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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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 Post subject: Re: KT120 Oddblocks
PostPosted: 09 Sep 2016, 01:44 
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Joined: 24 Oct 2010, 07:05
Posts: 312
You have checked only one place suggested. It may be 220 Ohm, not kOhm. You may have bad capacitor or LM555 itself.
Put the "spike diode" right, unsolder the 220k resistor (and check its value by the way) and switch the block on. If B+ activates immediately, then you have either short circuit in the output of the LM555 or LM555 itself is gone.
If not, check the value of the 220u capacitor.


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 Post subject: Re: KT120 Oddblocks
PostPosted: 09 Sep 2016, 12:08 
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Joined: 18 Jun 2016, 06:52
Posts: 28
Thanks. In the meantime I found the wrong connection I made (embarassed faced emoticon). The upside is all this hassle gave me time to realise I was using 5V dc relays instead of 12V ones. One keeps learning all the time.


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 Post subject: Re: KT120 Oddblocks
PostPosted: 09 Sep 2016, 14:42 
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Joined: 04 Jun 2008, 20:59
Posts: 4285
Location: Arizona, USA
Oops. :idea: I think all of us have done stuff like that. At least there was no smoke, fire or arcing. :eek:

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 24 Sep 2016, 10:32 
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Joined: 18 Jun 2016, 06:52
Posts: 28
I finished one monoblock. The sound is very powerful and impressive, but boy does the amp get hot! I don't have kt120s yet, so I tried kt88 (with a 6.8 Ohm cathode resistor as per the schematic). A pair of Svetlana winged Cs wasn't comfortable with this amp, the plates got red hot spots after some time. The circuit seems to agree better with JJs. Current measured at the test points was 0.092 A with both types of tubes. Is the amp going to get even hotter with KT120?


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 Post subject: Re: KT120 Oddblocks
PostPosted: 24 Sep 2016, 15:01 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, you can reduce the current to the winged Cs by changing the resistor to perhaps 8R or 8R2. What you have discovered is that not all tubes of a given type are equal in all areas. Even though the winged Cs are supposed to have the same specs as other KT88s, they clearly do not meet the same dissipation standards.

Yes the amplifiers do run pretty warm. Class A amplifiers generate the most heat at idle and are typically only about 35% efficient with the U/L design. Using KT120s and setting the idle current to the 130ma range or even a bit more will generate more heat. This is expected and normal.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 24 Sep 2016, 16:07 
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Joined: 18 Jun 2016, 06:52
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Thanks Bruce. I have 3 cm clearance between the KT88s and the transformers. I hope that is enough to avoid problems in the long run. My friend put the output valves in front and the 6SL7 in the middle. That was perhaps a wise move, but I thought my layout was nicer looking. So maybe I should have followed his example, but I hadn't expected the heat the amp is generating. Would it be an idea to use the 6R8 resistor for KT120 and 8R for KT88 as you suggest? I don't really need all that power. I installed a switch with 6R8 on one side (KT88) and two parallelled 9R1s = 4R55 (KT120) on the other.


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 Post subject: Re: KT120 Oddblocks
PostPosted: 24 Sep 2016, 17:39 
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Hi, 3cm is a bit tight, but probably OK. 5 or 6 would be better. If the trannies go over say 90 C, then I would probably put a small aluminum reflector in from of each. BTW the power trannies will get warm on their own and not to worry about it. Yes you can reduce the power on the KT88s and KT120s that way. One thing to watch for is how high the cathode voltages get. The lower the current on each type of tube (which actually is how it is biased) the higher the cathode voltage will be. You want to avoid going over about 45 volts. The LM317HVs will handle that easily and the AC swing on the cathodes will go both above that and below it when operating. I have seen peaks on one amp at a combined value (AC + DC) of just over 100 volts and there was no harm. Apparently the 317HVs can tolerate a lot more than the rated 57 volts. Still I feel it wise not to press your luck.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 24 Sep 2016, 18:18 
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Joined: 18 Jun 2016, 06:52
Posts: 28
This is very helpful. Thanks for your fast response and your patience. I'm glad something can be done if a problem arises.

Jan


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 Post subject: Re: KT120 Oddblocks
PostPosted: 28 Sep 2016, 02:47 
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Joined: 18 Jun 2016, 06:52
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Fully operational now.


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 Post subject: Re: KT120 Oddblocks
PostPosted: 28 Sep 2016, 07:47 
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Joined: 18 Aug 2015, 07:38
Posts: 176
Very nice


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