Mesa Boogie Bass 400+ power tube question

[TASBrendan]

Hi all,

First time working on a Mesa Bass 400+. It has 12 6L6GC output tubes. One of them shorted out the heater and took out the two 100R heater reference resistors and burned the filter cap next to them pretty bad. I've heard different things about how many tubes can be replaced at a time. Tube depot might be the only place that will match 12...but if the other 11 tubes are fine, how many need to be replaced? Just wondering if I can avoid the small fortune of replacing all 12.

Thanks,

Brendan

[Jon Snell]

You can replace in pairs if the idle current is near to the others.
My schematic shows a sextet of 5881s, not an octet.
These are nice valves and a reasonable price for todays shortages; https://www.hotroxuk.com/electro-harmonix-6l6gc.html

[TASBrendan]

Thanks, Jon! That's what I needed to know.
The 400+ does indeed have 12 power tubes. I think the schematic for the 400 is more commonly available and has only 6. The schematic shows 5881s but the one I'm working on has Mesa branded 6L6GCs, so I assume they're the originals. I'll check that shop out and see if I can't find ones that fit the same specs.

[g1]

Schematic with 12 x 6L6 attached.
If you replace only a pair, one should go to 'push' side and the other to 'pull' side. Depending on how they bias up, they will either do more or less work than all the other tubes.

[Helmholtz]

Does the amp work correctly with only 10 tubes (except a litte less power)?

A tube rarely develops a short between the heater pins and even if so it would not take out the heater balancing resistors.
More likely is a short between heater and screen. Especially check the screen resistor and the zener.

Also with that many averaging tubes you might get away with just replacing the bad tube.
The tubes don't operate in pairs here. Rather it's one sextet pushing and pulling against the other sextet.
At large output individual tube idle current becomes rather insignificant.

[TASBrendan]

Thanks for the schamatic, g1! I haven't checked the Zener. The tube most likely shorted to the screen, as Hemholtz said- heater reference resistors totally burned up, and blown mains fuse. The tube even blew through the glass at the pin 1 and 6 holes. Screen resistor and Plate resistor are both in tact, but the screen resistors have drifted high, so I'm going to replace those. I understand the concept of lots of tubes working less, averaging out but I've never dealt with it first hand. Running on 10 tubes would technically cause a slight impedance mismatch although the transformer could probably handle it.
The Mesa tubes have some hand written Sharpie values- I'm assuming idle current- written on top, but most of them are faded. I have yet to put them on my tester. I'll see how they idle up and if I can't get a pair that are close-ish.

Thanks all! I'm headed in the right direction.

[Helmholtz]

Why a pair?
I'd just try find one tube that idles close to the average of the existing ones.

One tube here doesn't have one specific partner.
Rather each tube has 5 partners on one side and 6 partners at the other end of the rope.

[TASBrendan]

I guess the theory is that you can easily get 2 matched tubes and put one on either side and it'll all balance out without having to match the others so closely.
Also, ain't no Zener in there! I've been scouring photos online, too and there's not even a spot on the board for one. Ah well...the 47uF/150k network is looking good.

[Delta362]

I had one of these come thru last summer. Unfortunately for him, 9 of the power tubes were shot and the fan circuit was not functioning. I was able to get a matched 12 pack from Eurotubes for him and give him the original good remaining tubes as spares, though retubing this amp is a pricey adventure.

[Jon Snell]

I guess the theory is that you can easily get 2 matched tubes and put one on either side and it'll all balance out without having to match the others so closely.
Also, ain't no Zener in there! I've been scouring photos online, too and there's not even a spot on the board for one. Ah well...the 47uF/150k network is looking good.

Zeners are supplying the screen grids of the output valves by dropping the screen voltage to around 400volts. Absolute maximum rating for 6L6, KT66 and 5881 as far as I can accertain. Not needed for KT88 versions though.

[TASBrendan]

I see it on the schematic, but it's not in the amp. No missing components on the board and not on the screen grid rails. Can anyone read that on the schematic or know what it's supposed to be? I could add one if the screen supply is above 400 but the amp has been running 6L6s fine for 30 years. Thanks!

[Delta362]

Mine did not have the zeners either. My SGRs were 2k2 instead of the schematic's 680. I've attached an overview photo of what came thru last year. I have some other photos if you need anything else more specific.

[TASBrendan]

Thanks for the pic Delta362- Good reference, looks the same. The one I've got has 680R resistors on the screens. One had previously been replaced with a cement, but the rest are the original carbon comps that have drifted so they're all getting replaced. I might up them if the screen voltage is too high.

[Delta362]

Just looked thru some old notes. I had 479 on the plates, 462 on the screens, and -51.9 on the grids. The owner typically puts several hours a week on these JJ 6L6GC tubes and is still loving it.