Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Archives of favorite topics => Topic started by: TubeGeek on April 01, 2006, 09:48:21 pm
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I just picked up an Epiphone Valve Junior amp at my local gtr center for $90(it was the floor demo). It is 5W and has 12ax7 and el84's in it. I notice that it does not have enough power supply filtering (it has 120Hz hum) so that will have to be changed. Just out of curiosity I googled this amp and there are already several websites dedicated to modding this amp! It looks like I have a fun weekend project to do.
One of those websites mentions a lightbulb mod that acts like an attenuator on the output. Sounds interesting.
I might buy a couple more of them at this price!!!
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Another pic
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and another...
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What a cool little project! Looks like plenty of room for a turret board if you wanted to go that far.
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OMG!
It's a Champ for $99!
You just have to fix some cheap and stupid details.
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There's some good suggestions for mods in a thread over @ 18w>>>>
http://www.18watt.com/modules.php?name=Forums&file=viewtopic&t=9053
(You must be a member now to even look at the forums now)
"zaphod_phil" has some very good suggestions thru-out the thread.
I'm going to be near a GC either Monday or Tuesday......I think I'm going to pick up one of the head units! I have a good # of cabs to run it thru.
~$100 for a cab, chassis & iron is pretty amazing. :o
Jeff
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There's also the Galaxy 25 with push-pull output. I got one one of these in the shop with blown power transformer and output tubes. Installed 7591 tubes (6V6 would have worked also) and added a few cathode bypass caps and Hammond power transformer and the owner was pleased as punch.
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When I went to gtr center I was there to purchase some picks and I ended up with this amp. I must say that I am still amazed that it is only $99. The cabinet, trannies, chassis, and tubes alone are worth more than that. The speaker is a Weber ceramic 8".
I am thinking of either modifying the stock circuit to correct its inherent problems or maybe building a complete new turret board and dropping that in. I don't want to turn this into a money pit project but just a simple, quick project. But now that I am thinking of this...which drop in circuit would be suitable for a marshall like dirty tone?
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What a cool little project! Looks like plenty of room for a turret board if you wanted to go that far.
Yeah I picked one up the other day too, now I'm just looking for a design to replace that pcb with. I need a miniz schem now :)
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here is a schematic of the circuit...I hope its ok that I post it here.
I post it to open up a discussion on what can be modified to make it sound better...
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If you add a filament transformer then you can add a 6.3V pilot lamp and another gain stage, maybe a pentode input. And add some negative feedback.
The 100K resistors that are shown on the filament may really be 100 ohms.
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I plan on modding this amp tomorrow so now I just want to iron out the details...
More power supply filtering is needed. I plan on installing two 100uF 350V spragues in parallel with C8 or C6.
Would altering the value of C1 and C2 coupling caps affect tone?
What would changing the values of r8 and r9 do? Will it affect gain by changing the bias on the tube? I know that fenders use 1k5 here.
Lastly...I have read that the filaments nees to be changed to either a dc filament design or a twisted wire above the sockets. This is supposed to help eliminate some hum. I will add the extra power supply filtering and then this if there is still hum.
The on/off switch has a pilot light inside of it.
I'm not sure I want negative feedback...I am looking for a dirty tone. Negative feedback would clean it up somewhat, correct?
The 100k's were a mistake and are 100 ohms in the actual circuit.
Can you tell me more about the pentode, more gain mod you speak of?
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The coupling caps are purposly small to reduce low frequencies so the amp does not sound too boomy (also called farty). If you want to increase gain, increase the triode plate resistors from 100k to maybe 220K max. Twisted filaments above the sockets is best. Negative feedback makes the amp sound more solid.
I have used Sovtek 7199 in a couple of amps with the pentode plate tied to the triode grid, then 100K triode cathode resistor. Output is off the the triode cathode, wired as a follower with the plate tied directly to tht same B+ that feeds the pentode. If you add this stage I recommend leaving the 100K resistors on the other triode plates or you will get too much gain. You may need to change the 12AX7 to 12AT7 depending on the sound you want. Players say the pentode gives good touch sensitivity, meaning a larger range of sounds depending on how hard you play and where the gain is set on the guitar. You will need to add another filter section to the power supply for the 7199, maybe 22k and 20uF.
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I think I will just remove one or two of the existing filter caps and install 100uF 350V in there. Hopefully that will be enough filtering. Is there a formula to calculate how much filtering is needed? I know there has to be but I have forgotten those small details...can someone refresh my memory?
I would also like to add a bright cap into the circuit, maybe a 120pF in parallel with r6. In fact there is a spot for this cap already on the pcb. I found this by reading another website about mods on this amp. Is this cap pretty much the same thing as the 120pF bright cap on a fender? By adding this cap are the high frequencies bypassing r6?
Thanks for the replies thus far.
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This looks to be a simple financially feasible project for me. This morning I discovered a box of stuff I had in a storage unit that contained several filament transformers. Looks like a good project to use one of them.
TubeGeek please keep us advised on what you do.
Peace
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Negative feedback makes the amp sound more solid.
more solid??? Do you mean stability in the circuit?
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Adding feedback will make the amp sound different.
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> which drop in circuit would be suitable for a marshall like dirty tone?
A Marshall.
You are asking how to make a Honda sedan more like a 1-ton pickup. It's not a drop-in. A single-ended amp is a very different beast than the monster Marshalls.
Epi has a push-pull amp at ~$299 that might be the core of a mini-BassMan/Marshall hack.
> More power supply filtering is needed.
Actually, 20uFd is cheap but not stupid.
The bulk of the hum and buzz appears to come from incredibly stupid (or cleverly commercial) heater routing.
Fender Champs work OK with AC heat: the difference is they don't run the heat right NEXT to the plate lead as the Jr does. You'll need DC heat at least on the 12AX7 to get recording-quality low-buzz; properly routed AC heat should be fine for casual knockabout amp use. I think Hoffman has a Champ layout with proper heater routing: cut the PCB pads away from the traces, run proper wires. Ground the now disconnected heater lines on the PCB, this will help suck-out general buzz pickup (since we know these traces work fine for injecting buzz, the reverse is true).
They seem to be conflicted which end of the chassis to ground to. The obvious quick-fix is an insulated input jack, and let ground happen at the output.
The wire from the pot wiper is very critical: if it picks up any signal from the EL84 plate, the amp will get unstable.
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The input impedance is absurdly low. It should suck-out the top end of the guitar pickup. The 68K from grid to ground "should" be 1Meg. However, nobody says the amp is dull. It may be that the speaker has a ringy top end, and "normal" pickup loading would make the result too shrill.
> Would altering the value of C1 and C2 coupling caps affect tone?
Not a lot.
C1 passes down to like 7Hz. If you wanted less bass you could reduce this a LOT, but you would get more buzz pickup at that point. Leave it alone.
C2 passes to ~50Hz. You need to shave the bass in a small amp or it will fart when overloaded. You could try 0.01uF for a thinner tone.
C4 and C3 are typical over-kill, flat to ~9Hz. Everybody uses 10uFd-47uFd here for the Normal channel. Brite channels often use 0.68uFd, that's the other obvious value to try for C4. I expect that changing both these caps to 0.68uFd would give no ballz at all.
C5, output cathode cap, looks too small to me. For hi-fi, that would be 100uFd or more. However, hi-fi would use 10 times the output iron; this undersize cathode cap may be needed to prevent fart-bass. OTOH, maybe it could be useful. Throw 100uFd >16V across the existing cap and try it.
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> What would changing the values of r8 and r9 do? Will it affect gain by changing the bias on the tube? I know that fenders use 1k5 here.
Very very slightly. Not worth hacking a PCB. If you toss the PCB and tagboard it, copy a complete Fender preamp.
The minimum-change for more gain is to replace 68K grid to ground with 1 Meg (no input pad and much less pickup loading), short R6, break R7. That's about all there is; it really should be more than enough. Increasing gain does not make sense until you fix the buzz (unless you like to play in the key of 120Hz).
Instead of a monster main cap to smooth the B+ ripple, it is more elegant to add another stage to the filter. The least-hack would be to break the output transformer lead and insert 100 ohms 2 Watts in series, 20-40uFd 400V to ground.
Increasing R13 10K to 22K or 33K might also reduce preamp hum.
R15 seems to be a last-minute hack. A resistor here is an old custom. Values from 220 to 100K give different stability and overdrive effects. For sure a try-it-and-see experiment.
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Is it wise to add dc filament heater wiring
Like PRR stated, for a knock about amp (Not for a studio recording amp), do a good old fashioned twisted wire for the heaters and get it off the pc board completely. That should bring the 120hz under control. If you're determined to leave the heater wiring on the pc board, then dc may be the only solution worth considering. But that involves a transformer, rectifier, and filtering. Pretty pricey solution for a $99 amp that only needs 2 feet of heater wire to work right.
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I have a champ clone I made where I used a sheilded twisted pair cable for the heaters. It's next to silent. One of my bands did some recording last week and you can here the results here:
http://thegreywolf.us/iijuicy/IIJuicy_Recordings.htm
Listen to "Blackbird". That's the amp with chorused guitar. No hum!
Dave
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Maybe I wasn't clear....I do want this amp to be useable in the studio as that is why I build amps in the first place. If I cannot use it the studio, then I don't want the amp. I am just trying to think of all the possibilities out loud and get opinions.
Now the modding begins...I will post my results soon.
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"Maybe I wasn't clear....I do want this amp to be useable in the studio"
It was clear to me.. I guess I wasn't clear enough. My point is that a sheilded, twisted pair with the sheilding grounded at one end is extremely quiet. I got the idea from one of the TOT books from Donald O'Connor and it works. To really go all the way, use the wire I mentioned . Replace the 2 - 100 ohm resistors with 2 - 50 ohm resistors connected to the outside lugs of a 100 ohm pot and connect the center lug to an approximately 40 volts supply (I use a voltage divider from the B+). It's kind of a pain but it's about as quiet as it gets.
The attached may help with the 40 volt part.
Dave
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It don't matter, just git 'er done!
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I just finished modding this amp and it sounds much quieter. There is far less hum. I installed a 100uF 350V cap in parallel with the c6. I installed a bright switch with a 120pF cap in the c3 spot. I also installed a gain switch. It gives it just a little bump in gain. I installed a bridge rectifier with a 1000uF cap across the + and - terminals. From those terminals I connect to the board. From that position on the board I wired the filaments of the tubes with a twisted pair. Pins 4 and 5 of the 12ax7 connect to pin 4 of the el84, pin 9 of the 12ax7 connect to pin 5 of the el84. I also replaced the 0.022 caps with equivalent mallory 150's. I replaced the 22uF electrolytics with sprague atoms.
I measure 5.9 VDC on the filaments.
The amp sounds different than stock. So far it seems that the fizziness of the stock distortion is gone. It has a nice distortion still. Now I will play it some more and see if I need to tweak anymore.
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Here is the front of the amp with bright switch and gain included.
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Here is the inside of the chassis view of the bright switch and gain switch
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I notice that there is still hum when the volume pot is maxed out. Maybe this is unavoidable??? I could replace the input jack with an insulated type and see if that helps.
I am through for tonight but I will try a couple more things that PRR suggested tomorrow.
I would still like to experiment with lowering the spl of this thing, It is still loud for 5 Watts!
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Just out of curiosity I measured the plate voltage of the el84 and it is 354 VDC. Shouldn't it be around 300 VDC?
If I need to drop the main transformer voltage down is the best way to install a 1k 5w resistor in series from the HV red wire of the PT? There are two red wires...do is matter which wire I install the dropping resistor to? How do I know which one to install the resistor to?
If I understand this all correctly this extra resistor along with the 100UF cap I added makes up an RC filter stage which is good for filtering. Does this resisor also drop the main voltage down within the el84's proper range, or is that going to be done with another resistor?
PRR said to use a 100ohm 10W resistor in series with the PT red wire...is this going to drop the main voltage? I asos read that the proper value is 1k here on another forum.
I am trying to sort all this info out and do what is proper. Please help!!!
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> plate voltage of the el84 and it is 354 VDC. Shouldn't it be around 300 VDC?
Plate-cathode or B+? (Though on EL84, there isn't much difference.)
If you want good long life from classic EL84, keep it down to 300V. I suspect that most current production EL84 are either cheap-junk, or use the same 400V-500V plate-metal as the bigger tubes.
If you want maximum power with shorter but still usable life, such as stage guitar, 350V is a fine value for classic EL84.
If you are selling a cheapo amp, you only care that it doesn't fail RIGHT away. What kinda warranty do you get on a $99-$139 tube amp? Even so, classic amp warranties didn't cover tubes.
> do what is proper
But what IS "proper" for a cheap little amp? Maybe "it does what I want"? Do you want 4 Watts or do you want 1 Watt? Do you even know? Awful hard to translate "still loud for 5 watts!" into a target.
Let's see. If an EL84 isn't red-plating at 350V, it must be running 35mA max. The entire output stage works like 350V/35mA= a 10K resistor. If we insert a 100 ohm resistor, 10K/100= 100:1, B+ drops about 1%, power drops 2%. A C-R-C filter with 100 ohms in the middle will do a lot for buzz, not much for excess volume. A 1K resistor will drop about 10% in voltage, about 20% in power, still not much less loud. You might be going to 5K or 10K to seriously reduce volume. At that point you may want to separate the power stage B+ and the preamp B+, so you can drop the power stage to maybe 150V yet keep 300V on the preamp.
> the fizziness of the stock distortion
Could be crappy caps. The electrodes in a cap want to vibrate, much like an electrostatic speaker. Good caps are tightly wound, little vibration. At this box's price-point, they are probably rolling those caps as quick as possible, no matter they come out loose as a roll of economy toilet-paper.
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> the new heater wiring...
Too late now.
My thought was to keep the heater wires WAY away from the audio. Take a foot of wire-pair and run from the heater supply the long way around to the back of the PCB and get onto the heater pins without crossing audio paths.
But yeah: even getting a little above the PCB, twisted-pair, with ground-wrap, is a ton better than the original plan. Even at hunret-bux, they could have done better layout. Makes me wonder if they WANTED it to hum, so it didn't steal sales from the $200 and $300 boxes.
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Something to think about, if you're getting 354 volts on the plate of the EL84, you better increase the voltage rating of that big sprague cap you installed to 450v, you're right at the edge of exceeding the voltage rating of that 350v cap, it wont be pretty if you blow it up.
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I made a couple of changes this afternoon. I relocated the filament wiring to the open side of the pcb. This way they have some space instead of under the board scrunched between the chassis and the board, seperated by standoffs. Better.
I removed the 100uF 350V filter cap and replaced that with a 20uF 500V rated Sprague cap. I also added a 470 5W resistor from the + of the 4k7 1W resistor on the pcb to the + lead on the new 20uF cap. The - lead of that cap connects to the - lead of one of the other filter cap grounds. Better.
I also experimented with r6 and r7, combinations of lower values and shorts and opens and I decided to leave those both at 1M values with r6 having a switchable 1M in parallel. This allows me to switch in a 1M resisotr with r6 which is 1M, thus having 500k when switched on and the gain is perceived to increase. Better.
I left in the switchable 120pF bright cap.
Thanks to PRR and the rest of you for helping me understand this. I know I am addicted to this forum! I just learn so much. :)
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Final...the front panel.
B= bright
N= normal
HI= 500k resistance
LO= 1M resistance
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That's just too cool! By my calculations you're still well under $200 total, you can't beat that!
J.
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I am sitting at $110 total. 8-) 8-) 8-)
I am going to purchase a couple more of these tomorrow. Fun weekend project.
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Looks like that 20uF 500V sprague was the most "expensive" of the parts ;D
What did you use for the HI LOW and Bright/Normal decals....are they regular stickers? If so they look like they're supposed to be there.. nice work.
J.
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The sprague was indeed the most expensive part.
I used black rub on lettering from a craft/hobby store. I have not decided whether to leave it as is or not.
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Maybe I'm being obsessive like Nikki Tesla.
> still hum when the volume pot is maxed out.
Some of that is of course the first stage, which has the most gain to the speaker, so the least hum will be audible when the Volume is high.
But I'm looking at your added cap and resistor switching at the top of the volume pot.
The top of the pot is by far the highest impedance in the whole amp. 1 Meg into a 1 Meg pot shunted by 1 Meg, allowing for V1a plate resistance, the top of the pot is 339K impedance, everything else is under 50K (at least at buzz frequencies) (oh, pot wiper is <188K). So the wires to the top of the pot are most vulnerable to buzz.
I think you have added two wires to this node. They are "only" 4 inches, but now you have almost a foot of buzz-catching wire hanging on a 339K node. Shielding will help and also hurt: a foot of shielded wire on this node will cut treble about enough to notice.
Ponder this half-baked plan.
Change R6 to very-low, a jumper or a 1K resistor. Put a new 1Meg R6 right AT the volume pot. If you think vibration is a problem (break the pot leg), use Shoe-Goo to stick the resistor to the pot body. Put your Brite and Gain switches close to the pot (they are probably OK now) and use the shortest possible wires to the top of the pot.
In the hasty-sketch, top is "now" and all the red wires are begging for buzz: several leads passing semi-near the heater supply. Bottom is "revised" and only the short wire(s) tight to the front panel are buzz-catchers.
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Your second shot at the heater wires look MUCH better. Did you cut the traces for the heater runs on the board? If not they will still be pushing AC around on the circuit board. Measure the traces, they should be dead open.
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Thanks.
I did cut all the traces and I do measure opens.
I cut the traces where they start and end on each trace.
The amp is super quiet except when at 100% maxed. I don't hear hum when I am playing only when it is idle.
I may try a few more things to get it quieter but not until after I record the amp.
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I think I might buy the head version of this thing, gut it, and build a Schedule 40 inside it.
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I whipped this up a while back for a GA-5, I'll try this instead of the PCB in the V-Junior.
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Do you have a full schematic of what you plan on doing?
I'd be interested in modifying a couple more of these different from each other.
I want to build another just like the one I now have but install a turret board instead of the pcb and keep the circuit components the same. Just so I can compare one to the other.
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Under "Mods and Projects" on the Hoffman Library of Information. Schedule 40 ....
Original 6V6 GA-5 Schematic:
http://www.hoffmanamps.com/projects/images/GA5.jpg
Schedule 40 schematic:
http://www.hoffmanamps.com/projects/images/Schedule40GA-5.jpg
Layout is there too, i think ....
Alan
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Everybody here is out of them. Good luck, and have fun. Post the results. You are gonna love the GA-5 (schedule 40) version.
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I noticed that the GA-5 used feedback from the speaker output back to the last half of the 12AX7 cathode by way of a 47K resistor. Is is possible to do this with the valve jr to help reduce some of the noise this amp makes?
So far I've changed the input jack to an insulated one, changed R2 to 33K, pulled R1, put a 1M across the input jack and jumpered the input ground to the pcb ground. I didn't notice any noise reduction but I am probably hearing heater hum. That fix is next on the list.
Thanks for any input.
Brad
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I would just stick with the Solid State rectifier myself. Keeps cost low. The Gibson GA-5 "Les Paul" Reissue uses SS rectifier. Look at how they did the reissue GA-5 here: http://www.blueguitar.org/new/schem/electar/modnotes/bridged-t_mods_for_lp_jr_amp.pdf
Compare them all, best of both worlds. Notice the Gibson GA-5 reissue doesn't show NFB loop, old GA-5 does.
My opinion, what makes the GA-5 better than like the Champ 5f1, is the 220K/2.2K plate load / cathode resistor and a little lower value input resistor. Gibson used twice the resistance on the NFB resistor, 47K as opposed to Fender 5f1 using 22K. I guess because the GA-5 has twice the plate load resistance on the preamp tubes. 100K on 5f1, versus 220K on GA-5. GA-5 is pretty crunchy!
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Awesome!
This will be next weekend's project.
I couldn't resist ordering another combo and a head today.
right on! Let us know how it sounds. I bought one of these with the intention of gutting and haven't figured out what to do with it yet.
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I was wondering how many messages it took before this board opened a second page...
> The iron will have to be different from the epiphone correct?
No.....
>> how the stock PT will work...it doesn't have a rectifier tap.
> I would just stick with the Solid State rectifier myself.
Agree. An ideal single-ended audio amp sucks a constant current, so there isn't any rectifier sag. A real amp's current varies some, especially in overdrive, but you'd have to smack the stuffing out of the amp to get enoug sag (or rise) to measure. The old amps used bottle rectifiers because they had no choice. You have a choice. Leave the bottles for collectors.
So no 5V winding needed.
Sticking with 1N4007 sand diodes: all the iron works. You have not changed anything that matters.
You might eventually want to try a bigger OPT than what comes in a $99 box; it's probably "junk". But all the old one-power-tube amps were "cheap junk", and sometimes junk is really treasure. Get one of the monster MagnaQuest OPTs and it might just be boring.
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> Gibson GA-5 reissue doesn't show NFB loop, old GA-5 does.
That will make a big difference. You will have to re-voice the amp when you add/remove negative feedbag. There is a "sound" in a naked pentode. There is also a sound in a low-feedback amp like the Bassman, and another sound in a high-feedback amp like Sunn or Ampeg V-40. All have been well-loved, but the tone-stack and speakers have to be selected for the basic amp-color.
> Gibson used twice the resistance on the NFB resistor, 47K as opposed to Fender 5f1 using 22K.
To compare dem apple to apples, the output tube and load impedance needs to be the same. If one tapped the 4 ohm output and the other the 16 ohm output, a 22K:44K change of NFB resistor would give the same result.
But in DIY-life, you can fiddle that NFB resistor and see what soundz you get. Values of 10K and 5K will give a tight clean hi-fi sound. 22K, 47K, 100K loosens the NFB, lets the cone shake more, lets more distortion out. Break the NFB resistor for naked-pentode sound.
Decreasing the NFB resistor reduces gain. Don't let that fool your ear: crank the volume when you decrease the NFB resistor.
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Oh, the choke is charming, but in this high voltage low current amplifier, I bet a 5K or 10K resistor would work as well, with less weight. The screen and preamp current is quite small. The plate current is small so we don't need a high screen voltage. If the screen voltage is low, we don't need heroic voltage on the driver. 250V-300V at the screen should be plenty. Since we start with 360V, we can afford a lot of resistor drop.
> I didn't notice any noise reduction but I am probably hearing heater hum.
That seems to be the #1 fault of this amp. Really stupid heater routing.
I'm surprised that a one-cap DC supply cleans it up. Instead of 6V 60Hz, we have 1V of 120Hz, 240Hz, etc: less voltage but at higher more-annoying frequencies. I once posted a plan for a recording amp, and builders had bad buzz until I added a second R-C filter to smooth the ripply "DC".
Getting the heater voltage OFF of the PCB, and routing the long way around, ought to get the hum/buzz very low. That's the way most studio and hi-fi gear was run: AC heat, just very carefully routed. If done right, it can be next best thing to perfect. I don't say it WILL work for you: there is an art and a knack to routing heater lines, and it is hard to describe. I think DC heat is an expensive cheat, but today the expense is low and results are usually excellent.
If you want a very hasty check of your hum: disconnect one heater line, and use clip-leads to connect a big 6V lantern battery. It'll run a half-hour or more. Since you have to have the chassis open for this hack (Don't get electrocuted!), you'll catch some room-hum, but you won't have heater hum.
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I have put together a website. This is my first attempt at a website. ;D
www.GlacierAmps.com
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TG: Very Nice!
Alan
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nice visual documentation of the mods!
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Hey,
I just bought a head version of this amp for $99. Haven't had much time to test it but it is darn quiet. More info when I test it more.
66merc
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I just bought a head version of this amp for $99. Haven't had much time to test it but it is darn quiet.
I think the head version has DC heaters (?)
EDIT:
Yep, here's a quote from MF>>>
This head version of the Valve Junior Tube amps includes features not found in its combo cousin. These include [highlight]DC filaments[/highlight] for both the preamp and power sections, reducing unwanted filament hum. [highlight]It also has 4, 8, and 16 ohm speaker outputs[/highlight] so you can drive any kind of extension speaker, from a single 8" box to a 4x12" cab.
Like the combo version, the Valve Junior Head delivers great tube tone with 5W of Class A power.
Epiphone Valve Junior Tube Amplifier Head Features:
5W
Single-ended Class A electronics
[highlight]DC filaments[/highlight] for both preamp and power tubes for reduced filament hum
[highlight]4, 8, 16 ohm outputs[/highlight] for speaker flexibility
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Well, I just ordered one (a head unit) from MF & it's on the way! I got a "returned" unit for $85! Added a few other items I needed to bring the total up over $99 & got free shipping!
WooHoo....can't wait! :D
Cheap thrill.... ;) Jeff
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ANybody know yet what kind of transformers epi used? When I get mine I am going to make a schedule 40, maybe even an additional socket for 6V6. Less the tube rectifier of course. What kind of OT would ya'll suggest using if the one in it isn't sufficient? Like one from a 5F2 build maybe?
Additionally - I have allot of these little turret boards from other projects. Think a layout like this will work? See photo.
Alan
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> Anybody know yet what kind of transformers epi used?
Cheap! The iron alone "should" sell for $50-$100; Jeff bought the whole box and bottles for $85. The factory cost of the OPT is probably under $5.
BUT cheap iron is an old tradition, and often has interesting sound. I don't see why you would replace it before you wring it out and see what it does.
> maybe even an additional socket for 6V6.
Interesting. It has lower heater demand than EL84, similar plate ratings. But you will need a different cathode resistor, roughly twice the value. Simple: jumper both sockets pin for pin, except give each socket its own cathode resistor and cap.
Gain will be a bit lower, but nobody says this beast is low gain, and there's a little more gain available by hacking around the volume pot.
> these little turret boards
Then do the whole thing properly. Trash the silly PCB, wire-up a 12AX7 Champ turret board, except use EL84 cathode resistor if you stick with the mini-tube.
Something to check... OK. The left-right order of the Jr is similar to an early Champ. If it were laid out right-left, you'd mirror-image the tagboard layout. But the Epi is laid out in Champ order, just rotated the tubes to the big side of the chassis (and omitted the rectyfier).
Hmmmm... the EL84 will "work" with a 6V6's cathode resistor. Power output will be lower, but you didn't buy a 5-Watt to rock the stadium, and "only" ~3 Watts might still be plenty.
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PRR...
Is there a way to get the output to 1 or 2 watts? If so how?
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OK....rec'd my "Blem/Return" VJ Head today from MF & opened it up to see what kind of mess I bought....It came in an un-opened, factory sealed box. I looked it over from top to bottom/inside & out....I can't find a "Blem" anywhere on it. It's perfect!
They have "improved" the filaments w/ a DC supply (a large bridge recto unit w/ a 4700uf cap beside) & reports I'm hearing are that it's way quieter than B4, so hopefully that's one less mod I'll need to do.
It also has the 4/8/16 ohm speaker jacks, so I can run it thru any/every cab I have!
The iron is larger than I expected...even "beefy" looking.
BTW, this cab is very nice...very retro looking in burgundy & black. It's even shielded inside over the chassis. Reminds me of what a "mini Matchless" would look like.
What a deal for $85!!! (I even got free shipping by ordering another item I needed to make it over $99)
I can't wait to get it home to try'r out!
Jeff
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Tube Geek, if you raise the value of the EL84's cathode resistor, it will limit the cathode current and plate current more and lower the output power. Watch the cathode to ground voltage when you do that, the max cathode to heater voltage is 100 volts, so I wouldn't go above about 75 volts on the cathode, just to be safe, you'll probably need to change the voltage rating on the cathode bypass cap to handle the increase.
It will probably go into distortion sooner with the same drive signal from the preamp.
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> raise the value of the EL84's cathode resistor.... It will probably go into distortion sooner with the same drive signal from the preamp.
But as you say, the cathode voltage will be higher, so shouldn't it need more signal to overload?
It is certainly worth a try. Whatever the math says, the sound might be useful.
BTW, you probably won't be able to get the cathode much higher than 20V and have anything like a "power" amplifier. To predict this, divide the working screen voltage by the Mu of the screen. Assuming 350V and Mu(g2) of EL84 is 18, that's 19V. The Mu is not perfectly constant, but by the time current is so low that Mu falls off, it is too low to drive a speaker even at table-radio volume.
Also you are increasing plate resistance. Output impedance of a guitar amp is important. The EL84 is a fairly soft pentode (by 6L6 standards) and damps the speaker a little. Shoving it toward cutoff will give less damping. Damping is darn low to begin with, so less damping may be all the same. Try it and see.
One trick for lower power without great change in operational balance is to just reduce the supply voltage. A 120VAC winding, substituted for the ~250V winding, would give around +160V B+ and around 1 Watt. You could add a 120VAC:120VAC transformer, if you can find one. You could do the old dirty-trick: use a 120VAC:6VAC transformer backward on the Jr's 6V heater winding to give 120V (more like 110V) AC to feed the rectifier.
And there is the brute-force approach: big hot resistor to throw-off half the power before it gets to the tube. This can also give improved B+ filtering, and can be bypassed with a switch for when you play the big room.
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Thanks I will give it a try.
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Let me see if I understand this correctly. I am refering to the schematic snippet above.
To decrease the output wattage of the amp you can increase R14 to say 440 ohms but in doing so you would want to increase the voltage rating of C5 to say 100VDC? That's interesting.
I've been playing around with a master volume mod to try and get some distortion at lower volumes. It's not overly distorted, kind of crunchy, but still needs work. Just wondering what the best way to attack this might be.
Thanks for sharing the knowledge!
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> increase R14 to say 440 ohms but in doing so you would want to increase the voltage rating of C5 to say 100VDC?
No. When you try to reduce a tube's current by increasing the cathode resistor, it fights you. You have to make a BIG change of resistor to get much change of current.
Here's a dog-ear from the EL84's transfer curve. I plotted cathode resistors of 220, 680, 2K, and 20K.
(http://www.hoffmanamps.com/Forum/yabbfiles/Attachments/EL84-bias.gif)
The corresponding operating points look like(*):
220 = 38mA, 8.5V
680 = 16mA, 12V
2K = 6mA, 13V
20K = 1mA, 17V
Note that a 3:1 change of R, 220 to 680, is just over a 2:1 change of current. A 100:1 change from 220 to 20,000 is just a 40:1 change of current.
Voltage does not rise to infinity. Basically because current is going down. Not as fast as resistance goes up, but the voltage tends toward Vg2/Mu, or for this amp roughly 300V/18= 17V. It will get a little higher if the cathode resistor is even larger than a few K ohms... but at just 20K we are passing less current than many preamp tubes! Shame to waste the big 12-watt EL84 for a teeny job.
680 and 2K (or anywhere between) might be interesting operating points. Power will be much smaller. The 20K plot is just to show where we are going: 15-25V max, not to voltage high enough to threaten the heater insulation.
(*) I don't say these are the exact values you will see. The data is not for the same supply voltage as the Jr. I've neglected the screen dropping resistor. Individual tubes vary +/-10%, even +/-40%, from the nominal parameters in the datasheet. But the trend will be like this.
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Second thoughts.
> big hot resistor
Splicing a big resistor between the Jr's rectifier and first cap, both of which are on the PCB, is awkward.
Putting a resistor between the transformer and rectifier is easier. And ya know, I think I like it better.
There isn't a lot of call for large-K 10 Watt resistors (all they are good for is wasting power). But at the bottom of the page you can find 5K 5W, and four of those makes a dandy 3-position power selector. Roughly 5W, 2W, 1W... never mind the numbers, a good selection of lesser "full power" settings.
The switch has to be sturdy. Not a lot of current, but over 300 volts, with some inductance in the break. Big toggles or similar.
Normal toggles are 2-way, but an on-off-on ("center off") switch is a common part. Heck, you may have ripped a death-switch out of an old amp; that will work. It is widely available new too. With the on-off-on switch, one way is 5W, the other way is 2W, and in the center is 1W.
If you prefer a rotary for a more logical progression than "up, down, center", the generic $2 rotary may not stand the strain. Our host has a sturdy 3-way: maybe "too good", but it is a lifetime investment you can use in other amps when you tire of the Junior.
These 5-watt resistors will run HOT. Use good terminal strips, put them somewhere the heat can escape.
These resistors have deadly voltages on them, so must not be mounted outside a finger-proof enclosure. If they make too much heat under the chassis, you may have to fabricate a perf-metal doghouse.
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Very interesting! My assumption about the cathode resistor change doesn't work quite the way I thought, but might be effective for a small change, obviously the bypass cap would not need to be more than 50 volt rated, I tend to like to go oversize voltage a bit for a safety margin, it doesn't cost much more.
I like the resistors in the power supply idea, that should work for other SE amps also, would the resistors need to be rated at 10 watts for a SE 6L6 amp? I've been working on a SE 6l6 amp using iron from an old PA amp ( I still need to order the power supply caps, I could get the switch at the same time)
Has anyone put a tone control in one of these Valve Juniors yet?
Is the chassis deep enough for terminal strips?
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Has anyone put a tone control in one of these Valve Juniors yet?
Not yet, but I soon will be trying some sort of tone stack
Is the chassis deep enough for terminal strips?
The chassis is pretty deep...looks to me like it could handle T-strips just fine (it's @ home now so I can't measure) IOW, it's not a shallow SS type chassis.
After firing the amp up last nite, I'm not too impressed w/ the tone. Pretty dark & lifeless w/ the strat until it's almost maxed, then it gets pretty "farty". Also, it's just not very punchy or loud at all.....just not my cup-o-tea, I guess.
So, looks like this one's going "under the knife".
First, I'll try some of the mods mentioned here & at some of the other forums. If that doesn't do the trick, then I think a mini "Train Wreck" w/ EL84's in P/P will fit nicely in this chassis. I've got a tranny set here that will work fine for that.
I guess $85 for a nice head cab & chassis isn't too bad! Plus, I'll have the the Tx set for a future project, or I can sell them on Ebay for a few bucks (or to someone here).
Woo-Hoo! Another amp to build!!
Jeff
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Second thoughts.
> big hot resistor
Splicing a big resistor between the Jr's rectifier and first cap, both of which are on the PCB, is awkward.
Putting a resistor between the transformer and rectifier is easier. And ya know, I think I like it better.
There isn't a lot of call for large-K 10 Watt resistors (all they are good for is wasting power). But at the bottom of the page you can find 5K 5W, and four of those makes a dandy 3-position power selector. Roughly 5W, 2W, 1W... never mind the numbers, a good selection of lesser "full power" settings.
The switch has to be sturdy. Not a lot of current, but over 300 volts, with some inductance in the break. Big toggles or similar.
Normal toggles are 2-way, but an on-off-on ("center off") switch is a common part. Heck, you may have ripped a death-switch out of an old amp; that will work. It is widely available new too. With the on-off-on switch, one way is 5W, the other way is 2W, and in the center is 1W.
If you prefer a rotary for a more logical progression than "up, down, center", the generic $2 rotary may not stand the strain. Our host has a sturdy 3-way: maybe "too good", but it is a lifetime investment you can use in other amps when you tire of the Junior.
These 5-watt resistors will run HOT. Use good terminal strips, put them somewhere the heat can escape.
These resistors have deadly voltages on them, so must not be mounted outside a finger-proof enclosure. If they make too much heat under the chassis, you may have to fabricate a perf-metal doghouse.
RIGHT ON! Hopefully I'll have time to try that soon
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tbeck,
The chassis measures 7" front to back, 12.5" wide, & 2.75" deep.....very roomy.
Jeff
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Thanks, Geezer, that's plenty big enough for a number of project ideas. If a person was to gut that thing, a new faceplate might be in order, especially with more controls being used.
2 EL84s in push-pull would save some space on top, as compared to 2 octal sockets.
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Alright, I'm done with this one..... :( It stays thin & lifeless no matter what I do.
I tried every mod possible, every tube in my collection....nothing can make me happy with this amp. SE is just not for me!
I'm still glad I got it, as the cab & chassis alone were worth it. I'm going to take some time to study the best locations for controls on the chassis & make a final decision on which circuit to build (some variety of 2xEL84 P/P).....I'm open to suggestions! ;)
Jeff
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Thanks for the explanation PRR, some of what you said made sense. :o It looks like I would have had to triple the value of R14 in order to obtain the effect I was looking for. I need to hit the books a little more to figure out the rest of what you explained.
My whole reasoning to decreasing the output level was to get a natural overdriven tone without rattling the windows. It would be a shame to do that to an amp, even a 5 watter. I am achieving good results with the mv control I put in so I will experiment with that some more.
Say Tubegeek, in reply #38 you installed a 470 ohm 5W resistor between the rectifiers and C6/R12. What did that drop the voltage down to? I'm looking to do that too but I don't have any spare power resistors laying around to try. Looks like another mail order.
If your looking to add some quick distortion, I did this over the weekend. While experimenting with a pot and different values for R6 and R7 to get more gain I ended up removing them and putting in two reversed 1N4007 diodes with a 330pf cap across them in place or R7; the simple circuit used in distortion pedals. A SPST switch enables it. I later switched the diodes to 1N4148 for a brighter distortion. It does pull the output volume down some but that was expected.
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bradleyt...sounds interesting...do you have a pic of your mods? They are worth a thousand words to me. I'd like to try the diode clipping in my amp. I am also interested in your MV addition.
The 470 ohm 5 watt resistor is something I have plenty of in the shop so I used that value. I was going to order 1k values but decided to use what I had on hand. I went off of PRR's suggestions in his post:
"Let's see. If an EL84 isn't red-plating at 350V, it must be running 35mA max. The entire output stage works like 350V/35mA= a 10K resistor. If we insert a 100 ohm resistor, 10K/100= 100:1, B+ drops about 1%, power drops 2%. A C-R-C filter with 100 ohms in the middle will do a lot for buzz, not much for excess volume. A 1K resistor will drop about 10% in voltage, about 20% in power, still not much less loud. You might be going to 5K or 10K to seriously reduce volume. At that point you may want to separate the power stage B+ and the preamp B+, so you can drop the power stage to maybe 150V yet keep 300V on the preamp. "
Since I used 470 5 Watt I 'd figure I have around 5% less voltage and 10% loss in volume. I cannot really tell there is a volume loss. I will measure the actual voltage later today...the amp is being used for a recording session today. I'll have it back late this afternoon.
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This thread has inspired me to buy one of those heads. It's crazy that you can build the ga-5 into this head because an RI ga-5 sells for $599 ($719 list).
http://www.musiciansfriend.com/product/Guitar/Amps?sku=481209
The tranys might not be as good but I bet you could get pretty damed close to the same sound out of the Epi. jr. and wire it PTP t'boot.
I am going to gut this thing right away and get to work on it with a few terminal strips. I think I will stick with the 1 knob ga-5 circuit. I am going to leave it in my friends recourding studio, who has been passing enough work over to me that I could quit my day job. Though I might go crazy hovering over an iron all day. It sure is a fun hobby.
kris
kris
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BOTH Guitar Centers in Minneapolis were clean out of them when I went to pick one up this past weekend :'( Looks like everyone's out. There is a similar thread on the sterilized 18w site that seems to have upped the demand for this unit too.
TubeGeek you opened a huge can of worms.............
J.
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I emailed my buddy this thread and he bought one.
I wouldn't mind hearing some clips of your modded one, TG!
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Sorry no pictures of the work, I don't own a camera. The diode mod was simple since I used the pads where R7 goes. It should be easy enough to piggyback the 2 diodes and cap off of the ground end of the resistor and a switch to the signal end. I didn't experiment with LEDs but if I get some time I might.
If it’s a whole distortion circuit you want to add then check out this site under the electronics section.
http://www.guitarheads.net/index.html It's a tube screamer circuit mounted to a pot. Pretty cool huh?
I read PRRs post about dropping the voltage and wanted to apply that in my unit. I tried to do the math but my results varied. I just need to order some parts and try it. Makes me wonder if the over voltage problem was fixed in the head version along with the heater noise problem.
As far as the mv control, I originally posted that at http://www.klugemusic.com/phpBB2/viewtopic.php?t=78 Broddi has done a lot with this amp and that helped fuel my interest in it. ;)
I used a 1M linear pot and some two conductor shielded cable. With the pot shaft facing me and the pins at the bottom, I number them (left to right) 1 2 3. The braided shield was connected to pin 1.
Drill a small hole in the pcb by R5/R15 (mounted on the underside)
Disconnect R5 and R15
Feed the cable through the hole and make all connections on the bottom side
Connect pin 1 to the ground end of R5 (the shield)
Connect pin 3 to the other end of R5
Connect pin 2 (the wiper) to R15
The pot isn't mounted but I left enough slack in the cable so it can dangle out the front with the chassis screwed in place.
Give it a shot. I'd like to know how it works for others. I borrowed the idea from the P1 and other mv amps. Hope it is electrically sound.
Brad
Long post... sorry.
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I just finished jammin out with the head and it does sound pretty darn good for $99. I connected up my modified ts9 tube screamer to the valve jr and cranked up some eric clapton blues albums on my studio monitring system and jammed out for a good 45 minutes while experimenting with settings. I know that the stock circuit sound is lacking in high end and a little around the bottom end but what do you expect for $99. I sure don't expect too much. But I like this little amp because of the modding possibilities it presents. Heck I was using my 2-12" weber texas alnico speaker cabinet I had built for my twin reverb clone and it isn't so bad. I could put it to use in the studio for a gig or two, just for fun!
If I opened up a can of worms on this project here on this forum I am glad. There were others before me that really started it all. I used their info and experience to work on my amps. Now it's up to the rest of us to see what we can come up with. :)
I had an idea that these amps would be difficult to come by soon and that is why I bought mine up asap. In fact in about 3-4 weeks I have to relocate across the country for 4 months and then relocate back across the country again...temporary work assignment with my new employer. Ugh This means that I may not have time to work on projects for that time :'( Who knows maybe I could use a break and come back at it in October with a fresh approach.
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Bassmanster...you can hear a recording clip of my modified valve jr combo on my website. See above posts for address and link.
You have to have quicktime installed on your computer....i use macintosh.
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If it’s a whole distortion circuit you want to add then check out this site under the electronics section.
http://www.guitarheads.net/index.html It's a tube screamer circuit mounted to a pot. Pretty cool huh?
That does look interesting and that is the type of distortion I am looking for. I think I may try one out in my valve jr's...maybe the head. Thanks for the link.
I will study your MV mod and maybe give it a try if I can get motivated. I have been lazy today and just chilled out. 8-)
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Hey TubeGeek,
Mercury Magnetics has a mod for sale $$$$. Kind of expensive but interesting. It's about 2 bills for a tranny set w/choke. Check it out-- http://www.mercurymagnetics.com/pages/specials/ValveJrPjt/EVJ-01.htm
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Gee I wonder why they wouldn't just make it the same as the ga-5 to start with? And why make an amp that hums so bad?
http://www.musiciansfriend.com/product/Guitar/Amps?sku=481209
I've heard of this trick before.
kris
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Alright, I'm done with this one..... :( It stays thin & lifeless no matter what I do.
I tried every mod possible, every tube in my collection....nothing can make me happy with this amp.
In all these mods, did you reduce the values of the preamp cathode resistors, and also the cathode bypass caps? Also, did you short out the series 1M resistor between the two preamp stages? What exactly are you looking for in this amp, tonewise? There's no reason at all for you not to get plenty of great tone from it, but the stock preamp design is badly flawed. The only thing a two-tube amp like this can't do that well, is give you is lots of sustain.
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Zaphod,
I tried everything! (I have followed your suggestions from the 18w thread)
Lowered/different cathode resistor & Cap values.....
Shorted/removed the 1meg (R6) & replaced the 1meg grid ref (R7) with a 1meg pot as a drive control....
33k on the grid of the input w/ a 1meg ref to ground.
I'm pretty comfortable gutting it, and I think I'm going with a "5F6-84" circuit.....5F6-A Bassman pre into 2xEL84's cathode biased. It'll be a lower power version of my "AC-Tweedy" (tweed pre into an AC30 power section) which, although my first build, is still one of my favorite amps.
Also, I think I'll try the Baxendal tone stack like Tubenit used on his newest build (with a concentric pot to save space on the front panel).
Jeff
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So what exactly were you looking for that you didn't get with this amp? Was it really about sustain?
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Tube geek, it sounds excellent! About halfway between Fender and Marshall. Close-miked w/ a small speaker into GB; very nice! What's your homemade mic pre, if I may ask?
Yes, you are using Tiger with iLife '06 because that's an iWeb site. I'm a Mac guy from 1986 on. I don't know why I missed the player under the image since I did the same thing for a while on my site...also made with iWeb. www.thesoundwaves.com
Cheers!
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Zaphod said:
So what exactly were you looking for that you didn't get with this amp? Was it really about sustain?
No, it just is thin, dull & lifeless....no "fullness" or "body" to the tone at all! Sounds like I'm playing thru a transistor radio w/ a 3" speaker, even though I've run it thru all my favorite cabs....a 15" closed back, 2x12 closed & open backs, 1x12 closed & open.
I've put JJ's in it, a nice 7189, 12ax7, 12ay7...along with all the mods.
Maybe there's a problem w/ the OT?
BTW, the EL84 plate voltage on mine is around 305v....much lower than other reports I've heard.
Jeff
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Maybe there's a problem w/ the OT? BTW, the EL84 plate voltage on mine is around 305v....much lower than other reports I've heard.
The OT's just about the only part that hasn't been touched yet. :) Yes, the OT could make a huge difference to the sound. A Hammond 125DSE would be perfect in this amp - or even an ESE, although that one would be somewhat overkill.
Also, if you find the amp a little too shrill, you can try a trick from Hot Cat's Mini Cat SE EL84 amp, which is a 100k resistor in series with a 2.2nF cap between the EL84's grid and ground.
Another trick for SE amps running at relatively high voltages, and therefore biased cold, is to use a very large cathode cap - ie 1000uF or bigger. This has the effect of stabilising the cathode bias voltage against transients caused by sudden increases in signal level,which can make the power tube go even colder and sound somewhat ratty. The big cathode cap will really smooth the tone of the amp.
I'm real surprised at your B+ voltage, when others have been reporting 350V to 365V. Or is that after installing the power supply mods with the large filter/dropper resistor?
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I've not done any mods to the power supply....that B+ is stock.
I have a 125ESE here on the bench...maybe I'll tack it in & see if there's any improvement.
Thanks! Jeff
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BTW Zaphod....You're doing your best to keep me from gutting this little thing, aren't you! ;)
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I've not done any mods to the power supply....that B+ is stock.
Interesting.... I'm guessing that maybe in the head version they've fixed the power supply, so that it doesn't hum and the B+ voltage is lower too.
I have a 125ESE here on the bench...maybe I'll tack it in & see if there's any improvement.
It shoudl be pretty easy to try for fun, and you've nothing to lose...
BTW Zaphod....You're doing your best to keep me from gutting this little thing, aren't you! ;)
;D The urge to experiment is irresistable! ;) :)
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I am planning on installing a GA-5 circuit in an epi valve jr. I found a 5V4G rectifier tube in my box of tubes so I figure what the hay I'll try out the schdule 40 adaptation...soon. ::)
Bassmanster...check out my website...I just posted pics of my tube and Jfet microphone preamps.
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Thanks, TG, I have been eyeing those Hamptones for a while now. What do you thinks? The reason I asked is that your sound, given that it's a close-miked 57, is excellent. I had read that 57s sound good with a good preamp and bad with a cheap one.
Well I guess this adds to the mounting evidence. How was the build on those Hamptones? Pretty straightfoward? I have built a scratch amp, so I wouldn't expect any problems.
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Here is my complete signal path....sm57...mogami quad cabling to the tube hamptone....more mogami quad cabling to my mbox2 input one set to line in...mbox2 connected to my mac via usb 2.0.
I used no EQ and no compression.
An SM57 is pretty standard for many recording applications. They are cheap and they sound good too. I do find that they sound better when ran through a quality preamp.
The Hamptone preamps are excellent IMO. They use solen fast coupling caps and xicon electrolytics. Resistors are metal oxide 1% tolerances. The input and output xformers are custom made and are the same xformers Walter Sears used in his consoles. The PS uses a torodial xformer. When I received the kits I was pleased with the organization and instructions. I built both of these preamps before building any amplifiers and they couldn't have been any simpler. I was just fresh out of electronics school and bought myself these to keep me occupied. The preamps do not come with the rear output jacks, that is an addition.
They beat out many other well known preamps in the studios I have worked in. Scott Hampton is a very nice person as well, I talked with him several times about his projects.
I definately recommend these preamps.
Now if I could only build my own microphones in kit form! I am still looking for this one!
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Thanks for the info. Yes, I have had a 57 (and a 58) for about 10 years and I know what it sounds like through a cheap mixer. Which is to say...not at all like your recording. Thus the queries.
I'm stoked to get one as soon as I save a few pennies.
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One more thing...with the Hamptone's I find it easy to really set the gain properly. I can hear the characteristics of the microphone in every detail. If I set up the microphone properly and dial in the tone of the amplifier then adding the 57 and preamp...I find it hard to get a bad recording. They tend to add to the signal instead of taking anything away. Similar to vintage Neve pre's but modern :o
For what you pay for these kits I think you are actually getting much more than what you pay for. They rank up with other preamps that cost 5X what these do.
I see a couple of little improvements that I could do on my hamptone's...thanks for motivating me to look at them again!
BTW Bassmanster...you have a great looking site too. I love my mac, it makes that stuff so simple to do. I need to find a .com domain name and hosting site...who do you suggest? I'd like to use iweb and publish to a folder and upload it to a host site.
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Thanks man!
I am using a friend of mine's hosting so he gives it to me for cheap, but if you wanna look it's out of barrierislandgraphics.com .
I have another site under imhosted.com. Not bad for cheap. It's easy to do stuff from the linux hosting control panel. Well simple stuff which is all I do.
I publish to a folder and upload using CyberDuck. At least one thing that I know of that won't work publishing to a folder: the hit meter. Otherwise I am happy.
Domain name: I use NetworkSolutions but I have done so for a long time so I am used to them. I suspect there are better bargains to be had. I register the name myself so I have separate control of it instead of through my host.
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Regarding installation of a gibson ga-5 circuit:
I would like to use the power transformer already in the valve jr. It has 6.3 VAC 260 VAC.
First question: With the schedule 40 ga-5 circuit...it has a 330 V PT secondary. Can I use the 260V secondary safely?
The 5V4 is rated to 500V 175mA.
Second question: I have a 9 Henry 120mA inductor lying around...can I use this in replacement of a Hoffman 125C3A?
Third question: What is the best way to get 5V from the PT...it looks like the stock valve jr has a 12V secondary...can I just rectify that down to 5V or is it best to use a seperate transformer?
Where can I find an output transformer for this amp that will have a 4,8,16 outputs?
Or should I just use another transformer? ::)
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> it looks like the stock valve jr has a 12V secondary
Looks like 6V to me. Powers the EL84 directly. When rectified onto a big cap. we get ~5.9VDC.
But you can't power most 5V rectifiers from that winding, if it is feeding your 12AX7 and EL84 tube heaters. The filament of most 5V rectifiers sits at the B+ voltage, ~+360V; the heaters of the 12AX7 and EL84 have to sit near their cathode voltages, essentially zero volts, or within about 100V of that.
You can try to find a heater-cathode rectifier with 400V of heater insulation and maybe 40mA current capacity. 6X4 will work (except the Epi's HV winding is wrong for a common-cathode rectifier; see below).
But I really think: you add another big bottle, you need to add heater power supply. If you are not going to replace the original main transformer, you will need an extra trannie just for the wrectifier. 5V at whatever current is ideal. A 6V winding may be easier to source: use Ohm's Law to figure a resistor to drop 1.3V at the rectifier's rated heater current.
Note that the 5V4's heater alone eats more power than the 12AX7 and EL84 heaters combined.
And you KNOW, for ~~$99, Epi didn't build any excess power capacity into that amplifier.
> output transformer for this amp that will have a 4,8,16 outputs?
The Head version has 4 8 16 taps.
> Can I use the 260V secondary safely?
That is a single winding, suitable for a Bridge (4 diode) rectifier. The 5V4, and most bottled rectifiers, are 2-diode with common cathode. What they really want is a center-tapped winding. Using the Jr's single winding, the only pure-bottle way to go is a Half Wave rectifier. Half of a 5V4 is plenty big for this ampifier, and the ~320VDC is fine, but the buzz will be bad and the power transformer will be unhappy about the half-wave load.
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Hey TG, first just build the GA-5 around the Solid State rectifier and the existing power supply design already in the amplifer. The 2 12AX7 stages have major differences that make a big difference in the amps sound/gain. I'm not sure where to tie the feedback if you decide to do so.
Alan
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That gives me some stuff to think over!
Thanks.
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> The 5V4
Why use a bottle rectifier at all?
In push-pull class AB, the power stage sucks more current as output power rises. This causes the rail to sag. This "should" be "bad", but it seems to be good in hard-worked guitar amps. So a high-sag bottle rectifier gives a different sound than a fatter bottle or a sand-state rectifier.
However, the current drain of a single-ended stage does not change up to full "undistorted" power. When it does go into overload and gross distortion, supply current can increase, decrease, or even both depending on the signal and how bad it is mashed. But it really changes very little compared to classic push-pull amps.
So: does the bottle rectifier actually add anything to the single-ended amp? Or anything we couldn't instead get by fooling with bias and load? I haven't smacked the stuffing out of an SE amp in a very long time. As I recall, a low-feedback SE amp can give a wide range of tones with hardly a wobble in its current.
Gibson had to use a bottle: no choice back in the day.
The Sch40 uses a bottle because it uses a power transformer designed for a bottle: with a sand-state rectifier the B+ would be awful high. Anyway the Sch40 is as much a visual statement as a musical tool, and more bottles is more kewl.
Within the confines of a 99-buck amp's case, I'm not sure what a bottle rectifier offers except more heat (and slightly lower max power output).
> 12AX7 stages have major differences that make a big difference
That's my guess too. The Jr's preamp is functional but may be uninspired. Try trashing the silly PCB and using Classic Amp values and details in the preamp. Let the output stage alone for now.
Oh: do try mis-loading the output. My guess is that it will be more soulful with a "too low" load impedance: 4 or even 2 ohms on the 8 ohm tap. Unlike transistors, you can't hurt this class of amp with "overloading". Power output will drop a bit, but so?
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I just ordered my parts from Doug to build the ga-5 with a bridge rectifier instead of a bottle rectifier.
I bought the 3 amp/1000Piv rating, probably overkill but oh well.
I am going to use a 9H, 120mA, 125ohm inductor.
I will drill holes and mount the turrets this week and hopefully build the amp early next week. As soon as my parts arrive.
PRR...I ordered some 5k 5W resistors so I can try the switchable outputs you suggest
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I forgot how to make an archive request... :-[
Can this thread go in? Lots of info on SE design.
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Hey bassmanster...speaking of the SM57....the latest issue of recording magazine has an interesting mod for it. It is basically adding a resistor between pins 2 and 3 of the xlr in order to lower its impedance and match to most preamps better. The article said that it changes the sound of the mic, better clarity. I thought this may interest you. I will probably try this out and match it to my preamp input Z. Another fun weekend project. ;D
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Thanks for the heads up. I see it's not online so I'll have to find a real mag stand in this two horse county.
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I have a couple other mods for 57's also. One involves removing the transformer. If you cannot find the magazine, email me and I'll hook ya up!
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GA-5 install completed. Check my website for details.
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Thread archived.
I hope everyone was done contributing.
Ken
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I wasn't satisfied with the B+ being around 350V so I added two 22uF caps with a 1k 10 Watt resistor in between those caps in the epi combo I have...decreased hum to almost zero and brought B+ to 305V....which I can live with.
Now I am really done playing with this amp!
::) ;D
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Hey guys........your "new" replies have been moved to (click>>) [link=http://76.162.4.197/Forum/yabb2/nph-YaBB.pl?num=1172398164]This Thread[/link] (<<click) so that more people will see & be able to comment & help.
Geez'r