Preamp not working after shipping. Mesa 400+

rockbobmel · April 06, 2018, 05:55:47 PM

I ended up selling my Mesa 400+ on ebay. I found an excellent double box. It got there in iin 1 1/2 days with no damage to the box. The power tubes light up but the preamp tubes don't. What course of action should I take. Should I have him ship it back? The amp worked flawlessly when it left here.

Thank You, Bob

Dave W · April 06, 2018, 09:31:39 PM

Considering the cost of shipping, I wouldn't have him ship it back. I'd have him take it to amp repair shop in his area and offer to pay the bench fee to see what's wrong. Better yet would be for you to contact a shop in his area first, have it all arranged for him. That way, if it's actually working fine and he's just trying to get a lower price from you, his bluff will be called.

I'm not saying he's dishonest, sometimes things happen in transit, but it's always good to be a little wary of claims like this.

Highlander · April 07, 2018, 06:22:22 AM

+1 on Dave's comment...

doombass · April 07, 2018, 02:17:22 PM

I agree also. I have'nt found any schematics of the 400+, only the 400. Since the 400+ uses 12 6L6 output tubes and the 400 use 6 6550 output tubes the heater supply might be divided somehow. What puzzled me at first was that the heaters on the output tubes are ok while the preamp tubes are not. That can be explained by Mesa using different supplies/fusing for preamp and output tubes respectivly. Hopefully it's just a shorted preamp tube which will not cost much and the only way to find out is having a qualified tech have a first look at it. Maybe PBG or amptech here have the correct schematics to validate or dismiss my theory?

Dave W · April 07, 2018, 02:46:53 PM

IIRC the preamp section of the 400 and 400+ are the same. I've never seen a schematic for a 400+, and there's no paper schematic inside the housing of mine.

doombass · April 07, 2018, 04:42:37 PM

Yes, I suspect differences would be mostly in the power supply and output stage. 12 output tubes as opposed to 6 would draw a lot more current from the heater supply so I'd recon they are quite different in that area.

amptech · April 10, 2018, 01:55:13 AM

Quote from: doombass on April 07, 2018, 02:17:22 PM
I agree also. I have'nt found any schematics of the 400+, only the 400. Since the 400+ uses 12 6L6 output tubes and the 400 use 6 6550 output tubes the heater supply might be divided somehow. What puzzled me at first was that the heaters on the output tubes are ok while the preamp tubes are not. That can be explained by Mesa using different supplies/fusing for preamp and output tubes respectivly. Hopefully it's just a shorted preamp tube which will not cost much and the only way to find out is having a qualified tech have a first look at it. Maybe PBG or amptech here have the correct schematics to validate or dismiss my theory?

Quote from: Dave W on April 07, 2018, 02:46:53 PM
IIRC the preamp section of the 400 and 400+ are the same. I've never seen a schematic for a 400+, and there's no paper schematic inside the housing of mine.

According to the 400 + schem., the heaters are not divided - nor are they fused, it appears. I would guess a simple solder/wiring fix. Being hooked up in parallel, one faulty preamp tube would not affect the others.

doombass · April 10, 2018, 02:10:09 AM

Quote from: amptech on April 10, 2018, 01:55:13 AM
According to the 400 + schem., the heaters are not divided - nor are they fused, it appears. I would guess a simple solder/wiring fix. Being hooked up in parallel, one faulty preamp tube would not affect the others.

That makes it a quite easy and cheap fix. The advice to have it checked by a tech will anyhow reveal any scam attempt to get partially refunded for nothing if that's the case.

amptech · April 10, 2018, 06:22:31 AM

Quote from: doombass on April 10, 2018, 02:10:09 AM
That makes it a quite easy and cheap fix. The advice to have it checked by a tech will anyhow reveal any scam attempt to get partially refunded for nothing if that's the case.

I just remembered... It might as well be another typical M/B problem; though I have not opened a 400 chassis yet - I´ve had many other M/B amps where the actual connection from transformer to PCB is the problem. The heater wires shares a common connector (along with HT wires) and can just barely take the heater current. Easy to see - the white plastic connector is brown (even black) around the (usually) green heater wires. I´ve had a couple of cases where the connector is broken/burnt, stripped end of wires corroded and the pins of the connector have loosened from the PCB. I dont´t care much for this design feature.

So, logically, if the preamp/driver tubes have their own connector on the PCB, that´s where you should expect a problem. At least, I´d start there if a customer described the problem like you did.

rockbobmel · April 19, 2018, 07:08:29 PM

Well I hooked up the buyer with a tech in WV. Turns out he is well respected with glowing reviews. He is Charlie Powers in Fairmont, West Virginia and goes by Guitar CPR. The guy got it fixed and running in no time. It turned out a wire came off during shipping. I offered to reimburse the buyer for the cost and he gave me a graceous pass. WOW! He was so up front and was thankful that I helped, he just said "we're good. I'm happy"
There IS a GOD!

Dave W · April 19, 2018, 07:34:54 PM

That's great news!

gearHed289 · April 20, 2018, 08:38:23 AM

Excellent. It's nice when humans are just nice.

doombass · April 21, 2018, 05:53:46 PM

Wow, what a relief!