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Vintage SVT (MTI) biasing procedure?

#21
I've never heard of anyone saying MTI's are 350 watts.
I've owned 2 of them and they have both tested clean output @ about 270 watts like all other 6550 svt's I have had and have worked on.

My old 70 6146 SVT runs 360 watts clean tested the same as all SVT's.

I think some old ads for MTI's may have said 350 but that was just marketing bs or something.
 
#23
By the way, I have in mine white-covered 12DW7 so channel 1 does not work. Can I leave it now for some time and use only channel 2 for playing? i mean, does it hurt the amp without one 12Dw7 in the preamp?

I think I have some MTI which is made from magnavox parts (some preamp tubes are still ge&magnavox, power tubes are GE and Philips - mixed but bias is stable around 70mA/side) - because it has 12DW7's all over the place and also on schematics printed inside. Can I change 12DW7 to 12Ax7 without mods? I read some about changing resistors, but what will happen ifI don't do that?
 
#25
OK.

The amp is dead quiet I mean in positive way, it does not hum or crackle... but The switches behave completely different than in SVT-CL. Turning the ultra low on is like perfect sound for playing with a pick, beefy but without "sub bas" and mud that CL had when i turned ultra low on it.

I actually like mti better with ultra low on than without (which sounds kind of "hard")

BTW. I was cleaning the amp and removed power tubes, then put them back, powered the amp and it went loud "booooooooooo" + strange smell. Two tubes had no light. I saw that almost every tube has no plastic ding in the center of the pins which sets it right on the tube socket, so I put the tubes in wrong way (!!!). But did this again in proper way and amp sounds good. Did I burn something?
 
#28
Can I replace 12Dw7s in driver and preamp for 12ax7s without mods? Schematic stickers inside the chassis says 12DW7 everyhwere... but if I read correclty EIA codes on transofrmator they;re 81 and 82 year so kind of late for "spare Magnavox-era part unit made by MTI" hm?
 
#29
boroman said:
Can I replace 12Dw7s in driver and preamp for 12ax7s without mods? Schematic stickers inside the chassis says 12DW7 everyhwere... but if I read correclty EIA codes on transofrmator they;re 81 and 82 year so kind of late for "spare Magnavox-era part unit made by MTI" hm?
Click to expand...

For best results when 12DW7 is replaced with 12AX7, there is a mod, involving changing some resistors so that circuit is appropriate for the 12AX7 in the places where the 12DW7 has the higher current tube portion (the part which is like a 12AU7).

You do not really "have to".... but it improves performance. Not quite to where it is with the right tube, though.
 
#30
Yes, the 12AX7 can be substituted for a 12DW7 in the power amp without any changes. You can do this because of the way the tube is being used in the power amp. It works out well if you don't have a spare 12DW7.

A 12DW7, like the 12AX7 is what they call a dual triode. The tube has two tubes built into one package. A 12DW7 is one half 12AX7 and one half 12AU7. The 12AU7 has less gain but more current drive capacity than a 12AX7.

You can substitute a 12AX7 for a 12DW7 in the pre-amp but it will change the sound a little. To make a 12AX7 work more properly in the pre-amp, there is a simple modification that you can perform. It involves changing some resistors as Jerrold said.

MTI had a procedure to convert the 12DW7 to a 12A7 in the SVT's because good quality 12DW7's were hard to find. Their procedure said that in the pre-amp, change resistor R25 to 4.7K, R6 and R7 to 220K, then plug in the 12AX7 in place of the 12DW7. The original SVT had R25 as 1.5K, R6 was 47K, and R7 was 100K. These changes affected tube positions V1 and V3; V4 did not require any changes and a 12AX7 could be substituted.

These changes are not perfect, the amp tone changes a small amount. You can try it and see if this is what you are looking for.

If you are changing resistors, I would look at you amp and compare the values of all the resistors with a schematic from the early SVT's (Invalid Link Removed, Invalid Link Removed). You never know if someone has modified your amp already.

PS: Boroman, I wasn't able to respond to your PM because you have receiving PM's disabled in your talkbass setup. You have to enable this feature in your configuration.
 
#31
Thank you both for the replies. beans-on-toast - I think we can discuss it here, but I will look for that features in talk bass setup - thanks for the info. I've searched a lot about this and found some info but I'm not an expert in soldering ;)

By the way, I inserted 12ax7 on the first channel instead of 12dw7. Its more "gainy" but also more balanced than the second channel with 12dw7 which sounds rather "contoured" with upper mid pronounce.

What really confuses me, that power amp 12DW7's. If they're completely interchangeable so why they used 12dw7 back then? It does not use the second (u7) half? What is the difference in the power amp of having dw7 vs ax7?

And the second question goes to "balance" pot. Can I just bypass it when setting up new power tubes? Or it is a MUST to do this? Tried to mess it with VERY LITTLE but I see no change in my voltmeter (v1+ground, then v2+ground).

And, if it's a must to do this, can I just generate 40hz sine wave and put it though input jack with an iPod device (I know, i know, but hey, I am not a tech in any means) and the 810 cab @ 4ohms. And I should be getting 0V between V1 and V2 (so, if I understand correctly, connecting voltmeter red to V1 and black to V2 - without the ground?)

I'm a little but confused, excuse me.... :(
 
#32
boroman said:
And the second question goes to "balance" pot. Can I just bypass it when setting up new power tubes? Or it is a MUST to do this? Tried to mess it with VERY LITTLE but I see no change in my voltmeter (v1+ground, then v2+ground).

And, if it's a must to do this, can I just generate 40hz sine wave and put it though input jack with an iPod device (I know, i know, but hey, I am not a tech in any means) and the 810 cab @ 4ohms. And I should be getting 0V between V1 and V2 (so, if I understand correctly, connecting voltmeter red to V1 and black to V2 - without the ground?)

I'm a little but confused, excuse me.... :(
Click to expand...

This is correct, no "ground" as you are checking for imbalance between the push and pull sides of the output tubes.
 
#35
boroman said:
What really confuses me, that power amp 12DW7's. If they're completely interchangeable so why they used 12dw7 back then? It does not use the second (u7) half? What is the difference in the power amp of having dw7 vs ax7?
Click to expand...

I suppose that the designers had their reasons for using the 12DW7 at the time. Some SVT amps have a 12AX7 in this position.

You can safely substitute a 12AX7 for a 12DW7 in the power amp and it will work without any modifications. You can do this because of the way that the tube is functioning in the design of this particular stage in the power amp. Both halves of the tube are being used. The output impedance of the 12AU7 part is not the same as that of a 12AX7. There will be some very small differences that can affect the performance and signal to noise level. It will not affect the power output.

Try it, listen, and see if you hear a difference then decide which tube is best.
 
#36
Thanks! Will do that today and see how it changes the sound.

And about DW7 vs AX7 in the preamp... You said that second part of dw7 tube is cathode follower, so in ax7 it will have different voltages etc so it will sound different. But does it harm the amplifier parts or tubes when set ax7 vs dw7 or it just be different sounding and thats all?
 
#38
If someone would like to have some gear porn - here are the shots and videos from the inside of my MTI (sorry for the quality, it was shot by the cellphone)

Download - Streaky Upload

I wonder about two things more. First one that 450v silver caps look like they blew up but it isnt an electrolyte. This is probably glue sticking to the plastic bound...? The second thing as you may see (better on video) is "standby" light placed in the front metal panel. And it lights when the amp is in bypass mode only (helpful from the standing point in the front), but it wasn't original to these models?
 
#39
You have a very nice amp. There are high quality components and it is built like a tank. The layout looks very similar to the early models but they have made a couple of improvements.

The brown stuff on the capacitors is old glue. In general you can carefully clean the dirt in the amp with isopropyl alcohol (IPA). It is often available at drug stores. Q-Tips, cotton pads, and pipe cleaners are useful when cleaning. Try using the alcohol to remove the glue. If you need to apply new glue to hold the capacitors in place, a silicon glue works well. Usually, just the plastic ties are sufficient though.

Some SVT's had a light for the standby next to the power switch. I couldn't see yours.
 

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