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FDP Forum / Miscellaneous and Non-Fender Topics / Please answer this tube amp question for me.


Southwest Florida

Alone in my principles
Dec 10th, 2017 07:11 AM   Edit   Profile  

I recently purchased a used tube amp. It's an 18 watt, 2 x 12 combo. The combination of low wattage going into two speakers gives me virtually no clean headroom. Is there anything I can do to at least give me a little clean before breaking up?

Contributing Member

Roisin, I wanna

fight your father
Dec 10th, 2017 07:25 AM   Edit   Profile  

First, all amp circuits are designed to do a certain thing; not all are clean at low volumes. Trying to shoehorn a tone into an amp that it's not designed to do can be frustrating--and expensive. It's like buying an SUV and then applying modifications to get 40 mpg out of it.

Depending on the amp you have there, the first inexpensive thing to try is to cool off the first gain stage by swapping a lower-gain V1. The majority of tube amps these days use a 12AX7 in the V1 position. Swap that one for a cooler tube like a 5751, 12AT7, or 12AY7.

Beyond that, other mods can get expensive. If you need clean tones, sell the growly amp and get something like a Princeton Reverb or other 'non-Marshally' amp.



Dec 10th, 2017 07:57 AM   Edit   Profile  

Not all 18w 2x12 tube amps use the same design.

There is no way to really be specific and answer your question without knowing what you have.

Contributing Member


Life makes a man tired.
Dec 10th, 2017 08:08 AM   Edit   Profile  

I’d swap V1 for a 5751. If that isn’t enough, go to the 12AY7 next. I’d skip the 12AT7. It’s a current driver and they usually don’t bias correctly in a socket set up for a 12AX7.

Guitar Fool
Contributing Member

Sunshine State

Just a pawn in someone else's game
Dec 10th, 2017 08:17 AM   Edit   Profile  

I have an 18 watt head and 2x12..both pre amp tubes are 5751...i had tried 12ax7's and it was a bit too aggressive......the 5751's tamed it quite a bit....sure it gets dirty, but its more subtle.... more like "edgy"...

it is the most gig friendly amp I have...just the perfect amount of speaker and wattage...

the only other thing I can think of that could give you more clean is different speakers but that is what I would do as a last resort..

just curious, what amp are we talking here?

my 18 watter

Contributing Member

Northern VA

The Fine Line Between Clever & Stupid
Dec 10th, 2017 11:05 AM   Edit   Profile  

I generally use a 5751 in V1 as well. Like others have said I wouldn’t put a 12AT7 in V1. I use them as a phase inverter in my HRD and for tremolo and reverb.
I have a Champ clone that I sometimes use a 12AU7 in when I want to get a little more headroom before breakup.
I also have a JJ 12DW7 in the V2 position of my HRD. These tubes have 2 triodes. One triode is 100%, the other is 20%.

Gain Factors
AX=100 mu
5751=70 mu
AT=60 mu
AY=45 mu
AU=19 mu

But you also have to consider:
Plate Resistance
AX=80K ohms
AT=10.9K ohms
AY=25K ohms
AU=7.7K ohms

Would help to know what kind of amp you have.
Is the amp fixed (adjustable) bias? If so, it could be off. If I’m remembering correctly an amp biased too cold will get crossover distortion and distort the signal too soon. Some people bias cold because they want to tubes to last longer, but as a result the tubes don’t run efficiently and that can adversely effect tone.

(This message was last edited by daveg at 01:13 PM, Dec 10th, 2017)


Southwest Florida

Alone in my principles
Dec 10th, 2017 11:53 AM   Edit   Profile  

Thanks for all the suggestions! The amp in question is a Crate V18. I couldn't pass it up for $80.00!

(This message was last edited by LHW at 01:54 PM, Dec 10th, 2017)

Contributing Member

Roisin, I wanna

fight your father
Dec 10th, 2017 12:04 PM   Edit   Profile  

Well THAT was a deal if everything works!



Buckeye Country, USA

Motorcycles, Guitars, and Golf
Dec 10th, 2017 12:11 PM   Edit   Profile  

That's a steal for sure.

I just checked online and your amp is not self-biasing, which means it needs to be rebiased after changing tubes. From the V18 manual...

"For the best performance and longest tube life, proper biasing is imperative. Bias is the negative voltage which is applied to the power tube’s control grid to set the level of idle current. We cannot over emphasize the difference in warmth of tone and dynamic response that come with proper biasing. If the bias is set too high (over iased), the sound from the amp will be distorted at all levels. If the bias is set too low, (under biased) the power tubes will run hot (the plates inside the tubes may glow red due to excessive heat) and the sound from the amplifier will lack power and punch.


Southwest Florida

Alone in my principles
Dec 10th, 2017 12:26 PM   Edit   Profile  

Thanks, guys!

Contributing Member


Life makes a man tired.
Dec 10th, 2017 01:33 PM   Edit   Profile  

I have a Jet City Custom 5/2 head and its pretty hot with 2 12AX7s. I currently run a pair of 5751s in it. Those give me nice cleans but still retain the 12AX7 tones, albeit tamed.

Contributing Member

San Francisco, CA

Jan 3rd, 2018 09:23 PM   Edit   Profile  

The best advice I can offer is to get yourself a nice vintage SFDR. It’s the amp that basically gives you everything you need for the rest of your life.


Contributing Member

South Florida

Jan 3rd, 2018 09:37 PM   Edit   Profile  

For $80 that's a nice pick up.

(This message was last edited by larryguitar19 at 11:38 PM, Jan 3rd, 2018)

Contributing Member


RonHalen Jokingly He Says
Jan 4th, 2018 06:57 AM   Edit   Profile  

Weren't Treble Boosters used to bring out some Clean? A TS-9 set asa Boost might work.



Jan 4th, 2018 08:53 AM   Edit   Profile  

OK, now we need to know what you mean by "headroom". I believe that it's a term that has different meanings to different people.

Meaning A: You plug in a guitar, and no matter what volume the amp is set at there's no clean. There's always at least a little grit. You are probably overdriving the input stages with your guitar (or, there may be something wrong with the amp). By the way, if this is the problem, adding a booster to the input will make it worse.

Meaning B: It's clean at low volumes at home. Then you and your 18 watter go to jam with a bunch of note mashers with 30-50-100 watt amps. There's no way you're going to keep up with them.

FDP Forum / Miscellaneous and Non-Fender Topics / Please answer this tube amp question for me.

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