svt pro 2 tube help

[leftychris]

hi all

right ill start at the beginning just got this amp last week it had a shattered 12ax7 in the preamp probably from being mis-handled by the courier !!!

it was making a deafening hiss so i replaced the valve now its alot quieter but there is still a hum will this type of amp ever be completely quiet

looks like none of the valves have been changed so started looking into it, keeping in mind this is all new to me

some sites say the v1 is the most important tube and others say its the phase splitter now wot i dont understand is the phase splitter in the pre amp section or the driver section !

depending on wot valve sites you are in there selling pre amp re-valve packages with one balances valve for the phase splitter position

so from bottom left on my svt 2 pro next the input jack is

12ax7 12ax7 12ax7 12au7 12ax7 pre amp yeah ?

and at the back behind the power tubes are from left to right are

12ax7 12au7 12ax7 Driver tube yeah?

so wot is V1 and wot is PS and wot needs to be balanced and wot doesnt

thanks in advance

[coreyfyfe]

V1 as you're referencing would be the first tube in the preamp section, in this case the 12AX7 nearest the input jack is likely the one.

The three near the back are the phase splitter and driver tubes. There was a thread recently where Jerrold Tiers (former Ampeg engineer) stated that the for the phase inverter it is not essential that the halves of the tube be matched/balanced, but that your two driver tubes should be matched for best performance. Just to clarify, in this quote V1 is referring to the PI and V2/3 to the drivers - different than V1 of the preamp.

Originally Posted by Jerrold Tiers

Both halves of V1 are used....... one is input amp, the second is phase splitter/inverter

Assume you mean V2/V3 are the matched ones, as they are the ones that can use the matching.

IF you have an OLD SVT, an MTI version, an SLM "SVT-HD" or an SLM or Loud "SVT-VR", there is a balance adjustment which much reduces the need for a matched pair of tubes for V2/V3.

And, of course, since the bias is separate for the two groups, that is no reason to match either..... You really only need groups of 3 outputs matched, and drivers need no be matched at all, because between the two bias controls and the balance, it's all covered.

For SVT-II, SVT-CL, SVT-2PRO, there is no balance control, and in those cases matched driver tubes may be of some advantage.

[coreyfyfe]

Also, your driver section should be one 12AX7 and two 12AU7s - the 12AX7 being the splitter, the 12AU7s being the drivers. I'm no expert, but being that 12AX7s are higher gain than 12AU7, if you have a 12AX7 as a driver, that could be throwing off the bias and causing some of your hum/hiss issues.