70s Ampeg V4B Blows Fuses

[vintagekiki]

Both these things are mostly due to your false belief; that you have no need to learn the english language to participate on this forum.

I don't understand what you mean by this.
How "my false belief" understood all what written on this forum, and home don't understand own language. Syntax, fellow syntax.
Too bad. I lived in the belief that this an international forum.
However ... ...
Respected, I did not give myself the freedom to comment on anyone's technical knowledge like ...
By the way, the translation is machine, the ignore command is always at your disposal.

PS
If the administrator deems it necessary, let free to delete all vk inappropriate posts.
thanks

[Helmholtz]

He is using a bias probe that has no resistor and requires a meter on DC current function. I suspect his fuse for that function is blown. I take it he set the meter to read volts after getting no reading on mA range. So at that time measuring DC volts at cathode with cathode disconnected from ground.

I don't think you can get plate voltage at the cathode if you lift it (except with a shorted tube).
The cathode voltage can only rise to the tube's cutoff voltage, maybe 100V.

So I think the probe actually measures at the plate in order to get pure plate current.
In fact Eurotubes offers probes that break the plate connection: https://www.eurotubes.com/store/pc/E...robe-6p149.htm

[vintagekiki]

I have a bias adapter from eurotubes...

This probe takes a direct DC plate current measurement by breaking the plate connection (pin #3). It does not use a reference resistor and it is not in the cathode connection.
https://www.eurotubes.com/store/pc/Eurotubes-Octal-Bias-Probe-6p149.htm

Connect both wires from this probe to DCA (200 mA range).
Pay attention
Both wires from this probe are under high voltage (> 400VDC) and do not connect any wire to the chassis (blow fuse in DCA)

[ca7922303]

Are you one full wall voltage, or variac? 520VDC is a bit low (also 6VAC instead of 6.3V at power tube heaters), but if AC outlet or variac is low, that would explain it.

V3 pin3 is too far off, but probably because 12AX7 instead of 12DW7.

V201 pins 2 & 3 not right. Recheck. Also check resistance of R202,R203,R204


The mV numbers are measurement error, re-do

Full wall voltage(121.8 vac),
R202(0.5k)
R203(6.3k)
R204(230k)

will post rechecks asap.

[g1]

I don't think you can get plate voltage at the cathode if you lift it (except with a shorted tube).
The cathode voltage can only rise to the tube's cutoff voltage, maybe 100V.

So I think the probe actually measures at the plate in order to get pure plate current.
In fact Eurotubes offers probes that break the plate connection: https://www.eurotubes.com/store/pc/E...robe-6p149.htm

Yes, you are correct. That info makes it very clear that it is measuring at the plate rather than the cathode.

[g1]

I don't understand what you mean by this.
How "my false belief" understood all what written on this forum, and home don't understand own language. Syntax, fellow syntax.
Too bad. I lived in the belief that this an international forum.
However ... ...
Respected, I did not give myself the freedom to comment on anyone's technical knowledge like ...
By the way, the translation is machine, the ignore command is always at your disposal.

You misunderstood my statement, and I can not understand much of your response.
That, is the problem. The translation is not reliable, and constantly causes communication breakdowns.

Yes it is an international forum, and the language used on the forum is the english language. You barely get by with your 'translation machine' but you could do 100 times more good here if you were fluent in the language. It is not personal that I say this. You have been doing this here for at least 15 years and have made no attempt to learn the language. And it sounds like you have no plans to do so. This is because you believe everything is ok in communication, and that is the belief I call false.

Imagine someone here for 15 years and never learned to read schematics. And they insist they do not need to because they use layout drawings. Yes, they can do it, but it is incredibly more difficult for everyone involved.

I am sure this post will be misunderstood because of the 'translation machine'. We will never fully understand each other, and that is a tragedy.

[vintagekiki]

You misunderstood my statement, and I can not understand much of your response.
That, is the problem. The translation is not reliable, and constantly causes communication breakdowns.

Yes it is an international forum, and the language used on the forum is the english language. You barely get by with your 'translation machine' but you could do 100 times more good here if you were fluent in the language. It is not personal that I say this. You have been doing this here for at least 15 years and have made no attempt to learn the language. And it sounds like you have no plans to do so. This is because you believe everything is ok in communication, and that is the belief I call false.

Imagine someone here for 15 years and never learned to read schematics. And they insist they do not need to because they use layout drawings. Yes, they can do it, but it is incredibly more difficult for everyone involved.

I am sure this post will be misunderstood because of the 'translation machine'. We will never fully understand each other, and that is a tragedy.

Your statement is understood exactly as you wrote it. (bold)

Anyone who is invited to fluent in the language, logically he should understand Simplified English (post #64)
The translation machine does a great job, the problem is the difference in the syntax of the two languages it translates, which one of the side does not understand (or won't to understand).

Respected, since you cannot understand translation machine, the ignore command is always at your disposal.
I would ask you to stop the unnecessary story, and return to the technical part of the forum.
Good night, and thank you for your time and space.

[ca7922303]

Are you one full wall voltage, or variac? 520VDC is a bit low (also 6VAC instead of 6.3V at power tube heaters), but if AC outlet or variac is low, that would explain it.

V3 pin3 is too far off, but probably because 12AX7 instead of 12DW7.

V201 pins 2 & 3 not right. Recheck. Also check resistance of R202,R203,R204


The mV numbers are measurement error, re-do

V5(28mA)
V6(28mA)
V7(28mA)
V8(39mA)

R52/53 no change(mv)
V2-#6(153v)
V3-#3(82v) no change
V201-#2(114v), #3(19v) no change
R202(0.5k)
R203(6.5k)
R204(230k)

[g1]

The numbers are all pretty good. The one power tube at 39mA is unmatched from the others and running a bit hot for this model amp. Ideally you would have a matched set of power tubes all running in the same 28mA range.
R52 and R53 I think you are not measuring at the resistors, but we don't need those measurements as the voltage is getting through to R51.

The differences for V201 and V3 are mostly due to the difference of the substitute tube types (6AC10 instead of 6K11, 12AX7 instead of 12DW7). The amp will work fine with those substitutes, like you said, it sounds good. It's up to you if you want to replace them.

The differences in the heater voltages (6.3VAC) for the preamp tubes may be due to poor solder connections. Those connections should be resoldered. I'll try to point them out on the layout drawing later.

[Leo_Gnardo]

The differences for V201 and V3 are mostly due to the difference of the substitute tube types (6AC10 instead of 6K11, 12AX7 instead of 12DW7). The amp will work fine with those substitutes, like you said, it sounds good. It's up to you if you want to replace them..

JJ makes a competent replacement for 12DW7 called ECC 832. Won't be found at every tube dealer. Try Antique/tubesandmore.com. They sell at a very reasonable price. Could well be a sonic improvement after swapping one in. OTOH good luck finding 6K11. Generally nonobtainium. Those that have 'em tend to want a small fortune. Even I have resorted to using the 6AC10 dodge. So far they have worked out just fine though amps using them might be missing out on a bit of gain - no big deal just dial the knob up a bit.

[ca7922303]

The numbers are all pretty good. The one power tube at 39mA is unmatched from the others and running a bit hot for this model amp. Ideally you would have a matched set of power tubes all running in the same 28mA range.
R52 and R53 I think you are not measuring at the resistors, but we don't need those measurements as the voltage is getting through to R51.

The differences for V201 and V3 are mostly due to the difference of the substitute tube types (6AC10 instead of 6K11, 12AX7 instead of 12DW7). The amp will work fine with those substitutes, like you said, it sounds good. It's up to you if you want to replace them.

The differences in the heater voltages (6.3VAC) for the preamp tubes may be due to poor solder connections. Those connections should be resoldered. I'll try to point them out on the layout drawing later.

Once again, I really appreciate all the terrific help I got on this. Extremely grateful. Thank you all.

[vintagekiki]

V4B preamp filaments copper side ¹⁾

[g1]

The differences in the heater voltages (6.3VAC) for the preamp tubes may be due to poor solder connections. Those connections should be resoldered. I'll try to point them out on the layout drawing later.

The drawing has been provided by vintagekiki in post #387 above. Thank you vintagekiki .

Resolder all the blue and pink circles in the diagram. Inspect the traces (blue and pink lines) for any damage. Be sure to also resolder V201 pins 1 & 12. For re-soldering, always add some fresh solder.