so i biased my amp... can you validate the values I got?

[bluesboy]

So my new tubes arrived, I put them in, and then I tried to bias them. The amp: 1978 Silverface Deluxe Reverb. The tubes, from left to right:

EH rectifier tube 5UGR somethign or other... i dont remember, two Tung-sol matched 6V6GT's, mullard cv4024, jj ecc83s, EH12ax7, mullard cv4024, tung-sol 12ax7, jj ecc83s.

 

First thing that seems weird to me was the plate voltage. I have a biasprobe tool that I got from ebay, and the manual says remove one power tube, stick black in slot 8 and red in slot 3. I got a reading of 484 DC volts. Isn't this too high (I think for some reason it is?)

 

then it says use this equation to calculate the proper bias current (the probe reads DC current directly).

 

I-bias = (RMS watts * 500) / (plate voltage * tube quantity)

 

so, here's what I plugged in:

 

I-bias = (22 watts * 500) / (484 * 2)

 

I-bias - 11.3.....

 

then it says I-max = I-bias*1.2 = in this case 13.5

 

So, I used the tool and set the bias to 11.5 mA

 

the amp sounds fine... althoug i didnt get to play ti for long becasue this took place at our practice space and it was late, people were sleeping, etc.

 

Im just curious what you people thing of these values, etc. ?

 

Any and all opinoins are welcome.

 

PS- also, the reverb and vibrato dont work (it didn't witht he old tubes either). Any ideas there?

[potaetoes]

where did you get the "* 500" part? i always thought it was (tube power rating)/(plate voltage)(.7)

 

anyway, 485 is probably dangerous for those 6V6s. JJs might be good at that voltage, but others will probably fry quickly. assuming you had some 6V6s that could handle that voltage, you'd be looking for an idle current in the ballpark of 20-23ma as a starting point.

[bluesboy]

where did you get the "* 500" part? i always thought it was (tube power rating)/(plate voltage)(.7) anyway, 485 is probably dangerous for those 6V6s. JJs might be good at that voltage, but others will probably fry quickly. assuming you had some 6V6s that could handle that voltage, you'd be looking for an idle current in the ballpark of 20-23ma as a starting point.

 

 

 

I dont know, that's what it says in the manual.

 

And as far as the plate voltage, I thought that was something intrinsic to the amp? Like it would be that way no matter what tube sI had in there?

[adiohead]

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

[potaetoes]

I dont know, that's what it says in the manual. And as far as the plate voltage, I thought that was something intrinsic to the amp? Like it would be that way no matter what tube sI had in there?

 

 

that's right... plate voltage won't be changing. 6V6GTs are only rated up to 350 on the plates, and a couple hundred on the screens. you might like 6L6s in that amp if it's running 485v B+

[Wolfeman28]

 

If you have a later silver-face or blackface Deluxe amp, the plate voltage will be very high--plugging low-cost Russian or Chinese 6V6s into such an amp is risky. Suggest NOS 6V6GTs or 7408s for such amps.

 

 

hmmmmmm

[bluesboy]

OK, so my amp has a really high plate voltage. what's the worst that will happen? The tubes will go bad? Or the amp will be damaged? Tubes I can always replace... I dont want to damage my amp though.

BTW, one tube socket gave me a rating of 477. Still high though.

[bluesboy]

hmmmmmm

 

 

where is that quote from? maybe you can provide more input than "hmmmmmm"

[Wolfeman28]

where is that quote from? maybe you can provide more input than "hmmmmmm"

 

 

Hell I dunno dude, that's why I said 'hmmmmmmm'.

 

http://www.thetubestore.com/inonfenlatbl.html

[bluesboy]

Hell I dunno dude, that's why I said 'hmmmmmmm'. http://www.thetubestore.com/inonfenlatbl.html

 

 

 

lol

[potaetoes]

odd that the voltage is different between sockets...

 

anyway, it won't necessarily hurt the amp. tubes might just die fast. if you get JJs next time you should be good to go. i've used them at 490v with no problems.

[bluesboy]

odd that the voltage is different between sockets... anyway, it won't necessarily hurt the amp. tubes might just die fast. if you get JJs next time you should be good to go. i've used them at 490v with no problems.

 

 

 

cool, thanks for the info. I wonder why the person I bought them from didn't consider that these amps have high plate voltage? Oh well, live and learn I suppose.

 

Also, when I first tried all this I had a loose tube or somethign which was when I got 477... after I rechecked all the tubes were tightly in place I checked B+ inthe second socket and got 484... I figured that was the more accurate reading

[Wyatt]

475-485 is a little on the high side, but not unheard of.

 

The DR were designed to be around 425 or so, most run hotter (450 or so). The rectifier tube may be more efficient than vintage. The SF DR's used a larger power tranny than the BF models, so they stepped down the rectifier (from 5AR4 to 5U4) to compensate. Perhaps your 5U4 doesn't have the same voltage drop as the amp was designed around. If you want to bring that plate voltage down, find a rectifier with more voltage drop (if you can't find a suitable tube, WeberVST will build a copper cap for any voltage drop).

 

The EH 6V6's were designed to survive plate voltages of 475 or so because, at the time, there weren't weren't any modern 6V6's that could even survive the 425. Those that don't die in infancy do perform well. The Tung Sol is latest version of that tube. I will assume they well handle it, Sovtek/Refector says they will, but you are butting up to their limits. The JJ is far more sturdy, but also isn't a real 6V6, but a small bottle 6L6.

 

By my calculations P=E*I, I come up with biasing to 20mA for 70% dissipation (maximum safe use, about 9.8 watts/tube) and 17mA for 60% dissipation (average, 8.4 watts/tube).

[bluesboy]

475-485 is a little on the high side, but not unheard of. The DR were designed to be around 425 or so, most run hotter (450 or so). The rectifier tube may be more efficient than vintage. The SF DR's used a larger power tranny than the BF models, so they stepped down the rectifier (from 5AR4 to 5U4) to compensate. Perhaps your 5U4 doesn't have the same voltage drop as the amp was designed around. If you want to bring that plate voltage down, find a rectifier with more voltage drop (if you can't find a suitable tube, WeberVST will build a copper cap for any voltage drop). The EH 6V6's were designed to survive plate voltages of 475 or so because, at the time, there weren't weren't any modern 6V6's that could even survive the 425. Those that don't die in infancy do perform well. The Tung Sol is latest version of that tube. I will assume they well handle it, Sovtek/Refector says they will, but you are butting up to their limits. The JJ is far more sturdy, but also isn't a real 6V6, but a small bottle 6L6. By my calculations P=E*I, I come up with biasing to 20mA for 70% dissipation (maximum safe use, about 9.8 watts/tube) and 17mA for 60% dissipation (average, 8.4 watts/tube).

 

 

OK thank you also for the info... so right now they are at 11.5mA, which is pretty low I guess. Next I go over there, Ill bump it up to 17mA

[Wyatt]

OK thank you also for the info... so right now they are at 11.5mA, which is pretty low I guess. Next I go over there, Ill bump it up to 17mA

 

 

Well, I am curious what your VDC is off the power transformer (don't go poking around in there after unless you really know what you are doing). Because you should be getting a 50V drop VDC from the 5U4GB. The SF PT is supposed to be around 475VDC before the rectifier.

 

Do you have the old rectifier still? Maybe you should do a measurement with that installed?

[Loghead]

Bump.

[tommythelurker]

 

where did you get the "* 500" part? i always thought it was (tube power rating)/(plate voltage)(.7)

 

 

It sounds like they substituted .5 for .7 and then multiplied by 1000 so that the result is in milliamps (i.e. 11.5mA) instead of amps (.0115A). The part that I don't understand is dividing by plate voltage * number of tubes. I think they meant to multiply the entire result by the number of tubes, if the probe is measuring all the tubes together. 11.5mA per tube definitely sounds low, I think it should be more like 23mA, and that's probably the lower end of the range since it's based on a factor of .5 instead of .7. edit: 23mA assumes 22W max per tube, which I now believe is incorrect. See the values Wyatt gave above.

[bluesboy]

It sounds like they substituted .5 for .7 and then multiplied by 1000 so that the result is in milliamps (i.e. 11.5mA) instead of amps (.0115A). The part that I don't understand is dividing by plate voltage * number of tubes. I think they meant to multiply the entire result by the number of tubes, if the probe is measuring all the tubes together. 11.5mA per tube definitely sounds low, I think it should be more like 23mA, and that's probably the lower end of the range since it's based on a factor of .5 instead of .7.

 

 

well, here's all I can add... the probe fits on one tube, Im assuming it only reads one tube. To bias the amp there a little screw done in a little hole under the power tubes on the chassis. There's only one srew, so i assumed that the value I adjusted to applied to both tubes. Maybe somebody can verify this.

 

I played the amp, and it sounded good even at 11.5mA. I think I will leave it at that until we can verify whatthe proper numbers should really be. so far, we've got:

 

11.5 from me

17

23

[potaetoes]

now that i think about it, i looks like the "* 500" thing is a way to take the max rated current in amps (a decimal like .0031) and convert it to milliamps at 50% rating (a whole number like 15 or so) ... which would explain the somewhat cold setting. the cold setting might also be a cautious number so the tubes will survive longer at that plate voltage... dunno.

 

most probes measure one tube. the adjuster adjusts bias for both at the same time - and you should check both with the probe to make sure they're both running where you think they are.

 

it's not uncommon even for "matched" tubes to vary slightly in current draw when they're actually in an amp. testers often don't test at voltages that high. so adjust one, put the probe on the other, and see where it's running. if it's the same, you're good. if they're different, try to split the difference so they average out to your desired setting.

[tommythelurker]

Well see now, where did you come up with 22W? The value I gave was based on 22W. After reading Wyatts post and looking up some datasheets on the web, it seems the 14W value Wyatt used is more accurate. I would go with his numbers.

 

My bad for not reading the whole thread more closely before posting.

[bluesboy]

Well see now, where did you come up with 22W? The value I gave was based on 22W. After reading Wyatts post and looking up some datasheets on the web, it seems the 14W value Wyatt used is more accurate. I would go with his numbers. My bad for not reading the whole thread more closely before posting.

 

 

 

I thought it was a 22watt amp? that's just the number I always had in my head

[tommythelurker]

 

I thought it was a 22watt amp? that's just the number I always had in my head

 

 

OH, now things are starting to make a little more sense. That's where the "number of tubes" part comes into the equation you were using.

 

The value that the rest of us are using is the max power rating of the tube based on the tubes spec. What you're doing is finding the power per tube based on the total power rating of the amp.

 

I've never seen that before and not really sure whether it makes sense or not to be honest.

 

bottom line, I'd say you could experiment anywhere below the max that Wyatt gave and go with whatever sounds best to you.

[bluesboy]

OH, now things are starting to make a little more sense. That's where the "number of tubes" part comes into the equation you were using. The value that the rest of us are using is the max power rating of the tube based on the tubes spec. What you're doing is finding the power per tube based on the total power rating of the amp. I've never seen that before and not really sure whether it makes sense or not to be honest. bottom line, I'd say you could experiment anywhere below the max that Wyatt gave and go with whatever sounds best to you.

cool, thanks for your advice, and everyone else's too

[Wyatt]

I thought it was a 22watt amp? that's just the number I always had in my head

 

 

The 6V6's *maximum* dissipation, per the old tube bibles, is 14 watts. That is 100% dissipation, which isn't healthy. The maximum *safe* dissipation is 70%. We multiply 14 by 70% (.7) and come up with 9.8. So, at 70% dissipation the tube should put off 9.8 watts.

 

We divide the 9.8 watts by the 484 vdc. This .202... amps, which is 20mA.

 

I came up with 17mA by figuring on 60% dissipation.

 

I would try 20mA and keep an eye on the tubes, if the plates don't burn red hot, they should be ok-dokey. I would not exceed 20mA.

 

As for the 22-watt RMS output of the DR? Well, that was figured out in some calculation by Leo that may or may not be accurate.

[Wyatt]

it's not uncommon even for "matched" tubes to vary slightly in current draw when they're actually in an amp. testers often don't test at voltages that high. so adjust one, put the probe on the other, and see where it's running. if it's the same, you're good. if they're different, try to split the difference so they average out to your desired setting.

 

 

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