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Post subject: Re: hotrod deluxe loss of power
Posted: Fri Jul 22, 2011 8:45 pm
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OK, if they are glowing then you have filament voltage, you indicated 0 volts, but it has to be there or the tubes wouldn't glow. Next, I notice that you only show -4 volts at pin 5 of V4, and you have -40 volts on pin 5 of V5, both pin 5s on the output tubes should be close in voltage. But if it really is at -4, and the other at -40 then with amp power off, unplug the output tubes and power up the amp and recheck those voltages at the output tube sockets to see if those two voltages become equal. If the voltages are closer with the tubes removed, then V4 or V5 is a bad tube, try another set. They can still be bad when received new, it happens. If the voltages at both pin 5s remain unequal with tubes removed, check the resistors R59, R60, R64, R65. :idea:

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Post subject: Re: hotrod deluxe loss of power
Posted: Sat Jul 23, 2011 12:22 am
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:oops: :oops: :oops: Sorry for the disturbance (thanks Retro I didn't understand correctly the sentence).

I suspect as Shimmilou :
v4
pin1 0
pin2 0
pin3 422v
pin4 421v
pin5 -4
pin6 0
pin7 0
pin8 0

The grille voltage is abnormal (should be rougly -40 ), follow Shimmilou advices
again sorry :oops:

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Post subject: Re: hotrod deluxe loss of power
Posted: Sat Jul 23, 2011 12:36 am
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I checked both pin 5s again and i must have misread them they are the same voltage


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Post subject: Re: hotrod deluxe loss of power
Posted: Sat Jul 23, 2011 12:49 am
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Just out od interest should pin 4 of the power tubes have such a high voltage on it it? What is its function?


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Post subject: Re: hotrod deluxe loss of power
Posted: Sat Jul 23, 2011 1:16 am
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Pin 4 is the screen grid. And yes, 421 volts seems a skosh high.

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Post subject: Re: hotrod deluxe loss of power
Posted: Sat Jul 23, 2011 7:22 am
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OK, after those two wild goose chases, it seems we are back to square one. Not clear whether or not you checked your coupling caps, sometimes they can go bad, not often but it happens. But,...

As 63supro originally suggested, output tubes (it could even be a preamp/pi tube). The fact that you can't get the bias reading at the test point to go above 75 mV, supports the notion that the output tubes are suspect. With 5881s this limit wouldn't be unusual, but with the JJs you should be able to get at least 85 to 90 mV at the test point, and more. The easiest thing to do would be to try another set of output tubes, and recheck bias reading, making sure to lower the pot before changing tubes so as to reduce the risk of red-plating the new set. (the JJs are idling at well under 50%, that can make the output distorted)

Next, as Retroverbial suggested, the OT. You can measure the DC resistance of both halves of the primary, and the secondary. The primary halves should measure approx 82 ohms on one half, and 118 ohms on the other half. The secondary should be close to 1 ohm (main speaker jack, speaker cable connected to jack, measure at speaker end of cable with one of the speaker wires disconnected). You should have an infinite (OL) reading from primary to secondary. If you have a megger, you can check the insulation, but if the OT doesn't smell funny, and the DC readings are good, odds are it's OK, but anything is possible.

You didn't really start at the beginning of the story. Is this your amp that developed trouble, and then you tried replacing the tubes? Or did it come to you like this, with supposed new tubes, or did you try some mod/upgrade and then the trouble, or.....?

If everything is as reported, and you haven't mistakenly put in your caps backwards, and you did a good job soldering, there isn't much left other than tubes and OT, and as you mentioned, sockets. :idea:

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Post subject: Re: hotrod deluxe loss of power
Posted: Sat Jul 23, 2011 11:21 pm
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okay so from the beginning this is what happened. my friend brought in the amp to me cos he thought i could fix it. it had been making loud crackling noises and was distorting at low volume but was still producing full power albeit mushy and distorted on the clean channel.

so i replaced r4 r11 r16 r22 r57 r58. i checked my supply voltage to the power stage and it had alot of ripple on it so i put in a new filtering cap. still no better, so i checked the bridge diodes and d7 was conducting in both directions, bad. so i replaced it with a 1A 1000v diode as i could not find the one from the schematic. after that the ripple was gone.
i put in a new set of jj 6l6gc s (the original tubes were jjs too) and bias them at 60mv at the test point. plugged in my guitar and the distortion was there but more prominent and messy, and the volume was significantly less, not even as loud as my 20w hk tube combo.

i checked the transformer and i am getting 183ohm across the brown and blue wires that come off pin 3 of the power tubes, and 0.6ohm on the speaker side. so it looks like it is okay.

so maybe i should go get another set of tubes for testing?
thats the story so far


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Post subject: Re: hotrod deluxe loss of power
Posted: Sat Jul 23, 2011 11:22 pm
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oh and i replaced all 12ax7s including the pi with jjs from one of my other amps that i know is working


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Post subject: Re: hotrod deluxe loss of power
Posted: Sun Jul 24, 2011 5:01 am
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okay so i did some tests with a osciliscope. with a pure 1khz sine wave in the power amp in socket the output to the speaker was clean no clipping until very high input voltage. however, when i plugged in my guitar there was massive clipping on the output even when playing softly. so i went back to follow the signal and it was fine until the output of the pi which was clipping alot. could this be loosing me power and why would it clip with such low signal?


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Post subject: Re: hotrod deluxe loss of power
Posted: Sun Jul 24, 2011 5:08 am
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Thelovman wrote:
... it had been making loud crackling noises and was distorting at low volume but was still producing full power albeit mushy and distorted on the clean channel...i put in a new set of jj 6l6gc s (the original tubes were jjs too) and bias them at 60mv at the test point. plugged in my guitar and the distortion was there but more prominent and messy, and the volume was significantly less, ...


Sounds like you possibly originally had two or three problems. Possible bad preamp tube or two and/or tube socket solder joints (crackling noises) as well as the bad power supply diode, and likely a bias that was too low. Or, the preamp tubes were OK and the noise was caused only by the bad diode.

You seem to have gone through the amp thoroughly and repaired the problems, but then maybe introduced another problem with the second set of output tubes (further compounded by the low bias setting). If possible, put in the original output tubes (or better yet, a new set) and see if you can get the bias up to 85 to 90 mV at the test point. 60 mV at the test point with those tubes will probably sound like crap.

I prefer to try only one thing at a time, to be more certain of what has been changed. Maybe try the output tubes, and the bias adjust first, then go from there. If you are unable to get the bias up to where needed, that would indicate a problem in the output section (tubes or sockets or solder joints), as the bias supply seems to be good. :idea:

Just saw your new post when I clicked "Preview". One thing at a time. I wouldn't think that the pi would cause your bias limit problem, as it has nothing to do with the DC from the bias supply.

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Post subject: Re: hotrod deluxe loss of power
Posted: Sun Jul 24, 2011 5:21 am
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im pretty cirtain the old tubes are toast as they are giving .2v at the test point at the lowest bias setting.


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Post subject: Re: hotrod deluxe loss of power
Posted: Sun Jul 24, 2011 5:24 am
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soo jj s need to be bias hotter?


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Post subject: Re: hotrod deluxe loss of power
Posted: Sun Jul 24, 2011 5:37 am
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The 60 mV indication on the schematic is universally known to be too cold even for the stock tubes, disregard that setting. No one knows why that is only 60 mV, other than for reference only.

The JJs are a 30 watt tube, and should be biased to at least 50% to 70% (15 watts to 21 watts). With a reading of 76 mV at the test point, with your Plate voltage, the tubes are idling at well under 50%, and would probably sound bad just because of that. 90 mV at the test point would be about 17 watts idle in this amp, right in the acceptable range at just under 60% idle. You could go higher, but start there.

But, yes, if your minimum bias with the old output tubes is 200 mV, you are correct, toss those tubes, providing that your bias supply is solid. Same with the new set, if the max bias is 76 mV this indicates something is wonky with those as well. :idea:

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Post subject: Re: hotrod deluxe loss of power
Posted: Sun Jul 24, 2011 11:17 am
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What is the level of you injected signal (1KHz) ?

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Post subject: Re: hotrod deluxe loss of power
Posted: Sun Jul 24, 2011 6:52 pm
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it was about 5v peak to peak at tp23 before the pi would start clipping, but much lower for the guitar signal


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