Perhaps the jankiest 6S33 idea ever (with MOT as output trafo)

So my class mate wanted some tube guitar amp we kept looking, the good old fender champ was the best candidate but then I realised the 6S33 is relatively cheap too and quite powerfull in class A.

Another story was the output transformer. Custom winding is out of option since eh..not cheap. So I was wondering if there is any relatively close regular line transformer (I know they aint suitable for audio but come on its a quitar amp).
I found nothing UNTIL. Microwave oven transformer.

The MOT is rated 230V to 2kV (roughly, perhaps 2,1kV). That tells me the the ratio of the MOT is about 8,7. Givving me 605Ohm which is JUST about perfect.
I am from europe and I purchase toroidal output trainsformers from Toroidy and I just checked their transformer for the 6S33 and turns out the MOT is JUST ABOUT BANG ON with the impedance for a 8 ohm setup if I power this thing with about 250-260VDC. The MOT sure can take it. 800W at the secondary at 2kv means 400mA roughly. It can take the DC current and being such a high power transformer I dont think the secondary resistance is all that much.

I thought sure someone had to notice this and try this out but I was wrong I can find NO THREAD on using a MOT as output transformer.

I do understand the the lamination thickness and all that stuff is going to effect the sound in pretty much all the negative aspects you can think of but hey, its a guitar amp...or rather bass guitar amp (because the rated frequency of a transformer and inductance and such.)

So the actual question is has anyone tried such jank and do you even think this would work :D This would make a pretty cheapo and powerfull guitar/bass guitar amp. (I dont have a schematic actually this is just an idea without a schematic so far but I might make this happen in the close future)
 
Microwave oven transformers are not gapped, so DC current may saturate the core. You'd have to measure the saturation characteristics to know how much DC current is allowable - being large device is in your favour here. The magnetic shunts are definitely going to be an issue - should probably be removed.

I'd invest in an extra rugged handle for the amp, its going to the last thing to vacate the stage in a hurricane with that lump of iron weighing it down :)
 

Sorento

Member
2008-03-12 9:59 pm
Microwave oven transformers are not gapped, so DC current may saturate the core. You'd have to measure the saturation characteristics to know how much DC current is allowable - being large device is in your favour here. The magnetic shunts are definitely going to be an issue - should probably be removed.

I'd invest in an extra rugged handle for the amp, its going to the last thing to vacate the stage in a hurricane with that lump of iron weighing it down :)

I was thinking about removing the magnetic shunts (nock'em outta there) and replace them with PVC spacers. Also rip out the aux winding from the thing. (perhaps use it as some kind of indicator but probably not)

This may sound dumb but I dont know how to measure saturation of the core. Im no transformer expert after all I just build amps and buy the transformers (but this amp needs serious cost cuttin lol).
So far all I seen on the net is based on a current ramp and measuring the DC current with a scope- which I can do because I have a old school digital HP scope. I just dont really want to fry it with the inductive kickback of the trafo.

Also now Im begining to wonder what kind of AC drive voltage would the 6S33 want so I can roughly test it.
 
small update: I dug up one of those old ones I have and well one side of the secondary is rivited to the case..bummer but I can deal with that later no problem. The secondary resistance is about 93 ohms which worries me because the 6S33 is rather the "more current less voltage" kind of tube.. if Im being optimistic pushing 100mA trough this thing is about 1W of heat and 9 V of voltage drop.
If I would desire to go further 200mA is 3W of dissapation and about 18V of voltage drop. That aint so nice is it. Tho I think the transformer can happily deal with the 3W which is silly compared to its normal operation :D. And the rivit..kinda forgot bout that but because how lazy I am I might go ahead and isolate the transformer from the chassy and run the HV right up into the chunka iron and not give a single damn as the amp wont be touched by anyone else unless they want a one way trip to the other world.
 

Paul Joppa

Member
2008-11-06 8:01 pm
You might consider parallel feed. A plate choke can be just a regular power supply choke - derate the current to 60%-70% of rating to allow for AC flux. Anything 5 henries or more is plenty at 600 ohms.

You can ground the output transformer primary (which is the MOT secondary) this way. Parallel feed capacitor will be around 30uF for a 5 henry choke.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
...MOT is rated 230V to 2kV...

And a Champ is 5V:250V

Working a winding at 1/10th or so of its rating means some kind of compromise.

Extra weight
Extra resistance
Extra longevity (an issue because the MOT is not rated to run all night, but half-voltage would live forever, and you are not even close to that).

I suspect your 230V side has more dead resistance than your speaker impedance. At least similar? So half your precious audio just heats the winding.

In this class of amp, what you want to find is old US tubes made for 110V B+. 50L6 and many others. Maybe a couple. Then find power transformers whihc can be wired 230V:15V and rated 3X the power you honestly expect from the tube (raw power delivery does not mind THD; guitars are not fussy but working AT the rating will give nasty bass).

MOT as a audio choke is discussed at "Build this MoFo!" in the Pass Labs section.
 

Gnobuddy

Member
2016-03-01 4:10 pm
I wasn't familiar with the 6S33, so I downloaded its datasheet and took a look. Mu is only 2.7, which is an indicator of problems to come when the intended use is audio (guitar) amplification.

The datasheet shows that weird things happen to the allowed maximum anode dissipation just above 240V DC, so that seems like a reasonable upper limit to B+ voltage.

With 240V on the anode, it takes (-100) volts (!!) of grid bias to bring anode current down to 100 mA (24 watts anode dissipation.)

This suggests you'd need about a 200 volts (peak to peak) input signal just to drive the 6S33 to full clean output. :eek:

Nobody uses guitar amps 100% clean in 2019, so you'd need considerably more drive signal than that, to overdrive the 6S33 even a little bit.

You might just barely pull that off if the driver is a small-signal pentode, running on the same 240 V B+. A driver triode is out, unless it has a separate and much higher B+ than the 6S33 itself.

With an input signal of some 240Vpp, and a maximum output signal at the anode of less than 480 Vpp, voltage gain of the 6S33 stage is less than 2. Add in the step-down ratio of 8.7:1, and voltage "gain" is about 0.23; call it -13 dB. It's an attenuator, not an amplifier.

Because the output stage attenuates rather than amplifies, you need massive voltage gain in front of the 6S33, to bring up the relatively tiny guitar signal to the whopping 200Vpp needed for full clean output, or more realistically, to maybe 300V - 400Vpp to allow overdrive.

Using the typical Fender spec of full output at 20 mV from the guitar pickup, you'd need a voltage gain of x20,000, or +86 dB. :eek: This would require a minimum of three cascaded half-12AX7 gain stages (or better, one 12AX7 followed by a small-signal pentode driver), along with a low insertion loss tone control design.

Even if workarounds can be found for the MOT-as-output-transformer problems, the design of the rest of the amp is a bit like an attempt to touch the back of your heels with your nose. It is not impossible (for a contortionist), but it would take a lot of hard work for rather small eventual rewards.


-Gnobuddy
 

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