50C5 conversion

[PRNDL]

I have a Fanton amp with a pair of 50C5's and two 12AX7's, which is about 5 watts in push-pull (2.5W per 50C5).

It's loud enough, but the hum is unbearable. I believe it is because of AC heaters and a AC ripple. It has a diode rectifier and 3 electrolytics.

Also, there's no power transformer, but I did put in a polarized 2-prong plug. (The grounding scheme is very weird, otherwise I would have put in a 3-prong).

I'm considering the following mods to make it suitable for guitar.

A P-T292 power transformer from TubesAndMore.com ($18), which has 140V (100 mA), 28V (20 mA) and 10.5V (2 A) secondaries. I'd connect the 28V to the 30C5's and the 10.5V to the 12AX7's.

The problem is the 30C5's have a huge heater current (150 mA each) and the heater voltage is wrong. The 12AX7's are only 150mA, so there's plenty on the 10.5V.

So, my question is what tube do you recommend replacing the 50C5's with so that this transformer (or another at that same price) would work. It's a 7-pin mini tube.

Thanks!!

[jrevillug]

DO NOT USE THIS AMP WITHOUT A TRANSFORMER , especially with a guitar. An isolation transformer can be had cheap (I believe allied do one for about $12) and doesn't add that much weight to an amp- Is your life really worth less than $12 and a tiny bit of inconvenience?

Even if you are using a polarised 2-pin plug, what it the socket is wired wrong- You may well have faith that the wiring at home is Ok, but what about at the venue where you go to play. Or a friend's house. If it is the wrong way round, the chassis is at 120v . Which also means that your guitar leads and pickups etc. are at 120v.

You really must earth it, too.

Safety warning aside, let's answer your question:

There is a 12c5, using .6A heater current- a beefy 12.6v heater transformer (1.5A@12.6v)+an isolation transformer for B+ would allow this to work.

However, let's try to get the best out of what you have:

Is your hum 120Hz or 60Hz? Are you half-wave or bridge rectifing? If 60Hz and bridge, the your problem is the heaters. If not, then it is B+.

Use an Isolation trasformer to run your existing circuit. Connect the 50C5s and the 12AX7s in series (=124v), and run it on AC from the transformer. Then build another supply for the B+- a well filtered one.

It sounds like you have only C filtering in your B+ supply, which will cause a horrid noise. You really need crcrc or clcrc or clclc filtering for a quiet amp. Use Duncan's Amps PSUDII- it gives good answers.

The 12ax7 and the 50c5 are indirectly heated- so there should not be a major problem with picking up hum from the AC heaters. Look at the B+ supply and the routing of the heater wires- try to run twisted pairs to the heaters, and keep them as far away from the signal wires as possible.

After you have built the amp with the isolation transformer, the well-filtered B+ supply, then, IF you still have hum, you should look at DC heaters.

Also, when you add the mains transformer, be careful not to let it magneticaly couple to the OPT- with the transformer connected to the mains, and the OPT to the speaker, if you get a hum, then it is a problem- I have this problem with an amp with ALL the valves out.

I am sure that someone more knowlegable than me will be along to shout at me too, but this is my input.

EDIT- what this post boils down to is:
-Put an isolation transformer at the input (safety)
-Earth it (safety)
-improve the B+ (hum)
If you still have hum after improving the B+:
-move the transformers about
-try DC heating.

[PRNDL]

Part of the problem is the amp is crammed point-to-point, which makes sorting out the circuitry almost impossible.

I really like the 12C5's and using the P-T292 power transformer, which has 2 amps for the 12V heaters.

My plan is to get the transformer and make a circuit board to mount above the tubes on standoffs.

I'll have to think about the rectifier section. I have another selenium rectifier in the parts bin.

Thanks!!

[tubelab.com]

I would consider using two transformers that could be mounted inside the case somewhere. A 120 to 120 volt isolation transformer like the Triad N68X ($12 from Mouser) and a full wave bridge with silicon diodes (1N4007, about 10 cents each) will make 140 to 150 volts. For the filaments use a 6.3 volt (Mouser 41FD030, $7) or 12.6 volt transformer (Mouser 41FG010 $6). If you go the 6 volt route use a 6CU5 output tube which is the 6 volt version of a 50C5 and wire the 12AX7's for 6 volt operation. If you go the 12 volt route use a 12C5 output tube and wire the 12AX7's for 12 volt operation.

Then you can and should install a 3 wire line cord with the ground lead connected to circuit ground.

I had a similar transformerless Electrolab guitar amp. I just added the N68X isolation transformer in the bottom of the cabinet (always powered up) and added a 3 wire line cord. The ground wire is connected to the ground side of the input jack. After replacing the main electrolytic cap the hum is minimal. You could do the same thing. It is a simple mod.

Finally there is no 30C5, and the 35C5 is not the same tube as the 12C5, 17C5, 25C5 and 50C5 (all the same except for filament). It has lower plate ratings and biases differently.

[PRNDL]

Thanks!!

I've put together a design based on the Fender Harvard.
The RC values, however, will change.

Does Mouser have a good blank circuit board for something like this?
(I really appreciate your recommendations!!!!)

[PRNDL]

Here's an updated schematic.

[tubelab.com]

A few comments:

You dont need a bunch of diodes in series in the power supply since you are dealing with a 120 volt transfomer. You do however need a full wave bridge circuit (4 diodes). Alternatively, you can turn the transformer around, wire the two primaries in series and use them as a secondary. Then ground the center tap and use the standard full wave centertapped circuit.

You need a resistor from the grids of the 12C5 to ground and a resistor from the cathodes to ground. The output tubes need to be biased. The screen grids are not connected on your schematic. The usual "guitar amp" connection is to wire them to point "B" in the power supply.

[PRNDL]

This is much harder without a circuit to copy!

Here's another update.

Thanks again!!

[tubelab.com]

Right idea, just a few changes.

Turn the diodes in the power supply around. You will get negative voltage as shown.

The grid resistors in the output stage (R8 and R25) should be 180K not 180 ohm.

The feedback resistor (R18) is probably too low in value. I would leave it out until the amp is working and then experiment with its value. I have even put a pot in this location for tonal flexibility. Look at the wiring of the presence control in an old Marshall amp.

Your schematic shows 12U7 tubes. I assumt that you are going to use 12AX7's since that is what you spoke of earlier. 12U7's have a 30 volt maximum rating and are not commonly available.

Expect to experiment with the value of R23 also. It sets the current through the output tubes.

[PRNDL]

OK, how is this for a plan.

Use a Triad N88X and 2 diodes (1N4007) to make a center-tapped full-wave rectifier. The previous owner put in new electroytics, which I can use.

Replace the 30C5 tubes with 12C5.

Use a 41FG010 for heaters, which will require rewiring since the heaters were connected to the AC.

Put in a 3-prong cord and connect the ground to the guitar jack.

I'll connect the wires now connected to the AC cord to the N68X output. This avoids problems with the grounding scheme of the original circuit, which I thought was odd the last time I looked.

The next step would be to replace the coupling caps, which I believe are ceramic disks. They are inexpensive, but should work fine for testing.

Ron