building a Vox AC30 from the Tino Zottola book!!!

[Curranproducer]

OK kept the MV down and powered the amp up, still has the plates are getting red. I am checking the voltages on the heaters at 2.8v over every pin. the AC off of the recto tube is 300v coming off the pin 8 of the recto tube i am getting 280v, the pin 7 on the el84's is giving me 260v-270v, pin 9 is giving me 260v-270v. The classic tone PT is within the 280-0-280v spec. the tito book is telling me I should be getting 350 volts on most of the HV parts of this circuit. so i am wondering if I have the wrong PT for the components for this amp? the pin 6 voltages for the preamp tubes all seem to be correct with in 10% or dead on. i have also checked continuity on the grounding of the MV and everything seems to be in line. when the MV is full CCW everything is grounded, when CW nothing is grounded

[martin manning]

Do you have a common 50 ohm cathode resistor for the power tubes? What is the voltage at the top of that resistor (cathode voltage)? High current draw will reduce the B+, so let's see where it is. Measure with the MV at zero and also all the way up. There should not be any apreciable difference.

[Curranproducer]

I have 10v on the pin 3of each el84, then I am also getting the same 10V on the 50r 50watt resistor cathode as well. I did move the resistor/cap stack from out from under the chassis, could it have been grounding out and causeing the red plate? it is a LARGE metal bodied resistor

[Curranproducer]

here is a shot of the cathode cap/resistor coming off of the 7 pins I moved it away from the chassis

[Curranproducer]

ok it looks like that resistor metal casing was causing the issue, what is the deal?!?!? I have it "floating" right now, it was fixed in place under the MV pot before. before i moved it I was getting a 0v reading off of both sides of the resistor/cap stack. So how do I afix this or rewire it so I dont have it loose in there?

[martin manning]

If the bypass cap's lead touches the casing and the casing is grounded to the chassis then the resistor is shorted out and tube current goes high. Insulate the cap leads and get them away from the case.

[Curranproducer]

OK!! I am going to get little rubber booties or something for this and then I should be good, just got done ripping on it a but in the loud room of the studio BUT I now have a definite grounding issue. when I unplug my guitar cable it buzzes. should I go back and shield wire the MV section? or is this because of LIFTING the cathode resistor/cap combo after the el84's?

[martin manning]

...when I unplug my guitar cable it buzzes. should I go back and shield wire the MV section? or is this because of LIFTING the cathode resistor/cap combo after the el84's?

That sounds more like the input jack is not shorting the input stage to ground when nothing is plugged in- a faulty contact issue.

[Curranproducer]

Looking at scopes, and these new fangled USB scopes that you can buy look interesting. would this be something to look into for the tube amp thing?

[martin manning]

I would much rather have a stand-alone unit. New digital scopes are selling for about $400, and you can pick up a nice used analog scope for much less.

[Curranproducer]

yeah I have been finding a bunch of older analog scope on ebay for under 100.00 just wanting to make sure I go the right way with the money. I also build studio preamps and compressors so I just want to be able to cover all the bases! Analog it is!!!

[martin manning]

Get some bandwidth if you can, 100MHz is good, and look for a scope with probes included, you'll need x10 type for the voltages found in tube gear.

[Bob S]

I'd take a look at your local craigslist too. Shipping can be just as expensive as the scope - analogue scopes are big & heavy. They are the way to go though.
+1 on x10 probes included. They are expensive when bought separately.

[Curranproducer]

I actually went scope crazy and scored two scopes on ebay for CHEAP!!!!! I got a HP Agilent 1715A 200mhz and a Tektronix 465 100mhz, both are dual trace analog, and they only need calibrated. So NOW I Need to find info on how to use them!!! I figured which ever one works best for tube amps and studio pre-amp/compressor build I do, I will keep. The other one I can use as a back up or drop up onto ebay for some one else!

[Bob S]

You won't be disappointed with the Tektronix 465 - good scopes.
Score !