Valve Advice - 68 CDR

colourofsound

Because I am bored at work, I have nothing better to do than pester this parish of knowledgable legends so i don't have to wade through a veritable mass of utter bullshit elsewhere online to find this information.



With that said, I have only ever had 2 channel valve amps, and the '68 Custom Deluxe Reverb is, from valve point of view, a different kettle of fish.

So, what would you recommend I revalve it with? Given that it is a noisy amp, 'low noise' valves for the Reverb and Trem bits would be good, if such a valve exists.

Should I stick to one brand? Mix and match? I already have a few valves left over that are already in it, namely an EHX 12AH7 and a Philips NOS 12AT7. The other valves are stock Groove Tubes which appear to be rebranded JJs after some inspection.

I've had the amp for about 4 months now; but I believe it is coming up to it's second birthday.

Bonus question: The glass fuse blew last night; not sure when, but I switched it off and then 10 minutes later came to switch it on and it was dead. The fuse had blown. Anything in particular I should be concerned about? My Dad is convinced 'fuses only blow for a reason'...

Thanks in advance. And maybe sorry in advance too

jpfamps

You can experience "nuisance" blowing of fuses, however if a fuse blows persistently then there is a problem. 

In a valve amp this is usually due to a power valve shorting.

I've not seen many 68s, but they don't strike me as that noisy.

Usually the valve at the start of the signal path have the greatest effect on noise.

In my experience their aren't any brands that are less noisy than others; if you want low noise valves then they need to be selected for low noise. You will of course pay extra for this.

As you have seen with Groove Tubes, many valves are simply rebranded from one of the 3 or so factories still making receiving valves.

Of new production I like EH-12AX7s in Fenders. I've also had good results with JJ 6V6s.

ecc83

Fuse value selection is more art/experience than science. The designer cannot allow for every eventuality.

If a fuse blows in a particular position a few times a year and there is no obvious cause it is usually in order to move up to the next preferred value but try to contact the manufacturer first to establish, 1) that you have the correct value and type in the first place. 2) They don't tell you absolutely not to do it!

Dave.

ICBM

Lower-noise valves will not make any difference in the reverb drive and tremolo positions (V3 and V5) although the reverb recovery/intermediate gain stage (V4) probably will.

Put the Philips 12AT7 in the phase inverter position (V6) if it isn't there already, it will have the most benefit there.

The main reason this amp is noisy is the reduced negative feedback compared to the original version though - it's intentional to create earlier breakup and a more 'driven' sound, but it also noticeably increases noise.

DJH83004

In the main your dad is dead right fuses blow because they are passing excessive current, though that said fuses can blow through age related reasons, but unlikely in your case. Normally as stated by JPF it is due to a momentary internal short on an output valve, and if they do it once, the tendency is they will do it again, so best to replace the pair, certainly if you are gigging. I am not a massive fan of NOS valves, as they can be a bit on the delicate side and subject to microphonics, but just my personal preference. I find JJs a good all round robust valve, which are great for repairers as they don't often go wrong, but tone wise I will leave it to the guys that have better HF response than me, my ears are shot to pieces.......      

ICBM

For what it's worth, yesterday I repaired an amp with what appeared to be a faulty - not blown - fuse. This is quite rare but I've come across it enough times that I wouldn't say "fuses *never* fail unless there is another problem".

This fuse looked perfect but tested open circuit. I replaced it with the same value and ran the amp literally wide open - gain and volume up full and tone where it seemed loudest - for about 5 minutes. No repeat blow even though the valves were looking really quite stressed.