I am currently working on a peavey classic 400 project that I acquired overseas. This is the USA built Peavey Classic 400 bass tube amplifier head made for Japan model. As you can see it is not in tweed but in black, covered in carpet. It dates back to 1996 and weighs in at almost 100lbs for the head only. The only issue with this “labeled non-functioning” head was a few burned out tubes. I am going to replace one capacitor that may have been effected by the burned out KT88. This project is on going and I am still waiting for some parts. I am going to locate a matching cabinet and then update pics, documentation and manuals. Please take a look at some pics before I post the final article.

The cabinets that were released during this period have a specific width and depth. Peavey is constantly changing their cabinet profiles but for this project I would like to see a solid block of both head and cabinet profiles.
The head will need this matching profile: (24.5″ Width & 17.25″ Depth)
The choice of cabinets are these: (210TX, 410TX, 115BX & RBS-2)

Well after much investigation I have some bad news. This amplifier needs a new output transformer. I was able to verify that all the tubes were operational and the preamp circuit was sending signal through. The tube were receiving 650vdc and everything on the main board was operational. I contacted Peavey and the output transformer for this model is not available. I reached out to Mercury Magnetics and I sent in my old output transformer to be reverse engineered. Hopefully this will mean that they will soon be offering these on their website. There is a three month turn time to tear down, inspect and reverse engineer my old output transformer. When it arrives it should be a pretty straight forward install and I will post pictures of the final rig. Not the repair I was hoping for but sometimes vintage gear repair is unpredictable.

So the results came back from Mercury Magnetics and the output transformer tested as good. “It passed 3,000 volt Hi-Pot and all secondary’s were tested and no fault found. We could not detect an intermittent issue nor a break down in insulation or any shorts primary to secondary.” Currently only the preamp signal is passing through to the speakers which means that I am back to the beginning of diagnostics. I will have to look into this further and it is turning out to be quite the difficult repair. So far, tubes, output transformer, primary transformer all test good. The preamp signal is coming from the send outputs and all the faceplate controls work. B+ supply voltage is reading around 675v and the signal gets to the power section. I feel like it has to be something minor I am overlooking.

Update: Still seeking the culprit, some transistors on the preamp circuit were replaced but that did not resolve the issue either. There may be a concern that the center tap voltage is not getting to the output transformer correctly. In the beginning that was an item that was measured as ok. I now have the help of Jeff Snider to help diagnose this repair and resolve it finally. Will update the results when they are in.

Ok the results are in: 08/28/21
This particular model has a primary transformer that is rated (Japan) at 100v. When running it on (US) 120V it also increased the output of the low voltage line to 17.5vdc. This in turn caused an operational amplifier on the preamp circuit to burn out LM3080. Ideally to run at 120V, this chip should be LM3080A which is rated at 22v max or LM3080N rated at 18v max. These chips are discontinued and hard to find. He was able to source an LM3080N rated at 18V max and recommended I run a 20A variac or step down transformer until I can source a higher rated chip. (This chip is located under the preamp board)

Next a resistor was bad causing high voltage line to not make it’s way to the power tubes grid plates. It has a burn in the center suggesting that the original component was burned over time. He increased the size from 15w to 25w since there was enough space. Ideally this power amp could use a filter choke in this location instead of a resistor. Unfortunately due to the size of the power transformer, a filter choke would have to be the size of a 50w output transformer and just wouldn’t fit.

Lastly there were preamp tube problems. There are 5-12AX7 & 1-12AT7. The AT7 was in the wrong location (no markings on the chassis, located back row towards the center of the amp v6). The forward bank v1-v3 (closer to faceplate) of 3 tubes have a dc filament voltage and 1additional tube at v4. The rear bank has 2 ac filament lines at v5 & v6 . 1 tube in the locations v1-v4 had a bad filament causing a break in the dc filament voltage which are in series. He changed all 4 of the 12AX7’s in this category. The filament was burning out due to the increased voltage from the transformer causing all v1-v4 filaments to go dark. Normal voltage should be +/-12.6vdc and the readings were +/-17.5vdc. It may be possible to source some JAN tubes that have a higher filament voltage rating so that the amp can run at 120vac. In this case the best solution will be to run the amp with a 20a step down transformer at 100vac.

The amp was final biased using a matched octet of new 6550 ruby tubes. Overall summary, not any easy repair to diagnose. The tweed models have the 120vac rated transformers, in my opinion they just don’t look as nice. I will post a pic of the full rig when I get the head back. Below are the documents associated with this investigation and additional photos.