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previous: Dave H Re: PT specs for Vox AC30 power amp schematic here? -- 6/30/1998 6:03 AM View Thread

Re: Was PT specs for AC30 (Questions ? )

7/2/1998 9:29 AM
Re: Was PT specs for AC30 (Questions ? )
This AC30 PT talk leads to other thing ,what I have been recently wondering and actually doing .  
I have Crate VC5115 combo , and it has  
physically AC30 / EL84 like setup on power section (4 EL84 ,except Class A/B ,fixed bias , about 25-30 mA /tube ,and there are 100 ohm screen grid resistors ,possibility to connect 220k signal grid to ground reference resistors to ground etc.)  
There are also , as I think you know , good sounding Class A 30 watt VC3XXX Crate models available (I don't mean VC5XXX series is bad ,only different . )  
above models are using same type PCB , so there are places for components needed to get  
Class A setup (huge 10W size cathode resistor , AC bypass cap , and possibility to cut/add jumpers needed for cathode bias)  
Only question mark is B+ , its about 375-380 volts , so I did mod's and haven't got guts to  
bias it not more than to 135-140 mA for 4 tubes (you guessed why) ,screen currents included , voltage between plate and cathode is about 360 volts , therefore what is needed is B+ drop way below ,under 300 , to get it safe with 50 mA idle per tube.  
To the question :  
Does anyone know in what way drop is done (if not ,at all ,only wondering) on socalled Class A Crate models ? different PT rating , resistor and/or zener voltage divider ? or anything else ?  
I have no schems or PT Specs for VC3XXX series , where I could do some spy work .  
On 120-130 mA PT gets quite hot ,as it did it before mods . When played , current jumps up to 210-220 mA ,and also grids are glowing inside tubes in frequency of playing dynamics  
as it did before , quite logic I think  
I have only tested it with power brake , and therefore have no idea ,how is it sound with real speaker load , yet  
BTW : What is B+ on AC30 , some sources claims that ,it is about 340 V ? , is it true ?  
All comments appreciated .  

Dave H 340V is about right for the AC30 B+... -- 7/2/1998 10:00 AM
Doc I have the schematics for the VC am... -- 7/2/1998 1:42 PM
Steve A. JR:    ... -- 7/2/1998 7:33 PM