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Post subject: Bias adjustment on a fender bassman50 silverface
Posted: Mon Sep 15, 2014 5:21 pm
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Hi all,

One of my dear friend's bass amp poped a fuse during practise last saturday.
I was able to replace one of the resistors on one of the output tubes, to me these seem to be the original ones.

My friend wants to put in new 6l6gc tubes, we think one of the tubes might be dead/very weak!.
The amp works fine for now but i need to know if i have to do a bias adjustment with the new tubes and need the procedure to do it.

if anybody can guide me into doing the bias ajustment it would be greatly appreciated!.

Thanks!
:)


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Post subject: Re: Bias adjustment on a fender bassman50 silverface
Posted: Mon Sep 15, 2014 5:38 pm
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If you don't know how to do a re-bias DO NOT attempt it, there is enough voltage in the Caps to Kill you! This is the time to take it to a qualified tech.

T2

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Post subject: Re: Bias adjustment on a fender bassman50 silverface
Posted: Mon Sep 15, 2014 7:21 pm
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Thank you for re-plying, BTW i am an electronics technician and i am fully aware of what is involved with these types of amplifiers and high voltages , although i have not done many repairs over the years with tube amplifiers, nonetheless i can do almost any repairs but just needed advise if re-biasing is needed when replacing the power tubes and the procedure, i do have all the electronic equipement at home in my workshop!.
regards , jean-pierre


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Post subject: Re: Bias adjustment on a fender bassman50 silverface
Posted: Mon Sep 15, 2014 8:52 pm
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You need to find the plate voltage on those 6L6GC. Adjust the idle bias current draw so that the idle dissipation is around 18-20 watts per tube (around 60% of 30 watt max).

Where watts = (plate voltage, in Volts) times (idle current, in Amps).


You will need a way to measure the idle bias current draw. There are devices that insert between the tube and the socket & can be measured with a DVM or have their own meter. Like the Bias Probe (this one also measures plate voltages) :


http://www.eurotubes.com/eurotubes-Bias ... oltage.htm



The bias adjustment screw/pot is behind the choke, on the chassis. You can see it next to the second tranny from the left, in the photo. This amp has been pulled from the cab and is upside-down, in terms of orientation --- compared to the amp in the cab.

Most 6L6GC Fender amps with a healthy PSU put around 450-475 volts onto the plates. At this range, the adjusted idle mA measured should be around 38-40mA (or 0.038 to 0.040 amps)

-----

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Post subject: Re: Bias adjustment on a fender bassman50 silverface
Posted: Mon Sep 15, 2014 11:38 pm
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Hi!
Thank you for the info and the help! i appreciate it!.

i will keep you posted when i get a chance to get around to actually do the re-biasing, i just want to make sure that the new tubes that are going to be installed operate at the correct settings in order to get maximum performance.
i also requested from Fender support a copy of the schematic for the fender bassman50 whith a silver front face.

BTW i was looking today at some 6L6gc from mesaboogy brand there selling for 52.99 matched pair any thaughts on those! or should i get something better!.


Regards

jean-pierre


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Post subject: Re: Bias adjustment on a fender bassman50 silverface
Posted: Tue Sep 16, 2014 2:11 am
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For modern tube , Mesa Boogie are very good . Better tubes? Yes vintage tube, but cost lots of money . Do you need those ?

For bias value , use weber Bias calculator , easy and fast


http://www.webervst.com/tubes1/calcbias.htm


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Post subject: Re: Bias adjustment on a fender bassman50 silverface
Posted: Tue Sep 16, 2014 6:24 am
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Jean-Pierre Ethier wrote:
Hi!
Thank you for the info and the help! i appreciate it!.

BTW i was looking today at some 6L6gc from mesaboogy brand there selling for 52.99 matched pair any thaughts on those! or should i get something better!.

Regards

jean-pierre


Mesa-Boogie does not manufacture tubes. They buy, test, and select tubes from other manufacturers. Usually, Russian or Chinese made tubes. Similar to Groove Tubes. You can save some $$$ by buying tube from distributors of the those Russian and Chinese made tubes. Go with distributors that have good reputation.

For your Bassman 50, check out these tubes:

http://www.thetubestore.com/Tubes/6L6-5 ... -6L6GC-STR

http://www.thetubestore.com/Tubes/6L6-5 ... 7581-6L6GC

In checking some background info, the Bassman 50 originally had specs of 50 watts. Or 25 watts per tube. You may want to use heavier duty 6L6GC types (like the STR-6L6GC or the 7581), if you are truly going to run the amp at 50 watts. In my experience, the Electro-Harmonix 6L6GC can take the higher plate voltages and idle dissipation in these amps. The Tungsol 6L6GC -STR and 7581 are tough tubes, that work well in these Bassman and even in later silverface ultra-linear Fender amps.

If I were you, I'd set the idle bias to around 18-20 watts/per tube (36-40 watts) and listen to the amp. Try adjusting the bias a bit hotter (to say, 23 watts) and see if that sounds better in all volume ranges. Check for red-plating, signs of overheating, and/or too early onset of overdrive/distortion.

In general, the hotter the bias --- warmer the tone and earlier the OD. Also, the shorter the tube's life.

Good luck! Keep us informed. :mrgreen:


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Post subject: Re: Bias adjustment on a fender bassman50 silverface
Posted: Tue Sep 16, 2014 4:30 pm
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Sorry to butt in here, but it looks to me like the Bassman 50 only has a bias balance adjustment. Just my 2 cents.

http://el34world.com/charts/Schematics/files/fender/fender_bassman50.pdf

Regards,
vinyl

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Last edited by vinyl on Tue Sep 16, 2014 4:39 pm, edited 1 time in total.

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Post subject: Re: Bias adjustment on a fender bassman50 silverface
Posted: Tue Sep 16, 2014 4:34 pm
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vinyl wrote:
Sorry to butt in here, but it looks to me like the Bassman 50 only has a bias balance adjustment. Just my 2 cents.

Regards,
vinyl



You are right ; No bias adjustment just balance between tubes


http://el34world.com/charts/Schematics/ ... sman50.pdf


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Post subject: Re: Bias adjustment on a fender bassman50 silverface
Posted: Tue Sep 16, 2014 6:04 pm
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Sorry about the confusion. Yes, balance pot. You may have a hinky tube. Or as Stratele stated, need to look at the negative voltage produced by the bias supply. Should be around (-)48 VDC at the bias pot wiper. With the pot a mid-point.

If not, you need to look at the components (resistor, diode, caps) in the bias supply. And any bad connections from the bias supply diode (on separate board) to the pot.

Good luck!


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Post subject: Re: Bias adjustment on a fender bassman50 silverface
Posted: Wed Sep 17, 2014 1:29 am
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A balance pot can be modify in bias pot .

Jean-Pierre Ethier , first check If the amp need a new bias with 2 news tubes .

Did you know how to read bias with a meter ?


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Post subject: Re: Bias adjustment on a fender bassman50 silverface
Posted: Wed Sep 17, 2014 6:32 am
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Hi all !

First i have to mention that when i pulled the chassis out of the cabinet i took a good look at everything and noticed a burned 470 ohm 1 watt resistor under one of the power tubes, so i proceeded to replace the burned resistor and noticed after measuring the other resistor that looked good! i was getting about twice de value of the value of the resistor so again i decided to replace that one too!!.

according to the schematic these 2 resistors connect to the choke and to pin 4 of each tube.
Anyhow after repairing the amp i proceeded to re-install the chassis and to power it up.

The amp works, but i noticed on the 2 power tubes, one of them shows a small amount of blue glow! but the other tube (the one with the burned resistor) hardly had any blue glow at all, i suspect that that tube is either very weak/gone!!.
No sign of red plate on any of the power tubes.

I'm planning on getting a bias probe from eurotubes this will make adjusting the balance bias easier to do .

And yes! by looking at the schematic this amp is equiped with a bias balance pot!.

i also noticed a note on the schematic saying " WHEN INSTALLING NEW 6L6GC, RESET HUM BALANCE FOR MINIMUM HUM!"

Regards
Jean-pierre


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Post subject: Re: Bias adjustment on a fender bassman50 silverface
Posted: Wed Sep 17, 2014 7:44 am
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"The amp works, but i noticed on the 2 power tubes, one of them shows a small amount of blue glow! but the other tube (the one with the burned resistor) hardly had any blue glow at all, i suspect that that tube is either very weak/gone!!.

No sign of red plate on any of the power tubes. "


Blue glow is normal in output (power) tubes. It's just contaminants, collected during manufacturing, burning off. You may see an unsteady idle bias during this burn-off process. The tube should settle down to a steady bias, once most of these contaminants burn-off.

The 470-ohm resistor from the choke to pin 4 of the 6L6GC is the screen resistor. This resistor helps to insure that the screen voltage remains below the anode plate voltage. 470-ohm resistor usually = about 5 VDC drop in voltage in most 6L6GC powered amps.

This resistor will pop if too much current passes through the tube. Like if the tube shorts outs. I like to put a good 5-watt wire-wound resistor on the screen. It's nice to have the screen voltage always present. Even in the most demanding situations. To help prevent the output tube from going ballistic.

You may also want to check the grid stopper resistors (the resistor on the socket that attaches to pins 1 to 5). These are often stressed at the same time the screen resistors pop.


Good luck! Keep us informed. :D


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Post subject: Re: Bias adjustment on a fender bassman50 silverface
Posted: Wed Sep 17, 2014 8:19 am
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If you set master vol. to 0 then you adjust the balance pot for minimum hum from the speaker do you get anything?


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Post subject: Re: Bias adjustment on a fender bassman50 silverface
Posted: Wed Sep 17, 2014 5:50 pm
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Pictures of the inside of the chassis, under the "doghouse", and the transformer numbers would be good here. It would help the experts here render the best advice possible. (I don't consider myself an expert, because most of my experience as an electronics technician stems from repairing UHF and microwave radio equipment, some of which contained tubes, and my schooling included tube theory. That doesn't make me an expert on old Fender tube amps, though!)

Your friend has a fine old piece of Fender history, why not restore it to good working order? That may require a little more than replacing some tubes. It may need a cap job, tube socket contact cleaning and re-tensioning, etc. I'm not saying turn it in to some museum queen, just turn it in to a good amp that will give years of reliable service.

Whoops, I digress here. If it's still out of the cabinet, take some pictures and post them here. You'll get better advice.

Regards,
vinyl


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