Amp biasing question... 5150 bias to run at half power? Recommended or ok to do?

[Virgil.]

Ok, so I recently picked up a 5150, but the 120w is just way too loud... I literally can't turn it up past '1' where I live. Sooooo, I was considering getting an Attenuator, but for a good one those seem to be $200+ even used that could handle the full 120w power load.

So, today, I called up a local amp repair tech nearby and needed to get costs anyway on getting it biased and cleaned, and he recommended that I have them remove a set of power tubes (leaving just 2 in the amp) and just have it biased to run at half power (roughly 45-60w) instead of the full 120w.

Has anyone here ever done something like that to one of their high-gain amps? I'm not sure how it affects tone, or if in the long-run if it does any damage to the amp. I am only using the amp right now for bedroom volume levels and recording, so having a 5150 head running less-than half power would be fantastic if it's safe and doesn't diminish the quality of the tone.

Thoughts?

 

[Wolfeman]

Running at 50 watts is going to be just as loud as running at 100 watts. It's a fraction of a difference.

You can test it out yourself. Just pull a pair of tubes(one from each side), set the amp to run on half the impedance of the cab, and try it out.

Your local tech is smoking crack if he thinks that biasing needs to change when pulling 2 tubes. They still get set to what it would be with 4 tubes.

 

[Virgil.]

I'm running it into a Mesa Boogie 2x12 at 8ohms.... so you're saying I can pull 2 tubes out, change the setting on the back of the head to 4ohms, and still run it into my cab and it be ok? Sorry I just want to make sure I'm understanding correctly.

 

[Virgil.]

I had a Peavey Vypyr tube 60w combo and I could usually have the post-gain/master set to about 3 or 4 before it got as loud as this 5150 at 1.

 

[Wolfeman]

I'm running it into a Mesa Boogie 2x12 at 8ohms.... so you're saying I can pull 2 tubes out, change the setting on the back of the head to 4ohms, and still run it into my cab and it be ok? Sorry I just want to make sure I'm understanding correctly.

Yes, that's all you do.

Even if you leave the impedance where it is at it will be fine, but for the sake of keeping everything happy plug into the 4ohm.

 

[Genius Gone Insane]

I didn't know you had to switch the ohmage if you pull tubes.

 

[Virgil.]

Yes, that's all you do.

Even if you leave the impedance where it is at it will be fine, but for the sake of keeping everything happy plug into the 4ohm.

Cool thanks!

Is there any particular reason I want to pull a tube from each side (i'm assuming you mean one far left and one far right) instead of 2 that are next to each other?

 

[Wolfeman]

Cool thanks!

Is there any particular reason I want to pull a tube from each side (i'm assuming you mean one far left and one far right) instead of 2 that are next to each other?

It's a push-pull power amp, so you need a load on both 'sides' of the circuit. As long as you pull in one of these combinations, it will keep the amp happy-

Original-
O O O O

Pulled-
X O O X
O X X O
X O X O
O X O X


It's usually just easier for the sake of things to pull the 2 outer tubes.

 

[Wolfeman]

I didn't know you had to switch the ohmage if you pull tubes.

If you want to keep everything operating the same way as before you do. When you pull 2 tubes, the load on the one side of the transformer changes, so naturally the load on the other side(speaker side) changes as well.

Some amps can handle the mismatch no problem, but it will affect the operating point of everything, which means it may change the tone a bit.

 

[Virgil.]

It's a push-pull power amp, so you need a load on both 'sides' of the circuit. As long as you pull in one of these combinations, it will keep the amp happy-

Original-
O O O O

Pulled-
X O O X
O X X O
X O X O
O X O X


It's usually just easier for the sake of things to pull the 2 outer tubes.

Awesome! Thanks for the explanation and idea on how to test this out!

 

[TheWinterSnow]

Awesome! Thanks for the explanation and idea on how to test this out!

Just pull the tubes, drop the impedance selector on the amp down one notch (4ohm) and that about it.

I really only came in to say that from personal experience pulling two tubes out does not make the amp any quieter, but reduced the volume slightly as to when the power tubes begin to saturate and reduces sag a lot making the amp way too stiff and sterile IMO. As always YMMV.

 

[Wolfeman]

Awesome! Thanks for the explanation and idea on how to test this out!

No problem. It's a lot easier to A/B the difference when you have a switch for it. At most you are changing the volume like 3db. So you will hear a change, but nothing that will help you get better use out of the master control.

I'm a fan of adding a few resistors (an L-pad) in front of the MV control to keep the signal at a lower level, giving the master volume control more usable range. On my amps, I call this a 'focus' switch.

Any decent tech that knows what they are doing should be able to do this for you. It's a more productive way of getting what you are trying to achieve.

 

[Wolfeman]

Just pull the tubes, drop the impedance selector on the amp down one notch (4ohm) and that about it.

I really only came in to say that from personal experience pulling two tubes out does not make the amp any quieter, but reduced the volume slightly as to when the power tubes begin to saturate and reduces sag a lot making the amp way too stiff and sterile IMO. As always YMMV.

I agree.

 

[Virgil.]

No problem. It's a lot easier to A/B the difference when you have a switch for it. At most you are changing the volume like 3db. So you will hear a change, but nothing that will help you get better use out of the master control.

I'm a fan of adding a few resistors (an L-pad) in front of the MV control to keep the signal at a lower level, giving the master volume control more usable range. On my amps, I call this a 'focus' switch.

Any decent tech that knows what they are doing should be able to do this for you. It's a more productive way of getting what you are trying to achieve.

So what would you recommend then me asking my amp tech to do if I take it in to him? The 5150 doesn't have a "master", so unfortunately only has a post-gain (as I'm sure you already know). Do you mean for this amp having them adjust the post-gain? Sorry for so many questions, I'm a total amp noob, let alone tube-amp noob.

 

[Virgil.]

I'm almost thinking just saving up for a used "hot plate" will get me what I'm after... saturated tube tone at a lower volume.
And it might be cheaper than an amp tech charging me up the arse for all these mods since I'm an electrical noob as well.

 

[Wolfeman]

The 'post gain' control is a master volume, they just use a different name.

The master volume control is in and among itself a simple L-pad that is adjustable. On the 5150, even at low MV setting, it's already letting a high enough signal through to drive the power section pretty hard.

By padding down the level a bit more before it reaches the MV, you will get more 'sweep' in the lower volume area.

 

[Virgil.]

Oh haha now I understand what you're saying. Is that a complicated task? I just want to know price-wise what to expect for something like that from a typical amp tech so I know when to negotiate them down more or walk away.

 

[Virgil.]

Ok, so the amp tech just told me:
"I wouldn't advise installing an l-pad. if you want an L-pad, just mount on in a box and plug in between the amp and speaker. no mod necessary."

Doesn't that get into basically just acting as an attenuator at that point? Any cheap recommendations for me aside from a THD?

 

[Wolfeman]

It's just 2 resistors and a switch.... $5 in parts and probably a $80 labor job.

I've never used attenuators because I think they sound shitty. But that's my personal opinion. Other may have more input on good/bad ones.

 

[guitarguru777]

I always run my 5150 with 2 tubes. I prefer the tone better, it tightens up the low end which is a HUGE plus on the amp Been running it this way for years with no issues.

Also if you find volume an issue run a 2x12 instead of a 4x12 this helps a bit as you dont get as much "throw" from the speakers.