Modding SLO30 Question

[Mr. Willy]

Is this possible? How hard/difficult would it be to convert the SLO30 from using 5881s to 6550s?

 

[BackCrack]

If the SLO30 is B.A.D.'s version of the HR25, then I would say no if using the stock iron. The HR25 runs its 5881's at reduced plate voltage, and that voltage is transformer supplied. It's hard wired in. The operating plate voltage for a pair of 6550's is way different. So you'd have to (1) experiment with 6550's to find the reduced plate voltage sweet spot, and then (2) get the proper specs to a transformer maker and get the iron made.

But, I may be way off, which wouldn't be the first time.

 

[Mr. Willy]

If the SLO30 is B.A.D.'s version of the HR25, then I would say no if using the stock iron. The HR25 runs its 5881's at reduced plate voltage, and that voltage is transformer supplied. It's hard wired in. The operating plate voltage for a pair of 6550's are way different. So you'd have to (1) experiment with 6550's to find the reduced plate voltage sweet spot, and then (2) get the proper specs to a transformer maker and get the iron made.

But, I may be way off, which wouldn't be the first time.

Thanks for the reply! My tech knowledge on this is minimal, but that’s primarily what I was wondering, if the transformers could handle it. I think if they could handle it, it’d just be a matter of changing out some other minor parts.

 

[fusedbrain]

Biggest issue going from 6L6 variant ( 5881) to el34 variant ( kt77, 6550 ) is heater current.
El34 variants will draw close to double the heater current of 6L6 variants. If power transformer is designed for 6L6 types only, installing el34 type tubes will likely destroy the power transformer.
Specs of power transformer must always be confirmed before attempting this mod.

 

[harddriver]

Is this possible? How hard/difficult would it be to convert the SLO30 from using 5881s to 6550s?

I think you might be better off just running 6L6's in place of the 5881's. 6550's are pretty much a larger big bottle 6L6 tonewise. 6L6's should work just fine in the SLO30 and give similar feel and tones of the 6550's IMHO.

The primary impedance of the SLO output transformer will be specifically for 5881/6L6's. Marshall output transformer's were spec'd for EL34's only. 6550's and 6L6's are a mismatch but they will operate. The big fear about 6550's was runaway bias so the Marshall engineers reduced the 220K PI grid leak resistors to 150K to lessen this possibility which ended up not being much of an issue in the end.

Mike Soldano uses the 5881 for specific reasons in the SLO's.

Usually the bias range will have to be set to a pretty wide range to run EL34's, 6L6 to 6550's, most amps use a 25K bias pot. You can use a 50K bias pot and then the bias circuit dropping resistors will need to adjusted to get the bias range on the high and low end where you want it.

An EL34 amp can run 6L6's but a 6L6 amp with power tube socket pinouts wired specifically for 6L6's cannot run EL34's aside from heater, screen voltages/current requirements.

 

[Mr. Willy]

I think you might be better off just running 6L6's in place of the 5881's. 6550's are pretty much a larger big bottle 6L6 tonewise. 6L6's should work just fine in the SLO30 and give similar feel and tones of the 6550's IMHO.

The primary impedance of the SLO output transformer will be specifically for 5881/6L6's. Marshall output transformer's were spec'd for EL34's only. 6550's and 6L6's are a mismatch but they will operate. The big fear about 6550's was runaway bias so the Marshall engineers reduced the 220K PI grid leak resistors to 150K to lessen this possibility which ended up not being much of an issue in the end.

Mike Soldano uses the 5881 for specific reasons in the SLO's.

Usually the bias range will have to be set to a pretty wide range to run EL34's, 6L6 to 6550's, most amps use a 25K bias pot. You can use a 50K bias pot and then the bias circuit dropping resistors will need to adjusted to get the bias range on the high and low end where you want it.

An EL34 amp can run 6L6's but a 6L6 amp with power tube socket pinouts wired specifically for 6L6's cannot run EL34's aside from heater, screen voltages/current requirements.

I will try this at some point. I’ve got an OG SLO that’s my primary amp. I’ve been running 2 5881s/2 6L6’s in it for quite some time and like it a lot. I tried all 6L6’s, but I lost a lot of aggression in that configuration.

 

[fusedbrain]

I think you might be better off just running 6L6's in place of the 5881's. 6550's are pretty much a larger big bottle 6L6 tonewise. 6L6's should work just fine in the SLO30 and give similar feel and tones of the 6550's IMHO.

The primary impedance of the SLO output transformer will be specifically for 5881/6L6's. Marshall output transformer's were spec'd for EL34's only. 6550's and 6L6's are a mismatch but they will operate. The big fear about 6550's was runaway bias so the Marshall engineers reduced the 220K PI grid leak resistors to 150K to lessen this possibility which ended up not being much of an issue in the end.

Mike Soldano uses the 5881 for specific reasons in the SLO's.

Usually the bias range will have to be set to a pretty wide range to run EL34's, 6L6 to 6550's, most amps use a 25K bias pot. You can use a 50K bias pot and then the bias circuit dropping resistors will need to adjusted to get the bias range on the high and low end where you want it.

An EL34 amp can run 6L6's but a 6L6 amp with power tube socket pinouts wired specifically for 6L6's cannot run EL34's aside from heater, screen voltages/current requirements.

Oh yeah, forgot about the tube socket pinouts too.
Good point.
Agree on the primary impedance of the output transformer as well, but the heater current issue and the tube socket wiring issue are potential catastrophic failure points that would need to be addressed.
I'd start rolling 6L6's to try and get the sound your after.