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Discussion Starter #1
Wild Bill I was reading your posts over in the open mic session a little off topic but I like the way that you talk about amps. This question is sort of looking for advice on whether I should keep my present amp, an Ampeg SJ-12R a re issue of the blue diamond series. I picked it up on Ebay for a reasonable price and it is a nice sounding amp. Of course there are a couple of issues that I would like to get around. The first is changing the speaker which I have already purchased on on ebay. The replacement speaker is an eminence legend gb128. I played a Carr amp a while ago and that is what was in it. Secondly I find that the amp breaks up early, about 4 on the dial and would like a little bit more clean headroom. Would changing the preamp tube type achieve that for me? The power tubes are winged C 6L6's. I like the fender clean sound, I use a good OD and boost to get the tubes singing.
Keep up your good posts i have found them very informative. Thanks Bill

Oh ya .... free beer ? well if your ever up my way...:food-smiley-015:
 

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HI Paul,
I know you were asking Wild Bill, but maybe I can help. If you want to get a little more volume out of the amp before breaking up, you could try a 12AT7 in the V1 position of the preamp. The AT7 is a lower gain version of the AX7 and usually works very well for what you are after. I've used it and seen it used very successfully in many amps to help stop early breakup. You could also try a 5751 (again a lower gain version of the AX7), they work very well in fenders for the same purpose.

KT66 power tubes would also increase your headroom, but if you are happy with the sound you have now, I wouldn't go that route.

Ripper
 

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Nice amp!

PaulS said:
Wild Bill I was reading your posts over in the open mic session a little off topic but I like the way that you talk about amps. This question is sort of looking for advice on whether I should keep my present amp, an Ampeg SJ-12R a re issue of the blue diamond series. I picked it up on Ebay for a reasonable price and it is a nice sounding amp. Of course there are a couple of issues that I would like to get around. The first is changing the speaker which I have already purchased on on ebay. The replacement speaker is an eminence legend gb128. I played a Carr amp a while ago and that is what was in it. Secondly I find that the amp breaks up early, about 4 on the dial and would like a little bit more clean headroom. Would changing the preamp tube type achieve that for me? The power tubes are winged C 6L6's. I like the fender clean sound, I use a good OD and boost to get the tubes singing.
Keep up your good posts i have found them very informative. Thanks Bill

Oh ya .... free beer ? well if your ever up my way...:food-smiley-015:
Hey Paul! That's a nice amp!

I've worked on a couple of them and they do sound reasonably close to the originals. I dunno if merely changing tubes will increase your headroom. C's are great tubes but headroom comes more from the preamp.

I use 12AX7EH's as my main preamp tubes. They're middling priced - not the cheapest but not silly, either. They seem to have a Mullard flavour, at least to my ears.

If I had the amp in my shop I would try to increase headroom by changing voltages on the preamp tubes. This is something that if you have to ask how to do it you likely don't know how to do it safely! :)

I haven't got the schematic handy but there's another possibility. Your amp probably uses what is called a negative feedback loop. It's a simple mod to defeat this loop and it opens up the headroom considerably. It also makes for a peakiness in the mids and a bit more distortion but it can make a great rockin' blues tone! Jazz guys don't like this mod much 'cuz they like it clean but SRV or Pat Travers fans love it.

I don't know if the Eminence GB128 is the best choice. It is the replacement for the original sound but you're asking to change that. I'd look at a Wizard myself. I should think that this speaker would not break up at as low a level and is pretty loud besides.

If the speaker choice does it then great Paul but if not then you'll need someone to do a bit of tweaking inside the amp.

Hope this is useful and like all free advice, if it doesn't work I'm hiding out in Nairobi!
 

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Hello again Mr. R!

Good to see you here, Mr. Ripper - oh fellow tubehead!

How are the dams holding up in flatlander country? I worry about you guys sometimes when I watch the weather channel in the spring.

I don't mind direct emails if you want to keep in touch.

[email protected]
 

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Hey how close are these amps to the old reverbrorockets? I just finished fixing one for a guy. Great sounding amp.

Hey Bill, things are going great here. We don't need to worry about dams too much right now in my part of the province, we haven't had any rainfall to speak off since June. it's been a super hot, super dry summer here. Way to dry. I'm right on the western side of the province.

Bill's right about the feedback loop. It can really make an amp sing the blues. I've also found sometimes it really adds to the treble too, so that might need to be addressed but it all depends on the amp. I use EH 12ax7s in most of my amps and have been very pleased with them. I also got my hands on some old Sylvania 12ax7s that I put into one of my Fenders and they worked nice. Try using a 12at7 before moding the voltage to the preamp tubes. It might just do what you want and allows you to switch back to the earlier breakup without any mods.

Hey Bill, I'll drop you a line direct for sure! Always good to talk to a fellow tuber.
 

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Discussion Starter #6
This great info, it is a good thing we have you two guy's around. Yes I like the sound but I find the original speaker gets farty in the bottom end . The amp as it is right now seems more towards the trebly side, when I use my strats I have to turn the bass to about 8 the treble to about 4 and play with the mid to find the sweet spot. With HB's I bring up the treble to about six and adjust the mids a bit.I'll give the 12at7 a try . As for changing the voltage on the pre amp tube, with the schematic and some general advice I would also try that. Bill inside the head there is a trim pot, is this for biasing the power tubes? As for the GB128 I picked it up at a very good price. If it doesn't do it I'll try something else, Have considered the scumbacks but I want to know if the amp is going to sound like i want it. I have jammed with numerous others that use fenders,marshalls and boogies but i like the general tone that I'm getting just needs a little refining.

Ripper the free beer offer is good for you also if your ever in this neck of the woods. Thanks guys....:food-smiley-004:
 

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"Maybe yes, maybe no!"

PaulS said:
This great info, it is a good thing we have you two guy's around. Yes I like the sound but I find the original speaker gets farty in the bottom end . The amp as it is right now seems more towards the trebly side, when I use my strats I have to turn the bass to about 8 the treble to about 4 and play with the mid to find the sweet spot. With HB's I bring up the treble to about six and adjust the mids a bit.I'll give the 12at7 a try . As for changing the voltage on the pre amp tube, with the schematic and some general advice I would also try that. Bill inside the head there is a trim pot, is this for biasing the power tubes? As for the GB128 I picked it up at a very good price. If it doesn't do it I'll try something else, Have considered the scumbacks but I want to know if the amp is going to sound like i want it. I have jammed with numerous others that use fenders,marshalls and boogies but i like the general tone that I'm getting just needs a little refining.

Ripper the free beer offer is good for you also if your ever in this neck of the woods. Thanks guys....:food-smiley-004:
Paul, Ripper's 12AT7 suggestion is a good one and certainly worth a try. That trimpot is almost guaranteed to be the bias pot but I'd really like to see the schematic again or the inside of the amp before I'd care to be held to it! I have a vague memory of the last one I worked on and I seem to recall it had a trimpot adjustment in the tremolo circuit. I could very well be wrong. I've crawled through a LOT of amps!

Funny, the brain worked fine before the kids came along. Maybe they have something to do with it? :)
 

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Wild Bill said:
Funny, the brain worked fine before the kids came along. Maybe they have something to do with it? :)
That is exactly what happens! They cause certain parts of the brain to shut down. I've got five and since they have started going off to university (3 of them so far), my memory and thought processes have started to return somewhat.
 

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Discussion Starter #9
wow three in university at the same time.... the thought process might be coming back but how is the financial.... I have one entering his fourth year and it certainly is costly. I feel for you....

Bill my model doesn't have the tremolo only reverb, I think I'll pick up a 12at7 and give it a test. Does this require any bias adjustment ? I have looked all over the net and have yet to come up with a schematic for the SJ-12R. I can find the older reverb rockets or the jets but none for this one. Would they be somewhat the same ? Anyway thanks for all the input on my amp...

Paul
 

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Discussion Starter #10
Bill, I was snooping around schematic heaven and although there was no SJ-12R schematic I found a drawing for a reverbrocket re issue 50W 1 - 12 speaker much the same as mine and there is a trim pot on the schematic (25K) that they refer to as bias adjust so I saved a copy for further study, ie: when I am looking at the drawing and the head components. Now I got to find one for an Ampeg B2, a bass player friend just brought his in and says it is not as loud as it use to be... :eek:
 

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Yeah the three of them are costly for sure...cuts into my available funds for GAS. Two stayed in this province one is in Dalhousie.

The 12AT7 requires no bias adjustment, only power tubes require that. The Reverbrorocket only has a bias adjust.

Biasing is best left to someone who has experience and only needs to be done when changing power tubes or when really trying to find tune your sound. For example, Garnet amps benefit big time from having their bias redone as Gar designed the circuits to run fairly "cold". It gives the power tubes longer life, but not necessarily the best sound. By adjusting the bias you can improve (in theory) the sound of your amp.
 

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Discussion Starter #12
Thanks for the info on the 12at7 Ripper, the trim pot has my curiousity now. I am an Industrial electrician by trade so electrical circuits are quite common for me it is just that even when I took my apprenticeship (early 80's) tubes were not in the picture it was at transistors at that time and I work more with communications and automation issues in the industrial enviroment. I have tinkered with pedals, modded and built, guitar wiring and setups now amps have my interest. I know enough to respect electricity for what it can do, early in my apprenticeship I took a hit off of one leg of 600V which would be about 347V and from then on I have been very respectful of it's power.
 

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Hey that's cool. Then you are probably more than able to do the biasing of your amp should it need it. There is some great documentation on the web on how to properly set bias and what the bias ranges etc for different tube types are. It can be a tedious process as you adjust and play and listen and start over again, but it can be very worthwhile. There are a couple of different methods for biasing.


http://members.shaw.ca/house-of-jim/Html/bias_tables.html This is pretty good for tables for tube bias tables. Again, as Jim states on his page, these come from old manuals, and some of the newer tubes from may need to be set a bit more conservative.

http://tone-lizard.com/Biasing.htm This is a really great article on biasing and some myths etc.

Hope some of this helps. Oh by the way, you can also stick in a 12AU7 if you find the 12AT7 doesn't clean things up enough.
 

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Discussion Starter #14 (Edited)
Ripper - thanks for links and the vote of confidence. In Bancroft there are no guitar techs or amp techs so over the years I have had to become resourceful and teach myself, it has paid off because I saved a few buddies trips to the city and I get to play with a lot of neat gear sometimes. :food-smiley-004:

One more quick question is there a preferred brand of 12AT7 or 12AU7 to use ?
 

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Is there a preferred flavour of ice cream?

PaulS said:
Ripper - thanks for links and the vote of confidence. In Bancroft there are no guitar techs or amp techs so over the years I have had to become resourceful and teach myself, it has paid off because I saved a few buddies trips to the city and I get to play with a lot of neat gear sometimes. :food-smiley-004:

One more quick question is there a preferred brand of 12AT7 or 12AU7 to use ?
Paul, I've found that 99.9% of tonal differences in the preamp come from the very first tube in the chain, or V1 as it's termed. This is the one farthest from the output tubes. After that not only are my ears not good enough to notice any differences but I've never known anyone else that can either. I'm not saying there's not somebody with mutant bat ears that could do it out there somewhere, just that if I were you I wouldn't worry about it! :)

Besides, it's mostly personal taste and how can one make an absolute judgement on what someone ELSE would like?

I would caution that if you try a 12AU7 it will fit and appear to work but don't totally write it off if you don't like it. This tube requires different values of parts feeding it than a 12AX7, 12AT7, 12AY7, 5751 or whatever to run properly. So to give it a fair shake you can't just plug it in. You really should at least change the plate resistors in the phase inverter circuit to maybe 22K or so instead of the much higher values you'll likely find.

Come to think of it, Ampeg often used a phase inverter circuit with a gain of unity (no gain at all) anyway. So changing tubes at this position might be a waste of time.

I hear a lot of mojo spread around about different brands of tubes, Paul. I agree there are some differences but they tend to be rather subtle. Since most people don't know enough to make circuit changes they tend to seize on doing easy things, like swapping tubes. It's easier to get all anal about such stuff rather than learn some real theory. These ideas are often championed by guys who may really know better but they're selling something they want you to buy!

The hifi world is just full of stuff like this. I still vividly remember a hifi house in Oakville that in their catague offered "non-oxygenated" speaker wire, at $400 per foot! US dollars! They would sell you a 12AX7 tube for $50 that came with its own "Testing Certificate" suitable for framing. If you actually understood the theory behind tubes you'd quickly see that there was nothing special about the tube. It was merely an ordinary brand that they tested and not surprisingly, found it to have exactly the specs you would have expected anyway!

As Kurt Cobain said, "Just play the damn thing!"

:)
 

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I'm bad. I meant to put 12AY7 not 12AU7. Sorry about that. I had just been working on an old tube stereo amp with 12AU7's in it so I guess I had it on the brain.

I'm with Wild Bill and not sure I have noticed any difference in the actual tone between preamp tubes. The differences are subtle to say the least. If you have trouble finding a 12AT7 let me know.

I hear what you are saying Bill about the "hifi audiophiles". Hollow core oval silver wire power cables etc. Makes me chuckle.
 

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Discussion Starter #17
No problem Ripper I've done the same thing, get on one train of thought and part of the brain stays there...
I hear you Bill, I've read a lot of threads and articles about tube types, makes and the term NOS. The NOS one gets me if it's new "old stock" where have they been hiding them. There are a lot of audiophiles out there that do take things to extreme's, if you need instruments to read the difference how will the ear be able to pick up on it. One thing I like to ask Bill is about speakers, you mentioned the wizard, I picked up the eminence Legend GB128 on Ebay for 42.00 US including shipping, I have seen them used in boutique amps such as Carr and the Gretch Amps all use the Legend. What would be the differences between the two or is that strictly up to the individuals style and his ear. I have been doing a lot more clean playing lately and relying on the tone of the guitar. Yep Kurt had it right same as Zappa "shut up and play the guitar" a great box set.... Again thanks for the info guys ....:food-smiley-015:
 

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Discussion Starter #18
Hey Bill and Ripper just wanted to let you know how things were working out. I got the Legend in there finally and there was a lot of change, it handles the bottom a lot nicer and tamed some of the icey highs. I am not sure what make was in there an ampeg custom ?? but the eminence has made a big improvement. I have yet to get an 12at7 to try but that is next. Thanks guys for all the great help....
 
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