un-loud 6aq5 amp

a fine line between stupid and clever

Postby nyazzip » Mon Mar 30, 2009 11:26 pm

I am still unclear if your plate voltage reads high when you pull out all the tubes and falls when you put them in.

Is there a feedback loop around your power tubes?

In the event you have toasted a 5y3 on account of my advice
(I assumed If no load, no current, and thus no need for a filter reference to ground) I will gladly refund your dough or replace it with one of my own.

In terms of the speaker I was more concerned that you were operating your amplifier into a speaker that was too high of a wattage (and thus too clean and not lively enough) to do justice to the amp.

Your proposed impedance mismatch may be good, but may just as easily be damaging at high volumes. Without a look at the amp I can't tell. If you are going to mismatch your OPT to speaker impedance do two things. First don't mismatch by more than 100% (no lower than 4 ohms if your builder was telling the truth). Second start quiet and see if anything sounds weird, proceeding slowly to louder planes.

I have mismatched many times with no ill effect, but I play Fenders, a notoriously tolerant of mismatch amp. I have seen several Vox and Marshall amps release their magic smoke on account of an impedance mismatch.
----------------------------------------------------
i just checked that out a second ago...with all the tubes pulled B+ shot up to 380vdc! the transformer is a "chicago standard transformer' which i understand later became staco. it has a nice helpful label on it, and i just noticed it is rated at 40mA. is that too low?
regarding impedance mismatches, i discovered that i had in the past miswired a cab at 32 ohms and had been using it sporadically for about 6 months! i even cranked my reissue marshall thru it. my poor old 4 ohm traynor, too. YIKES. but that to me was a lesson that maybe its ok not to trip out over impedance too much.... :D
one last thing, how do you check filament voltages? my probe to ground readings just now were all only 3.5 vac. could this be my problem finally? bad 6.3 v heater supply? would tubes even glow or operate at 3.5 vac? the 5 v is ok.
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Postby cartoonweirdo » Tue Mar 31, 2009 7:05 pm

Heaters are measured from one side of the 6.3v winding to the other. In most cases the heaters are referenced to ground via a center tap in the winding or a pair of 100 ohm resistors. Suffice to say that one side of the heater to ground ought to land you right in the neighborhood you measured. Try measuring across the two heater ins on the tube socket (normally from the yellow wire to the green wire), and you should read 6ish volts. A bit over or under is no cause for alarm, but half the value would be.

Now about your plate voltage.... We are at a point in which we have determined that with NO load your power stage puts out just fine, but with a load on it you loose around 200 volts DC.

I don't know if you can make a schematic or if I should bombard you with a million questions as to values of componants and etc. Perhaps if you could link us to photos of the guts it would be a nice compromise. What I suspect is that the design of the amp (or more likely the conflicting designs of several amps put together in a DIY build) are causing your problem.

As to "magic bullet" solutions (a bad part for instance), I want to know if you have tried changing the non rectifier tubes. If you don't have any of those types on hand, measure the plate voltage as you put them in one at a time. Expect the 6aq5's to lower the plate voltage by somewhere between 30 and 80 volts, and the combination of all the preamp tubes and the driver to lower the plate voltage between 20 and 50 volts. If you did your math you can see what I am driving at (the 60 or 70 volt difference between actual loaded voltage and projected loaded voltage). If any one tube causes a huge drop in plate voltage we know to suspect that tube or it's attendant circuitry.

Since this was a DIY build, by an inexperianced builder I am suspicious of small simple mistakes and wonder if a full schematic might be best. Until one appears I want to know what your pre amp and driver tubes are, what the cathode resistor values are, if the cathodes have a bypass cap, and if the amp has a negative feedback loop. If you aren't sure what all these parts are let me know and I will explain how to find them.

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Postby nyazzip » Tue Mar 31, 2009 10:31 pm

Heaters are measured from one side of the 6.3v winding to the other. In most cases the heaters are referenced to ground via a center tap in the winding or a pair of 100 ohm resistors. Suffice to say that one side of the heater to ground ought to land you right in the neighborhood you measured. Try measuring across the two heater ins on the tube socket (normally from the yellow wire to the green wire), and you should read 6ish volts. A bit over or under is no cause for alarm, but half the value would be.

Now about your plate voltage.... We are at a point in which we have determined that with NO load your power stage puts out just fine, but with a load on it you loose around 200 volts DC.

I don't know if you can make a schematic or if I should bombard you with a million questions as to values of componants and etc. Perhaps if you could link us to photos of the guts it would be a nice compromise. What I suspect is that the design of the amp (or more likely the conflicting designs of several amps put together in a DIY build) are causing your problem.

As to "magic bullet" solutions (a bad part for instance), I want to know if you have tried changing the non rectifier tubes. If you don't have any of those types on hand, measure the plate voltage as you put them in one at a time. Expect the 6aq5's to lower the plate voltage by somewhere between 30 and 80 volts, and the combination of all the preamp tubes and the driver to lower the plate voltage between 20 and 50 volts. If you did your math you can see what I am driving at (the 60 or 70 volt difference between actual loaded voltage and projected loaded voltage). If any one tube causes a huge drop in plate voltage we know to suspect that tube or it's attendant circuitry.

Since this was a DIY build, by an inexperianced builder I am suspicious of small simple mistakes and wonder if a full schematic might be best. Until one appears I want to know what your pre amp and driver tubes are, what the cathode resistor values are, if the cathodes have a bypass cap, and if the amp has a negative feedback loop. If you aren't sure what all these parts are let me know and I will explain how to find them.
--------------------------------------------------------------------------

heres a couple pics if anyone wants a look:

http://s265.photobucket.com/albums/ii229/nyazzip/?action=view&current=6aq5bttm.jpg

its a 5y3gt, a 12ax7, a 12ax7 PI, and two 6aq5.

i did indeed start to figure i was probably seeing only half of the 6.3vac...

the power transformer runs a little warmer than any other guitar amp i have messed with, which amounts to...not many. but not as warm as my st-35 mind you, before i put the big aluminum block on it. actually i measured it with the nifty thermocouple that came with my probe, and the laminate surfaces were 115-120F. no biggie i guess but on most of my other amps the PTs run stone cold.
anyway today i was unable to hear the parasitic nasty overtones; my theory is that all the pulling and reinserting tubes cleaned up some contacts and possibly solved that. but, the amp is still quiet, and the B+ is still 188-190v or so....too low
i put in good 12axs but i don't have any 6aq5 to play with. i hate to buy new tubes for this thing at this point, not knowing if its going to be a good amp or not....but i guess i might have to, in order to answer that question :D
regarding that, is it ok to insert only one power tube to get a quick B+ measurement?
i did yank out the line out stuff and determined that the amp still makes sound, but have not given it a proper audition with that "mod" :D
anywho, thanks alot for the help thus far, i have really learned a lot of hands-on stuff. finally. i'm just waiting to get blasted from something one of these days, probably that old Rauland PA is gonna git me :D

speaking of that, i was able to get some sound from it tonight as well, which is fantastically encouraging! even with black tar coming out of the 65 year old caps! wow. and, believe it or not, it doesn't hum! i ended up bypassing all the input wiring and output wiring and rigging speakers directly off the OPT, and clipping a makeshift guitar lead right to the preamp pins. which makes for a horrible "boom" when i switch pickups. and will probably end up with my guitar strings becoming electrified...ah but thats a different story. now i know the thing has potential
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Postby battradio » Tue Mar 31, 2009 11:16 pm

Hi ,

My RCA tube manual says 70 ma zero signal plate current for push pull 6AQ5 or 6V6 . A 40 ma transformer is way too small , sound like you need a larger power transformer .

Mark
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Postby nyazzip » Tue Mar 31, 2009 11:52 pm

Hi ,

My RCA tube manual says 70 ma zero signal plate current for push pull 6AQ5 or 6V6 . A 40 ma transformer is way too small , sound like you need a larger power transformer .

Mark


is that so? makes sense why it would run warm. how does one calculate a appropriate transformer? i have a couple of other ones that i could try. the best is probably from a 2x el84, 5y3, with one 12 ax7(a little power amp for an old VOM extension speaker, i understand).
i also maybe could use a gutted lafayette 2 el84, 6x4 rectifier, that used a couple of oddball preamp tubes. in fact i have the original photofact manual right in front of me with all the tranny specs....but i don't understand it fully. "secondary 1" is rated at 580 vct@ 0.085 amps. then the filament windings are all listed, and they all add up to about 3.6 amps....thats 3600 mA right? i assume i just look at the 580 @ 85mA.
anyway, guess the voltage is too high on that one.
so what should i look for at Hammond? :D
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Postby battradio » Wed Apr 01, 2009 12:21 am

Hi ,

The 580 center tapped transformer is just fine it will give you about 330 volts with a tube rectifier at 85 ma . Half of 580 is 290 volts times 1.414 =
410 volts DC minus about 80 volts for the tube rectifier = 330 .Then the droping resistors and your right where you need to be with your vlotage .

You will have to use the 6X4 or 6X5 rectifier ,with a 6X5 you can reuse the socket of the 5Y3


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Postby nyazzip » Thu Apr 02, 2009 4:41 pm

so i ended up getting a new power tranny from triode (MAG-6452 ) and a couple of matched 6aq5s(they brought out the hickock and did it right in front of me)with my dwindling funds. great guys.
the tranny is a direct replacement for a fender deluxe(two 6v6). the specs are 330-0-330 @ 100 mA...
just wondering about the CT ground wires, the striped ones: does it matter which goes to which ground point? can they both go to the same point? i'm going to wire it exactly like the old one, but i was just curious.
this is a "laydown" style transformer which is a bummer, because i have no room ro cut a giant hole to accomodate a bell. which leads me to another question:
awhile back i found a some sheets of pretty heavy brand shiny new stainless (looks like it anyway). its hard as hell...i can't cut it even with a cutoff wheel on an angle grinder or dremel. well i could, but it would take a half hour and 3 wheels to go 6 inches, with a white hot working surface....i think its probably some weird hi-tech alloy.
i was hoping to be able to use it for plates to fill holes with, or even to make a chassis with. i doubt i can, but does anyone have any tips for working with stuff like this? i'd guess its 1.5mm at least, very rigid stuff.
looks like this little 6aq5 amp is going to escalate into a chassis build now
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Postby battradio » Thu Apr 02, 2009 5:29 pm

Hi,

You can allways use angle brackets to make it a surface mount if it has two end bells .

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Postby nyazzip » Thu Apr 02, 2009 10:46 pm

i got the new tranny installed(its currently loosely strapped to the chassis with trashed speaker wire (lol); (we used to have a term like that, the last word is "rig" but it would be inflammatory and very distasteful for me to say it), and i was able to verify that yes, the amp emits sound, and yes it will be much much louder! i couldn't turn it up, at all, tonight, due to the non-rock and roll lifestyle parameters i currently find myself immersed in.
the "parasitic/ring modulation" effect is still evident on the bass notes, but i think it may be a low-volume phenomenon. as far as the Sprague can caps go, i couldn't ID one of their values due to the ink being worn off, but they appear to be different based on the other numbers that i can read.
i tried the new matched power tubes also, and i imagined maybe i could hear more EMF hum with them- but because the transformer mount is very temporary, i did ZERO wire "dressing", and also i figured maybe a matched set of power tubes is able to transmit more hum whereas the worn/unbalanced set may have choked it off...? i do see that they exhibit bit of "plasma" or blue glow now which at least looks cool. that is starting to seem to me to be a signature of a properly operating output tube; maybe "plasma blues" is just one notch under "red plate fever".

anyway, my plate voltage is up to about 220, and thats with the Full Monty of the mighty 124vac line. the cathode resistor now has 13+ volts on it. so if i want more output tube plate voltage i can use a lower value cathode resistor, is this right? the current one is 270 ohm.
oh, anyone need a 325-0-325@ 40 mA transformer? lesson: don't be shy to crank up an amp when buying it off of Joe Schmoe
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Postby cartoonweirdo » Mon Apr 06, 2009 10:30 pm

So what ever became of the new pwr tranny? 220V on the plates is up a bit; nothing to shout over but into the range that should be expected.

My best bet is that now (provided you want more gain) you should tinker with the actual design.

Great places to start:

Increase or remove the feedback resistor around the power stage (if you have one)

Slowly bump up cathode resistor values.

Ditto for plate load resistors.

Increases the resistor to ground for the "mid" knob

Are your grid load resistors at least 1 meg (volume knobs too)? They should be (tone stack is an exception).

Make sure your driver is at least as "gainy" as a black face fender (if you see a bunch of low resistor values to ground you can bet it isn't loud)
.... you can go up from there too.

if you have two channels or an extra tube section, dump one channel into the other (gain stages in series) or jumper the jacks with a small cord (channels in paralell).

If you are tired of goofing with the amp you can overdrive the whole thing with a pedal! If you've an inclination to solder (bet you do) check out a book called "Electronics Projects for Musicians" by Craig Anderton. I got into this stuff via this book (more years ago than I want to admit). If you would like them, PAIA electronics (Not to be confused with PIAA of car part fame) still mail orders some of the kits to you for cheap with all the parts and a PCB.

If you want to plug and play I love the sparkle-drive from voodoo labs (I have unfortunately paid retail price for a total of three so far). They stand by their warranty, (for the record dumping beer in them is not covered). The clean boost actualy boosts, the tube screamer section sounds good, and if you like you can mix them together.

I think any small amp can be made into a "screamer" with a bit of dedicated monkeying. Do keep in mind the following things.

Current increase sounds better than voltage increase.

A bigger cabinet is a better cabinet.

You have the rest of your life to make it sound good.

Do only one of the mods at a time, so that if you make a mistake, or it sucks you know where to look.

Take or draw a picture first so you can put it back the way it was.

Try to distribute componant value increases in small amounts throughout the entire circuit rather than pumping up only one part of the whole thing.

If you don't like the clean sound you won't like the fuzz sound.

I think that's all the advice I have (and I bet that's a good thing).

I hope it turns out great!!

Carl
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Postby nyazzip » Sat Apr 11, 2009 11:02 am

last week i rigged up a beefier power transformer. bit more volume but thats it. last night i swapped out the output transformer for some ancient Midwest Coil that was in a 2xel84 amp. it was risky not having any idea what the specs are and what the specs should be. but it works at least, so it was a useful exercise.today i'm going to hopefully try it out at a decent volume

question: if you mismatch an output transformer, what are the first physical problems you will see(other than poor audio)? in other words what should i keep my eyes on... nothing sizzled or redplated or glowed or smoked so far (lol)

also cartoon: can you describe where the feedback resistor, if present, would likely be?
thanks
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Postby cartoonweirdo » Sat Apr 11, 2009 4:55 pm

The feedback resistor in question (there can be other ones) is from the speaker back into the power stage or phase inverter (probably the phase inverter). Check for a resistor or a resistor/cap combo coming from the speaker jack and going somewhere else (if you have a presence control it is part of this circuit). You can start by adding larger and larger value resistors or you can take it out and see what happens. If the amp begins to oscillate, or sounds like crap you will have to put some kind of resistor back, or lower the gain of your amp.

In general too high of a value of feedback resistor (not enough feedback) is going sound sloppy, particularly in the low end, or have a poor balance across the frequency spectrum. Too low of a value (too much feedback) tends towards stiff, and sterile. I prefer as little as possible (which is no resistor at all most of the time), but other people have different ideas. You'll have to fool around with it yourself.

My other question is:
What kind of guitar are you using? Some amps that sound great with a loud guitar (a Les Paul, or any number of high gain "lead" guitars) don't quite do it for quieter ones (a strat or a tele). Perhaps bumping up the input sensetivity is the key.

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Postby nyazzip » Mon Apr 27, 2009 5:51 pm

i don't know what the problem is with this little 6aq5 amp; for some reason the voltages in the tubes are all low. apparently it has no NFB loop. i'll get it figured out one of these days...
but....i did take your advice and i've been playing around with the NFB on my old Traynor guitar amp...man, what a difference! with no NFB it is incredibly loud and very lively sounding, almost like a single-ended sound. but i also like it with lots of NFB....it sounds "tight" and more Marshall-esque, darker and mysterious... and a bit quieter in the noise department. depends on what you're playing.
i have it rigged to a 25k A pot; sounds like greater than about 15K not much NFB gets through....so i'm going to try to find a 20K linear pot to mount permanently somewhere. fun stuff
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Postby cartoonweirdo » Tue Apr 28, 2009 6:01 pm

That's great!! Don't be afraid of bypassing the NFB resistor with some caps, or a cap/resistor pair in series so as to only effect certain frequency ranges.

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Postby cartoonweirdo » Tue Apr 28, 2009 6:04 pm

Another idea on the voltage front. What about subing the tube rectifier for some silicon? The currents and the voltages indicate to me that you can remove the tube and temporarily solder some diodes across the socket pins and see what the deal is. Could be great, could be crap, but it's fast to reverse and cheap to try.

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