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Post by ChrisK on Apr 13, 2006 23:10:09 GMT -5
Anybody tried them?
Seems like a cheap basis for fooling around w/ low power tube amps. Tube set is a 12A?T7 and an EL84.
The combo is 5 watts single-ended w/ an 8" speaker and a volume knob. It's $119.99 at GC.
The head is the same plus DC filament power and 16/8/4 Ohm output jacks. It's $99.99 from MF (it's not available at GC).
I have 3/4 of a mind to get the combo, rip it apart, and see if a 10" speaker will fit. I bet I can convert it to triode as well (if not already). Seems to be plenty of room for two tone controls and an additional gain stage.
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Post by sumgai on Apr 14, 2006 15:27:36 GMT -5
Chris, 5 watts on an EL84 may already be in triode mode. Not sayin' it's for sure, mind you, but that is a low number for this tube, even in a full-on Class A circuit. Yep, it's worth experimentin' with. You're elected!  sumgai |
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Post by ChrisK on Apr 14, 2006 18:36:05 GMT -5
It was $100 out the door today!
It looks like it's screwed (and not glued) together. A 10" speaker likely won't fit.
As far as triode mode, most amps w/ EL84's run about 15 Watts push-pull in pentode mode. A single-ended EL84 in pentode mode ought to do about 5 Watts.
Triode mode should do about 2-3?
sumgai, I sent you the design brief on the boosted triode mode, I just may give it a try here.
Once I verify that the amp works (too much power chugging afoot in the GC today), I'll rip it apart.
Hmmmm, an additional preamp tube, 2-3 tone controls (Fender stack), and who knows...lots of chassis room.
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Post by sumgai on Apr 14, 2006 23:41:05 GMT -5
Chris, Good grief, I don't think I replied at all to that, did I? Curses, sorry 'bout that. But don't worry, my meds have kicked in full-force by now, so it should be a swingin' time in the ol' town tonight!!! ;D So, you could well be right about that, expect for one thing.... EL84's can easily keep up with EL34's, tube for tube. A pair of them in Class B push-pull at only 15 watts, they're purely loafing, they are.  And if my reference books are any indication, the norm for pentode versus triode power curves is not just 5 watts down to 2 or 3, but more likely it'll be down to 1 watt. Just a quick guess, you understand.  Cabinet, schmabinet. Be unique, build your own, and shove this chassis into it! ;D BTW, have you ever seen the AX84 project website? Uses just one 12AX7 and an EL84, and that's it. It's an "open source" project, meaning that lots of contributors have pitched in. Might be worth your time to check it out, who knows?  sumgai |
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Post by ChrisK on Apr 15, 2006 17:14:25 GMT -5
sumgai,
Thanks, I've had the ax84 link for a while.
This amp is indeed a twin triode and an EL84.
I took it all the way apart last night and traced the PCB schematic. There are some SERIOUS electrical safety issues afoot, from my perspective as an engineer that has been submitting products to U.L for two decades.
One is that the PCB gap between the B+ trace (the output supply) and its common trace (chassis) is on the order of .020" to .025". Whisker growth (metal migration) anyone? Which just might be related to the warranty period! Dremmeling is in order. Only a fool depends on the solder mask for insulation.
Another is that a chassis mounting stud hits the screen grid trace near the output tube, but is covered by a thin layer of tape on the PCB. The standoff will be rudely pliered about and removed.
A third is some decidedly non- standard-compliant minimal spacing between hot circuits and the chassis standoffs due to the tolerances (slop) built into the PCB mounting holes. Again, more Dremmeling.
Any circuit that has over 42.4 volts or 8 amps within is Class 1, and requires spacings stated in the applicable standard (which are desk standards written by U.L.). Some changes have occurred recently, I (and U.L) have known for some time that most spacings are larger than actually needed, but liability abounds.
The unit was listed by ETL. I'll give them a call........
I would NEVER design, and certainly NEVER sell such. I'd fire anyone that would.
There is no market for discount parachutes....
BTW, I did take several digital pics of the process and will CAD 'up the schematic as well.
I will also proceed with some mod GeThunkin'.
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Post by sumgai on Apr 16, 2006 22:05:12 GMT -5
Chris, Could I interest you in this fine Emergency Door for a Submarine? Won't cost you a dime over factory invoice, I swear! ;D  Agreed with all you said about safety. I left the "design to UL specs" portion of the industry a long time ago, but I suspect that the mindset hasn't changed so much. Safety is safety, and electrons don't change their habits just because some yoker in a cheapskate operation thought he could cut some corners and get away with it. Tape on a PC board at a mounting point?!?!  Ye Gawds, what'll they try next?? Glad to see that whomever is on watch (that's you!), is indeed watching out. Thanks for that. +1 for your diligence where it counts most. sumgai |
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Post by ChrisK on Apr 16, 2006 23:50:27 GMT -5
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servant
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Post by servant on Apr 17, 2006 0:57:44 GMT -5
There appears to be a whole community popping up of Valve Jr. Modders (and Rockers? ;D). Here are some links: Valve Junior #1Valve Junior #2Valve Junior #3Note that much of the noise problem experienced with the combo have been addressed with the head model. There is also a long thread over on 18watt.com, but you have to register to view the posts: Valve Jr thread at 18watt.com |
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Post by UnklMickey on Apr 17, 2006 11:29:52 GMT -5
Chris,
i think you've figured out by now, shoe-horning a 10 into that cabinet is not a good plan.
i hear that the stock 8" is actually a pretty good sounding speaker.
i remember reading somewhere that someone tried a mid to high priced replacement, but went back to the better sounding stock speaker.
Servant,
those are good links. i looked at them a while back.
while the information in them should be taken with a grain of salt (like everything on the web), they do contain some valuable info.
unk
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Post by ChrisK on Apr 17, 2006 18:06:39 GMT -5
Thanks guys,
I'm gonna take a look inside the Valve Special (5 Watt + DSP) and the Valve Standard (15 Watt push-pull + DSP) to see what transformers are used therein. I'm pretty thinkfully sure that the other 5 Watt one is using the same transformers, so an extra twin triode should be a given. Also, the 12-0-12 AC winding is most likely for the DSP and solid state stuff, so some power abounds there also.
I'm gonna fish for info on the Valve Special to discover if someone has captured the schematic for the additional gain/tone stack stages.
Epi has the Blues Custom toob amp which is a 30 Watt Class AB or 15 Watt Class A, 5AR4 tube rectifier, reverb, two channels, DC powered filaments, in a multiply birch cabinet with 2 12" Eminence Lady Luck speakers. Anybody got any feedback on this?
I'm in love. Epiphone has published their 2006 catalogue with actual real meaningful data therein. Nut width, pickup specs (resistance AND inductance @ 1kHz), and other date in an actual real logical table format.
Fender, Gib$on, and most others....SHAME. (Of course many merchants of Bright Shiny Objects don't actually want educated consumers.)
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Post by ChrisK on Apr 18, 2006 20:15:53 GMT -5
Well,
I beat on it for a while today. It breaks up at around "3" w/ my SG Faded, and at around "5" w/ my PRS Soapbar SE (P-90's).
Man, is this thing LOOOOUUUUUDDDDDD for 5 Watts.
I'm liking the posts for modding. May even try a light bulb in series w/ the B+ for some sag. Hmmm, a string of several Christmas lights protruding thru the front panel anyone (holy light organ Batman!)?
Were they really called "light organs" back in the day?
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Post by UnklMickey on Apr 18, 2006 20:57:06 GMT -5
...So, you could well be right about that, expect for one thing.... EL84's can easily keep up with EL34's, tube for tube. A pair of them in Class B push-pull at only 15 watts, they're purely loafing, they are. ... mebbe ahm missin sumpin here, i'm pretty sure 15 watts for a pair of EL84 / 6BQ5s is mostly all they'll do. a pair of EL34 / 6CA7s should be good for about 40 watts. "Were they really called "light organs" back in the day? "yeah Chris, i think i read something about that in a history book. LSH... unk |
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Post by ChrisK on Apr 19, 2006 17:45:07 GMT -5
I've seen a pair of EL84's pushed (?pulled) to 25 Watts in push-pull, but that's about the limit.
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Post by Runewalker on Apr 19, 2006 19:36:29 GMT -5
Well, Were they really called "light organs" back in the day?Tempting, but no, I must be mature....... Maturity like working for a living are both completely overrated. - T. Hornet |
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vjmodman
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Post by vjmodman on May 25, 2006 8:38:10 GMT -5
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Post by UnklMickey on May 25, 2006 12:07:52 GMT -5
hi Daniel,
welcome to Guitarnuts2.
thanks for the link to that forum.
i'm sure some of our members will find that very helpful.
VJs are quite limited in stock form, no tone controls and all.
but when you consider what you get for the money, they are quite worth it.
the combo is a great place for a serious modder to start.
you get a cabinet, a decent 8", a chassis, and transformers.
and a couple of tubes that could be used as spares for emergency use.
(BTW, are they chinese tubes, or are they actually something decent?)
even if you just pitch the printed board, and install a few chassis-mount sockets, and a turret board, the stuff i mentioned above would have cost way more if you bought it separately.
i think it's funny how a "low power" amp is still too loud, to get some evil sounds without rattling the windows.
i wouldn't worry too much about burning out the light bulb in the one mod i've seen.
he's using a 12v 7w bulb.
that would be 20 ohms (if you could get it hot enough with the output of the VJ), even if you had your speaker shorted.
so the REAL problem there wouldn't be burning out the bulb, it would be whether or not the OT was loaded well enough to begin with, prevent damage.
that brings up an interesting issue.
if you had a bulb of the correct voltage and power rating that would begin to incandesce, would it have an effect similar to sag.
since a filament has increasing resistance the hotter it gets, it would tend to add a bit of "compression".
my guess is, to get a serious reduction in power, you would need to do a series/parallel arrangement of bulbs. the speaker would have a bulb in parallel with it, and that would be in series with other bulbs.
unk
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Post by ChrisK on May 25, 2006 12:25:42 GMT -5
vjmodman
1105 1014
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vjmodman
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Post by vjmodman on May 25, 2006 12:48:34 GMT -5
Hey unklmicky thanks for the welcome. BTW I was a member of the "original" guitar nuts forum. Thats what got me started in those whole guitar electronics mod/building/amp mod craze ;D
I agree that these little amps are a great investment. they sound good stock, but are amazing little mod platformes for dirt cheap price.
and yes the tubes that come with it are sovtek crap tubes. Even if you don't do any mods I recomend a quick change to JJ. I have a big bag of NOS 6v6GT, 6L6GA, and tons of 12AX7 and 12AT7. I have around 70 tubes so I won't be running out any time soon. Currently I'm running a Tung-Sol 6v6GT but earlier I popped in a 6L6GA just for kicks and that tube sounds awesome clean.
Chris, since yours is one of the "older" style amps (like mine) I HIGHLY recomend you do broddis mods in the sticky topic on the forum link I posted. It cuts the hum down to almost nothing. at first I thought I did somethign wrong and it wasn't working because it was so quit.
Just make sure you discharge those filter capacitors by playing with the amp turned off -than- sticking a screwdriver or the like between the + side of the capicitors and ground. One of the times I took it apart I diddn't play throught it with the amp off bleeding out most of the voltage. When I touched the screwdriver to ground to discharge I got a big spark that was bright enough to make me see a nice little black spot for a few minutes and loud enough to make me jump back five feet and make my ears ring for a few minutes. I guess 350+ voltage doesn't like to get discharged to ground instantly. I have a burn mark on the chassis to prove it! Just measure across the cap leads and make sure thers not 300+ volts still in there. There will be around 50mv you can't get rid of any time soon, but don't worry about that. It won't hurt you or even spark.
You don't need to be scared to work on these amps, just make sure you exercise the right caution measures and know what your doing. Seeing that spark made me realize just how much I DONT wan't to be electricuted.
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Post by ChrisK on May 25, 2006 13:10:57 GMT -5
unk,
Methinks that mine has at least one SOVTEK tube (EL84).
Well, all hard drives fail, so all light bulbs burn out. I suspect that this will be "helped" by the wide frequency range of induced vibration from the speaker.
Yeah, but when cold it will be about 1-3 Ohms. An incandescent bulb is a positive temperature coefficient device (the secret of its success).
I suspect that a tube rectifier (emulation is a sincere form of flattery also) is a fixed transconductance device. In other words, it becomes a current limiter whence current saturated (and gets hot due to the increased V in the VI plate (heat) dissipation. Up to the saturation point (well, region really), it's a crappy diode. Semiconductor diodes in comparison have hard conduction thresholds.
In "The Tube Amp Book" by Pittman, he shows the rise time of the B+ supply as a function of tube rectifier type. While NOT conclusive due to the uncontrolled test environment (no inclusion of transformer primary, secondary, and reflected mains impedance; no indication of the B+ capacitance [The "C" in the time constant]; no mention of whether a choke or capacitor input structure is afoot; and no correlation of B+ voltages), it does show the (most) general nature of the beasts.
The 5U4-G, 5R4-GYB, AND 5Y3GT all show a modified exponential response curve. The solid state diodes and the 5AR4 rectifier show a straight line for the first 50% of the realized B+ voltage, which then becomes fairly exponential in nature. I'd hope that this in the 5AR4's case is transconductance limiting, but it may well be the aforementioned transformer/choke issues as well. As for the diodes, who knows. Much of the visible curve deviation may also be a result of the transcribing of the observed data.
Under controlled conditions ( the same B+ voltage from the same transformer into the same input structure), actually measurable design info might be gleaned.
Typically, the filament isn't varied in excitation level (except by inherent voltage reduction within the power transformer as it approaches full power).
While incandescent lights and rectifiers may be somewhat similar over empirically determined parts of their transfer functions, they're not the same. After all, we're futzing w/ the load impedance in a non-linear manner here, and not w/ the rectifier.
If one is design literate, for a rectifier emulator, I would surmise that a LM317 regulator set up as a current limiter would be but one component used. Additional components will be needed to "soften up" the transfer function.
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Post by UnklMickey on May 25, 2006 14:15:29 GMT -5
... I have a big bag of NOS 6v6GT, 6L6GA, and tons of 12AX7 and 12AT7. .... Daniel, two questions: 1 - what is your address. 2 - when will you be on vacation. just kidding! i there seems to be quite a few "old forum 'newbies' " showing up these days. great stuff that! i share your opinion on the JJs. i think the Sovteks are a big improvement over the typical Chinese tube, though. ...Well, all hard drives fail, so all light bulbs burn out. I suspect that this will be "helped" by the wide frequency range of induced vibration from the speaker... Chris, i guess i'm guilty of not shifting gears very well, going from the example using a 7w 12v bulb, to that other stuff. in the case of the 7 - 12, do you think that the VJ would be able to heat it enough to cause enough evaporation of the tungsten, to cause it to fail, even with the vibration? (i wonder if it will have more than the slightest glow?) i'm sure, eventually when our grand-children are ancient relics. but i'm guessing not any time soon. on the other hand, if you do run a bulb in a fashion that would have it near it's normal operating range, failure would be a concern. (or if i'm wrong about the 7w - 12v, it would be more of a concern than the 7 -12) you kind "teched me out" with all that about rectifier emulation. i wasn't suggesting that a bulb in series with the speaker would sound exactly like sag. just that, given the positive temperature coefficient, would it tend to "compress", since when heated, the resistance would be greater, causing an increase in the proportion of voltage dropped across it? and when cooler (playing softer) the resistance would be lower, so in proportion it would have a lesser voltage drop? i don't think trying to emulate a rectifier with a light bulb in series with a speaker is a path i would suggest anyone take. i was just wondering if there might be an interesting side-effect that would be present when using a light bulb as a way to reduce the power to the speaker. unk |
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Post by ChrisK on May 25, 2006 15:43:46 GMT -5
I do think that there will be some sag induced by a light bulb in series w/ the speaker. I would recommend that this is a safer place to experiment (as opposed to the B+ supply).
Having resistance in series w/ the speaker will affect the load impedance, and hence the tone (why we have $300 dummy loads).
I would decide on the drive reduction desired (ie 20%) and pick a light bulb that would give this result NEAR FULL BRILLIANCE. This will be difficult, but we want as much temperature difference/thermal lag (for cooling) as necessary to mimic the sag response desired.
We can actively dump heat into the bulb, but can only passively cool it. For compression to work, we must not only reduce the output drive for increasing amplitude signals, but shed heat quickly for decreasing amplitude signals. This infers minimal thermal inertia/maximum temperature differential. And hence, burnout is.
A 1.5 VDC flashlight bulb may be a good choice if it can handle 1 1/2 amps. If not, use several in parallel.
If more headroom is needed, use the taped-off 8 Ohm output wire with the 4 Ohm speaker and 3 VDC Christmas bulbs.
Since response time is important, if we don't want a bulb running near full brilliance, use a bulb that has a cold(er) resistance near the design goal, and BREAK THE GLASS ENVELOPE (get rid of the vacuum insulation). After all, many an anemometer has been realized this way.
AND, see below....
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Post by JohnH on May 25, 2006 22:17:45 GMT -5
Heres a worry,
with a lightbulb in series with the output, if that bulb blew, the output would be open-circuit. Since that is most likely when the amp is driven hard, could that result in power-stage destruction?
John
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Post by ChrisK on May 25, 2006 22:25:28 GMT -5
Yes, which goes to my Christmas bulb recommendation, many new ones fail shorted.
Or, pad the bulbs with a parallel power resistor equal to the speaker impedance (as a guess).
Such as in using the 8 Ohm tap with the 4 Ohm speaker.
A broken envelope cold(er) filament bulb won't fail as soon.
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Post by knuckledragger on Jul 22, 2006 11:00:05 GMT -5
I have just the valve head and this little thing rocks .running two 12" vintage 30's great old rock and roll sound. starts to get good at around 3 gonna get tweaked soon new tubes for sure those mod sites are good for these.go for it $99 bucks are you kidding me?
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Post by ChrisK on Jul 27, 2006 13:49:56 GMT -5
Due to the extreme value in the Epi Valve Junior, I can't wait to get one of their new Blues Custom amps (MF lists them now). 30 Watts class AB (pentode), 15 Watts class A (triode), EL34's tone stacks, 2 12" speakers, tube rectifier, all for $500. No tremolo (vibrato to the Fender mis-informed), but that's only fun for aboot 15 minutes anyway. Sounds possibly Marshall'ish. www.musiciansfriend.com/product/Epiphone-Blues-Custom-30-Watt-2x12-Combo-Amp?sku=480288The 65' Deluxe Reverb will wait for now (I was gonna mod it for high gain options and stuff anyway, who needs four input jacks anyway). Oh-oh, PnS is having their first August super sale, maybe I can get that Pro Reverb for $700'ish (I passed on it for $850 during the March Madness sale). All tube and it does 50/12 Watts and medium high gain. My Mesa Mark IV covers the rest. The Blues Custom weighs the same (65 to 70#) as the Pro-Tube series. |
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And if my reference books are any indication, the norm for pentode versus triode power curves is not just 5 watts down to 2 or 3, but more likely it'll be down to 1 watt. Just a quick guess, you understand. 
