Help Adding an Inductor to a Yamaha T-50-C Amp

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DCtoDaylight

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Joined 2007
#1
Hi all! a bit of background: I've no problem working on solid state amps, but am less knowledgeable when it comes to tubes, but love the sound of a tube amp for my guitar...
Here's the issue: my Yamaha T50C has more hum than I would like, particularly when running in "Overdrive". I read an interview with Mike Soldano where he mentioned that one of the changes Yamaha made to his design, which he didn't like, was removing the inductor from the power supply. He recommended putting it back, and that's what I'm thinking of doing!
So my question: how should I spec the inductor? More Henry's is better, no problem there, but less so is the DC resistance and the current rating. This isn't a "cost is no object" build, but I don't want to cheap out either! For a 50 watt amp, with a 500 vdc supply, running a pair of 6L6's, about what current rating should I be targeting? And how much resistance would be "too high"?
Cheers, Dave
 
#2
No one can suggest the current rating for the inductor in a 50 watt tube amp supply? Well, maybe someone can help me understand how the output transformer works... With a 500 volt supply on the center tap of the output transformer, does that effectively double the voltage swing as seen by the transformer? In other words, do we have an effective peak to peak voltage swing of 1000 volts? and an effective RMS voltage of 350? Or am I over thinking it, and the peak to peak is still 500 volts, or 175 Vrms? From there, with an ideal efficiency, the 50 watt amp would draw either 140 ma, or 280ma? Then I size up for the lower efficiency, and some safety margin...
 
#5
The amp has 2 6L6GC power tubes, say 50mA each, and 7 preamp tubes, say 5mA each, so ~150mA, and a b+ of 500v. The choke must be able to handle the voltage and current. A quick look at the Hammond website shows that all 500v-rated open-frame chokes are the large size C14H - make sure you have space in your cabinet.

Chokes tend to be more useful in Single-Ended amplifiers because they don't inherently cancel out mains hum the way Push-Pull amps do. I would look at a re-cap first, get everything up to spec before you go cutting wires.
 
#6
dj and dub: the amp is getting on, I will certainly check the filter caps, out of the circuit (and may just replace them anyway).
dub: thanks for the sizing help! As I mentioned, the amps designer (Mike Soldano) had specified an inductor power supply, but Yamaha nixed it to save money. He still recommends it as an upgrade, and that's part of the reason I'm giving it serious thought. I agree though, with getting the amp working as original is always best before cutting wires!
printer: Thanks for the links and suggestions. If I substitute the 500 ohm resistor for the inductor, it will only supply the 12ax7's, and the bias to the 6L6GC's. Is this where they are normally inserted? I had been thinking to add it before (or after) the standby switch...
Thanks for the suggestions/idea's guys! I really appreciate them!! Dave
 
#7
Finally found my copy of "The Tube Amp Book", with all it's schematics of classic amps. Not sure how it got mixed into the fantasy fiction, must be a wizards work! 🙂 Anyway, several of the amps show exactly what you suggested, replacing the resistor with the inductor, and leaving the center tap of the output transformer connected directly to the first filter caps. Makes sense, quieter supplies for the lower level gain stages. Cheers, Dave
 
#11
Hang on now. The 500ohm resistor in the power supply of that schematic is after the b+ goes to the OT. HT comes from the BR, is filtered by a pair of 80uF caps in series, then the Standby Switch, then it becomes A, and A goes straight into the output transformer. The 500ohm resistor drops the b+ before it goes to the screens and then becomes a Shade rail(?).

If you wanted a choke-filtered power supply you'd put the choke where the standby switch is, with another filtering cap, before b+ goes to the OT (A in the upper part of the schematic.)

If you must have a standby switch, put it on the lower leg of the HVAC going into the bridge rectifier. Switches are much happier with HVAC.

Chokes' inductance is determined by the current passing thru it; as the amp demands more power you'll get more voltage sag.
 
#12
There's only one filter stage before the b+ goes to the OT, the pair of 80uF caps next to the BR in the first page of the schematic. The way they are "stacked" with the resistors in parallel increases the voltage rating beyond the individual caps' ratings, which is good as they prob see >600v when the standby switch is open.

Since those are the only filter caps between the rectifier and the OT, they are probably the source of hum. You should replace those 80uF caps, and the pair that filters the screen supply (at the bottom of the schematic.)
 
#13
Time to replace the electrolytics in the amp, the filter goes where the 500R is. Don't worry about the standby switch, better not to use it at all though. If you want a mute function there are better ways of doing it. Otherwise there is no good reason to turn off the power.
 
#14
In the old schematic's I've been looking at, I see both implementations, sometimes the inductor is where the 500R resistor is, sometimes it's directly after the first filter caps. As I'm hoping to make this amp much quieter, I'm leaning towards putting it after the first filter caps.... And as for those filter caps, I have a half dozen 100uFd 500v caps that I can use, so I'm good that way.
 
#15
a pair of 100uF will give you 50uF, the OG PS had 80+80=40uF, close enough.

assuming you're on a pcb, much easier to remove the 500ohm resistor and connect the leads from the choke to the pads. if you replace the 500ohm resistor with the choke, it can be much smaller as it will pass <50mA of current. cheaper and smaller, easier to mount.
 
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