Twin Reverb Bias...

[JMadson]

I recently modded a AA270 Twin Reverb to make the bias pot a "real" bias pot, (like the AB763. I've had to change some of the resistor values to make it work. My question is: As long as I'm getting the current to measure the way I want with the bias probe, the bias is set right, correct. I mean, it doesn't matter how you get to the bias voltage, or whether it's anywhere near the -52 volts that Fender shows on the schematic. As long as your setting idle current to say, 32ma per tube, (if that's what you want), it takes into account the variation in things like B+ voltage and resistor tolerances, etc. right?
Thanks in advance...
Jason

[TD_Madden]

Offhand, I'd say yes...if the resultant power output falls within the tubes limits.

That said, i'm wondering about your question...did you do something really odd to get it to bias properly?

jus' wonderin'

Regards.

[Tom Phillips]

The bias voltage listed on the schematic is just the original "nominal" reference voltage that Fender picked for the tubes of the day. Just a starting point really. There is no reason that you need to set the referenced bias voltage.

I'm not exactly sure what you meant by "it takes into account the variation in things like B+ voltage ... etc. right"?

I reality you set the bias voltage to get a bias current which will result in the desired idle plate dissipation. Plate dissipation is the product of plate voltage times plate current. I think that 32ma is a good value if the plate voltage is around 450V but it is too low if you have one of the low B+ TRs.

[JMadson]

Not odd, just off-stock...

The circuit is an AA270, I converted the bias section to AB763 specs, only to find that all I could get was around 8ma per tube! So I had to change the 470 ohm resistor in between the transformer and bias cap to a 1K (like the AA270 was originally), and still maybe 10ma per tube. So I ended up with a 15 on the bias pot rather than a 27K as per the AB763. This gets me really nice range of about 68-43 negative volts on the wiper. I was just wondering about my mixture of resistor values to get the desired results. Part of my problem was that I had around 70 volts AC at the bias tap on the transformer, secondly, I the 10K bias pot only measured around 8K at the most. I was just thinking that the dynamics get too compicated to calculate when you start to factor in all the voltage variables in an old Fender amp, so I was hoping that if you could just center the range of the bias pot on about -52 volts, you could just sort of adjust your way to bias-happiness, it's just that putting a 15K resistor where a 27K should go along with a 1K where a 470 ohm should go seems pretty extreme! But the bias on an amp is more like a ratio game than pure numbers, I guess.
Thanks to all for your help and input...
Jason

[Thorny]

You should have just left the bias control as it was, and replaced that resistor on the bias pot with a 25K trimmer pot. Then you could adjust the bias side to side with the normal included pot and overall up and down with the trimmer - WORKS GREAT.

But - yeah I have done what you did when I didn't have a trimmer pot handy. It moved left the current value resistors on the board but placed them just like the older circuit (but left the same values that were in there). I did wind up adjusting the bias resistor to give me a right range - probably just like you but I don't recall exactly what values and am too lazy to look them up at the moment

You will probably want to set your 6L6s for around 15w or so static dissipation - so take your voltage times the current and when you get around the 15-16w range per tube I would stop. So, if you had 400v X .04 (current) you would get 16w static dissipation. You would go less current if your voltage is more. Check it after you change it because your voltage will drop some when your current rises. Don't go over 18W for a good 6L6GC tube. There are other ways to bias it, but this has worked well for me and my customers for years.

[MickG]

Why do these bias problems occur?

So....I have just completed the same mod on my AC568 circuit....all values changed to blackface specs.....I have yet to rebias, (not sure how yet, tempted to buy a bias rite) anyway I have heard of this problem...having to change some things back so it'll bias correctly....what gives? There is no difference in a converted AC568 and a AB763 right? I mean, my AC568 had the bias pot already stock wired the same as a AB763, so all I had to change was resistor values and removal of cathode resistors and some caps. All should be well....why are so many people finding that after they convert to blackface specs they cannot bias correctly and have to replace the 15k resistor or 470ohm resistor...!? What gives?----Mick

The circuit is an AA270, I converted the bias section to AB763 specs, only to find that all I could get was around 8ma per tube! So I had to change the 470 ohm resistor in between the transformer and bias cap to a 1K (like the AA270 was originally), and still maybe 10ma per tube. So I ended up with a 15 on the bias pot rather than a 27K as per the AB763. This gets me really nice range of about 68-43 negative volts on the wiper. I was just wondering about my mixture of resistor values to get the desired results. Part of my problem was that I had around 70 volts AC at the bias tap on the transformer, secondly, I the 10K bias pot only measured around 8K at the most. I was just thinking that the dynamics get too compicated to calculate when you start to factor in all the voltage variables in an old Fender amp, so I was hoping that if you could just center the range of the bias pot on about -52 volts, you could just sort of adjust your way to bias-happiness, it's just that putting a 15K resistor where a 27K should go along with a 1K where a 470 ohm should go seems pretty extreme! But the bias on an amp is more like a ratio game than pure numbers, I guess.
Thanks to all for your help and input...
Jason

[ctcpete]

The current (April, 09) issue of Guitar Player magazine has an article on this exact issue (adding a trim pot and a fixed resistor in addition to the existing balance pot)written by Gerald Weber , Kendrick Amps. I'll be attempting something like this as well on my SF Twin in the near future assuming I don't chicken out and bring it to an amp tech for the blackfacing. One thing I have seen from Weber, (he has a chapter on Blackfacing in his book) he seems to leave the 50u 70V Electrolytic cap that comes from the hum balance pot in the circuit while the Blackface amps did not have this cap. I know other techs remove this cap. Any thoughts?

[ctcpete]

Any comments on the advisability of leaving the bias balance pot (75 Silverface Twin,aa 270 circuit) as is and adding a 25k trim pot in series with a fixed resistor to adjust bias? Thorny posted earlier and has had success with this. Gerald Weber seems to like the idea as well. It seems like there would then be a hum adjustment as well as a bias adjustment.

When connecting the trim pot, would 2 legs (one would be the variable/wiper leg) be connected together to the fixed resistor (which would then go to ground) while the other leg of the trim pot would go to the appropriate lug of the existing bias balance pot?

I want to thank all the members for their assistance. My cap job, new ac cord with proper circuit changes and assorted damaged component replacements have gone very well. I am considering some black facing next.

Thanks again, Pete

[ctcpete]

Any advise...Silverface Twin...Hum balance pot convers

This is a few week old thread, there where no replies to my last message.
Has anyone tried to add a bias adjust pot in addition to the existing hum balance pot? It seems to be a good concept. Also, what are the pros and cons to removing the electrolytic cap that comes from the pot and goes to ground?

[Enzo]

There is no reason you can;t have a bias level control and a bias balance control if you want.

[Rick Erickson]

[why are so many people finding that after they convert to blackface specs they cannot bias correctly and have to replace the 15k resistor or 470ohm resistor...!? What gives?----Mick]

If you are dropping BF bias circuit values into a SF amp it is because the power transformers have different voltages on their B+ and Bias windings. As for the Bias+Balance pot - I like to set up the SF amps this way. It's a simple way to add a bias adjust without having to un-ass(emble) the SF Balance pot wiring.

RE

[ctcpete]

Thanks for the info. Do you leave the electrolytic cap that goes from the hum balance pot to ground (is it for hum control?)or do you remove it? I've already replaced the other cap in the section with a 100u.

Thanks again.

[ctcpete]

One more question (from an earlier post). When connecting the trim pot, would 2 legs (one would be the variable/wiper leg) be connected together to the fixed resistor (which would then go to ground) while the other leg of the trim pot would go to the appropriate lug of the existing bias balance pot?

[ctcpete]

Worked great...

Hello, Finally had a chance to install the trimmer pot in conjunction with the existing hum balance pot. I installed a 6.8k resister to grnd in series with the trim pot (25k) which goes to the pot pin the old 15 k resister(removed) was attached to. I left the electroltic cap in place (replaced it with a 100u). Not sure if that was the way to go (leaving both electrolytics in the bias circuit) but assumed that if I was leaving the hum balance function as is the cap coming off the pot would serve a hum reducing function. Please correct me if I'm wrong.

I've never biased an amp in my life. I used the method of measuring resistince from the output x former center tap on both sides of the power tube circuit (power off) then measuring the dc V with power on and dividing the amps by the V. Using the hum balance feature I was able to balance the values to some degree to come up with closer amp readings side to side. I am still using the old tubes to learn on. They sound fine after doing all the earlier refurb (Caps etc) stuff.

Thanks for all the help during this process. Black facing in progress as time permits.