70s Ampeg V4B Blows Fuses

[ca7922303]

V4(#3-31v, #8-31v), V3(#3-110v, #8-2.5v), V201(#3-185v, #4-140v, #6-3.1v). All measurements are vdc with variac @100vdc. Amp is out of standby with 2-5881 power tubes installed. Have not installed new preamp tubes with the measurements. Appreciate the help.

[vintagekiki]

V4(#3-31v, #8-31v), V3(#3-110v, #8-2.5v), V201(#3-185v, #4-140v, #6-3.1v). All measurements are vdc with variac @100vdc. Amp is out of standby with 2-5881 power tubes installed. Have not installed new preamp tubes with the measurements. Appreciate the help.

V4B cathode voltage	schematic	ca7922303	Check
V201 (6K11) pin3	22V	185V	R201, R202, R203,R204
V201 (6K11) pin4	140V	140V	OK
V201 (6K11) pin6	1.26V	3.1V	~ OK or R206, R207
V3 (12DW7) pin3	200V	110V	R18, R19, R20
V3 (12DW7) pin8	1.8V	2.5V	OK
V4 (12AU7) pin3	9.7V	31V	R28
V4 (12AU7) pin8	9.7V	31V	R28

https://irationaudio.files.wordpress.com/2016/02/ampeg-v4b-schematic.jpg
https://irationaudio.com/2016/02/27/ampeg-v4b/
https://www.ampegv4.com/images/schematics/V4B.jpg
https://www.unbrokenstring.com/ampeg-v4b-bass-amp-refurbishment/

The V4B for the US is designed to operate on a 120V AC main voltage (look rear panel), so the variac is unnecessary.
All voltages on the schematic are given for the predicted AC main voltage.
In order to do any correctness analysis the V4B must be connected to the intended AC main voltage.
You must repeat the measurements for the correct AC main voltage (120V)

All measurements are vdc with variac @100vdc.

Probably a typo

Have not installed new preamp tubes with the measurements.

You can see that there are large deviations of measured voltages from the factory values. Unfortunately you will have to get reliable V201 (6K11), V3 (12DW7) and V4 (12AU7). At least for testing (voltage measurement)
As there is a large voltage deviation from the factory values, check the values of the resistors marked in the table with Check with an ohmmeter.
If the value of the resistor is as on the schematics, and the voltages are still not as on the schematics, the tubes need to be replaced.


I know that you know, but let's remind ourselves ... ...
- When measuring resistors, AC main voltage is disconected from the wall.
- When PT (5881) are installed, the amplifier must be connected to the speaker so as not to damage the OT.

[ca7922303]

Everything checks out except for R203(111k). Schematic shows 7.5k and R28(1.5m). Schematic shows 1.5k. The K on schematic looks like a M so I ordered the wrong resistor. Will replace both. Will pickup at radio shack today. What wattage resistor will work and they are telling me they don't have a 7.5k but have a 5.6k or a 10k. With no space to solder two in to make 7.5k, which do you recommend for R203 temporarily until order comes? Thanks for all the great help.

[vintagekiki]

Everything checks out except for R203(111k). Schematic shows 7.5k and R28(1.5m). Schematic shows 1.5k. The K on schematic looks like a M so I ordered the wrong resistor. Will replace both. Will pickup at radio shack today. What wattage resistor will work and they are telling me they don't have a 7.5k but have a 5.6k or a 10k. With no space to solder two in to make 7.5k, which do you recommend for R203 temporarily until order comes? Thanks for all the great help.

All resistors in the preamp are 0.5W (See Notes bottom left)
R28 is a 1.5k resistor

About R203 (7.5k)
You measured 111k but need 7.5k. This says that the resistor R203 (7.5k) is faulty or changed value(111k) and therefore on pin3 V201 (6K11) instead of 22V you have 185V
Ideal replacement for resistor 7.5k is 2 resistors 15k connected in parallel. If you don't have them, the closest value is 6.8k or 8.2k. Ultimately try with 10k.
If it is easier for replace R203 (7.5k or 2x15k in parallel) resistor(s) can solder from the solder side the PCB.

[g1]

With no space to solder two in to make 7.5k,

You can easily make a combination resistor fit in a single resistor space, either using series or parallel arrangement.



The voltage shown on the schematic for V201 pin3 is wrong. It's not uncommon to find these kind of errors on Ampeg schematics. When you get the proper resistor for R203, the pin3 voltage will probably be somewhere around 5V.

[ca7922303]

Installed R203(10k), and R28(1.5k), now have much louder hum and the following readings: V201(#3-99mv, #4-380mv, #6-0.0mv), V3(#3-1.5mv, #8-0.0mv), V4(#3-0.0mv, #8-0.0mv). Rechecked resistance on R201,202,203,204,206,207,18,19,20,28 and they are still reading correct resistance per schematic.

[g1]

Those readings don't make sense. Nothing should have changed for V3. Try to redo them.

[ca7922303]

Here is every pin on V3-#1(572v), #2(13mv), #3(1.5mv), #4(80mv), #5(78mv), #6(555v), #7(2.9mv), #8(0.0 mv), #9(79mv).

[Enzo]

Are the tube heaters even glowing in that tube?

[ca7922303]

Used two(2) different dmm's. Getting same readings on both?

[ca7922303]

All preamp tubes and V201 we're glowing and just noticed that one of the power tube sockets does not light up tube when installed in it. Installed two power tubes next to each other and one started to red plate. Shut amp down , restarted and now preamp tubes and V201 do not light up? Extreme hum also is gone.

[vintagekiki]

...V201(#3-99mv, #4-380mv, #6-0.0mv), V3(#3-1.5mv, #8-0.0mv), V4(#3-0.0mv, #8-0.0mv) ...

Something happen with your measuring instruments, or you are wrong somewhere.
How do you explain the difference in your measurement results.

...V201(#3-185v, #4-140v, #6-3.1v), V3(#3-110v, #8-2.5v), V4(#3-31v, #8-31v)...

Check the condition batteries in your measuring instruments. Are they new or as new (as-as)

[ca7922303]

Installed new batteries in both dmm's now measurements are: V201(#3-27v, #4-114v, #6-2v), V3(#3-103v, #8-2.3v), V4-(#3-10.8v, #8-10.8v).

[vintagekiki]

Well done master

[ca7922303]

Now had a loud pop near V5 while at 120v and all measurements have dropped back to millivolts.

[ca7922303]

C16 now reading 278v & 374mv. C15/C23(+0.00 & -63.7v), D6(377mv & -64v).