SVT Newbie Question about 12DW7

[jakejoseph]

I just bought an MTI era SVT from a reputable used company. Sounds great, but I have a couple of questions:

1) I know that they changed from 12Dw7 to 12AX7 at some point in time. Since mine has no tube replacement chart for the pre amp tubes - what are the preamp tubes from left to right facing the back of the amp? Right now, it is 6C4, 12AX7, 12AX7, 12AX7, 12AX7. Is there any way to tell if this was a 12AX7 amp without opening it up? Did someone jamalam 12AX7a's where 12Dw7's should go? The phase splitter on this is a 12AX7 - and the tube chart says this. DOes this mean that all preamp tubes are 12AX7?

2) Which way does the fan blow on this? What CFM fan do I need? It is LOUD as loud.

3) I can only turn it up to 3 before it is too loud to listen too - is that normal?

[beans-on-toast]

I don't know which version your amp is.

Here is a copy of an early 70's tube chart. One these amps, you can use a 12AX7 in place of the 12DW7 in the V1 position in power amp without any changes.

In the pre-amp, you need to modify the amp to properly use a 12AX7 in V1 and V3. For V4 you can substitute a 12AX7. It won't hurt to simply plug in 12AX7's but half the tube has a higher gain than that of the 12DW7. It will be more distorted as you increase the volume. Some people prefer this. The only way to tell how your amp is set up is to look inside and check the resistor values. To use a 12AX7 in the pre-amp, change resistor R25 from a 1.5K to a 4.7k, change R6 from 47K to 220k, and change R7 from 100K to 220k.

The fan spec is 120 VAC, 60 FCM, 120 mm X120 mm X 20mm. Airflow into the amp. Yes it is loud. Quieter replacements can be installed.

Loud at a volume setting of 3 is normal. SVT's are loud amps. They like to be driven that way.

[jakejoseph]

Thank you sir.

Did MTI make amps with 12DW7's in the pre and 12AX7A as the phase inverter tube? Or when they converted the phase inverter to 12AX7A (which my amp lists as the correct tube) the convereted all of the pre-amp tubes to 12ax7?

Very helpful info.

J

[beans-on-toast]

MTI made some amps with 12DW7's and eventually switched to 12AX7's, as you stated.

As I recall, when they first switched to 12AX7's in the pre-amp they didn't modify the amp. The resistor mods that I mentioned above came from MTI. So they must have eventually modified the amps to better accept 12AX7's. As far as I know, they always had a 12AX7's in the phase inverter.

Checking the resistors is the only way to know for sure what is in the amp. If the 12DW7 resistors are there, that still isn't going to tell you what tubes came in your amp originally. If the 12AX7 resistors are there and they are original, then that is the tube that your amp probably came with.

I wouldn't worry about what came in the amp. Put whatever you think sounds best in the amp. If you want to tone things down a bit, try the 12DW7's in the amp. You can also experiment with lower gain tubes from the 12A?7 family in place of the 12AX7's. Examples include 12AU7 (gain(mu)=17-20), 12AV7 (37-41), 12AY7 (42-44), 5965A (45-47), 12AT7 (60-70), 5751 (70), 12AZ7 (59-60), 12AX7 (100-115).

[rickdog]

Originally Posted by jakejoseph

I can only turn it up to 3 before it is too loud to listen too - is that normal?

I almost sprayed my beverage across my keyboard... try playing it in a bigger room!

[jakejoseph]

Originally Posted by rickdog

I almost sprayed my beverage across my keyboard... try playing it in a bigger room!

I thought everyone would enjoy that...

[TomB]

Definitely try 12AU7's in place of 12DW7 in V1 to help tame the beast. It should give you more usable range in the gain stage, IME... still plenty of heft, though. I've heard a 12AT works well too but have not tried it.