Switching on an amp with all power valves removed - problem?

wabster

Hello kind folks, I'm looking for a bit of reassurance.
The effects loop on my Peavey JSX won't engage - I suspected the relay in the effects loop board, changed it out, but no difference - I think maybe the supply to the coil is at fault. Next step I think is to power up the amp and see where the 17V supply is / isn't on the effects board. But, to do this, I need the amp upside down, and don't fancy having it balanced on the power valves. If I power the amp up with no power valves in (at zero volume), is there a risk I could pooch the amp? (I have no intention of sticking fingers anywhere near the dangerous bits, just the effects loop board). 

If nothing else works, plan B is to take out the relay and hardwire the effects loop "on", since I never switch it off

Thanks for any wisdom!

wabster

EDIT - title should have read "all power VALVES" removed. Damned autocorrect....

De_Batz

The post title would present a tricky challenge - you can get power transmission without wires, but it would need something a bit special to do the 300W or so a valve amp consumes!

Seriously, though… the main issue is exposing the high voltages in the innards, so be careful with that. Taking power valves out doesn’t hurt the amp - but it may unbalance some of the voltages. Does the 17V line come off the transformer or some other part of the circuit?

wabster

Thanks! That's what I figured, but I didn't want to make an expensive mistake! Pretty sure the 17v is tapped off the transformer and rectified - didn't print off all the sheets of the schematic so I can't check for sure at the moment.

ICBM

Just put a couple of blocks, thick books or something, under the transformers. Although it’s fine to balance it on the power valves if it’s stable like that and doesn’t fall over.

But yes, it’s safe to power up with no power valves in. But not with no preamp valves, so leave them in - you can damage the filter caps if the preamp valves don't pull down the voltage at the far end of the B+ chain.

wabster

Gotcha! Thanks very much

LionAquaLooper

For future reference, you can still edit the title of a thread you created after you’ve posted it

ecc83

ICBM will bring this old timer up to date if wrong but not having the current pull of the OP valves could cause HT to rise high enough to damage the main filter caps. This was a problem years ago in all sorts of valve gear. Modern gear REALLY should be designed to avoid it but, 'it'appen. 

You can invert the chassis 'cold' and croc clip to the 17V point then stand back and power up. But really, if you wear some surgical gloves (I never did) and are not silly you should be safe. And... FFS make a FRAME to hold the chassis! Bits of ply and 2x1 and bob's yer whatsit. With a bit of thought it can be "modular" for other chassis. 

Just thought:relays are often driven from transistors so this could get clever/tricky?

Dave.

wabster

Thanks - yes, the relay coil is hard wired to +17v and switched to ground by a transistor, that'll be the next angle to investigate. Luckily the effects / footswtich board is on a ribbon cable and easily accessible without having to delve in deep among the HV nastiness...

wabster

LionAquaLooper said:

For future reference, you can still edit the title of a thread you created after you’ve posted it

Thanks! I figured that should be do-able, and now I've found the button to do it!

ICBM

ecc83 said:

ICBM will bring this old timer up to date if wrong but not having the current pull of the OP valves could cause HT to rise high enough to damage the main filter caps.

Very rare - the voltage rise from not having the power valves in is only a few volts usually, and if the caps are that close to the limit they’re really too close anyway. I do know just one amp where this was an issue, which is an old cathode-bias (genuine) Class A amp with a large current draw from the power valves… Selmer Zodiac. I don’t know any other guitar amp it applies to.

However the voltage rise from having no *preamp* valves in can be very substantial at the far end of the B+ chain - from around half the full HT right to the full voltage in some cases - and the caps are sometimes lower spec there, so it’s a very bad idea to pull all the preamp valves and power up.

ecc83 said:

And... FFS make a FRAME to hold the chassis! Bits of ply and 2x1 and bob's yer whatsit. With a bit of thought it can be "modular" for other chassis.

I always used to just put the amp cabinet face down on the bench and rest the chassis across the back of the cabinet at an angle. That way if it’s a combo you can still easily connect the speakers, reverb tank etc. Even easier if it’s a head.

ecc83

"I always used to just put the amp cabinet face down on the bench and rest the chassis across the back of the cabinet at an angle. That way if it’s a combo you can still easily connect the speakers, reverb tank etc. Even easier if it’s a head."

Good point. I was dealing with a lot of naked, pre prod chassis. (pretty sure Bs use full rated caps all through?)

Dave.

ICBM

ecc83 said:

pretty sure Bs use full rated caps all through?

Most manufacturers do now, helped by the easy availability of 500V-rated caps, rather than the old 450s - annoyingly just too low for most higher-powered amps which have B+ in the 470V range usually, and why most of those use stacked 350V caps for the first couple of filter stages.

springhead

You could always pull out the HT fuse if it has one. Then the valves can come out. I’m assuming your 17V rail is derived from its own tap. 

ecc83

Just thinking IC, now that many modern amps use sstate rectification the early stage caps must be subject to near peak rectified HT until the heaters get going? Electrolytic caps can be reformed to a slightly higher voltage so I would aver any 450V pre amp caps in such an amp would not mind 500V or so? But of course it pays to be careful!

Dave.

ICBM

ecc83 said:

Just thinking IC, now that many modern amps use sstate rectification the early stage caps must be subject to near peak rectified HT until the heaters get going?

Yes, if you don’t use the standby switch .

I think this is the reason the last cap in the chain seems to fail most often in Fender Hotrods - they’re not particularly good caps anyway, but it is noticeable that it fails more than the others. It’s probably in other amps too, I've just noticed it most in those.

ecc83

"Yes, if you don’t use the standby switch ."

True, just causes an early death to valve rectifiers!

Dave.

ICBM

ecc83 said:

True, just causes an early death to valve rectifiers!

Only if the switch is in the wrong place in the circuit. Fender, who were the first to fit one as far as I know, got it right - after the first filter cap - but many other manufacturers don't.

wabster

Happy ending to report - having followed the sage advice above, I determined that, although everything seemed to be connected  properly, the relay still wouldn't pull in. Maybe it's because the +17 volt supply to the board was a bit low at 14.5V, and not quite enough to pull the relay in? (The -17 volt supply was bang on). Anyway, took the relay out, put jumpers across the signal lines and now my FX loop is hard-wired on. (The only time I turn it off during a gig is by mistake when my foot misses the tuner...) Curious as to what was the root cause of the prob, but not curious enough to do any more probing. Thanks for the advice, amigos!