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KT120 Oddblocks
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Author:  sampleaccurate [ 16 Apr 2011, 23:52 ]
Post subject:  Re: KT120 Oddblocks

Geek wrote:
ax84.com is a good place for geetah ideas :)

Cool site.

My only suggestion for the preamp is that it be capable of clean all to way up to saturated heavy distortion. Heavy saturation and distortion are used on a lot of songs, not just heavy metal.

Musical instrument amplifiers are one area where analog SS electronics will never rival tubes IMHO. I've never heard a good sounding analog SS amp when it comes to producing distortion, nor have I ever played an analog SS amp with the dynamic touch sensitivity of a tube amp. If you want realistic tube sound from SS electronics it must be digital and right now the cost is about $1600 for something that sounds realistic - ouch! :shock:

Author:  gofar99 [ 18 Apr 2011, 14:06 ]
Post subject:  Re: KT120 Oddblocks

Hi All, I have been to the ax84 site, good stuff. The design have is a class AB PP and will be a new thread when I get it going. Separate power and preamp sections so you can build one or both as needed. Transformer coupled driver. All new ground for me, but at least on paper it will work fine. :idea:

Good listening
Bruce

Author:  gofar99 [ 20 Apr 2011, 17:26 ]
Post subject:  Re: KT120 Oddblocks

Hi All, The bigger trannies are enroute :thumbsup: Edcor shipped them today. They will be here in a day or so. Soon it will be possible to see what these big tubes can do.

Good listening
Bruce

Author:  gofar99 [ 02 May 2011, 14:52 ]
Post subject:  Re: KT120 Oddblocks

Hi Everyone, Trannies installed.... even without treaking the amps are monsters. With zero time on the tubes the ouput is clean to 22 WRMS with 24 more on tap with a bit more THD than I like. I'll be fixing that. It should not be too hard. I will have these amps (other stuff too) at the Lone Star Audio Fest in Dallas May 13-15. Google it and if you can visit ... cool. 8-)

Good listening
Bruce

Author:  poty [ 03 May 2011, 05:54 ]
Post subject:  Re: KT120 Oddblocks

gofar99 wrote:
Hi Everyone, Trannies installed....

Have the CCS voltage rising disappeared?
gofar99 wrote:
the ouput is clean to 22 WRMS with 24 more on tap with a bit more THD than I like. I'll be fixing that. It should not be too hard.

46W total in the future? Sounds promising!
For me it seems the bias should be higher. The output tubes have finite amplification, so to achieve 150-160mA swing in anode you should apply around 25-28V swing voltage to the grid (in ultralinear stage).

Author:  gofar99 [ 03 May 2011, 12:20 ]
Post subject:  Re: KT120 Oddblocks

Hi Poty, The voltage on the CCS drops with the increased current. If you think about it this is expected. The voltage swing on from the driver is in the +/-50 range and is sufficient to push the tubes to max (they run class A1) however, I do expect to have to add a mod I use on the little amps to increase the cathode voltage slightly to keep the swing a little further away from the 4 volt minimum on the 317

Good listening
Bruce.

Author:  poty [ 03 May 2011, 15:23 ]
Post subject:  Re: KT120 Oddblocks

gofar99 wrote:
Hi Poty, The voltage on the CCS drops with the increased current. If you think about it this is expected.

My question was not about biasing, but about
gofar99 wrote:
The DC voltage across the LM317 increases with output when you start to push the tubes.

I think this was the primary reason for ordering the new transformers.
gofar99 wrote:
The voltage swing on from the driver is in the +/-50 range and is sufficient to push the tubes to max (they run class A1)

The mentioned amount of the driver's voltage swing is practically useless if the following hasn't changed:
gofar99 wrote:
... 300mA for a pair... The voltage at the cathodes is in the 37-42 volt range on the 120s

You cannot exceed the cathodes voltages with the driver's swing.
gofar99 wrote:
I do expect to have to add a mod I use on the little amps to increase the cathode voltage slightly to keep the swing a little further away from the 4 volt minimum on the 317

To achieve more headroom it is preferable to raise B+ instead. It is practically inevitable to do so anyway.

Author:  gofar99 [ 03 May 2011, 20:44 ]
Post subject:  Re: KT120 Oddblocks

Hi Poty, True, as far as it goes. A quirk with using a LM317 as a ccs in the cathodes is that the cathodes can not swing all the way to zero. Effectively this restricts how much drive the lower (grid to ground) tube can see as drive. In this case about 38 volts -4.5. When you get close to this value the 317 does a few funny things, like sense it is being starved. The waveforms go down the tubes (nice play on words) rather abruptly and distortion goes way up. The solution to get full drive sounds odd, but when you think of how a tube is biased (the difference between grid and cathode) it makes sense. Just add a bit of positive voltage to the grids (through a high impedance). The amount can not be too much as it adds to the voltage across the cathodes and thus increases the heat dissipated by the CCS. Just increasing the B+ will not have the same effect in this particular application. You could trade off current for B+ and use less of one and more of the other (and still stay within the ratings of the tubes) but then if you increase the B+, in order to maintain the new current level the voltage on the CCS has to go up. I find that the best window for operation seems to be in the 400-450 B+. The trade off in dissipation in the CCS is more favorable than at other settings.

The actual reason for the new transformers was that the older ones had a lot of resistance in the windings and were not rated for the amount of current I was using.

Hope this helps explain how it works.

Good listening
Bruce

Author:  poty [ 04 May 2011, 06:03 ]
Post subject:  Re: KT120 Oddblocks

OK, It seems I can't ask my questions clearly! :)
I'm wondering about:
gofar99 wrote:
I have run into a quirk on the KT120 version I have now up and running. The DC voltage across the LM317 increases with output when you start to push the tubes.
Has the problem gone?

-------
Then:
gofar99 wrote:
Just add a bit of positive voltage to the grids (through a high impedance). The amount can not be too much as it adds to the voltage across the cathodes and thus increases the heat dissipated by the CCS. Just increasing the B+ will not have the same effect in this particular application. You could trade off current for B+ and use less of one and more of the other (and still stay within the ratings of the tubes) but then if you increase the B+, in order to maintain the new current level the voltage on the CCS has to go up. I find that the best window for operation seems to be in the 400-450 B+. The trade off in dissipation in the CCS is more favorable than at other settings.

Sorry for the big quotation. I think in the message above you forgot about rising the voltage on the CCS in case of adding small positive voltage to the grids too. The bigger dissipation level in that case is caused just by the voltage rising.
Increasing B+ on the other hand could help to linearise the characteristics of the output tube as soon as it is moved to the area where bigger swing is allowed. 450V B+, according to my counting, is the minimal appropriate voltage for the tube in that implementation.

Author:  poty [ 13 May 2011, 05:34 ]
Post subject:  Re: KT120 Oddblocks

gofar99 wrote:
...Just add a bit of positive voltage to the grids (through a high impedance). The amount can not be too much as it adds to the voltage across the cathodes and thus increases the heat dissipated by the CCS.

Playing with the schematic I've come to another couple of thoughts.
If we use the current schematic of the KT88 output stage (with some tweaking of the resistors' values of course) we have different resistors' values in the halves' grids (to ground). Earlier you mentioned adding two 1-2 MOhm resistors to the grids from B+. As soon as the grids-to-ground resistors differs, the resulting addition of the voltage will differ too for each grid. First thought out of it - made the grid-to-ground resistors equal for both grids = the resistor's value for "primary" tube. In the real situation there is no signal current through the grid-to-ground resistor of the "slave" tube, so the value of the resistor is not important. Then we could use identical B+-to-grid resistors symmetrically.
The second thought is about the 25 Ohm pot. In the KT120 version there is as much as (25/2)*0.16 = 2V voltage dropping on the each halve of the pot. It is additionally to the around 4.5V safe margin for LM-enabled CCS and some insignificant voltage dropping on 1 Ohm resistors for measuring balance. The main point - the voltage on the 25 Ohm pot can't be compensated by rising the voltage on the grids, so it mainly wastes power. As soon as we adding the B+-to-grid resistors for rising the grids potential it is preferable to transfer the balancing pot here. Then we'll recover the 2V from cathode circuit, 4.5V from the CCS, have balancing pot in the circuit without signal currents and as a side effect could measure the balance directly on the cathodes instead of comparing voltages on each 1 Ohm resistor.
What do you think about this?

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