AMT - Legend Amp Series 2  [traced]

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Post by bajaman »

Hi ice-9 - what value are c20, C21, and C22 please?
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Post by bajaman »

R34 should connect to the junction of C24 and C8 not the drain :wink:
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Post by bajaman »

How sure are you that VT9 source connects to ground ? It will not bias correctly for me with my simulation unless I add a 3.9k resistor in the source to ground :hmmm:
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Post by Ice-9 »

bajaman wrote:Hi ice-9 - what value are c20, C21, and C22 please?
C20- 21-22 are all 150p polystyrene caps. (150j 30v) markings

thanks fixed R34 on my drawing
The source of VT9 is connected directly to ground through a via which I have marked in green on the picture.

Could it be that the drain resistor needs altering to suit the jfet simulation model to correctly bias vt9. Could I ask what simulation software you use Bajaman, I have TinaTI which I am just learning to use.
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vt9-traces.jpg
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Post by bajaman »

Thanks ice-9 :D
I am using NI multisim V14. I still cannot understand how the first and third jfets in the cabsim section are biased though. the second jfet is biased by the 3.3m resistor connected to the dedicated half rail voltage 100k/100k, and the last jfet derives it's bias through the third stage drain resistor and the 1.5m series resistor to it's gate. there is simply no way the first and third jfets can be biased on with both sources connected to ground and no -ve bias on the gates :shock:
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Post by Ice-9 »

I see what you mean, I was thinking VT9 was biased through the 5M1 R30 from gate to gnd but as the source is directly connected to ground as well it can't bias ?? and VT4 I can't understand how that will bias at all.

I am currently drawing it into Tina with the correct values but I still have not completed the sim schematic yet,

I have been trying to find VT6 and VT8 so that I can complete this part of the circuit.
I think
VT8 is a 2SC2883 npn
VT6 is a MMHZ5261 Z- diode.
but I am a little unsure on these2 components.
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Post by bajaman »

VT6 and VT8 - i used a j208 for the top and j177 for the bottom - biased up fine but I suppose an NPN PNP combination would work as well. What are the smd codes on these parts ?
Incidentally, substituting BC857 npn bipolar transistors for the first and third stages biases properly as emitter followers, although the codes clearly show JR (2SK208) in your photos.
Also i cannot see an obvious connection for R25 and C22 to ground - other side of the board perhaps ?
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1:51am here - off to sleep now.
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Post by Ice-9 »

Yes both R25 and C22 are connected to the gnd plane on the top side of the pcb.

I have used n and p type jfets for the for VT6-8 as you suggested.

VT6 = 61S C
VT8 = GY P

I think I have completed the bottom PCB with cabsim, so posting this schematic for now. I think I have all the correct values in place :scratch: (J201's marked on schematic should be 2SK208)
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AMT S2 BOTTOM PCB.JPG
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Post by Ice-9 »

The clean boost chl is pretty much correct as in the schematics above, here is a hand drawn version with the correct component values. I won't be able to look at anything else until I get back from the TT races in about a week so this is it for now. :(
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clean-ch.jpg
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Post by bajaman »

Hi ice-9 :D
two corrections - R43 is 100k and C12 on the cabsim is 10n
I changed the VTO value in my spice model for the 2SK208 to more accurately model the low idss type fets that AMT are using. The standard model is -2.5v - i lowered it to -0.65v and now the cabsim biases correctly plus i get a much better response curve and output level. For anybody contemplating actually building one of these - the JFETS used are CRITICL to the performance - you simply cannot just "bung" any old J201, 2N5457, MPF102 ,2SK117 jfets in and expect the same sound or response characteristics UNLESS you are prepared to spend a lot of time adjusting the capacitor and resistor values to suit them. AMT use mostly the 2SK208-R type jfet which has the lowest idss classification (as well as some other types like the J177, 2N5459 ?). It looks like AMT use the smd version of the 2N5459 for the first stages of the high gain amp and clean channel sections and possibly one half of the cabsim output buffer section along with the J177 :hmmm:
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Post by Ice-9 »

Hi Bajaman

Thanks for those corrections, C12 in the cabsim should actually read C13 and is as you say 10n while C12 is a 100uf electro and is connected to the switched power for the cabsim fets. I will have to check over the drawings again to make sure all components have the correct labelling and values as the pcb's, and will udate that when I get back.

It is good that you can now get the fets biasing correctly by adjusting the spice model, did you use the specs directly from the data sheet of the 2SK208 for the R selected fet. Also are you taking any other spec into the spice model like lambda and Beta ? I have a handful of 2SK208-R smd fets so I might give it a go sometime.
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Post by bajaman »

On second thoughts - the 2N5458 is the correct jfet (code: 61S) - not the 2N5459 (code 61T) :oops:
Of course being smd on the AMT board it is MMBF5458 :wink:
Yes - I just adjusted the 2SK208 spice model in the multisim library - I did not change any of the other dat as it was really only the vgs and lower idss that needed to be adjusted. i think the multisim library component more closely matches the 2SK208-Y (or even the GR). The 2SK208 is available in smd as JR, JO, and JY - i could not find a reference for the JG or JGR code. JR has an idss spread of 0.3mA to 0.75mA - i adjusted my model to 0.65mA (although the circuit biases well with anything from 0.4mA to 0.75mA - 0.65mA gave the nicest response curve for the cabsim section.
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Post by bajaman »

just for fun I tried replacing the 2SK208-R in the high gain amp section immediately following the gain control with a J201 (in multisim :wink: ) The J201 has the same spice model VTO as my 2SK208-R spice model, BUT it has twice the BETA value and 20% of the Lambda value, so quite different transfer characteristics obviously :wink:
My simulation showed (as expected due to the double BETA (transconductance) value) a huge increase in gain across the whole bandwidth - typically 10db or more :!: AND a slight increase in low frequency gain below 70Hz. After adjusting the source resistor and capacitor values I was able to get similar response characteristics to the 2SK208-R. The original source values were 1k8 and 1uf in parallel to ground. The modified values that gave best results with the J201 were 2k7 paralled by a series connection of 1k and 560n from source to ground. I did not bother trying the 2SK117-Y ( VTO: -0.43v) because its BETA was over twenty times that of the 2SK208-R :shock: and this factor alone would require major component changes including the drain resistor etc.
Clearly these AMT pedals have been specifically engineered for the use of the 2SK208-R
I am looking forward to receiving my E2 pedal and will post pictures very soon. In the meantime it certainly looks like xbananov has totally redesigned these series 2 Legend Amp pedals and it looks and sounds like he has done a great job :applause: at a most affordable price too :D
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Post by bajaman »

After more research it appears that there is a direct through hole substitute for the 2SK208-R that allows the use in these circuits without the need to modify the source resistors. The device is the 2SK30ATM-R. Please note: It MUST be the R type idss NOT the O, Y, or GR for it to work correctly.
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Post by J0K3RX »

bajaman wrote:After more research it appears that there is a direct through hole substitute for the 2SK208-R that allows the use in these circuits without the need to modify the source resistors. The device is the 2SK30ATM-R. Please note: It MUST be the R type idss NOT the O, Y, or GR for it to work correctly.
cheers
bajaman
Yes, that's correct.. I believe you can also use 2SK879-R but good damn luck finding any of them.. I found 10 2SK30ATM-R on ebay by a US seller a long time ago but he only had 10 and was asking a stupid high price for them..

10 pcs.. for $5.80 not bad at all but kinda suspicious of ANYTHING coming from China! I guess I could take the hit if they were fake... not like $5.80 and shipping would send me to the poor house.
https://www.ebay.com/itm/10PCS-TOSHIBA- ... 5d40ebcea2

By the way, sorry for the jacked up schematics, found them on guitar-gear.ru and didn't even really look at them before posting :roll: At least if anything it sparked interest and we may get a good schem out of it eventually especially with bajaman and Ice-9 on it! :wink:

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Post by bajaman »

Hi JOK3RX :wink:
No apologies necessary for the schematics mate - it appears that they are 99% correct - just missing the cabsim power supply muting circuit and switch wiring etc. When I say "correct" I am referring to the schematic NOT the component values - these do not seem to bear any resemblance to the correct component values - cannot understand why :scratch:
If you find any of the other series 2 schematics, please do post them, regardless of incorrect component values - cheers bajaman
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Post by Ice-9 »

bajaman wrote:Hi JOK3RX :wink:
No apologies necessary for the schematics mate - it appears that they are 99% correct - just missing the cabsim power supply muting circuit and switch wiring etc. When I say "correct" I am referring to the schematic NOT the component values - these do not seem to bear any resemblance to the correct component values - cannot understand why :scratch:
If you find any of the other series 2 schematics, please do post them, regardless of incorrect component values - cheers bajaman
As Bajaman says no apologies necessaryfor the schematics, they are very close and proved a great start to work with.

As for them all having the wrong values, that is most likely down to whoever drew they schematics just used a copy and paste for each component when adding them to the schematics and never changed the values., notice quite a lot are all the same value.

I am back from hols so as soon as I can get a chance I will get back onto finishing the S2 trace. :)
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

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Post by bajaman »

Great news - my E2 arrived today - initial play test very encouraging - very low noise level too, no hiss monster :wink:
Very well constructed pedal and well worth the price - i bought it online on Ebay from a seller in St Petersburg.
I will wait until Ice-9 finishes his trace of the S2 before posting pictures and schematic for this one - the cabsim, power supply and clean amp section will be identical and i suspect the topology (but not component values ) will be the same as the E2 in JOK3RX's first pot in this topic.
cheers
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Post by Ice-9 »

@Bajaman - no need to wait until I have finished as having just got back from hols I have to catch up on work orders so it will be a couple of days before I finish the S2, so when you are ready to post your E2 just go ahead.

I agree also they are well built pedals and well worth the purchase cost.
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

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Post by Mbas974 »

Guys, I can not clearly recognize the power " supply muting circuit " on the circuit...
Can you pls highlight it please ?

will It be usable in any High Gain circuit like could be a BSIAB ?

Tx

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