svt vr preamp valves question

solitario · #1 12-01-2011, 02:04 PM

hello, I have an svt vr, it is supoused to have 3 x 12AX7, 2 x 12AU7 valves in the pream section but looking inside I can only see 3 preamp tubes and the 6 power tube. where are the other 2 valves?

coreyfyfe · #2 12-01-2011, 02:08 PM

The ones you see from the back and the phase splitter and driver tubes. The actual pre-amp tubes are in the upper chasis section, which has to be removed from the amp casing to access them. Take out the large screws on the top of the head and it should slide out.

beans-on-toast · #3 12-01-2011, 02:25 PM

The tube sockets are mounted on the circuit boards inside the pre-amp as Corey said.

If you open the pre-amp, be careful, there are several small cables with connectors. You don't want to accidentally pull one out or push it against the tube.

There are five small tubes in the pre-amp.

solitario · #4 12-01-2011, 03:45 PM

hold on hold on...driver tubes? im sorry but what's that? dose it has preamp tubes power tubes and driver tubes?

I mean these 3 tubes I see from behind are driver tubes but are they the ax7 tubes?

if there are 5 tubes in the preamp and I cant see the preamp but I see 3 small tubes on the back... how many valves has the vr? I thought it has 6 power tube 6550 and five preamp tube 3x12 ax7 2x12au7, if there are 5 tubes in the preamp what kind of tube are those driver tubes I see? are they part of the 5 tubes in the preamp? what are they for?

my amp is a usa model and it has a horrible hiss in the chanel 2 if you add highs or ultra highs, almost like a radio out of tune noise, I think it can be a problem with tubes thats y I'm asking how many tubes it has and where are they located

thank you for your help, it is my first tube amp and I want to learn a bit more about them

coreyfyfe · #5 12-01-2011, 03:57 PM

The driver tubes transmit the signal from the pre-amp into the power amp. The SVT-VR uses one 12AX7 and two 12AU7s as the PI and drivers, those are the ones in the power amp chasis near the 6550s. All the other tubes should be up in the pre-amp chasis. I'm not sure how many the VR uses exactly, but that should be the locations.

According to the ampeg website you should have:

Preamp: Tube (3 x 12AX7, 2 x 12AU7)
Driver Tube(s): 1 x 12AX7, 2 x 12AU7 Power Amp: Tube (6 x 6550)

Thus 8 total small tubes, 6 large tubes. As long as they're laid out like they are on the vintage amps, there should be 5 small ones up top, three down bottom. I haven't seen the inside of a VR so I can't be sure of the exact layout.

solitario · #6 12-01-2011, 11:54 PM

thank you! where will you start looking at to find the hiss noise in chanel 2? preamp or driver tubes? I have seen that svt cl has only 1 channel and 2 12ax7 preamp valves instead of the five valves the vr has. so the question is, any of the 5 preamp tube or driver tubes in the vr are just for chanel 2 or all of them work together with chanel 1 and 2?

I cant understand why chanel 1 is perfect and channel 2 is so noisey is almost useless. this vr amps are supoussed to be used bridging channel right?

Thank you

coreyfyfe · #7 12-02-2011, 04:34 AM

According to the schematic JohnK posted in this thread, the "V5" 12AX7 is for channel 2 and would be the one to check. That's if I'm reading it right, it's on page two of the attached schematic in John's post.

beans-on-toast · #8 12-02-2011, 05:59 AM

Quote:

Originally Posted by solitario

where will you start looking at to find the hiss noise in chanel 2? preamp or driver tubes? I have seen that svt cl has only 1 channel and 2 12ax7 preamp valves instead of the five valves the vr has. so the question is, any of the 5 preamp tube or driver tubes in the vr are just for chanel 2 or all of them work together with chanel 1 and 2?

I cant understand why chanel 1 is perfect and channel 2 is so noisey is almost useless. this vr amps are supoussed to be used bridging channel right?

I have experienced the same problem with hiss on channel 2. A better set of pre-amp tubes can help.

You have it correct Corey. In the SVT-VR pre-amp V1, V2, and V3 are just for channel 1. V5 is just for channel 2. V4 is shared by both channels, each one uses half of the tube.

V1 and V5, both 12AX7 tubes and both are the first tube in the signal chain for each channel. It helps to use a low noise tube such as a 7025 in these positions. Sometimes you have to try several tubes to find what works best.

My preference is a good new old stock tube made in North America, Europe, the UK, or Japan. I have had good results with the JAN-Philips or RCA 5814A, which is a military version of the 12AU7. You can never go wrong with a good Telefunken ECC803, ECC803S, or E83CC which are 12AX7 tubes. Unfortunately, Telefunken tubes are very expensive. ECC83 is the European equivalent of the 12AX7. Amperex from Holland is also very good.

staindbass · #9 12-02-2011, 06:11 AM

driver is just a term for another preamp stage.

Jerrold Tiers · #10 12-02-2011, 07:17 AM

Quote:

Originally Posted by staindbass

driver is just a term for another preamp stage.

Not quite, they are actually "IN" the power amp...... and are part of that "feedback loop" in amps that use feedback (not all do) but if you "chop logic" just right, your statement can be "sorta-truish".

Nominally they are for "driving" the grids of the power tubes (which can require some actual power to drive), and one or more may be used as a "phase inverter" to split and invert the signal for the 2 sets of power tubes (that handle opposite halves of the signal).