Teisco Checkmate 50 redplating

[Resonator Guy]

Although it is too hot to think about firing up a soldering iron here, it is time. I bought this amp a few years ago, replaced all the electrolytics, fixed the reverb and got it going. It sounded good for about half an hour and then the 6L6s started redplating. It was way too much for me at the time and may still be, but I would like to fix it. Most of the reading I have done refers to screen and grid resistors on the 6L6s.
The only resistors on these are 3000 ohms between pins 5 and 6 on both tubes and one from pin 6 to ground on the first power tube that I can't read the code on. I believe it is ? yellow green.
The amp has a bias pot with a 30K resistor off both right and left lugs and a balance pot (?) however I can't determine if they are actually doing anything.

Any advice on where to start here? It may be difficult to get voltage readings as the tube redplate quickly and I would like to do my testing with them rather than wreck a new pair. If someone can advise what to start looking for and what readings would be acceptable that would be a great help.
With my bias meter I get readings of about 1.7 ma on the first tube and an off the chart overload on the second.
There is a schematic all over the web for this amp but it is not terribly similar to mine in that it doesn't have the pots.
I am posting some gut shots.

[52 Bill]

Any advice on where to start here? It may be difficult to get voltage readings as the tube redplate quickly and I would like to do my testing with them rather than wreck a new pair. If someone can advise what to start looking for and what readings would be acceptable that would be a great help.

Pull the power tubes and read the voltages on pin 5 of both sockets. There should be a negative voltage there. If it isn't check the bias supply to see that it's working and the coupling caps from the driver tube for leakage.

If the negative voltage is there, then try different output tubes.

[Enzo]

And before we go nuts looking for resistors and volts, consider it might just be a bad tube. Oh, Bill covered that.

[Resonator Guy]

Thanks, guys. The tubes are or were okay. I pulled them from a National head that I was refurbishing and kept them for this purpose.
I took voltage readings from pin 5 of both sockets with the power tubes out. I did this with the head on standby.
Tube one read -13.7 volts and steadily falling. Wait, it's negative so is that rising? Anyway the number, disregarding the sign gets smaller.
Tube two read +57 volts and rising. I didn't wait to see where they might end up. Should I?
How do I check the bias supply? I will check out the coupling caps, or just replace them.

[52 Bill]

I took voltage readings from pin 5 of both sockets with the power tubes out. I did this with the head on standby.
Tube one read -13.7 volts and steadily falling. Wait, it's negative so is that rising? Anyway the number, disregarding the sign gets smaller.
Tube two read +57 volts and rising. I didn't wait to see where they might end up. Should I?
How do I check the bias supply? I will check out the coupling caps, or just replace them.

Well there's the problem, now to fix it. You need to have a negative voltage on both pin 5's. Because there is a negative voltage on one side, you can assume that the bias supply is at least working to some degree.

The +57 volts lead me to believe that the coupling cap is leaky. Lift the end that connects to the output tube socket. Now read the voltage at pin 5. Also carefully read the voltage at the open end of the cap.

This data should point you to the answer.

[g1]

Generally the pin 5 voltage reading should be taken with the amp out of standby, in "run" condition. This will make sure there is high voltage on the coupling caps so you can see if the leakage is affecting the bias voltage.
Not sure about your readings, it could be that this amp has a different standby method, but you may want to double check your readings out of standby.

[Resonator Guy]

g-one, the readings with it in "run" position were similar. Tube one -7.6 and falling, tube two , 68.3 and rising.

52 Bill, there is only one cap related to either 6L6, a .1uf on pin 6 of tube one.

[52 Bill]

52 Bill, there is only one cap related to either 6L6, a .1uf on pin 6 of tube one.

Looking at photo 2 in you earlier post, there are two 0.1uF @400v caps in the upper left hand side. These are the coupling caps from the phase inverter tube. One connects to pin 6 of the left power tube socket and the other connects to a terminal strip which connects via a yellow wire to pin 6 of the right power tube.

Each pin 6 is connected to pin 5 via a 3K resistor. These are the grid stoppers. Pin 6 is an unused pin, so they use it as terminal point.

So like I said before, lift the end of the cap that has the +50 volts on it and see what happens to the voltage at pin 5 of the power tube.

[Resonator Guy]

Bill. we are making some progress. When I went to lift that cap this morning I found the terminal strip rather wobbly. It has been broken off its mounting ring at some point and someone has tried to solder it back down - not me! Since I can see at least one resistor that should be grounded there, this may be part of the problem. I am going to see if I can get it solidly grounded first, or I will have to replace it. I don't think I have one in my parts drawers.
I will post again when I have made some progress. I still have to check those .1mf caps and of course I don't have any on hand. It only takes about 3 weeks to get stuff from AES, so this may take a while.

[Resonator Guy]

Bill, I have regrounded that terminal strip. In pic 2 you can see a light blue wire coming from the centre of the strip to ground - sort of. That wire was looped between two chassis holes and secured with a blob of solder that didn't stick to the chassis. I put a small nut and bolt with a star washer and fixed it tight. Maybe it was grounded before but it definitely is now.
I lifted the end of the cap at the strip and turned the amp on standby with no power tubes. Pin 5 of that tube now reads -40.8 volts. the other end of the cap reads +464 volts. What comes next? It feels great to be getting somewhere with this.

[52 Bill]

Pin 5 of that tube now reads -40.8 volts. the other end of the cap reads +464 volts. What comes next? It feels great to be getting somewhere with this.

The -40 volts is a very good sign, but I don't understand which side of the cap you are talking about. The side that connects to the driver tube should have a high voltage, +464 volts sounds high to me but let's not worry about that right now.

The basic function of the coupling caps is to carry the ac sound signal from one stage to the next. They also serve the function of blocking any dc voltage from the earlier stage from getting to the input of the next stage. That's why they will be referred to as either coupling or blocking caps. These caps can fail in many ways, but two of the ways that are most important to us are they will decrease in capacitance or they will become leaky. Either they will go open or they will short. When a cap goes open they will cause the ac signal that they pass to decrease in volume and frequency. When they short they will begin to allow dc voltage to pass through them.

The test of the coupling cap that you need to do is to see if any voltage leaks through it, so you should measure the voltage on the end of the cap that you just lifted from the terminal strip. Since the bias voltage is back to a negative number, I will assume that it is leaky and will need to be replaced.

As there is more that 400 volts there, you should up the voltage rating to be safe. Replace both of the phase inverter caps when you do it. In fact you may want to go through the amp and test all of the coupling caps to see if any others are leaking, so that you can place a single order for parts.

[Resonator Guy]

Thanks again Bill, I realized I had read from the wrong side of the cap that time. The free end, the end that was removed from the terminal strip reads 169 v. The end still attached to the PI reads 464.
I am going to order parts today, as AES is offering a discount. I will indeed order new coupling caps throughout, just in case.

[Resonator Guy]

Bill I happened to find, after turning my cap drawer over, two big orange .1mf caps at a higher voltage rating. I put them in and it seems to be getting better. Voltages at pin 5 were -41 and -42. No redplating with about 10 minutes playing. It isn't very loud yet and sounds farty, but a lot of that may be the tired, probably mismatched 6L6s. I thought I would wait a few hours before trying better ones. The 12ax7s I used all came from my pull pile too, just in case.
I learned a good bit from you on this one, and I want to keep plugging away at this. Need something to fill the days. I may actually understand some of this eventually.

[52 Bill]

Voltages at pin 5 were -41 and -42. No redplating with about 10 minutes playing. It isn't very loud yet and sounds farty, but a lot of that may be the tired, probably mismatched 6L6s.

Congrats, great to hear! Now you need to check the rest of those caps.

With the output tubes installed, I would assume that the B+ voltage has come down from the 464 volt level. Have you tried setting the bias of the output tubes yet?

[Resonator Guy]

No, I haven't tried setting the bias yet, however I will soon. Got to go out and play tonight and then head to the cottage tomorrow. So Sunday I hope. I also want to hook it up to a better speaker. The one in my shop is just to see if things make roughly appropriate noise. I did put a set of matched NOS JAN 6L6s in for about 15 minutes and my wife could tell it was working better.