AMPEG V4BHU - need service manual or shematic

[MHO]

Dear All,

Need shematic or service manual for an AMPEG V4BHU ( 06A441-01 Rev 4) Fuse 1 (2A) and Fuse 4 (0,250 A) are defect. Whant to check the amp and will also change all tubes. Current tubes are 3* 12AX7 / 1* 12AU7 / 4 * 5881WXT.
Anybody an idea where the restistors for bias adjustment are located (R?)

Many Thanks in advance.

Martin

[mozwell]

These may help

[g1]

Unfortunately, those schematics don't include the power supply. The bias circuit & trim pots would probably be on the power supply drawing.
Fuse F4 should be for the power tube V4 according to the schematic.
Someone mentioned there is a hole in the chassis on the rear panel where you can access the bias adjustment.
The "U" designation after V4BH is probably for European units? Are you on 240V? If so, then the 2A fuse is probably the mains fuse?

[mozwell]

The file Ampeg V4BH_2_ has the power supply & bias supply on sheet 2......
If the amp is not "known" to be working, pull the power tubes & power it up using a light bulb limiter.
If all ok, power it up normally & measure all the voltages the power supplies and on the power tube sockets before powering off & installing power tubes.
As this is a valve amp, always have a speaker or dummy load connected when power tubes are installed.
As i don't know your technical experience, if you are not sure about electricity & electronics, don't go poking around inside, as there are voltages which can kill you inside valve amps.

The schematic says to adjust bias using mains current draw for a 115VAC supply.
There may be other methods to bias a quad of 6L6GT, i am sure others here with more knowledge than me will be able to let you know the normal bias voltage on 6L6GT pin 5, the schematic Ampeg V4BH_1_ says approx -50V for bias volts......

g-one, appreciate the poppy , i miss Lemmy.....

[flyingdutchman]

Unfortunately, those schematics don't include the power supply.

The PSU is right there, on page 2 of the 2nd pdf file.

If so, then the 2A fuse is probably the mains fuse?

Exactly.

[flyingdutchman]

There may be other methods to bias a quad of 6L6GT,

Since this particular amp has fuses in series with each output valve cathode (F2 through F5) there's an easy method: temporarily replace the fuses with resistors of a convenient value, e.g. 4.7 Ohms/1W. Before setting the standby switch to "ON", first set the bias pot so as to get maximum negative voltage on pin 5 of the output valve sockets. Then carefully set the bias such that the cathode current on each valve is about 20mA which is a good rule of thumb for the 6L6 in most amps. You'd measure the voltage drop across the aforementioned resistors, with 4.7 Ohms this should be approx 95mA. Current through all 4 valves should be approximately equal (within +/- 10% or so). If one ore more valves pass little or no current, check the screen grid resistors.

[MHO]

Dear all,

many thanks for your fast support and posting the relevant shematics. Now I have a chance to find out whats wrong.
Yes, the V4BHU is an export model, with 240V. I'm from Germany.
The fuse F1 is the main fuse and F4 seems to be for protection within the circuit of V4 in direction to the output transformator.
First I will check the circuit including the ELKOS within power supply. @mozwell: many thanks for your advise.
Poti for bias should be the blue one in the middle of the power board. Will check today in the evening. Feedback follows.
All the best

Martin

[MHO]

Dear all,

Before starting to check the V4BHU main board, I followed mozwell's advise, using a Light Bulb Limiter, due to defect fuse F! and F4. Today I build my own Light Bulb Limiter (see picture :-)))) Not so easy, because europeans are absolute crazy. They prohibit 100W light bulbs.
The Poti for setting BIAS is the blue one, named AP1. It is located in the middle of the main board (see pictures). Will now start testing mainboard and give feedback tomorrow.



Martin

[MHO]

Hi g-one,

Yes it is an european version of the V4BH. You are right, there is a hole in the chassis to adjust BIAS external. The hole is not on the rear panel, it's on the bottom near the driver tubes. (I've labled it, see picture)



Martin

[MHO]

Dear mozwell, dear all,

I did some mesasurements of the power supply (file AMPEG V4BH_2, sheet 2) using the light bulb limiter, which did a great job. I pulled all power tubes. THX for that advise.

Here are the results:

Conditions: Light Bulb Limiter on / AMP power switch on / AMP standby switch off / standby LED red
Light bulb behavior: dark lamp

1. Measurement of the blue circuit: +10V and - 10V at C24 (OK)
2. Measurement of the blue circuit: +14,5V at D9 and - 14,5V at D10 (OK)

3. Measurement of the orange circuit: 56,9V at D15 and 40,5V at R40 (OK)

Conditions: Light Bulb Limiter on / AMP power switch on / AMP standby switch on /standby LED green
Light bulb behavior: when switching on standby switch: glare light bulb which will be dark after some seconds.

4. Measurement of the red circuit (HT Circuit)
- On measuring point A : target = 460V / actual = 424 V (not ok)
- On measuring point B : target = 456V / actual = 210 V (not ok)
- On measuring point C : target = 362V / actual = 135 V (not ok)
- On measuring point D : target = 350V / actual = 121V (not ok)
- On measuring point E : target = 334V / actual = 119V (not ok)

I measured currently not using light bulb limiter in direct mode, due to fault within red circuit.
Looking on the results, it look like a problem with C13, C14 or maybe C15 or C16, due to high voltage drop between MP-A and MP-B. I will now check these ELKOs.

Best regards

Martin

[flyingdutchman]

Conditions: Light Bulb Limiter on / AMP power switch on / AMP standby switch on /standby LED green
Light bulb behavior: when switching on standby switch: glare light bulb which will be dark after some seconds.

4. Measurement of the red circuit (HT Circuit)
- On measuring point A : target = 460V / actual = 424 V (not ok)
- On measuring point B : target = 456V / actual = 210 V (not ok)
- On measuring point C : target = 362V / actual = 135 V (not ok)
- On measuring point D : target = 350V / actual = 121V (not ok)
- On measuring point E : target = 334V / actual = 119V (not ok)

I measured currently not using light bulb limiter in direct mode, due to fault within red circuit.
Looking on the results, it look like a problem with C13, C14 or maybe C15 or C16, due to high voltage drop between MP-A and MP-B. I will now check these ELKOs.

I don't think the electrolytics have a problem.
Instead, note the voltage differential between point A and B, which is more than 200V. This is across R31 a 100 ohm resistor. If Ohm's law adds up, there would be a current of 2A flowing here, and the poor resistor would have to dissipate more than 400W. Also, with more than 2A in the HT circuit, the light bulb limiter would glow quite brightly. Since this isn't observed, something else must be the matter here.

That something else is R31 being defective and having a much higher than nominal resistance. This is a common failure mode for the type of resistor involved (supposedly the rectangular wirewound type).

The cause of R31 failing may have been an output valve shorting (which was sort of obvious because fuse F4 was found to be blown) and the screen grid circuit pulling excess current.

Remedy:
1) Check R31 and replace if defective
2) Check R205...R208 (screen grid resistors) and replace if defective

The voltage on point A being a bit low is caused by the light bulb limiter dropping the mains voltage seen by the amp a little bit, and is nothing to worry about.

[mozwell]

The light bulb limiter is just to be used to make sure there are no "gross" faults & you don't do more damage to the amp if you repeatedly power it up with a fault.
The light bulb goes bright when you power the amp on, then goes dim / dark, great this means there are no gross faults & we can now power the amp without the light bulb limiter.
I see you have a switch to short out the light bulb, great, once you power it up on the limiter, then short out the bulb with your switch & take the voltage readings again. Don't insert the power tubes yet.
The voltage readings you have taken with the light bulb limiter in, will not be exact, as the amp is powered up in series with the light bulb filament. This will reduce most of the voltages you measure, but all we are looking for is "generally" we get some non zero voltage reading (and hopefully more than (say) 50V on the HV dc string...... so we know that we don't have shorted diodes, or capacitors.

Measure the main high voltage DC points, anode & cathode voltages for all 12A*7 tubes, and voltages on the power tube socket pins. Set the bias to the maximum negative voltage. Once we have confirmed everything else is ok, we can put the power tubes in & set the bias.

I would install the 4R7 resistors across the fuse on the cathode of each power tube, so we can confirm the current sharing is equal, and only once all this is confirmed as correct, we remove the 4R7 resistors & put the fuses back in.

F4 was a power tube fuse, so it may be that one of your power tubes is faulty

For now, lets just do the voltage measurements on the HV dc & power tube sockets, and adjust the bias to the maximum negative voltage.

[MHO]

Dear all,

many thanks for your support.

Checked all parts of the red circle, inclusive the screen grid resistors R205 - R208. All parts are in tolerance and ok. I only changed C18 and C19 due to 20% difference. (used equipment: Peak tech 4380 Multimeter and Digital Capacity Meter 7001 M )

Will finally check the voltages tomorrow.

Best regards,

Martin

[MHO]

Hi Everybody,

I've new tubes. They are currently on the way. Will hopefully finish the repair than and give also a full update to the voltages I've measured & the failure (report) .
Will use a TAD Bias master for BIAS adjustment .
Update will follow at the beginning of next week.

Have a nice weekend.

Best regards Martin.

[MHO]

Dear all,

today I received some spare parts. I changed C13 and C14 (220uF/350V) C13 had a problem under working condition, the PSU was buzzing. And now its silent. C13 was arced at the top but the capacity was in tollerance.
I measured the voltages again.

Measurement of the red circuit (HT Circuit)
- On measuring point A : target = 460V / actual with light bulb = 426V (ok) / actual without light bulb = 469 V (should be ok)
- On measuring point B : target = 456V / actual with light bulb = 425 V (ok) / actual without light bulb = 467 V (should be ok)
- On measuring point C : target = 362V / actual with light bulb = 336 V (ok) / actual without light bulb = 370 V (should be ok)
- On measuring point D : target = 350V / actual with light bulb = 330 V (ok) / actual without light bulb = 364 V (should be ok)
- On measuring point E : target = 334V / actual with light bulb = 307 V (ok) / actual without light bulb = 336 V (should be ok)

The AC Voltage is 232 V

The DC Voltages are a little bit high, but looks like ok.

The Anode voltages of the power tubes are all 472V also a little bit high. (should be maximum 460V).

Measured without installed power tubes.

Best regards

Martin