New Airbrake III

[traktop]

I´m going to build an airbrake III, (the one with the gold 8ohms 25watts resistor and no rheostat like the picture below), for taming my ´73 hiwatt dr-103 but I´ve got some doubts:
-I use to play with the master volume set at 9 o´clock, and I just want to rich the 12 o´clock to get more output stage saturation, (not planning on cranking the amp, but one never know...), but I readed somewhere over here that using resistors with double the power of the amp would be convenient for safe operation. I was wondering if I should look for 25 ohms 200 watts instead of 100watts for the variable resistor and shunt.
-I was planning on not using the lorling switch because it cannot handle high voltages. Instead I would use this one above, but I´m a little confused about how to "clone" the lorlin connection to the new switch, since I see 2 lugs in some of the possitions. What are them? Are they like the poles on the lorlin? Anyway, If someone could confirm that the Lorling works with no problem, I would go for it.
-Since Mouser doesn´t have the shunt on stock, and I would need to attach another lug to it, I´d like to know if another 25 ohms adjustable resistor, (like the one that goes with the 3 lugs), would do.
-The gold 8ohms 25watts resistor and the new lug on the shunt are for better resistive matching, (to 8ohms I guess). Since I´ve got a 2x12 350watts 16 ohms cabinet, what amp tap should I use? 8ohms tap maybe?
-Since I readed that some experienced a high frequency roll off, I was thinking on introducing a bright switch with a couple of caps with different values, so I could use it anytime . I was wondering what place should I introduce it on the chain.
Thanks in advance.

[img768]http://i209.photobucket.com/albums/bb15 ... G_1160.jpg[/img]
[img:800:598]http://binrock.net/permanent/2012/0625_airbrake/03.jpg[/img]

[Littlewyan]

Do you know the manufacturer? Make? As sometimes they have diagram's on their website or the supplier will have a datasheet/diagram available. Have you tried testing for continuity with your meter to see which pins are connected to what? That's your best bet, sometimes there will be 4 pairs of pins connected to each other or 3 pairs if you understand what I mean. I'm not sure about the two pins together though.

[traktop]

Thanks.
I took a look to the pdf data sheet, but It looked confusing to me. I finally ordered the Lorlin switch, (since there are many diagrams and pictures of that build), and 2 adjustable resistors, (I hope that will work...).
The placement of the cap is what isnt so clear to me.
Cheers, Gilberto.

[Littlewyan]

The cap on the original airbrake was for the bedroom control only and goes across the rheostat/pot in the same way as a bright cap in a Marshall or Express. Did you want it for the entire attenuation range?

[traktop]

The cap on the original airbrake was for the bedroom control only and goes across the rheostat/pot in the same way as a bright cap in a Marshall or Express. Did you want it for the entire attenuation range?

Well, having a 3 way switch with a couple of caps of different values, so I could choose anytime between normal/bright/brighter, was what I had in mind. I readed about people getting a treble roll-off on last steps of attenuation, but it seems that the gold resistor and the new adjustable lug on the upper tubullar resistor helps in that matter, so maybe I won´t need it.
Best regards.

[Littlewyan]

Yer I would see how you go without it first.

[traktop]

Ok, already got mouser parts and It´s almost done, but although the lugs are at the right place, (6, 11 and 16 ohms), I didn´t do any general measurement yet. I heard that I need to measure with the the cabinet connected to the output of the attenuator, that way I could check what the amp would see on every click, right¿. What 2 pins do I have to measure¿.
I got the new alpha lorlin with a switch trick washer/latch that allows reduce the number of clicks to 5 for example. Since I realized that all diagrams share clicks 1 and 2 , (att. off for the first 2 clicks), I guess that I could erase number 2, remaining 4 levels of attenuation and 1 bypass click. Or maybe I could use 1 remaining adjustable lug, (I ordered a couple more than needed), for an additional attenuation level between the 11ohm and 25 ohm lugs.
It is supposed that the "no rheostate lay-out" favours a 8 ohms load. I´m a little confused, because my cabinet is a 16ohms load, and I readed someone bringing up the original airbrake instructions, which said that I should use the 8ohms tap of my old hiwatt on the 2 first levels of attenuation, (but maybe was because there were big mismatches on the original airbrake design, and this new design that I´m using solves that).
The thing is that I´m currently running the amp with 2 tubes pulled, so in order to match the cabinet impedance the amp tap is on 8ohms. I guess that I should use the 4ohms tap for using it with the airbrake, right¿

[traktop]

Ok, already got mouser parts and It´s almost done, but although the lugs are at the right place, (6, 11 and 16 ohms), I didn´t do any general measurement yet. I heard that I need to measure with the the cabinet connected to the output of the attenuator, that way I could check what the amp would see on every click, right¿. What 2 pins do I have to measure¿.
I got the new alpha lorlin with a switch trick washer/latch that allows reduce the number of clicks to 5 for example. Since I realized that all diagrams share clicks 1 and 2 , (att. off for the first 2 clicks), I guess that I could erase number 2, remaining 4 levels of attenuation and 1 bypass click. Or maybe I could use 1 remaining adjustable lug, (I ordered a couple more than needed), for an additional attenuation level between the 11ohm and 25 ohm lugs.
It is supposed that the "no rheostate lay-out" favours a 8 ohms load. I´m a little confused, because my cabinet is a 16ohms load, and I readed someone bringing up the original airbrake instructions, which said that using a 16ohms load I should use the 8ohms tap of my old hiwatt on the 2 first levels of attenuation, (but maybe that was because there were big mismatches on the original airbrake design, and this new design that I´m using solves that).
The thing is that I´m currently running the amp with 2 tubes pulled, so in order to match the cabinet impedance the amp tap is on 8ohms. I guess that I should use the 4ohms tap for using it with the airbrake, right¿

[vibratoking]

It appears there is quite a bit of confusion on this build. You should be aware that the max power rating on the wirewound resistor only applies if you use the whole resistor. When you slide the taps to lower resistance points, the power rating has to be derated as well.

[traktop]

It appears there is quite a bit of confusion on this build. You should be aware that the max power rating on the wirewound resistor only applies if you use the whole resistor. When you slide the taps to lower resistance points, the power rating has to be derated as well.

I´m following this schematic that seems to work well indeed:
http://ampgarage.com/forum/download/file.php?id=11515
I didn´t take the readings from the input to the output of the attenuator with the speakers connected yet, but I´m not shure to understand what you said. Do you mean that using just a 100 watts resistor for different levels of impedance woud be wrong? That would be better using just fixed resistors with selected wattage at a given impedance?

[vibratoking]

Sorry, I didn't mean to confuse you. The wattage rating of the adjustable resistor only applies if you use the whole resistance. For instance, if you use half the resistance of a 100W resistor, then the rating must be reduced by half. It can only support 50W in that configuration. I only mentioned this because I thought you were trying to design the circuit to handle a higher power amplifier.

[traktop]

Ah ok. Well, the amp is a 100 watts one, but I'm using it with 2 tubes pulled. The original air-brake lay out used 2x100 watts tubullar resistors, (using the complete upper resistor), and the instructions says that is perfectly safe using it with a 100watts amp, (although for old el34 amps recommends a 1:2 mismatch, to reduce current I guess). But I heard that a vintage hiwatt can deliver almost twice the power when dimed. The thing is that I just want to turn up the master like 1/4 more, (but one never know...). Do you think that I should use the complete upper resistor and not just half? or get 200 watts tubullar resistors instead of 100?
Cheers, Gilberto.

[traktop]

Work done.

[traktop]

I measured the tip and the ground of the input on each click with the speakers connected, so I can find out what the amp would see:
For a 16 Ohms load (11,8 ohms actual reading)

1- (Full Bypass)- 11,8 ohms
2- (5,8 ohms) - 9,8 ohms
3- (10,8 ohms) - 8,5 ohms
4- (16 ohms) - 9,1 ohms
5- (19,8 ohms) - 9,5 ohms
6- (25 ohms) - 9,8 ohms

When I measured the cabinet load, (16ohms), I expected a little less of that, but not 11,8 ohms, Is this normal?
Except for the first click, it clearly favours a 8 ohms load. I Think I´m gonna adjust the first 2 adjustable lugs to be as near as possible to 8 ohms

I avoided the original "click 2", (bypass through the upper resistor), just because I think there was no point on having 2 bypass clicks; well, I think there is... The second click is bypass but through the circuit, so although I didn´t check it, I guess that It would safely keep the impedance closer to 8ohms, right? So maybe I better change it. Instead of that click, I installed another lug, (19,8 ohms), so I have one more options.

The sound: Better than expected!!! I´m ok with the 3rd click, but I cranked the amp for 5 minutes on last attenuation click and although It became a little fizzy I had lots of fun. Tubes seemed ok and nothing smelled burnt... I could definitely hear a little of "air loss", but just a tiny bit. I tried adding some presence on the amp, but it´s not really about that, (I now miss the old "click 2", so I could compare). Although I didn´t ordered the rheostate I did the 0.1 uF cap, so I will use it for a bright switch. But I still don´t know the best placement for this... Maybe between A and C poles on the lorlin switch...?

Now that I´ve got attenuation going on, this thing is definitely asking for the 2 tubes pulled back, (100 watts), so I guess that all I would have to do is skiping the upper resistor middle lug and use the complete wirewound resistor measuring again to see what load favours; Any word of wisdom about that?

One last thing, the box didn´t get as hot as expected after 20 minutes of playing, but I´m running the amp with 2 tubes pulled, (maybe playing it at 100watts, the thing changes...).
Cheers, Gilberto.

[Mark]

I have a Weber Mass at home and it just destroys the tone of my 18 watter, I also found it rolled the bass off my Boogie Mark 1 too.

I've tried a Tom Holmz power soak many years ago and I found the tonal trade off too much for me.

I'm glad to hear you are happy with the results of your Airbrake III. I'm more inclined to go with VVR in an Express rather than a master volume or an airbrake. I know this isn't your situation, I'm just thinking out loud and giving you an idea of my perspective.