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Post subject: Bassman Reissue Bias Pot Question...
Posted: Fri Dec 24, 2010 5:14 pm
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Hi All!
I have a 1994 Bassman reissue. She's due for a tube change, but I'd also like to add a bias pot to the amp, instead of leaving it with it's current fixed bias setup.
Can anyone give some direction on how do do this mod?
I have some modest amp repair experience and all the tools and parts necessary to do it safely and correctly. But I do not have an engineering degree! lol
If you could tell me what components I would need to replace, the values etc, and maybe a pic of the area of the circuit board I will modifying I can get her done!
:D

Thanks in advance for your help!!!
-Chris


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Post subject: Re: Bassman Reissue Bias Pot Question...
Posted: Fri Dec 24, 2010 8:24 pm
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Hi ctyoung,

Is this the schematic for your amp (59 Bassman)? If not, do you have a link to your schematic? I could maybe help if we both have the correct schematic. :)

http://support.fender.com/schematics/guitar_amplifiers/59_Bassman_schematic.pdf

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Post subject: Re: Bassman Reissue Bias Pot Question...
Posted: Fri Dec 24, 2010 9:55 pm
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Hi,
Yes, the schematic I have. I was thinking since it is dated 1990, it's probably accurate for my 1994 model.

Thank you!!


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Post subject: Re: Bassman Reissue Bias Pot Question...
Posted: Fri Dec 24, 2010 10:46 pm
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Cool! In that case look at the bottom of page 1, look for an arrow labeled "C-", TP14 with the voltage measured of -54.5 VDC. That is the bias voltage circuit that sets the idle dissipation for the tubes. The more negative the voltage, the lower the idle wattage of the tubes. One way to add a bias pot is to remove R41 (56K) and replace with a pot and resistor in series with each other. Maybe a 50K pot and 30K resistor, in place of the existing R41. With the 50K pot adjusted to the mid position, the bias voltage will be close to what it is now, and allow adjustment up or down. You could also just use a 100K pot in place of R41, but that could allow an accidental adjustment too low which could damage the tubes, so the series resistor will keep the voltage from getting too low in case the pot is turned all the way down. I added a bias pot to my BJr, I drilled a hole in the circuit board between the existing holes from the removed resistor (in your case R41), and used a 10 turn pot as a variable resistor like this one (link below) on top of the circuit board, and put the series resistor on the underside of the board for a nice neat install. This is provided that there isn't a circuit trace in this spot on the circuit board. You can also use a small circuit board mounted to the chassis and put the components on there and run wires to the main circuit board, keeping the wires as short as possible.

http://www.mouser.com/ProductDetail/Bourns/3299Y-1-503/?qs=sGAEpiMZZMuz2E8dTn0VFfo4R8fTsagshrcvCSgT0EA%3d

You also don't have a test point to measure the current through the tube. You can add a 1 ohm, 1 watt, resistor between the Cathode of both tubes and ground to use as a test point, or just measure the resistance of half of the primary of the output transformer and the voltage across the same half of the transformer to determine the idle current (I = E/R), or use a bias probe to measure Cathode current. But, you must please keep us updated with what you decide to do. As always, be sure that the power supply capacitors are safely discharged before modifying the amp. :)

A picture of the bias pot that I installed on the BJr.

Image

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Post subject: Re: Bassman Reissue Bias Pot Question...
Posted: Sat Dec 25, 2010 8:48 am
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shimmilou wrote:
Cool! In that case look at the bottom of page 1, look for an arrow labeled "C-", TP14 with the voltage measured of -54.5 VDC. That is the bias voltage circuit that sets the idle dissipation for the tubes. The more negative the voltage, the lower the idle wattage of the tubes. One way to add a bias pot is to remove R41 (56K) and replace with a pot and resistor in series with each other. Maybe a 50K pot and 30K resistor, in place of the existing R41. With the 50K pot adjusted to the mid position, the bias voltage will be close to what it is now, and allow adjustment up or down. You could also just use a 100K pot in place of R41, but that could allow an accidental adjustment too low which could damage the tubes, so the series resistor will keep the voltage from getting too low in case the pot is turned all the way down. I added a bias pot to my BJr, I drilled a hole in the circuit board between the existing holes from the removed resistor (in your case R41), and used a 10 turn pot as a variable resistor like this one (link below) on top of the circuit board, and put the series resistor on the underside of the board for a nice neat install. This is provided that there isn't a circuit trace in this spot on the circuit board. You can also use a small circuit board mounted to the chassis and put the components on there and run wires to the main circuit board, keeping the wires as short as possible.

http://www.mouser.com/ProductDetail/Bourns/3299Y-1-503/?qs=sGAEpiMZZMuz2E8dTn0VFfo4R8fTsagshrcvCSgT0EA%3d

You also don't have a test point to measure the current through the tube. You can add a 1 ohm, 1 watt, resistor between the Cathode of both tubes and ground to use as a test point, or just measure the resistance of half of the primary of the output transformer and the voltage across the same half of the transformer to determine the idle current (I = E/R), or use a bias probe to measure Cathode current. But, you must please keep us updated with what you decide to do. As always, be sure that the power supply capacitors are safely discharged before modifying the amp. :)

A picture of the bias pot that I installed on the BJr.

Image



Shimmilou....Beaubs here.....hey you had asked me (in the recent past) about the idle wattage(?) of the tube bias on my Blues Jr. after my mods were done. I did ask my tech and he did not give me a definitive answer but I do know this...the volts using the TO20 transformer are set at 3.2 - 3.4...per instruction from Bill M. How would I check the idle watts????

Please advise.

Beaubs

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Post subject: Re: Bassman Reissue Bias Pot Question...
Posted: Sat Dec 25, 2010 9:31 am
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Hi Beaubs,

I believe that the 3.2 to 3.4 is the DC voltage measured across half of the output transformer primary, at idle. With the amp turned off and unplugged from electricity, and the caps discharged, measure the resistance of the same half of the output transformer primary that you measure the voltage on (example CP2 to CP3). Mine is right at 100 ohms for each half, yours will be different with a different transformer. Divide the voltage reading, from CP2 to CP3, by the resistance reading from CP2 to CP3, to get the Plate current for one output tube (I = E/R). Then measure the DC voltage from ground (chassis) to CP3, this is the Plate voltage. Then simply multiply the Plate current that you calculated above and the Plate voltage that you measured to get the idle wattage (P = I x E). You can take the same readings for the other half of the output transformer primary, which is for the other tube, to see the balance between the two output tubes. There will be a slight difference between the two, which is OK. Let me know what you find. :)

P.S.
The amp should be warmed up for a few minutes before taking the voltage readings.

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Post subject: Re: Bassman Reissue Bias Pot Question...
Posted: Tue Dec 28, 2010 3:25 pm
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Shimmilou,
Thanks a ton for your help. I greatly appreciate it. :-)
Sorry for the delay in writing. I've been under the weather with strep throat.
I did manage to get the parts ordered, so I can do the mod to add a bias control. They should be here in a few days.
The main reason that started me down the road of new tubes and a bias control was, the bassman I have, I just recently acquired.
Here's the issue....
When the amp is plugged into either the normal or bright channel and plated at speaking level, there is a sound in the background that is about 1/4 the loudness of the main sound coming out of the amp, that sounds like a "fuzz pedal". It's really annoying.
I thought maybe new tubes might solve the problem. But after the new tubes were installed, there was no difference. I have a bias-rite current meter and plugged it into the power tube sockets and put the 6L6's back in. The bias current is is only 17ma. I'm new to Fender amps, but tried to take a plate voltage with a volt meter, by touching the meter to one of the power tubes pin #3 and the negative probe to the chassis. The plate voltage was 453v. So the bias current should be something like 36ma really, right?
At this point I'm thinking what I may be hearing is some type of crossover distortion from the cold bias. But why is it coming out as a ghost note/sound instead of just the main amps sound? It doesn't really get any louder when you turn up the amp. And the main tone of the amp sounds pretty good.
Any ideas??

Thanks again,
-Chris


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Post subject: Re: Bassman Reissue Bias Pot Question...
Posted: Tue Dec 28, 2010 6:03 pm
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Yeah, 17 mA sounds way too low for the idle current, just under 8 watts at 453 volts on the Plate. That sounds unusually low for any 6L6 tubes. I knew that Fender biased some amps on the cool side, but that is way too cool by any ones standards. :? You might check your other test points to make sure that there isn't another problem. The power supply voltages seem to be lower than the schematic listing, but that is probably OK. That current reading is just for one tube, you should also check the other one's current and voltage, as sometimes they can be unbalanced, but they shouldn't be hugely different. Depending on the amount of imbalance between the tubes, you can find a spot where only one side is at your desired current, and the other side is a little low. For the idle current, check which tubes you have first. If you have the 6L6WGC 25 watt tubes (Sovtek 5881WXT), then the idle current should be between about 28 mA (50%) and about 39 mA (70%), at 450 Plate volts. So I would agree that 36 mA would be a good setting in this case, on the high end of the range. If you have 30 watt tubes, then 36 mA would be about 55% idle, on the cool end of their range, so you're good either way at 36 mA.

If you get the chance, can you check the C- voltage at TP14? I'd be interested to know if the listed voltage is correct on the schematic. Also, be careful around the tube socket connections on the circuit board, it is too easy to slip and touch two of the connections at the same time, which could result in a big flash/damage/injury. There is a point on the circuit board where the output transformer wires connect that might be a better point to check the Plate voltage. You are correct, reference your readings to ground (chassis). Let me know how things turn out. :)

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Post subject: Re: Bassman Reissue Bias Pot Question...
Posted: Wed Dec 29, 2010 1:58 am
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Just to help to understand;

Does the buzz still present when the amp is on and the guitar is not connected and played?
- If yes, there is no issue with the current zero-crossing;
- If no, it could be the crossover.

Does the buzz is hum or always the same sound ?

When the guitar is unplugged, does the buzz is amplified when you turn the volume potentiometer ?

Definitively, 17mA is very low, as Shimmilou said, check negative voltage on the power grilles. Check also supply decoupling.

Take care about voltages that can be fatal for human, even the amp unplugged!

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Post subject: Re: Bassman Reissue Bias Pot Question...
Posted: Fri Dec 31, 2010 11:04 am
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Hi,
I did get the new bias circuit installed last night. Thank you so much for your help and instructions!!
:-)
I did do a quick bias job after installing the pot. Using my bias-rite meter, I brought the bias current up from 17ma to 35ma.

But I do have another question for you. I am somewhat familiar with getting the bias current for a Marshall, but am new to bias a Fender amp like the bassman. In the past with a 50 watt Marshall, I would get the plate voltage from pin #3 of a power tube, then put that into 25(watts) 25/457= .0547
Then multiply .057 x .70(%) = .038ma (bias current).

For the Fenders I understand there is a difference because the rectifier tube is in series with the power tubes. Plus I had read somewhere that instead of 70% idle the Bassmans use 60%?

Could you let me know how to properly calculate the bias current for the Bassman? The plate voltage is 457v. The Rectifier tube is a JJ GZ34, the power tubes are JJ 6L6's.


Oh, and the fuzz sound is still there....
It's really only noticeable at very low "speaking level" volumes, but it's still there. It is not a buzz or hum. It actually sounds like a fuzz pedal. You have your regular sound coming out of the amp nice tone and everything. Then in the background, like a ghost note at about 10% of the main sound you can hear a this fuzzy ghost sound of whatever you play. It's almost like having a fuzz pedal going through a parallel effects loop setup up at about 10% of the original signal.

Sample Sound:
http://www.zshare.net/audio/8459303840efbdfc/

Side note......
Ya know what? I think one, possibly two of the speakers are blown or have issues....
There are 4, 10" 8 ohm speakers in the cab. For grins and giggles, I laid the amp on it's face. While playing some notes on the guitar, then gently pressing on the back of the cones of the two speakers I think I am hearing the fuzz come from, the fuzz stops....
If I press on one of them, on say the left side of the back of the cone, the fuzz gets more intense. If I press on the right side, the fuzz completely disappears.

At this point, I'm pretty convinced one or two of the speakers are blown.....

I'll keep you posted....

Thanks!
-Chris

-Chris


Last edited by ctyoung on Fri Dec 31, 2010 12:20 pm, edited 1 time in total.

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Post subject: Re: Bassman Reissue Bias Pot Question...
Posted: Fri Dec 31, 2010 12:16 pm
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You are correct in your method of calculating the bias setting, it is the same for any similar tube amp, P = I x E, so it always depends on the Plate voltage and the tubes rated Plate dissipation wattage. As far as where to set it, it depends on the breakup point that you want. Most people like to set their bias between 50% and 70%, 50 for very clean sound with high headroom, 70 to get breakup/distortion earlier. I'm sure that some people even use a higher setting. I could believe that for a Bassman 60% would sound good, with the tube rectifier you get some sag and maybe wouldn't need to bias it very hot, but I don't have one so I really don't know the best setting. Generally, running the tubes hotter can also contribute to shorter life, it's a trade-off.

Just from your description of the "ghost note", it almost sounds like something loose on the amp, giving you a sympathetic vibration. First, make sure all of the chassis screws are very tight, and tighten all other screws too. It could be a preamp tube also, but look for something loose first. :)

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Post subject: Re: Bassman Reissue Bias Pot Question...
Posted: Fri Dec 31, 2010 12:33 pm
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shimmilou wrote:
Just from your description of the "ghost note", it almost sounds like something loose on the amp, giving you a sympathetic vibration. First, make sure all of the chassis screws are very tight, and tighten all other screws too. It could be a preamp tube also, but look for something loose first. :)


+1

Be sure to check the speaker screws as well.

As for the 38mA on the bias, that's fairly accurate for the typical 70% of max plate dissipation figure that most of us rely on. I typically bias vintage NOS 6L6s to between 35 and 37 mA to help extend their service life but 38mA is not excessive IMO.

YMMV.

Arjay

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Post subject: Re: Bassman Reissue Bias Pot Question...
Posted: Fri Dec 31, 2010 3:47 pm
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Also try replacing your Phase Inverter tube (last tube before power tubes).
I think it's a 12AT7 on the Bassman (or possibly a 7025 - Low Noise 12AX7 on the Reissue).

Snowy


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Post subject: Re: Bassman Reissue Bias Pot Question...
Posted: Fri Dec 31, 2010 4:26 pm
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No457 Snowy wrote:
Also try replacing your Phase Inverter tube (last tube before power tubes).


+1

This dual-triode needs to have as close to absolute balanced transconductance as possible -- otherwise, and the precision bias setting is of little value.

Arjay

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