Valve Junior mods

[Ol Boy Rivers]

Sorry i havent replied folks, Thanks for the welcome to those who did. Allright, Got half of the money for the amp but now i need som advice. PREAMP TUBES! I am very glad my grandmother has endless valvestate crap in her garage, And ive come across antique tubes. 6CG7...Looked a little bit on it, And aparently this GE tube has some history, SO! I need to know if i can put it in the slot for the preamp tube of the valvejunior head. Next question: Anyone heard of that Lightbulb compression thing-a-ma-bob? Im really interested in it, And plan on entergrating it in to the stanby switch so i can switch between non-attentuated, And the lightbulb ( Besides, If flashes and changes glow when you play..Shiney things excite me 8-D ) Last question: Ive heard talk of people taking out the EL84 and replacing it with the 6V6 and the EL34 tubes. Now...I see how you would need to do a little mod work on the tube sockets but wouldnt the tranny's have trouble pushing those tubes? Once again a little brain storm: It would be killer to beable to change from the EL84 to a 6V6 for that more of a preamp drive sound. REPLY BACK! Country folk like me have trouble trying to program a VCR much less Mod an amp, So help a honkie out. Thanks yall.

[Ricochet]

The 6CG7 has a different pinout than the 12AX7 and won't work without rewiring the socket. You'd do better to simply plug in a 12AU7, which has a similar amplification factor. Why do you want to drop the gain so much?

 

As for "history," certainly the 12AX7 and 6BQ5/EL84 have as much of that as any of the other types you mentioned.

 

I'd recommend against trying to convert the EL84 to a 6V6 or EL34. You're really looking for trouble making that sort of change. Don't start unless you really know what you're about. (Doesn't sound like it.) You'd likely need to replace the power and output transformers as well as enlarging the socket hole in the chassis, and rebalancing the preamp to provide the different levels of drive needed for these tubes. You're talking about replacing much of the amp, and it'd be easier just to design and build a similar amp to use these tubes from scratch.

 

I posted some comments about light bulb attenuators on the valvejunior.com forum today. I won't try to repeat them here now.

[Ol Boy Rivers]

Thanks for the advice on the tubes,Ricochet. Yeah, I really dont know much about the scene, Though i have a fairly good share about the guitar instead of the amp. For the powertubes, Thats a #@%&load of work that i dont intend on doin', just poking around with curiousity i guess. As for the Preamp, In all honesty i didnt really know what the 6CG7 did to the sound, Just knew old tubes are supposed to be praised, But for the gain facter: I would like a lower distorted sound then i know the 12AX7 is going to give me. I play the blues and a little bit of a rock-a-billy scene, So i dont really want an extremly dirty sound when i want to crank on that knob. But hey, We'll see what going on when i actually get the amp and start messing with my small asortment of pedals and guitars. HOWEVER, With the same Oddball idea with the powertubes i bet a switch to switch between two preamps would be awful nice, Though ive heard something about a resister being replaced with a switch inside of the board to beable to limit and add more gain. Well hell, Too many posibilities with guitars, But i guess its a never ending thing. Thanks again Ricochet, Truelly appriciate some advice from a master!

[Rock Soldier]

Hey Ric, the 12au7 is less gain? I have an Electar Century DR30 hybrid amp with a 12au7 preamp tube. The distortion is wimpy. Might I try throwing a 12ax7 in there to boost my gain? Ooops ... sorry .. didn't mean to get off the topic of VJ mods.

 

LPCustom, I like that cabinet mod with the 2 10's. I just got a VJ Head and use a 4 x 12 cab but am tempted to build a smaller cabinet to go with it to make it more portable. Your project makes me feel I can make a small cab of similar style to match the head.

[Ricochet]

There's a whole series of 12A*7 tubes that have the same pinout and can be freely swapped in most guitar amps. The 12AX7 is the highest gain, with a nominal amplification factor of 100. The 12AU7's the lowest, at about 18. The 12AT7's rated at 60, the 12AY7 (original input tube of the tweed Bassman) at 45. The 12AZ7's very similar to the 12AT7. There are a boatload of industrial types with 4-digit numbers, like the 7025 equivalent of a 12AX7, that you just have to look up.

 

A while back I mentioned one, the 12DW7, a.k.a. 7247, a.k.a. ECC832, that has 1/2 of a 12AX7 and 1/2 of a 12AU7 in an envelope.

 

Then there's the 12DF7, a close equivalent of the 12AX7, the 12AD7's another, as is the 12BZ7.

 

These tubes designed for TV applications often can be had for little or nothing, but they'll work great in your amp.

[Rock Soldier]

Thanks Ric! I'm learning more about this stuff every day. Next time I got that amp out I'll have to try that. It's not gonna be no Valve Junior though as the power amp is solid state, but might be more to my likeing. It's a versatile little amp. It has an effects loop, chanel switching, digital reverb, ext. speaker jack, a 12" speaker and its small and light.

[Greg B.]

I am doing the gain mod with a switch to go between high and low gain. I alligator clipped the wires to R6 as a test to make sure that I was happy with the sound (and boy was I ever!!!), but when I switched back to low gain, there was a nasty noise and a little bit of a squeal. But only on the low gain. Could this be because of the alligator clips? I unhooked them, plugged in, and turned up- no noise at all. Suggestions?

 

Also, after bleeding the caps, is there any possibility of getting hit(maybe some voltage left in the caps?) when touching R6 to solder?

[Ricochet]

The higher the gain of an amp, the more sensitive it is to feedback that results in oscillation. A loose wire lying around connected to part of the circuit is an excellent "antenna" to pick up signal from another part of the circuit, producing this feedback.

 

I like to leave a lead clipped across the filter cap terminals for a while after discharging them. Electrolytic caps can actually redevelop a bit of voltage after being quickly discharged. (It occurs due to ion diffusion in the electrolyte.)

[AeroG33k]

-double post, sorry-

[AeroG33k]

Originally posted by LPCustom:

Originally posted by Xplorer:

LPCustom: Nice job dude! That looks sweet! Any other plans to cover the wood, piping etc...?

Thanks! I wish I had more time to work on it.

 

I think I'm going to cover the "wood" (it's actually MDF) with a cocoa brown tolex. You can see a swatch of that here:

 

http://www.mojomusicalsupply.com/cgi-bin/mojotone/7310000.html

 

The swatch here is too dark and too red for what it actually looks like.

 

I put Marshall cane on it as the grill cloth but I don't care for the looks of it now that it's on. I think I'm going to change it to this:

 

http://www.mojomusicalsupply.com/cgi-bin/mojotone/6301004.html

 

I also have some of this. I can't make up my mind about this. This isn't exactly what it looks like but it's what they say it looks like. This is far too light in color. What I have is darker and more of a wine color.

 

http://www.tubesandmore.com/images/inv/S-G302.JPG

 

I also have some white piping. I'll probably try that out to see if I like the look with it. The original cab has piping. So I probably will put piping on it.

 

I just took a couple of pictures of it. Sorry about the poor picture quality.

 

Here's what the front looks like, now:

 

http://www.crystalblack.com/valvejuniorcombo2x10_front.jpg

 

And here's a shot of the back.

 

http://www.crystalblack.com/valvejuniorcombo2x10_rear.jpg

 

I'm actually considering rebuilding it in pine for two reasons. The first is that I made a mistake on the back when I cut the speaker cable channel (got it on the wrong side). The other is that The MDF is too fragile for my taste and too heavy. The amp weighs in at 43 pounds, now. MDF is heavy stuff.

 

I have inserts to cover the amp section screws on the top. But I'm not going to install them until I get tolex on it.

 

I don't have the proper wiring installed for the speakers right now. I'm testing a stereo/mono and multi-impedance jack arrangement for my other speaker cabinets in it. That's what all the red and black wires are. The plate is not in view, I think it's in the corner of the back.

WOW! Amazing Job.

 

I was thinking about (not actually seriously planning to...yet...so it's really more like dreaming) get a Valve Junior Head and make a small cab with 4 really small speakers (probably 8") as opposed to a single 12". I'd imagine it'd sound a bit fuller than a single speaker and a little brighter due to the smaller size?

 

Anyway, when you get done with the new one, I can have the old one, right?

[Greg B.]

Originally posted by Ricochet:

The higher the gain of an amp, the more sensitive it is to feedback that results in oscillation. A loose wire lying around connected to part of the circuit is an excellent "antenna" to pick up signal from another part of the circuit, producing this feedback.

 

I like to leave a lead clipped across the filter cap terminals for a while after discharging them. Electrolytic caps can actually redevelop a bit of voltage after being quickly discharged. (It occurs due to ion diffusion in the electrolyte.)

So, if I am understanding this correctly, the alligator clips are acting as antenna and causing the noise. Once I solder the wires it should be gone, correct? Thanks for the info, I'm kinda new to messing with amps.

 

That cabinet is killer, isn't it.

[Ricochet]

You've got to be really careful with "lead dress." Often a wire can cause feedback if it gets too close to another part of a circuit. Troubleshooting amps for oscillation often involves simply pushing leads around with an insulated "chopstick."

 

Yes, that cabinet is gorgeous!

[Greg B.]

Thanks for the info, Ric.

[Ricochet]

You're welcome.

 

Generally, keep your connections as short and direct as possible.

[Greg B.]

I soldered it up and most of the noise went away. It is still a little bit noisy on the low gain, but not really too much. The high gain is really quiet. But WOW! that thing sounds amazing on the high gain. I love it even more now. My Classic 30 is going to get really jealous!

[Ol Boy Rivers]

Howdy yall, Ol'boy hear. Got more money for the VJ and now i have another question. Just how loud is this sucker? I know lots of people are saying its loud enough to keep with a band with the right speaker or loud enough to put the neihbors in a bad mood...So in relivity how loud is it? Compare it to something or something so i can percept this thing with my redneck mind! Thanks yall.

[Bibleboy3]

Hi all,

this is my first post here.. has anybody added an extra gain stage to the VJ ..

[Ricochet]

I don't think the power transformer is stout enough to handle adding another tube. It'd be pretty crazy anyway, in terms of the work. Just plug in an overdrive pedal in front of it and you've added a gain stage.

[teleamp]

A few threads up Myles Rose told of the down side to DC filaments.

 

Ricochet wrote: And why would that be, Myles? Got a reference?

 

 

If Ricochet would have followed the links given by Myles in his post, it would be pretty evident that Myles knows what he is talking about. Any west coast session player or pro guitarist should have recognized his name. And any thing I can learn from a true amp guru (MR is one), I take to heart and try to show a little more respect.

 

And properly implemented AC filaments along with proper lead dress will not induce noticeable noise.

 

The DC filament mod is a simple, quick fix that may have a price as Myles has pointed out.

 

MikeY

[ChewingAluminumFoil]

I don't think Ricochet was picking on Myles, just asking for a reference.

 

A quick search turned this up:

 

DC vs. AC Operation

As tungsten atoms evaporate from the filament, a very small percentage of them are ionized by the small amounts of short-wave ultraviolet light being radiated by the filament, the electric field around the filament, or by free electrons that escape from the filament by thermionic emission. These tungsten ions are positively charged, and tend to leave the positive end of the filament and are attracted to the negative end of the filament. The result is that light bulbs operated on DC have this specific mechanism that would cause uneven filament evaporation.

 

This mechanism is generally not significant, although it has been reported that light bulbs sometimes have a slight, measurable decrease in lifetime from DC operation as opposed to AC operation.

 

In a few cases, AC operation may shorten the life of the bulb, but this is rare. In rare cases, AC may cause the filament to vibrate enough to significantly shorten its life. In a few other rare cases involving very thin filaments, the filament temperature varies significantly throughout each AC cycle, and the peak filament temperature is significantly higher than the average filament temperature.

That's from http://www.bulbs.com/lightingguide/tech_i_history.asp

 

Doesn't seem to corroborate Myles' 5x claim. Anyone find anything else?

 

Btw, I personally did the DC conversion on this amp, and on this amp it took hum way down. I've done it on others and it didn't help much.

 

CAF

[teleamp]

Myles, I've been playing with circuit variations and OT's. Can you give me an answer to why my VJ combo when wired as a GA-5 and using the Mojo 771 OT does not have the bottom end of the GA-5? Can the difference in cabinet construction (particle board/MDF vs. plywood) and speaker baffle density and mounting be having that much bearing on speaker response.

 

http://i70.photobucket.com/albums/i115/teleamp/Resizedcircuit.jpg

 

That photo does not show the latest changes: The circuit has been changed to the MM modified circuit, the PT has been replaced with a Hammond 270DX, the OT has been replaced with a Heyboer HTS 8215, and a custom 3 henry choke from Heyboer has been installed.

 

http://i70.photobucket.com/albums/i115/teleamp/ResizedHTS8215.jpg

 

The amp sounds similar to the MM clips, the clean tones are spectacular with lots of bottom. The overdriven tones are OK (IMHO).

 

I think I prefer the 5F1 circuit with a 6V6 (much better distorion IMHO). I have bought the MM mod kit, but will use it to do a DIY build instead of installing in a VJ.

 

MikeY

[Rock Soldier]

Originally posted by teleamp:

A few threads up Myles Rose told of the down side to DC filaments.

 

Ricochet wrote: And why would that be, Myles? Got a reference?

 

 

If Ricochet would have followed the links given by Myles in his post, it would be pretty evident that Myles knows what he is talking about. Any west coast session player or pro guitarist should have recognized his name. And any thing I can learn from a true amp guru (MR is one), I take to heart and try to show a little more respect.

 

And properly implemented AC filaments along with proper lead dress will not induce noticeable noise.

 

The DC filament mod is a simple, quick fix that may have a price as Myles has pointed out.

 

MikeY

Seemed like an innocent question to me. Nothing rude about Ric. He's a class act. I know myself I want to know why about everything. "Just because" isn't a valid answer. Nothing wrong with that at all. The reference ChewingAluminumfoil posted was informative. It explained it although did not support five times the wear. Then again Myles is an expert and the voice of much experience. I on the other hand know very little about the topic. I just know Ric is not rude.

[teleamp]

Sorry, not trying to be a pain to anybody.

 

MikeY

[Ricochet]

I'm not trying to be rude, and I'm certainly not denying that Myles is an amp expert. He's got a tremendous amount of experience with amps. It is appropriate, however, to be prepared to show evidence of a claim such as the one he made about DC heater power shortening preamp tube life drastically.

 

I have (and have fully read) a fairly extensive collection of books on tubes from the "tube era." None mention such an effect of DC heater power on tube life. I can imagine no physical effect by which DC power per se would shorten cathode life in preamp tubes with indirectly heated cathodes. The quote above about filaments does not apply. That's referring to directly heated cathodes. In the tubes we use in guitar amps ("preamp" and "power" tubes, not rectifiers such as the 5U4 series), the heater is a coiled or folded wire, coated with aluminum oxide insulation, inside the cathode which is a cylinder of nickel with a coating of activated barium and strontium oxide that emits the electrons for the tube's operation. The heater serves only to heat the cathode to operating temperature, and its voltage does not communicate to the cathode except through capacitance across the insulation, which is a source of hum in cathode-biased amp stages without or with insufficient cathode bypass capacitance. This cathode structure is also known as the "unipotential cathode," the resistance of the cathode sleeve being small enough that the potential of the cathode surface is effectively equal everywhere, unlike the filament cathode that drops the full DC (or AC) supply voltage from one end of the filament to the other, producing the potential difference mentioned in the post above regarding light bulb filaments. Our tubes are rated for an optimal voltage of 6.3V to the heaters. That's either AC (the rating being RMS) or DC voltage. The heating of the heater and cathode is the same, whichever source of heater power is used at those ratings.

 

Now, there are different ways of supplying DC heater voltage. One, which we've been discussing, is to use a rectified AC source. Another, which has been used as a cost saving measure in some amps of the past, is to use the heaters of the "preamp tubes" as part of the cathode bias resistance of the "power tubes," the average cathode current drawn by the power tubes being designed to closely approximate the nominal heater current of the "preamp tubes." This system, though economical to manufacture, does NOT closely regulate the voltage and current of the heaters in the "preamp tubes." It causes a large surge of current through the heaters on turnon, which is a factor known to shorten the life of tubes. Also, in amps using a DC source of higher voltage than a tube uses, with several tubes strung together in series, the voltage often does not balance out equally between the tubes in series, particularly if they do not all warm up at the same time. The faster warming ones reach operating temperature first, the resistance of the heater wire in those tubes rises as they reach full temperature, but the full current drawn by the full string of heaters, some with cooler wires and lower resistance, has to flow through them. The early warming ones overheat before the rest can catch up, and this can be seen as a visible "flash." This happens with AC heater strings, too, and was considerably lessened by the adoption in the 1950s of tubes with controlled warmup times that would more closely match.

 

I suspect that Myles, with his vast experience with amps, knows that some amps using such DC heater power sources have shortened preamp tube lives. (But it's not because they have DC heater power, it's because the DC is poorly regulated.) That's why I'm wondering about the source of his info.

 

One of the best books addressing the issues of tube life and of tube failure modes is Robert Tomer's 1960 Book "Getting The Most Out Of Vacuum Tubes." A scanned PDF file of this book is available at: http://www.pmillett.com/Books/Tomer_1960_Getting_the_Most_Out_of_Vacuum_Tubes.pdf

It is well worth reading.

[Rock Soldier]

My new Valve Junior Head was manufactured with DC filaments. I hope it doesn't mean it's gonna be a tube eater. It sure is quiet though. Perfectly silent operation. No hum at all, not that the hum of the origional VJ bothered me all that much.

[teleamp]

Originally posted by trguitar:

My new Valve Junior Head was manufactured with DC filaments. I hope it doesn't mean it's gonna be a tube eater. It sure is quiet though. Perfectly silent operation. No hum at all, not that the hum of the origional VJ bothered me all that much.

I don't think you'll have any problems. Tubes should last for years if all is well.

[ChewingAluminumFoil]

Yeah, I wouldn't worry about it at all. I had a Princeton I'd modded to run DC filaments to make it as quiet as possible for recording work, I ran it long and hard, and never noticed any tube wear issues.

 

CAF

 

www.valvejunior.com

[Junior 1]

I did a comparison last night of my VJ combo and VJ head using the speaker from the combo. They sound different! Their clean sound was pretty much the same, but they broke up differently. To my untrained ear the combo seemed to sound fuller. The head seemed a little more harsh in it's break up. Both have stock tubes and I did it with a Squier Tele.

[audiofreakme]

can anyone tell me how to install a "triode switch", that will knock the amp down to 1/2 output power? Ricochet mentioned in another post that "you'd be switching the screen grid of the EL84 from its own fixed-voltage power supply on the second filter cap to connect it directly to the plate. " can anyone diagram or explain it step by step me? seems like another useful mod

[Ricochet]

Well, that pretty much was a step by step explanation. To convert the EL84 to triode operation, you simply disconnect the screen grid from its voltage source (which is the second filter capacitor in the power supply) and connect it to the plate. That could be done with a single pole, double throw switch, the moving pole being connected to the screen grid terminal at the socket, the other two poles being connected to the screen grid supply and the plate terminal.

 

The screen grid is socket terminal 9. The plate is 7.

 

My VJ is securely buttoned up and I'm not looking at it. You may have to break a trace on the printed circuit board to disconnect the screen supply. Can't remember just how it's set up.

 

I don't have handy any data on the 6BQ5/EL84 operated as a triode. I think it'll cut the output power by more than half, probably about 2/3.