6550s in a Mesa D180? That's Not a Thing, Right?

[fu22ba55]

Saw this ad on Reverb:

Mesa Boogie D-180 (No Graphic Equalizer) 6550s! | Reverb

This D-180 is a transition model from the early eighties. It doesn’t have a graphic Equalizer as you can see from the pics, which makes this particular head a rarity! It’s fresh from the bench having a fresh sextet of 6550s! This rockin amp is amazingly loud and clear. If I were to keep it, I’d ...

reverb.com

I'm not in the market, and there are some others already on sale here on TB and elsewhere on Reverb,

But 6550s and 6L6s are NOT interchangeable, right?

This is either a typo or a surefire way to damage this amp?

Does anyone see mods in the pictures that would enable this kind of substitution?

 

[Wasnex]

AFAIK the D180 was originally designed to run 6550 (see image below).

Not 100% certain, but my impression is the factory bias on some D180S and 400s may be set to a middle ground that sort of works for 6550 and 6L6GC. The bias in my D180 is -59V and it is running a 6L6 variant. There is a hand drawn D180 schematic that shows bias at -56V. The schematic for the 400, which is a very similar amp, shows bias at -63V.

Changing the bias should be a matter of swapping out a single resistor in the bias supply. Shouldn't be hard if you know what to do. Some users mod the amp to have variable bias.

 

[mikeswals]

The non eq ones are not the rare ones...

 

[mikeswals]

AFAIK the D180 was originally designed to run 6550 (see image below).

Not 100% certain, but my impression is the factory bias on some D180S and 400s may be set to a middle ground that sort of works for 6550 and 6L6GC. The bias in my D180 is -59V and it is running a 6L6 variant. There is a hand drawn D180 schematic that shows bias at -56V. The schematic for the 400, which is a very similar amp, shows bias at -63V.

Changing the bias should be a matter of swapping out a single resistor in the bias supply. Shouldn't be hard if you know what to do. Some users mod the amp to have variable bias.

View attachment 7457309

It seems back then Mesa wanted to try making a production 6550 amp. I got my Bass 400 in 91 (and 2 yrs later added a D180), immediately went straight to Mesa/Hollywood since a tube blew in the 400 and it was only a 2.5 hr drive away for me. Even back then the guys there said Mesa didn’t have a consistent supply of 6550s so would just run 6L6 in everything.
In 35 years -while I've obviously not seen all the D180s out there- I've still never seen one stock with 6550s. The Hollywood guys told me the D180 was 6L6 only, and the 400 would run either one.

I've had several pieces of flyers and literature from back then there was always oddities in pre-production units, just like that white D180, I saw that pictured back in those days but have never seen one pictured by any owners.

 

[agedhorse]

I would sure want to be sure that whatever work was done to the amp was done correctly (including the bias). I don’t know if the stock power transformer can safely supply the 6550 heaters (which draw more current than 6L6 tubes). I don’t have access to any of those original documents.

 

[Wasnex]

I would sure want to be sure that whatever work was done to the amp was done correctly (including the bias). I don’t know if the stock power transformer can safely supply the 6559 heaters (which draw more current than 6L6 tubes). I don’t have access to any of those original documents.

I don't have my D180 and 400 open right now, but my notes suggest both amps have the same transformer P/Ns: OT (0180322) and PT (1905550 .

 

[agedhorse]

I have notes that suggest that earlier amps may have used a different PT, or the PT was beefed up knowing that the 400 was in the queue. We often did running changes using the same part number as long as the new part was backwards compatible.

The challenge is that it’s an old amp, and factory PTs are no longer available which means that a resulting repair will get really expensive in a hurry.

 

[coreyfyfe]

I don't have my D180 and 400 open right now, but my notes suggest both amps have the same transformer P/Ns: OT (0180322) and PT (1905550 .

I have p/n 561180 in my 400, looks like this may have been common to a number of amps if this list is accurate

FS: Mesa Boogie power transformer (fits various models)

Technical specifications: * Mesa Part no #561180 (identical to #561203, #561200) * Serial number EIA606-915. * A drop-in replacement for the following Mesa Boogie amps: * Stereo Simul-Class 295 poweramp. * Bass 400 / 400+ amp * Baron poweramp * Buster Bass...

boogieforum.com

On the heater supply - somewhat of an aside, but I looked at the voltage in my 400 with 6550, 6L6GC, and a set of verellen octals, the readings were 6.5, 6.7, and 6.9VAC, respectively. Certainly more current being pulled with the 6550s and dragging that down a tad, whether the D-180 can handle the current draw is a different question. I've considered putting the 6550s in my 400, but honestly I like the sound and feel with the 6L6GC so I opted not to make any changes.

 

[agedhorse]

I have p/n 561180 in my 400, looks like this may have been common to a number of amps if this list is accurate

FS: Mesa Boogie power transformer (fits various models)

Technical specifications: * Mesa Part no #561180 (identical to #561203, #561200) * Serial number EIA606-915. * A drop-in replacement for the following Mesa Boogie amps: * Stereo Simul-Class 295 poweramp. * Bass 400 / 400+ amp * Baron poweramp * Buster Bass...

boogieforum.com

On the heater supply - somewhat of an aside, but I looked at the voltage in my 400 with 6550, 6L6GC, and a set of verellen octals, the readings were 6.5, 6.7, and 6.9VAC, respectively. Certainly more current being pulled with the 6550s and dragging that down a tad, whether the D-180 can handle the current draw is a different question. I've considered putting the 6550s in my 400, but honestly I like the sound and feel with the 6L6GC so I opted not to make any changes.

Take that information on transformer part numbers from the Boogie forum with a large grain of salt. Those 3 part numbers are not identical.

 

[Wasnex]

I have p/n 561180 in my 400, looks like this may have been common to a number of amps if this list is accurate

FS: Mesa Boogie power transformer (fits various models)

Technical specifications: * Mesa Part no #561180 (identical to #561203, #561200) * Serial number EIA606-915. * A drop-in replacement for the following Mesa Boogie amps: * Stereo Simul-Class 295 poweramp. * Bass 400 / 400+ amp * Baron poweramp * Buster Bass...

boogieforum.com

On the heater supply - somewhat of an aside, but I looked at the voltage in my 400 with 6550, 6L6GC, and a set of verellen octals, the readings were 6.5, 6.7, and 6.9VAC, respectively. Certainly more current being pulled with the 6550s and dragging that down a tad, whether the D-180 can handle the current draw is a different question. I've considered putting the 6550s in my 400, but honestly I like the sound and feel with the 6L6GC so I opted not to make any changes.

I believe this is what is in my amps.

• D180 and 400 PT = 190555
• 400+ and Buster PT = 561200

AFAIK the 190555 PT runs a bit higher voltage on the HT winding than 561200