Bassman 300 Pro, Ohms...

[Count Bassie]

Might seem like a dumb question, but since I don't know, it's less dumb than not asking...

I've got this amp, see, and it's switchable between 2, 4, and 8 ohms. Now on any other head I'd go ahead and use a single 8 ohm cab because it's pretty obvious you can do that, even if the head has a 4 ohm rating.
Now if I were to run the Bassman into an 8 ohm cab, while the amp is set to the 4 ohm setting, will it be fine? With my ss heads I'd just not have access to the headroom available at 4 ohms. I am figuring it's the same with the Fender tube head.
But also... would my Fender tube amp 'break up' a little sooner set at 4 ohms, into the 8 ohm cab?

Thanks for your time.

 

[Wasnex]

Running an 8 ohm cab on a 4 ohm output is an impedance mismatch.

Tube amps use the output transformer to match the high output impedance of the tubes to the low output impedance of the speakers. In other words that load presented to the tubes by the transformer remains the same as long as the amp sees the expected load.

If you show the amp an impedance mismatch, the transformer will not present the output tubes with the expected load. This will reduce how much power the amp can make, and it also increase stress on the amp and tubes.

Whether running an 8 ohm load on the 300 Pro 4 ohm output is safe, I can't say. Some amps can tolerate a minor impedance mismatch and some can't. Some amps are safe when the impedance is one order too high, but not one order to low. While other amps are safe if the impedance is low, but not high.

Since you have a 4 ohm loud, why not simply connect it to the 4 ohm output.

 

[Drredplate]

Tube amps are pretty tolerant of load. (They however prefer a short circuit to an open circuit.)
The amp would probably survive ok but won’t perform its best as wasnex stated.
Ime fenders are more tolerant of load than Marshalls.
You should always match the load as previously stated

 

[Count Bassie]

Running an 8 ohm cab on a 4 ohm output is an impedance mismatch.

Tube amps use the output transformer to match the high output impedance of the tubes to the low output impedance of the speakers. In other words that load presented to the tubes by the transformer remains the same as long as the amp sees the expected load.

If you show the amp an impedance mismatch, the transformer will not present the output tubes with the expected load. This will reduce how much power the amp can make, and it also increase stress on the amp and tubes.

Whether running an 8 ohm load on the 300 Pro 4 ohm output is safe, I can't say. Some amps can tolerate a minor impedance mismatch and some can't. Some amps are safe when the impedance is one order too high, but not one order to low. While other amps are safe if the impedance is low, but not high.

Since you have a 4 ohm loud, why not simply connect it to the 4 ohm output.

That's the kind of informed answer I was hoping for, thank you.
I do have the ohms switch properly set for the fridge it's sitting on top of. I was asking to see if setting that ohm selector to 8 ohms while on the 4 ohm cab might cause the head to 'break up' sooner...
This particular head has been modded to be able to use KT-120 tubes, which are currently in it. There's lots of headroom, but in order to get it to do that break-up thing it would have to be louder than a whiskey-swilling nun with a bullhorn at a farm-league hockey game, and no one needs that...

 

[Wasnex]

That's the kind of informed answer I was hoping for, thank you.
I do have the ohms switch properly set for the fridge it's sitting on top of. I was asking to see if setting that ohm selector to 8 ohms while on the 4 ohm cab might cause the head to 'break up' sooner...
This particular head has been modded to be able to use KT-120 tubes, which are currently in it. There's lots of headroom, but in order to get it to do that break-up thing it would have to be louder than a whiskey-swilling nun with a bullhorn at a farm-league hockey game, and no one needs that...

I haven't played this amp, but I have read about it quite a bit. My impression is it excels at big, clean. Probably not the best choice for OD. My understanding is the OD channel is not generally considered great (YMMV).

@BassikBrad had one of these a few years ago and did some experiments with tube rolling. Maybe he will comment.

 

[Drredplate]

I would be interested if anyone come back with tube rolling info they have tried.
You could try swapping the 12at7 phase inverter for a 12ax7.
I have tried this on other amps resulting in a more aggressive tone.

 

[Count Bassie]

I've been using it with a stoner/doom jam project and it really does deliver a big, clear, but heavy, tone. Through 8x50-watt 10" Jensens it's fairly compelling: big three -dimensional notes and phrases come rolling into the room, it's pretty darn satisfying.
I've actually always been a "clean" guy, but the ability to run into breakup mode is a very attractive idea. That element as part of a dynamic formula is something I'd like to be capable of.

 

[Drredplate]

Might seem like a dumb question, but since I don't know, it's less dumb than not asking...

I've got this amp, see, and it's switchable between 2, 4, and 8 ohms. Now on any other head I'd go ahead and use a single 8 ohm cab because it's pretty obvious you can do that, even if the head has a 4 ohm rating.
Now if I were to run the Bassman into an 8 ohm cab, while the amp is set to the 4 ohm setting, will it be fine? With my ss heads I'd just not have access to the headroom available at 4 ohms. I am figuring it's the same with the Fender tube head.
But also... would my Fender tube amp 'break up' a little sooner set at 4 ohms, into the 8 ohm cab?

Thanks for your time.

There are no dumb questions only dumb answers imo.
A valve amp with a transformer behaves quite differently than a ss amp. Quite complex. A good reference would be to look up “valve wizard” on the internet and have a read. Suffice it to say this is unlikely to deliver the result you want. it may not sound any different. Meant to say this in my previous post

 

[agedhorse]

I would not recommend it. It's possible that it will increase the odds for a failure, and a failure of this type on that amp will be very expensive.

 

[Count Bassie]

I'm getting the idea that I ought to do well playing by the rules with this amp. And so it shall be.

Thanks everyone!

 

[Drredplate]

I would not recommend it. It's possible that it will increase the odds for a failure, and a failure of this type on that amp will be very expensive.

I bow to your superior knowledge sir!
I have only done this on amps running 4 el34s (which worked)

I will be interested to see what tube rolling has actually been tried and tested

 

[Drredplate]

I'm getting the idea that I ought to do well playing by the rules with this amp. And so it shall be.

Thanks everyone!

That might be a wise and cautious move.
Sounds like the amp would be a good pedal platform given the clean headroom

 

[Freight Train]

As I'm sure has been said, always set the switch to the proper impedance. I forgot which is which regarding it being set to a lower or higher impedance than the speaker rating, but one way will wear the power tubes out quicker, the other way overheats the output transformer to the point of a possible meltdown. Whichever, you don't want either of those. set it to the correct impedance.

 

[Count Bassie]

Great info, thanks everybody for your time and wisdom.

I got one more...
This thing is set up with a modification for KT-120s. It's designed for KT-88s or 6550s. Any advantage to returning to 6550s? It wouldn't require any reversal of the mod.
I'm getting the idea that this amp is just going to do its clean thing, whatever power tubes are in it.
I'll go read the Tube Wizard next!

 

[Count Bassie]

That might be a wise and cautious move.
Sounds like the amp would be a good pedal platform given the clean headroom

I do use a SansAmp Bass Fly Rig, an EHX Cockfight (fuzz/wah kind of gadget), an MXR Phase 90, and a Broughton HPF in front of the whole mess... Seems copacetic, the amp does a nice job of sending it all into the room mix.
I use the SansAmp's DI when we record, with a mic on the cab.
After all the learning I've gotten here I'm thinking I'll go ahead and be happy with what I have to be happy with.

Thanks again all.

 

[Wasnex]

Great info, thanks everybody for your time and wisdom.

I got one more...
This thing is set up with a modification for KT-120s. It's designed for KT-88s or 6550s. Any advantage to returning to 6550s? It wouldn't require any reversal of the mod.
I'm getting the idea that this amp is just going to do its clean thing, whatever power tubes are in it.
I'll go read the Tube Wizard next!

I believe the mod to allow the amp to run KT120s involved installing an extra heater transformer. KT120 pulls a bit more heater current than 6550 or KT88. It's possible an additional modification was needed so KT120s will bias properly in the amp. I would ask the person who sold you the amp if there is documentation of the mod and what sort of change was noted when the amp was converted to KT120.

My understanding is the power transformer and output transformer are the limiting factors that most determine how much power an amp can make. You might get a little more with a different tube type, but probably not enough to make much of an audible difference.

The various comments I have seen suggest KT120 may sound a bit fuller. So you may experience a small change in the basic voicing of the amp. You may also experience some difference in the character and tone of drive when the tubes are pushed into saturation.

Probably the biggest advantage of running the original tube (6550) is they are much cheaper and there are many more varieties to choose from. AFAIK there is only one common KT120 variant.

 

[agedhorse]

I bow to your superior knowledge sir!
I have only done this on amps running 4 el34s (which worked)

I will be interested to see what tube rolling has actually been tried and tested

Sometimes it works and sometimes it doesn't. It depends on your "luck of the day"

Great info, thanks everybody for your time and wisdom.

I got one more...
This thing is set up with a modification for KT-120s. It's designed for KT-88s or 6550s. Any advantage to returning to 6550s? It wouldn't require any reversal of the mod.
I'm getting the idea that this amp is just going to do its clean thing, whatever power tubes are in it.
I'll go read the Tube Wizard next!

If the power transformer's heater winding is not rated for the KT-120's, moving back to the correct tubes will improve the reliability.

 

[JeezyMcNuggles]

Might seem like a dumb question, but since I don't know, it's less dumb than not asking...

I've got this amp, see, and it's switchable between 2, 4, and 8 ohms. Now on any other head I'd go ahead and use a single 8 ohm cab because it's pretty obvious you can do that, even if the head has a 4 ohm rating.
Now if I were to run the Bassman into an 8 ohm cab, while the amp is set to the 4 ohm setting, will it be fine? With my ss heads I'd just not have access to the headroom available at 4 ohms. I am figuring it's the same with the Fender tube head.
But also... would my Fender tube amp 'break up' a little sooner set at 4 ohms, into the 8 ohm cab?

Thanks for your time.

No. You could drop your transformer.

Simple rules for tube amps:

Never turn on a tube amp without a load attached.

Always use the designated taps, or switch settings for the appropriate load.

Do not attempt to fix it yourself. Besides rolling tubes, take it to a tech.

 

[Count Bassie]

I believe I have the paper the guy who did the mod drew up, and I was assured that he'd done his homework and did it correctly.
I did have an issue where one of the tube sockets had vibrated loose and pulled up some traces. The guy I take my amps to point-wired the connections and used some kind of cement to fasten the socket back down to the board. He checked all of them while he had it opened up.
Other than that there've been no problems.
The DI seems to be noisy, but I use the SansAmp.
Now I'll go look for that plan he drew up...

 

[agedhorse]

That amp uses double-sided PCBs, so if traces or pads got lifted, it was from improper work techniques. The solder acts like a rivet that bonds the pins through the PCB with the plated holes. I have used this mounting technique for many years with zero issues (other than a tech that did not have the necessary skills).