un-loud 6aq5 amp

[nyazzip]

hmm whats up with the new "emoticons"....i think i prefer the "lo fi" ones.

anywho, i picked up a homebuilt PP 6aq5 guitar amp from some tinkerer. it employs two 12ax7s and a 5y3 rectifier. it has a volume, super high treble(or presence?), a mid, and a low control. it also has a couple of toggle switches, i think one of them jumpers pins on the power tube sockets for trying out different tube types, the other one i don't know. everything is unmarked.
anyway, the amp sounds pretty good, it doesn't buzz or hum at all. but it is very quiet at full volume, seems to be about 1/3 of what my 5 watt valve junior does, and on the low notes i can hear a semi-tone, almost like an octave thing going on. its subtle but definitely there on the lower guitar registers. i'm very pleased with the amount of low end, and the amount of tone control i have over the mids and highs too.
i'm wondering, could that parasitic/overtone effect have an effect on volume? the way i see it a PP (2)6aq5 should comfortably put out well over 10 watts.
the output transformer is physically tiny, the smallest i have seen i guess.
anyone have any off-the-cuff ideas why its so quiet? the dude gave me a schematic but i don't think it applies; the preamp tubes are different on the drawing, among other things.
i realize i didn't supply enough hard data here for troubleshooting; i'm just soliciting a quick idea or two
regards :sick: 8) :? }:) :D :)http://s265.photobucket.com/albums/ii229/nyazzip/?action=view&current=IMG_6328.jpg

[Geek]

Hi,

The 6AQ5 is a close 6V6 match and should net you at least 3.5W...

The semi-tone in there along with low volume seems to indicate to me:

1) Bias is horribly wrong (if cathode biased, did the resistor drift up?).
or
2) OPT is a crummy match.
or
3) the 6AQ5 is on its last legs.... errr....pins?

Do you have a little 8K anode load transformer there you can try?


As for the smilies, they're a little saner, we just need more

Cheers!

[nyazzip]

The 6AQ5 is a close 6V6 match and should net you at least 3.5W...

i did see that the 6aq5 specs are the same as the 6v6....thats why i went ahead and bought the amp. but i thought those tube types dissapate about 12 watts, theoretically...? how do you get the 3.5 watt number?

[quote]The semi-tone in there along with low volume seems to indicate to me:

1) Bias is horribly wrong (if cathode biased, did the resistor drift up?).
or

highly possible. at least the plates aren't glowing
2) OPT is a crummy match.

i like that theory alot
or
3) the 6AQ5 is on its last legs.... errr....pins?

i dunno. i'm under the running assumption that all tubes are good, just because i have no way to test them, and thats a happy thought :D
maybe i will try to find another little opt. i have one i can pirate from an old 2x el84 PP amp, but it is riveted to it's chassis, so i kinda don't want to go there, yet....
anyway thanks for the input

[EWBrown]

3.5 watts would be about normal for a single-ended 6AQ5 running in pentode mode. Maybe up to 4.5Wwith a good tailwind and moderate THD

If it sounds weak and "weedy", I d suspect that the tubes have run their course...

6AQ5s, and their JAN equivalent, 6005 can be found for cheap on e-bay, I've seen sleeves of 5 NOS 6005s go for 10-15 bucks, These tubes don't have the "audio panache" as the more populer EL84s, 6CA7s, 6L6GCs, etc.

Tubes can, and do go "flat line" with long service hours.

FWIW, I have an octet of "flatline" Sovtek EL84s that came out of a Peavey 50/50 that once belonged to the band Aerosmith.

The filaments still light up, but they don't bias worth a poop, and their power gain is extremely low. I ended up retubing the amp, re-adjusting the bias, and it is currently proudly serving in a local "youth activities" hall.

FWIW, some of the old style metal 6V6s actually contained a 6AQ5 inside, along with a 7 pin to octal base converter. (I corrected this from a typo'ed 9 pin adapter).
These were "good enough" for their original intended purposes, but shouldn't be hammered real hard in a guitar amp.

Also, check your B+ voltage, it should be around 250V to 275V. It can be less, as low 225 and even sneak up close to 300V.

Typically, properly operating PP 6AQ5s are good for 10 watts with 250V, and up to 14W with 275 V. Normally these are run cathode biased, and with a shared cathode resistor. Chec the resistor to see if it is within rasonable tolerance to its original value, and that the bypass cap is good.



The teeny OPT :sick: would mean poor bass response, almost any semi-decent 8K to 10K CT OPT would be a great step up.

HTH

/edB

[nyazzip]

3.5 watts would be about normal for a single-ended 6AQ5 running in pentode mode. Maybe up to 4.5Wwith a good tailwind and moderate THD

yes, but it's a push pull; at least that is, theres two 6aq5s. thats why i am disturbed that it is much quieter than my 5 watt single ended valve junior...according to a GE data sheet on the 6aq5, it states that for push-pull class AB1, in a two-tube setup, "maximum signal power output" =10 watts...not sure if my amp is AB1 though. but my point is, no matter what, it should be at least as loud/louder than a 5 watt SE el84....right? also when it is cranked it is still clean. so i surmise for some reason the power tubes arent being driven like they could....

If it sounds weak and "weedy", I'd suspect that the tubes have run their course...

i think the sound is actually well balanced and quite good; there is ample low end, and as much high as i could want. however, it is quiet, and the weird overtones on the bass are not my imagination. its a bit subtle but i'm definitely not imagining it, sounds a bit like an "octave pedal" for you guitar people. i guess i would call it parasitic oscillations.

6AQ5s, and their JAN equivalent, 6005 can be found for cheap on e-bay, I've seen sleeves of 5 NOS 6005s go for 10-15 bucks, These tubes don't have the "audio panache" as the more populer EL84s, 6CA7s, 6L6GCs, etc.

Tubes can, and do go "flat line" with long service hours.

yeh i guess it wouldn't hurt to hunt down another pair of 6aq5 equivalents just in case...


Also, check your B+ voltage, it should be around 250V to 275V. It can be less, as low 225 and even sneak up close to 300V.

Chec the resistor to see if it is within rasonable tolerance to its original value, and that the bypass cap is good.

alas i don't see any shared resistor, just a 1.5 K on each "pin 1", or what i would guess might be pin 1...the sockets are not marked. said resistors both(remarkably) read 1.468 K ohms. i don't really understand tube pinout diagrams but from what i saw pin 1 on a 6aq5 looks like a screen grid. pin 2 is most certainly the cathode, 3 and 4 the heater, and 5 the anode....
wait, i just now measured B+, or plate to ground on each tube, and i got 189VDC...so theres my problem, right...? but why...?



The teeny OPT would mean poor bass response, almost any semi-decent 8K to 10K CT OPT would be a great step up.

so smaller physical size equals less windings thus less resistance? thats simple enough to understand i guess...
thanks much

[EWBrown]

Small (cheap) OPTs have less turns on the primary winding, which means lower inductance, which translates to less bass frequency response.

The bass "overtones" are the result of the B+ supply voltage (and current) being too low, which results in higher harmonic distortion. If it is second harmonic, it doesn't sound bad (think of a 12-string guitar, with the "octave" tuning.

Third and other odd-order harmonics are far more dissonant, and are considered the worst distortion.

6AQ5 data sheet here:

http://frank.pocnet.net/sheets/127/6/6AQ5.pdf

/ed B

[nyazzip]

any ideas why the plate voltage would be so low? the power transformer is rated at 325-0-325v, rectified by a 5y3gt
i'm guessing there are some incorrect resistor values somewhere, or could worn out power tubes show low voltages? the thing is they both read 189vdc on the plates, so that would be suspiciously even "wear".....

[EWBrown]

Carefully measure the AC voltage on the plates (Pins 4 and 6) of the 5Y3GT, to see if they are indeed 325-0-325, or perhaps a bit higher (which is OK).

If that checks out, then check pin 8 and check the DC voltage, this should be around 350VDC though it doesn't hav to be exact. THis may seem high for 6AQ5s but then this is a guitar amp, and the rules can be bent but not totally broken.

If it reads a lot lower than that, then chances are that the 5Y3GT is weak.


Other than that, check the caps and resistors dow the B+ line, maybe something is fried or way out of tolerance. Also check the voltage at the center tap of the output trannie, this is generally a red lead, and goes back to the electrolytic caps in the PSU.



The "shared resistor" I mentioned earlier would be a shared 6AQ5 cathode resistor, somewhere around 270 - 330 ohms, 5W atts typically. If the amp is fixed bias, then there would be no resistor, cathodes connected straight to ground, or perhaps a 10 ohm resistor in order to facilitate measureing the bias current.

[nyazzip]

Carefully measure the AC voltage on the plates (Pins 4 and 6) of the 5Y3GT, to see if they are indeed 325-0-325, or perhaps a bit higher (which is OK).

i have two 5y3gts laying around, the first pin 4/6 measurements were 323/320, the second were 325/321...i have the line AC in dialed at about 120v with a variac; i like to drop it by a few volts because any time i have checked it has been around 124vac

If that checks out, then check pin 8 and check the DC voltage, this should be around 350VDC though it doesn't hav to be exact. THis may seem high for 6AQ5s but then this is a guitar amp, and the rules can be bent but not totally broken.

so this part is way off: i measured only 220/222vdc, pin 8 to ground, with both rectifier tubes. could it be a bad socket?

If it reads a lot lower than that, then chances are that the 5Y3GT is weak.

ah, but two of them...?


Other than that, check the caps and resistors dow the B+ line, maybe something is fried or way out of tolerance. Also check the voltage at the center tap of the output trannie, this is generally a red lead, and goes back to the electrolytic caps in the PSU.

center tap of the OPT measured 189/190vdc, just like the 6aq5 plates.


The "shared resistor" I mentioned earlier would be a shared 6AQ5 cathode resistor, somewhere around 270 - 330 ohms, 5W atts typically. If the amp is fixed bias, then there would be no resistor, cathodes connected straight to ground, or perhaps a 10 ohm resistor in order to facilitate measureing the bias current.
ok, it looks indeed like the cathodes share a 240 ohm resistor...though its a carbon composite, it measures out bang on. dang you're good!
it took me awhile to figure that one out because this amp has a toggle switch that alternately either jumps or opens pins 2 and 7, for trying different 7 pin types. the guy i bought it from kind of explained it, but i'm not sure he understood it either. yes, the toggle works, and yes, i have it in the right position.... :D
for the record this amp is a contemporary build, not an antique or anything, other than the trannies and tubes. all the components look shiny and good....
thanks for all the help, this has really been great, i have actually learned a few baby steps.
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[EWBrown]

The low (220V) B+ is kinda suspicious...

Now it's just a process of elimination, to find the culprit.

First, measure the DC voltage across that 240 ohm resistor - that will tell you how much current the 6AQ5s are gobbling up. It should be somewhere around 10-14VDC, which would translate to 42-58 mA for both tubes. The higher the measured voltage, the more current is being consumed. This could be as high as 17V, with the tubes taking up 70 mA (35 mA each) but that would be the extreme upper limit, before meltdown commences :D

If the 5Y3GTs aren't bad, then something downstream is consuming excessive current, but generally that would make something get very warm, or even smokin' hot (and, no, I'm not talkin' about Faith Hill)

Run it a while, shut it off then feel for any overly warm/hot electrolytic caps, power resistors, etc. If something was really getting cooked, the aroma of electrical demise would be quite obvious :sick:

I'm making the presumption that the 5Y3GT filament is actually getting 5VAC, you could carefully check that, but keep in mind that there is also HV DC riding on that, referenced to ground.

Best way is to turn off the amp, then clip the meter leads on to pins 2 and 8, then turn it back on and check the AC voltage. 5VAC +/- 0.3V is normal, if it is really low (like 4V or less) than there is a problem, the filament isn't heating sufficiently, so less electrons are being emitted, etc...

If these are old layabout 5Y3GTs, they could be old, tired and worn out (sorta like me) I've run into more than one low emission rectifier tube, and they light up nice and pretty, but little or no DC comes off pin 8. They "flatline" in a tube checker.

The OPT should be OK, the usual failure mode would be an open winding or an internal broken connection.

With the power turned off, and the amp unplugged, You can do a DC resistance check from the primary's center tap to each plate, the measured sresistance can be just about anything from 50 to 300 ohms per side, and there is usually a 10-15% difference in the resistance, which is normal - it's just a result of multiple layers of windings, so one half of the primary has more wire than the other.

Sockets really don't go "bad" (unless there is physical damage) but they can get loose contacts, corrosion, or some junk gets inside the contacts (or on the tube pins). If you were losing around 100VDC on a tube contact, there would be some sizzling, sparking and an odor. :D

Loose contacts are simple to fix, a careful squeeze with tweezrs or fine tip needlenose pliers.

Corrosion can generally be taken care of with a small round file, or a toothpick and some contact cleaner ,or even WD-40 in tiny precise applications, don't just "drown" the tube socket with it.

I'll generally relpace really old sockets, and those "cheapies" with the two-point "fork" contacts, and any of the "wafer" type phenolic sockets - they were made "bad" to start with, IMnsHO...

/ed B

[cartoonweirdo]

I am going out on a limb here and making two assumptions. The first is that your amp is hand wired (no pc board). The second is that you have a relatively simple design on your hands.

Why not remove things from the circuit until you find your culprit?

First thing to do, cause it's easiest, is pull out all the tubes but the 5y3 and check B+. If it's still low the problem is in the rectifier (your pwr tranny was kicking out 325-0-235 right?) or in the filter section. Turn off the amp and bleed the caps and unsolder the wire that feeds the filters. Check pin 8 of the 5y3. If the problem still persists it's got to be the 5y3. If not it's in the filter stage.

If the voltage is good without the tubes in but falls when you insert them the question is a bit more complicated. In that case I want to know how big your pwr transformer is and if it gets excesively hot when you run it. Also I wonder what size your filter and coupling caps are. Too big is accurate all the way down to frequencies your guitar will never use.

Last of all: When you plug this amp into the same speaker as your valve junior does it "come alive"? Or conversely if you plug your valve junior into the speaker from your new amp does it flatline?

[nyazzip]

ok....i blew a fuse in my DMM somehow so that was down for a couple days. anyway: i was perusing the 5y3 data sheets, and it says using "capacitor input filter"(that just means filter caps right, as opposed to choke?) AC supply voltage= 350 vac
DC output voltage at filter input= 360 vdc.
number one, my power tranny is only rated at 325 vac, and it seems to actually put out a few volts less than that, AND each side is a bit different. i would guess that this is somewhat normal. but, does the 5y3 require 350 vac? because its only getting about 320.
secondly, how can the rectifier increase voltage from 350 ac to 360 dc? surely i don't understand the chart....http://www.dreamtone.org/pdf/5Y3GT.pdf

The low (220V) B+ is kinda suspicious...

Now it's just a process of elimination, to find the culprit.

First, measure the DC voltage across that 240 ohm resistor - that will tell you how much current the 6AQ5s are gobbling up. It should be somewhere around 10-14VDC, which would translate to 42-58 mA for both tubes.

i measured just under 11 vdc...

If the 5Y3GTs aren't bad, then something downstream is consuming excessive current, but generally that would make something get very warm, or even smokin' hot (and, no, I'm not talkin' about Faith Hill)

hmm can't say i know what she looks like. but i'm a pop culture dropout :D anyway, i went to Triode and got another NOS 5y3gt, just to eliminate that possibility. no change, still outputs about 220vdc.

Run it a while, shut it off then feel for any overly warm/hot electrolytic caps, power resistors, etc. If something was really getting cooked, the aroma of electrical demise would be quite obvious

yeh i've kept my eye on it and nothing seems to be frying. and i know what toasted CC resistors smell like...yech

I'm making the presumption that the 5Y3GT filament is actually getting 5VAC, you could carefully check that, but keep in mind that there is also HV DC riding on that, referenced to ground.

that checked in at about 4.98 vac. just to compare i tried the same measurement with another 5y3 rectified amp i have and i got exactly the same reading in that circuit....

The OPT should be OK, the usual failure mode would be an open winding or an internal broken connection.

With the power turned off, and the amp unplugged, You can do a DC resistance check from the primary's center tap to each plate, the measured sresistance can be just about anything from 50 to 300 ohms per side, and there is usually a 10-15% difference in the resistance, which is normal - it's just a result of multiple layers of windings, so one half of the primary has more wire than the other.

i measured 161 ohms on one and 180 ohms on the other...um....what percentge difference is that? that would almost be getting into the range of being "intolerable" wouldn't it?


and cartoon:

I am going out on a limb here and making two assumptions. The first is that your amp is hand wired (no pc board).

that is correct, sir

The second is that you have a relatively simple design on your hands.

well..."simplicity" is relative. but it has what i was told a "line out", a toggle switch for jumping/opening pins on the power tubes, and an unknown toggle on the back. i got bold and operated it in the "other" position and the voltages were even lower.....

Why not remove things from the circuit until you find your culprit?

i'm about to. starting with the line out.

First thing to do, cause it's easiest, is pull out all the tubes but the 5y3 and check B+. If it's still low the problem is in the rectifier (your pwr tranny was kicking out 325-0-235 right?)

(actually a few volts under 325)

or in the filter section. Turn off the amp and bleed the caps and unsolder the wire that feeds the filters. Check pin 8 of the 5y3. If the problem still persists it's got to be the 5y3.

(i will try this tonight but i have prettymuch ruled out the rectifier tubes. so its cool to operate the amp without filter caps?)

If not it's in the filter stage.

If the voltage is good without the tubes in but falls when you insert them the question is a bit more complicated. In that case I want to know how big your pwr transformer is and if it gets excesively hot when you run it. Also I wonder what size your filter and coupling caps are. Too big is accurate all the way down to frequencies your guitar will never use.

Last of all: When you plug this amp into the same speaker as your valve junior does it "come alive"? Or conversely if you plug your valve junior into the speaker from your new amp does it flatline?

i have an assortment of speakers to mess with; i was told the load should be 8 ohms but i don't know how to verify this. maybe i'll plug it into a 4 ohm and see how that is....

[battradio]

Hi ,


Quote ( ok....i blew a fuse in my DMM somehow so that was down for a couple days. anyway: i was perusing the 5y3 data sheets, and it says using "capacitor input filter"(that just means filter caps right, as opposed to choke?) AC supply voltage= 350 vac
DC output voltage at filter input= 360 vdc.
number one, my power tranny is only rated at 325 vac, and it seems to actually put out a few volts less than that, AND each side is a bit different. i would guess that this is somewhat normal. but, does the 5y3 require 350 vac? because its only getting about 320.
secondly, how can the rectifier increase voltage from 350 ac to 360 dc? surely i don't understand the chart )

The AC voltage is measured as RMS voltage , what that means is if you have a light bulb that needs 100 volts DC to be 100 watts you need a higher AC voltage to heat the fillament to the same temperature 141.4 volts peak to peak . 100 AC volts RMS is equal to 141.4 AC volts PP on your meter , so when the 5Y3 rectifies the 100 AC to DC it is 141.4 volts minus the voltage drop across the 5Y3 . So if yo have 325 volts AC times 1.414 it equals 460 volts DC minus about 80 volts across the 5Y3 equals 380 no load . So 360 volts is good considering yor transformer is a bit low and your 5Y3 isn't new .


How much voltge drop is there from the center tap of the output transformer to the plate of each 6AQ5 .

Mark

[EWBrown]

You need the filter caps... The first one (connected to the 5Y3GT pin 8 should be 29049 uF, then the choke (or resistor) then the second cap which can be 20-40 uF, and up to perhaps 80 uF without straining the 5Y3 too much. A choke input filter will develop significantly lower B+ voltages
than a CLC (or CRC) filter.

As far as the power trannie is concerned, the VAC output isn't absolute, there is usually a 5% fudge factor built in, and I've seen some DVMs which read a bit off on AC and especially RMS volts, I have a Fluke 87 which seems to vary with the phase of the moon, with its RMS readings.
The actual AC waveform can als afect the RMS readings, if it isn't a clean sine wave.

The DC voltage drop across each side of the OPT is just a few volts, just measure the DC resistance of each side, and factor in the plate current (in this case, 23 mA each), or you could very carefully measure from CT to each plate. Careful, becaus the DVM will be "hot" at the B+ voltage. Typically small OPTs like that will be around 100 ohms per side, perhaps as low as 70 and high as 200, depending on the primary wire gauge and number of turns.


I'm also proudly among the "pop culture dropout" category myself. Just seen Faith on a TV show and man she is hubba hubba hubba... 8) 8) 8) 8) (ther are no dirty iold man smileys yet)... She's 100% prime USDA country goods..

5Y3GT can run with lower or somewhat higher voltages, no sweatie dah The RCA tube manual data is just a guideline, and actual tubes can and will vary by 10% or more if good, and a lot more out if they are weak or dead :sick:

11V across 240 ohms indicatres around 46 mA, or 23 mA per 6AQ5 if they are fairly closely matched. They can go up to 30-35 mA each, before getting the red plate fever. That would translate to 14-17 V across the resistor.

/ed B

[cartoonweirdo]

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 :sick:
(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.:D

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.

Let me know what happens!!

Carl