Tube substitutions for a tiny little amp: 12av6, 50c5, 35w4

[inverseroom]

OK, this is for all the amp geeks. I got a little practice amp on eBay that sounds cool but distorts way too early for my taste. I want more headroom! The tubes are old radio transmitter tubes: 12av6, 50c5, 35w4. I'm going to get myself some NOS RCA tubes and try those, but perhaps there are substitutions that will allow me to get a clean sound. Forgive me if this is too "guitar" for this board, but where else can I turn?

If I were a harmonica player, I probably would be very, very satisfied with the little guy.

--John.

[brakeshop]

Hmmm, I'll just throw in my two cents even though I'm not an expert. The 35w4 is part of the power supply(the rectifier) so there probably isn't any reason to change that. You might want to change the 12av6 with a 12au6, or 12at6, (I think those are the right numbers), those might have a lower amplification factor so it should be cleaner sounding. I don't know what replaces the 50c5, but the 12av6 is your preamp, and the 50c5 is your power amp. Lots of small amps have the same tubes, so it shouldn't be too big of a problem to find something, I have 2 small amps with the same tubes. I changed the sovtek 12ax7 tube in my microphone preamp for an old silvertone tube that I had lying around, and it sounds much nicer, with less distortion, so every manufacturer isn't the same either.

[SeventhCircle]

Actually, if what you really want is more headroom, the rectifier diode might be the first thing to substitute, and not with another tube. My RCA tube manual says a 35W4 will drop 15V at its maximum forward current of 100mA, which is a pretty big percentage of the supply voltage, I'll bet. If you can find a plug-in silicon rectifier substitute, or cobble one together out of an octal plug and a 1N4004 diode, you'll probably get noticably more output before distortion. Whether or not it'll be enough for you, who knows? Re-capping an old tube amp with modern low ESR electrolytics can stiffen the supply voltage as well. Just some things to try.

This is really one for NewYorkDave...

-Tim

[inverseroom]

OK, I'll try the 12au6's--but where on earth could I find a plug-in silicon rectifier substitute? Are there sources for such things? I would love to try recapping the amp but I'm a little nervous about getting a bad shock--do I have to worry about discharging the filter cap on such a small amp?

[NewYorkDave]

Those are not transmitter tubes, but tabletop radio tubes usually used in AC/DC sets. What this means is that your amp most likely does not have a power transformer, which makes it dangerous indeed. Before worrying about tube substitutions, the first thing you should do is obtain an AC isolation transformer. Jameco, for instance, sells a 2.5 amp model for less than 40 bucks, if memory serves.

You're not going to get a whole lot of headroom out of that amp in any case because the 35W4 rectifies the raw AC line voltage, which will give you a maximum of ~170 volts DC, not counting the forward drop of the rectifier. Using silicon diodes will gain you a few more volts, but I recommend against it because you'll lose the "soft start" characteristic of the tube rectifier, and tube and capacitor life will be shortened.

The 12AV6 is the 12-volt filament version of the 6AV6, which is a high-mu triode/dual diode, the triode section of which is equivalent to 1/2 of a 12AX7. It's a pretty high-gain tube. The 12AU6 is a pentode with a different pinout, and will not substitute for the 12AV6. The 12AT6, however, is a high-mu triode/dual diode with the same basing as the 12AV6, but a slightly lower amplification factor. The heater current, plate current and bias characteristics are very similar, so it should make an excellent lower-gain replacement.

Small amps such as yours are usually very simple circuits running open-loop and they do tend to distort; in fact, they're often prized for that fact. A skilled technician could investigate adding a little negative feedback to the circuit, but that's getting into the realm of redesign, and is fraught with perils of its own.

Lastly, it's generally a good idea to replace the old filter capacitors. Use caps of the same or higher voltage rating; and if you can't find the exact value as the original, it's OK to go UP slightly in capacitance. When possible, I recommend caps with a temperature rating of 105 degrees-C instead of the more common 85 degrees-C caps.

[inverseroom]

Great, thanks a lot! I will try the 12at6. By the way, there is a power transformer. (Whew!)

[brakeshop]

Are you sure its a power transformer, and not the output transformer? There should probably be two transformers on the chassis.

[inverseroom]

The fact that there were two
Was exactly how I knew
Do dee do dee do

Perhaps this thread is going on too long--or rather I'm posting too much on it. Seriously, the amp is apparently based on a circuit from a Beltone practice amp, the caps look good, the solder points are solid, I just want to get the most out of it. I had a guy make a little tweed cab for me and he put this giant CUT-OUT HEART on the back panel--I love it, it's completely nuts. And if breaks up like a little sonofabitch.

John.

[ciminosound]

Be very careful with that amp. It may be you have an isolation transformer in there already since you have two transformers there. Some of those little amps connected directly to the AC line with no iso. Some had an iso on the mains. There's one schematic in the GT book that shows an iso on the filament of the preamp tube only with the rectifer and the power tube connected directly.

A solid state rectifier won't work there without further mods. The filaments in those tubes are connected in series to the AC line. 12 volts (12av6), 50 volts (50c5), and 35 volts (35w4) for a total of 97 volts the remaining voltage (approx 20v) was dissipated by a dropping resistor.

The general circuit design came straight for RCA or GE. You can find it in the back of most tube manuals.

The 50c5 is generally operated at around 2 watts.
http://hereford.ampr.org/cgi-bin/tube?tube=50C5