DIY Audio Projects Forum

(above quote from this post that touched on the subject)

Chris,

That article really caught my attention, so I searched on "Tim Mellow" and found this thread.

This is an interesting amp. The only bad thing I see about it is the high heater power required, but SMPS to the rescue (the purists cringe)! This amp will get about as hot as the GM70 I'm building, but it will weigh 22 lbs. less. and have a lower output impedance and better damping factor than the GM70 or the Oddwatt with more power output than the Oddwatt into very low impedance loads. The cost of the tubes is more (possibly - depending on what brand you use), but that's offset by the OTs (or lack thereof).

After studying the circuit I think I understand it. I redrew it in an easier to understand format that more clearly shows the separate stages and it seems like a very clever idea. I have almost all the parts but the driver and output tubes and a chassis.

This is a "one of these days" projects that I will CERTAINLY build given enough time. It's the easiest to construct OTL I've ever come across, it's not that expensive to make, and the only OTL I feel I really understand quite honestly out of the others I've looked at. The 6C33C operates at low voltges and high currents. It's the closest thing to a solid state device in the vacuum tube world that I've ever come across. It handles massive current compared to other tubes using anode supply voltages that are close to non-lethal. (ANY voltage should be treated with respect and surely the 150 volts used in Tim Mellow's amp could kill, but compared to the 1300 volts in some high power SE amps it's not quite as risky).

I'm definitley building this amp. Without the OTs this would be an excellent candidate for PSPICE analysis, however, I know of no models for the 6C33C.

Are there any drawbacks to this amp that I don't know about?

Here's my redrawn schematic:

_________________
Stephen

Hi!
Those tubes really look weird, but may be some good performers. Although I have heard that they are very unstable in means of anode current. But I have no experience with them.

Looking for PSPICE 6C33C model?
Maybe this can help: http://www.dmitrynizh.com/ecc99-6c33c-se.htm

Cheers,
Miguel

WOW! Yes it helps, thanks. If I can get that model to work I'm in business - just need an EF86 model now or even just something close.

Mellow claims this circuit is very stable even after changing tubes.

_________________
Stephen

I have just checked this guy´s webpage. Try to take a look of all his links. It´s full of precious information, also with PSPICE tricks.

There is also a Java tool, which I guess it´s intended to create models? I´m impressed with all the theory/calculations revealed behind his projects.

But these... OMG!!! Look at his speaker system!!! http://www.dmitrynizh.com/labaffles.htm

Cheers,
Miguel

I can't think of any drawbacks really. It DOES run quite hot, of course; I've built in a quiet fan, and made a few holes in the top of the chassis, and that helps quite a bit.

By the way, you have a misprint in your schematic where you state the output power. The 25W into 8 ohms is correct, but it should say 40W into 16 ohms, not into 4 ohms. I checked mine feeding in 8 ohms resistive load, and indeed I was getting about 25W when clipping set in. I measured the output impedance, and I got about 0.35 ohms, which is about what Tim Mellow says. Out of interest, I measured the output impedance of the Oddwatt amp (mine uses KT77 tubes), and I got about 4.5 ohms. So indeed, OTLs can have much lower output impedance than tube amps with output transformers. (I did the measurements by measuring the output voltage into an 8 ohm resistive load, and then measuring again with an additional 8 ohms in parallel.)

Chris

Cool,

the original article from AudioXpress (Tim Mellow - 25W OTL Tube amp) can be downloaded from this site (the site where you're logged in now ) :

The PDF includes a BOM and the PSU...

Have fun .

_________________
Cheers,
Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.

The schematic looks fine.

But the tube placing is so bad!

How hot do these tubes get?

FWIW, an IR thermometer pointed at the plates of the 6C33C tubes registers about 380 Fahrenheit, after they've warmed up. Each tube is pushing out about 70W quiescent; 40W for the heaters, and 30W plate dissipation. So allowing a few watts for the drivers, and transformer losses, the stereo amp puts out 300W or so in heat, as the base level.

I've built mine on a 17" x 10" chassis, which is quite a bit bigger than the one Tim Mellow used, and allows a bit more space between the tubes. Also, I've installed a fan. I don't think the temperature gets too excessive.

Chris

Your right, I copied it wrong, the article says 40 watts into 16 ohms, not 4 ohms. Now I just need to wrap my solid state brain around the counter-intuitive idea of higher power into higher impedance loads instead of the other way around. It makes sense to me only for an amp with a very high output impedance, but this one is supposed to be very low as you mention.

How did you measure the output impedance?

_________________
Stephen

The reason is that current is the limiting factor. These tubes can be pushed hard; with the anode current able to peak at something of order 1.75A maybe (way above the 0.6A continuous average indicated in the data sheet). But, that is about the limit. So, with the peak current constrained like that then power, being the product of resistance times current-squared, gets bigger roughly in proportion to the speaker impedance. (At least over the range of impedances around 8 ohms or so.)

The impressively low output impedance of the OTL is a consequence of the negative feedback. I think I saw an article saying it would be about 11 ohms for the "modified Futterman" type circuit using 6C33Cs, if there was no feedback. With feedback, this is reduced to about 0.3 ohms or so.

The output impedance can be defined by assuming the output of the amplifier can be modelled as a perfect voltage source (zero impedance) with a series resistor Rint which represents the output impedance of the amplifier. To measure Rint, we first set the amplifier to drive a resistive load (say 8 ohms), and measure the voltage across it for a fixed input signal. Now, keeping the input fixed, add an additional resistor in parallel with the 8 ohm resistor, and measure the output voltage again. It will now have dropped, of course. A simple calculation from these measurements allows us to determine Rint.

I did my measurements by first using an 8 ohm load, and then paralleling another 8 ohms across that for the second measurement. So if we say that the perfect voltage source is producing V0 volts, and that we measure V1 across the 8 ohm load, and V2 across the 4 ohm load of the two paralleled 8 ohm resistors, then

V1 = 8*V0/(8 + Rint) and V2 = 4*V0/(4 + Rint),

so having determined the ratio V2/V1, we can solve for Rint.

Of course, the idealisation described above is only valid as long as the current is never required to exceed the maximum the tubes can pass. So the impedance is low as long as the amplifier is still within its current capabilities. I made my impedance measurements with the input signal low enough that the output currents were way below the limits.

But if, for example, I had made the first measurement using a level that would imply 25W power dissipation into 8 ohms, then the output voltage would have collapsed down dramatically upon paralleling an additional 8 ohms across the load, and the resulting measured "output impedance" would have been way higher.

I hope this helps!

Chris