Orange AD200 Bass MK III not working

[beatnik]

Hi everybody

my Orange AD200 bass amplifier doesn't work anymore

The problem is it sounds like it's going through an heavy gating effect - only if the strings are played hard the sound briefly comes through, but without sustain

I opened the amp and checked for burnt components - everything seems fine and all the fuses are intact

Suspecting a valve issue I changed the three preamp tubes with a spare set i had, but that didn't solve the problem

I don't have a spare set of output tubes available to make a test and before i spend 100 bucks on a new set of tubes i would like to be sure that's really the problem

What is bugging me is that there is actually sound coming to the output - shouldn't be completely dead if there is a fault on the power tubes ? Also, all the protection fuses are fine, shouldn't they burn when the tubes fail ?

I am wondering if I could power the amp with just one pair of output tubes at once - this way i could identify if there is a bad one

Any suggestion is highly appreciated. I am confident working with high voltages, let me know if there is some measurement i should do

Cheers !

[52 Bill]

Amps fail in many ways. just as tubes fail in many ways. Not all failures are catastrophic enough to blow fuses or burn parts up.

Your problem could be mechanical in nature or electronic. Trying to replace the tubes is a smart approach to finding the problem. As for the power tubes, you can remove them one at a time and see if there is any change. And you can run the amp with just two power tubes during testing and cause no real damage to the amp. Just remember to keep a speaker or a dummy load connected to the amp at all times.

Do you have the skills and equipment to take dc voltage readings on the amp?

[beatnik]

thanks for your reply

so i can play the amp with just one tube at once ?

this would make possible to discover if there is one or more faulty output tubes

[beatnik]

i can perform voltage readings without problems, just i don't really know what to measure

maybe should i start by checking power supply voltages ?

[52 Bill]

i can perform voltage readings without problems, just i don't really know what to measure

maybe should i start by checking power supply voltages ?

Yes, power supply voltages are always a good place to start. I'd pay close attention to the output tube bias voltages.

The amp will work with only one power tube, but it won't sound very good. I'd run with one tube missing or in pairs to see if it makes a difference.

Does this amp have an FX or power amp in/out jack? Do you have a schematic?

[beatnik]

here are the schematics

[beatnik]

tomorrow i will test the power supply voltages and the bias voltage and will report back

[52 Bill]

Oooh, separate plate and screen supplies. There's a good thing to check, if either one shuts off there is a problem. I'd watch out for the high tension on this amp, it's probably way up there in the 550vdc range.

[beatnik]

you mean V3 and V4 ?

[52 Bill]

you mean V3 and V4 ?

Yes, V4 is the plate supply and V3 is the screen grid supply.

[beatnik]

Finally had a chance to have a go at this one

I have taken some measurements

With no tubes :

- Output tubes screen grids 393V
- Output tubes plates 690V
- Preamp V1 and V2 plates 360V
- PI V3 plates 390V
- bias voltage -35V / -50V on the two ends of the trimpot

With preamp tubes inserted

- V1 plates 118 / 110 V
- V2 plates 188 /182 V
- V3 plates 293 / 293 V
- V1 cathodes 2.7 V
- V2 cathodes 33V
- V3 cathodes 190V

I couldn't find a schematic that has the actual reference voltages, but these looks like at least the power supply and preamp section is working fine

I am thinking the problem could be indeed bad output tubes

Let me know your thoughts, I would like to be 100% sure everything is fine before i put in a fresh set of output tubes

[52 Bill]

I am thinking the problem could be indeed bad output tubes

Let me know your thoughts, I would like to be 100% sure everything is fine before i put in a fresh set of output tubes

Wow 690vdc on the plates!

I'd suggest re-installing the output tubes and take voltage measurements with the tubes in circuit. That will tell you a lot more about how the circuit is working.

[beatnik]

I finally found time and patience to get back to this

As suggested i put the power tubes back in circuit and taken voltage measurements

This is what i read on all the output tubes

plates 675V
screen grids 385V
cathodes 0.01V
grids -45V

These look about right don't they ?

Maybe i have a bad output transformer ? although i suppose that would throw the voltages all over the place

I really need a little bit more help with this one, hope some one can give a hint

Thanks in advance

[Jazz P Bass]

Those are 'static' voltages.
And, yes, they look good.

Now you need 'dynamic' voltages.
It would be best to test with a dummy load. (save your hearing)

With a 100mv signal in & all of the knobs cranked, you want to see a nice healthy Volts AC signal at the grids.
50 Vac for a 50 watt. output section.


Once you have that, then you can tell if the OT is good or bad.

Quote"Maybe i have a bad output transformer ? although i suppose that would throw the voltages all over the place"
A faulty OT usually will not affect the static voltages.

[beatnik]

thanks for your reply, i've learnt something new

i am gonna perform the testing you suggested, the only thing is i don't have a dead speaker to use as a dummy load, if i use a resistor what would be the appropriate value ?

the amp is rated at 200W output !

i am also thinking i could try testing the OT by sending an ac signal to the secondary and monitor the reading on the primary. that can be done without a load on the secondary

do you think this is as effective as the method you suggested ?

i am also thinking there could be some issue in the preamp stage, static voltages seemed fine, but maybe there is a cold soldering somewhere

i was thinking to feed the input with a signal generator and follow the circuit path with the oscilloscope

maybe there is something in between the two gain stages and phase inverter