Build This MoFo!

[nick696]

You want a microwave oven transformer, they look very similar but bigger. They are about 60mH and 10A. That's what I am using and works great.

You use the primary side of the transformer?

 

[nick696]

Cathode-Follower Power

The circuit from tubecad.com only use 2mH inductor, so may be 5 to 8mH is good enough.

 

[xrk971]

You use the primary side of the transformer?

It's the one with the thick wire and measures about 1.5ohms. I do not know microwave oven operation well enough to know if that is primary but assume so as the 120vac is stepped up to 1200v for the magnetron. Theoretically, you can get a 10x step up voltage out of the secondaries so be careful and insulate those leads to avoid shock.

 

[xrk971]

MoFo with Melbourne preamp

I finally connected my MoFo (which has been in storage for over 5 months) to my new Karlsonator speakers (~93dB/2.83v 8ohms). I decided to test my new Melbourne preamp (Aksa Lender with mu-follower output and dual rail supply and DC-coupled output) to drive it. Wow, what a fantastic combo! Sounds heavenly, absolutely stunning resolution and bass authority. No noise hiss or hum whatsoever. Source is Amazon Alexa Dot and works well for a superb kitchen radio.

For Melbourne, I am using a 20VAC 15v toroidal to power a Mark Johnson BJT cap Mx (Prasi SMT layout with TO-220 BJTs and BC850/860's). I only had my nice 2SA1837/2SC4793 Toshiba's on hand for TO-220's, so it is kind of a waste on a cap Mx, but works. Power is very clean and noise free.

 

[Eric]

I used microwave oven transformers for my MoFo - got them from ebay for $25 each. They are "MD-803AMS-1" transformers. When you find a specific transformer, just do a google search on the model number - this will lead you to the specific microwave oven that it came from and you'll find the wattage rating for the oven. These transformers were used in a variety of brands of microwave oven, all of which were rated at 900w (7.5A AC). The primary winding (1/4" metal faston tabs) measures about 0R54 at DC and ~56mH and is wound with 15g solid core wire.

These transformers are heavy and wound with decent wire (15g), though I would expect a microwave oven is designed with the intention of only running for several minutes at a time as opposed to continuous duty. I would bet that the transformers are not air gapped.

After about an hour running at 19.5v and 3A bias, these transformers exhibit a ~10c temp rise. This makes me think they are plenty robust for this application.

 

[Hook]

Would these transformers work as choke?

I have a pair of transformers from a pair of SWTP Univeral Tiger ampliers, I am wondering if the transformers can be used as chokes for Mofo amplifiers.
I think they are the 62v versions from the chart. I measured 85v across the + and - outputs of the power supply. Any ideas on if these could work as chokes for a MOFO? Would it work to simply wire these into a Mofo and see if it performs well? If that route is possible would the primary or secondary side be the best side to use?



Thanks, Jim

 

[Zen Mod]

only secondary could possibly have wire fat enough for MoFo current ........ but - likely not enough turns and core is gap-less

so bummer

trade those Tigers unmolested and put money from them in pair of Hammonds

 

[Soundhappy]

Merry Christmas Sir Rothacher

 

[CLiu]

just replace mosfet on bad channel and rebias

take care of proper torque and isolation to heatsink

always use BIG washer for mosfet , and always use spring washer

Hi Zen Mod,

I am happy to report that after replacing the mosfet on the bad channel, my MoFo is up and running again. As advised, I have used a bigger washer (a drilled coin) and also added spring washer. I have also dialed back a little on the bias, ~2.3A now.

Just want to add an observation to the sound: with the bias set a 2.3A instead of 2.5A, the bass is very marginally less extended, but still tight and overall balance is still maintained. Never imagined Iq would have this kind of effect on the sound. Very interesting. This doesn't affect the enjoyment of music though.

Anyway, thanks for the help and advice from Zen Mod and everyone else.

Best regards
Liu

 

[Zen Mod]

 

[flocchini]

Garage1217

All

First of all - Happy Holidays

Just wanted to report that I have used the MoFo with a Garage1217 Ember II headphone amp/pre with great results for the digital side of my system. Vinyl always had enough gain. Ember II is a tinkerer's dream with auto bias for the tubes used.

Best

Bob

 

[Puggie]

Having a festive rummage through the loft and found a pair of old (car audio) DLS ZM-1 Z-Match autoformers. So basically tapped inductors? I poked a cheapie lcr meter over the crusty terminals to see if these could be any use for a MoFo build? Going from the common through connections 1 up to 4 I get the following inductance and iffy impedances (the terminals are a bit grotty) measurements were taken at 1khz.

40mH 0.4ohm
85mH 0.35ohn
180mH 1.5ohm
425mH 2.1ohm

These weigh in at about 3kg each. I could also measure up the winds between the other combinations of terminals (say 1 and 4 losing the first 40mH). I will clean the terminals up to get some proper dcr readings if these look worthy of a build.

Would these be any use for a MoFo build?

They are car audio (originally) 'Mobile MoFo' anyone? They would probably drive my old JBL2118 midbass and ID horns to significant levels in a car!

 

[Eric]

The key for being appropriate for a MoFo amp is current capability. Ideally, you want something that can handle 3A without too much of a temperature rise. Are you able to measure the diameter of the wire used for the windings? You want a fairly heavy wire here to handle the current.

A trial and error approach would be to build the amp and start with the bias point at a low setting and very gradually raise the bias level with the pot and monitor the temp rise on the transformer. Set to 0.5A and let it sit for an hour before adjusting the bias again. Measure the temp rise and see how things go. I would try to keep temp rise below 20-30c.

 

[Puggie]

They are a hefty old beast designed for subwoofers use and state '500 watt power handling'. I could pry a lid off and have a look at the windings.

They would be worth a go for a MoFo build though which was my main query.

Would the 180mH tap be a good start point?

 

[Zen Mod]

find datasheet of choke Mr. MR originally use, then compare and let us know what are your thoughts

 

[kp93300]

With regard to the 20V zener diode, can i use a 12 V 0.5 W zener diode instead ?
I have several 12V but no 20 V zener in my collection.

 

[Eric]

0.5w zeners are not strong enough to protect the output transistor in the event of improper power disconnect. I learned this the hard way (repeatedly). Go for something more like 3-5w zener. Just about any voltage rating is fine (9.1, 12, etc).

 

[Hook]

I have a pair of transformers from a pair of SWTP Univeral Tiger ampliers, I am wondering if the transformers can be used as chokes for Mofo amplifiers.
I think they are the 62v versions from the chart. I measured 85v across the + and - outputs of the power supply. Any ideas on if these could work as chokes for a MOFO? Would it work to simply wire these into a Mofo and see if it performs well? If that route is possible would the primary or secondary side be the best side to use?
Thanks, Jim

Thanks Zen mod for the advice, I suppose there may be a market for those Tiger amps, especially working ones. Still trying to be cheap and hoping for a lower cost alternative to the big Hammond chokes.

Since I first enquired, I see that microwave oven transformers may be the among the most readily available low cost chokes. From the last few pages of the forum it looks like MOTs have an inductance of around 60mH and can handle around 9-10 amps. They can handle enough current for the 24v version of the MOFO, but is the inductance enough at 60mH? Does the 24v version of the MOFO need more inductance along with current capacity, or is the larger Hamond 193V choke used just to handle the higher current? Still trying to find a cheaper substitute for $80 Hammond chokes. (and that doesn't count shipping them)

Thanks, Jim.

 

[PRR]

> is the inductance enough at 60mH?

60mH is 8 Ohms at 21.2Hz.

For any commercial recordings (many low-cut at 50Hz), this is more than ample.

If the inductance were lower, the frequency response would still be "flat" (the MOSFET Source is far under 8 Ohms) but the amount of clean power possible at 20Hz would be lower.

Since few of us have enough cone area to pump audible amounts of 20Hz with the dozen+ Watts of this amplifier, it doesn't matter.

The primary of the larger Tiger PTs may pass 1 Amp DC without overheating, but the inductance will fall way off. The microwave transformers are different because the cheapest (thus only) way to feed a magnetron is a quasi half-wave circuit which puts large DC in the core. The core is "gapped"... not like audio chokes but by laying a thin weld along the non-interleaved laminations.

 

[Eric]

60mH is 8 Ohms at 21.2Hz.

PRR, would you be so kind as to indicate a formula or reference that can be used to calculate this? I've never seen this conversion before and I'd like to learn more. Thanks.