Resistive to Reactive Load Box Upgrade

[clipless bumper]

what makes that reactive - the capacitor?
I thought you needed a speaker motor (magnet and coil) to make a reactive load??

[Sascha Franck]

I thought you needed a speaker motor (magnet and coil) to make a reactive load??

That's how the Weber does it.

Sorry but that is NOT how the Weber does it and your whole idea is simply not going to work.

[Sascha Franck]

Well, you can go through with it but in the end i'm going to say 'told you so'.

[Sascha Franck]

Childish.

[Zozobra]

I'm interested in why you think it won't work.

[Sascha Franck]

Look at the numbers near resonance resistor.
Or do you want me to spill it out?

[clipless bumper]

Spill it Sasha

[Zozobra]

I'd prefer a rational explanation if that's alright with you. I've modelled similar circuits in LTspice and the impedance responses look fine to me.

[Zozobra]

That inductor is unexpectedly small

[Zozobra]

I think most people just use a heavy duty switch. Maybe parallel the terminals on a DPDT to make it a bit more robust? It should only be problematic if the switch is thrown while there is signal but even then I don't think it's a big deal. I've yet to get the parts together to build a reactive load though. I do have a bunch of 50W dale or arcol aluminium clad resistors to build a resistive load but I've not had the need to put it together yet and I have other projects to clear first. I've been dicking around in LTspice modelling the circuit to fit what I coils I can buy easily. The resonance network of 220µ/20mH can be replicated pretty well with 440µF/20mH with just a small loss of Q and I'm having difficulty finding suitable 20mH coils in euroland without paying extortionate shipping. I could always wind one myself I guess...

[Zozobra]

I'm having difficulty finding them without paying out of the ass to get them to my door. I have found a uk winder that winds on 1mm and 1.25mm diameter wire which is a little less than 18 and 16 AWG respectively so they may do the trick.

[Zozobra]

Yeah I was looking there a few days back and got annoyed by that. It's still possible to open the image in a new browser but it's a pain in the ass to do.

[Zozobra]

I'm gonna try a 200W resistive just to see if it's worth the effort or sourcing the damn coils If it's okay in the mix I'm happy.
I do have a some smaller amps I can build a cheap reactive load for too. A 4W ecc99 and a 15W 6v6 amp wont need big coils at least.

[Zozobra]

I need to summon the motivation to finish rebuilding the ecc99 amp. I've built it a whole bunch of times using a toroidal PT but the chassis was cramped and a pain in the ass too work in so I bent a simple chassis to play around in. I'm using a ZVS boost converter to produce the B+ from a laptop PSU here.

[Ostinato Rubato]

Sascha disapproves.

[Zozobra]

This has got me looking at what I can pick up from euro component houses again. I was searching for info on the AMT power eater, as that is a little more involved than some of the other options out there as it features selectable resonant peaks and presence profiles, which I guess they'd achieve by switching in an extra coil for the presence and caps for the resonance. This dropped me on a russian page discussing the unit and someone posted a picture of the business part of a DIY reactive load using Visaton air core and ferrite x-over inductors, both of which can be sourced fairly cheaply from cpc/farnel/rs etc.



0.47mH air core, 1mm diameter wire is approx 16AWG, so should be sort of equivalent to the ERSE stuff, £6.68
http://cpc.farnell.com/visaton/5009/ind ... dp/FT01417

10mH ferrite, 4.1A saturation current, £11.22
http://cpc.farnell.com/visaton/3708/ind ... dp/FT01404


This might be useful to you if you want to use a 1% power passthru to a cab. It's in russian but google will take care of that:
http://forum.amtelectronics.com/index.php?topic=33.0

[Zozobra]

Yeah i was planning on homebrewing my own bipolars using electrolyrics.

I did some LTspice modelling using the inductors I can get cheaply from cpc. I went with the option of 2 6,8mH, 7.5A resonance in series so I can switch one out so I can move the peak from 95Hz to 130Hz.

[Zozobra]

Nothing in particular, just working with what I can get off the shelf. I probably need to use a 1mH for the presence inductor with a 16 ohm load box come to think of it. May use two 0.47mH in series so I can switch one out to get two presence controls.

[Zozobra]

It becomes a rabbit hole of switches pretty quickly if you're not careful. I just want something functional for minimal cost.

I'm thinking of three switches; 1 to bypass the reactive elements all together; 1 to switch in a series presence inductor; and 1 to switch in a resonant peak inductor.

That should give me enough to dial in as I need to suit the amp and IRs.

[Zozobra]

I'd be amazed if any of the units out there used anything particularly fancy. Just don't throw it while running a Rat into a dimed model T

[Zozobra]

Tube town do 50 & 100W reactive load kits for a pretty fair price. No enclosure, but they're cheap enough.
They seem well received from what I can see and those inductors looks pretty small so I'm guessing they're ferrite core.

https://www.tube-town.net/ttstore/Kits/ ... anguage=en

[Zozobra]

Nah, we can get mouser stuff here. I ordered some real small eyelets from there a few years ago to mount relays on turret boards.
I get most of my stuff from either RS (who sell chinese valves incredibly cheap) or cpc/farnell.

RS sell these real slick Takachi enclosures which have the front/bottom part painted black, and the top part brushed and anodised. I'll probably use one of those. I want to make some ZVS based preamps in them because they'll look tits

[Zozobra]

I'm gonna risk it with the visaton x-over parts I think. I'm not going to be running a cranked NMV amp into it so I don't really need a huge amount of current handling by the inductors. My AOR at about noon is probably only passing 30W RMS with the odd big ass transient which shouldn't saturate the core. The 1mH air core should be okay since it'a about the same AWG as the ERSE that everyone uses. The 10mH ferrite coil is good for 5.2A before saturating, which unless I'm passing 100W at 100Hz should handle it. if not then maybe I'll try and import the super Q.

I'm currently pondering the best way to skin the main load resistor. The huge ass 150/200/300W resistors usually have fairly shabby power handling if they're not mounted to a heatsink so I may be better off using 6x10r 50W in 3xseries/parallel for 15r. I'll probably spec enough space to add a fan to get some forced convection going in any case.

[Zozobra]

Yeah, that'll do it

[Zozobra]

Looking forward to hearing the results man.