It plays! Break in / bias point (SimpleSE) - diyAudio
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Old 15th May 2008, 06:51 AM   #1
Zap is offline Zap  United States
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Default It plays! Break in / bias point (SimpleSE)

Earlier today my SimpleSE made sound for the first time! With tubes that haven't been broken in yet and some more wiring to complete I'm still very impressed and anxious to listen more, though I have a few lingering questions.

There is a faint buzz that seems to come from the power transformer. I'm not sure if this is an unresolved grounding issue... any hints there would be helpful. The chassis is wood with an Aluminum plate, so I am trying to make sure all the grounding is there. I presume the board needs to be grounded by connecting one of the ground pins to the central ground lug as well?

Secondly, I'm curious to know approximately how long the tubes should be broken in for proper sound to begin. I have EH KT88 & 6CA7 to try, Chinese 6L6GC coke bottles, and Philips and Mullard 12AT7s.

Additionally, I'm wondering what bias point is required to get the EH KT88's to fluoresce. I'm not seeing any blue glow and I presume it either happens or it doesn't. The bias resistors are 560 ohm. Do I need something different? Does voltage/current affect this? Is it possible to clip in parts to adjust the bias for testing?

Also, is there an easy way to determine the B+ that I'm running?

Thanks a bunch - this has been a great learning experience!
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Old 15th May 2008, 10:29 AM   #2
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try to operate a 'star grounding' scheme - that should sort any potential difference on the ground plane.

B+ can be sought with a multimeter. Just put it on the anode leg of the valve (ever so slightly out of hte socket). BE CAREFUL!!

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Old 15th May 2008, 07:36 PM   #3
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I had a buzzing initially too. It turned out to be a ground loop that I fixed by putting in a 10k ohm 5w resistor in the earth ground that runs to the speaker and input grounds. I'm running triode, no feedback. I thought it was the transformer at first. The Hammond 274BX just likes to buzz on it's own, but it doesn't affect the sound.

Some reading if you're interested
Link
Me getting a clue

George (Tubelab.com) suggested paralleling the 560 ohm with another 5w resistor to increase the current for KT88's. I don't recall the appropriate values. I'll dig around. I used alligator clips, but it was a very tight fit for me and probably not very safe in my case. I'll solder in a switch eventually. I'm using 6BG6's (with adapters) that fluoresce like crazy at 560 ohms, but I got very little from 6L6GC's.

Be sure to test your B+ hands free! A DMM with clips on the probes are a good thing.
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Old 16th May 2008, 01:31 AM   #4
Zap is offline Zap  United States
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Two other questions have come to mind in addition to the ones remaining from above. Thanks for the input so far, though!

Are the approximate figures I've seen quoted for the SimpleSE of 10-14 W in UL per channel or total? Same for the 5-7 W in triode?

Whatever the case may be, my initial listening was in UL without feedback and it gets quite loud!

Also, I've got the "big Edcors" CXSE, 5kohm/8ohm. Would I effectively be running the tubes harder at 2.5k primary if a 4 ohm set of speakers were installed?

Finally, if anyone has suggestions for good speakers for a system like this (or even more efficient, as I eventually would like to build a DHT low power amp) they would be appreciated!

Thanks
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Old 16th May 2008, 03:26 AM   #5
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I believe the power output estimates are per channel. I've never tested mine. Too busy just listening.

As for the OPT primary resistance, my guess is that you wouldn't be running the tubes harder, but they would operate less efficiently. You may even get less wattage and more distortion. The only way to find out is to test it yourself. Tubelab.com has some good info. 5k gives good results in my opinion.

I'm using fullranger speakers that are pretty darn efficient, and a good bang for the buck. Commonsense Audio Nirvana Super 6.5's in a Fostex BK-16 back loaded horn cabinet. It seemed harsh at first, but now that I'm accustomed to them I like them very much. I even disabled my subwoofer. There are better speakers out there. I was offered a pair of extremely nice high end Tannoy's but they didn't pass WAF and are verboten to me.
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Old 16th May 2008, 11:04 PM   #6
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Default Re: It plays! Break in / bias point (SimpleSE)

Quote:
Originally posted by Zap
There is a faint buzz that seems to come from the power transformer. I'm not sure if this is an unresolved grounding issue... any hints there would be helpful. The chassis is wood with an Aluminum plate, so I am trying to make sure all the grounding is there. I presume the board needs to be grounded by connecting one of the ground pins to the central ground lug as well?
Some power xfmrs (especially Hammonds) sometimes like to buzz. Just make sure that the bolts that hold the lams together are tight. Then don't worry about it.

Quote:
Secondly, I'm curious to know approximately how long the tubes should be broken in for proper sound to begin. I have EH KT88 & 6CA7 to try, Chinese 6L6GC coke bottles, and Philips and Mullard 12AT7s.
Depends on type/manufacturer. It shouldn't take more than a few hours for new finals to settle down.

Quote:
Additionally, I'm wondering what bias point is required to get the EH KT88's to fluoresce. I'm not seeing any blue glow and I presume it either happens or it doesn't. The bias resistors are 560 ohm. Do I need something different? Does voltage/current affect this? Is it possible to clip in parts to adjust the bias for testing?
That blue glow is due to cobalt contamination in the glass (and sometimes the plate). If you don't have any Co, then no glow. I've seen that glow from mainly RCA types (807s, 6BQ6GTBs, 6AQ5s) but not Sylvania or Westinghouse. Shouldn't take more than a couple hundred volts DC to make an observable glow.
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Old 19th May 2008, 03:44 PM   #7
Zap is offline Zap  United States
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What is strange, is last night I was watching the tubes as they came on - the blue glow as seen in some of the pictures posted here showed up, and then it tapered off once the tubes stabilized to their normal operating condition. There was some glow left, but not what was there before.

I still haven't been able to find the bias adjustment, but I know its out there.. know George is busy, but if he gets a free minute I presume he should have the answer...
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Old 19th May 2008, 04:11 PM   #8
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Resistors R17 and R27 are the cathode bias resistors. This resistor value is adjusted to increase or decrease bias current. Decreasing the value from 560 ohm to 430 ohm for a KT88 will lower distortion, but that's about all. I think I used a 1.2k resistor in parallel to bump up the current for a KT88.

Check out this chart which will give you an idea of how to brew up your config:
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Old 24th May 2008, 06:14 AM   #9
Zap is offline Zap  United States
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As an update to this thread, I made sure the bolts were tight on the endbells - I'm running an Edcor power transformer, so I don't know how they compare to the Hammonds. The buzz is still present, but the actual speakers are whisper quiet without any input so it could be worse. I'm fairly certain I have all the grounds correct at this point. I also found a circuit that blocks DC from the mains from getting to the core and reduces buzz and heat. I may see if that has any effect on the final result at some point.

The tubes seem to exhibit more of the blue glow now without any bias point adjustments. I'm guessing it might be a function of some settling out or something of the sort, as they were run a fair amount prior to this happening. I suppose if I would push them harder the "light show" effect that George talks about would occur.

I've finally got all the mode switches installed, so it is possible to switch feedback and mode on the fly. Quite interesting the audible difference between all the choices. Out of curiosity, does anyone know if the 5AR4 should be removed when wanting to use the SS rectifiers?

I believe I am one of the first to use the big Edcors and while I have nothing to compare them to, the amount of sound produced by the SimpleSE and the amount of bass with fairly weak efficiency speakers is impressive. The sound is just incredible as well. I'm already trying to decide what to do next, LOL. I should probably build a set of speakers that compromises efficiency, good highs from a tweeter as good or better than my Telefunken speakers, and decent bass response. I haven't found a design that seems to embody these traits yet. When I do, I'll probably consider a design like the TubelabSE and have some fun with DHTs.
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Old 25th May 2008, 12:09 AM   #10
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Quote:
I believe I am one of the first to use the big Edcors
I got one of the early pairs that were painted metallic blue. They do absolutely ROCK. I had them in a SimpleSE but I liked them so well that they currently reside in a 300B version of a TubelabSE.

Quote:
Would I effectively be running the tubes harder at 2.5k primary if a 4 ohm set of speakers were installed?
Yes, hooking 4 ohm speakers up to an 8 ohm transformer loads the tube with half of the rated transformer impedance which would be 2.5K in this case. This would usually raise the power output and the distortion somewhat. In reality the speaker impedance varies a lot with frequency and power level. My "8 ohm" Yamahas vary from 6 ohms to over 20 ohms and are 8 ohms only at 2 points across the audio range. If the amp sounds good with 4 ohm speakers hooked up, then it is probably safe to use them. The EH KT88's can deal with a 2.5K ohm load OK, but may produce too much distortion.

The easiest way to determine your B+ value is to measure it. I have not used the Edcor power transformers, so I can't come up with a good guess.

The "blue glow" that I photographed in my KT88's was not as bright as it seems since the room was dimly lit (no lights on, just the light from the computer monitor). I have had the same pair in my SimpleSE since I built it, and sadly to say much of the glow has faded. I have not yet tried the other EH KT88's that I have.

I "boosted" the current in my tubes by putting 1 K ohm resistors in parallel with the 560 ohm cathode resistor. This raises the tube current to over 100 mA which runs the tube at (or slightly above) its maximum dissipation rating. This is not generally recommended and may cause short tube life. I use this same pair of tubes for my general experiments. Last week I had them in a push pull amp cranked up to 550 volts at 100 mA each, no glow.

Quote:
Out of curiosity, does anyone know if the 5AR4 should be removed when wanting to use the SS rectifiers?
It does not need to be removed. If the 5AR4 is left in the amp it will still draw about 2 amps of filament current from the power transformer. If it is removed this power is not drawn from the transformer which will allow it to run cooler. In theory the transformer will be slightly more efficient and may produce slightly more voltage, which could produce more power. In my amp with an Allied (Hammond) power transformer (like all Hammonds, hot) the transformer does run cooler, but the voltage only goes up 3 volts, which does not make a measureble difference in power.
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