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Old 23rd April 2013, 10:12 AM   #1
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Default Single ended KT88 / 6SN7 not enough power

Hi

I used the chassis and ironwork from an earlier build 6B4G amp to get a more powerfull and integrated KT88. Testing the new amp I really miss power. I do have to turn the volume to max to get the about the power of my laptop !

Anyone any idea what goes wrong ? Or do I have to swap the 6SN7 tubes for 6SL7 with much more gain.

Thanx for your help
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File Type: jpg KT88-SE.jpg (48.7 KB, 665 views)
File Type: jpg Amp ext.jpg (508.3 KB, 634 views)
File Type: jpg Amp int.jpg (511.3 KB, 574 views)
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Old 23rd April 2013, 12:02 PM   #2
45 is offline 45  Italy
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I think there is a mistake in the PSU. To drop 115V with 1K8 resistor your SRPP should work at about 64 mA anode current!!

Just to give you some ideas.....I would not go for the 6SL7, I would rather use the 6SN7 sections as single ended common cathode stages in cascade (DC coupled) in a similar way to the Sun Audio design. Having more than 400V supply you can play with this a lot. You can bypass the cathode of first or the second stage, both cathodes or none them to get the right gain and more imprtantly the right drive! As you are RC coupling to the KT88 you might also try a CCS plate load for the second stage and find its load by tweaking the KT88 grid resistor. Having the two stages DC coupled you might get very low distortion because of cancellation with some tuning. Then most of THD would be that of the KT88. You might also by-pass both capacitors and use some feedback.
You might also use the two 6SN7 sections as RC coupled voltage amplifiers if you want to make it simple. I think you have many options to try before swapping the 6SN7 with the 6SL7 and the SRPP is your main limitation.
Using about 1K for decoupling the supply you should be able to get 400V for the the 6SN7.

Last edited by 45; 23rd April 2013 at 12:04 PM.
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Old 23rd April 2013, 01:17 PM   #3
45 is offline 45  Italy
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One other thing which is not good in your schematic is the heater supply. There is no reference to groun. However even in this case, using SRPP or DC coupled stages, you need to lift the reference voltage for the heater of the upper tube. You can use one tube shared between the two channels as the lower valve and the other as upper tube in the same way.
The 6SN7 has a max rating of 100V DC heater-to-cathode voltage (twice this value for DC + peak). The 6SL7 a bit less. So, even with 300V supply you are way above max ratings with the SRPP for sure. The result will be that the heater-to-cathode insulation will fail suddenly. This will be the first limitation for the tube life. A compromise could be that you lift the single heater supply you have to a voltage midway between the upper and lower cathode voltages (for example, the heater supply is not grounded but is connected to a reference voltage of 80V if your SRPP is working at 300V where the lower cathode is few volts above ground and the upper cathode at about 160V). You can create the reference voltage from the anode supply using two or more sistors in series after the last capacitor and bypass the 80V rail with a small capacitor. You only need little current (1 mA should be enough) through the series of resistors.

Last edited by 45; 23rd April 2013 at 01:21 PM.
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Old 24th April 2013, 08:09 PM   #4
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Default retry

Hi 45

Thanx for your reply
The 1k8 in the power-suply was already replaced by 46K.
I did some homework and changes the schematics
Do you think this will work beter ??
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Old 24th April 2013, 10:39 PM   #5
chip647 is offline chip647  United States
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Default If you don't want to reinvent the wheel.

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Old 25th April 2013, 12:52 AM   #6
45 is offline 45  Italy
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Quote:
Originally Posted by Ilikemusic View Post
Hi 45

Thanx for your reply
The 1k8 in the power-suply was already replaced by 46K.
I did some homework and changes the schematics
Do you think this will work beter ??
The second tube will work at lower current than the first in your schematic. I would change that 22K resistor in the supply. You could use another choke, in case.

For example, in a similar way to the schematic posted above, if you have 405V for B++ you can use:
1) input tube -> 750R for the cathode and 75K for the anode
2) second tube -> 22K for both cathode and anode.
This way the first tube will work at about 4 mA with 3V bias and second at about 5 mA with about 6V bias.

You might try 75V anode voltage for the first tube with low bias, as in your schematic, but you should increase the current in the second with enough bias. I have never tried such low voltage (and bias) with the 6SN7.

You still need to lift the heater to a reference voltage midway between the two cathodes. In the example above the first cathode is at +3V and the second at about 116V. So the heather should be at about 56-57V. It could be a bit higher because the second tube will swing more voltage. To do this from B++ =405V you put 330K + 75K to ground. This will give 1mA current and you will get 75V in between the two resistors and this will be the reference voltage for the heater. Bypass the 75K resistor with a small capacitor.

Last edited by 45; 25th April 2013 at 01:07 AM.
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Old 25th April 2013, 01:15 AM   #7
TheGimp is offline TheGimp  United States
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To answer your original question, I would look at your overall gain first.

6SN7 - mu = 20
KT88 - mu = 8
5K:8r transformer gives 25:1 step down ratio

So the total system gain is on the order of:

(20*8)/25 = 6.4

It's not that you don't have enough power, you don't have enough gain to get the power out for the level of input signal you are driving it with.

Your options are (1) add a preamp, or (2) redesign the front end to get more gain.

The suggestions have been for option 2.

If you are going to direct couple the two gain stages, be sure to re-calculate Pd of the cathode resistor in the second stage.

Increasing the anode resistor as 45 proposes will serve several purposes including reducing distortion, increasing gain, and reducing power dissipation in the cathode resistor as well as the anode resistor. (warning, run-on sentence)
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Old 25th April 2013, 09:53 AM   #8
timpert is offline timpert  Netherlands
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What are you driving it with? if you use the headphone out of your iPod as a source, then the gain of your front end is insufficient. A CD player on the other hand should be capable of driving your amp to full power.
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Old 25th April 2013, 10:20 PM   #9
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Thanx for helping me all
following 45's usefull remarks, the final scheme should look like this ?
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File Type: pdf KT88 amp def.pdf (56.2 KB, 138 views)
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Old 26th April 2013, 03:09 AM   #10
45 is offline 45  Italy
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Yes, it looks ok. One more thing, increase the by-pass capacitor for the second 6SN7. However you can do this when you build the amp....
Now you only need to use resistors of appropriate power, especially the 75K anode resistor. This will see about 4 mA and thus will dissipate 1.2 W.
You can use 5W carbon film resistors (Kiwame or KOA SPR, depending on where you are...) and you can also get 75K using two 150K resistors in parallel.

P.S.
Are you sure about the 390R cathode resistor for the KT88?

Last edited by 45; 26th April 2013 at 03:17 AM.
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