- Jul 27, 2009
- 1,408
- 0
- 0
could anyone help me out? i know what im doing with the pcb now but what is a safe value to set the bias too?
Peavey 5150 Bias Values
- Thread starter RichMinerva
- Start date
lolzgreg
Cereal Shipping Sneapster
It depends on the tubes... Have you installed a bias pot, or are you switching a resistor to allow more range with the fixed bias in those.
18mA per tube is my favorite.
18mA per tube is my favorite.
That's pretty cold. You could safely bias up to 40mA per tube (80mA per OT primary side). Plate voltage is around 450 volts, right? I did a bias mod on a friends 6505+, which sounded nice to me at around 35mA.
Never owned a 5150 myself though, just Dual Rectifiers
And if the OP didn't know, you should measure plate voltage from the OT primary (around 400-450 volts usually). Then measure resistance over both sides of the primary (from center tap to anode). Plate voltage / (OT resistance *2) = plate current per tube. Don't go over 0,7 * 30W / plate voltage.
+1, exactly this. It varies a bit from amp to amp based on these readings, but I think I have mine biased at ~36mA.
- Jul 27, 2009
- 1,408
- 0
- 0
Right guys 
got that onboard thank you
Now I've recently fancied putting a kt88 in position 1 and 4 what are people opinions on this and how could I go about that?
got that onboard thank you Now I've recently fancied putting a kt88 in position 1 and 4 what are people opinions on this and how could I go about that?
Right guys
Now I've recently fancied putting a kt88 in position 1 and 4 what are people opinions on this and how could I go about that?
It is NOT safe to put KT-88s in a stock 5150. You will need to change out the screen grid resistors and modify the bias circuit in order to bias the tubes in pairs, not to mention, it could be detrimental to the power transformer. Here's a few quotes from some amp gurus on the subject:
"KT88s draw roughly twice the heater current of 6L6s. i don't know what the stock PT is rated for, but i'd be very hesitant to keep those tubes in there without a new PT or a filament transformer to run the heaters. There's other issues, as well. the grid to bias resistors need to be different for KT88s than 6L6s, and the info about impedances is also relevant."
-Titan Amps
"You'll have to have the screen grid resistors changed and the bias circuit modified to use the KT88's. I do those mods. To be honest I don't think it's worth it. I have tried 6L6's, EL34's, 6550's and 6V6's in the 5150 and to my ears the 6L6 works best. The EL34 is too crunchy and with the huge gain the 5150 preamp has the EL34's sound like pure mud. The 6550's break up a little more than the 6L6 and aren't as tight and clear as the 6L6. The 6V6 isn't bad, but the 6L6's sounded best for a tight, clear, defined tone. KT88's are very similar to 6550's.
Upgrading the output transformer gives the amp a more defined and clear balanced tone. It gives you a more responsive tighter feeling amp."
-FJA Mods
Here's a few threads on it:
http://acapella.harmony-central.com/showthread.php?1467402-kt88s-6550-tubes-in-a-5150
http://acapella.harmony-central.com/showthread.php?2557286-5150-with-KT88-6L6-mix
http://acapella.harmony-central.com/showthread.php?2044097-KT88-s-in-a-5150
- Jul 27, 2009
- 1,408
- 0
- 0
Thanks dude you've saves me some work anyone got any links on how to correctly measure the bias etc?
anyone got any links on how to correctly measure the bias etc?
lolzgreg
Cereal Shipping Sneapster
I would never bias at 40mA, nor would I at 35mA. High gain amplifiers start to sound mushy with the tubes running that hot, and say goodbye to your tubes since the percent plate dissipation is close to if not over 70% at that point. I've owned a lot of 5150s, and they sound better biased cold.
TheWinterSnow
Den Mørke Natt
Thanks dude you've saves me some work
Its called a bias probe, get one.
Its all preference, I love my 5150 at 42mA (which is exactly at 70%). That additional sag makes the amp sound a bit more pissed off and compressed on the palm mutes which I absolutely love. I don't get what you mean by mushy though, as I have yet to experience that.
That's your opinion - Personally, I think they sound leaps and bounds better biased at the correct voltage, cross-over distortion is gross. Mushy? If anything, my 5150 sounds much fuller, maybe that's what you mean by mushy? Amps with properly biased tubes (~70%) last years without a tube changes, I don't see how the 5150 would be any different
You might as well not even install the bias mod then if you prefer the amp cold.
- Jul 27, 2009
- 1,408
- 0
- 0
TheWinterSnow
Den Mørke Natt
::XeS::
Member
I maxed the bias pot in my 6505+ and it reach 35mA for tube. It sounds nice.
I did a 6505 vs 6505+ comparison 2 weeks ago and the 6505 was very very cold... with the post gain at 3 it had the same volume of mine at 2.
I did a 6505 vs 6505+ comparison 2 weeks ago and the 6505 was very very cold... with the post gain at 3 it had the same volume of mine at 2.
Are you sure it wasn't 35mA for the pair of tubes? I had to add a resistor to get even close to that on my friends 6505+.
+1, maybe the 6505+ was modded for a bigger sweep? Even with the pot turned all the way up, the voltage should still be too cold for those tubes..
::XeS::
Member
No it's not modded. I have the bias probe and it measures the current for the single tubes (in fact every tube has a different current).
PaoloJM
Member
I've modded a few and i prefer to bias somewhere around 25mA personally for 5150s.
Anything below the 70%** rule is "safe".
** (W*.75)/Vp where W = valve wattage (25 W for most 6L6 and 5881s, 30W for 6L6GCs) and Vp equals the plate voltage with respect to ground.
That's going to be 44mA or less for 6L6GCs and 36mA or less for most other valves, based on the typical Vp measured in most 5150s. Vp should always be measured and not assumed. These are max values, not target values.
A touch of cross over distortion in a guitar amp is great, despite the myth.
A non master volume Marshall has a good helping of crossover distortion when it's dimed, it's part of the sound.
Cranked master volume amps like the JCM800 etc. have it in spades, dimed AC30s have it too.
It gives that real aggressive, "hairy" sound.
If you don't like it bias higher, within limits of course.
People seem to believe cross over distortion is bad because most of our valve technology books are written for HiFi where is sure is a bad thing.
It's not the case for guitar amps.
Cranked power amps have cross over distortion. Most people like cranked guitar amps. Therefore most people like cross over distortion in a guitar amp.
Anything below the 70%** rule is "safe".
** (W*.75)/Vp where W = valve wattage (25 W for most 6L6 and 5881s, 30W for 6L6GCs) and Vp equals the plate voltage with respect to ground.
That's going to be 44mA or less for 6L6GCs and 36mA or less for most other valves, based on the typical Vp measured in most 5150s. Vp should always be measured and not assumed. These are max values, not target values.
A touch of cross over distortion in a guitar amp is great, despite the myth.
A non master volume Marshall has a good helping of crossover distortion when it's dimed, it's part of the sound.
Cranked master volume amps like the JCM800 etc. have it in spades, dimed AC30s have it too.
It gives that real aggressive, "hairy" sound.
If you don't like it bias higher, within limits of course.
People seem to believe cross over distortion is bad because most of our valve technology books are written for HiFi where is sure is a bad thing.
It's not the case for guitar amps.
Cranked power amps have cross over distortion. Most people like cranked guitar amps. Therefore most people like cross over distortion in a guitar amp.
lolzgreg
Cereal Shipping Sneapster
That's your opinion - Personally, I think they sound leaps and bounds better biased at the correct voltage, cross-over distortion is gross. Mushy? If anything, my 5150 sounds much fuller, maybe that's what you mean by mushy? Amps with properly biased tubes (~70%) last years without a tube changes, I don't see how the 5150 would be any different
And the other 5000 engineers in the world who own stock 6505s, have no issues producing amazing tones with them off the shelf? Peavey has sold a billion of these amps, and with the stock lack of bias adjustment, it really hasn't been a significant issue. I've had 6505s modded, and heard several other modded 6505s belonging to other people, and in my honest, educated, and experienced opinion, just about nothing beats a stock 6505 with a good boost for delivering awesome metal tones.
TheWinterSnow
Den Mørke Natt
This falls into the "if everyone else is doing it you should do it to" category, just because some people like one sound, doesn't mean its the end all say all. everyone is entitled to their opinion, I know of many people that bias mod their amps and still keep them relatively cold, it all comes down to what you like and if the mod is worth it.
Exactly.
lolzgreg, I could care less what other "engineers" do or like, because what they like might not work for you or me. I didn't say that what you like is wrong, nor did I say what I like is right. You gave your opinion, I gave mine.
However, while I realize that this is more of a music production oriented forum, since you're the one who spoke about the "other 5000 engineers in the world who own stock 6505s," how many of these "5000 engineers" have actually tried a bias modded 6505 to compare to their stock amps? Not many, I'd guess.
Again, I'm not attacking your opinion, but go say that on any other guitar gear-related forum on the internet and I'm sure most will disagree with you. Of course, I applaud you for having your own, educated, opinion, which is refreshing to hear, however, I've also found this forum to be completely opposite on many tone-related subjects on guitars from the rest of the the internet community of musicians, of which I myself neither completely agree with, nor disagree with.
I have formulated my own opinions, as well, by owning over 70 different tube amps, not counting the ones I've had multiples of (which includes at least 14 amps from the 5150 family). So, you can throw out your own credentials to make it seem like I don't know what I'm talking about to every one, and to belittle my opinion, but in the end, I highly doubt you've had more experience with most guitar amps than I have. I'm not trying to convert you to my opinion, hell, I could really care less what you think - If you're happy with your amp, that's all that matters - I've just been merely stating my own
