How to properly bias an express......

[marcoloco961]

Just for conversations sake, my bias was at -31.3 with the cathode current @ 45mA. After bumping up to 49mA at the cathode reading, my bias was at -30.4.

All that said, still within the -30 to - 33 range suggested and it sounds freakin great. That 4 mA brought this thing to life. It only takes a second to try, and you might just like the results. It improved the punch and the dynamics quite a bit to the naked ear. Well worth the couple of minutes to try it, IMHO. I personally will sacrafice a little tube life to get the sound I want.

[redshark]

Thanks Rooster, paulster and everyone that came to give their opinions.
Yes indeed is in the little details. That is why "nobody can make a TW that sounds like a TW but Ken". I'm starting to believe that.
I finished my amp (in december), took me 2 weeks to build but I knew at that moment that it was just the beggining. I've been tweaking the amp for 4 months.
Tonight I took the amp to a blues jam and for the first time I heard it in a live situation. Cuts through great and sounded better than the other amps. I'm proud of the results and all that I've learned with this adventure. I will experiment with bias settings and I will try all that has been said here.

I'm ready now for the rocket!!

[rooster]

Er, Firestorm, I missed thanking you. I can't read the O'Connor take on this because i don't have his books. But I get that he has covered this, too. Maybe someday. That's the thing about Aikens, he really puts his thoughts right out there for free, which sometimes pays off I think. In this case, yes.

[passfan]

Great stuff in this thread right here. I boosted mine by 5ma and it sounded so good I broke out the calculator and ran them on up to 85% at 55ma and 400 volts. Played her for around 2 hrs checking for signs of redplating and found none. Wow what a difference. I guess I'll spring for a new set every year or so for sound like this.

[Firestorm]

Er, Firestorm, I missed thanking you. I can't read the O'Connor take on this because i don't have his books. But I get that he has covered this, too. Maybe someday. That's the thing about Aikens, he really puts his thoughts right out there for free, which sometimes pays off I think. In this case, yes.

I always loved Aikens' site; as you say, he just puts it out there. If you get around to buying books, even before O'Connor you should grab Merlin Blencowe's wonderful little book on preamps. Completely self-contained as opposed to O'Connor who constantly refers to things covered in one of his other books, so you can't always get what he's saying unless you happen to have that one too.

[paulster]

+1 on Merlin's book.

Hopefully a) he's listening since he's a member here and b) he writes a second one on power amps.

[sst4270]

Ahh... The Valve Wizard. I love his articles. And I also too purchased his book.

[paulster]

I think we may all have to agree that this is always going to be a grey area, this knowing exactly how our particular clones would sound if Ken biased our amps with the tubes that he chose. Regarding the *correct* biasing method, Ken has left the building in a trail of crumbs. redshark, you get the 'Best Detective' award today by pointing out the -32 number. I hadn't heard that before.

Going back to this point, from Ken's 'Trainwreck Pages' the 'correct' bias for an Express was stated as being -30V at pin 5 on the output tubes, for both 6V6 and EL34 tubes.

This would be with Siemens tubes for the EL34s and who knows what for 6V6 tubes? (Aside here: does anybody know what the preferred 6V6 tubes were?)

This would be pretty hot and certainly around or above the 70% mark depending on the particular pair of tubes used.

Unless we know for certain who'd biased Kelly at -33V it's difficult to say whether it was at optimal bias at the time, or how the tubes were holding up compared to when Ken had biased it, for example.

It's safe though to say that a figure around or slightly above 70% plate dissipation is where he was shooting for, and that seems to tally with what everyone finds when they get the bias into the sweet spot.

Fortunately the plate voltages are comparatively low, so it's not going to suffer red-plating like a Marshall would when the bias is pushed towards the hot side.

[redshark]

It would be interesting here to know why Ken changed the value of the resistor from 56k to 47k and how does that affect the bias settings.
Yes I tried the -30vdc with my Siemens pair but that set the current like 54mA on my bias probe -6mA screens would be like 48mA at 408vdc on plates. Way hotter than 70%.
My 70% should be like 43mA.... so 49mA on probes.

[redshark]

OK, I tried it, raised my bias to 49mA - 6mA = 43mA that actually matched with -31vdc. Played it for 20 minutes and....

I decided to take it again where it was...-32vdc 43mA in my probes...

For some reason the amp started being more agressive and turned slightly brighter by getting it hotter, some people call that "bite".. the most beautiful thing about this amp is the transition "clean to mean" on the warmer side.

[paulster]

It would be interesting here to know why Ken changed the value of the resistor from 56k to 47k and how does that affect the bias settings.

If you mean Ken changing the value of the resistor from a regular 50W Marshall bias circuit 56K to 47K then it's simply the bias pot range resistor.

Because he (and Marshall) chose to use 20K bias trimmers there isn't a lot of voltage range available, the the 47K or 56K resistor sets the extreme of this range.

It's quite common in Marshalls to have to replace this resistor depending on the tubes you drop in, as the range is so limiting.

Ken probably just found that for the voltages he was using (much lower than Marshall) and the fact that he was allowing for biasing 6V6 and EL34 tubes that this value gave the most acceptable range.

Bear in mind that it is also affected by the dropping resistor (220K), which again was lifted straight from a Marshall 1987 50W plexi.

[steviea55]

I have my express project build bias set at -33vdc and it sounds good to my ear at this setting. I have varied it slightly each way and this is the spot for the tubes I'm using. I am running a set of old EL34's, so I'll be tweeking it agin when I get new tubes. I believe a lot of the issues folks have with this build are with the grounding, but that's for another thread. No dis-respect to anyone but, oh yea, If I ever hit it BIG I am going to buy an Original Express and then we will have more pic's,data, etc.

[Cliff Schecht]

My pot maxes out at -31V at my house with a 56k resistor and a 125V wall voltage. I probably need to turn it just a bit so that I'm not pushing the power tubes too hard on a normal 120V line but I haven't done this yet, it sounds good as is .

[passfan]

OK, I tried it, raised my bias to 49mA - 6mA = 43mA that actually matched with -31vdc. Played it for 20 minutes and....

I decided to take it again where it was...-32vdc 43mA in my probes...

For some reason the amp started being more agressive and turned slightly brighter by getting it hotter, some people call that "bite".. the most beautiful thing about this amp is the transition "clean to mean" on the warmer side.

I tend to agree with you after listening to mine the last couple of days. I'm going to cool it down and see where it goes.

[redshark]

Yes, there's an optimal point for the tubes to be run at. IT DOESN'T necesarily has to be 70%. Every pair of matched tubes is capable of drawing certain current according to the voltage applied. That is what basicaly the groove tubes rating is and that started in the 80's.
When Ken was building amps in the 80's he was mostly using certain batches of Siemens EL34's that were labeled and graded by groove tubes because he was a dealer so that is why he set the bias by negative voltage. It was easy to say...."get a pair of grade 6 el34 and run -30vdc"
Most of the tubes labeled grade 6 were gonna react in a similar way to the negative voltage applied.
In bias columns written by Ken he never refered to any 70% formula. He said from this mA range to this you can bias.
It takes a good ear and patience to set the bias in an amp to reach that "sweet spot" and make it inspire your playing.
"Let's see how she wants to sound..."