Amp volume issue

english_bob

lincolnblue said:

Thanks. I assume it's safe to jst take out the valves, have a look around etc?

I don't think it's necessary to take the valves out, but I'd wait for @ICBM to give you the valve amp safety 101 before you go poking around inside the amp. You should definitely unplug it.

ICBM

With this one it should be as simple as turning off and unplugging it. Although there is a theoretical risk that the filter caps could retain a charge, in practice they will discharge almost completely through the valves before they cool down fully. There may be some small residual voltage but it won't be dangerous.

The thing *not* to do is to remove the valves then turn it on. If you do that it will guarantee the caps won't discharge afterwards.

lincolnblue

Ok, thank you. It's been unplugged since Tuesday so I guess it's nice and ok then. As an amp novice, what should I be looking for and where should I be looking?

ICBM

Things that look like this, although not always with as many wires.

http://www.molex.com/mx_upload/superfamily/wire_to_board_connectors/wire_to_board_billboard.gif

Give them all a wiggle, or if they're not fixed with hot-melt glue, unplug and re-plug them.

lincolnblue

ICBM said:

Things that look like this, although not always with as many wires.

http://www.molex.com/mx_upload/superfamily/wire_to_board_connectors/wire_to_board_billboard.gif

Give them all a wiggle, or if they're not fixed with hot-melt glue, unplug and re-plug them.

Thanks. I'll have a look tonight and see what I find. Very much appreciate your help on this

lincolnblue

I've just taken out the amp bit from the chassis and the white connectors were all glued in but I gave them a wriggle anyway. I took out the valves and then back in again making sure they're in properly. I assume it's bow safe to plug back in again?!

ICBM

lincolnblue said:

I've just taken out the amp bit from the chassis and the white connectors were all glued in but I gave them a wriggle anyway. I took out the valves and then back in again making sure they're in properly. I assume it's bow safe to plug back in again?!

Yes.

lincolnblue

Thanks. It started off at its usual volume. All good I thought. But then the volume seemed to gradually reduce. It's useable but just quieter than it should be. Do you think changing the power tubes would be the next option?

lincolnblue

Hmm.switched it off for 15 mins and it's back to normal again...very frustrating!

ICBM

lincolnblue said:

It started off at its usual volume. All good I thought. But then the volume seemed to gradually reduce. It's useable but just quieter than it should be.

Ah - that sounds like it might be the 'thermistor fault', which is a known problem on these amps. The symptoms you gave before didn't sound like it was…

Do the valves look like they're glowing dimmer than they should be when it's quiet?

lincolnblue said:

Do you think changing the power tubes would be the next option?

Probably not, since they're still unlikely to be the cause, but if you have a spare set it's worth trying.

lincolnblue

Ah - that sounds like it might be the 'thermistor fault', which is a known problem on these amps. The symptoms you gave before didn't sound like it was…

Do the valves look like they're glowing dimmer than they should be when it's quiet?

.[/quote]

I'm not sure to be honest. No idea how power valves should glow? Is the thermistor fault likely to be an expensive fix?

I don't have any spare output valves so can't check without buying some

ICBM

It will be all the valves if it's the thermistor. You should be able to see the filaments glowing much less brightly than normal as the amp gets quieter.

It's not a very expensive fix, but not one you can do yourself.

A spare set of valves isn't a bad thing to have anyway, so it may be worth buying some - sometimes different ones will draw slightly different amounts of current, and that could be just enough not to trip the thermistor into overheating… although not very likely.

lincolnblue

Thank you for your help. Really appreciated. Will get so valves and in the meantime try and find a local amp tech

DJH83004

As ICBM says, sounds like the thermistor in the heater circuit, does the power-on LED gradually dim in conjunction with the volume fade? as this is fed off the heater circuit - though may not be that noticeable unless you look carefully.

lincolnblue

DJH83004 said:

As ICBM says, sounds like the thermistor in the heater circuit, does the power-on LED gradually dim in conjunction with the volume fade? as this is fed off the heater circuit - though may not be that noticeable unless you look carefully.

The LED has always been a bit intermittent to be honest - so it doesn't seem to relate to anything else. It's usually off actually. I used the amp at church yesterday and it worked perfectly. I'll get some new output tubes anyway and look for a tech

DJH83004

lincolnblue said:

DJH83004 said:

As ICBM says, sounds like the thermistor in the heater circuit, does the power-on LED gradually dim in conjunction with the volume fade? as this is fed off the heater circuit - though may not be that noticeable unless you look carefully.

The LED has always been a bit intermittent to be honest - so it doesn't seem to relate to anything else. It's usually off actually. I used the amp at church yesterday and it worked perfectly. I'll get some new output tubes anyway and look for a tech

If you are located anywhere near Nottinghamshire (North Notts actually) be more than happy to have a proper look at it, just let me know.

lincolnblue

DJH83004 said:

lincolnblue said:

DJH83004 said:

As ICBM says, sounds like the thermistor in the heater circuit, does the power-on LED gradually dim in conjunction with the volume fade? as this is fed off the heater circuit - though may not be that noticeable unless you look carefully.

The LED has always been a bit intermittent to be honest - so it doesn't seem to relate to anything else. It's usually off actually. I used the amp at church yesterday and it worked perfectly. I'll get some new output tubes anyway and look for a tech

If you are located anywhere near Nottinghamshire (North Notts actually) be more than happy to have a proper look at it, just let me know.

Thanks! I'll try new valves and if that fails, I'll get in touch. Appreciate that!