6V6 push pull amp overheating

[ms660]

Have you measured the voltage with respect to chassis on g1 on each of the 6V6's (pin5)

Lawrence.

[Levente]

Hi Lawrence, not yet. I am just about to swap the cap...I am getting a bit confused about that pin 1 6v6 pin as it is connected to ground and now I am connecting the +ve side of the cap to a ground....

[ms660]

The -ve of the 100uF is connected to pin 1 of the 6V6 from what I can see in the photo, if the +ve is connected to pins 3 and 5 of the 6SJ7 and pin 1 of the 6Vg is connected to chassis then it's connected correctly.

Lawrence.

[Levente]

yes, on the photo, the positive of the cap is connected to pin 3-5 on the 6SJ7 and the negative of the cap is connected to the pin 1 on the 6V6 which is connected to ground.

So is this correct? I am confused. so the first drawing is OK?

[ms660]

The first drawing is wrong so far as I can make out, the second drawing is correct.

Lawrence.

[Levente]

please ignore this...i am starting from the begging..i am mixing stuff here,

the first drawing is wrong but the first pictures are correct...the drawing there and the photo are different.

[Boater Sam]

You are not alone being confused.

Electrolytic caps are polarity conscious, if you have the -ve wire connected to the chassis, even if by a circuitous route, its OK provided that the other +ve end is connected to a + voltage.

Change one part at a time, then test, and then if you make an error you know where.

[Levente]

Okay,

I have tried with another cap= same results.

I have even tried to connect the +ve to pin 8 on 6v6= same results.

Swapped tubes around, one at the time, all of them (3) really overheating, I can not touch them with my fingers, so hot.

I am unsure now where to go from here. ( I know, will measure the voltages)

the 300 Ohm ceramic resistor is it normal if it is hot?

[Boater Sam]

You said at the outset that one 6v6 was cold, the other hot. I asked you to swap them over to see if the same valve got hot or if the same socket was the one that got hot.
So?

The valves are 6v6GT, small ones. They always are too hot to hold when in use.

I'm thinking that the hot resistor is the combined cathode resistor for the output valves. If it really is overheating, then one or the other output valves is drawing too much current.

I asked for some voltage readings too, so?

[CambridgeWorks]

His first post has a picture showing 6v6 that are metal. Hence pin 1 is earthed,bit is the metal case.
Rob

[cathoderay57]

The two 6V6 are biased using a common cathode resistor which means you ought to be using a matched pair of valves. This means that the test results for the valves should show both drawing approximately the same anode current for the same value of grid bias. If the emission of the 2 valves is widely different then the grid bias will be incorrect and potentially the valve drawing the higher current could be under-biased and overrun. According to the 6V6 valve data you should have each valve drawing around 32 to 35mA (anode + grid2) with a common 300R cathode resistor giving a grid bias of -19 to -21v for both valves. You will be passing up to 70mA through the 300R cathode resistor so yes, it is likely to run hot. It is only dissipating 1.5W but should be rated at about 5W which will run slightly cooler but maybe still hot enough to burn your fingers!

[Levente]

Originally Posted by Boater Sam

You said at the outset that one 6v6 was cold, the other hot. I asked you to swap them over to see if the same valve got hot or if the same socket was the one that got hot.
So?

The valves are 6v6GT, small ones. They always are too hot to hold when in use.

I'm thinking that the hot resistor is the combined cathode resistor for the output valves. If it really is overheating, then one or the other output valves is drawing too much current.

I asked for some voltage readings too, so?

Hey

Okay, here are the readings I have:

AC

at rectifier with the 250 Ohms resistor ( i think thats 4 and 8)

Starting at 680/620 and then a few minutes later 652/600

pin 8 on both 6v6 are equally 63/ 63
pin 1 on both 0

DC

at rectifier 297/ 274

pin 8 on both 6v6 are equally 29 / 29

pin 1 on the one which is really hot is 0

the lesser hot tube pin 1 goes crazy. My DMM goes down to negative 56 and up to positive voltages and back...

With regards the overheating tubes:

when both 6v6 are plugged, only 1 gets hot, the other one not so.... When one tube is plugged in, no matter which one, both sockets get hot. At least that is what I've gathered.

[PJL]

You should not be powering up this amplifier until the 2 capacitors marked in the picture are replaced. These are the output grid coupling capacitors and if they are leaking (they will be) then the voltage on the grids is raised and the valves will run with excessive anode current causing damage to the valves and transformers. Any voltage measurements you make with the old capacitors will be meaningless.

The first thing to do with any new project before powering up is to measure the resistance of the transformer windings to confirm they are all OK. This should include the output transformer primary and the HT windings. You can measure the output transformer windings by measuring the resistance between the anodes of the 6V6 output valves (pin 3) as shown in yellow.

You can do further testing if you disconnect the marked capacitors from the 6V6 grids.

[Levente]

Thank you for your message advise!

I have replace those two capacitors you have marked with 47nF 480V cap's.

Now the measures are completely different:


AC at the rectifier 735/ 700
DC at the rectifier 330/315

AC on both 6v6 pin 8 is 41
DC on both 6v6 pin 8 is 19

AC/ DC at pin one on both 6V6 is 0

Now the voltages after the replaced capacitors going up into the 6SL7 pins ( i think its pin 5 and 6)

now these are different:

157 going to pin 6 and 412 AC going to pin 5 / 75 going to pin 6 and 195 DC going to pin 5

Now both of the 6v6 tubes are equally hot and no smoke coming from anywhere.

Not sure how good is the reading...I've a junction point in the middle of this chassis with 600 V+ in there just there...wires around it...quiet scary. I would need to isolate that somehow.

Please advise on the readings...maybe I should carry on replacing those yellow/black cap's....

thanks!

[whiskas]

Wow 600V+ in a 6V6 amp sounds very high!
If that's really correct, then definately stick with 600 or 630V capacitors for replacement.
Personally I would change ALL of the capacitors, they're probably all in a compromised state, if not completely faulty, and it will be easier to find any other fault issues once they are out of the way.

[Silicon]

The high reading of 600V may be an error due to measuring a DC voltage when the multimeter dial is set for AC.

Please measure the DC voltage on pins 3,4 and 8 for each valve.

[cathoderay57]

Referring to my previous post, you should have 19v DC on Pin 8 (cathode) of both 6V6 (actually -19v with respect to the grid). Changing those coupling capacitors has done a good job because you have reduced the cathode voltage from 29 to 19. That means before you changed the caps the 6V6 were drawing about 44mA each and now they are correct at about 32mA per valve. I agree with the previous post that most of the capacitors are probably leaking and worth replacing.

[Boater Sam]

Sorry for misidentifying the valves, 6V6 are metal, very robust but they are being driven extremely hard with 600v on the anodes. I like metal valves, I've seen them run so hot that the paint comes off.

I am unaware of the "stancor" power supply, is it just a step down transformer?

It would seem that your main problem was leaky capacitors on the grids of the 6V6s, good progress.

[ms660]

Stancor manufactured transformers, including audio, used a lot in vintage stateside stuff.

Lawrence.

[PJL]

Do you have a speaker attached to the amplifier? You must always have a speaker attached to a valve amplifier or you can damage the output transformer and other parts due to excessive high voltages from fly-back. The alternative is a dummy load using a large power resistor.

As the voltages are looking OK now, I suggest you replace the remaining capacitors. As you do not have a circuit drawing you should take lots of pictures so you can see exactly where each part was connected.

PS: Don't worry about taking AC readings anymore as the meter is including the DC voltage.