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Shacky

8 OHm L-Pad Mod to Mimic 16OHM Pot

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I have some 8 OHM L-Pads I orginally ordered for a 2AX restoration but later discovered the variance from oritinal 16 OH AP Rheostats. Now I have a pair of AR 5's that have pretty bat pots that didn't clean up well and thinking about using the 8 OHM L-Pads with modification I recall reading here.

Can someone point me to the modification thread or teel me what I need to do with the L-Pads to make them act more like the original rheostats? I don't want to spend $80 on new rheostats for the 5's. But I can tell you my 3a's sound wonderful with them!

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I have some 8 OHM L-Pads I orginally ordered for a 2AX restoration but later discovered the variance from oritinal 16 OH AP Rheostats. Now I have a pair of AR 5's that have pretty bat pots that didn't clean up well and thinking about using the 8 OHM L-Pads with modification I recall reading here.

Can someone point me to the modification thread or teel me what I need to do with the L-Pads to make them act more like the original rheostats? I don't want to spend $80 on new rheostats for the 5's. But I can tell you my 3a's sound wonderful with them!

OK - should have taken the time to review the 3a restoration guide prior to asking this question. That guide is invaluable!!!

Jim

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I have some 8 OHM L-Pads I orginally ordered for a 2AX restoration but later discovered the variance from oritinal 16 OH AP Rheostats. Now I have a pair of AR 5's that have pretty bat pots that didn't clean up well and thinking about using the 8 OHM L-Pads with modification I recall reading here.

Can someone point me to the modification thread or teel me what I need to do with the L-Pads to make them act more like the original rheostats? I don't want to spend $80 on new rheostats for the 5's. But I can tell you my 3a's sound wonderful with them!

Hi Jim,

There has been some confusion and debate about this topic, so this is a good place to clarify some things. There are really two primary issues...The first has to do with how to connect l-pads instead of pots, and the second is the addition of a parallel (25+/- ohm) resistor to the circuit to make the l-pad act like the old pot *for typical AR pot settings*.

When using an l-pad, it is important to make sure the original polarity is observed. Below are photos showing the original AR-3a midrange pot and a replacement l-pad with the appropriate connections. The original pot terminals and the replacement l-pad terminals are labeled differently (1, B, 2 for the pot and 3,2,1 for the l-pad). I have indicated the corresponding pot tabs on the l-pad. This is how l-pads would be connected with or without the added resistor.

Adding the resistor as shown between the l-pad's #2 and #1 tabs is the same as placing it on the back of the driver (mid or tweeter) and connecting it between the + and - terminals. The resistor simply changes the circuit impedance to make the l-pad duplicate pot settings between *just off* maximum to about 1/2 revolution back. The "white dot" setting is easily emulated, and the required 15 to 16 ohms across the + and - side of the level control is maintained. (The resistor can also be placed between the tabs on the front of the cabinet for front-wired models.) My personal preference is a straight l-pad on the tweeter (no resistor) for slightly greater tweeter output, and the original pot or l-pad/25 ohm resistor arrangement on the mid.

The only real concern when using l-pads is that the "maximum" setting removes the series and parallel resistance, consequently directing more power to the drivers, so care must be taken if the user's preferred setting happens to be maximum/fully increased. I have not found this to be a problem in actual use. I have spoken to Larry Lagace (Ebay's "Vintage AR") about this at length. He and AB Tech have sold literally hundreds of l-pads to AR users, and have had no reports of problems resulting from their use.

Roy

post-101150-1235423764.jpg post-101150-1235423737.jpg post-101150-1235423752.jpg

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Roy,

Thanks so much!!! After using new Memcor 16 OHM pots on my 3a project I really noticed the scratchiness of the cleaned up AP pots on this pair of AR 5's. One of the mid pots is really bad to point where I either have to have it way too low or way too high. I have some extra AP used old stock but it's nice to have flexibility of dialing in the right level without having to make do.

And I really appreciate you drawing this out for me. I remember setting thes up on one of my beater 2AX's and the pot was in reverse - clockwise rotation lowered sound. :lol:

And since I need to order the 25 OHM resistors I like your option of front loading it so I can get these working while I way for shipment.

I may pre-order a pair of the HiVi 6 OHM tweeters while I'm at it. I looked on PE and I don't see a 0.07 mH inductor - just 0.05. Is that close enough or is it critical?

Jim

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I may pre-order a pair of the HiVi 6 OHM tweeters while I'm at it. I looked on PE and I don't see a 0.07 mH inductor - just 0.05. Is that close enough or is it critical?

Jim

Jim,

An easily obtainable .1mh coil works best with the HiVi Q1R tweeter. Unfortunately, Parts Express and Madisound are both sold out of the Q1R at the moment.

I am currently restoring a pair of 1971 3as for someone using l-pads for the HiVi Q1R tweeters (reversed polarity) and l-pad/25ohm combos for the original mids. They sound great.

I just sent you a PM...

Roy

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An easily obtainable .1mh coil works best with the HiVi Q1R tweeter. Unfortunately, Parts Express and Madisound are both sold out of the Q1R at the moment.

I am currently restoring a pair of 1971 3as for someone using l-pads for the HiVi Q1R tweeters (reversed polarity) and l-pad/25ohm combos for the original mids. They sound great.

For anyone needing a .07mH coil, they can be ordered from Erse (www.erseaudio.com). Price each is reasonable, but there's a $25 min on orders.

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I have some 8 OHM L-Pads I orginally ordered for a 2AX restoration but later discovered the variance from oritinal 16 OH AP Rheostats.

Can someone point me to the modification thread

Jim:

If you deal with pictures better than text, as some of us do, compare Fig. 3.8 (pots) with either Fig. 3.15,16, or 17 (L-Pads) in the restoration manual. Examine the wiring and simply convert the pot number or letter to the L-pad number at the same circuit node.

As Roy and I have discussed, adding 23-25 Ohms in parallel with the tweeter section is not needed if you have ferrofluid tweeters, as they can take more power than the original tweeters.

Commercial L-Pads are made in an inexpensive, simple to manufacture way using a not very "ideal" parallel winding taper. The purpose of an 8-Ohm L-Pad is to make its input resistance 8-Ohms for all rotations of the pad shaft, provided that an 8-Ohm resistive load is connected to the Pad's output. Were that the case, the value of the parallel leg would have to vary in a non-linear manner with shaft rotation starting at 0 Ohms fully CCW, and increasing to infinity at 320 degrees (maximum) CW shaft rotation. That would be an expensive taper to manufacture. So the units we all purchase use what I was taught to call a "piecewise linear" profile: The practical parallel winding is a linear resistor that begins a 0 Ohms fully CCW and becomes ~40 Ohms at 300 degrees rotation. Then the winding ends, the rotor looses contact and the resistance becomes infinite from 300 to 320 degrees rotation. This gives a reasonable approximation to matching an 8-Ohm load, but unfortunately for our aging, original tweeters, it allows full power to go to the tweeter for that last part of the shaft rotation. ... take an old one apart and you will see how it is made.

Cheers,

John

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Jim:

If you deal with pictures better than text, as some of us do, compare Fig. 3.8 (pots) with either Fig. 3.15,16, or 17 (L-Pads) in the restoration manual. Examine the wiring and simply convert the pot number or letter to the L-pad number at the same circuit node.

As Roy and I have discussed, adding 23-25 Ohms in parallel with the tweeter section is not needed if you have ferrofluid tweeters, as they can take more power than the original tweeters.

Commercial L-Pads are made in an inexpensive, simple to manufacture way using a not very "ideal" parallel winding taper. The purpose of an 8-Ohm L-Pad is to make its input resistance 8-Ohms for all rotations of the pad shaft, provided that an 8-Ohm resistive load is connected to the Pad's output. Were that the case, the value of the parallel leg would have to vary in a non-linear manner with shaft rotation starting at 0 Ohms fully CCW, and increasing to infinity at 320 degrees (maximum) CW shaft rotation. That would be an expensive taper to manufacture. So the units we all purchase use what I was taught to call a "piecewise linear" profile: The practical parallel winding is a linear resistor that begins a 0 Ohms fully CCW and becomes ~40 Ohms at 300 degrees rotation. Then the winding ends, the rotor looses contact and the resistance becomes infinite from 300 to 320 degrees rotation. This gives a reasonable approximation to matching an 8-Ohm load, but unfortunately for our aging, original tweeters, it allows full power to go to the tweeter for that last part of the shaft rotation. ... take an old one apart and you will see how it is made.

Cheers,

John

Thanks John. After waving the no- L-Pad flag for years I broke down and decided to give them a try on my AR 5 project. Just hooked one up with intention of adding the resistor to front panel when it comes in. After a quick test where I thought I heard the tweeter working I put everything back together and... no tweeter :lol: Can I check for juice on front wires with DMM? If so what setting?

Jim

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Guest oldfart

another approach might be considered ,I guess,

if you have determed the optimal setting for your listening room, what you might do is the following, mark the setting on the potmeter,

open the speaker enclosure, take an ohm meter, and measure the resistance between, the three lugs, disconnect all wires first, because else you will make a faulty

measurement, omce you succeeded, determing the values between one end and movable contact and the same for the other....

THEN you would be able to make A "fixed" l-pad for your needs, thus, you lose the abillity to adjust , but , with a couple of good resistors, you must be able to fix this problem,

but leave the old lpad in its place, becouse otherwise you will introduce an air leak, and that you do not want either....

kind regards luke ; ye olde fart

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Can I check for juice on front wires with DMM? If so what setting?

Jim

You could, and should see some small ac signal, but I will bet you knocked a tweeter lead loose when restuffing. That would be apparent when pulling the tweeter. I have done that when the push-on did not grasp the tab firmly. Sometimes they need to be crimped to hold tightly. I thought I posted this reply this morning, but must have omitted hitting the post button!

Cheers,

John

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Well I pulled out an extra EPI tweeter I had on hand and ran some wires to front leads. Sure enough - it's working. So this AR tweeter is now dead. May it rest in peace. I wonder if the added current through the L-Pad was neough to put it over the edge.

I either need another 2AX/5 tweeter or think about going with HiVi rpelacement.

With HiVi replacement, wouldn't the 8 OHM setting in the 5 require a different inductor value than the 4 OHM application for 3a?

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