Tonefiend Forum

Best I can figure this was a table top hi-fi kind of amp with Low and High inputs (mono), a single volume knob, and a woofer/tweeter speaker arrangement.

Here’s a not very clear photo of the schematic:

The tubes are:

12AT6 input
12AB5 power
12X4 rectifier

Clearly this is going to be a low power single ended amplifier.

The 12AT6 is called a “double diode triode”:

https://www.r-type.org/pdfs/12at6.pdf

However the Webcor schematic only shows the triode section hooked up.

I’ve gotten the Webcor amp basically working already. My next goal is to see what can be expected of it with different kinds of guitars, etc.

Commitment.

That is the point where you take the wire cutters and start hacking. Oh sure, take a few photos first. Leave some crumbs in the forest.

Nip delicately at first.

Then just start hacking. Of course, I clipped the wires off right where they were soldered to the capacitor’s terminals, so there will be enough length to reconnect to the new capacitors.

Wow, it’s easier than I thought it would be. A few twists of the capacitor locking tabs and they broke right off!

“Non, je ne regrette rien” – Edith Piaf.

Meanwhile, the no-suitable-replacement-found scratchy volume/power switch mocks me:

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Oh, I forgot to mention that what I am doing here is replacing the electrolytic capacitors – step 1.

OK I am sure hoping somebody out there who actually has some bench experience with tube amps can help an old SPICE head.

Amp works fine for about 3 minutes after it warms up. Then there is a sudden volume drop accompanied by crackling. It kinda cuts in and out if I gently wiggle the single power tube. I have changed the power tube for another one and the same thing happens with the other tube.

I thought maybe the socket contacts were cruddy so I sprayed some Deoxit on it and also pinched together the contacts from the bottom side so that the pins would have to scrape through any crud on their way in. But it didn’t help a bit. However, based on the crackling when I wiggle the tube, I still get the impression it’s a bad connection down there.

I remelted all the solder joints on the power amp tube socket and likewise it did not help.

All the resistors have already been replaced, along with one of the 0.01 uF ceramic caps.

Ideas?

Quote from Digital Larry on November 13, 2012, 08:06
Last night I ripped out the old socket and split it open. (Insert photo). Carbonized? Uh-yup.

Took awhile to wire the new socket. As the mounting holes were not at the same radius as the old socket, this made rewiring the point to point wires a bit challenging, since the new socket had to be rotated a bit in order to drill new holes.

I thought it would be brilliant to use one of my boys’ strong magnets to retrieve the filings from drilling the chassis. Well, it got a lot of them, but after it stuck itself to the side of the chassis, it magnetized the chassis and the filings started sticking to it! I blew the rest out with some compressed air.

Double checked everything and plugged it in. Started humming like a mofo in proportion to the volume setting, which it had not done before. See, this is why I generally prefer software. As I lay in bed, having given up for the time being, I thought this might be the result of having moved the yellow wire from the volume pot wiper to the power amp’s grid right next to the high voltage supply’s first filter cap. There’s quite a bit of line frequency current right there as the caps take the inrush from the rectifier tube. I’m going to see if I have some thin coax, to replace this wire and ground it as close to the power tube as possible.

I’m glad I started with such a simple amp. Plenty of frustrations already – also known as learning opportunities.

My final plan for this little sucker is to put a switch to allow the power tube turn run in triode mode. I’m sure this change the sound as well as reduce the power. Any of my tube wrangler homies want to offer some advice? Apparently a screen current limiting resistor is in order.

I usually just use the same screen grid resistor as normally used. In this case since it’s single ended you need a separate one looks like a 22K would the trick. In push pull I put the switch between the supply and the regular screen grid resistors.

Well in the case of most amplifiers the screen grid resistor is selected to limit the amount of current to the screen grid. If there is no limit the screen grid will try to act like another plate and melt down. Since you are connecting the screen grid to the xfrmr plate winding for triode mode this is almost like connecting it to the power supply at the first filter cap. The voltage difference between the first cap and the plate is for all practical purposes zero volts at idle. So basically the resistance will need to be the same as the resistor between the first two filter caps. The data sheets usually gives you a maximum screen grid current so you can calculate the minimum resistance required to keep the screen grid current in a safe region. Too much screen grid current will destroy tubes very quickly.

You can actually limit the power output of the tubes by increasing the screen resistor’s value which limits screen grid current. You would need to put a fairly large resistance value as the screen grid resistor to hear a difference. I spent some time many years ago trying to reduce amplifier power output by reducing the screen grid current. I found that in most push/pull amps that it took 250K ohm pot in series with the screen supply. That would allow you to make the amp start clipping at output power as low as 0.5-1 watts with the pot maxed out and full power with it minimized (you need a pot that is rated for the voltage and at least a few watts of power and keep in mind the elements of the pot are at B+ voltages which are a bit dangerous). This sounds like a great idea but I found that the sound was more like a hard sounding fuzz than an amp running hard which was not what I wanted. It gave the amp a cold sound when you reduced the screen current too much since the power supply was not sagging. The one good thing about the setup was that it increased the life of the output tubes dramatically when you limited the screen grid current. Carvin used to use this technique in some older amps (late 70’s early 80’s) to get a half power setting.

The other thing I would like to point out is that many modern tubes don’t seem to be as robust as the originals (especially Asian made tubes but many Russian brands as well). So when I rework an old Fender or Marshall I usually replace the 470 ohm 2 watt resistors with 1K ohm 5 watt resistors this increases tube life with no impact on tone. If you decide to play around with screen grid resistors be sure visually watch the tubes for the screen grid glowing (which is a sign there is too much current flow) when running it near full power in to a dummy load. The increase of resistance limiting the screen current also allows the tubes to handle a bit more plate voltage as well.

One final point is that many old amplifiers were designed to operate on 110 VAC but today we have 120 – 125 volts or more (I have measured up to 135 VAC at the outlets in clubs and other venues). The power company does this because it allows them to sleaze by rather than add the extra xfrmrs and lines required to do the job properly. That extra voltage might not sound like much but it can make the B+ voltages too high for the filter caps. When I redo an amp I put 500 volt caps in to replace the old 450 volt caps to get a safety margin.