BLONDE BASSMAN HUM

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BLONDE BASSMAN HUM

Postby GRIDBIAS on Sun Jan 08, 2017 11:29 am

I recently found an old Bassman head which had been modified with an extra choke and power transformer as the original ones were blown. Funnily enough they left the old ones in !
I replaced the PT with a Mojo 762EX as the original was an export model with the voltage selector pin and socket arrangement inside the chassis.
The choke was replaced with a new f 036486 part. The doghouse caps were leaky and bubbled so I replaced them with 22uf 500v f+t's, the 25uf's on the fibreboard were gone so I replaced them with Sprauge's as well as the bias cap and its associated resistors.
Powering on I get high voltages running on 240vac (GB Based),straight from the rectifier 493v, -58 bias voltage. Using a variac down to 225vac I get
460v and -54 which is what I have been running it on.
The amp works but it has a loud hum clearly audible with the two volume controls on zero, the hum gets louder when the volume's are turnned up but not a great ammount. Slightly louder on the bass input.
I have checked the input jack grounding switches and the blue poly caps for any dc leakage by lifting the low voltage end but they all read to zero.
I spent all day on this checking everywhere and most of the resistors and voltages are close enough to the schematic but for pins 1 and 6 on v2 7025. The sheet says +150 vdc but I am getting only 63vdc. I am getting 207vdc at the junction of the two 100k's
Any idea's anyone ?
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Re: BLONDE BASSMAN HUM

Postby Baron Von Machinenmann on Sun Jan 08, 2017 2:19 pm

Hum with volume down indicate trouble is in the power section, if the components check out it could also be tube related.
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Re: BLONDE BASSMAN HUM

Postby stratele52 on Sun Jan 08, 2017 3:48 pm

Filter caps are reverse or bad, Bias cap are reverse or bad.
Tubes heater center tap from power transformer do not work.
Primary wires from output transformer are reverse if OT was replaced.
Output tubes are unmatched by far, one is dead.
Noisy preamp tube(s)

Read AC volts on all DC supplys, you must have less than 0.1 volts AC
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Re: BLONDE BASSMAN HUM

Postby Vortexion on Sun Jan 08, 2017 10:27 pm

stratele52 wrote:Filter caps are reverse, Bias cap are reverse.

No. If any electrolytics were reversed, they would explode as soon as the amp was turned on.

stratele52 wrote:Primary wires from output transformer are reverse if OT was replaced.

Again, no. If these wires were reversed, you would hear a deafening screech as soon as the amp was turned on, because the negative feedback loop would now be acting as a positive feedback loop.

I agree with the other possibilities, though.
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Re: BLONDE BASSMAN HUM

Postby stratele52 on Mon Jan 09, 2017 10:22 am

Hum; check for bad solder if you are a beginner in soldering
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Re: BLONDE BASSMAN HUM

Postby GRIDBIAS on Mon Jan 09, 2017 11:08 am

Thanks for the replies, I am away at the moment and wont get a chance to check it out till later in the week. Heres what I do know.
I swapped all the tubes with a working bassman so thats not it.
The Mojo transformer does not have a centre tap for the heater voltage ( unless its internal )only the other red/yellow wire for the bias ground is earthed.
I replaced all the electrolytics, observing the polarity carefully, could be a bad new one though, unlikely but does happen, is there any easy way to check the doghouse caps ?
Output transformer is original checks out fine resistance wise and has not been touched.
Been soldering professionally for 40+years.
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Re: BLONDE BASSMAN HUM

Postby Vortexion on Mon Jan 09, 2017 11:39 am

GRIDBIAS wrote:The Mojo transformer does not have a centre tap for the heater voltage ( unless its internal )only the other red/yellow wire for the bias ground is earthed.

You could check to find out whether the heater winding is internally earthed by disconnecting both legs of it from the rest of the circuit, and - with the amp fully switched off and caps dumped, of course - do a DC resistance test between the two legs of the winding and again between each leg and chassis/earth. If there's no centre tap to earth and the transformer is healthy, on this latter test you should get an infinity reading. If you can find a measurable resistance to earth from either/both legs, then either (a) it's faulty! or (b) it's got an internally earthed centre-tap (although, TBH I've never come across a tranny made like that). As you know, all heater supplies must be tied to signal earth at some point, so if the winding hasn't got a centre tap you can create an artificial one using a couple of low-value resistors. A fuller explanation with diagrams is available on this page:

http://www.valvewizard.co.uk/heater.html - just scroll down to the paragraph headed "Artificlal Centre Tap".

GRIDBIAS wrote:I replaced all the electrolytics, observing the polarity carefully, could be a bad new one though, unlikely but does happen, is there any easy way to check the doghouse caps ?

Not really, unless you've got an old-school capacitor tester (I don't mean the "find out what value it is" type you get in a modern multimeter), which can test the component's Equivalent Series Resistance (ESR) at or near its working voltage. ESR is the only genuine test of capacitor health. As you say, though, the likelihood of a brand new cap being bad is pretty slim.
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Re: BLONDE BASSMAN HUM

Postby GRIDBIAS on Mon Jan 09, 2017 4:22 pm

Trip cancelled so had a go at the amp.
Checked out the heater winding and there was no reading to ground so I did the 100 ohm resistor to ground mod and the buzz is now gone completely ! Thanks guys !
The original transformer must have had a centre tap to ground, it was in such a bad state I couldnt tell as the wires were cut tight at the neck of the cover. I recently tried the 100 ohm trick on a Marshall 1930 10 watt and it made no difference, that amp still buzzes like hell so I didnt think it was worth trying it on the Bassman, I wasted a lot of time checking the rest of the amp because of that.
The normal channel on the Bassman is at least half as loud as the bass channel, I dont think this is right. I think it could be those low voltage readings on pins 1 and 6 on v2 7025. The sheet says +150 vdc but I am getting only 63vdc. I am getting 207vdc at the junction of the two 100k's.
Any 6g6b owners out there taken readings from theirs to compare ? Do any UK owners run their amps on 240vac with 490vdc from the rectifier ?
I will try and post some pics tonight but it looks like the site only takes very small images unless I can find a way around that.
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Re: BLONDE BASSMAN HUM

Postby stratele52 on Mon Jan 09, 2017 5:37 pm

Two 100 ohms is a must if you don't have heater center tap.

This do not fix all hum issue, you have to find where hum come from first.
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Re: BLONDE BASSMAN HUM

Postby GRIDBIAS on Wed Jan 11, 2017 3:30 pm

Quick pic.
B2.JPG
Heres a rough pic before the 100 ohms went in.
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Re: BLONDE BASSMAN HUM

Postby GRIDBIAS on Fri Jan 13, 2017 2:44 pm

Made an error, the BASS channel is the quiet one, about half the volume of the NORMAL one, any suggestions ?
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Re: BLONDE BASSMAN HUM

Postby stratele52 on Fri Jan 13, 2017 6:17 pm

Bass Channel ;

Wrong volume pot resistance ? Too low, or short on tab to ground
Tube ?
Heater voltage too low.
Read voltages at tube

Schematic;

http://ampwares.com/schematics/bassman_6g6a.pdf
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Re: BLONDE BASSMAN HUM

Postby GRIDBIAS on Mon Jan 16, 2017 12:50 pm

Found the second 7025 was down on output and all volumes are the same now with a new tube in although the voltages on the pins are still low.
Strange when the rest of the voltages are high with 493vdc from the rectifier. I suppose its within the +or- 20% stated on the schematic ?
Does anyone else have such high readings. Bias is still -56.
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