Low end clipping on SL 18 conversion

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geoff 1965
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Re: Low end clipping on SL 18 conversion

Post by geoff 1965 »

can you post the SL18 schematic you converted the lite to?
it might just be a case of attenuating the preamp signal if you have cascaded the preamp.
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Re: Low end clipping on SL 18 conversion

Post by JimbpGaff »

I did wonder that.
I will change mine over to the design on here and see if that works.

I may try bypassing the tone stack also to see if the problem is in there.
I will also post a schematic.

Thanks for the advice.
Jim
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Re: Low end clipping on SL 18 conversion

Post by JMPGuitars »

JimbpGaff wrote:
Sun 05/03/20 1:48 am
I did wonder that.
I will change mine over to the design on here and see if that works.

I may try bypassing the tone stack also to see if the problem is in there.
I will also post a schematic.

Thanks for the advice.
Jim
Take a look at the latest version: files/JMPGuitars_18_Watt_Superlite_TMB_Schematic.pdf

Compare that to yours. From how you describe the issue, if it isn't related to the Ruby mod, then you likely need to attenuate the signal in the preamp. In my example above, that's done with the 470K/470K voltage divider/attenuator between the two halves of the first preamp tube.

But the other question is, what is your power tube cathode capacitor value?
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Re: Low end clipping on SL 18 conversion

Post by JimbpGaff »

Cathode capacitor is 47uf.
I will check voltages tomorrow .
Should it read around 12 volts?

I have now removed the tone stack entirely and removed the 1m resistor before the MV pot and put the two 470ks after the pot, one to ground.
It is now much better and usable.
There is a small amount of clipping when I hat the low E really hard but the tone is so much better.
I rather like it as it is but may just go back to a generic tone knob see how that goes.
Thanks as always.
Jim
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Re: Low end clipping on SL 18 conversion

Post by JMPGuitars »

Your power tube cathode cap is 47uF? That could be part of the problem. ZP recommends at least 1000uF, and prefers 2200uF. I use 2200uF also (I tried after his recommendation, and it makes a difference).

Yes, power tube cathode voltage should be about 12VDC.

Thanks,
Josh
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Re: Low end clipping on SL 18 conversion

Post by JimbpGaff »

Thanks Josh

I will see if I have one and change it over or I will order one from RS.

Jim
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Re: Low end clipping on SL 18 conversion

Post by JimbpGaff »

The 2200uf cap has turned up now so I will hopefully change it over later.
The biasing reads 7.7 volts , I assume to raise the volts to 12 I should increase the resistor size ?
I currently have a 150r fitted .
As always , thanks Jim
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Re: Low end clipping on SL 18 conversion

Post by JMPGuitars »

JimbpGaff wrote:
Thu 05/07/20 2:53 am
The 2200uf cap has turned up now so I will hopefully change it over later.
The biasing reads 7.7 volts , I assume to raise the volts to 12 I should increase the resistor size ?
I currently have a 150r fitted .
As always , thanks Jim
Perhaps you should post all your voltages. 12VDC is the target for higher cathode resistor values biasing higher voltages in standard builds dealing with high wall AC. In your conversion's case, it might be a little different, so you'll need to calculate the correct bias point.
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Re: Low end clipping on SL 18 conversion

Post by JimbpGaff »

Ok .
Good advice.
I will check them out later.
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Re: Low end clipping on SL 18 conversion

Post by JimbpGaff »

I have now installed the 2200uk cap, reverted to the original tone control, fitted the sag resistor and installed the bright switch.
All works ok but the clipping is still there ( slightly ) and the overdrive is only light.
I will measure and post my voltages later.

To be honest, wiring wise, it has all got a bit messy in there now and hum has become an issue.

I feel I may have to strip it out and start a re build using the recommended layout and circuit.
If I do, would you recommend I go with a better power transformer?
Mine is the Ampmaker ( Danbury ) one with the 2 /9 / 18 watt switch option. I want to get it how I want it so I will have to dig into the savings pot.
I also have F&T filter caps - 47/22/22/22 UF - would you recommend I change the first two for a 32/32 double?
I also would need the passive loop before the phase inverter.

Thanks as always, Jim
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geoff 1965
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Re: Low end clipping on SL 18 conversion

Post by geoff 1965 »

hello Jim,
can you post some pics with your voltages? i had a danbury PT in my lite 2 and with a 100 ohm sag got some very good tone so i wouldn't be concerned with that, looking at the schematic the first thing i would do is fit a 10-33K ohm grid resistor into pin2 of V1, did you use shielded cable for the input?
SL18preamp.jpg
regarding the effects loop i have a series passive in my lite2 but if you are going to rework the circuit the one in the downloads is best, Josh has included it in the new layouts if you want to check it out.
You do not have the required permissions to view the files attached to this post.
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