Funny sound as note decays?

MajorNut1967 · 10-17-2009, 12:28 AM

Ok amp guru's heres a chance to prove yourself. Model 1987 Marshall: as a chord or single note is in the final stages of Decay it starts to crackle and sound like it is breaking up, the single notes are never a clean note. Sound like when you are continue to play after you have shut off the mains, but left the standby on! If you bash a power chord sounds great till the decay! Any takers?

Wilder Amplification · 10-17-2009, 04:37 AM

Quote:

Originally Posted by MajorNut1967

Ok amp guru's heres a chance to prove yourself. Model 1987 Marshall: as a chord or single note is in the final stages of Decay it starts to crackle and sound like it is breaking up, the single notes are never a clean note. Sound like when you are continue to play after you have shut off the mains, but left the standby on! If you bash a power chord sounds great till the decay! Any takers?

Crossover distortion.

Doesn't sound like that at normal amplitude since the signal peaks are much higher than the crossover notch. Once the note decays, the peaks become closer to the amplitude of the notch, causing the notch to become more audible.

This is my theory on it anyway.

Is this a 1987 reissue? Where is the bias set at?

MajorNut1967 · 10-17-2009, 11:08 AM

Quote:

Originally Posted by Wilder Amplification

Crossover distortion.

Doesn't sound like that at normal amplitude since the signal peaks are much higher than the crossover notch. Once the note decays, the peaks become closer to the amplitude of the notch, causing the notch to become more audible.

This is my theory on it anyway.

Is this a 1987 reissue? Where is the bias set at?

I'll get you some numbers in a minute and it a real 1968 50 watt.

MajorNut1967 · 10-17-2009, 11:58 AM

ok just to fore warn you this amp has aways had real low B+ & plate voltages, but always sounded great. ok B+ 419, start winding plate 375v @ 50mA, finish winding plate 376 @ 42mA Bias voltage at the split -30.75

Wilder Amplification · 10-17-2009, 12:12 PM

Quote:

Originally Posted by MajorNut1967

ok just to fore warn you this amp has aways had real low B+ & plate voltages, but always sounded great. ok B+ 419, start winding plate 375v @ 50mA, finish winding plate 376 @ 42mA Bias voltage at the split -30.75

Low B+ compared to most Marshalls yes. However if you look at the tube data sheet you'll see that 420 volts is actually the recommended for Class AB1 operation so you should be able to bias out the crossover notch.

However I am seeing quite a bit of imbalance between the two tubes. When tubes are matched they should be within 2-3mA of each other, whereas you've got an 8mA difference between yours. Also, the one that's biased at 50mA is WAY too hot. Bias on that one shouldn't be any higher than about 45mA @ 375VDC going by the 70% plate dissipation rule.

What's the DC resistance of your OT from the HT fuse to pin 3? With a 45 volt difference between plate and main B+ it sounds to me like the valves are drawing a bit more current than that, but not sure since I don't know what the DC resistance of your start and finish primaries are relative to the center tap.

Also, how are you measuring your bias current?

MajorNut1967 · 10-17-2009, 12:36 PM

Quote:

Originally Posted by Wilder Amplification

Low B+ compared to most Marshalls yes. However if you look at the tube data sheet you'll see that 420 volts is actually the recommended for Class AB1 operation so you should be able to bias out the crossover notch.

However I am seeing quite a bit of imbalance between the two tubes. When tubes are matched they should be within 2-3mA of each other, whereas you've got an 8mA difference between yours. Also, the one that's biased at 50mA is WAY too hot. Bias on that one shouldn't be any higher than about 45mA @ 375VDC going by the 70% plate dissipation rule.

What's the DC resistance of your OT from the HT fuse to pin 3? With a 45 volt difference between plate and main B+ it sounds to me like the valves are drawing a bit more current than that, but not sure since I don't know what the DC resistance of your start and finish primaries are relative to the center tap.

Also, how are you measuring your bias current?

bias probe for the tool. CT to start 39.4ohms finish 40.4ohms

Wilder Amplification · 10-17-2009, 02:49 PM

OK that plate voltage is way off. With 40mA going through the 40 ohms DC resistance of one side of the OT, you should have about a 1.6 volt difference between the plate and the CT give or take (40mA x 40 ohms = 1.6VDC). That 45 volt difference (419 and 375) just does not seem right at all.

MajorNut1967 · 10-17-2009, 03:14 PM

Ok new info! I bypassed the preamp stages and injected signal directly into the PI and the sound is immaculate no problems at all! so its some where in the Preamp or tone stack. More later. And you are right the power tubes are not matched well and they are about a year old now (TUNG-SOL EL34)

I goofed you earlier I meant the B+ was 383.2 not 419 sorry that was with No-Load at all. I have replaced all the preamp tubes. I'm going to test the preamp stand alone later. you guys notice the regular Guru's aren't chimning in on this LOL

Wilder Amplification · 10-17-2009, 06:35 PM

Quote:

Originally Posted by MajorNut1967

Ok new info! I bypassed the preamp stages and injected signal directly into the PI and the sound is immaculate no problems at all! so its some where in the Preamp or tone stack. More later. And you are right the power tubes are not matched well and they are about a year old now (TUNG-SOL EL34)

I goofed you earlier I meant the B+ was 383.2 not 419 sorry that was with No-Load at all. I have replaced all the preamp tubes. I'm going to test the preamp stand alone later. you guys notice the regular Guru's aren't chimning in on this LOL

Do you mean no LOAD or no SIGNAL? Just FYI so you don't lose an output transformer, I'll mention that it's not good for a valve amp to be operated without a speaker or a dummy load hooked up to the amp.

In the case of it being in the preamp...sounds to me like you may have a filter cap or a coupling cap that's about to go south on ya.

MajorNut1967 · 10-17-2009, 09:01 PM

Quote:

Originally Posted by Wilder Amplification

Do you mean no LOAD or no SIGNAL? Just FYI so you don't lose an output transformer, I'll mention that it's not good for a valve amp to be operated without a speaker or a dummy load hooked up to the amp.

In the case of it being in the preamp...sounds to me like you may have a filter cap or a coupling cap that's about to go south on ya.

When I say "no load" I mean: no tubes, no speakers, no signal.

Wilder Amplification · 10-17-2009, 09:25 PM

Quote:

Originally Posted by MajorNut1967

When I say "no load" I mean: no tubes, no speakers, no signal.

Oh...no tubes = nothing drawing current = voltages mean nothing. That makes sense.

I think the next tech article I'll do will be on how to open a tube...of toothpaste that is.

MartyStrat54 · 10-17-2009, 09:28 PM

Major, I know that you are very capable of working on your own amps (I mean you have built some). Do you think that you isolated it in the tone stack? I just hope it isn't filter caps. Maybe it could be something as simple as a microphonic tube in the tone stack.

MajorNut1967 · 10-18-2009, 02:12 AM

Quote:

Originally Posted by MartyStrat54

Major, I know that you are very capable of working on your own amps (I mean you have built some). Do you think that you isolated it in the tone stack? I just hope it isn't filter caps. Maybe it could be something as simple as a microphonic tube in the tone stack.

Ya Marty this thing is trying me. I tested the circuits in isolation. Individually they all work fine, put them together total crap. I checked all the coupling caps they are all fine. Next gona run down the Filter Caps.

Wilder Amplification · 10-18-2009, 02:18 AM

Quote:

Originally Posted by MajorNut1967

Ya Marty this thing is trying me. I tested the circuits in isolation. Individually they all work fine, put them together total crap. I checked all the coupling caps they are all fine. Next gona run down the Filter Caps.

Yeah I'm bettin' on filter caps as well. Sounds like intermodulation distortion since they work fine when they're apart and go to shit when they're together. Are those the original caps? If so, I'd DEFINITELY change them out.

MajorNut1967 · 10-20-2009, 02:22 AM

Hey further updates: I changed out all the filter caps and its doing the same thing. It gets worse from about 75% to 100% on volume settings. its oscillating slightly when sitting flat on the bench. It's Fxxxxxx weird. lol I put a sub OT in still sound like its crapping. All voltages are constant and fluctuate normally according load. and the cap were replaced with F&T's about 3 years ago. head scratching over here?

Wilder Amplification · 10-20-2009, 02:33 AM

Quote:

Originally Posted by MajorNut1967

Hey further updates: I changed out all the filter caps and its doing the same thing. It gets worse from about 75% to 100% on volume settings. its oscillating slightly when sitting flat on the bench. It's Fxxxxxx weird. lol I put a sub OT in still sound like its crapping. All voltages are constant and fluctuate normally according load. and the cap were replaced with F&T's about 3 years ago. head scratching over here?

Have you tried the amp on a different cab yet?

MajorNut1967 · 10-20-2009, 02:49 AM

Quote:

Originally Posted by Wilder Amplification

Have you tried the amp on a different cab yet?

Yes on three different cabs: a 4x12 1960, a 2x12 Bassman, and a 4x10 Acoustic cab.

Wilder Amplification · 10-20-2009, 03:39 AM

Quote:

Originally Posted by MajorNut1967

Yes on three different cabs: a 4x12 1960, a 2x12 Bassman, and a 4x10 Acoustic cab.

Since you didn't mention ever changing power valves, I would try that next before going any further. Let's rule those out as the culprit.

PaoloJM · 10-20-2009, 06:04 AM

Major,
Your B+ does seem quite low to me.
What are your reading on the PT secondary in Vac, before the rectifier with no valves installed? We can then guesstimate what the B+ should be with valves installed. It might be ok but I think you should check.

A complete voltage chart would really tell a story here.
Have you done one, especially for the preamp? Can you post it?
My guess is that your excess voltage drop and this problem may be linked, eg resistor(s) somewhere in the preamp drifting in value etc.

The other thing to look for is wires in the preamp that may have been moved or disturbed. Not having heard the sound I can't tell, but the sound you describe could be some parasitics being picked up from a signal wire routed in a bad spot.

Can you post some gut shots too?

Wilder Amplification · 10-20-2009, 06:19 AM

Quote:

Originally Posted by PaoloJM

Major,
Your B+ does seem quite low to me.
What are your reading on the PT secondary in Vac, before the rectifier with no valves installed? We can then guesstimate what the B+ should be with valves installed. It might be ok but I think you should check.

A complete voltage chart would really tell a story here.
Have you done one, especially for the preamp? Can you post it?
My guess is that your excess voltage drop and this problem may be linked, eg resistor(s) somewhere in the preamp drifting in value etc.

The other thing to look for is wires in the preamp that may have been moved or disturbed. Not having heard the sound I can't tell, but the sound you describe could be some parasitics being picked up from a signal wire routed in a bad spot.

Can you post some gut shots too?

I agree also with this. For all we know, the amp very well could have had a low B+ the whole time he's owned it like he mentioned, but could also be a sign of a power transformer going south. Usually when they do they'll short and blow the mains fuse immediately upon power up, but I have seen stranger things.

Another thing too...I wonder if 1 of the diodes in the rectifier circuit may have blown open, essentially turning itself into a half wave rectifier, which would also cause a low B+.

MartyStrat54 · 10-20-2009, 05:21 PM

Quote:

Originally Posted by Wilder Amplification

I agree also with this. For all we know, the amp very well could have had a low B+ the whole time he's owned it like he mentioned, but could also be a sign of a power transformer going south. Usually when they do they'll short and blow the mains fuse immediately upon power up, but I have seen stranger things.

Another thing too...I wonder if 1 of the diodes in the rectifier circuit may have blown open, essentially turning itself into a half wave rectifier, which would also cause a low B+.

Say this is the case, I think the PT would have dumped on itself way before now. I guess a lot has to do with the amount of operation time. Also, the Major does like to crank his amps.

I would guess that the Major has already scoped his power supply. A blown diode would show an increase in ripple. However, it is a good thought.

I think John's idea of doing a voltage flow chart could help get a better understanding of how the amp is working. There are a multitude of things that could be causing these symptoms. I also agree that all of the tubes need to be certified as good.

MajorNut1967 · 10-20-2009, 06:26 PM

Quote:

Originally Posted by Wilder Amplification

Since you didn't mention ever changing power valves, I would try that next before going any further. Let's rule those out as the culprit.

I subbed some brand new NOS Sylvania 6CA7's in it and crapped!

Wilder Amplification · 10-20-2009, 07:12 PM

BTW...can we get some photos of the insides of the amp? Something tells me that something is amiss in the power transformer, the rectifier, or the multitap primary is miswired or the voltage selector on the back is on the wrong setting. 383.2 volts B+ with no valves installed is quite a bit low, even for a 50 watter.

MartyStrat54 · 10-20-2009, 07:34 PM

Quote:

Originally Posted by MajorNut1967

I subbed some brand new NOS Sylvania 6CA7's in it and crapped!

Are you saying, "Crapped" in a good way or a bad way? Those particular 6CA7's can take a lot of voltage. They'd be hardly working with what your amp is putting out.

BTW-I scored me a NOS pair of these, Philips brand, double getters.

MajorNut1967 · 10-20-2009, 10:19 PM

Quote:

Originally Posted by MartyStrat54

Are you saying, "Crapped" in a good way or a bad way? Those particular 6CA7's can take a lot of voltage. They'd be hardly working with what your amp is putting out.

BTW-I scored me a NOS pair of these, Philips brand, double getters.

Hi Marty,

I meant the bloody thing sounds like it fell out of my arse this morning!

MartyStrat54 · 10-20-2009, 10:25 PM

Wow! I'd hate to see that.

Sorry that it's still sucking. Those 6CA7's are nice tubes. Maybe you should take them out until you get this figured out. I'd hate to see you ruin those nice tubes.

MajorNut1967 · 10-20-2009, 11:15 PM

Ok here Numbers and pics later!

Input voltage: 125.41vac
PT “no-load” outputs:
• HT 275-0-275 vac @ 173mA
• Filaments 6.7 vac @ 6amps
Rect. Voltage no cap 363.12vdc
Rect. Voltage with 100uf cap 383.26 “no load”
Standby on:
• Plate voltage start: 376.45vdc
• Plate voltage finish: 375.69vdc
Then the Marshall standard test points as follows:
1. .966v
2. 186.4v
3. 1.63v
4. 220.2v
5. 236.1v
6. 263.5v
7. 1.25v
8. 150.3v
9. 143.2v
10. 295.6v
11. 22.3v
12. 184.7v
13. 175.9v
At the bias split: -30.78

Wilder Amplification · 10-20-2009, 11:29 PM

Quote:

Originally Posted by MajorNut1967

Ok here Numbers and pics later!

Input voltage: 125.41vac
PT “no-load” outputs:
• HT 275-0-275 vac @ 173mA
• Filaments 6.7 vac @ 6amps
Rect. Voltage no cap 363.12vdc
Rect. Voltage with 100uf cap 383.26 “no load”
Standby on:
• Plate voltage start: 376.45vdc
• Plate voltage finish: 375.69vdc
Then the Marshall standard test points as follows:
1. .966v
2. 186.4v
3. 1.63v
4. 220.2v
5. 236.1v
6. 263.5v
7. 1.25v
8. 150.3v
9. 143.2v
10. 295.6v
11. 22.3v
12. 184.7v
13. 175.9v
At the bias split: -30.78

Well, based on the incoming VAC RMS x 1.414 rule you're rectified/filtered "no load" voltage is within 2% of my calculation based on an RMS of 275VAC (I calculated 388.85VDC out filtered). Is that transformer original?

Also, what schematic are you going off of when you refer to the "standard Marshall test points"?

MajorNut1967 · 10-20-2009, 11:33 PM

Wilder Amplification · 10-20-2009, 11:37 PM

WTF!? Your screen resistors are missing.

10-17-2009, 12:28 AM	#1 (permalink)
MajorNut1967 Senior Member Join Date: Jul 2009 Location: South Pacific Posts: 710	Funny sound as note decays? Ok amp guru's heres a chance to prove yourself. Model 1987 Marshall: as a chord or single note is in the final stages of Decay it starts to crackle and sound like it is breaking up, the single notes are never a clean note. Sound like when you are continue to play after you have shut off the mains, but left the standby on! If you bash a power chord sounds great till the decay! Any takers? __________________ MJN1967 aka BA (Bad Attitude) 1968 model 1987 50watt. A Baby Major (Half-Power) I built from the ground up. JTM 45 Clone combo Ground up build. JCM 800 Clone Ground up build. Custom Built Warmoth Strat 75' Fender Tele-Deluxe '70 style strat custom build w/Custom Shop 69's PU's

10-17-2009, 11:58 AM	#4 (permalink)
MajorNut1967 Senior Member Join Date: Jul 2009 Location: South Pacific Posts: 710	Re: Funny sound as note decays? ok just to fore warn you this amp has aways had real low B+ & plate voltages, but always sounded great. ok B+ 419, start winding plate 375v @ 50mA, finish winding plate 376 @ 42mA Bias voltage at the split -30.75 __________________ MJN1967 aka BA (Bad Attitude) 1968 model 1987 50watt. A Baby Major (Half-Power) I built from the ground up. JTM 45 Clone combo Ground up build. JCM 800 Clone Ground up build. Custom Built Warmoth Strat 75' Fender Tele-Deluxe '70 style strat custom build w/Custom Shop 69's PU's

10-17-2009, 03:14 PM	#8 (permalink)
MajorNut1967 Senior Member Join Date: Jul 2009 Location: South Pacific Posts: 710	Re: Funny sound as note decays? Ok new info! I bypassed the preamp stages and injected signal directly into the PI and the sound is immaculate no problems at all! so its some where in the Preamp or tone stack. More later. And you are right the power tubes are not matched well and they are about a year old now (TUNG-SOL EL34) I goofed you earlier I meant the B+ was 383.2 not 419 sorry that was with No-Load at all. I have replaced all the preamp tubes. I'm going to test the preamp stand alone later. you guys notice the regular Guru's aren't chimning in on this LOL __________________ MJN1967 aka BA (Bad Attitude) 1968 model 1987 50watt. A Baby Major (Half-Power) I built from the ground up. JTM 45 Clone combo Ground up build. JCM 800 Clone Ground up build. Custom Built Warmoth Strat 75' Fender Tele-Deluxe '70 style strat custom build w/Custom Shop 69's PU's

10-17-2009, 09:28 PM	#12 (permalink)
MartyStrat54 Senior Member Join Date: Feb 2009 Location: Formerly Phoenix, now north of KC Posts: 2,583	Re: Funny sound as note decays? Major, I know that you are very capable of working on your own amps (I mean you have built some). Do you think that you isolated it in the tone stack? I just hope it isn't filter caps. Maybe it could be something as simple as a microphonic tube in the tone stack. __________________ MARTY 2009 Gibson Explorer-Hell Raising Machine 1989 Fender Strat Plus USA 2008 Fender Tele USA 2004 Schecter Elite w/DiMarzio's TSL100 with custom 4x12 cab. TSL122 with Man 'O War Speakers. TSL602 #1 with JBL D123 Speakers and 602 #2 with Altec 417 Speakers. "Oh so sweet my friends." DSL401 #1 with Red Fang Speaker and 401 #2 with JBL D123 Speaker. Tubes: Let's just say they are NOS that kick ass.

10-20-2009, 02:22 AM	#15 (permalink)
MajorNut1967 Senior Member Join Date: Jul 2009 Location: South Pacific Posts: 710	Re: Funny sound as note decays? Hey further updates: I changed out all the filter caps and its doing the same thing. It gets worse from about 75% to 100% on volume settings. its oscillating slightly when sitting flat on the bench. It's Fxxxxxx weird. lol I put a sub OT in still sound like its crapping. All voltages are constant and fluctuate normally according load. and the cap were replaced with F&T's about 3 years ago. head scratching over here? __________________ MJN1967 aka BA (Bad Attitude) 1968 model 1987 50watt. A Baby Major (Half-Power) I built from the ground up. JTM 45 Clone combo Ground up build. JCM 800 Clone Ground up build. Custom Built Warmoth Strat 75' Fender Tele-Deluxe '70 style strat custom build w/Custom Shop 69's PU's

10-20-2009, 06:04 AM	#19 (permalink)
PaoloJM Senior Member Join Date: Jun 2008 Location: Cork, Ireland Posts: 534	Re: Funny sound as note decays? Major, Your B+ does seem quite low to me. What are your reading on the PT secondary in Vac, before the rectifier with no valves installed? We can then guesstimate what the B+ should be with valves installed. It might be ok but I think you should check. A complete voltage chart would really tell a story here. Have you done one, especially for the preamp? Can you post it? My guess is that your excess voltage drop and this problem may be linked, eg resistor(s) somewhere in the preamp drifting in value etc. The other thing to look for is wires in the preamp that may have been moved or disturbed. Not having heard the sound I can't tell, but the sound you describe could be some parasitics being picked up from a signal wire routed in a bad spot. Can you post some gut shots too? __________________ www.youtube.com/paolojm

10-20-2009, 10:25 PM	#26 (permalink)
MartyStrat54 Senior Member Join Date: Feb 2009 Location: Formerly Phoenix, now north of KC Posts: 2,583	Re: Funny sound as note decays? Wow! I'd hate to see that. Sorry that it's still sucking. Those 6CA7's are nice tubes. Maybe you should take them out until you get this figured out. I'd hate to see you ruin those nice tubes. __________________ MARTY 2009 Gibson Explorer-Hell Raising Machine 1989 Fender Strat Plus USA 2008 Fender Tele USA 2004 Schecter Elite w/DiMarzio's TSL100 with custom 4x12 cab. TSL122 with Man 'O War Speakers. TSL602 #1 with JBL D123 Speakers and 602 #2 with Altec 417 Speakers. "Oh so sweet my friends." DSL401 #1 with Red Fang Speaker and 401 #2 with JBL D123 Speaker. Tubes: Let's just say they are NOS that kick ass.

10-20-2009, 11:15 PM	#27 (permalink)
MajorNut1967 Senior Member Join Date: Jul 2009 Location: South Pacific Posts: 710	Re: Funny sound as note decays? Ok here Numbers and pics later! Input voltage: 125.41vac PT “no-load” outputs: • HT 275-0-275 vac @ 173mA • Filaments 6.7 vac @ 6amps Rect. Voltage no cap 363.12vdc Rect. Voltage with 100uf cap 383.26 “no load” Standby on: • Plate voltage start: 376.45vdc • Plate voltage finish: 375.69vdc Then the Marshall standard test points as follows: 1. .966v 2. 186.4v 3. 1.63v 4. 220.2v 5. 236.1v 6. 263.5v 7. 1.25v 8. 150.3v 9. 143.2v 10. 295.6v 11. 22.3v 12. 184.7v 13. 175.9v At the bias split: -30.78 __________________ MJN1967 aka BA (Bad Attitude) 1968 model 1987 50watt. A Baby Major (Half-Power) I built from the ground up. JTM 45 Clone combo Ground up build. JCM 800 Clone Ground up build. Custom Built Warmoth Strat 75' Fender Tele-Deluxe '70 style strat custom build w/Custom Shop 69's PU's

10-20-2009, 11:33 PM	#29 (permalink)
MajorNut1967 Senior Member Join Date: Jul 2009 Location: South Pacific Posts: 710	Re: Funny sound as note decays? __________________ MJN1967 aka BA (Bad Attitude) 1968 model 1987 50watt. A Baby Major (Half-Power) I built from the ground up. JTM 45 Clone combo Ground up build. JCM 800 Clone Ground up build. Custom Built Warmoth Strat 75' Fender Tele-Deluxe '70 style strat custom build w/Custom Shop 69's PU's

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