Ampeg v-6b multitap inductor replacement ideas?

[Alex R]

Hi all. Here is the schematic for an Ampeg V-6B. L1, in the preamp near the inputs top left, is a multitap inductor. It has been removed from the amp at some point, it's just not there. So the mid range selector switch beneath it, and the mid sweep pot, don't work, obviously. Any ideas for getting them working again?

[Alex R]

might these be of use?

http://docs-europe.electrocomponents...6b80eb1ca1.pdf

[nickb]

I think you should be looking at something like this:http://www.bourns.com/data/global/pdfs/70F_series.pdf
Try digikey or mouser

For 300 Hz with 0.68uf you need about 414mH or 206mH if wired in series with the 19mH and 200mH chokes (use 200mH)
For 900 Hz with 0.15uf you need about 209mH or 200mH if wired in series with the 19mH
For 3000 Hz with 0.33uf you need about 9mH use 10mH

[J M Fahey]

Since it's working on an Op Amp circuit, meaning it lives in a +/-15V world, if you can't get a real inductor you can replace it by gyrators.
To be more precise, you can build a small PCB with 3 (single transistor or ICs, your choice) sections out of a graphic EQ; the ones matching your 300 / 600? (unreadable) / 3000 Hz bands, and select them with your current switch.
Thet circuit *is* a (switchable) single band graphic EG, after all.
You couldn't pull this stunt on Tube amps, where ypu can have dozens of RMS volts around, but there, no problem.

[MarkusBass]

Please note that:
- there is a mistake on the schematic: C4 is not 0.33uF -> it is 0.033uF (33nF) - please check it on the board,
- the middle frequency is 800 Hz and not 900 Hz,
- calculated inductance values are: 414 mH with 0.68uF (300 Hz), 264 mH with 0.15uF (800 Hz), 85 mH with 0.033uF (3 kHz).
Knowing Ampeg amps I'm sure that they used even inductance values and the taped inductor was 100+200+100 mH. With this information you can easily wind the inductor on your own (probably using a different core). Or, you can use inductors mentioned by Nick.


Mark

[52 Bill]

- calculated inductance values are: 414 mH with 0.68uF (300 Hz), 264 mH with 0.15uF (800 Hz), 85 mH with 0.033uF (3 kHz).

That puts it in the range of Cry Baby wah inductors. There are parts all over the place to wind your own, and adding two taps shouldn't be that hard.

I wonder if any of the guys that wind wah inductors would take it on as a custom job.

[mozwell]

Several years ago now, i built an SVT2 pre into my 400W SS bass amp (wow what a change).
As part of this i hand wound an inductor, you may be able to do something similar.
N67 core material has an AL of 3300nH
Inductance is turns (N) squared x AL value
So for AL of 3300nH, a total of 245 turns gives an inductance of 0.2H

If you know the frequencies & capacitor, you can work out the inductance needed if it is different to my sample above.
0.68u and 0.2H will give a freq of 430Hz F = 1/ [2 x pi x sqrt (LC)]

So with the information above, you can make your own inductor.

enjoy

[MarkusBass]

Please note that in SVT2 the total inductance was 800 mH (I think it was 100+200+500 mH). In this case we need 400 mH (100+200+100 mH) <- this should be even easier to wind.

Mark

[oc disorder]

Here's a page to put in your "inductors" folder.
Someone posted this earlier I just added the translation and a typical Ampeg schematic,
This may be of some use.

[Alex R]

Thanks so much guys . Must learn to do my own sums... one day...

I found these 100mH chokes: Buy Leaded Inductors Radial lead inductor 100mH 10%; 0,07A Murata Power Solutions 19R107C online from RS for next day delivery.

Four of these in series, taps as appropriate, what do you think?

[MarkusBass]

For me it's OK. The only drawback is that it does not look like the original.

Mark

[52 Bill]

Based on the specs listed the only thing that may be a problem is the resistance of each coil. I have no idea how the original one was wound, but each of these is listed at about 90 ohms. It may not matter at all or it might make for a weak filter.

Give it a try and let us know how it works.

[J M Fahey]

Wow !! It sure brings back old memories !!
Once I made a classic SVT '72 preamp for some good customers, and handwound the inductor, on a Siemens core.
Had no data whatsoever, but I wound 100 turns and measured inductance; after that it was easy getting what was needed.

Here you can see the Ferrite Core in the middle of the eyelet PCB.



The rack chassis was handmade out of aluminum sheet , plus all holes, painting and screen printing.





It drives an AB Systems 1500W and a QSC900W rack amps, into four Ampeg SVT 8x10" cabinets.



My customers play LOUD !!
Which is justified, here they are setting up sound at a 48000 people Football (Soccer in USA) Stadium.

Those are my kids (then 9 and 6), literally looking at the Sky , because one of the singers was "flying" 15 meters above the stage, hung from a steel wire and clad as an Angel, complete with huge feathery wings and carrying a flaming sword.

[woodyc]

I have one I could send you for shipping plus whatever you think its worth. Send me a private message if you want it...

[Alex R]

Mate that is so kind! I am in the uk though... The little chokes I bought should be here on Monday, if the 90ohm thing makes a problem I will get back to you! Thanks again.