Orang bass amp keeps going pop!

therose789 · October 2014

Hi guys,

Wondered if i can get a little bit of advice. I have an AD200mk3 Bass amp and it keeps blowing bothe output valve fuses.

When the amp is on it plays fine for about 15 mins then tubes 3 and 4 start to glow really intensely and then 'pop' both fuses go!

Does this just mean that it needs re-valving or is there likely to be another problem that is causing the protection fuses to go?

Im going to take it to a repair dude but thought i would ask before i do!

ICBM · October 2014

Either 3 or 4 is faulty, or there is a bias failure on that side of the amp. (And both 3 and 4 may now be damaged anyway, even if it's the bias.)

If you're going to take it in anyway there's not much point in going further with trying to find which it is. The good(ish) news is that even if it needs a re-valve - which is best done as a full set - 1 and 2 should be safe to keep as spares. But don't just change the valves without being sure it's not the bias.

ecc83 · October 2014

Just beat me to it IC!

A set of 6550s is well over £100 for JJs I see so you do not want to risk anything.

Dave.

DartmoorHedgehog · October 2014

jpfamps · October 2014

ecc83 said:

Just beat me to it IC!
A set of 6550s is well over £100 for JJs I see so you do not want to risk anything.

Dave.

I've been advised off JJ 6550s due to relatively high failure rates with the early samples (in Orange amps).

I use EH 6550 with good results in SVTs, which is a good test for power valves!

These are a bit cheaper than the JJs, but you would still be looking at around a ton for a set.

When all the valves on one side of the amp red plate it's worth getting the amp checked out as bias failure is likely, and simply replacing the valves won't rectify this.

therose789 · October 2014

jpfamps said:

ecc83 said:

Just beat me to it IC!
A set of 6550s is well over £100 for JJs I see so you do not want to risk anything.

Dave.

I've been advised off JJ 6550s due to relatively high failure rates with the early samples (in Orange amps).

I use EH 6550 with good results in SVTs, which is a good test for power valves!

These are a bit cheaper than the JJs, but you would still be looking at around a ton for a set.

When all the valves on one side of the amp red plate it's worth getting the amp checked out as bias failure is likely, and simply replacing the valves won't rectify this.

Thanks for the advice! I currently use Sovtek 6550 tubes which are reasonably cheap on googling! How much of a difference in tone do the output valves have? I love the sound of the amp as it stands at the moment.

Cabicular · October 2014

Could it be a burnt out screen grid resistor?

ICBM · October 2014

Cabicular said:

Could it be a burnt out screen grid resistor?

Not by itself, that will just shut down the affected valve. It could happen as a result, but then one of the valves will stop glowing red.

If both valves on one side are redplating, it's a bias failure on that side - either caused by the bias supply circuit itself (bad solder joint, failed resistor, or most likely adjuster pot), or by a short in one of the valves pulling down the bias for both of them. Or more rarely - usually in older amps - a failure of the coupling cap from the phase inverter/driver stage which will send high *positive* voltage into the bias feed on that side.

Cabicular · October 2014

Thanks I'm a bit of a beginner
I have a 22w two rock that is red plating on one side that started With a low level persistent hum
I thought about measuring the screen grid on that . Then I sobered up and realised I only have a vague idea of what I'm doing

ICBM · October 2014

If you only have a vague idea of what you're doing, it's probably best to keep away from the 400V inside it

.

And even if you're joking, never ever work on an amp when you're not entirely sober

.

Is it a 2-valve amp? (I assume so at 22W) If so, swap the two power valves round and see if the fault follows the same valve or stays with the same socket. No risk to you doing that, if you wait until they've cooled down.

Cabicular · October 2014

I'm kidding about the sober I'm tee total

I'm also very wary of capacitors
It's a 2 power valve amp
I tried brand new valves and still had the same issue
I have a valve tester as well and ran all the valves through it with passes (although experience tells me not to 100% trust the test)
It is confined to one side which is why I was thinking screen grid?
User replaceable? (Not bad with an iron)
Anything else I should look at or is it off to try and find a tech?
Appreciate the advice

ICBM · October 2014

Cabicular said:

I'm kidding about the sober I'm tee total

lol

Cabicular said:
It's a 2 power valve amp
I tried brand new valves and still had the same issue
I have a valve tester as well and ran all the valves through it with passes (although experience tells me not to 100% trust the test)
It is confined to one side which is why I was thinking screen grid?

It could be the grid stopper resistor - which is not the same as the screen grid resistor - that supplies the bias voltage to the valve. If it fails or has a bad joint on it, there will be no bias on the valve and it will redplate. A failed screen resistor will simply stop the valve working though - it's always the result of a valve failure, not the cause.

You're right that valve tester results aren't always conclusive for valve amps - they test at much lower voltages - but if new valves show the same problem it's not likely to be the valves unless you're extremely unlucky and have two faulty valves… very unlikely but not totally impossible! Swapping the two valves will confirm it, but it does seem almost certain to be the amp.

Cabicular said:
User replaceable? (Not bad with an iron)
Anything else I should look at or is it off to try and find a tech?

Ideally you need to take some voltage measurements inside the amp with it running, to find out where the problem is before you change anything. There's no real way around this, you don't really want to just start changing components even with a good idea where the problem is. If that sounds too risky for you, you do really need a tech.

Cabicular · October 2014

Thanks Appreciate it and many apologies for the thread hijack

Howdy, Stranger!

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Orang bass amp keeps going pop!

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