Would some one like to shed some light on this?

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PostPosted: 27 Jan 2012, 19:25 
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The 20kΩ for 200kΩ substitution for the 6SN7 grid resistor will have two effects.

1. The lower value will have a serious effect on the voltage transfer from a preamp to the input stage. Midband gain of the coupling is Rg/(Ro+Rg) where Rg is the input stage grid resistor and Ro is the output impedance of the preamp. For example, if you were using a 4S universal preamp at an 80% setting (Ro is approximately max at this setting; see this post) the Ro is in the neighborhood of 65kΩ to 70kΩ (see this post) and the coupling gain factor is approximately 0.23 or -12.8dbV. This is a significant loss.

2. The input impedance of the amplifier (which consists largely of the 20kΩ resistor) appears directly across the preamp load in the midband. This has the effect of rotating the preamp load line clockwise about the bias point by some amount. When the ratio of preamp stage load resistance (typically Rl) to Rg is high, the actual gain of the preamp can be significantly affected (i.e. reduced).

These two effects together can significantly affect the overall chain gain. But the effect will only be apparent where the input grid resistor is small (i.e. in that channel). Keep us informed.

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PostPosted: 27 Jan 2012, 21:26 
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Matt thanks for info. I knew the load on the tube preamps would be to heavy and this would effect the overall performance of the system. I'm going to change the resistors today. I will let know if I can pickup any audible changes. This change may allow me to go back to the tube preamps which in the past I have found did not have sufficient gain. Now maybe we know why.

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PostPosted: 28 Jan 2012, 07:13 
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I attempted to replace the 20K resistor on the grid of the 6SN7 with a 200K resistor. I rushed down to my local electronics store and bought 8 X 220K resistors for 46 cents. Yep I meant to buy 200K and bought 220K and soldered them in. It has to be better than 20K. I need Matt now to tell me 200/220K it doesn't matter. Does it?

Yet to have a listen. I have the bonnet up on the amp and are going to check some voltage and current levels around the cct.

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PostPosted: 28 Jan 2012, 11:48 
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200kΩ or 220kΩ; it's all good. Shouldn't really make a lick of difference.

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PostPosted: 28 Jan 2012, 21:41 
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The Dragon under test only produced about 5W. Matt thought 7.5W should closer to the mark. I fixed the input resistor and took some voltage readings around the amp:

6SN7 plate voltage =150V, Cathode voltage = 5.3V (both respect to GND)
300B plate voltage = 440V, filament voltage = 52V (respect to GND)
HT = 440V and HT to 6SN7 = 280V
300B current draw (52/750) = 69mA
6SN7 current draw = 2.5mA

These values are all down on what they should be and this may explain the low power reading. Solution = higher HT!!!
By reducing the HT limiting resistor to 4.7ohm at 10W see new readings:

6SN7 plate voltage =164V, Cathode voltage = 5.8V (both respect to GND)
300B plate voltage = 455V, filament voltage = 56V (respect to GND)
HT = 470V and HT to 6SN7 = 294V
300B current draw (56/750) = 74mA
6SN7 current draw = 2.8mA
6SN7 plate to cathode voltage = 156V
300B plate to filament voltage = 401V
Total current draw = 157mA.

Raising the HT has brought the amp closer to Matt's calcs (in some places very close). I would say the amp is well within specs. But (Matt) how to get more current through the 300Bs? Is there another 20mA in there? I'm about to measure power of the amp.

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PostPosted: 28 Jan 2012, 23:10 
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I ran some power checks and frequency range measurements:

Into 5.6ohms (my speakers are 6ohm): 5.8W RMS
frequency range: 10hz < -3db > 58kHz (excellent) @ 1W

Into 8.2omhs: 3.4W RMS
frequency range: 10hz < -3db > 50kHz (good) @ 1W

When looking at the FFT results distortion @ 1W into 5.6ohms is very, very low. The worst amp I every sore for an FFT result was my 6T9 amp. The Dragon barely had a bump at the 2nd and 3rd harmonic. The results were nearly dead flat. Of course a little more at the 2nd than the 3rd but that's tubes for you.

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Projects: "Carbon” - MM phono preamp | 180W RAW Class D amp | "Illusion" - 6SN7 SRPP preamp in a plastic jiffy box | “Salt Cellar” - Mono 807 triode integrated amp |


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