Choosing new valves/biasing without death

imalrightjack

I've finally settled on an amp after some labouring and actually bought back an Egnater Rebel 30 Mkii. It takes pedals really well, although I'm not touching the second channel. It hasn't been revalved and it makes sense to have backups so I'm looking to replace the stock ones and keep them as spares.

This is a recommended package (which is look to get from the UK):

http://www.thetubestore.com/Shop-by-Amp/Egnater-Rebel-30-Amp/Egnater-Rebel-30-Premium-Package

Anybody any thoughts on it?

Also, I have a multimeter and have already pulled the amp out of the head for a look. Ideally I'd like to re-bias it myself, while avoiding death. 

There's a schematic here:

http://www.rig-talk.com/forum/download/file.php?id=14221

And this is apparently what needs done:

There are two trim pots and 3 test points. The bias voltage is printed on the circuit board (40mV +/- 4mV) and you measure between TP1 and TP2 and then TP2 and TP3 while adjusting the appropriate trim pot. It is printed on the PCB which test points to use for each trim pot. Re test after 5 mins and you're done.

I've identified these bits. Plug in a speaker when doing it, yes? Safety advice or simply go to an expert?

ICBM

You can do it easily yourself if you're sensible and you have a multimeter, no need for a tech.

Yes, you need a speaker connected really - just for the unlikely event that the amp is unstable with no load. Or you could use a deliberately shorted plug in the speaker jack if you prefer, that's completely safe for the amp.

Keep your hands off the amp while it's on - place or support the chassis so it can't wobble or fall over. Most important of all, never hold the chassis with one hand while poking around with the other, even with a tool. If you slip and touch something live the current will then travel up one arm, across your chest and down the other, which is the most likely to kill you. If you simply touch something live when the current doesn't have anywhere else to go at the other end, it will hurt but it's unlikely to do any real harm - this is why you will sometimes see old-school techs working with one hand in their pockets, just as a precaution. But if you're sensible you won't be touching anything live anyway.

I think that's about it, other than don't get distracted or allow yourself to be surprised by something, that's when accidents tend to happen.

imalrightjack

Thanks, @ICBM The amp actually has a silent record mode, which allows it run without a speaker. Would that work? Not that it makes much difference. 

All good advice. How much do you need to turn a trim point usually? Fractional adjustments, I imagine?

ICBM

imalrightjack said:

Thanks, @ICBM The amp actually has a silent record mode, which allows it run without a speaker. Would that work? Not that it makes much difference. 

All good advice. How much do you need to turn a trim point usually? Fractional adjustments, I imagine?

It depends. It might put the output stage on standby and derive the signal from earlier in the circuit, in which case it won't work. Or it might be like the Mesa DC series one which mutes the input to the power amp, in which case it will.

Usually - if the bias circuit is well-designed - the trimmer isn't too 'touchy', but some are...

imalrightjack

Thanks again, @ICBM Any recommendations on valves? 

ICBM

TAD or JJ, depending on your preference. TAD are not actually a maker, but seem to do the testing better than anyone else. JJ seem quite reliable anyway. 

DJH83004

I would just like to make a couple of points, ICs safety precautions are bang on the money as always, but I do look after a couple of these for customers and so have quite a bit of working knowledge. First of all I would seriously doubt you need to go to all the expense of a full valve swap, as you know they run EL84s and 6V6s or a blend in between, on fixed (adjustable bias) so don't get the torture most cathode biased amps get. I have yet to change the output valves on the two I look after. Equally the preamp valves (x5) don't get massively overworked (I have changed a couple, I think) so in summary to change the full set seems way over the top, and would be worth your tech checking them out whilst he is tweaking the bias. Regarding the bias, whilst first glance it looks straightforward, the whole amp design is very very compact and there are lots of components close to hand with high voltages on them, so be very careful, equally the trimmers are very sensitive, so take your time, adjust / leave / adjust leave etc.
Just thought it was worth a mention.     

imalrightjack

Thanks, @DJH83004 The stock valves seem to get slated a bit and I thought it's worth having backups so was going to put them in and keep the old as backups. They'll come in at some stage, I'm sure. 

I think I might get somebody to show me the biasing. Being dead might be crap. 

ecc83

To amplify (Boom! Boom!) ICBM's point about 'distraction'. No kids, no animals and stone, cold, sober.

Ideally you should have another 'responsible' adult within earshot.

I like to tell peeps to use croc leads to connect into the circuit, can be done cold, then power up and tweak. I see there are two bias points and so that is bit more of a faff. I cannot find an actual schematic but I assume there is a test point and setting for each OP valve and so getting the readings the same value is more important than getting the Absolute value bang on.

Dave.

imalrightjack

@ecc83 There are three test points. More of a faff?

ecc83

imalrightjack said:

@ecc83 There are three test points. More of a faff?

Not really if, as I suspect the centre is common/earth and the others k1 and k2. The faff is that you need to swap the meter between the two but I guess since the connection can be well clear of the chassis, it could safely be 'hot swapped'?

Dave.

DJH83004

The centre test point is common, and the left sets the bias for the EL84s and the right for the 6V6s, . You could treat yourself to a bias probe kit (TAD Biasmaster for example - but you would need two sets of probes for the valve bases), you would get more accurate results directly in mAs mind.
Sorry to reiterate , but hate to see people spending money unnecessarily, a full set as backup seems a bit over the top, as you have the option of EL84s or 6V6s on the output stage and 5 common pre-amp valves, ultimately your call of course      

ecc83

Ah! Not finding a map I did not realize the OP valve options. Ok so, no faff, just croc up for the valve of choice, if having both you will obviously power down for the swap. 

Dave.

RandallFlagg

My new Victory V40 Deluxe has bias points and the adjuster on the outside of the chassis, points for V6+ Ground- and V5+ plus adjustment trim.

Is this safer, with less danger of death?

ecc83

RandallFlagg said:

My new Victory V40 Deluxe has bias points and the adjuster on the outside of the chassis, points for V6+ Ground- and V5+ plus adjustment trim.

Is this safer, with less danger of death?

Almost certainly. I would need a schematic but suspect the test points come out via high value resistors and are thus  not capable of delivering lethal current (the source voltage is very low anyway) .

BTW. ICBM and I agree to differ about external bias facilities. He is pro, I feel they invite fiddling and are an unnecessary added expense. But then maybe I am biased (ha!) because I would have been the poor slob that had to sort shedloads of maladjusted amps!

Dave.

imalrightjack

Is there any mileage in changing valves to break up earlier/later?  I understand comments on not wasting money (DJH83004) but, by all accounts, people get much smoother drive and less noise with new valves in this amp.

DJH83004

imalrightjack said:

Is there any mileage in changing valves to break up earlier/later?  I understand comments on not wasting money (DJH83004) but, by all accounts, people get much smoother drive and less noise with new valves in this amp.

Fully understand your thinking, but having played around with the various permutations on this amp, the output valves have a lot less influence than you may think, so for starters, why not try different preamp valves in the first stages. if you did want to go the whole hog, I would suggest JJs for the EL84s and either EH or TAD 'Summer of '69's for the 6V6s - but I may get shot down in flames      

imalrightjack

Which of the five pre-amp valves are most worth changing, @DJH83004 ? Thanks!

DJH83004

imalrightjack said:

Which of the five pre-amp valves are most worth changing, @DJH83004 ? Thanks!

This link will probably best explain the influence of each pre-amp valve, I still think it is applicable to your model 
http://www.egnateramps.com/EgnaterProducts/Rebel/Rebel30/Rebel30Specifications.html

imalrightjack

Thanks, @DJH83004 ;

Now to try and understand it properly

DJH83004

If you look at the overview, V1 most influential, main pre-amp gain stage for both channels, V3 channel one, V2 channel two, V4 send / return and V5 PI, trust that makes sense