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Author Topic: blues deville got the blues  (Read 14085 times)

ilyaa

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Re: blues deville got the blues
« Reply #15 on: March 20, 2014, 02:25:27 AM »
Quote
Did you specifically resolder all the pins of all the power tube sockets?

i resoldered the phase inverter pins and i THINK i did the power tubes but i dont remember now. i will go over the whole tube board again, though, just in case!

and ill do those readings -

Roly:

Quote
could be the OP bottles getting tired, but I always test before replacement

what do you mean test? is there a way to tell the power tubes are bad other than with your ears? any actual, measurable indicators?

Roly

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Re: blues deville got the blues
« Reply #16 on: March 20, 2014, 12:05:01 PM »
Quote from: Roly
could be the OP bottles getting tired, but I always test before replacement
what do you mean test? is there a way to tell the power tubes are bad other than with your ears? any actual, measurable indicators?

Well I have a dummy load with a built-in wattmeter which I find very handy, but a cobbled up bunch of power resistors, voltmeter and calculator to apply the Power Law works too.

P = ERMS2/R

Adjust for the onset of sine clipping and measure the voltage across the load.

Power bottles generally come in two classes, adequate, and clearly stuffed.  If a "60 watt" amp makes 50+ watts I'm happy.

I'll give you a protip; the human ear is a truly lousy judge of output power because it has inbuilt Automatic Gain Control.  I have been asked to look at big valve amps that were still loud but "sounding a bit weak" to the owner that have turned out to be literally making only a watt or three.  This deception can be compounded by preamp gain and speaker efficiency.  The only reliable way to know if a valve amp is making power is to measure it.


If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: blues deville got the blues
« Reply #17 on: March 20, 2014, 01:33:59 PM »
Agree.
FWIW I have personally measured 100W Twin Reverb, still with their Factory Installed 80's 6L6, and Marshall JCM900, still with their 80's EL34 .... and both were still putting out around 40W RMS.
Complain was: "it does not have "authority" any more, I no longer drown the drummer, something must be wrong, although amp still sounds and plays perfect" .
So there you have a clear example of tubes which needed replacing.
In that case, human ear was acceptable, because it was not "measuring" power (it can't tell 30W one day from 50W other day) but because it was *comparing* , as in "the amp is louder than the drums, played at the same time ... or not"
Similar example to: you can't tell temperature of a bucket of water with your hand with any precision beyond warm/cold/hot/unbearable , but you can quite accuretely *compare* 2 buckets, by immersing your hand back and forth.

ilyaa

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Re: blues deville got the blues
« Reply #18 on: March 20, 2014, 01:46:27 PM »
so the procedure is:

plug in dummy load - turn on amp - feed signal (knobs all the way up? halfway up?) - measure voltage across dummy load?

J M Fahey

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Re: blues deville got the blues
« Reply #19 on: March 20, 2014, 02:00:10 PM »
*Basically* yes, tone controls on 5, master on 10, feed 100mV 1KHz at input (or 400 or 440 Hz), start raising volume until it starts clipping, then back up alittle.
Scope is best.
If not available, an old trick is to put a piezo tweeter in parallel with load, start of clipping can be heard through it as an annoying buzzy sound, raise/lower volume to zero its start.
Then measure voltage across load and do the math.
Where to get those tones?
Download the MP3 and play through some portable MP3 player or your phone.
Make a headphone>guitar plug cable.
You can join both (stereo) headphone wires into a single (mono) one because signal is mono and headphone outs 99% of the time have a series resistor.
400Hz: http://users.skynet.be/fa046054/home/P22/track55.mp3
1KHz: http://users.skynet.be/fa046054/home/P22/track57.mp3
For the full Monty: http://www.testsounds.com/
Warning: not a virus but they offer a get rich quick or earn money without working scheme, simply ignore it.

ilyaa

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Re: blues deville got the blues
« Reply #20 on: March 21, 2014, 01:21:42 AM »
yeah i have a scope to put on there...

am i measuring DC volts with the meter? or p-p volts that the scope is showing? i assume the latter....

Roly

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Re: blues deville got the blues
« Reply #21 on: March 21, 2014, 04:57:17 AM »
I normally feed in a fixed signal and use the amp volume to find the clip point, but it amounts to the same thing.

Quote from: ilyaa
am i measuring DC volts with the meter?

ilyaa!!!  What is the amp outputting to the speaker?  {musta bin a late night post I guess  :loco }


Peak to peak, half for peak, divide by root(2) for RMS, P = ERMS/R
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 #22 on: March 21, 2014, 11:51:10 AM »
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ilyaa!!!  What is the amp outputting to the speaker?  {musta bin a late night post I guess  :loco }

hehehe....yayaya i was just making sure!

i know: DC on the speak = death
« Last Edit: March 21, 2014, 04:28:59 PM by ilyaa »

ilyaa

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Re: blues deville got the blues
« Reply #23 on: March 22, 2014, 02:00:32 AM »
Quote
a cobbled up bunch of power resistors, voltmeter and calculator to apply the Power Law works too.

as far as a dummy load goes:

can i just wire together a bunch of power resistors in parallel?? if i am just interested in power output.

say i wanted 100W @ 4 ohms, would 10 40 ohm/10W resistors do the trick? do i need to simulate inductance somehow? or is that not crucial to this application?

(and the reason i asked about DC volts across the load is cause you said voltmeter up there ^^.....)

Roly

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Re: blues deville got the blues
« Reply #24 on: March 22, 2014, 09:02:40 AM »
Quote from: ilyaa
say i wanted 100W @ 4 ohms, would 10 40 ohm/10W resistors do the trick? do i need to simulate inductance somehow? or is that not crucial to this application?

(and the reason i asked about DC volts across the load is cause you said voltmeter up there ^.....)

Yes.  No.  No.  They will get hot so an old metal computer power supply case (blah blah).

A secure connection is vital for valve amp testing.

"Voltmeter" = Multimeter on AC range.















Note: the circuit and this build are only similar, not identical, but I'm sure you will get the idea.

HTH
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: blues deville got the blues
« Reply #25 on: March 22, 2014, 09:03:00 AM »
Yes, 4 ohms total impedance, 100W dissipation is fine.
Most (99%) amps are tested with resistive loads.
Inductive/reactive ones, simulating complex impedance shown by real world speakers, are needed to test complex short protection circuits, which may seem to work with resistive loads, and self trigger, with *horrible*  buzzy/farty sound when driving speakers.

ilyaa

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Re: blues deville got the blues
« Reply #26 on: March 22, 2014, 11:11:00 AM »
coool - ill make one today.

is the diode section just for powering the fan? ooooo i see is it just stealing a bit of the load's AC, rectifying it, filtering it, and powering the fan?? so clever!
« Last Edit: March 22, 2014, 01:25:59 PM by ilyaa »

phatt

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Re: blues deville got the blues
« Reply #27 on: March 23, 2014, 06:47:35 AM »
Why yes,,,any dummy can build one,,
 but it takes very clever people to build a better dummy load. :P
Phil.

Roly

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Re: blues deville got the blues
« Reply #28 on: March 23, 2014, 10:11:30 AM »
...and for my next trick, a linear scale wattmeter using germanium diodes.



{after I pull mine apart to see how I did it}
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 #29 on: March 25, 2014, 07:56:03 PM »
resoldered power tubes.

the clean channel sounds good, actually, except it gets kinda congested when it starts to break up.

but the drive channel still sounds really bad. hollowed out and totally unclear.

i get ~200VDC on pin 1 of the PI and ~400VDC on pin six. so thats bad. should i check the resistors feeding them, coming from point Y on the schematic?
« Last Edit: March 25, 2014, 07:57:12 PM by ilyaa »