Proper operation of a VTM 120- Concerned about overheating

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deltabratwurst
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Proper operation of a VTM 120- Concerned about overheating

Post by deltabratwurst » Sat Sep 10, 2022 6:09 pm

Hello everyone. I am new to the world of tube amps and guitars and could use some help. I recently purchased a used Peavey VTM 120 power amp on a wim (It was a good deal). I was able to test the amp before purchasing. The amp was hooked up to a very large and high power four speaker cabinet and it sounded really great. I brought the amp home and hooked it into the power amp slot of my Boss Katana 50 MKII via the 8 ohms port on the back of the Peavey (using the boss amp as a speaker cabinet). Guitar was plugged into the high gain input jack on the front of the Peavey. I turned the unit on and let it warm up for about 2 minutes before turning on the standby to play. I was able to play for approximately 30 seconds before I started to smell a hot dust/ burning smell. I peaked around the back and saw one of the tubes starting to glow orange unevenly. I also started to hear a buzzing coming from my Boss katana that was steadily growing louder. I hit the standby switch and the tube began to cool down and the buzzing stopped. I am concerned that I was causing the amp to overheat. I think its safe to say I am dealing with equipment that is above my skill level and if I continue to mess around with it I am going to risk damaging it. Any advice on what I could be doing wrong would be greatly appreciated.

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JamesPaul
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Re: Proper operation of a VTM 120- Concerned about overheating

Post by JamesPaul » Mon Sep 12, 2022 7:30 pm

Welcome to the forum.

The VTM manual is available here.

https://assets.peavey.com/literature/ma ... 300704.pdf

I am not familiar with the Katana, but most Power Amp inputs are expecting a Preamp Output signal. That is a line level signal, likely 1V or less. I would expect that jack to feed into the Katana's power amp, which then drives the Katana's speaker. Review your Katana manual.

The 8 ohm output on your VTM is a Speaker Output. It is designed to supply the full power of your VTM to an 8 ohm speaker load. Like the 4 speaker cabinet it was hooked to when you tested it before purchase.

This is likely why your Katana was humming. It was being overdriven by your VTM. Possibly even damaged.

You should connect the Booster Output/Remote Standby jack of your VTM to the Power Amp In jack of your Katana, with the disclaimer below.

Tube amps, like your VTM, use an output transformer to match the power tube's impedance to the cabinet's/speaker's impedance. If there is not a load on the transformer when you use the amplifier, it can easily damage the transformer, power tubes, and circuit.

This is likely why the uneven orange glow on the power tube, which may have eventually turned red (red plated).

At this point, check out your Katana by itself, and ensure it is operating normally.

For the VTM, you need to connect a speaker output to a cabinet, or speaker, and ensure it is operating normally.

To use your Katana speaker with your VTM, you should disconnect it from your Katana, and connect it to a Speaker Output on your VTM. Or connect a dummy load to a Speaker Output on your VTM, and use the Booster Output... connection as described above.

Let us know how your amps check out.

Edit: I reviewed the Boss website and the Katana's Power Amp In is for line level signals. It lists in the Main Features, "Power Amp In for preamps, modelers, and multi-effects". 30 seconds worth of a bad condition, may not have been fatal. Still a couple take aways are always know what level signals you are connecting, and never operate a tube amp without a speaker or load attached.
Enzo wrote:I find if the amp is working, that is a good point to stop fixing it.
James Paul's Peaveys
Decade, Classics, Ecoustic, Windsors, VYPYR, Triple XXX, XXL, VKs, Bandit, JSXs, VIP, Piranha and a Penta.

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