70s Ampeg V4B Blows Fuses

[ca7922303]

Read as it is written.
Don't philosophize.

Three(3) power tubes measured 1.2mv on pin#8, 4th power tube burnt up R28. Replacing R28 or can I leave R28 out to test more power tubes?

[vintagekiki]

Three(3) power tubes measured 1.2mv on pin#8, 4th power tube burnt up R28. Replacing R28 or can I leave R28 out to test more power tubes?

Burn R28 (cathode resistor V4 / 12AU7) 1k5 has nothing to do with measure 4th power tube (6L6).
Replace R28.
If measured voltages as in your post #180 [# 3 (586v), # 4 (596v), # 5 (-63v)] and on pin # 8 tube V5 you have 1.2mv these are unusable tubes .
You should expect a voltage > 20mVDC on pin # 8 of tube V5.
What is the value of the sensor at pin # 8 tube V5?
Please upload photo V5 tube socket.

[ca7922303]

Burn R28 (cathode resistor V4 / 12AU7) 1k5 has nothing to do with measure 4th power tube (6L6).
Replace R28.
If measured voltages as in your post #180 [# 3 (586v), # 4 (596v), # 5 (-63v)] and on pin # 8 tube V5 you have 1.2mv these are unusable tubes .
You should expect a voltage > 20mVDC on pin # 8 of tube V5.
What is the value of the sensor at pin # 8 tube V5?
Please upload photo V5 tube socket.

Any idea why 4th power tube tested, caused R28 to burn?

[vintagekiki]

Any idea why 4th power tube tested, caused R28 to burn?

leave philosophy. read #197

1)
How much did you help to make R28 burn.

[ca7922303]

Burn R28 (cathode resistor V4 / 12AU7) 1k5 has nothing to do with measure 4th power tube (6L6).
Replace R28.
If measured voltages as in your post #180 [# 3 (586v), # 4 (596v), # 5 (-63v)] and on pin # 8 tube V5 you have 1.2mv these are unusable tubes .
You should expect a voltage > 20mVDC on pin # 8 of tube V5.
What is the value of the sensor at pin # 8 tube V5?
Please upload photo V5 tube socket.

Are you asking me to get a voltage measurement without tube installed on V5 pin#8?

[ca7922303]

leave philosophy. read #197

1)
How much did you help to make R28 burn.

The three(3) power tubes that I tested were tested in a tube tester at a amplifier service in Nashville, TN before I began working on this V4B. They tested good. Three(3) other power tubes I had with me also tested bad. I'm curious what could have caused them to be bad now?

[ca7922303]

After you get that socket working, try this test again to see if power amp & supply are still basically working: https://music-electronics-forum.com/...556#post929556

Had tubes in V6&V7, ran signal to ext. amp jack, have output. Have a loud hum, but can hear signal. Used two tubes that we're used in one at a time test in V5 today.

[ca7922303]

Also tested two(2) tubes in V5&V8, same result, can hear signal thru ext. amp jack and loud hum.

[vintagekiki]

Are you asking me to get a voltage measurement without tube installed on V5 pin#8?

Where is sensor resistor (1 Ohm) from pin 8 (cathode) tube V5 to the chassis???



Did you look at the schematic.

[vintagekiki]

I'm curious what could have caused them to be bad now?

Let go of philosophizing.
Where is resistor 1 Ohm ?
Do you have any idea what you measuring ?

If you are really interested in the cause, get educated, study, read technical books.
This is just a waste of time.

[vintagekiki]

Also tested two(2) tubes in V5&V8, same result, can hear signal thru ext. amp jack and loud hum.

You're wrong somewhere in test.

[DrGonz78]

Let go of philosophizing.
Where is resistor 1 Ohm ?
Do you have any idea what you measuring ?

1ohm resistor? I have been trying to follow this thread but have found it difficult to pinpoint exactly what we are fixing. Anyhow did we advise to install 1 ohm precision resistor(s) for testing? In my head I was picturing a huge SVT with those 1 ohm resistors on the board somewhere but this V4 beast does not have the same setup.

[vintagekiki]

1ohm resistor?

Only one 1 ohm resistor, temporarily in the air ...
https://music-electronics-forum.com/filedata/fetch?id=930419&d=1619849061

[DrGonz78]

Only one 1 ohm resistor, temporarily in the air ...
https://music-electronics-forum.com/filedata/fetch?id=930419&d=1619849061

Oh okay I missed that schematic with the suggested install of the resistor and testing points. Pretty long thread and tough to navigate what is going on. Thanks Vintagekiki!

[g1]

Only one 1 ohm resistor, temporarily in the air ...
https://music-electronics-forum.com/filedata/fetch?id=930419&d=1619849061

I don't know how you expected him to understand this. It is beyond his experience level. I didn't even catch it til I blew up that schematic. It's just a tiny thumbnail that looks like a copy of the regular schematic.
Why are you chasing power tube quality, it is not important now. It was determined earlier (post #97 & 98) that the power amp was working and no signal was getting through the preamp.

Appreciate that. Ran signal generator to ext. amp jack and am getting output. So preamp is at fault.

Unable to hear signal from V4b ext. Jack to other amps input.