Hot Rod Deluxe re-valve/biasing - help please

BenSirAmos

I love my Fender HRdlx - it does exactly what I want it to.

I got it second-hand and I think it is stock. On Thursday it started distorting badly on lower notes (normal channel) which disappeared when I rolled the volume back a bit.

I thought it is probably a preamp valve going and, as I don't have reliable spares, I thought I might as well get a set and treat it to new ones, using the old ones as spares. I don't have time to fiddle about trying to work out which valve is the dodgy one anyway - and I can't reproduce the problem at home volumes.

So I thought I would check the biasing prior to changing anything - and it is around 85mV. I think it should be between 70 and 90, but should I bias the new valves the same way at 85mV or should I go lower? I like the way it sounds now but I don't mind a bit of experimenting. I've ordered matched  JJ 6L6GC to replace the original GT6L6B.

As I won't get much opportunity to identify the faulty 12ax7 can anyone give me a clue which might be faulty to save me carting it around as a spare?

ICBM

You're mixing up power and preamp valves. Power valves need biasing, preamp valves don't.

The fault could be either, but sounds more like a power valve to me - have a look and see if either isn't glowing properly. Orange glow when the amp is powered up and faint blue glow when fully running - most visible when switching from standby to on usually. If both show the blue glow they're working.

To find a faulty preamp valve, just replace them one at a time until the fault goes away - if it doesn't, it's not a preamp valve.

It may possibly not be a valve at all though - there's a known fault with these amps which gives the same symptom as a power valve failure but is caused by a resistor in the phase inverter circuit.

You should always carry spare valves! How big is a spare 12AX7?

BenSirAmos

I wasn't sure what/which it was - that's why I decided to buy a set.

I carried a spare for so long without using it and then broke it before I got to use it - after that I used a pod as a back up and it didn't seem worth carrying spare valves. But as I've grown keener on the HR I've decided I'll find a safer place for a couple of valves. 

If it is that resistor that's gone, it's off to a tech - but I at least I'll know (more or less) it ain't the valves. Would the PI resistor be intermittent? I can't reproduce the fault at home.

ICBM

BenSirAmos said:

If it is that resistor that's gone, it's off to a tech - but I at least I'll know (more or less) it ain't the valves. Would the PI resistor be intermittent? I can't reproduce the fault at home.

Unlikely. I've never come across one that does anything other than fail permanently.

Intermittent power problems on these are usually bad contacts in the power valve sockets - or sometimes actually in the valve base itself... it shows up as the filament not lighting (no orange glow) in the affected valve.

An old plastic harmonica case and some tissue paper is good for storing spare preamp valves in - tuck it behind the reverb tank under the speaker where it's protected .

BenSirAmos

Thanks.

Earlier I couldn't see any problems with the filaments not lighting but I'll look again now I know that's what I'm looking for. And I've tested for bad contacts with an "engineer's tap" to no avail. 

And I haven't got any old harmonica cases (well, not empty ones) but that's reminded me of similar things that would do the job. Cheers.

randomhandclaps

ICBM said:

 

You should always carry spare valves! How big is a spare 12AX7?

Although when it comes to ampy things ICBM is usually right I have to disagree with this bit.  A lifeguard at the local baths ejected me once for carrying spare valves in my trunks and when I thought about it afterwards it probably wasn't a necessary scenario. 

BenSirAmos

randomhandclaps said:

ICBM said:

 

You should always carry spare valves! How big is a spare 12AX7?

Although when it comes to ampy things ICBM is usually right I have to disagree with this bit.  A lifeguard at the local baths ejected me once for carrying spare valves in my trunks and when I thought about it afterwards it probably wasn't a necessary scenario. 

There's another idea right there. Where is my old Non Doctor?

BenSirAmos

So one power valve isn't glowing as brightly as the other - so I guess that's faulty although I still can't reproduce the distortion. I'm going to hunt to see if I've still got some 6L6s around here. 

BenSirAmos

That's disappointing. Put in new valves - bloody great hum. Works fine except for the hum.

BenSirAmos

Replaced old valves. That hum is still there. Put in new valves: hum is still there. Try it on drive channel - hum goes away.

Any ideas? It is usable with just the drive channel - but I need a fix so suggestions welcome before I take it to a tech.

ICBM

Bugger. Sounds like it's not a valve. Could be a failing filter cap - which is another of the known faults. For some odd reason it's almost always the one at the input jack end.

It is worth trying a spare preamp valve before you do take it in though - just possible it could still be that, although not very likely since it should affect both channels.

BenSirAmos

Been swapping the preamp valves to no effect. I shall hunt out a tech. Thanks for the advice. I just don't want to have to use my back up - it's a Twin and I'm feeling feeble.

Danny1969

The biggest prob on these amps tends to be the 2 big 470 Ohm  5 watt resistors that in conjunction with some Zenors  lower the voltage to run the op amps that switch the channels and the reverb  via the relays. The resistors get red hot and cause all kinds of problems when their solder  joints on the PCB go bad. Check those and also check the spade connectors from transformer to PCB are tight. Also check ribbon cable joints from PCB to valve base PCB as those can  go bad

BenSirAmos

Cheers, Danny. I'll re-check that I haven't knocked anything out because I didn't notice the hum before and that wasn't the original problem.

jpfamps

Is it 100 Hz hum or 50 Hz hum (100 Hz is around G# on the bottom E string).

If it's 100 Hz hum it's coming from the HT supply, if it's 50Hz hum it's coming from the filament supply.

I would check that the filament supply is correctly referenced to ground via the two 100 ohm resistors (R80 and R81). If one of these has gone open circuit your amp will hum.

The fact there is no hum on the drive channel could be because the induced hum is being cancelled due to being induced in opposite phases in the drive channel pre-amp valves.

BenSirAmos

Thanks

I'll listen again, but I thought it was 50Hz. Not sure I can check the reference to ground though. Would another engineer's tap suffice?

ICBM

jpfamps said:

I would check that the filament supply is correctly referenced to ground via the two 100 ohm resistors (R80 and R81). If one of these has gone open circuit your amp will hum.

True, and if a valve failed that can happen easily - my 'firework' Custom Shop Fender repair did that as well.

jpfamps said:

The fact there is no hum on the drive channel could be because the induced hum is being cancelled due to being induced in opposite phases in the drive channel pre-amp valves.

That should only work if More Drive is selected - the plain Drive only uses the first half of V2.

BenSirAmos

It's affecting both channels now. It's getting worse - but I'll look on the bright side - at least it's not an intermittent fault anymore. 

I'm going to take it to John Kelly - he's a nice bloke to visit anyway

Budwiser

The correct bias level on these amps are 60mv..during my service time at the local guitar shop it seemed to when these amps came for repairs after some tampering finger's made it worse....

The spec Manuel from memory clearly says 60mv...also there are some fault's around pre-amp supply lines.. 

Budwiser

Channel switching is also a case fault plus cracked solder joints on those 6l6 power valves...

BenSirAmos

It is currently with the tech - although he has been unable to find what is wrong so far. I can't remember where I got the specs from but I'm pretty sure I read 70-90mV. Before I took the old ones out, I checked and got a reading of 85mV so that is how I adjusted the new ones - because it was sounding just right like that.  

Of course, I might have screwed it up - but I'm not sure how.

It hums on the normal channel with everything turned down. But on the drive channel it doesn't hum with everything on zero - but it hums when the master volume is turned up. I'll report back if / when we find the fault.