Valve pre-amp warm-up behaviour

[thinman]

My Hughes & Kettner Bassbase 250 amp seems to be exhibiting strange behaviour after switch-on.

After the initial warm-up of 20-30 seconds it seems to work OK for a couple of minutes, then has a period where the gain fluctuates considerably for a few minutes until it eventually stabilizes.

At first I thought this was a fault (and it may still be) but I'm also wondering if it's just a quirk and that it needs to have warmed-up thoroughly to be stable.

I've changed the valve and also all of the electrolytic caps (it must be around 20 years old), re-soldered most of the PCB joints but to no avail. I'm yet to go through the other components.

Has anyone else experienced similar strange valve amp behaviour?

[Al Heeley]

could be a faulty valve or intermittent contact - are they all seated well and snug?

[ahpook]

[quote name='Al Heeley' timestamp='1337204263' post='1657233']
could be a faulty valve or intermittent contact - are they all seated well and snug?
[/quote]

it certainly souds like it could be a contact heating up and moving around.

[BILL POSTERS]

I'm not familiar with this amp, but I'd start by monitoring the anode volts on the valve. Preferably while the fault is present ( pins 1 & 6 of its an ECC83 ) or as its old, even swapping the anode load resistors specially if theyre a bit discoloured. Always a good starting point, .

Of course, thats assuming it is a pre amp fault. Have you tried feeding a signal into the effects return - if there is one - see if the faults still present ?

[thinman]

[quote name='BRANCINI' timestamp='1337215066' post='1657366']
I'm not familiar with this amp, but I'd start by monitoring the anode volts on the valve. Preferably while the fault is present ( pins 1 & 6 of its an ECC83 ) or as its old, even swapping the anode load resistors specially if theyre a bit discoloured. Always a good starting point, .

Of course, thats assuming it is a pre amp fault. Have you tried feeding a signal into the effects return - if there is one - see if the faults still present ?
[/quote]

Thanks - I'll try an external pre-amp into the effects return. I have assumed it's a pre-amp problem primarily so maybe need to look wider now. I've checked the voltages across the resistors on the anodes and cathodes as the amp warmed up but didn't see them drift at all.

It's difficult to tell for certain but it seems that the level drops off after hitting a note hard as though the power supply needs a bit of time to recover! However, there's no distortion or hum that I thought would go with the power supply filter caps going - just a real weakness to the sound for a bit.

It's a pain to find because of the short window of time that there's a problem. I don't want to use the amp gigging because I've lost a bit of trust!

[thinman]

Tried a signal into the FX return and that seems stable so that eliminates the power stage and a good chunk of the pre-amp.

What would be the most likely component to be affected by temperature excluding the valve? Dry joint or resistor?

There's a couple of IC op-amps in the circuit but would they behave as described?

I tried measuring the anode voltage when the problem was present but a small bit of clumsiness with a probe meant I popped the 50mA fuse to the HT side so need to need to get a replacement. Knowing there's 300v knocking around there focuses the mind a little....

[BILL POSTERS]

Have you checked the heater volts ? Not usually a problem with valve amps, but if its DC there could be an issue.

Op Amps tend to go noisy. but it sepends what they are. My way would be feed a signal in from something like a tuner or CD player, play it at low volume, and heat em up with the tip of a soldering iron, same with any transistors - they dont usually melt, and if theyre duff it'll show up before you get em very hot anyway, but note the type numbers first in case the printing comes off. .You'll hear a variation more at low volume, even better if you have a scope, that way you van see it as well.

You could use freezer and a hairdryer, but in 40 years of repairing stuff, I can count the faults I found that way on one hand, hundreds & hundreds with a soldering iron though.

If its single sided print, for what Op Amps cost, you might as well just swap them tbh.

[u][i]Only do it live if your competent to so so, and keep one hand in your pocket. Dont hold a Bass with earthed strings while the tops off. Hence the Tuner or CD player.[/i][/u]

[thinman]

Heater voltage is 6.2v and seems stable - doesn't seem much to go wrong there - it's got its own transformer winding with a pot across it. I'll double check it though.

I think I'll replace the anode resistors next as they're a little difficult to get at when the valve is in to measure the voltage when powered-up. I'll try your soldering iron tip at some point too.

Op-amps are NE5532 and TL08 - so very cheap.

[thinman]

Well, finally fixed the thing. A can of freezer spray applied to the solder-side of the board showed up the problem having failed to find any component at fault. There was a solder joint to a small board at right-angles to the main board on which the preamp valve is mounted which was the culprit with output falling away when frozen.

I had previously gone over these joints with the soldering iron to no avail so removed all the solder and re-did the joint. Problem gone. Still, the amp has been completely re-capped, a few IC pre-amps changed plus a failing gain pot changed.

Hopefully good for a few more years as I love this amp. But then I'm contemplating getting an EBS Riedmar ...