fender silverface mod bias question

[kwm488]

hi

i own 1976 silverface twin reverb. i mod it to blackface. but i have question of the bias problem. original , it is balance bias system. i mod it to blackface bias, so i can tune bias.
but the layout show that the PT bias tap is 48vac, my pt is 60vac. should i still need to replace the 1.2k resistor to 470ohm like original layout? but i am worry, the wire which go to the 2.2M resistor. i am afraid the voltage will too higher if i lower it to 470ohm.

please help



http://www.el34world.com/charts/Schematics/files/fender/super_reverb_ab763_schem.pdf

[Fresh_Start]

If you have an AB568 Silver Face Super Reverb, it looks like the bias range resistor should be 1K.  

If your bias range resistor is 1.2K, I would leave it there to start with.  Turn on the amp without the power tubes in place.  Does the bias adjustment cover a range from roughly 45 to 60 volts (negative)?  If so, OK.  The range may be wider than 10 volts and that would be good.

If the bias voltage range is too low (too close to zero), you will need to reduce your "range resistor" value, that 1.2K meg, by trial and error.  I can almost guarantee that 470 ohms will be too small.  Tacking on temporary resistors parallel to the 1.2K meg is the easiest way.  For example 1.2K meg in parallel with 4.7K meg works out to 955 ohms.

If the range is too high, I would test it with the power tubes in.  Make sure you set your bias adjustment to the highest (most negative, e.g. -58) before turning the amp on with the power tubes in.  Their current draw should pull down the voltages in the amp slightly, including the bias voltage.  Still too high?  Then you need to increase the value of the 1.2K meg range resistor.

Hopefully that makes sense. PLEASE ask more questions if it does not.

I apologize, but I do not understand the comment about the 2.2 meg resistor.  I do not see one on either schematic.

Cheers,

Chip

EDITS:  1.2K, not 1.2 meg!  Too late at night but at least I was somewhat consistent...

[kwm488]

hi
thank you for your reply. i really need help

please see this photo. the white wire connect from the diode (for bias) to 2.2M resistor. i remember the 2.2m resistor for reverb or tremelo. because my PT bias tap is 60vac, original is 48vac. i am worry reverb or tremelo parts will blow if i change the 1.2k resistor to 470ohm like this layout.

now, i can't tune the power tube to my desire bias. this layout have 27k resistor in bias pots. original is 15k. 15k , bias too high for me, 27k , bias too low for me

if possible, i hope to change this amp same with this AB763 layout totally.

last, should i need to switch the bias cap from 100uf to 50uf like original layout. what different if i higher the bias cap value?

[sluckey]

i am worry reverb or tremelo parts will blow if i change the 1.2k resistor to 470ohm like this layout.

Stop worrying. The trem circuit will still work fine and the reverb circuit doesn't use the bias voltage.

[kwm488]

hi
so should i need to replace the 1.2k resistor to 470ohm?

also , why the white wire connect to the bias? they need bias voltage?

[Fresh_Start]

First, the 100uf filter cap is fine if the rating is for 70 volts or more.

When you say that the bias is "too low" with a 27K resistor going from the bias pot to ground, do you mean that the cathode current on the power tubes is too low?  

If I understand your situation, you can either reduce the value of the 27K resistor or increase the value of the 1.2K meg range resistor.  It does not matter which one, but you already know 27K and 15K bracket the value you need.  Why not try a 22K resistor from the bias pot to ground?

PLEASE make sure I understood correctly what you meant by "the bias too low" correctly before changing parts.  I had to guess based on the values you referred to.

You are not going to get exactly the same layout or schematic as an AB763 because the voltage of your power transformer's bias tap is different.  Do not worry - just find resistor values that get you a workable range of bias voltage.  

Do NOT put a 470 ohm resistor in place of the 1.2K meg.  Your bias voltage will be too high (absolute value too high).  If you keep the 27K resistor from the bias pot to ground like the AB763, it sounds like you actually need a higher value bias range resistor - more than 1.2K meg.

Finally, trust sluckey!  That white wire connects to the Vibrato footswitch jack.  If I remember correctly, it provides a grid return resistor when the footswitch is open (off) for the triode that wobbles to create the vibrato.  In any event, do not worry about it - just leave it there.

Step back and look at the AB763 schematic.  The combination of the bias range resistor (1.2K meg in your amp), the bias pot (10K divided by the wiper) and the 27K resistor to ground create a voltage divider.  The diode between the 1.2K meg and the bias pot complicates the math a lot, but it still is a voltage divider.  If you need a lower bias voltage (closer to zero), you either reduce the value of the 27K going to ground or you increase the value of the 1.2K meg range resistor.  

If you need a higher bias voltage (absolute value - farther from zero), the exact opposite is the case - higher than 27K or lower than 1.2K meg.

Hope that helps,

Chip

ARGH!  Consistently wrong is still wrong! 

[kwm488]

i use 70uf cap, not 100uf

and my meaning is , i check the 1ohm resistor through pin 8 to ground. it is just 30ma even i tune bias pots max. i need 42ma.

sorry for my bad english.
but i think you misunderstand. my amp use 1.2k (not 1.2m) which after bias wire.

is it mean even more voltage go to the white color wire, my amp will not get any problem?

i think sch will show what function of white wire, but i don't know how to read it

[sluckey]

is it mean even more voltage go to the white color wire, my amp will not get any problem?

Did you even read my post? The bias voltage on that white wire goes to the trem circuit, but it has nothing to do with the OPERATION of the trem circuit. It's only function is to make sure the trem oscillator is biased OFF when the trem footswitch is OFF. Makes no difference if the voltage is -40 or -60.

[kwm488]

hi

sorry, sluckey. i read your post, but i don't understand . i know i still need to learn, please forgive me.

so i can replace the 1.2K resistor to 470Ohm like AB763 layout even my PT bias tap is 60vac (original is 48vac), right?

thank you for your help

[sluckey]

so i can replace the 1.2K resistor to 470Ohm like AB763 layout even my PT bias tap is 60vac (original is 48vac), right?

Short answer, yes.

Long answer, yes, but...

[kwm488]

but..............?

what problem?

[sluckey]

Chip already covered that.

[kwm488]

HI

I think i am not smart enough, what the meaning of clip already coverd that?

thank you for your help. you help me alot before

[Willabe]

I think i am not smart enough, what the meaning of clip already coverd that?

No, -- Chip -- already covered ( talked about, answered) that.  

If you have an AB568 Silver Face Super Reverb, it looks like the bias range resistor should be 1K.  

If your bias range resistor is 1.2K, I would leave it there to start with.  Turn on the amp without the power tubes in place.  Does the bias adjustment cover a range from roughly 45 to 60 volts (negative)?  If so, OK.  The range may be wider than 10 volts and that would be good.

If the bias voltage range is too low (too close to zero), you will need to reduce your "range resistor" value, that 1.2K meg, by trial and error.  I can almost guarantee that 470 ohms will be too small.  Tacking on temporary resistors parallel to the 1.2K meg is the easiest way.  For example 1.2K meg in parallel with 4.7K meg works out to 955 ohms.

If the range is too high, I would test it with the power tubes in.  Make sure you set your bias adjustment to the highest (most negative, e.g. -58) before turning the amp on with the power tubes in.  Their current draw should pull down the voltages in the amp slightly, including the bias voltage.  Still too high?  Then you need to increase the value of the 1.2K meg range resistor.

Hopefully that makes sense. PLEASE ask more questions if it does not.

I apologize, but I do not understand the comment about the 2.2 meg resistor.  I do not see one on either schematic.

Cheers,

Chip

EDITS:  1.2K, not 1.2 meg!  Too late at night but at least I was somewhat consistent...

First, the 100uf filter cap is fine if the rating is for 70 volts or more.

When you say that the bias is "too low" with a 27K resistor going from the bias pot to ground, do you mean that the cathode current on the power tubes is too low?  

If I understand your situation, you can either reduce the value of the 27K resistor or increase the value of the 1.2K meg range resistor.  It does not matter which one, but you already know 27K and 15K bracket the value you need.  Why not try a 22K resistor from the bias pot to ground?

PLEASE make sure I understood correctly what you meant by "the bias too low" correctly before changing parts.  I had to guess based on the values you referred to.

You are not going to get exactly the same layout or schematic as an AB763 because the voltage of your power transformer's bias tap is different.  Do not worry - just find resistor values that get you a workable range of bias voltage.  

Do NOT put a 470 ohm resistor in place of the 1.2K meg.  Your bias voltage will be too high (absolute value too high).  If you keep the 27K resistor from the bias pot to ground like the AB763, it sounds like you actually need a higher value bias range resistor - more than 1.2K meg.

Finally, trust sluckey!  That white wire connects to the Vibrato footswitch jack.  If I remember correctly, it provides a grid return resistor when the footswitch is open (off) for the triode that wobbles to create the vibrato.  In any event, do not worry about it - just leave it there.

Step back and look at the AB763 schematic.  The combination of the bias range resistor (1.2K meg in your amp), the bias pot (10K divided by the wiper) and the 27K resistor to ground create a voltage divider.  The diode between the 1.2K meg and the bias pot complicates the math a lot, but it still is a voltage divider.  If you need a lower bias voltage (closer to zero), you either reduce the value of the 27K going to ground or you increase the value of the 1.2K meg range resistor.  

If you need a higher bias voltage (absolute value - farther from zero), the exact opposite is the case - higher than 27K or lower than 1.2K meg.

Hope that helps,

Chip

ARGH!  Consistently wrong is still wrong!  

Updated Chip quotes.


                  Brad

[kwm488]

thank to let me know.

but fresh start always mention 1.2m resistor. i get confirm. it should 1.2K resistor,right?

and i test, 27k resistor in bias pot is too high, 15k is too low. i should use 22k, right?

thank

[Fresh_Start]

I am very sorry for typing "1.2 meg" when I meant "1.2K".  I have corrected my earlier posts typed too late at night but can't change the quotations Willabe put together. 

Other than that consistent error, I did understand what you meant by the bias being "too low".  Too low plate & screen grid current through the cathode with a 27K resistor going from the bias pot to ground.  So you need a lower bias voltage (closer to zero).  Either reduce the value of the 27K resistor or increase the value of the 1.2K resistor.

Try 22K.  You need to experiment until you get a useful range of bias voltage.  You don't want the bias pot at one extreme giving you just enough bias voltage, for example.

Are you moving the bias pot through its full range when testing?  You should start with the highest bias voltage possible and gradually move downward until you get your target bias on the power tubes. 

Forget the white wire - it's fine as is.  You can't always trust me but you can always trust sluckey.

Cheers,

Chip

[kwm488]

hi

i believe you and sluckey all the ways.

and i have another question. i add the presence pot to my twin reverb. but it is not function.

i follow here to do , is this design have problem?

http://forums.birdsandmoons.com/forum/showthread.php?p=385301

and this is 1978 silverface twin reverb. it is 100w OT. i plan to change it to mecury 85w blackface OT. is it worth to do this job? in fact, what sound different of 100w and 85w OT

[sluckey]

AB763 does not have a presence pot.

[kwm488]

hi
i know ab763 no presences pot. but my amp is silverface master volume amp, i remove master volume. i get one hole here. i hope to install presence to this amp to cover the hole.

i did a lot of mod in this amp.

1. change all wire to western electric cloth wire. but i don't feel sound have a big different

2. change speaker cable to kimber. a lot of different here, fender use rubbish wire for speaker cable even in 1978.

3. change more thick power cord cable. i get this idea from MM. and my friend , he is tube hifi player. he suggest me. but i can't hear big different too.

4. the old reverb OT is blow by my accident connection. i install MM reverb OT, reverb better much too much.

5. change some signal wire to audioquest. sound is better.

and i plan to mod follow
1. RCA cable for the reverb tank. will use audioquest and switchcraft rca jack
2. MM blackface design 85w OT. but is it worth to do this?

Thank

[Fresh_Start]

Are your power tubes biased properly now?  If so, what made it work?

What do you want to change about your amp and its sound?  What tubes are you using?  What's the rest of your signal chain?  Guitar, effect pedals, speaker & cabinet.  We need to know all of that before recommending any tweaks, OT replacement, etc.

Honestly, I'm impressed that the amp works after all that wire substitution.  If the amp sounds better afterward, I really doubt the wire type is the cause.

Cheers,

Chip

[kwm488]

hi
i can tune my power tube bias, but the result is 30ma max in 1ohm resistor. i need 45ma. i will change the bias resistor and do again.

i am using RCA preamp tube. and SED 6L6GC. i think this power tube sound like RCA, because i also own RCA 6L6GC

i just use Klon to boost this amp clean sound. i don't use much effect in this amp. speaker are original oxford.

as the kimber speaker cable and audioquest signal wire. it make the sound better.

in this movement, i hope to know why my presence pot no reponse even i follow the forum to do

thank

[kwm488]

hi
i follow everyone advice , i fix the bias problem.

now, i just need help about presence problem. can anyone tell me will this presence mod work?

http://forums.birdsandmoons.com/forum/showthread.php?p=385301

[Fresh_Start]

Exactly what fixed the bias problem?  It would be very helpful to know what worked.

I suggest that you start a new thread to ask about adding a Presence control.  The resistor values in the long-tailed phase inverter and the negative feedback resistor are all interrelated.  For example, the 5F6-A Bassman has an LTPI and Presence control, but the resistor values are completely different from your circuit.

Chip

Chip

[kwm488]

thank for your advice

[sluckey]

But what fixed the bias problem?