Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jayfitt on July 14, 2018, 09:18:39 pm
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Hi guys,
I just added a clean channel using LND150's to my project amp (basically a hot rodded jcm800). Relay switching works great and the clean channel sounds good BUT it's way too loud. It pushes the output tubes into breakup at around 1.5, past 2-3 it starts to feedback. It's a very simple circuit with a tone and volume pot - I've attached the schematic.
Any advice? I'm still new at this and I'm thinking its something simple I'm overlooking.
Thanks,
jay
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Yeah not really up to speed with this, but sub the 220k/220k voltage divider with a 500k pot and dial it down see if that helps.
If it does you can replace the pot with resistors.
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I'd just replace the bottom 220K with a 47K. Decrease the 47K even more if still too loud. Increase the 47K if not loud enough.
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Hi guys,
I just added a clean channel using LND150's to my project amp (basically a hot rodded jcm800). Relay switching works great and the clean channel sounds good BUT it's way too loud. It pushes the output tubes into breakup at around 1.5, past 2-3 it starts to feedback. It's a very simple circuit with a tone and volume pot - I've attached the schematic.
Any advice? I'm still new at this and I'm thinking its something simple I'm overlooking.
Thanks,
jay
I couldn't help but notice your name jayfitt - using j-fets... coincidence??? Or are you the j-fet jedi??? :icon_biggrin:
By the way how do they sound compared to the tube's sound? Used in an amp situation rather than a pedal are they as "tube-like" as reported?
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You have way too much gain. I could overdrive the first stage with just a guitar and the second stage looks like it will clip before the power tubes (Unless you use a really high voltage power supply).
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I couldn't help but notice your name jayfitt - using j-fets... coincidence??? Or are you the j-fet jedi??? :icon_biggrin:
By the way how do they sound compared to the tube's sound? Used in an amp situation rather than a pedal are they as "tube-like" as reported?
I need to do some tweaking but it sounds good. I went with the lnd150 because it's in a prefabbed 16.75" JTM45 style chassis and there's not much room for another tube.
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You have way too much gain. I could overdrive the first stage with just a guitar and the second stage looks like it will clip before the power tubes (Unless you use a really high voltage power supply).
Thank you! Have you built this? What's the purpose of the zeners?
jay
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Have you built this? What's the purpose of the zeners?
I haven't built the whole thing and put it in an amplifier, but I have built each section by themselves and I am dead certain that they will work when combined as shown.
The manufacturer states that the Gate-to-Source has a maximum of + or - 20V. I gave it a try and, sure enough, it destroyed the device when I exceeded 20V. The Zeners prevent this method of destruction in the event that somebody is inventive enough to exceed the maximum. The Zeners are nowhere near breaking-over during normal operation, so they have no effect on the signal.
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Hi,
How about this one? It is well proven circuit, at least by me;)
Actually, this is german T-T schematic.
It works within SE amps, as well.
My friend provided it into TW replica with success.
Regards,
Adam
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Note that the fx return gain stage Q2 has an inverting amp feedback loop around it, formed by R5 and R4.
That will reduce the gain a bit, help to linearise things, but also reduce the input impedance to ~ R4.
A basic LND150 gain stage only needs to comprise Q2, R6, R7, R8 and R9, C5 and D3 and D4; plus whatever dc blocking caps are necessary.
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Note that the fx return gain stage Q2 has an inverting amp feedback loop around it, formed by R5 and R4.
That will reduce the gain a bit, help to linearise things, but also reduce the input impedance to ~ R4.
A basic LND150 gain stage only needs to comprise Q2, R6, R7, R8 and R9, C5 and D3 and D4; plus whatever dc blocking caps are necessary.
To tell you the truth, I was sceptical very much, when heard this idea of mosfet fx loop.
I am not that electronic analyst as you are, but I can reflect myself refering to my ears and they tell me that this loop works fine, at least...
I was looking for solution of loop dedicated to SE amp.
I've made T-T schematic based pcb and applied it into the amp.
I did not notice any loss of an amp's tonal character. Both reverb and delay boxes within this loop sound very good. Contrary, the amp seems to be better with loop on(switch on/off).
I spite of it, ther're two pots on board-send/return. With these two, the signal through the loop can be easily adjusted.
Same circuit works quite well in TWExpress of my friend, as well.
So, I still recommend this loop, just as the experiment, nevertheless your remarks;)
Regards,
Adam
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Yes, the TT fx loop is fine for its intended purpose, my comments were in regard of how its circuitry might be adapted in the context of the thread, ie jayfit was looking to use LND150 gain stages to construct a useful clean channel for his amp :icon_biggrin:
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Upsss...you're right. Sorry. Silly me;)