My G*S*G progress

2nd harmonics for the masses

Postby Ty_Bower » Tue Jun 23, 2009 6:53 am

Thanks for the compliments. So far, it sounds really good. I hooked it up to a pair of Klipsch Chorus II and the two seem to play very nicely together. The bass is rich, warm, and deep. The highs are clean, airy, and magical. With no input signal, I get only the faintest of 120 Hz hum from the woofer if I put my head right into it. The tweeter and squawker are dead silent - not a trace of hiss or noise anywhere. It is a remarkably clean and quiet amplifier. I didn't even have to pull any tricks with the grounding. The ground wire from the IEC goes straight to the extra ground lug on the board's terminal strip.

I took a few preliminary measurements from the amp before I installed the tubes. The Edcor power transformer makes 695 VAC with no load. The DC voltages at the filament pins of the power tube sockets were exactly at 8.00 volts. I'd like to take a few more measurements to verify operating values, but I was too busy listening to it. Can I operate the 6B4G tubes on their sides? Usually I'd flip the amp on its edge to check the voltages while it's running, but I don't know if that would be a good idea considering the way their filaments are constructed.

I'll have to admit I experienced a little bit of disappointment with the amplifier's visual appeal. There's nearly no visible glow coming from the filaments of the 6B4G. You have to peek down through the slot in the top of the mica to see the inside, where you can make out the dull orange glow of the W shaped cathode. I suppose the minimalist approach to my chassis building doesn't help any in the looks department either. One of these days, I swear I'm going to get off my duff and finish the chassis plates of my amplifiers. I've got some nice hardwood I should use to build the box. Oh well.

Tom's advice about Front Panel Express is good. I'd give them a try if I weren't such a cheapskate. I usually manage to find scrap pieces of aluminum plate for nothing, and I don't mind drilling a few holes in them. Another big plus for FPE is that you can get your plates with a anodize finish on them.

Here's another photo with a little better lighting, and another of my attempt to catch the 6B4G:

Image Image
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby TomMcNally » Tue Jun 23, 2009 11:33 am

I suppose the minimalist approach to my chassis building doesn't help any in the looks department either.


Minimalist ? Nah ... you used fine Phillips deck screws to hold
it together, not 10 penny nails.

;)
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Postby Ty_Bower » Tue Jun 23, 2009 8:18 pm

I took some more measurements tonight. I'm particularly interested to see how well the Edcor power transformer works on this design, as I suspect most people have used the PA774. First I checked the DC filament voltage on the 6B4G output tubes. One side was sitting at 6.1 VDC, the other at 6.0 VDC. Good enough, I suppose.

Next I checked the B+ voltage. I only saw ~100 VDC. What the? I checked the high voltage off the PT secondary, and got roughly 345 VAC per side. Checked the B+ again, and still saw ~100 VDC. Tested the cathode voltages and saw about 10.5/16.5 VDC at each end of the filament. Went back to check the B+ a third time, and also tested around the choke leads. Still ~100 VDC.

I went into my big box 'o tubes, and pulled out another 6AX5GT. I stuck in a different RCA, and now things look more reasonable. B+ is around 312 VDC. I checked the cathode voltages at TP1 and TP2, and saw 46.3 volts and 46.9 volts. Much better now. I calculated plate dissipation around 13.7 watts.

Visual inspection shows much more of the typical (?) Russian blue fluorescence than I saw last night. I guess having a working rectifier makes a difference. I'm surprised at how good I thought it sounded with only 100 volts on the plate. So much for my golden ears. I'll try to get a photo of the blue glow. It's kinda weird - not like the blue glow on my 6p14p, nor like the 6p3s-e. On those tubes the blue sticks to the glass, but on these it sticks to the plates. I'm hoping it's not gas.

Edit: here's the photo. Sorry for the grain, but my digicam has a teeny sensor. It's definitely not gas - the blue isn't in the space, it's on the plates. I just haven't seen a tube do this before.

Image
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby Ty_Bower » Wed Jun 24, 2009 4:42 pm

:(

I bought a pair of nice Sylvania 6SL7, and one of them arrived with a broken ring getter. Somehow the ring snapped off, and now it's running around loose inside the tube. I can't use it like this, can I? There is too much risk of the getter shorting out something.

Image Image Image
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Postby mesherm » Wed Jun 24, 2009 5:14 pm

My EH 2A3s have a trace of that blue plate glow. I think its Chernobyl remnants in the Russian tubes. =:o
I would say that your loose getter tube is not usable. There is nowhere that loose piece of metal can go that won't do damage. You could cut the glass off and take it out. Remember to clean the inside of the glass real good. (lol)
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Postby Ty_Bower » Wed Jun 24, 2009 5:29 pm

I wonder if the getter ring is steel or some other magnetic material? I've got some small, strong, neodymium magnets. Maybe if I placed one on top it could hold the ring up out of the way? Of course, any magnet strong enough to hold the ring is going to have some effect on the electron field as well...
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Postby Roger P. » Thu Jun 25, 2009 1:38 am

Tom, Thank you for the Top Plate info. I live appx. 40 miles from Seattle. I should go see them.

Ty, Thank You for the Detailed and interesting description of your build. Good read. I am sorry about your fallen halo (LOL). any recourse from the supplier? That blue glow looks interesting and a bit Si Fi. Also looks a bit like the glow of the Sophia 300B's I have seen photos of.
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Postby ecir38 » Thu Jun 25, 2009 6:18 pm

Good to see you get her done. I kind of dig the chassis and the simplicity of it.

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Postby Ty_Bower » Thu Jun 25, 2009 7:54 pm

Any comments on my filament voltages? At 6.0 and 6.1 volts, they're just a tiny bit low. I'm thinking of swapping in a different value at R19 and R20 to bring the voltage up closer to the specified 6.3. Or should I just leave well enough along? I suppose the tubes will last quite a bit longer with the slightly lower filament voltage.
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Postby ecir38 » Thu Jun 25, 2009 8:18 pm

It wouldn't hurt playing with it just to see the difference it would make for experience purpose. I don't see a problem with where it is at now.

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Postby Ty_Bower » Tue Jul 14, 2009 11:46 am

Has anyone tried an RCA 5691 "Special Red" in the V1 socket of their GSG? It seems like no data sheet specifications will be exceeded. The 5691 should be a happy tube here, right?
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby mesherm » Tue Jul 14, 2009 3:54 pm

A 5691 should work fine. Maybe even better. I wired mine with a loktal socket so I could use 7F7s since I have a bunch of them NOS.
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Postby Roger P. » Tue Jul 14, 2009 4:13 pm

Good to know. I just missed one on e-bay. I have been thinking of the same thing. Thank you.
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Postby TerrySmith » Tue Jul 14, 2009 4:20 pm

There's also the 6SU7, very close to the 6SL7, I've been meaning to try one someday.

I wouldn't worry about a blue glow, but a pinkish - orange glow inside the plate usually indicates a gassy tube.Image
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Postby Roger P. » Wed Jul 15, 2009 2:49 am

Man, I thought the RCA Red Label was expensive! I remember the blue glow inside my Dads Harman Kardon tuner amp combo. Fascinating. Pre plasma globes for sure.
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