To Standby or Not Standby, that is the question

[RayL]

This is an interesting thread! I'm using 'valve technology' brain cells that haven't been exercised since the 1970s and 80s when I did repairs for Cascade Music in my spare time.

Replying to Roger Cook's point about where the HT feed for the output valves is taken from (before or after the choke), the answer seems to be 'it depends'. I went to my dog-eared folder marked 'Amplifier Circuits' and flipped through at random.

The Vox AC30 (4 x EL84) and AC100 (4 x EL34) both take their feed to the centre-tap of the output transformer from before the choke. Likewise the Marshall Major (4 x KT88), the Marshall Bass 100 (4 x EL34), the Sound City 120 (6 x EL34) and the Fender AB763 Twin-Reverb (4 x 6L6). This technique obviously reduces the size of choke needed, and the push-pull output valves will tend to hum-cancel.

On the other hand, Selmer owners were expected to be STRONG, since the Treble & Bass 100 (4 x EL34 and two GZ34 rectifiers) fed all the HT through what must have been a massive choke. Likewise the Zodiac Twin 50.

Down-market, the VAMPOWER 100 (great name, wonder what happened to them?) (4 x EL34) economised by having no choke, but fed all the HT through a high-wattage resistor. That '100' might have been a bit optimisitic.

All this is great fun, but do I use a valve amp these days? No thanks! My Fender 100 watt semiconductor amp has that great clean Fender sound, 2 x 12" speakers, real spring reverb, is very loud when it needs to be, but can be picked up easily using the single strap handle.

I'll leave the fun and games with valves to you other guys.

Ray

[Amanda]

Hi,

The Vox AC30CC series has the switch in the centre tap of the Ht winding,
having measured the peak instantaneous current through the GZ34 rectifier
when switching the standby switch on after warmup it is around twice the
maximum current rating of the valve, and I've actually seen GZ34's flash over
minutely as the standby is switched on, but still work afterwards for so long.

I suppose if you can afford to replace GZ34's at the price decent ones are then that is o.k.

One other point as two Hankies members will tell you, turning an amplifier of whatever make
on and off quickly to plug something in, is just as good at blowing rectifiers, one was an AC30
Heritage and the other was an AC15 heritage!!

Amanda

[petercreasey]

Hi,

The Vox AC30CC series has the switch in the centre tap of the Ht winding,
having measured the peak instantaneous current through the GZ34 rectifier
when switching the standby switch on after warmup it is around twice the
maximum current rating of the valve, and I've actually seen GZ34's flash over
minutely as the standby is switched on, but still work afterwards for so long.

I suppose if you can afford to replace GZ34's at the price decent ones are then that is o.k.

One other point as two Hankies members will tell you, turning an amplifier of whatever make
on and off quickly to plug something in, is just as good at blowing rectifiers, one was an AC30
Heritage and the other was an AC15 heritage!!

Amanda

All very technical Amanda and way above this simple old guitar player's head but WHY do the manufacturers tell us to use the standby switch and then the technical minded folk on forums tell us not to?
Who is right?
Oh and it's not about if you can afford to change the valve it's about it being reliable enough to do it's job. In these days of technical expertise is it not reasonable to expect things to work? Then we can get on with the much more important bit....... playing the music!

[BrianD]

Well I certainly seem to have sparked off some interesting discussions with this question. I have read, with interest, the technical bits but confess they pretty much went over my head and I don't know exactly where the StandBy switch is in the system in relation to Ray's points.

I let my local store look at the answers that have appeared in this thread and they were quite interested and commented that they had certainly heard a number of arguments in recent years stating that the StandBy should be ignored / left in the ON position although they certainly aren't advocating that this is the best practice. When in doubt, follow the manufacturers' instructions then you can at least kick them if it all goes pear-shaped. I also posted a similar query on the BlackStar forum but so far the only responses have been from other users who have experienced similar valve reliability problems to those I have experienced.

As a result of all this I have just bought a secondhand Fender Blues Junior (15 watt and NO StandBy Switch!) to use hear at home and will keep the BlackStar's purely for gigs - they will get very light use!

Thanks for all the responses

Brian

[chas]

Anyone have any advice for my late '90's AC30 (without manual)?

Chas.

[Amanda]

Hi Chas,

If I remember correctly the late 90's AC30 uses a GZ34, so if it was mine
I would leave the standby switch in the on position. but it's entirely up to you!

Amanda

[BrianD]

There still seem to be two schools of thought. As I mentioned earlier in this thread I forwarded many of these comments to an electronics engineer based in Brighton. He is the guy who recently looked into the problems with my Blackstar. He is a fellow guitarist, runs his own small electrical appliance store and, I recently found out, he designs and hand builds amps for a few other guitarists. His comments are as follows -

in answer to your question, you should really preheat valves to get the best out of them, at least 5 - 10 mins it ensures the thermeonics of the valves are at the correct temprature, this is why a standby switch is fitted to amps, but like all things - if your not going to use the amp for a long period of time then just power down, but if your doing a normal set with a beer/fag break, then switch to standby for that period.


I will continue to follow his advice and that of the manufacturers and use the StandBy switch in the prescribed manner.

Brian