Hondo
195 posts
May 04, 2013
10:15 AM
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I bought a 94' Tweed Pro Junior this morning. It has 12AX7's in it and I have some 5751's here to change too. I also have a Tone Plus pedal which helped the feedback a little.
Any suggestions would be appreciated- volume/ tone settings, Tone Plus settings? I know that alot of you guys have experience with these amps. THANKS
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Rick Davis
1742 posts
May 04, 2013
10:24 AM
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Lots of info at The Blues Harp Amps Blog.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
Tip Jar
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Kingley
2589 posts
May 04, 2013
10:36 AM
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Hondo - Try changing V1 (the farthest valve on the right looking at the amp from the back) to a 12AY7. If it still feeds back then try try dropping it to a 12AU7. Keep the tone rolled off, keep the amp on the floor and make sure you have a good cup on the mic. With a 12AY7 in V1 you should be able to get the volume up around 5-6. With a 12AU7 in V1 it should get to around 8 on the volume. However the amp could lose a lot of it's livelier tonal qualities with a 12AU7.
Last Edited by Kingley on May 04, 2013 10:41 AM
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Hondo
196 posts
May 04, 2013
10:40 AM
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Thanks guys. Rick, can you tell me more about the disconnection of the negative feedback? Is it something that I can do?
Last Edited by Hondo on May 04, 2013 10:40 AM
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Kingley
2590 posts
May 04, 2013
10:49 AM
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Unless you have extremely good soldering skills, I'd be wary of doing any mods on the PCB of the Pro Junior as it's very crowded and there's not a lot of space to work with. They are notorious fiddly amps to work on.
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Hondo
197 posts
May 04, 2013
11:01 AM
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OK. I better skip that Kingley. I appears that the 12DW7 and the ecc832 are the same. I have one, so can I use it in place of the DW7?
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Hondo
198 posts
May 04, 2013
11:37 AM
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Something isn't catching up here on the forum.
Well it looks like I have scratched up two JJ EL84's, one JJ ECC832 and a Jan-Phillips NOS 5751 and Tone set to 4. What do you think? Anything else?
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Kingley
2593 posts
May 04, 2013
11:47 AM
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If the 12DW7 is indeed the same as an ECC832 then I can't see any reason why not. I'd leave the stock power valves (the EL84's) in for now though and concentrate just on the preamp valves. try the ECC832 and the 5751, then try switching their positions to see how that affects the tone. I'd leave the tone on zero (at least for testing purposes). If you still find the amp feeds back too early then try a 12AY7 in V1. The other thing you could do is to change the speaker to an Alnico speaker. That's a fairly expensive mod though, so for now I would suggest you concentrate solely on those preamp valves.
Last Edited by Kingley on May 04, 2013 11:48 AM
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Hondo
199 posts
May 04, 2013
12:13 PM
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Is V1 the socket next to the Power tubes
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Kingley
2594 posts
May 04, 2013
12:23 PM
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No. V1 is the socket farthest to your right when you look at the amp from the back.
Last Edited by Kingley on May 04, 2013 12:27 PM
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Hondo
200 posts
May 04, 2013
12:30 PM
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Kingley, You are officially my favorite guy today. THANKS man.
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Kingley
2595 posts
May 04, 2013
12:35 PM
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My pleasure Hondo. Just glad I could be of some use today.
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MN
255 posts
May 04, 2013
12:49 PM
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Apropos of nothing -- I once used a guitar player's PJ during a rehearsal. All stock, 12AX7s and all. I plugged an SM57 in with only the lo/hi z converter, no pedals. I turned tone all the way down and cranked the volume as high as it would go before feedback. I was REALLY pleased with the result. Great crunchy tone and a lot of volume as well. The PJ's a damn solid little amp.
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Hondo
201 posts
May 04, 2013
12:54 PM
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So according to the picture- V1 5751, next socket to the left- 12DW7 ( or ECC 832) then two EL84's? Or would it be_ right to left- 12DW7 then 5751?
I hate to be so dumb on anything.
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Kingley
2596 posts
May 04, 2013
1:09 PM
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First I'd try V1 5751, V2 (next socket going left) 12AX7 and just leave the two EL84 power valves alone for the moment. Then try V1 5751, V2 ECC832. Then try switching them round (for example V1 ECC832, V2 5751) and see which you prefer the sound of. Don't worry too much. As long as you don't touch the power valves you can't really damage the amp by switching over preamp valves. Just remember to switch off the amp each time before you remove any of the preamp valves (V1, V2). Also make sure not to touch any of the circuit board (especially the capacitors) to avoid getting an electric shock.
MN - Yes, I agree the Pro Junior is a good little amp for harp. I gigged with one for years.
Last Edited by Kingley on May 04, 2013 1:10 PM
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Rick Davis
1743 posts
May 04, 2013
2:01 PM
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Disconnecting the negative feedback circuit is easy. Just clip the violet wire, insulate the ends and tie it off. No soldering if you don't want to. Here is a photo. Follow the violet wire to the speaker tap and clip it or unsolder it there.
BTW... the negative feedback circuit doesn't have anything to do with the acoustic feedback we sometimes hear howling in our amps. It is a circuit Fender uses in its amps to get their signature sound of shimmering highs in guitars. Disconnecting it will give you a slightly grittier sound.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
Tip Jar
Last Edited by Rick Davis on May 04, 2013 3:28 PM
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5F6H
1620 posts
May 04, 2013
5:08 PM
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Disconnecting the NFB loop will increase highs and make the amp sound tighter. Fender used it to even out frequency response, not to increase high end.
Most amps that sound OK without a NFB loop are cathode biased, not fixed like the pro jr, but if you have made other changes it might work for you. I'd actually be more inclined to incease NFB by reducing the value of the NFB resistor.
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www.myspace.com/markburness
http://www.facebook.com/markburness
Last Edited by 5F6H on May 04, 2013 5:09 PM
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Rick Davis
1746 posts
May 04, 2013
6:11 PM
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Mark, not according to Aiken amps:
"The use of global negative feedback does several things: it flattens and extends the frequency response, it reduces distortion generated in the stages encompassed by the feedback loop, and it reduces the effective output impedance of the amplifier, which increases the damping factor. All of these things affect the tone in some manner.
The flattened, extended frequency response obviously changes the tonal character by removing "humps" in the output stage response and producing more high and low end frequencies.
The distortion reduction makes the amp sound cleaner and more "hi-fi", up to the point of clipping, with less output-stage generated noise."
The smoother frequency response and increased production of highs is that "shimmering highs" and "Fender cleans" thing the Fender guitar players are so in love with. It is not so good for harp.
I experimented with the NBF loop in my silverface Champ and it produced a nice tone, so I tried it in the Pro Jr and liked that as well. I could hear the slight increase in distortion and reduction of clean highs in both amps.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
Tip Jar
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arnenym
134 posts
May 04, 2013
7:02 PM
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Do not disconnect the NFB loop on that amp.
It works good for harp on cathode biased amps.
i have done it on Pro juniors and Blues Juniors for guitarplayers. It "open up" the sound and make this small PP amps sounding as small fender twins. Bright, open and superclean. But don't do them harp friendly. They be terrible feedback monsters for us.
I've tried it...
Do a switch if you want to try.
Last Edited by arnenym on May 04, 2013 7:04 PM
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Rick Davis
1748 posts
May 04, 2013
9:38 PM
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It is easy to do, and to undo. I encourage you to try it for yourself. It sure can't hurt anything, and I liked the result. You can always re-connect the circuit if you change your mind.
You can listen to the tone clips on my blog and decide for yourself if the amp sounded bright and clean and harp-unfriendly. Feedback was not an issue and that little sucker was LOUD and had decent tone. I was impressed.
Lots of readers have duplicated the project with there Pro Jr amps and none have ever written me that the NFB mod was a problem.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
Tip Jar
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Kingley
2597 posts
May 04, 2013
9:39 PM
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Hondo - I can't comment on the NFB loop thing as I've never tried it. The one thing I will say though is this. When modding any amp for harp, it's always best to start with the most simple things first. In this case that is the preamp valve and the phase inverter (V1 and V2). At this stage I would not recommend that you try any other mods. Until you know for certain the effect that simply changing the values of those two valves has on the amps performance. If it does then need further mods (which I seriously doubt it will), I would recommend you simply change the speaker. I would change it for either a Jensen or Weber Alnico speaker.
Last Edited by Kingley on May 04, 2013 9:57 PM
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Rick Davis
1749 posts
May 04, 2013
9:48 PM
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arnenym, a Fender Twin has a NFB circuit, and that is the classic Fender tone. How can disconnecting the NFB circuit make an amp sound like a Twin? That makes no sense.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
Tip Jar
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Kingley
2598 posts
May 04, 2013
10:00 PM
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Hondo - Here are my thoughts regarding modding amps.
One thing to bear in mind is that Rick can no doubt always go to Bruce Collins at Mission amps if he makes a mistake and damages something on the circuit board when modding the amp (and probably get "mates rates" on the cost of fixing it). Mark Burness (5F6H) is a world renowned amp tech so he can obviously sort any problems out himself. Arnenym has been modding circuits for a lot of years now (often with advice from Mark) and has gained a lot of knowledge on the subject. You may not have this knowledge or the luxury of a local amp tech or even be easily able to afford to pay an amp tech. I'm saying this not to put you off of trying circuit mods, but merely to give you an insight into things. The best way with any amp mods is always to try the most well known, simple, cheap and less dangerous things first. Which in this case is the V1 and V2 valves. If more mods are needed further down the road and you venture into the circuit board. I would strongly advise that you seek the guidance of Mark Burness to point you in the right direction. If that time comes I'd also suggest joining a good amp forum where you can then get highly detailed technical advice on modding amp circuits.
Last Edited by Kingley on May 04, 2013 10:00 PM
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Rick Davis
1750 posts
May 04, 2013
10:22 PM
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LOL.... whatever. It is a very small thing, not nearly so dangerous as you make it out to be, Kingley. I like tinkering with amps. I can see why you do not.
Bruce Collins has never had to "fix" anything I did because I never make difficult, dangerous, costly, or complex circuit changes. The NFB circuit mod falls perfectly into that category. It is basic. Even you could do it.
The NFB mod is a small part of the conversion to make a Pro Jr amp more harp friendly, and it is optional. Don't do it if it is too scary. The biggest improvement comes from the speaker swap, which is true of most of these conversions. In this project I was trying to keep the total cost under $300 including the amp, and I did. That means I used a low-cost speaker. If you have a higher budget for the project I would strongly recommend that you use the Eminence Lil Buddy speaker instead of the Weber Sig speaker. It has much better harp tone.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
Tip Jar
Last Edited by Rick Davis on May 04, 2013 10:24 PM
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Kingley
2599 posts
May 05, 2013
12:05 AM
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Rick - I didn't say that Bruce has had to "fix" anything you have done. Nor did I say that people shouldn't try these things. I was merely outlining some processes that I believe would be beneficial to the owner of the amp in question. I was also pointing out the fact that you, Mark and Arnenym all have knowledge of modding amps and that you in particular have more easy access to a well known amp tech than most people do, from whom to seek advice and/or assistance if needed.
As for the NFB mod to a Pro Jr, yes you are correct even I could do it. That's really not the point I was making though. I didn't actualllyy refer to the NFB in that last post at all. Which if you reread my last post correctly and in it's entirety, you will clearly see. What I did do is too make some suggestions on a course of action. Which have been proven by a significantly large number of people to be beneficial in modding amps and the Fender Pro Junior amp in particular.
Now as to the issue of the NFB itself. Regardless of your experience with it, personally I would be inclined to heed the advice of Mark Burness. Simply because I believe he has much more experience in these things generally speaking. I'm not saying I would never try it. I'm simply saying that in this instance I would trust in Mark's guidance. Because I'm more than well aware of his credentials when it comes to working on amplifiers and harp specific modded amplification in particular.
With regards to speakers with the Pro Junior amp. I can't comment on the Lil Buddy as I have never used one. I'll take your word though that it's a good speaker as I know you have done quite a lot of testing with various speakers in different amps. The Jensen speaker that I used in my Pro Junior had a preferable sound over the stock speaker to my ear. Sorry I can't recall the model number as it was quite a long time ago.
Last Edited by Kingley on May 05, 2013 12:50 AM
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Hondo
202 posts
May 05, 2013
10:59 AM
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A couple of things-
1. I hear ya about the speaker swap, but I thought these 94's with the blue frame were good and made by Eminence.
2. I changed the tubes to- JJ EL84's, Put a Jan-Phillips NOS 5751 in V1 and a 12AX7 Gold Pin in V2. The feedback went away! BUT it hummed like a SOB. So I swapped the 12AX7 for a JJ ECC832 and nothing changed. Then I tried what Kingley suggested and swapped the ECC832 into V1 and the 5751 in V2. The feedback was still gone and the hum was completely GONE! Not a very dirty sound but LOUD w/ no FB or hum.
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Kingley
2608 posts
May 05, 2013
11:10 AM
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Hondo - If yours has the blue frame Eminence then I wouldn't change the speaker at all. Those are very good for harp. The later Pro Junior amps had different speakers in them. Try putting the 12AX7 back in V2 and leave the ECC832 in V1 and see what happens. If it's still not dirty enough, then I'd suggest either a 12AY7 in V1 and a 12AX7 in V2. I have had problems in the past with 5751 valves being noisy and much prefer a 12AY7 in V1 most situations. If it's still not dirty enough after that look at your mic cupping techniques as they can make a big difference to the sound.
Last Edited by Kingley on May 05, 2013 11:11 AM
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Rick Davis
1753 posts
May 05, 2013
11:18 AM
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Honda, yes, the blue frame speakers are good. Keep them if you like them. I have two of those in my Bassman and two of the Eminence Lil Buddy speakers. Sounds awesome.
As for the ECC832 it is dual triode with unbalanced gain: Half of it is equal to a 12AX7 and the other half has much less gain, equal to a 12AU7. It sounds cool when used as a phase inverter in that amp because it drives the two power tubes unequally, giving it an interesting tone.
If you use it in V1 -- the preamp socket -- one side is the preamp and I believe the other side is the secondary gain stage that recovers lost gain in the tone stack. I'm not sure which side of the tube is doing which job. I would be curious to know.
Here is the most important rule for these projects: If it sounds good then it is good. Don't be afraid to try things. If the tube combination you have in there sounds good then you are good to go.
I am a little concerned about that hum. If it were me I would chase that down just for the sake of finding out what it was.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
Tip Jar
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Kingley
2609 posts
May 05, 2013
11:23 AM
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I'd suspect that hum is caused by the 5751 in V1. I have experienced that exact same thing before using a 5751 valve in V1 on a number of amps. I suspect the valve may be going bad. As long as the hum doesn't come back with other combinations of valves that do not include the 5751 then i wouldn't worry unduly. If it does occur with other valve combinations. Then as Rick says I'd be a little concerned and investigate the issue further.
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Rick Davis
1755 posts
May 05, 2013
11:30 AM
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I've never had a problem with an NOS JAN Philips 5751, but you never know. You may need to spray some Deoxit in the tube sockets and re-insert all the tubes several times. That is the first thing I'd try.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
Tip Jar
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arnenym
135 posts
May 05, 2013
11:43 AM
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Rick wrote."arnenym, a Fender Twin has a NFB circuit, and that is the classic Fender tone. How can disconnecting the NFB circuit make an amp sound like a Twin? That makes no sense."
There is some other differences between those two amps!
One , of few things they share from the beginning is a NFB loop.
This "twin statement" is done by two different guitarists who has played for many years and they both have owned Twin's. And both of them loved their old Twin's.
But i build my opinion about cutting the nfb loop in this two amps on my own experience. I made them switchable and when you could do this A/B listening you really hear the difference and see the advantages and disadvantages when you do a modification.
IMO a fixed bias Longtail PI amp is no good for harp without NFB loop.
IMO The NFB loop made the amp more feedback resistant and give a better bass response.
IMO PP amps with paraphase PI or selfsplit is the best amps without NFB loops.
Last Edited by arnenym on May 05, 2013 11:52 AM
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tmf714
1755 posts
May 05, 2013
11:47 AM
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From Bill M Audio-
Hi bill,
i was wondering if anybody ever did a test comparing the values a new pro junior that hum and one who don’t hum ( or as low as a normal blues junior).
I am sure these do exist ( non humming pro jr) , i tried two in stores and if i look on youtube half of the video featuring a pro junior have a non-humming one.
I do ask you that because i think there must be some caps too much off of the writen value or power transformer that do something wrong or i don’t know what in the bad ones…
Reading all the blogs on the internet i realize the problem people gets is always the same : A constant hum that you can hear at any volume when the amp is on . the hum dont go up or down when you change the volume or the tone.It sound kind of like a microwave.
so if we bring the values of the humming amp to the values of the non-humming amp , this could be the perfect mod to keep it stock.
So, for now, I will try to change the preamp resistors for light blue ones as you did and keep dreaming
I dont have the money to buy another pro junior and i dont understand enough how this amp work to do the tests
Sorry for all those questions and comments
Log in to Reply bill says:
April 3, 2013 at 10:19 pm
Most Pro Juniors hum because one of the preamp cathodes is not bypassed. You won’t find any measurable difference in components as a cause of the hum. Doing the heater offset mod on my site raises the DC voltage on the heater so that the cathode doesn’t pick up hum from the heater.
PJs are very sensitive to the tubes installed and you can sometimes mix and match 12AX7s until you find a quiet pair.
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Rick Davis
1758 posts
May 05, 2013
11:56 AM
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tmf, the cathode bypass issue is interesting. One feature of the Mission 32-20 amp is a mini 3-position toggle switch on the bottom of the chassis that allows the player to vary the amount of bypass. It is useful when using different mics or if you just want to alter the tone in a subtle way.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
Tip Jar
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Hondo
203 posts
May 05, 2013
12:00 PM
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Wow. What a lot of great information.
Kingley- I'll put a 12AX7 back in V2 (instead of the 5751)and see if it gets dirtier.
Rick- Is the Phase Inverter V2? If so, you want me to try putting the ECC832 in V2 and a 12AX7 in V1.
tmf714-Thanks, I'll look at that heater offset mod.
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