Bias control, a point not to be taken lightly

[Humbucker]

Bias with Express seems to raise an eyebrow or two, sometimes even neck hair.
When I was cobbling my clone my knowledgeable friend, who has background in electronics, made an adjustment to Kelly schematic.

The problem was the same mentioned on this site: not enough juice. So my friend checked his build and measured a bit here and there and came up with this: bias should be taken from somewhere else.

I attach his words and a drawing as a Word doc below. Hope it helps to clarify the bias question - or at least shows us where we left the trail to Holy Grail

Keep on rocking.

[paulruby]

Here's a very solid bias circuit I use for all my amps...

[Allynmey]

Paul, it is very strange to me how 10 of us could build the same amp and some not have the range in the Bias. I like the design you put in above. I wonder if we could replace the 180K with a 2W locking pot and call it "range" next to the Bias pot and initially set it to whatever the schematic calls for and then adjust until our range is set then lock it down.

What do you think?

Allyn

[UR12]

The bias supply on the Kelly schematic is wrong and this has been known for at least a year now since the Franny pics came out. It has been pointed out (By myself and others) in numerous posts in the past but the schematic has never been corrected. I am sure if you do a search on this forum you will get numerous hits on the subject.

Paul
That looks pretty much like the Marshall style layout. It is rock solid but in the event of a failure in the wiper of the pot you will loose bias to the tubes, redplate and blow a fuse and / or tubes. I know that it has been argued that the failure rate is slim to none on the bias pots but they can fail. Using the circuit that Ken uses protects the tubes from failure if the wiper opens up on the pot by applying a max negative bias voltage on the tubes thus protecting them. Both circuits will work fine.

[paulruby]

Excellent point: I forgot a wire...

And, no need for a variable resistor for the 180K. In only needs tweaked for alternative PTs that deliver drastically different VAC. It is not all that critical.

[mac0611]

I've thought it strange also. I started at 220K off of the secondary tap, and ended up at 47K to get into that range, although I have about -47VDC at the high end. I biased the EL34's at about 40mA@390VDC on the plates, which sounds good to me.

[Humbucker]

...and I just thought somebody would tar and feather us...
but yes, the Kelly schematic has an error and we stumbled upon it without knowing about the thing. Sorry if we wasted time and bandwidth but the bias thing just kept popping up.
After correcting the bias circuit the bias worked fine, plate voltage was remarkable and, what keeps me crossing my fingers, idle hiss is low and hum virtually nonexistent.

Yes. Clips are coming when I get the chance. Might record some rhytm tracks with my friends TW clone (which has two extra components to lower the gain a bit) and and use mine for riffs and solos, for comparison.

Thank you all for support.

[UR12]

...and I just thought somebody would tar and feather us...
but yes, the Kelly schematic has an error and we stumbled upon it without knowing about the thing. Sorry if we wasted time and bandwidth but the bias thing just kept popping up.
After correcting the bias circuit the bias worked fine, plate voltage was remarkable and, what keeps me crossing my fingers, idle hiss is low and hum virtually nonexistent.

Yes. Clips are coming when I get the chance. Might record some rhytm tracks with my friends TW clone (which has two extra components to lower the gain a bit) and and use mine for riffs and solos, for comparison.

Thank you all for support.

Humbucker

I didn't mean for it to sound like we should tar and feather someone just that after a year I think we should look at correcting the schematic so people don't run into this problem. The Kelly 90 schematic was done prior to all of the Franchesca pics. I would be glad to change it if we could get Omar to post the revised schematic. There are a couple of other problems with the Kelly 90 schematic also. The fact that you brought it up again isn't wasting any bandwith but just serves to drive the point home that we need to change that schematic. I'm just glad you had a knowledgeable friend to help you get through this but I wonder how many people will have trouble with this again down the road.

Great to hear yet another Express has come to life and I can't wait to hear your clips!

[Zippy]

Paul, it is very strange to me how 10 of us could build the same amp and some not have the range in the Bias.

Might that not be just a function of the variation in the values of the power resisters? Or is everyone using high precision resistors?

[Allynmey]

No tar and feathering with the esteemed gentlemen who answered your post! Just great knowledge and a polite answer with these guys! I'm glad to see the forum taking on a great gentlemen like tone lately. Seems more like a a year and a half ago!

Allynmey

[Omar]

Guys,
If you could edit the Kelly90 schematic with the corrections I would be glad to replace it. I'm not aware of all the corrections otherwise I would do it.

Thanks,
Omar

[UR12]

Omar

Here is a doctored up version but it has all the corrections that I know of. If anyone sees anything else that needs changing then let me know.

[Humbucker]

What can I say? Great forum, if I've ever seen one!

[paulruby]

Omar

Here is a doctored up version but it has all the corrections that I know of. If anyone sees anything else that needs changing then let me know.

What bias range does this give? At a glance, it doesn't look like it would have enough range for all types of power tubes. KT66 needs -50V but 6V6 might only need -20V. The circuit I posted above gets full range.

[UR12]

Omar

Here is a doctored up version but it has all the corrections that I know of. If anyone sees anything else that needs changing then let me know.

What bias range does this give? At a glance, it doesn't look like it would have enough range for all types of power tubes. KT66 needs -50V but 6V6 might only need -20V. The circuit I posted above gets full range.

Hey Paul

The bias range of the pot is generally -28 - -42. Of course this is dependent upon the actual voltage of the tranny that is used. I think that Moose's trannys fall into this range and they are supposed to be a copy of Ken's. Having said that, some people have had to adjust the value of the 220k resistor to a 150k to get the pot in the correct range. It is believed that Ken set his bias voltage at -33v and it was noted by him in his literature that the amp was set up to use either EL34s or 6V6s "without the need to change the bias". These are the only tubes he mentioned.

The schematic changes I made corrected the circuit as drawn by Mr. Brown. After Allyn and Greg posted the pictures of Franchesca, we found that the schematic didn't match the bias circuit in the actual amp that Ken built. The circuit and the component values are taken from the pictures posted of Franchesca and IMHO should be a starting point for anyone wanting to clone an Express. Anything other than the exact circuit used should be considered a 'Mod'

Personally I like to switch the values of the 47k resistor and the 25k pot around. In other words I like to use a 47k - 50k pot and a 25k - 27k resistor, but that's just me. Your circuit will also work flawlessly and give you more range. The TWs have always been mysterious because so few pictures existed and it would seem that there are a lot of people guessing and putting out various schematics of their own version over the years.
Since we have had so much interest generated by these amps in the last year maybe it would be fitting to create a Mods section in the Files area for people to post their mods. While I have never seen one or heard of an Express running KT66s, hey it might sound great and there would be a need to modify not just the bias circuit, but maybe a few other wiring changes also. Some people are adding a "Touch Sensitivity" switch (Split plate resistors) with some great results.

There are a lot of guys on this forum like you and me that could build this amp in our sleep using the data that is now available. I just wanted to attempt to get an Express schematic posted without errors to document a real Express to give some of the guys that are just now getting their feet wet something to go by .

Someone stated that Ken had always asked the question "what have you done to make it better" so maybe a Mods section in the files section would be just the ticket.

Sorry for such a long winded answer

Below is a picture of the actual PS board in Franchesca. Note that the bias is taken from the White wire