Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bakerlite on January 17, 2011, 11:20:09 am

Title: can anyone help ac30/6 problems
Post by: bakerlite on January 17, 2011, 11:20:09 am: hey folks i have an old 1965 vox ac30 normal layout with added vox top boost on an l plate issued by vox,
this problem ihave is that the vib/trem channelll while being perfectly loud in volume ithe sound of the actual effect is very very feint., it is a tad more audible for half a second when you engage the footswitch but only just,
i have changed the ecc82 and two ecc83s in v7 v8 and v9 and itmakes no difference , i also checket the footswitch is functioning whch it is.
all l know is that this is the type of vib trem that uses two stages of tubes modulating against each other to create the effect but idont know where to check next totroubleshoot, can anyone help?
thanks
Title: Re: can anyone help ac30/6 problems
Post by: PRR on January 17, 2011, 09:29:19 pm: Schematic somewhere?

I smell an open grid resistor, but I may be full of it.
Title: Re: can anyone help ac30/6 problems
Post by: bakerlite on January 18, 2011, 06:49:29 am: thanks but late last night i traced it down to a bad solder joint - to be honest i was lucky to come accross it.
The last thing wrong is the wimpy Brilliant channel - this is the channel that also has the L Bracket with top boost mod issued by vox before they started top integrate it - basically its a mid and a bass control with an ecc83 at the back of the amp.

http://members.optusnet.com.au/~glen325/topboost.html info on the mod if you are interested

i have used the new voltage data aquasition page which is brilliant! :

http://www.el34world.com/charts/valve/ValveData.php?e=view&f=10370

v1 is the pre amp and first stage of vib/trem
v2 is the modulator for vib trem
v3 is the oscillator of vib trem
v4 is the pre amp for normal and brilliant channels
v5 is the TOP BOOST mod tube
v6 is the phase inverter

maybe someone can point me in the right direction as i dont know why this channel is low volume and muddy and the other two (normal and trem/vib) are ok - especially as this channell should be the epic super loud channel!!
Title: Re: can anyone help ac30/6 problems
Post by: bakerlite on January 18, 2011, 08:59:05 am: also - it seems that when plugged into the brilliant channel (the onbe with the wimpy volume trouble) and NO patch cables i get more volume by turning up the Normal Volume.
I know the way these amps work it is common but i am actually getting like really LOUD volume of the normal when plugged into the brilliant channel like way more than what the actual brilliant volume can fo at max.

thanks again
Title: Re: can anyone help ac30/6 problems
Post by: archaos on January 19, 2011, 05:04:12 pm: Can you post pics please ?
Title: Re: can anyone help ac30/6 problems
Post by: bakerlite on January 21, 2011, 02:24:11 pm: turned out to be a faulty 500pf grid capacitor , read 2.6 pf on my meter! the make of the capacitor is hunt , a small axial type. apparently many 40 year old hunts bite the dust.
gotta say , love the ac30 sound!
thanks again
Title: Re: can anyone help ac30/6 problems
Post by: archaos on January 21, 2011, 03:34:05 pm: It seems that cap can sometimes be faulty in AC30s ; you're definitely not the only one having that kind of breakdown. Glad to see that all is fine now.