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PostPosted: 26 Feb 2009, 09:55 
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Joined: 03 Jan 2009, 10:44
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:cop: Split from: K-12 Tube Amp Build Thoughts

Bruce wrote:
the manufacturer built them sideways.

What an image! Or did they put the pins on the wrong end, maybe?

To Bruce's point, then, if the tubes are the problem, then the amp should be salvageable with a different tube configuration. Right?

If so, what would be a good starting point to begin exploring a change that would transform my K-8 into something more than a (passive) dust collector? I realize this is a dumb question, but am quick to acknowledge my ignorance.


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PostPosted: 26 Feb 2009, 11:05 
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Maybe it is just the tube itself? What brand are they?

I looked at the 6005 specs:

http://tubedata.itchurch.org/sheets/093/6/6005.pdf

and it was a ground up designed for audio tube, seems odd since we have gotten better results with tubes specifically designed for other purposes.

The 5670 doesn't seem like a bad choice either:

http://tubedata.itchurch.org/sheets/084/5/5670.pdf

Gain is pretty high...maybe check the schematic vs. their reference schematic and see if something is wonky in the preamp design.

If something is severely limiting bass I would imagine it would be in the pre design. Bruce did you ever try bypassing the preamp?


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PostPosted: 26 Feb 2009, 11:19 
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The tubes are GE JAN-6005W. I'm going to go back and take a look at the schematics. Thanks for the lead!


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PostPosted: 26 Feb 2009, 11:44 
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I have schematics for several 6aq5/6005 SE amps but no luck on a push-pull as of yet.


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PostPosted: 26 Feb 2009, 13:41 
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Hi Guys, I split off the K-8LS discussion into here to keep it separate from the K-12G tube amp.

Here is Bruce's testing of the S-5 Electronics K-8LS Tube Amp Kit.

I also think that the problem may well be in the driver stage, but another thing to check is the load of the audio output transformers.

Here is a scan of the K-8LS schematic that Bruce made:
Image

A side note about the heaters, there are four 6005 @ 0.45A + two 5670 @ 0.35A = 2.5A so the power transformer is a little on the small side (assuming it is actually rated at 2A as the schematic shows).

From the datasheet, the tube choices seem like good robust tubes. I'll try check the preamp stage when I get a chance.

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PostPosted: 26 Feb 2009, 15:00 
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Some interesting observations (yes I am bored at work)

1) 10gv8 is a 10v heater valve yet it is being run at 12v...have you guys experienced shortened life from this?

2) The 10gv8=6gv8=lcl85= (sort of) pcl85 (heater voltage diff.) and the pcl85 is where you will find specs and graphs.

2a) The PCL85 triode is very high mu, 60-63 while the 5670 is only 35. They have similar transconductance but wildly different max current and output ratings.

I would summize that the 5670 is not a good driver tube for the 6005.

All of the SE schematics use 1/2 of a 12ax7 or a 6au6 as driver.


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PostPosted: 26 Feb 2009, 18:01 
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The 10GV8 heaters are actually 11.6V.

Also, note that the K-12G kit that comes with the 10GV8 does not have any driving tubes, just four 10GV8.

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PostPosted: 26 Feb 2009, 18:41 
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The 10gv8 has a triode and a pentode so it does in fact have driver tubes...they are just internal. :mrgreen:

Interesting about the heater.


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PostPosted: 26 Feb 2009, 20:54 
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Hi Everyone, I almost promised myself never to get into this fray on this amp again, but to help others I'll do it. The K-8 was supposed to be the follow on of the rather good K-12/K502 amps. I got an early one and was greatly disappointed with it. Mine was so far off the mark that I immediately did lots of things to try to salvage it. Eventually I gave up as it was costing more fix than it was to build my bigger amps. That all said here are some of the things I discovered / concluded. BTW I don't claim to be a super expert, just an avid diyer with about 40 years experience in electronics. The output tubes and basic output circuitry are OK. I don't use that configuration and personally think there are ones that give better results. But that it can be implemented with good results. I stopped testing my K-8 before I got my new DSO and distortion analyzer (the amp is now is pieces in a box in my shed so it really can't be resurrected). The problems always pointed to the driver / voltage amp stages. The topology is not unusual, but doesn't seem to function as intended. Distortion sets in way too early and it doesn't drive the output tubes symmetrically. Frequency response was limited as well. There are two avenues of approach I would take now. First and easiest is to increase the current flow through the driver / phase inverter. It is rather small now. This would require reduction of the cathode resistors and anode resistors. I believe I would start by halving both and see what happens. It is important to get the phase splitter to operate in a linear manner. The values for its anode and cathode resistors are a bit high to my thinking as well. Dropping them to about 2/3 would also seem a good idea. From there it would be a test and adjust procedure to get the proper drive for the output tubes. You can do the sims/number crunching if you chose, but follow it up with signal generator and scope testing. The second possibility and , my personal approach at this point would be to change the driver tube to one that has known ability to drive such a load. My preference would be a type 5751. This is pin compatible with the 12AX7, 12AT7 and 12AU7. It has slightly different characteristics that govern my choice. It has fairly high (but not too high) gain (70), uses modest current flow (1-2ma typically) and is linear over a large region. It is a popular tube for guitar amps and is rugged to boot. The Russian ones work fine. I use some to directly drive KT77s and KT88s. The rewiring of the circuit board would not be extensive and you could initially try the circuit values that were OEM. A benefit here is that you can also try the 12AX7. The other 12A_7 tubes would need much higher idle currents to be linear (5-10ma). Either culd work with such an adjustment. I hope I didn't muddy the water too much. Please let us know what you all do and how it works. I'm sure there are others out there that are unhappy with the K-8 as well.

Good listening
Bruce

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PostPosted: 26 Feb 2009, 20:59 
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Hi All, I see that I didn't answer one of Gio's questions. The actual output transformers did not seem to be the problem. I had both old and new type ones and it didn't seem to matter. I was going to try some ones from Edcor but quit first. The guitar amp series and class X ones from Edcor would be my choices. The ones in the 8k and 10K sizes would be the choices. Either type and either value have advantages.

Good listening
Bruce

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