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'Modding AmpMaker donor to HO / L2L - advice needed'
Author:tristanc (registered user: 162 posts )
Date: Mon, Aug 10th, 2015 @ 02:49 ( . )

I'm in need of some advice for modding my amp to HO or L2L spec. Given the lack of AX84 kits readily available, the information here may be useful for others.

The starting point is an AmpMaker N5X, very similar to a P1-eX, but 5W with VVR and being able to use both EL84s and octal valves.

This is a great amp as stock, perfect for living room playing. But I found myself needing to use an EQ pedal and boost in front to get the level of OD/distortion I wanted. Ideally, I'd have this in the amp and be able to dial in cleaner sounds with the guitar controls - my dream is to have just amp, cable and guitar. I play blues, classic rock, 90s rock, indie etc. Nothing too extreme - anything else I can use pedals.

Of the AX84 designs, both the HO and L2L seem to fit the bill. So the challenge is to modify the N5X to have the additional gain stages of these designs.

I've had a go at grafting the circuit designs of the HO and L2L on to the N5X. The two files show the new or modified areas shaded in pink.

L2L: [link] this involved inserting the 3rd stage between the existing 2nd stage and tone stack, then the 4th stage after the tone stack.

HO: [link] the 3rd stage and CF are inserted between the 2nd stage and tone stack. NOTE - I've copied the wiring ciruit for the 1st gain control from the L2L schematic, R51 connecting lug 3 to just after C54.

There are a few component changes that would need to be considered (R_ks and pot values).

I've the following questions intially:

- How do I supply power to the new / existing stages? Can I run a connection from 'E'? Or do I add a resistor & cap (R60 & C53) and make 'D'? The former is easier!
- Are the bright / fat selector and tone stack bypass more or less redundant?
- Given the voltages indicated, would there be additional bias changes needing to be made to get a similar sound to the 'stock' L2L / HO?
- Is the change in HO wiring described above a good idea? I like the idea of keeping the volume similar whilst OD increases or decreases (again, perfect for living room playing).

Thanks for any advice!

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'Modding AmpMaker donor to HO / L2L - advice needed'
Author:tristanc (registered user: 162 posts )
Date: Thu, Aug 13th, 2015 @ 00:40 ( . )

An update and request for further help.

I've had extensive input from Brian S and Barry at AmpMaker answering some of the questions above and more. An adjusted schematic is here: [link]

I've gone down the L2L route, with the tone stack at the end (like the Super eXtreme). There are slight component value differences (e.g. 4th stage bypass cap value).

The power for the extra stage is taken from a new node inserted between R19 and R20 giving the same voltage to the entire preamp but the stages decoupled - this can be altered easily in the layout to allow a drop if this doesn't sound good.

The advantage of my current schematic is that I could potentially return to the stock N5X design by:

1) tapping off the signal between R7 and VR1
2) adding back the original 470k log pot (so another control on the front panel, a 'low gain circuit'?)
3) re-inserting the signal after R64

In addition I'd need to move the cathode bypass switching from V1b to V4b. This has the advantage of making the layout slightly easier with respect to valve locations and keeping wiring to a minimum.

The questions I have are:

1) Grid stoppers - I've kept 15k resistors on V1a and V1b. I included an additional 15k to V4a. And was wondering if it was suitable to add a 15k to V4b? Would I then reduce the value of R64 accordingly?

2) If I forget about keeping the original circuit as an option, what cathode bypass switching options would be recommended? I can re-use the existing switch, but which stages to apply it to?

3) If I do want to keep the lower gain circuit and do the tapping described above, do I need to worry about the un-used V1b and V4a when they are switched out?

Thanks

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'Modding AmpMaker donor to HO / L2L - advice needed'
Author:Merlin (registered user: 5235 posts )
Date: Thu, Aug 13th, 2015 @ 04:17 ( . )

If it were me I would try to keep things as simple as possible. i.e. keep the N5X circuit exactly as it is, then simply bolt the two extra gain stages on the front (with a gain pot between triodes 1 and 2). Yes, this would mean you end up with two gain pots, but that also means a lot of tonal flexibility.
This woudl make it trivial to go back to the original N5X. You could even have a switch to toggle the input jack between the original N5X or the new 4-stage design.

Anyhoo,
1) Why would you need to add a 15k stopper to V4b when R64 already does the same job?
2) Eat your heart out, but I would say second stage has the most effect.
3) Not likely if you ground the input to the unused amp section.

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'Modding AmpMaker donor to HO / L2L - advice needed'
Author:tristanc (registered user: 162 posts )
Date: Thu, Aug 13th, 2015 @ 05:27 ( . )

Thanks Merlin - point taken on the grid stoppers. The 15k was an effort to keep the circuit close to the N5X.

Interesting point on where to insert the 'new' bits. In outline fashion:

Stock N5X
Input -> V1a -> G1 -> V1b -> TS -> MV

My schematic, with Brian's L2L feedback from G1 to before G2
Input -> V1a -> G1 -> V1b ->[ G2 -> V4a -> Voltage Div -> V4b ]-> TS -> MV

Merlin's suggestion, L2L feedback from G2 to before ?
Input -> [ V4a -> G2 -> V4b -> ? ]-> V1a -> G1 -> V1b -> TS -> MV

I guess the "?" would be a voltage divider giving 2 controls as in the HO & L2L, except the 2nd is later on in the path. Or "?" could even be another gain pot to match the L2L better? That way there'd be 3 interstage controls. Could even make G1 switchable in conjunction with selecting the lower gain path.

My efforts to re-draw a layout, keeping spatial constraints & valve locations etc in mind, are ongoing. Without making major changes (I've no suitable tools to make chassis mods etc) I fear it will come down to what fits in the space I have, trying to keep to as many good practices as I can!

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'Modding AmpMaker donor to HO / L2L - advice needed'
Author:Merlin (registered user: 5235 posts )
Date: Thu, Aug 13th, 2015 @ 23:40 ( . )

Three gain pots might be overwhelming even to the most discerning tone tweaker! I would go with a voltage divider. Basically, I was bending the topology towards the UHG preamp from my book.

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'Modding AmpMaker donor to HO / L2L - advice needed'
Author:tristanc (registered user: 162 posts )
Date: Wed, Aug 19th, 2015 @ 12:02 ( . )

I've had another go implementing Merlin's suggestions: [link]

All the component numbering has changed for the new sections, and I've added in heater elevation (today's distraction, which hasn't lowered my 50/100Hz hum, but oh well: [link] ).

The two remaining concerns:

1) The value of R2 - I've kept it at 15k, so with S4 switching out the 1st two stages we are back to stock. However, Barry S's suggestion for voltage divider values would make it 470k (following the Super eXtreme schematic). The UHG circuit doesn't have grid stoppers following a voltage divider. What would be an appropriate value? Or is there a bit of trial and error here?

2) The switch S4 - I've copied the HO R/L mod to get me back to stock. But will this still work given the grid of V4b isn't grounded? Or will the feedback via R56 allow this? Any other considerations for this switch?

If I can get these bits (and any others that pop up) then I can start on making it all fit in the space I have: ~250mm by 50mm of turret board.

Thanks,

Tristan

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'Modding AmpMaker donor to HO / L2L - advice needed'
Author:Merlin (registered user: 5235 posts )
Date: Mon, Aug 24th, 2015 @ 01:25 ( . )

On 08/19/2015 @ 12:02, tristanc wrote :
Barry S's suggestion for voltage divider values would make it 470k (following the Super eXtreme schematic). The UHG circuit doesn't have grid stoppers following a voltage divider. What would be an appropriate value? Or is there a bit of trial and error here?

I would suggest you install 470k. Then you can tack in another resistor in parallel during testing, to refine the total value to whatever sounds best.


: 2) But will this still work given the grid of V4b isn't grounded?

Hmm, I would suggests using a 2-pole switch and use the second pole to short out R60 when in stock mode. This should prevent any signal bleed across the switch from the unused circuit block.

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'Modding AmpMaker donor to HO / L2L - advice needed'
Author:tristanc (registered user: 162 posts )
Date: Mon, Aug 24th, 2015 @ 03:56 ( . )

Thanks Merlin - really appreciate the input.

I've updated the schematic with what I think you mean [link]

I guess the switch could be the other side of R60 too, which may help out with layout considerations. I've also indicated the initial shielded wires (following HO / L2L schematics) and corrected an error on the heater elevation (resistors in divider wrong way round).

I can see I'll need to alter the values of R2 as you suggest. But I also guess there will be an element of experimentation with values of R_k for stages 1 and 2 and perhaps R_p. Reading other posts on the UHG design, it goes Warm, Cold, Cold, Warm. Whereas the L2L seems a bit 'warmer' throughout. Another layout consideration to make sure I can easily clip in other resistors initially.

I'm still iterating my turret layout to fit everything on. Then we'll get on to the build! I'll try and document it as best I can for the benefit of others potentially wanting an HO / L2L / UHG design from a readily available kit.

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'Modding AmpMaker donor to HO / L2L - advice needed'
Author:Merlin (registered user: 5235 posts )
Date: Mon, Aug 24th, 2015 @ 23:38 ( . )

On 08/24/2015 @ 03:56, tristanc wrote :
I've updated the schematic with what I think you mean [link]

Almost: S4b should be in parallel with R60. [link]
You don't need C9 (although it won't do any harm either).

I can see I'll need to alter the values of R2 as you suggest. But I also guess there will be an element of experimentation with values of R_k for stages 1 and 2 and perhaps R_p.

Yes, definitely. You can't predict the best values on paper alone.


Reading other posts on the UHG design, it goes Warm, Cold, Cold, Warm.

Sort of. It's kinda: warm > cold > less cold > warm, but that doesn't mean exactly the same thing will work in your case.

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'Modding AmpMaker donor to HO / L2L - advice needed'
Author:tristanc (registered user: 162 posts )
Date: Wed, Aug 26th, 2015 @ 02:25 ( . )

Thanks Merlin - that makes more sense. Learning this as I go! I'll likely clip/tie in values for R_k on stages 1 and 2 and for R2 for the 1st while and see how I get on.

The below is an updated schematic, with a proposed wiring of S4 indicated. (I've left the extra cap in to remain 'stock' if there is no harm...)

[link]

Two things trouble me:

1) Power supply to the 1st two stages - I have it to be the same voltage as stages 3 and 4 (D = C). However, I can't find a single instance of other schematics doing this. I did it this way to try and keep the voltage levels to those in the schematics I was aiming for.

I guess this voltage lowering is done to vary the load line / bias point location for tonal reasons? Mix it up a bit?

It's a very easy change to make so could be (yet) another regime of multivariate testing. But I'd rather not buck the trend needlessly!

2) Shielded wire grounding locations - is it OK to tie the ground from the wire to any close ground location? e.g, the pin to ground on S4 could be used to have the grounding for the wires from input and from R59. Things are going to get pretty cramped if so. I was thinking I could run the shielding to the ground rail on the board with a short stretch of jumper. Or can the shielded wires from the input jack (running to S4 and R51) both be connected together at the jack? In effect, making one long stretch with a gap in the middle.

I've also had a go at a layout: [link]

It took a while but I managed to fit everything in. The take-off points to the valves are all on the top with turrets for the grid resistors close to each valve, so no need for extra tag strips. Grounding is along the bottom. The light blue holes are for routing wires under the board and bringing them up close to where they need to be. Hopefully this will make it easier for me to alter components later on without burning through stray insulation...

There is ~6cm of cantilevered board stretching beyond the mounting points (orange) on the right. I reckon this is OK structurally (using PTFE - if there is a large degree of flex over the years it will be towards the cabinet, and the power caps insulation will hit it first) but there is space to add new mounting holes if required (not sure how I'd get new holes in the chassis with the equipment I had to hand...).

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'Modding AmpMaker donor to HO / L2L - advice needed'
Author:Merlin (registered user: 5235 posts )
Date: Thu, Aug 27th, 2015 @ 01:16 ( . )

On 08/26/2015 @ 02:25, tristanc wrote :
1) Power supply to the 1st two stages - I have it to be the same voltage as stages 3 and 4 (D = C). However, I can't find a single instance of other schematics doing this.
: I guess this voltage lowering is done to vary the load line / bias point location for tonal reasons?

No it is usually arranged as a chain of PSU filters to get better ripple reduction, and ease of layout. It probably doesn't matter either way though, in this case. The exact voltages have little effect on tone if the supply is above 250V or so.


: 2) Shielded wire grounding locations - is it OK to tie the ground from the wire to any close ground location?

In a small amp like this, yes it is probably OK to use random grounding onto your bus bar (but keep away from the reservoir cap).


: I've also had a go at a layout: [link]

It looks OK to me, although the link to the circuit seem to have stopped working for me?

Incidentally, I have found my turret board material for you.

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'Modding AmpMaker donor to HO / L2L - advice needed'
Author:tristanc (registered user: 162 posts )
Date: Thu, Aug 27th, 2015 @ 05:55 ( . )

On 08/27/2015 @ 01:16, Merlin wrote :
: It looks OK to me, although the link to the circuit seem to have stopped working for me?


Hmm, strange - I've uploaded a newer version just now: [link]

In the interests of keeping the preamp supply voltages closer to 250V I think the way I have it may be best (both stages at 235V).

For the grounding - there are 6 differing grounding points; the reservoir cap(s) C16 / C17 are on '4', whereas I'll keep any of the shielded runs linked to 1 (at the input) or 2 (serving the preamp).

As this is a tad more complex as my first N5X build I'll be taking my time and getting things as organised as I can before start soldering things!

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