1970s Ampeg V4 Low Distorted Output

[g1]

R55 is open - replace it.
But you might have power tube with a shorting screen which caused R55 to blow.

There are individual screen R's at each power tube. I think they usually blow first but you never know.

[g1]

These have those lovely access panels cut into the chassis. How about a picture of the component side of the board in that area.

[Helmholtz]

There are individual screen R's at each power tube. I think they usually blow first but you never know.

I just assume there must have a reason for R55 to blow.

[g1]

I just assume there must have a reason for R55 to blow.

That's why I'd like to see the component side. Looks like this amp has had a fair bit of work done to it, for all we know the individual screen R's may be increased wattage or bypassed. Schematic shows them as 1 watt type. This model is also supposed to have plate resistors; those are usually what blows.

[ca7922303]

That's why I'd like to see the component side. Looks like this amp has had a fair bit of work done to it, for all we know the individual screen R's may be increased wattage or bypassed. Schematic shows them as 1 watt type. This model is also supposed to have plate resistors; those are usually what blows.

[ca7922303]

That's why I'd like to see the component side. Looks like this amp has had a fair bit of work done to it, for all we know the individual screen R's may be increased wattage or bypassed. Schematic shows them as 1 watt type. This model is also supposed to have plate resistors; those are usually what blows.

Can you I.D. exact year while your in there? Thanks.

[Enzo]

Look at your controls. They will usually have numbers stamped into/onto them A typical one might be 1377133. I made that up, but the 137 means CTS, the company that made it, and the rest are a standard date code. In this case 33rd week of 1971. You can thus find out when your controls were made. The transformers ought to have date codes as well.

If you want to know the exact date it left the factory, I can't help you, but when you know when the major parts were made, then the amp is slightly less old than the newest part.

Myself, I don't care about age, I care about condition

[The Dude]

......Myself, I don't care about age, I care about condition

Are you still talking about amps?

[Enzo]

Being of Scottish descent, I always liked...

What is worn under a Scotsman's kilt?

Nothing is worn, it is all in good working order.

[ca7922303]

That's why I'd like to see the component side. Looks like this amp has had a fair bit of work done to it, for all we know the individual screen R's may be increased wattage or bypassed. Schematic shows them as 1 watt type. This model is also supposed to have plate resistors; those are usually what blows.

So R55(470 ohms 6 watt resistor), is the schematic I'm seeing. Can you verify correct part to order? Thanks.

[g1]

So R55(470 ohms 6 watt resistor), is the schematic I'm seeing. Can you verify correct part to order? Thanks.

The one that is in there now is a 10W. My v4 has a 7W there. I think the 6W on the schematic is an error. I would go with a 470R 10W. The one in there now is hanging off the board but you can probably find a newer one that is a bit smaller and will be a better fit.
Did you check that big resistor right at it's leads? It looks like the burn marks may be from a previous one that burnt and was replaced. It's solder joints don't look good, so if you didn't measure right at the leads it's possible it might just be a bad solder issue.

Check those 4 green resistors right in front of the bad one. They should all be 470R. Also check the four 3.6R beside those. If you find any of those 8 bad, remove the tube that it connects to and set it aside. It will most likely be bad.

I don't know any way to date them aside from what Enzo said. Whether it has metal or plastic knobs gives you a rough idea.

[ca7922303]

The one that is in there now is a 10W. My v4 has a 7W there. I think the 6W on the schematic is an error. I would go with a 470R 10W. The one in there now is hanging off the board but you can probably find a newer one that is a bit smaller and will be a better fit.
Did you check that big resistor right at it's leads? It looks like the burn marks may be from a previous one that burnt and was replaced. It's solder joints don't look good, so if you didn't measure right at the leads it's possible it might just be a bad solder issue.

Check those 4 green resistors right in front of the bad one. They should all be 470R. Also check the four 3.6R beside those. If you find any of those 8 bad, remove the tube that it connects to and set it aside. It will most likely be bad.

I don't know any way to date them aside from what Enzo said. Whether it has metal or plastic knobs gives you a rough idea.

R55(6m ohms)
R41/42/47/48(.47k ohms)
R39/40/45/46(3.6 ohms)

[g1]

So the big one (R55) is the only one blown. The individual screen resistors R41/42/47/48 are not going to blow if there is a bad power tube. I checked my amp and it has 5W resistors there as well.
Like Helmholtz said, if there is a bad tube R55 will just blow again.

[ca7922303]

So the big one (R55) is the only one blown. The individual screen resistors R41/42/47/48 are not going to blow if there is a bad power tube. I checked my amp and it has 5W resistors there as well.
Like Helmholtz said, if there is a bad tube R55 will just blow again.

Outside of buying a tube tester to check power tubes, what's a good way if any to weed out the bad power tube? I read online somewhere that a DMM can be used to check for shorts. Is this possible or is there another way if I don't have any extras? Is it possible to test with 2- power tubes? If so, which 2? Also, until I get 470r 10w resistor, would it be advisinle to test with 470r 5w resistors I have extra to find bad tube? Maybe 2-connected together? Thanks.

[Enzo]

No. A tube tester can tell you a tube is bad. But it cannot tell you a tube is good. Tubes that test OK under the weak load of a tube tester may fail as soon as you stress them. Also, tubes can be noisy or microphonic and a tester won't tell you. The best tube tester is...a working amp. Yes you can generally test an amp with just one pair of powr tubes. One for each side ot the output transformer.

Using a meter? A lot of "shorts" only show up at high voltage and with current flowing. About all I can do with a meter is verify an open heater, which is darn rare.