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| | |-+  Has anyone tried the JTM45/100?		 « previous next »
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Author Topic: Has anyone tried the JTM45/100?  (Read 44587 times)
seeker
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« Reply #15 on: June 13, 2009, 02:57:22 PM »
No,Nik said when he gave me directions on biasing that he liked it to be between 38 and 40 so I biased to the hottest tube to keep in range..
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lightice
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« Reply #16 on: June 13, 2009, 03:07:59 PM »
Ah alright, thanks.
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cmoore
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« Reply #17 on: June 14, 2009, 06:44:38 PM »
I think "lightice" may be leaning in the right direction.  But I am pretty confused at this point, probably my own fault.  So if you don't mind me going back over some common ground......

1.  I am not familiar with these amps at all, so pardon me for asking.  This amp uses 4xKT66 in the output and is SS rectified, is that right?

2.   Are you saying that you see 550VDC on your plates, and your current is between 34-40 MA?

3.  I am not sure if the KT66 is a 25 or 30 watt tube, but at 34MA your power is almost 19 and at 40MA your power is up at 22.  Even if the 66 is a 30 watt tube, your 70% mark would be 21 watts, obviously at 25 watts that mark moves down to 17.5 watts.  So I am thinking that maybe you are pushing the limits of what would be an advisable bias.  But it all hinges on what numbers you are seeing at the plates.

Can you confirm you plate voltage one more time?
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seeker
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« Reply #18 on: June 15, 2009, 02:44:39 AM »
hired hand just got back in town and saw your question so i doulbe checked Nik's e-mail.
 He did say he likes to see between 38-40ma on the kt66 tubes.I know just enough that I can bias them lower to see if that makes a difference but as far as plate voltage if you want to give me instructions on how to check it I'll be more than happy to and give you the results.
  I'll try biasing them lower  and crank my amp tomorrow if I have time and see if that makes a difference and let you guys know.
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lightice
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« Reply #19 on: June 15, 2009, 03:17:50 AM »
If you're using Shuguangs, which are real KT66s. Here is the datasheet from analogmetric .

It's a 25W tube, but max plate voltage stated is 500V. So I'm not sure exactly what Nik's got on the plates for his 45/100.



« Last Edit: June 15, 2009, 03:19:43 AM by lightice » Logged
cmoore
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« Reply #20 on: June 15, 2009, 03:26:48 AM »
Hey seeker -
How is it possible you know how to read plate current, but not voltage? Smiley Smiley Smiley Smiley
Which makes me wonder, are you measuring plate current from the cathode that is grounded via a 1 ohm resistor?
Anyway... to measure plate voltage, set your meter to VDC, clip on to chassis ground, put your probe on pin #3 and see what you get.  This will give you the other half of the power equation.  
Plate voltage or plate current by themselves is somewhat "useless".  It is important to know both numbers.  So let's see what you get......
« Last Edit: July 16, 2010, 07:50:31 AM by hired hand » Logged
lightice
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« Reply #21 on: June 15, 2009, 03:41:19 AM »
Actually Nik can make external bias probe sockets to read bias current easily.

I can do that on the JTM45, but I'm rather apprehensive about opening up the chassis and checking anything, especially probing around knowing there's 400+ v in the circuits. Due to my lack of electronics knowledge, I rather not get shocked to death.

If there's anything wrong, I can usually just bring it up to Nik, but as yet nothing's gone wrong.
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seeker
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« Reply #22 on: June 15, 2009, 06:26:42 PM »
Thanks hired hand for the info for reading plate voltage,and what lightice is saying is one of the mods I had Nik do for me which is an external bias(probe sockets and an adjustable pot on top of chassis) since when I say i'm new at this I really mean it,so I'm really trying to live another day.
  What I'll do is bias it colder and see if that makes a difference report back and then follow your instructions on reading plate voltage..
 But like I said earlier the ghosting is very faint, only occurs at extreme volumes and not annoying,but this troubleshooting might be of help to someone else because it's definitely giving me a better undertanding of how this amp works...
 Just did a quick read on Metros' forum in the tube section and if I read correctly 400v*40ma would give me 16watts which would be 64% on my 25 watt Shuguangs.If my understanding is correct then that would mean that the 38-40ma range that Nik recommended would be just right.
 Hopefully you guys will check my math and let me know if i'm missing something. Embarrassed
« Last Edit: June 15, 2009, 07:18:51 PM by seeker » Logged
lightice
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« Reply #23 on: June 15, 2009, 07:24:05 PM »
The max plate voltage that the shuguangs can take is 500V , from the data sheet.
So I'm not sure, I guess the only way we can find out is to actually take a measurement on the plates itself.

If Nik made the amps run at 500V, then its a tad too hot. If I'm not wrong the shuguangs are very robust tubes. But I think Nik would probably run the amps lower. I haven't e-mailed him about them, although I'm sure he's already measured them. He probably has the values off his head, but if you know how to check, it'll be great.

Please be careful and ensure all safety precautions are taken, there are highly dangerous voltages in the amp that can kill, even when switched off, as the caps still have high voltages unless drained.

Btw, seeker, thanks for the constant updates. Its a great learning process.
« Last Edit: June 15, 2009, 07:48:07 PM by lightice » Logged
cmoore
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« Reply #24 on: June 15, 2009, 09:58:01 PM »
seeker -
Your math is correct, if your assumption about a 400V plate voltage is accurate.  I have a feeling, however, that you will find it is higher than that.
Let Us Know
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seeker
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« Reply #25 on: June 16, 2009, 01:42:31 AM »
Just checked :Plate voltage 485 which would mean that altough I might be running a little on the hot side of the bias it's not bad.
 Also tried running it biased colder but could still hear the faint ghosting. But do remember I only bearly hear it and then it's only at stadium levels,and i'm not joking we're talking very,very loud.
   It also sounds slightly better in the 38-40ma range.
 So, in the end I rebiased to Niks recommendation.



 
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lightice
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« Reply #26 on: June 16, 2009, 01:47:35 AM »
I see, thanks seeker.

I guess next to look for is what hired hand has recommended, to increase the value of the filter caps.
Have you discussed with Nik the possible recommendations on his part?
« Last Edit: June 16, 2009, 01:54:06 AM by lightice » Logged
seeker
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« Reply #27 on: June 16, 2009, 02:04:48 AM »
I think that for the moment i'm going to leave it alone and enjoy playing it.
 If you knew how loud it has to be to hear the ghosting and how faint it is you would see why I say i'm not going to worry about it.
 I'm just going to keep reading to gain more knowledge and ENJOY!!
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lightice
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« Reply #28 on: June 16, 2009, 02:31:03 AM »
Have you considered less efficient speakers?

Something in the 97db sensitivity range. The G12H Heritage are 100db sensitivity.

If volume was an issue, I'd go with less efficient speakers.
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seeker
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« Reply #29 on: June 16, 2009, 02:44:41 AM »
Volume,s  not an issue at all, I love loud...
 Now it's just time for me to sit back and enjoy the tone and the volume....
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