• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

VTL MB-300 MB-450 Amp help

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I am hoping someone here will have knowledge on VTL vintage amps as I have some questions and perhaps learn more about amp maintenance and what I have. This weekend I bought a pair of VTL MB-450 amplifiers. I was told that the amplifiers were upgraded from MB-300 to MB-450 around the 2000 time frame. These amps were in storage for years and the owner lives outside the country. A friend was selling them for him and did not know much about them.

A manual came with the amps but the procedures and test nodes seem quite different on my amp. Here are some questions I have:

1. The amp front plate has an an ammeter and an 8 position switch, see picture (I assume for measuring the bias on the tubes). The manual says nothing about this.

2. In the center of the amp between the tubes appears to be another switch (see picture) between the red test nodes. This also is not in the manual.

3. There also appears to be 8 adjustment pots in the back (I assume for each of the tubes).

4. In the manual it says that you should never turn the amplifier on without a speaker being connected. Is the amp unstable without a speaker connection?

5. It has two driver tubes a 6350 and a 12AT7WA. I believe some of the other models used a 12BH7 (instead of a 6350). Does this mean I have a series l or series ll amplifier?

I would appreciate it if some has the correct manual for me to use or can give knowledge on what I should do in bringing these back to life.

Best Regards (and thank you for any help).
As these units have been in storage for years, be careful not to power them up without precautions.
Use a variac or put a light bulb in series with the mains.
Power supply caps can fail catastrophically when they have been without voltage for a long time and get full voltage at once.
A quick Google showed that these amps run at 550V and have a lot of capacitance and current capability.

My apologies as I thought my pictures were downloaded to the thread.


  • MB-450 Cover Plate.jpg
    MB-450 Cover Plate.jpg
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  • MB-450 Switch.jpg
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  • VTL MB-450 switch and test nodes.jpg
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yes bring them on a variac, but before you do, connect either an 8 ohm power resistor to the speaker terminals or a cheap speaker. either will work. most likely the main filter caps are still good.

if you see a high current draw beginning to form on your variac, then shut it down.
The VTL have a big caps on Vrail so it is better to start with a strong variac.
The Idle current is around 27-30 mA each power tubes; this type of amp with meter is the first I have seen.
In every case normally VTL put a 10 ohm resistor from cathode to ground of each power tube to check the current,
The two test point and the trimmer is about the balance on phase splitter; a scope will help
But if oyu send an email to VTL att. Luke mybe you will have little help.
The 6350 hasn't a sustitute; the only one close to it is 6463

Thank you for the help. I was able to resolve the issue. VTL was unable to help me. As it turns out, Manley Laboratories was able to help me. Manley Labs split off from VTL a long time ago to go after the pro audio market. The switch is used to bias each of the outputs. The other test node is for an AC signal check. I was able to get it up and running and it sounds excellent.

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