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PostPosted: 21 Jul 2009, 09:32 
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Post-ultimate question... How out of balance can a matched pair of Gold Lions get? One of my monoblocks is 75% at one side, the other is 50/50 as expected. Also, the one at 75% is 77mA per tube and the other is about 90mA. Is there something quirky going on here? I read your comments about a dying tube which made me think I may have one here...


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PostPosted: 21 Jul 2009, 13:05 
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Hi, First verify that after about 10 minutes the unbalance is still the same. During the warm up period some tubes get up to temp quicker than others. Then check after about 30 minutes. Next if it is still unbalanced, try swapping the tubes around. First just the two out of balance. Then (mark them first so you know where they came from) try ones from the other set. Some individual tubes are picky about having the grid go to the ground. There is probably a tech explanation for it, but I do not know what it is. And as you mentioned it could be a tube going soft (probably the one with the higher current as the control voltage is not enough to control the idle current). The amp will work with unbalanced tubes, but one tube will get hotter than the other and depending on how much you can exceed the dissipation limits for the tube. Unbalance of up to 5 ma is OK. Another possibility is to put the unbalanced pair in the other amp to verify that there is no wiring error in the first amp. As far as I know, GL KT88s have always been OK in the circuit if all are the same age and condition. Used tubes with unknown hours or conditions, may or may not be suitable.

Another idea, check the heater voltage. Post back on the value you measure.

FYI the balance circuit effectively adjusts the idle bias on the tubes from equal to -2 volts on the one needing more control. This range is usually sufficient. If the tubes are off by more than that then they will most likely not be very good in any other push pull configuration. They could only (realistically) be balanced in a circuit that has fixed bias adjustments. Still OK for single ended though .

Good listening
Bruce

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PostPosted: 21 Jul 2009, 22:09 
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Ok, so I swapped the tubes about and the imbalance followed as expected. Checking the heater voltages was a bit concerning though. I had trouble finding an appropriate SMPS, and am using made-in-China Wii PSU copies, which are rated 3.7A at 12V. However, I get 4.83v on one heater, and 5.08v on the other. The well balanced amp also has missmatched low voltages at 5.07v and 4.9v.

Although everything works, presumably this is very bad? Have I damaged the tubes? The amps have done about 2 hours so far (and sounded fantastic btw!).

My only real option for a better SMPS is RS, and I just found a very nice one here: http://malaysia.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0130647. pricey at about $45, but adjustable and can set it to 12.6v. Should fit too!

Cheers,


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PostPosted: 22 Jul 2009, 10:59 
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Hi, The heater voltages are a bit low. There have been two reports of balance problems that I know of and both were because the heater voltages were too high - over 6.5 volts per tube. I would conclude from your experience that the opposite could take place as well -too low. I design around 6.0 per tube with a range of + 0.3 to - 0.1. (5.9 to 6.3). With the SMPS I use in some projects (but surprisingly not in the kits) the voltages are quite close to spec. I do not know how easily you can get parts, but Jameco on the web has many SMPS and probably will ship your way. The one you mentioned would work also. I would personally replace the existing SMPS. I doubt that any harm has taken place regarding the tubes. I'm glad you find the sound to your liking. I love mine, but then I should. Let us know how it pans out.

Good listening
Bruce

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PostPosted: 24 Jul 2009, 00:12 
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Hi Bruce,

I decided to get the RS SMPSs in the end as I am fed up waiting for overseas shipments to come in. Voltage came up to about 10v, and on max with the adjustment it got just shy of 12v (specced to go from 10.8v to 13.2v at 0.5% tolerance) . Thought this was a bit strange, but carried on anyway, did the bias test (results not much different from before). Didn't run it for long.

This morning it suddenly dawned on me, that my multimeter is clearly having issues. There have been too many occasions where a voltage was reported lower than it should have been, including my preamp power supply which was supposed to give15v but only measured 13v. I adjusted that to what I thought was 15v, which is probably actually around 17v :eek:

I just tested a couple of brand new duracell AAs and they gave just over 1.3v, so this is clearly a problem. It is a cheap digital model (MEET U.S.A), and I don;t think it can be recalibrated, so will probably need to buy another. Unfortunately it means I wasted money on the new SMPSs, and also over-voltaged the heaters last night (I am guessing over 7v on a couple of the heaters). It wasn't for long (20 minutes maybe), so hopefully no damage done :worried: Will post back when I have more info. How long can tubes last with such high heater voltages??
Cheers,


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PostPosted: 24 Jul 2009, 08:21 
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Ok, time for an update. Bought a new multimeter and confirmed the errors in old one, opened it up and recalibrated via a dodgy wobbly trim pot, but will never trust it again! The new one is just as dodgy, but these are the only ones you can get readily (and they are very cheap)! Will get a real one from RS very soon.
Heater voltages are at 5.92/6.16 and 5.95/6.26 (allegedly) and the same pair as before balanced ok, while the others did balance, but the balance point slowly moved to the very limit of the 25R pot (not beyond as yet). Being cautious I swapped in a pair of JJs, which balanced ok near the middle.

After letting the JJs warm up, I did a sound comparison between the two channels, and the difference is remarkable. There is a whole extra layer of top end on the JJs and bottom end on the GLs - they couldn’t be more different! GLs sound much better with my setup though.

So, should I run with the GLs, until I can’t balance them any more? Or is it best to pre-empt failure and get a new pair?? Presumably I don;t want to try to mix and match with my other pair (bias points 35 v 52)?

Oh yeah, and still concerned a bit about the PT heat. They both get hot, but one always gets a bit hotter (marginally too hot to touch for few seconds) and I have no idea why. :confused:

Cheers,


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PostPosted: 24 Jul 2009, 11:13 
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Hi, As you noted the heater voltage can influence the tubes performance. As long as they balance, there really is no issue. As much as 10% difference is not audible (or measurable on my distortion meter and scope). I would just keep a watch on the one that runs hot and see if after a while it decides to behave. Since you have an adjustable SMPS, you may try to drop the heater voltage by .1 to .2 volts and see what happens. It is unlikely that you will cause a tube failure as they are run well below max (at about 80% actually). I would leave them in place. You are quite correct about the difference in sound between the JJ KT77s and the GL KT88s. Rather dramatic. The JJ KT77s have a truly lovely sound. Very detailed, quite tube like. The GL KT88s pack a real wallop (love those techie terms). A cheaper alternative to the GLs is the JJ 6L6GCs. The sound is almost the same, just a little less power. I switch between the 77s and 6L6s depending on the music I'm listening to. Both use the same settings and both balance easily. I am presently auditioning some new Sovtek 6L6WXTs. They have a higher rating than the regular 6L6GC (20%). My initial impression is that they are inferior to both of the JJ types. Neither the bottom end nor the sweet top end.

PT heating? I'm not sure what you are asking here.

Good listening
Bruce

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PostPosted: 24 Jul 2009, 20:52 
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Thanks, I'll just keep an eye on them then and see how it goes. All looking good now :thumbsup:

By PT heat, I was just curious as to how hot the Power Transformers should be running. They don't seem to reach a stable temperature, and just slowly get hotter and hotter. I assume this is fine, but just wanted to be sure. I have no way of measuring it, but I'd guess the it's over 70c after 30mins. All this tube stuff is new to me and I have no reference point, so excuse me if I'm asking silly questions. I had thought that as we're running easily inside the 250mA rating they should run cool. Maybe not!

Cheers,


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PostPosted: 24 Jul 2009, 21:32 
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Hi, OK, that is what I thought you meant. I contacted Edcor on the same question on a different project some time ago and they indicated that the transformers were rated at 85C and really could go quite a bit over that without problems. When used within the ratings they said the heat level was normal. My own experience is that the ones in the KT88 Odd Blocks run at about 140F to 150F (60-66C). I used an IR meter on them. At that temperature they really feel hot, but it is well within spec. So now I just don't touch them after they have been on for an hour or so. The actual load factor on the transformers is about 75% of the rating at worst case. With many of the amps now nearly two years old and not a single problem, I have to guess they are fine.

Good listening
Bruce

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PostPosted: 26 Jul 2009, 22:23 
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Great news, thanks, I'll stop worrying now :D

Back on the different tubes, the JJs I have are actually KT88s, not KT77s. This is a pair I used initially before deciding to invest in the GLs. They seem to have similar tone characteristics you describe in the JJ KT77s, but seemed quite harsh to me. From your comments I am tempted to try out some JJ KT77s though, given their low cost, but am a little concerned about the lack of gain.

I notice your kits are rated at 15w for KT77s and 25w for KT88s, and my preamp is usually at about 2 o'clock for the KT88s (87db/w speakers). So are the KT77s really only suitable for sensitive speakers?

Also, as a side note, I recently stumbled on a spec sheet for the KT88 which specified a distance of 4" between the centre of adjacent tubes, whereas mine are only about 2.5" apart (leaving about 1cm gap at the closest point). I was thinking of putting a small table fan near them to avoid any heating issues, but have just noticed from your kit photos that those look really close too (though it is hard to tell from that angle). Is tube spacing really anything to worry about??

Cheers,


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