Deluxe Reverb RI Bias questions as not same as my BF Super Reverb and Problems

[Newguy]

Forum Members

With your help I was able to check/adjust bias with DVM on my BF Super Reverb a few months back:
http://music-electronics-forum.com/t22832/#post191653

I just replaced power tubes in my Fender Deluxe Reverb RI and went to check bias (Over all as only one adjustment pot..just like my Super Reverb I think as only one pot.).

Problems:

1. I was going to go the 1 ohm resister method, but then saw there is not a GND wire, or any connection to pin 8 of the 6v6 sockets (My BF Super Reverb Had a wire from pin 8 to chassis ground)? So I did not install 1 ohm resister.

2. I figured I would just check voltage (pin 3 on 6v6 socket red probe and chassis ground black probe.). Power on ..OK, then power standby on, and 2-3 seconds it blows fuse? Pin 3 is orange wire BTW. Never any glow from the 6v6.

I am going by numbers on the sockets themselves to find pin 3, so I just now thought maybe socket is mislabeled? I figure better explanation is I am doing something wrong.

I have amp apart on kitchen table now. So please if any one can help respond as soon as you can. I will have to put amp away soon as wife will want kitchen table back.

Can some one link me to a guide on how to set bias on Deluxe Reverb RI (prob over all bias voltage), Please?

I can not figure out what I am doing wrong here?

I want pretty basic information (however I can not find it),

1. just where to put probes on the 6v6 for voltage (I figure around 425vdc), as DVM probes to pin 3 and chassis ground blows fuse in amp! And

2. Where to put put 1 ohm resister.

Thanks for any help.

What am I doing wrong?

[Newguy]

I think maybe I was on pin 4. However, moving it down a pin (brown or green wire), I still get no voltage? It was on long enough to see 6v6 glow. After a few seconds, it blew fuse also?

Amp works fine, I just want to check overall bias and voltages.

[Newguy]

Never mind on pin 3, setting on my meter causing that.

About pin 8 though, it has a yellow wire on pin 8 that goes to pc board (I think).

So this amp is NOT wired from pin 8 directly to chassis ground. Do I still solder 1 ohm resister from pin 8 to chassis ground? If not where do I insert the 1 ohm resister?

Thanks...I am embarrassed by the pin 3 mistake but no harm to amp.

[MWJB]

Yellow wire from pin 8 just goes to ground on the PCB. You can still add the 1ohm resistors, you can use a solder tag under one of the tube socket retaining nuts for your ground connection. Remove the yelllow wires completely.

Or, assuming that pins 1 & 8 are tied together with the yellow wires, or that pin 1 is unused (check this - they should be as the grid stoppers - R60/R61 - that run from pin 1 to pin 5 on the 60's BF are mounted on the PCB board in the RI), you can mount one leg of the 1ohm through both pin 1 & pin 8, leaving the resistor body & a small loop sticking out (away from other components, chassis & wires etc.) then reattach the yellow wires to the free end of the 1ohms. Place your meter probe on the bridge between pin 1 & pin 8 to take readings, rather than stressing the actual resistor.

Or, buy some bias probes, then you can rebias without dropping the chassis.

[Newguy]

Thanks Old Timer, lots of information for me in such a short post of yours. I get it, a couple options, thank you kind sir.

I put it back together late last night but would like to get some new Tung Sol RI tubes soon when I can afford them, and will put resisters in then.

I was just able to take voltage on pin 3. I noticed the range on the voltage was pretty narrow (411-450). I did it by ear and set it at 435vdc. It was set stock at 425 when I checked it, the small boost in voltage seems to make it bloom at lower volumes (which I play in house). More then 440 and it started to distort...and not so much in a good way.

I think it is pretty neat that the bias adjustment can change the tone so much. I have only done it on one other amp (Super Reverb BF from above link). Since it had a Bandmaster replacement, so top bias adjustment was still on the very cold side, however, it sounds so good, I left it alone, as i would have had to replace resister on the bias pot. Prob will do when I get money to buy some nice fresh 6l6 tubes (or sooner since only a resister).

[52 Bill]

I'm confused here, but that's easy to do. Are you setting bias by reading the plate voltage at pin 3 of the output tubes?

[Newguy]

Bill

Well kind of. I know the proper way is to use formula from link in my first post, and use the mv (ma) and voltage readings, but I had it apart last night and I had to put it away. The only thing I could do was measure vdc at pin3 on each tube. Stock they were running 425vdc.

I knew I could put it back to 425vds. I hear some people do it by ear. I plugged in guitar and kept tweaking the pot from 411vdc to 450vdc. Interesting to see the different tones.

It sounded better at 435 then 425, so I left it there. The tubes are older any way and would like to replace them. If they burn out quick...no big loss, but amp sounds better.

Now with the post above, I can install the 1 ohm resisters and bias correctly, but it sounds so good I plan to just enjoy it for a while.

I will keep an eye on the power tubes, to make sure they do not start glowing or be aware of other problems, but the amps sounds better at lower volumes now, fuller, more touch sensitive, so the small tweak from 425vdc to 435dv seemed to help out with the sort of brittle sound the amp had. Other forums say it is not uncommon for plate voltages to be 440 to 450 for a deluxe any way.

[Jazz P Bass]

Setting the idle bias by reading the B+ voltage.
That is a new one.
I think what you saw was the B+ going up as the bias was set colder (more negative).
Which is o/k. Better cold than way too hot.

[Newguy]

Setting the idle bias by reading the B+ voltage.
That is a new one.
I think what you saw was the B+ going up as the bias was set colder (more negative).
Which is o/k. Better cold than way too hot.

Yeap, when I did the bios on my BF Super Reverb, when I turned pot to increase ma, the plate voltage went down. And vise versa.

I do not know why it sounds better to my ears at a lower ma (across resister)/higher plate voltage, but it does???

Seems to not go along with hotter bias sounding better (most threads). I figured I did not have the terminology. If the lower ma reading is a colder bias, then my amps sounds better with a colder bias?

I know this is not the right way to set bias, I had it apart and was just playing around, as again, I took stock readings of 425 plate voltage, so I could set it back. But when the amp warmed up and seemed more alive, I figured I would leave it alone. I will put 1 ohm resisters on soon and see what % dissipation (hope right spelling/term is correct. My guess it will be pretty much in spec.

I am a newbe, but now that I have messed with the bias pot, I think by ear is the way to go for me, as long as in spec.

I read on one forum guy said he had bias to 95-100 percent dissipation with amps that will be played at low levels. He said after that he had no problems with tubes blowing.

However, I feel I am going in the other direction, if I understand correctly, and my dissipation will be on the low end...

Now, can any damage be caused by a very cold bias setting? That is a question I would like forum members to chime in on. Also why would the lower ma and higher plate voltage make the amp sound better to me?

[Jazz P Bass]

Output Tube Idle Bias

The most misused word as far as tube amps are concerned.
First off, your amp output section is classified as Class A/B. Push/ Pull.
The remark about 90-100% is NOT Class A/B.
Anyway.
There is a graph that shows the performance of a class A/B output tube.
The Loadline Graph.
The idea is to set the line up on the most linear portion of the graph. (which is a curve by the way)
Output tube amplification is NOT linear.
Class A/B with feedback helps, but it is not perfect.
Close.
When class A/B push pull tubes push & pull, the handoff can have a notch.
Class B.
It is commonly referred to as crossover distortion.
Good choice of words.
The main purpose of idle bias adjustment is to increase the idle just enough to get rid of the crossover notch. This ultimately results in a low (ie: not quite cold) current draw.
Class A/B.
That is how the RCA manual (among other pubs) advises setting the idle bias on push/pull outputs.
That is it. Done deal.
Perfect amp. (think hi-fi)
Enter Guitar Amps.
Clean sucks.
Mess up the signal & it sounds great. (overdrive this, mid-cut that....)
Back to the loadline graph.
By adjusting the bias anywhere beyond (or below) the recommended "get rid of the crossover notch" you have effectively moved the tubes on the performance curve.
Why setting the bias hot as Hades sounds good is a matter of serious discussion (& contention)
There's that curve again.
Peace.

[Newguy]

Jazz , I hear ya. I take it to see the notch one would need a scope?

I may be way off base, tell me how I am thinking wrong, but there is one pot...turn it to where it sounds best for your playing style/volume you want. If in spec ma/plate voltage, should be OK right?

If I was a Pro and playing out, I would probl bias hotter (higher ma lower plate voltage), as it seemed when I had plate voltage a 425 stock, it was meant to play a gig (would get louder before having a full sound with touch). Playing at home the the 435 plate voltage sound much, much better (to my ears, fuller, alive)

I may have misused the term, but found article I was talking about:

http://reitzel.com/schematics/Fender...ifications.pdf

A
6V6 tube has a maximum designed plate dissipation of 14 watts. In a
fixed bias, class A/B amp the standard practice is to bias the tubes for
70%~80% of the maximum plate dissipation. The stock tubes were
biased at 58% of maximum plate dissipation. With the new Electro
Harmonix tubes installed I adjusted the bias for 27mA, yielding 11
watts of plate dissipation (0.027 amps * 407 volts = 11.0 watts) That
works out to 78% of the rated maximum. For amps that are used at
lower volume settings I’ve found biasing the tubes closer to 90% of
maximum gives a little more warmth. With the Electro Harmonix tubes
as well as the RCA black plates I have biased some amps as high as
100% with no detrimental effects.

[Newguy]

Jazz , I hear ya. I take it to see the notch one would need a scope?

I may be way off base, tell me how I am thinking wrong, but there is one pot...turn it to where it sounds best for your playing style/volume you want. If in spec ma/plate voltage, should be OK right?

If I was a Pro and playing out, I would probl bias hotter (higher ma lower plate voltage), as it seemed when I had plate voltage a 425 stock, it was meant to play a gig (would get louder before having a full sound with touch). Playing at home the the 435 plate voltage sound much, much better (to my ears, fuller, alive, at a lower volume).

I may have misused the term, but found article I was talking about:

http://reitzel.com/schematics/Fender...ifications.pdf

A
6V6 tube has a maximum designed plate dissipation of 14 watts. In a
fixed bias, class A/B amp the standard practice is to bias the tubes for
70%~80% of the maximum plate dissipation. The stock tubes were
biased at 58% of maximum plate dissipation. With the new Electro
Harmonix tubes installed I adjusted the bias for 27mA, yielding 11
watts of plate dissipation (0.027 amps * 407 volts = 11.0 watts) That
works out to 78% of the rated maximum. For amps that are used at
lower volume settings I’ve found biasing the tubes closer to 90% of
maximum gives a little more warmth. With the Electro Harmonix tubes
as well as the RCA black plates I have biased some amps as high as
100% with no detrimental effects.

Like you said....meant to be messed with

[vavlehead]

Setting the idle bias by reading the B+ voltage.
That is a new one.
I think what you saw was the B+ going up as the bias was set colder (more negative).
Which is o/k. Better cold than way too hot.

Hey Jazz..
Do I understand this correctly???
Are you suggesting that the B+ on power tube plates changes with bias settings??
I'm kinda new to this but I thought B+ is constant going to the the center tap of OT.

[JoeM]

Yeap, when I did the bios on my BF Super Reverb, when I turned pot to increase ma, the plate voltage went down. And vise versa.

I do not know why it sounds better to my ears at a lower ma (across resister)/higher plate voltage, but it does???

Seems to not go along with hotter bias sounding better (most threads). I figured I did not have the terminology. If the lower ma reading is a colder bias, then my amps sounds better with a colder bias?

I have a DRRI too. I also dont like it biased real hot. If that's were it sounds best to you I cant see a problem.

[Jazz P Bass]

B+ voltage will remain constant if the current draw remains constant.
You cannot have a higher current draw (ie: higher plate idle dissipation) without a voltage drop.
The reverse also holds true.
The B+ varies as you are playing.