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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sun Jun 16, 2013 3:28 pm
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shimmilou wrote:
Since you say that you have the same distorted sound when plugging into the Power Amp In, that would seem to indicate a problem in the power section, or maybe even in the reverb section, as the preamp is bypassed when plugging into the Power Amp In. But, it couldn't hurt to verify the preamp section as you said, by plugging Preamp Out into another amp. I am still thinking about the PI tube, or the circuit around the PI, especially since you had a bad Plate resistor (82 K).


The OP did say earlier that he swapped in a new set of tubes.

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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sun Jun 16, 2013 5:08 pm
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Yeah, I wasn't sure if he meant just output tubes or all of them. I do occasionally get bad new ones, and it can sometimes throw me for a loop. Maybe the circuit around the PI?

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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sun Jun 16, 2013 5:22 pm
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shimmilou wrote:
Yeah, I wasn't sure if he meant just output tubes or all of them. I do occasionally get bad new ones, and it can sometimes throw me for a loop. Maybe the circuit around the PI?


Not much else you can do checking DC voltages and resistance. I really think its time for some AC signal tracing to pinpoint the problem area..

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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sun Jun 16, 2013 8:36 pm
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shimmilou wrote:
Since you say that you have the same distorted sound when plugging into the Power Amp In, that would seem to indicate a problem in the power section, or maybe even in the reverb section, as the preamp is bypassed when plugging into the Power Amp In. But, it couldn't hurt to verify the preamp section as you said, by plugging Preamp Out into another amp. I am still thinking about the PI tube, or the circuit around the PI, especially since you had a bad Plate resistor (82 K).


that's where i'm at with it. I only want to try from the preamp out just to definitively rule it out.

@bluesky636, thanks for the heads up on the oval vs square. that makes a lot more sense now. that explains why i was reading high voltage on all of those early TP points. Looking at the schem, high DC voltages passes through one resistor and then you are supposed to measure mV at TP2.. AC makes much more sense and it's a good 'duh' moment. That should satisfy my questionable thoughts about the front stage.

Just to clarify on on the tubes, here's the run down on the chain of events: at practice, jammin, volume drops out, output is distorted. before i go poking i decide to call up Bob Plekka at eurotubes and get a whole new set because the tubes that were in there were installed by the previous owner and i had no idea how long they have been in there. So i get a complete set, balanced PI, matched output. drop those in, no dice, same low distorted sound. those are the tubes currently installed, i did swap them out with a known working set out of my deluxe just to check and see if my new ones were somehow damaged in shipping.

I don't have a true signal generator or oscilloscope, but i can get my hands on one if we need to go poking in the front section. from tracing the signal path, i am pretty convinced the issue lies in the power section, but that really is just my educated guess, but seems logical to me, and it seems you guys concur given the information i've presented thus far.

I think that may be enough thinking outloud from me for one night. you guys have fed me a lot of good info today, i really appreciate it. i'll get back to it tomorrow after work and post another 100 thoughts.


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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sun Jun 16, 2013 8:52 pm
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The thing is, I think you have done everything you can by just checking DC voltages and resistors. I really think you need to send a signal through the amp, even if it is just into the PI and look at waveforms to see what is distorting and where.

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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Mon Jun 17, 2013 6:08 pm
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bluesky636 wrote:
The thing is, I think you have done everything you can by just checking DC voltages and resistors. I really think you need to send a signal through the amp, even if it is just into the PI and look at waveforms to see what is distorting and where.


yeah, I think i'm to that point. I've satisfied all my other concerns. I took the preamp out and ran it into another amp and everything sounded fine.

My next step would be to put a 1kHz sine at the input, correct? and see what the signal looks like starting at TP23 and so on? I'll start tracking down a signal generator and an oscilloscope. is there a poor mans test method I can try in the mean time? say an e-bow and a signal probe, hah.


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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Mon Jun 17, 2013 9:06 pm
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spint3 wrote:
bluesky636 wrote:
The thing is, I think you have done everything you can by just checking DC voltages and resistors. I really think you need to send a signal through the amp, even if it is just into the PI and look at waveforms to see what is distorting and where.


yeah, I think i'm to that point. I've satisfied all my other concerns. I took the preamp out and ran it into another amp and everything sounded fine.

My next step would be to put a 1kHz sine at the input, correct? and see what the signal looks like starting at TP23 and so on? I'll start tracking down a signal generator and an oscilloscope. is there a poor mans test method I can try in the mean time? say an e-bow and a signal probe, hah.


I usually start testing at the far end opposite the input, like at the speaker, but either way works. Do you have a dummy load? If not, I would put the speaker on a long cable or face it away from you in case things get loud.

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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Mon Jun 17, 2013 10:01 pm
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Since you verified the preamp already, start with the signal injected into Power Amp In, 1 kHz at about 1 volt (variable). :idea:

I still think that you can figure it out with just a meter, and I'd bet money that it is something simple, maybe even so simple that you overlooked it. Solder joint or broken trace perhaps. :?:

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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Tue Jun 18, 2013 3:11 pm
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Let me preface this by saying: I will gladly except a smack in the face for the following procedure, i know it is the correct way to do it, but while I'm waiting for a signal generator, I did the following.

So I took my microkorg and played a 1k tone out of it and plugged it into the power amp in. starting with the volume off, i cranked it up until I hopefully read 971mV at TP23. Well, the little guy didn't have enough juice to get me up to a volt. It topped out at 730mV. So with 730mV applied I took measurements at all the test points there after.

TP23: 730mVAC
TP24: 17.5VAC
TP25: 17.4VAC
TP26: 41.1VDC
TP27: 13.3VAC
TP28: 15.3VAC

Is this semi usable data? I'm well aware it's not ideal, or even decent for that matter. But I figured something is better than nothing until i can get the generator out of storage. you can respond by saying "::sigh:: be patient and wait for the correct equipment before I make you eat lead paint"


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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Tue Jun 18, 2013 6:19 pm
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That is exactly what you would do with a dedicated signal generator, seems perfectly acceptable to me. It seems that your output tubes aren't producing enough voltage swing, but the signal at TP24 and TP25 look about right. So, something between TP24/TP25 and TP27/TP28. That narrows it down quite a bit.

What is the negative bias DC voltage, and the AC voltage at pin 5 of each output tube?

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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Wed Jun 19, 2013 7:44 am
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The negative DC bias voltage (pin 5) is:
V4: -52.9VDC
V5: -53.3VDC

The AC is at 17VAC on both.


Last edited by spint3 on Wed Jun 19, 2013 7:53 pm, edited 1 time in total.

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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Wed Jun 19, 2013 6:43 pm
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That tells me that the coupling caps are good, and the bias circuit is good. Yet, the output from the tubes seems too low. Are you sure that the voltage at the screen grid (pin 4) of both tubes is correct? At this point, it seems that you aren't getting the correct screen voltage at the tubes, which should be approx 480 VDC. Be sure to check at the tube socket solder connection. Maybe your initial problem was the Plate resistor on the PI, then something went wrong with the screen circuit when you replaced those resistors?

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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Wed Jun 19, 2013 7:42 pm
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Pins 4 on both tubes read 473VDC

when i said 4 and 5 earlier it was meant to say V4 and V5, to list each tube separately since there was a slight difference.


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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Wed Jun 19, 2013 8:12 pm
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So upon looking at the schematic for the last fifteen minutes and processing things in my head, what is there left to be bad? those couple diodes or the output transformer? solder looks good and i've reflowed them the other day just to be sure. before its asked, i did blast the socket with deox-it, and it looks brand new, overall the amp doesn't have a spec of rust or corrosion anywhere.

There is a slight chance something was dead shorted when the amp failed. i found one of the board screws laying in the bottom of the chassis. it was the furthest one above the output jacks, i figured gravity took it straight down, and it is not over the board. there were no signs of a short on the board. I should have said that earlier, but i didn't really think about it until now.


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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Wed Jun 19, 2013 9:03 pm
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Since the AC voltages are low on the primary side of the output transformer, I would say the OT is OK. The diodes are to prevent damage from flyback voltages. The amp will operate without them. I too wonder if they could be bad. Easy way to find out is to just disconnect them.

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