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Old 9th April 2011, 02:47 AM   #1
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Default Tube replacement causes bias problems.

Hi all,

I just replaced the tubes in my Beard P100 amp from Sovtek KT88 to Gold Lion reissue KT88. With the Gold Lion in place I can not bias the amp to 40ma anymore and the minimum bias current I get when the bias pot are turned all the way down is 50ma. The Beard operates with plate voltages of 580-600v. Given that I have two questions.
1. Is it safe to operate the Gol Lion tube at 600v and 55ma bias current?
2. Why has the minimum bias current changed and is there anything I can do to fix this?

This is a link to the Beard Service Manual as a reference:
Beard_P-100_Mk2_service_manual.pdf - Windows Live

Thanks in advance,
Ronen
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Old 12th April 2011, 03:07 AM   #2
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Anyone?
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Old 12th April 2011, 04:07 AM   #3
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Hi Ronan: I am not able to open the PDF in your link....

You can post the PDF directly here under "manage attachments".

Here's my .02....

The power dissipated for 600V @ 55ma is 33W. KT88's are typically rated for 42W dissipation, so you seem to have some margin there.

600V on a reissue KT88 is probably a bit much.........someone with more experience than I have will hopefully chime in here, or search the forum for threads like this one:

JJ KT88s and other octals

According to KevinKr in post 9, you are over the ragged edge.

The reason that you are running out of bias pot adjustment is that the Gold Lion requires a bit of a different voltage for a given idle current than your old Sovteks. You need to be able provide the tube a more negative voltage than your current bias circuit provides. You can usually fix this by changing a resistor value or two in the bias circuit to get more negative voltage to the pot. That requires some soldering. The value of the pot in the bias circuit determines the range of voltages available, so changing out resistors just moves the adjustment range more or less negative.
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Old 12th April 2011, 08:49 AM   #4
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Thank you Boywonder,

I attached the schematic of the bias circuit. At its maximum it provides -85v which I understand should be enough to bias the GL KT88. The adjustments pots are not in great condition and I will replace them this weekend. It might be that they are not covering the full adjustment range. I will check.
As you said I do not feel very good about driving these tubes at 33w or event 31w plate disipation and will try to fix this.

Regards,
ronenash
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Old 12th April 2011, 03:34 PM   #5
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Quote:
Originally Posted by ronenash View Post
I attached the schematic of the bias circuit. At its maximum it provides -85v which I understand should be enough to bias the GL KT88.
Looks that way to me too. The pot should be able to adjust between about -51V and -85V which should be plenty. Pull the tubes, and measure the voltage at the wiper of the pot at each end of it's adjustment.
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Old 12th April 2011, 05:15 PM   #6
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Strikes me odd that -85V is insufficient to drive your KT88 below 50mA even with 600V B+. I suspect it might be time to replace the filter caps in your bias supplies depending on the age of the amps. As suggested checking the bias voltage range at the socket is a good idea. It might be that the transconductance and mu factor control grid to screen grid of these tubes differs substantially from what would be typical for the KT88.
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Old 13th April 2011, 12:33 AM   #7
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I found the same - the Gold Lion re-issue KT88 definitely bias at a different point than the JJ and the Sovtek KT88.
BUT what are you worried about?
KT88 Anode dissipation is 42W maximum.
For most stable bias and tube longevity (also allows larger Rg1 value) you should limit dissipation to 35W maximum.
600V and 55mA => 32W
Keep the idle currents to 58mA or less for Pa < 35W
The 600V is also not a problem Va = 800V max and Vg2 is 600V max BUT would probably be OK above 600V in Ultralinear Mode where the Vg2 is never higher than Va.

I also found that the GL re-issue sound MUCH better than the JJ.

Cheers,
Ian

Last edited by gingertube; 13th April 2011 at 12:35 AM.
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Old 16th April 2011, 01:48 PM   #8
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I replaced the bias pots with new ones but in the process found that the bias circuit only has only about -79v at the top of its range which biases the GL at ~52mA. It works OK but the amplifier sounds strained. I am sure it sounded better with the Sovtek KT88. I guess the GL's are not the best match for this amplifier. I will keep the GL's to use in future amplifiers.
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Old 11th May 2011, 11:41 AM   #9
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Quote:
Originally Posted by boywonder View Post
Looks that way to me too. The pot should be able to adjust between about -51V and -85V which should be plenty. Pull the tubes, and measure the voltage at the wiper of the pot at each end of it's adjustment.
Hi Folks,

I have done some more testing and it seems that because of the small current flowing through the bias circuit there is ~5v voltage drop on the 330K ohm resistor. I was thinking of changing the 330K ohm resistor with a 100K resistor. I have seen this done in other designs.

Do you see a problem with this?

I am attaching the full schematic of the amplification circuit as reference. The bias circuit is in a previous post.

Thanks
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