12AX7 + 12BH7 instead of 5751

for Dynaco Mark II/III/IV and DIY PP monoblocks

12AX7 + 12BH7 instead of 5751

Postby notben » Mon May 24, 2010 9:14 pm

Have any of you been happy with this combination compared to ax7/au7 or 5751/bh7?

I think I am going to populate my boards for this combination first as a I have a nice supply of matched pairs of each of these.

I was thinking of only adjusting the plate resistors R10 & 11 to 47k from the 68k for the 12bh7 and leaving R6 as 121k for the 12ax7.

Or does anyone think it is best practice to raise the input plate resistor to 150k and use a 5751 when using 12BH7's, or should I just leave the circuit as Shannon has it printed and just plug in the 12bh7's with the 68k resistors?

I may eventually try them all, but in my mind I just want to run ax7/bh7.

Thanks,
Ben
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Postby Ty_Bower » Tue May 25, 2010 6:46 am

Welcome to diytube. Nice to have you around.

I would study Ned Carlson's tubezone Poseidon Manual. I purchased my kit (boards & parts) from Ned, and assembled them per his schematic. The differences are minor - I think he simply tried to optimize the loads for his preferred 5751/12BH7 combination. Those are the tubes I use in mine and although I haven't made any effort to test them in any scientific manner, I feel they perform brilliantly.

I did try to study Ned's selection of load lines and attempted to plot his circuit against the published curves for the 12AX7 and 12AU7. I recall that I decided his component values would work perfectly well with them. As luck would have it, my Mark III are currently in rotation. I have buckets full of old Sylvania (?) 12AX7 salvaged from Baldwin organs. If I'm in the mood, maybe I can swap a pair in for the 5751 and see how it sounds.
"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 notben » Tue May 25, 2010 8:52 am

Thank you Ty.
Yes, Ned's manual is where I got the values for the 12bh7 and 5751. I figured he or others here had done a nice load line analysis of those operating points. My assumption is that there is probably a good reason Ned chose the 5751.
My limited analysis thinks it must be because the anode resistance of the 5751 is lower than the 12ax7, since the amplification factor of the BH7 is actually lower than the AU7. Is it because the lower resistance of the 5751 allows enough plate dissapation to drive the BH7 and operate the BH7 at a better load line?

I have been doing a lot of reading the past year and am trying to get better at circuit analysis. I am very grateful to every one who participates on all of these DIY forums.
What I am not sure of is how leaving the value of the anode resistor for the 12AX7 will affect the impedance that the 12bh7's will see. I have been doing a lot of reading the past year and am trying to get better at circuit analysis. I am very grateful to every one who participates on all of these DIY forums.

I would love to know if you think you here a difference between running the 5751 and 12AX7 with the 12bh7's.
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Postby Ty_Bower » Tue May 25, 2010 8:26 pm

I stuck a pair of Baldwin labelled 12AX7A into my Poseidons. Overall, the sound isn't much different from the 5 Star GE 5751. It might have a little more bite, with a touch less liquid if that makes any sense. I won't claim a clear preference for either tube.

I took a few voltage measurements around V1 and J, K. The current draw through V1 appears to be ~1.07mA. Here's the raw voltage readings:

plate = 170 v
grid = 1.17 v
J = 332 v
K = 420 v

I'm now careful to keep the line voltage around the Dynaco design voltage of ~117 volts AC. I use a bucking autoformer to tame my relatively high 126 volt line.

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 notben » Tue May 25, 2010 9:06 pm

Thanks for the update.
I take liquidity as being smooth with out a grainy midrange with a generally flat and clearly defined eq and no harsh spikes in the mids or no grainy artifacts (with guitar amps I would add no ghost notes)
Good thing is for comparison I have tightly matched pairs of Tung Sol 5751's and Tung Sol long plate 12AX7's. To me, both of those are liquid.

I guess now I just need to decide on R6. I think I will stick with 121k and try it with both for about a month and then switch to 150k and see if I hear a difference.
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Postby Ty_Bower » Tue May 25, 2010 9:31 pm

Ned's schematic shows 150K for R6. He did seek Shannon's approval prior to publishing his drawings, and Shannon approved. I've got 150K in my amp for R6, and it works perfectly well with 12AX7. Finally, Ned's schematic states "5751 or 12AX7" for V1. Anyway, I can't see much difference in 121K versus 150K. The higher valued resistor should offer a hint more gain, at the possible expense of a little more noise due to drop across the resistor. I've foolishly (?) chosen to go with carbon resistors in my circuit, so if resistor noise is going to rear its ugly head, it'll do it in mine. I'm just kidding about the foolishness part - I think my amp sounds great.

To me, liquid is more of a smoothness in the upper mids/lower highs. Too much "zizz", and I'll quickly start to complain. Maybe that's the same thing you hear, but from your words I'd position it slightly lower than where I perceive it. Words are so awkward for describing what we hear.

In any event, this amplifier has a lot of feedback in it, and that is going to neutralize a lot of differences between various tubes. Build it out with R6 wherever you like - it's got to work out just fine.
"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 notben » Mon Jun 07, 2010 7:40 pm

I finished one amp and have one more to go. I built it completely with the DIYtube specs to get acquainted with it first. Tried a few different combos of 12ax7's and 12au7's over the weekend and then dropped in a 12bh7 and just loved it even more.
Spent the weekend with The Beatles new mono catalogue in rotation through the one amp.
Sounds fantastic so far.

I sandblasted the bell covers and then painted them with engine block primer and paint. The chassis I just washed with Eagle chrome wash and then polished it with Flitz, although I barely touched the original lettering with any Flitz. I taped up the bare transformer, lightly brushed the laminations, blasted them with compressed air and sprayed them with some MarHyde rust converter.

Image
Image
Image
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Postby Ty_Bower » Mon Jun 07, 2010 8:28 pm

Wow. Really nice rebuild work you've done there. New sockets, new connectors, New Sensor Gold Lions, K40Y caps, everything is redone. The chassis cleaned up perfectly, and the transformers look every bit as good as they must sound. I'd figure these amps are ready to go another forty years.

I really have fun with my Mark III. They are as close to a bottomless pit of audio power as I will ever get. The big Dynacos don't shy away from anything, they never seem to run out, yet still have a tangible delicacy when played gently. It's sorta like the proverbial iron fist in a velvet glove. I hope you enjoy yours as much as I do mine.

I might suggest you look at where the UL screen taps connect to pin 4 on the power tube sockets, and think about adding a two or three watt 100 ohm carbon composition resistor there. There's a thread or two around about the screens and the abuse they must suffer. If you've upgraded the supply capacitance, adding a hundred ohms at the screen might be the difference between a short unhappy tube life and many thousands of hours of audio bliss. Use carbon comp resistors here because they can't catch fire and burn like a film resistor might. Keep an eye on the line voltage, too, if you can. I think the Dynaco manual said 117 VAC was design center. A high line voltage not only affects B+, but also heater voltages.

The only other thing I might add would be UF4007 diodes at the rectifier socket in series with the tube's plates. I meant to put these on mine and haven't suffered any ill consequences, but it is a simple, cheap, and inaudible method of extending the life of expensive 5AR4 rectifier tubes.
Last edited by Ty_Bower on Mon Jun 07, 2010 8:51 pm, edited 1 time in total.
"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 notben » Mon Jun 07, 2010 8:49 pm

Thanks for the feedback and the compliments.
I wanted to take some of the best iron on the planet and breathe some new life into them. I have a range of paradigm, sansui and gallo speakers, but I plan on building some 4pi's :)

Thanks for the tip about the screen resistors -- some would call these grid stoppers, no? I just read one of those threads and was very curious about that. I did use the Dynakits 80/40/30/20 can, so yes the filtering is increased quite a bit.
Do you think it is best to connect those in series with the taps or tack them on to ground?

I have seen a diode off the gz34 of my Marshall amp and was wondering if that was to help with the in-rush current. Sounds like it is worth it considering I will eventually put some of my Mullards in there, especially what they cost to replace these days.
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Postby Ty_Bower » Mon Jun 07, 2010 9:08 pm

notben wrote:I have a range of paradigm... speakers

I've long been a fan of the Paradigm speakers. I once owned a set of 11se Mk2, back when I was in college. I still regret having sold them because I thought they were too big and heavy. I still have a set of 7se Mk3, but sheepishly admit I prefer my current Klipsch Chorus II.

Thanks for the tip about the screen resistors -- some would call these grid stoppers, no? Do you think it is best to connect those in series with the taps or tack them on to ground?

It's not really a stopper resistor in the traditional sense, as you would see on a control grid. The grid stopper works in conjunction with the tube's inherent capacitance to form a (very) high frequency low-pass filter. They keep out parasitics, RF, and they might help limit blocking distortion. The screen resistors have a slightly different purpose. I'll refer to Eric's post for a better explanation:
http://www.diytube.com/phpBB2/viewtopic ... 8&start=22

For more reading on screen resistors, check out page three in this article:
http://www.audioxpress.com/magsdirx/ax/ ... ie2544.pdf

There's no ground connection anywhere near the screen resistors - considering the voltages involved, that would be a "very bad thing". They go in series between the transformer leads and pin 4 of the tube sockets. Look at the 82 ohm parts (grey-red-black) in my amp:

Image

About the UF4007 diodes on the rectifier, look here:
http://www.diyaudio.com/forums/tubes-va ... stion.html
Refer to post #12 for the diagram, and #13 for a strong recommendation.
"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 notben » Mon Jun 07, 2010 9:28 pm

Excellent information, thanks for the link and your picture.

Now to track down some 2 or 3 watt carbon comps, not always so easy. Mouser, digikey, angela, parts express and parts connexion don't have any 100ohm. There is a great old electronics store local that may have some. If not, I can just use a small wirewound.

Chorus II's eh? Very nice. I very much want something in a similar vein as the Klipsch, Altec or JBL horns, which is why building a pair of 4pi will be perfect for my need to build things and for my budget, although I need to sock away some $ so I can get the nice JBL drivers.
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Postby Ty_Bower » Mon Jun 07, 2010 9:39 pm

notben wrote:Now to track down some 2 or 3 watt carbon comps, not always so easy. Mouser, digikey, angela, parts express and parts connexion don't have any 100ohm. There is a great old electronics store local that may have some. If not, I can just use a small wirewound.


Don't use a wirewound, that's not the right type for this spot. A one watter would almost certainly be OK if that's all you can find. 100 milliamps over 100 ohms is one watt - should the screens draw more than that? Looking back, I'm guessing the parts I used were only one watters as well.

If you really get stuck, send me a PM and I'll dig through my box and see if I've got four more of those 82 ohm parts left.
"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 notben » Mon Jun 07, 2010 9:51 pm

Thanks again. I will stop by the electronics store. He has a lot of older stock and seems to come through with odd stuff when I need it. Plus, he has 50 years of experience with tube amps and helps me with things like this.
I appreciate the help.
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Postby junkwaffel » Tue Jun 08, 2010 7:07 pm

Your completed amp looks very nice. Are those the GL reissue KT88s? They are a really nice sounding Kt88, and compare very favorably to my old Tung-Sol 6550s. And the russkie KY9-40 PIO caps are very nice, that is what I am using as well. As Ty recommends, along with Dave Gillespie and Jim McShane (refer to my "bad luck" thread), please do install those sceen control resistors in series between pin 4 and the transformer leads. I wouldn't wish the problems I had on my worst enemy (with exception to politicians, lobbyists, and certain corporate CEOs ;) BTW, I am using 5751s and 12BH7s on mine, with the Uncle Ned tweeks.
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Postby notben » Tue Jun 08, 2010 8:42 pm

Thanks. Yes, they are the Gold Lion reissues care of Mr. McShane.
I am definitely going to install the screen control resistors.

I am going to do the next one with Ned's circuit changes and then compare them side by side to see which one I prefer.
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