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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: 07 Sep 2012, 22:21 
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Mark;

The 124B will give you a 3:1 voltage step-up. This is just about perfect for the 6SN7 driver loaded at 10kΩ.

With the 300B loaded at 750Ω your bias should be about 71 volts with the B+ stated. This means that the swing on the 6SN7 plate needs to be 71/3 or 23.7v. With the 6SN7 biased as stated, it has a bypassed gain of ≈11.9. So the input voltage needed to fully drive the 300B is 23.7/11.9 or 1.99v peak. This is good chain gain for this type of amp.

I will recommend one addition to the design however. It used to be that amps were designed with interstage transformers with "high" output impedance. They were wired directly into the grid of the next stage and it was just assumed that the reflected impedance seen by the driver was approximately correct. In this scenario the distortion in the driver can get excessive at some frequencies and the amp dynamics can suffer. The solution is to resistively match the interstage transformer so that the driver always sees the correct load impedance in normal operation. What all this means is that you should include a 90kΩ grid resistor on the 300B. There is no loss of power from this as the transformer is properly matched and the 300B in class A operation has a very high input impedance.

Other than those little tweaks I'd say go for it. If you've got the 300Bs and the interstage iron you're already half way there.

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PostPosted: 07 Sep 2012, 23:17 
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You have me excited. One amp on the kitchen table at the moment but should be done in the next for days. If not I'll have the wife on my back. 90K grid resistor is 90K from grid of the 300B to ground prior to the 3k3 grid stopper? Correct?

What about the 15KHz frequency limit of the inter-stage trannies? Do you think this will cause a "airless" amp?

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PostPosted: 09 Sep 2012, 11:26 
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mwhouston wrote:
90K grid resistor is 90K from grid of the 300B to ground prior to the 3k3 grid stopper? Correct?
Yes.
mwhouston wrote:
What about the 15KHz frequency limit of the inter-stage trannies? Do you think this will cause a "airless" amp?
Well, according to the Hammond website the 124B has this performance...
Hammond Website wrote:
Minimum frequency response 150 Hz. - 15 Khz (+/- 1db max. ref. 1 Khz.) at full rated power.
Now according to my calculations, the 6SN7 will be delivering approximately 100mW to the input of the transformer; 17dBw below the reference point of 5w. Assuming that the transformer has a single pole frequency rolloff, this would mean that the upper limit would move up almost a full decade. This will obviously be limited by leakage inductance and normal capitative coupling, but I would wager that the upper response point at this level is at least 20kHz and possibly higher. The simplest test would be to wire up a 6SN7 driving the terminated transformer and check the frequency range. In fact I think this would be an interesting experiment. The lower end performance might also surprise you.

Personally I doubt that frequency response of this transformer will be an issue. I seriously doubt that the transformer will be in hard cutoff anywhere near either 20Hz or 20kHz. If the end to end frequency response does suffer at all, it is a simple matter to put a little tone shaping in the response curve. It would make the amp slightly less sensitive but that's the normal tradeoff.

Keep us posted on your progress.

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PostPosted: 09 Sep 2012, 18:22 
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Thanks Matt. Definitely an amp for the bucket list and maybe a "different" amp to show off those 300B-Zs.

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PostPosted: 07 Dec 2012, 17:48 
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When I first built the Silver Dragon I used bipolar bypass caps across Rk on the 300B. While in the hands of its owner one cap shorted. I replaced the bipolars with normal electrolytics but on later listening never though the amp had its original excellent sound. In a recent EL34 build I also used bipolar bypass caps and the amp was astonishingly good.

I have put back some 68uf bipolar caps but being worried about another short have added a fuse from the OPT to the plate of the 300B. Thinking about it tubes with an RRP of $1200 for the TJ Full Music carbon plates SHOULD have protection. I don't expect the tubes to fail but a cap or resistor etc. may. The fuse is a fast blow 100mA. The amp currently draws about 70mA. The tubes can handle 100mA put if that is exceeded it will blow the fuse cutting HT to the tube.

I didn't want the fuse holders visible and don't expect to ever have to change a fuse. The fuse holders and soldered directly to the 300B socket. I am currently using the Dragon with my new Heavy Metal 6N7 tube preamp.
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Fuse.jpg


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PostPosted: 07 Dec 2012, 19:56 
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Looks good. I did mention here (http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2699&start=310) that I would normally use a pigtail fuse in this context, but this works as well. It's good protection for those big expensive tubes.

How does it sound with the "Heavy Metal" driving it?

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PostPosted: 08 Dec 2012, 00:29 
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Suncalc wrote:
Looks good. I did mention here (http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2699&start=310) that I would normally use a pigtail fuse in this context, but this works as well. It's good protection for those big expensive tubes.

How does it sound with the "Heavy Metal" driving it?

It may be a few days before I get the chance for a big session but when it happens I'll post back. At least now a have a tube preamp with lots of gain and with the Dragon I can wind up the volume. Before I only could use about 10% of the volume control because the chip amp had so much gain. Now I have much better control over vol level. On a few brief listening sessions it all sound natural and smooth.

I'm going to a Classical meet with 7 other to have a listen to double paralleled 300B in dual SE config. It is not a PP amp but uses two SE trannies and 4 X 300Bs per monoblock. The speakers are $26K 400lb Osborns lead lined with two 12" Focal woofers.

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PostPosted: 08 Dec 2012, 08:56 
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The 300B I listened too (again) is an Audio Space "power one" I think. 60W. It is a PP not SE.

I really didn't think much of it for $13K a pair. The Cyber 845s he had before I think were better for $5K a pair.

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PostPosted: 08 Dec 2012, 10:32 
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mwhouston wrote:
... 60W. It is a PP not SE.
I really didn't think much of it for $13K a pair.
I know that I'll probably step on a bunch of toes, but I don't see the point in running big triodes in PP AB1. You get a lot of power but it destroys the thing you use a triode for in the first place; nearly pure 2nd order harmonic distortion. The minute you go PP the 2nd order harmonics get mushy and when you bias for AB1 you get extended harmonics. In PP operation, I much prefer the pentodes and BPTs run in UL. For the big triodes, I'll aways trade power for sound and run them SE class A only.

I'll get off my soapbox now. :soapbox:

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PostPosted: 08 Dec 2012, 12:57 
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I agree with you Suncalc.
If power is the BIG issue go for something big in PP - KT88/90/100/120 ...

However I have all parts for a GU72-SE (25W SE pentode). :blush:

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