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Author Topic: blues deville got the blues  (Read 13461 times)

ilyaa

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blues deville got the blues
« on: March 07, 2014, 03:43:05 AM »
i like this amp a lot (the 2x12 combo) and it sounds good (usually)

this is a 93-ish deville (schematic attached)

lately ive noticed a sort of muffled/congested/sometimes even grainy quality to the drive channel. normally i have no complaints with this amp's tone, but this one is bugging me. it's sometimes crackly verging on staticy. why is it so hard to explain?! bottom line is, i keep wanting to turn it up but it just doesn't seem to break through the graininess. on top of that, sustained notes in particular will kind of crap out into a cheap sounding fuzz.

i got new power tubes. put them in. its fixed bias but i checked it out anyway and they seem fine. running about 37 mA at 474 on the plates so 17.5 watts. that's not too cold is it? (6L6GCs).

control grid voltage is only at -43 or so - should it be more like -50? im deliberating whether or not to put a bias pot in....

most voltages around the amp seem okay EXCEPT:

1) TP26 on the schematic (cathodes of the third pre-amp tube) should read 34 VDC but only reads 22 VDC
2) point Y, off of big filter caps, going to plates of third pre-amp tube and control grid of power tubes read 440 VDC should be 413
3) point X (going to plates of first two pre-amp tubes) reads over 400 VDC and should be only 364 VDC

now i know the high voltages have some leeway, but what about 1) ?? i cant help but feel like the problem is preamp related, seeing as new power tubes that seem to be running at the right bias didnt do much. if so, would a low voltage at preamp cathodes maybe cause the issue? ive swapped around preamp tubes with no luck yet (but i only have one known good one that im switching in and out)

any thoughts?

should i put in a bias pot and try to run the power tubes a bit hotter, see if that fixes it?
« Last Edit: March 07, 2014, 03:44:19 AM by ilyaa »

DrGonz78

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Re: blues deville got the blues
« Reply #1 on: March 07, 2014, 06:09:00 AM »
As they always have told and I have read before, Divide and Conquer. Don't just guess that new tubes means the output side of the amp is not to blame, but rather put it to the test. Take a preamp out from another amp and plug it in to the power amp in. Test to see if the power amp side is the problem. Then if it is not the problem we have just removed 50% of where to look.

Crackly and static like sounds could be any number of problems related to that symptom. First declare positively that it is the preamp that is to blame.
“A person who never made a mistake never tried anything new.” -Albert Einstein

Enzo

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Re: blues deville got the blues
« Reply #2 on: March 07, 2014, 09:20:27 AM »
You like this amp a lot, but lately it sounds wrong.  So NOW you think it needs a redesign?   You liked the way it was before.   Just fix what is wrong with it, don't redesign it.

This amp doesn't have a bias test point, and since it isn;t adjustable, it doesn;t need one, but in the very similar Hot Rod DeVille, the factory spec is 60ma, which is 30ma per tube.  B+ there is 485, not much different.  14 watts.

Besides, bias is not some super-critical thing, being "off" a volt or two will not harm anything or even usually be audible.

Quote
control grid voltage is only at -43 or so - should it be more like -50? im deliberating whether or not to put a bias pot in....

Well do things for the right reasons.   You are concerned that the amp is too cool already.  It isn't, but you can still prefer it hotter.  But now you are proposing to increase bias voltage to 50v from 43v.  Well, that is adjusting it cooler, the opposite of what you are wanting.

This amp is made so most any set of 6L6 tubes will work just plugging them in.

Gonz is correct, first determine where in the amp the problem lies.   Plug the guitar into the Power Amp In jack, still sound funny?   And run a cord from Preamp Out to some other amp and speaker.  SOund OK?  Or still has the issues?

TP26 is not a preamp tube, it is part of the power amp.  That is the cathode of the phase inverter.  Since your voltage there is low, it leads us to think one side of the phase inverter might not be running, thus reducing current through the cathodes.   So look at pins 1 and 6 of that tube and see if B+ is missing from one of them.  Suspecting an open plate resistor.

Roly

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Re: blues deville got the blues
« Reply #3 on: March 07, 2014, 11:36:07 AM »
ilyaa there is a logic to fault-finding amps, it's not magic or dumb luck.

"Divide and Conquer".  Is your problem before or after the main volume control (or Fx loop if fitted, which generally amounts to the same thing, splitting the whole amp half-and-half, into pre or main amp)?

The general approach is to first check that the power supplies are basically functional, that the voltages are roughly what they should be, then we proceed by binary division, first half or second half of the signal chain, pre or main, then splitting the faulty half in half, and so on until we come to the faulty stage, then the faulty component.

{assuming that the problem is in the main amp}

You find that the cathode voltage of the phase inverter V3a&b is low.  This means that the current in the cathode bias resistor is low, as Enzo says.

That leads him to think you may have an open anode resistor (R50, R51 or R52).  It leads me to think that the valve itself may be on the way out, possibly losing cathode emission.  It does not lead directly, or even indirectly, to replacing the output valves.  Either fault could lead to the amp sounding odd, but they obviously have quite different remedies.

You measure point Y at 440V instead of 413V.

Now stop for a minute and think what effect a high supply voltage would have on the phase inverter cathode voltage - would you expect it to be higher or lower than spec?

When you find an odd cathode voltage like this you then check the voltage(s) on the anode(s).  Low cathode and high anode means the valves isn't drawing as much current as intended (Ohm's Law x2), and you could perhaps try a new 12AX7; in this case one anode low or very low and Enzo is right, you have an open or high anode resistor.

One anode almost at the HT supply?  Are both sides lighting up?   No?  Then try rocking the valve in its socket to see if one of the heater connections is dodgy; pull the valve and measure the heater resistance of both sides to see if a heater is open, or is it at the socket?


You say the output stage grid voltage is -43V, but the grid voltage isn't specified, the bias voltage is specified at point -C, and they have a good reason for doing it that way.

Your meter has resistance, typically 1 megohm, so it draws current.

If the voltage at -C is -49.6V and you connect a one megohm resistor from one of the output grids to ground, what do you expect to read?

Well there is a 220k and a 1k5 resistor between point -C and each grid, making 221.5k in series.  Assuming your meter has an internal resistance of one megohm it forms a voltage divider of;

1000/(1000+221.5) = 0.81866558

49.6 * 0.819 = 40.6224V

If your mains is 6% high;

40.6224 * 1.06 = 43.059744

Well shucks, -43V.  What a coincidence.

There is a very important lesson here - your test instruments change what is being tested.  Mostly it's negligible, here it isn't.


What do you measure back towards the rectifier, i.e. point Z, and B+/SP1?  Are they high too?

First check your meter against a known voltage (or another meter), cheaper DMM's are not known for their accuracy.

What is the state of the battery in your DMM?  As the battery goes flat the meter loses accuracy.

But how far off is it?  440/413 = 1.0653753 or 6% high.  What is your mains voltage, 'coz a variation of 5% is quite common?

Did you read the Notes?
Particularly;

"LINE VOLTAGE 120VAC 60HZ"

Is it?  Exactly?  If not you have to correct for it.

Then;

"SUPPLY AND BIAS VOLTAGES MAY VARY +5%, -10%"

If you have a few percent error in your meter, a few percent mains voltage high, and take their error band, you could easily read 440V and still be within spec.

Did you have the controls set as specified?

We normally assume (and it is often stated in the notes) that voltages are measured at idle, however since they don't specify it is possible these voltages are under drive conditions which would explain why they are high at idle.

The control grid is grid one, where the signal and bias goes.  The grids that are connected to the HT via resistors are the screen grids.

Have you measured the output voltage at clip into a resistive dummy load and calculated the power?  Does it work out to be within 10% of rated power?

If it is you have demonstrated why professionals don't use "scattergun servicing" techniques.  Replacing a good set of output valves for $100 when it might only be a 5c resistor (or even a $20 12AX7) would either quickly make you go broke, or build a lousy reputation for replacing expensive components and still not fixing the fault complained of.

It's your amp and you're rich so you don't care?  Fine, but the bottom line is that it still ain't fixed.

Methodical, step by boring step, checking everything, and building up a mental picture from your voltage (or CRO) measurements.  When I get stuck I've found that printing out the circuit and writing the voltages found at each point on it can quickly expose what I've been missing.

Unless you are going to fit current sensing resistors in each output valve cathode so you can actually measure the voltage, thereby calculate the valve current, then with the supply voltage calculate the plate dissipation, I agree with DrGonz and Enzo - don't mess with the bias.
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: blues deville got the blues
« Reply #4 on: March 08, 2014, 06:51:11 PM »
when i take a signal from the preamp out, shouldnt it short out the power amp?

its not doing so - makes it difficult to hear if anything is changing because ive got two amps playing....


DrGonz78

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Re: blues deville got the blues
« Reply #5 on: March 09, 2014, 01:39:59 AM »
Yeah just using the preamp out is not going to shunt off the output speaker. In fact, actually those amps use those connections like it is an effects loop as I recall. Do you have a dummy load that you can attach to the output of the amp to cut out the speaker?
“A person who never made a mistake never tried anything new.” -Albert Einstein

ilyaa

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Re: blues deville got the blues
« Reply #6 on: March 09, 2014, 04:48:28 AM »
i dont have a dummy load yet - planning on making one soon.

what if i put a dummy lead into the power amp in? with nothing on the other end - will that be safe? wont that just fool the power amp into thinking its getting no signal in - therefore none out. and then i can take the preamp out and do with it what i wish. is that flawed thinking?

J M Fahey

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Re: blues deville got the blues
« Reply #7 on: March 09, 2014, 05:23:17 AM »
Quite close   :cheesy:.
Make a shorted plug and plug it in the Power Amp in, this will mute the Deville power namp.
It will not affect the Pre out.

Roly

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Re: blues deville got the blues
« Reply #8 on: March 09, 2014, 09:49:36 AM »
Quote from: ilyaa
when i take a signal from the preamp out, shouldnt it short out the power amp?

Not normally because the Pre Out often gets used as a signal pick off, say to the PA or a recorder, and you would want the whole amp to stay active.

Putting a plug into the Main In however you would typically want the preamp de-selected so the main amp could be used as a slave to another preamp, or the return from your Fx loop, so as JM says simply plug a shorted plug into the Main In and the main amp input will then be disconnected from the preamp due to the switch contact on the Main In socket, and shorted and silenced by your shorted plug while the preamp signal will still be available at Pre Out.
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: blues deville got the blues
« Reply #9 on: March 14, 2014, 04:59:26 AM »
alright so i did some mixing and matching and have come up with the frustrating conclusion that the problem is maybe in both the pre and the power amp (is there a possibility of it being kind of between the two? the phase inverter??)

IF i take the preamp out and connect it to another amp, i get the same kind of boxy and congested, even grainy sounding break-up.

IF i play my guitar directly into the power amp in, the amp sounds better but still has an element of the problem - a kind of unpleasant breaking up/distortion that verges on crackliness.

basically it seems like more of the preamp acting up, but its hard to be sure that the power amp isnt also a part of the problem, somehow....

additonal detail i forgot to mention: some months ago the amp was doing this weird thing where it would make and sustain a loud hum whenever id hit an F#. it would sustain the note in a loud, hum similar it 60Hz power stuff, but not exactly (it would just hold the F#!). the third preamp tube (phase inverter?) would amplify the noise if i tapped it (microphonic?). i figured it was the problem so i switched it out for a new one. the problem went away BUT strangely enough when i switched the old tube back in (out of curiosity) it did NOT come back, nor was it microphonic anymore.

ill try some tube swapping just to see if that changes anything, but otherwise:

does it make sense to troubleshoot the phase inverter given the symptoms?

DrGonz78

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Re: blues deville got the blues
« Reply #10 on: March 14, 2014, 06:47:42 AM »
Perhaps it's the phase inverter solder joints? Perhaps the whole amp needs some soldering re-flowing. The fact that you have problems on both sides of the amp makes it hard to conquer and divide. On the preamp side could be solder joints too, but I would be checking the coupling caps as well. Make sure you don't measure DC voltage on the pots that follow the coupling caps. Like after C2 on Volume R7 or after C9 Master Vol R21. I am sure other's here will have better advice soon.
“A person who never made a mistake never tried anything new.” -Albert Einstein

Roly

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Re: blues deville got the blues
« Reply #11 on: March 14, 2014, 07:44:09 AM »
Quote from: ilyaa
boxy and congested

I asked about a dummy load full power test because output valves, like car tyres, wear out.  "Congested" could be the OP bottles getting tired, but I always test before replacement, so you need to get on to building that dummy load.


{constructional shots also on Photobucket}


Valve microphony is not the only sort; it is possible for e.g. a valve socket/pin to be mechanically sensitive, but a microphonic valve does not self-cure, it has a clear high tinkly quality when the glass is gently tapped.  Pops and splats are dirty/loose socket connections, not microphony.
If you say theory and practice don't agree you haven't applied enough theory.

g1

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Re: blues deville got the blues
« Reply #12 on: March 14, 2014, 12:34:40 PM »
  Along with resoldering the tube sockets like Dr.Gonz suggested, I would also suggest resoldering the main filter caps and the 2 power resistors in the low voltage power supply.
The power supply is the only thing really common to both the preamp and the power amp, so if indeed the problem is in both sections, the power supply could be the culprit. 
The only other thing common to both is the speaker.
But you said the clean channel is fine.  If the clean channel is fine, then going straight in to the power amp should also be fine.
Do more testing to see if the problem occurs when straight in to the power amp or when using the clean channel.

ilyaa

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Re: blues deville got the blues
« Reply #13 on: March 19, 2014, 04:14:08 AM »
alright heres where im at:

did some resoldering - power supply, phase inverter - even put in new preamp tubes and a new phase inverter tube - problem persists.

after some extensive mix/match (including other amps and speakers) testing today, im pretty convinced the problem is in the deville's power amp.

if i play the amp's clean channel, it sounds good, but it still breaks up in a boxy,jagged,unpleasant kind of way. the drive channel, on the other hand, is not only quieter, but also kind of hollowed out (no low end) and much much much more jagged/boxy. not pleasant tube saturation but kind of gnarly, clipping, farty and grainy kind of distortion.

if i plug in a diffeent preamp direct into the deville's power amp, i get the same problem - gnarly, boxy, congested distortion

these are new powers tubes, as well.

testing wise, the only voltage that looks off is the phase inverter cathode voltage - its about 10 volts too low. could that be causing the symptoms? seems like it could, since the phase inverter directly feeds the power amp....

where is the cathode bias on that tube coming from???
« Last Edit: March 19, 2014, 04:15:18 AM by ilyaa »

Enzo

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Re: blues deville got the blues
« Reply #14 on: March 19, 2014, 10:05:25 AM »
Did some resoldering?   Did you specifically resolder all the pins of all the power tube sockets?   Not touch up the ones that looked suspicious, I mean solder every darn one of them?

My schematic says 33v on the PI cathode, and you get 23?  Pins 1 and 6 are the plates, what is on them?   They both ought to have about the same voltage.  What voltage?  I don;t know, somthing like 250-270 comes to mind  But what I don't want to see is zero volts nor do I want to see 400v on those pins.

And when taking readings, did you do the power tubes?  And was there good B+ on pin 4 of each as well as pin 3?