felip
43 posts
Jul 05, 2014
7:00 AM
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Hi:
I have a tube amp Fender Pro Jr, with two previous 12AX7 tubes and 2 EL84. It sounds pretty good with harmonica, the problem is that it is loud as hell, when I play with my band if I set the volume at 2 (it can be set up to 12) you can only hear the harp.
I would like to ask you what tubes could I change to still get a dirty sound but at a lower volume. Thanks you very much.
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Greg Heumann
2764 posts
Jul 05, 2014
9:02 AM
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Here are the 12A?7 series tubes and their gain factors:
GAIN FACTOR 100: 12AX7 - aka ECC83, 7025, ECC803, E83CC, 6681
GAIN FACTOR 70: 5751
GAIN FACTOR 60: 12AT7 - aka ECC81, 6201, 6679, 6829
GAIN FACTOR 45: 12AY7 - aka 6072, 6211
GAIN FACTOR 41: 12AV7 - aka 5965
GAIN FACTOR 19: 12AU7 - aka ECC82, 5963, 5814, 6189
I would recommend pulling the second 12AX7 and replacing it with a 12AU7 to start. If that doesn't let you get your volume knob to at least 5, then reduce the gain of the 1st tube as well - a 12AY7 would probably be a good choice.
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***************************************************
/Greg
BlowsMeAway Productions
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Joe_L
2485 posts
Jul 05, 2014
12:33 PM
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When I owned a pro Jr, I swapped the first 12AX7 with a 12AY7.
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Kingley
3626 posts
Jul 05, 2014
12:43 PM
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I did the same as Joe when I owned one too. I could turn it up to four and a half on the volume with a 12AY7 in V1.
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markdc70
149 posts
Jul 05, 2014
8:01 PM
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I believe that what felip is looking for here is an attenuator. What everyone has suggested so far will give more sweep of the volume knob, but it will still have the same tone at a relative volume level. An attenuator requires you to drive the amp harder to achieve the same volume, thereby potentially giving him: " a dirty sound but at a lower volume".
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Barley Nectar
432 posts
Jul 05, 2014
8:22 PM
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I don't care for attenuators. Good way to blow up an amp. It's like standing on the gas and the break at the same time.
Felip, that must be a mellow band. Tube it down is a option. Less efficient speaker is another option. That amp may have a linier taper pot on the VC. Changing it out to a CTS audio taper of the proper value will give you more control. That is a fantastic guitar amp. You may want to step down to a Vibrochamp or something similar, dare I say, Memphis Mini..............BN
Last Edited by Barley Nectar on Jul 05, 2014 8:24 PM
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Littoral
1115 posts
Jul 06, 2014
6:34 AM
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I played a Junior recently and had a whole lot of volume range using my LW Octave pedal. Level works like a pre-amp and nearly doubles the effective range of useful volume, acoustic to full band (2-3 numbers over normal feedback) The sound was great at all levels, with tweaking, -MUCH better than the amp without the pedal.
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jbone
1680 posts
Jul 06, 2014
6:56 AM
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If you can afford it, why not look at a smaller amp for that band? I'd keep the Pro for other opportunities and use maybe a Champ or other small tube amp for a quieter outfit.
I've owned and played through everything from a battery powered tiny little amp up to a Bassman and a lot in between. Currently wife and I use Silvertone 1482's, which at 12 watts are comparable to a Princeton. We're a duo or a low volume 3 or 4 piece, we both avoid the wall of sound we've heard for years.
We also just got a 5w vintage Vibro Champ, which has tone for days but only goes so loud with the single 8" speaker. You may do well to opt for a small solid state amp and run it with effects like a harp delay.
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Greg Heumann
2768 posts
Jul 06, 2014
9:53 AM
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@Barley - I know you have lots of experience so I'm confused about your statement about attenuators. My understanding is that they're simply a big 8 ohm resistor that dissipates the energy as heat. Surely the amp doesn't know the difference between a speaker and an attenuator?
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***************************************************
/Greg
BlowsMeAway Productions
See my Customer Mics album on Facebook
Bluestate on iTunes
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5F6H
1808 posts
Jul 06, 2014
11:30 AM
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It doesn't at typical settings, but using very high levels of attenuation can potentially lead folks to drive the amp much harder/at higher settings than normal...this effectively raises average plate current, creating more heat, more risk of burning out transformers.
Sensible use shouldn't be an issue with an attenuator, a simple resistive attenuator for the Pro Jr shouldn't be hard to knock up, but the rub is: If the amp needs lots of attenuation you may lose tone, some folks limit to around -9dB.
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felip
44 posts
Jul 06, 2014
2:03 PM
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I think I'll try with a 12AY7 first and look what happens. This amp is too loud, here I am playing with the amp between 1 and 2, it is an open space and playing just a little louder would let nobody hear the rest of the band http://www.youtube.com/watch?v=8NpzsHNQd-E&list=UU5Kuk0GUaGAF-Xca40tihrA
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rogonzab
569 posts
Jul 07, 2014
1:30 PM
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Maybe a pedal?
Try the tube down aproach, and try an overdrive pedal (lone wolf) to get crunch whit less volume.
I own that amp and is LOUD!
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J_Bark
11 posts
Jul 07, 2014
5:15 PM
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I want to semi-hi-jack this thread a bit and try to learn a little more about amps, so here goes:
I do understand the basic functions of the various tubes of an amp. What I do not know is WHERE in the amp the "overdrive/distortion and/or tone" comes from? is this introduced in the pre-amp tubes, in the output tubes, some of each?
It would help me to better follow these type of discussions if someone could post a brief explanation. I do realize that ALL of the components work together to create to overall sound of an amp but clearly some components must be far more "involved" than the others.
Cheers,
Jerry
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5F6H
1809 posts
Jul 08, 2014
2:59 AM
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-------Hi Jerry,
Tone comes from you, the amp transmits that to the air around you, obviously anything that is heard coming from the amp is subject to enhancement & limitations from the amp/signal chain...if that's not a "distinction without a difference"? ;-)
You play into the mic, mic sends signal to the amp, preamp tube catches the signal from the mic & sends to the other voltage amplifier tubes ("preamp tubes" working with signal volts AC in the single digit territory - but powered by hundreds of dc volts!) & the phase inverter/driver tube (bridges the preamp circuit & power tubes). The earlier stages handle more of the amp "character", later stages are more concerned with bumping up the signal to a strength that will drive the power tubes (tens of volts AC, but again, deadly dc volts make them function - signal is AC and travels from input jack to output transformer, but the tubes need high voltage dc to work, travelling from the rectifier to the preamp tubes - opposite direction).
Preamp tubes are mostly "single ended" (think one man sawing a tree, inefficient but cheaper than 2, still gets the job done) & tend to get raspy/fuzzy quick. Some amps (nearly all those with 1 power tube - Champ style) are also single ended at the power stage, good for quick overdrive, raspy/papery/crunchy tone.
Lower mu preamp tubes have less gain so reduce the crunchy/fuzzy element of single ended character, giving a sweeter/smoother tone, with less harshness...go too far and you can get a muddy tone.
The phase inverter's main job is to split the preamp signal in to 2 identical signals, 180degrees out of phase WRT each other, to drive a push-pull output section.
More powerful amps (15W+) tend to be push-pull output (2 power tubes working in see-saw, now you have 2 men sawing down a tree) this more than doubles power output over 1 tube single ended. Push-pull smooths out some of the harmonics when working optimally...but sometimes for tonal purposes we might run sub optimally because we like the sound...this is when the power tubes may contribute more to the character of the amp, more than the loudness (primary function).
As you drive an amp harder, it eats up more current than the power supply (power transformer, rectifier) may easily handle and we get sag/compression.
Normally, because we batter the preamp tube with big signal from a mic, we get a lot of the character from the preamp, this sends a dirty signal to the output, so even if the output isn't overdriven itself, it's not transmitting a clean signal...there are ways to increase power tube contribution to distortion, but typically we are not overdriving them to the same degree as the preamp, or as much as guitar players do.
Really, as you already suspect, it's usually a question of finding a balance between desirable character/pleasing distortion & volume out, with every stage working in harmony. Changes to the preamp might require subsequent changes in the power amp & vice versa.
Really, I wouldn't fret too much as to where what is coming from, you're typically limited as to what you can do about it, focus on what changes you can make to get a nice sound/best use from the amp.---
www.myspace.com/markburness
Last Edited by 5F6H on Jul 08, 2014 6:41 AM
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J_Bark
12 posts
Jul 08, 2014
5:30 AM
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Thanks for that great summary 5F6H, it really helps me out and makes good sense. Well done.
Most of my experience with electronics lies in building single purpose instrumentation amplifiers for the physics lab in grad school. The good old days, as it were.
now these various threads about tube swaps and whatever will be more interesting for me at least.
As for my own purposes and playing, well I have a lot bigger fish to fry than tweeking up an amp. I do have a nice little amp that gives me a nice "acoustic" sound when I use it. When I do get the mic cupped just right I can definitely hear the desirable change in tone so I know what I am working toward. I do not play with the mic much as yet so I have just been focusing on playing the music when I have a mic and not trying to perfect the cupping.
Thanks again,
Jerry
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