Express rewire "correct" sockets still some hum

[KGW]

I rewired my express again using the tube sockets from Mojo.
Here are some pix: http://beggingdogrecords.tripod.com/TWEX.html.

The amp sounds good, but is still not hum free. I had my doubts that using the "correct" sockets would make any difference since the hum is always there. Note, it is not terrible hum, but I'm having doubts that I can get rid of all of it. So I guess I'll leave it as is for now -- yeah, like that ever happens

I'll try to record some clips if I can.

P.S. working on my Dumb/Oogie (ODS built in a boogie MkIII chassis/iron) this winter, then a Rocket

[rooster]

KGW - Cool. Much better tube layout, Eh? One question: What the h#!@ are those 220K 1 watt resistors attached to and what are they doing there?

Also, on the sheilded wire leaving the input jact, do you have a resistor at pin 6?

[KGW]

KGW - Cool. Much better tube layout, Eh? One question: What the h#!@ are those 220K 1 watt resistors attached to and what are they doing there?

The 220k are the bias feed resistors - read TUT1 and the Garnet book for an explanation. Basically, it improves the life of the EL34s and (IMHO) is a neater layout.

Also, on the sheilded wire leaving the input jact, do you have a resistor at pin 6?

Do you mean pin 7 (grid)? No there is no resistor there. But there is none on Frannie.

[rooster]

KGW - Oh sorry, yeah, pin 7. Well, whatever the Franny is, she is an old gal. Glen uses a 10K carbon film resistor attached to pin 7 - you owe it to yourself to try it. I run this on mine, FWIW.

BTW, pull V1 and tell me if you amp is quiet?

[rooster]

KGW - You have heard Glen's youtube clips, of course. In another post I do a sound clip of my express: "hammond 1650g".

This is running the 10K resistor. I was just quickly trying to throw a Hammond OT clip up so its not too organized. However, there are moments where I don't play gtr - and - the gtr (Strat) volume is on '10' the whole time except for the very very end. My amp is not noisy is my point, and if yours sounds like mine then you are maybe overreacting - noise is sometimes a subjective thing, you know?

[KGW]

Thanks rooster - I appreciate the suggestions.

The hum is always there - power supply. volume, tone controls, etc, do not affect the hum.
That is why I was skeptical about the tube sockets being the culprit. But I thought, what the hell.
I tried pulling V1 and the hum doesn't go away.

And yes, I'm picky as hell about great tone AND getting rid of hum.
From the express clips I've heard (lots), my amp sounds like the clips.

My KGW-ized 1987 clone really is hum free and sounds great (though obviously less gain than an express). I spent lots of time on that one - different coupling caps (finally settling on premium sozos), wiring, grounding .... It uses a 2 ground buss scheme (like Aiken's)- preamp (grounded near imput jack) and power (grounded at PT ). I tried this on the Express and it has around the same as amount of hum as Alan's ground scheme.

BTW, the tubes in my express are New Sensor tung-sol and mullard re-issues.[/url]

[rooster]

KGW - OK, do me a favor? If we've done this before tell me...

Pull V1 and V2. Is the hum still there? Are your tubes biased to 40 mas? Do you have a good set of EL34s? Sometimes, especially if you have hum with V1 and V2 pulled, you could have a single bad pwr tube making the hum.

OK, if all of this is hum-free, put V2 back in. Does the hum come back?

OK, here's a problem. If it does, this would be very unusual. Most hum in these circuits come from V1. Now, if the hum was still there with V1 - AND V2 - pulled, you are dealing with an very strange beast....

........which leaves us with that 220K bias supply thing you have installed. Here I am blind. But it does occur to me that you might have one pwr tube taking a bias - and the other not. Is there a way - using an external bias type device - that you can check the bias of EACH tube?

For that matter, as a last ditch effort here, pull all the tubes. Now put 1 pwr tube in. Does the amp hum? Pull that tube and put the other pwr tube in. Does the amp hum? If good, then plug them both in. Hum? Switch sockets now and start over with 1 pwr tube, etc.. Hum?

I have to think, and only because what you have done here is different than what I have done - and the fact that you say the hum is quite obvious. It could be a pwr tube issue, related to biasing.

OK, get back to me on this? Oh, one last thing? Are you running with the Tonesluts trannys? When you turn you amp on, but not switching the 'standby' function, does the pwr tranny hum?

[Jana]

if it hums with all the tubes pulled it could be that the transformers are not oriented properly.

[KGW]

KGW - OK, do me a favor? If we've done this before tell me...

Pull V1 and V2. Is the hum still there? Are your tubes biased to 40 mas? Do you have a good set of EL34s? Sometimes, especially if you have hum with V1 and V2 pulled, you could have a single bad pwr tube making the hum.

OK, if all of this is hum-free, put V2 back in. Does the hum come back?

Yes it goes away with V1 and V2 pulled. There is about 4.5mV difference in the bias between the output tubes. I have tried adjusting the bias and the hum level barely changes.

Also, I tried switching the 12AX7s and have them in the quietest combination. I also tried some other 12AX7s (JJs). The Tungsols are way better than the JJs.

However, there is a difference between the tubes as far as hum is concerned.

OK, here's a problem. If it does, this would be very unusual. Most hum in these circuits come from V1. Now, if the hum was still there with V1 - AND V2 - pulled, you are dealing with an very strange beast....

........which leaves us with that 220K bias supply thing you have installed. Here I am blind. But it does occur to me that you might have one pwr tube taking a bias - and the other not. Is there a way - using an external bias type device - that you can check the bias of EACH tube?

Get over the 220k placement - I've done it both ways. Also, I've had this amp for 2 years now.

For that matter, as a last ditch effort here, pull all the tubes. Now put 1 pwr tube in. Does the amp hum? Pull that tube and put the other pwr tube in. Does the amp hum? If good, then plug them both in. Hum? Switch sockets now and start over with 1 pwr tube, etc.. Hum?

Of course no hum with both pulled. Hum with either or both.

I have to think, and only because what you have done here is different than what I have done - and the fact that you say the hum is quite obvious. It could be a pwr tube issue, related to biasing.

"quite obvious?" well ... it is there. Much better than most amps. Still very playable. Maybe a recording would help. Sorry, can't do that right now though. Maybe measuring it on the scope would be a better metric?

OK, get back to me on this? Oh, one last thing? Are you running with the Tonesluts trannys? When you turn you amp on, but not switching the 'standby' function, does the pwr tranny hum?

Heyboer trannies (I think I got them from toneslut - it was a while back).

There if hum (not though the speakers) in stadby mode.

[rooster]

KGW - OK, so V2 causes some hum then - did I hear this right?. Man, this does not happen in my amps, no matter what tube I put in there, NOS, new JJ, whatever.

So if you are still saying that this is loud enough to be a problem, then I got nothing for you. All I can say is that there has to be something wrong in the V2 section of the circuit - or you are over reacting to the hum. Without hearing it I can't tell you, sorry.

Going back to the TS PT, due to an unacceptable hum while the amp was in 'standby' mode, I talked with Chris and we swapped this out. One tell tale sign in mine was a very warm PT. As in HOT, actually. I also know that another member had a similar issue and made a swap for better results. ?? You might want to consider this. Mine was so loud in standby mode you could walk in the room and know that the amp was on - no pilot light required.

OK, I'm here but I think Ive exhausted my ideas...

[geetarpicker]

That is an incorrect rumor that I run a "10k resistor on pin 7". I don't have that in either my original nor two clones of her. I'm not sure where that rumor started, however I've never needed to add the part nor experimented with such.

I did however experiment with raising the PI fizz cap value in my clones, but eventually put that part back to stock. A little felt in front of my Scumback 12s helped warm them up just fine for my "clone" setup.

On the hum subject. It might be the way you have your grounds on the main filter caps. In the original there are three separate ground wires for those caps, that all terminate at the ground lug where the two big caps stand up from. In your pics it appears you may have a ground lead running from your pots over to the PS caps? Not that I understand Ken's logic with what he did, but it might be worth checking into as yours appears different in that department. Maybe you have some ground loops? That said, it's a little hard to tell as your pics aren't really close up so maybe you got it right.

Cheers,
GK

[KGW]

On the hum subject. It might be the way you have your grounds on the main filter caps. In the original there are three separate ground wires for those caps, that all terminate at the ground lug where the two big caps stand up from. In your pics it appears you may have a ground lead running from your pots over to the PS caps? Not that I understand Ken's logic with what he did, but it might be worth checking into as yours appears different in that department. Maybe you have some ground loops? That said, it's a little hard to tell as your pics aren't really close up so maybe you got it right.

Cheers,
GK

Actually, if you right click on the image and choose "View Image", you'll get a hires version.
The 3 40uF caps are hooked together and connected to the ground point where the bleeders go.
The 20uF cap next to the PSU board is grounded to the lug by the presence pot. The other 2 20uF caps are wired together and grounded at the lug by the middle pot.

Is this possible: ken used big CC resistors for the 100k bleeders and I used MO. Since metal film and metal oxide resistors can pick up RF, maybe I should try CC.

Also, I have tried moving the wired around with a chopstick and this does not seem to make a difference.

Or maybe, I need to find some really low noise tubes. However, the new sensors (that I have) were highly praised on this forum. They definitely sound better than the EH tubes I had before.

[KGW]

BTW, I got the iron from Paul Ruby in 2006

[rooster]

Glen - Well it may be mostly me that presents that rumor.... In the PDF file of the Express build, there's some quotes from you that deal with your PI cap. Yes, I see where you finish your statement - and then another quote comes along from someone else talking about the grid stopper resistor on pin 7. Somewhere else in the PDF file Paul Ruby says some things about grid stop resisitors, maybe Aikens, too, and then there seems to be just a list of quotes that aren't attributed to anyone in particular... that says some things about it.

Also I think maybe you may have suggested it to someone in a post? The 10K value? Eh, its seems like you said you tried in one of your builds, but I guess not. At any rate, sorry for attributing this to you, Glen.

KGW, I do use this on mine, but, not to be too obtuse here, the amp doesn't need it for noise per se. However I do realize that there have been grid stoppers on any amp that I have ever played or built - and hence my buy-in. For further example, in a couple of SRV's Fender amps, the Vibroverbs for certain, not clear about the SRs, Diaz said that he changed the 68Ks to 100Ks, making the value of the first input 50K. This is really not much different to the tube than the stock value, 33K, considering the tube really doesn't need to see anything for proper function and good headroom - but both parties seemed to notice a difference.

Anyway, with this as a background, and the understanding that some resistance here can stop some potential noise, I run my Expresses with a gridstopper resistor on pin 7. Funny thing about it is, it changed the 'feel' of the amp, too, something I was looking for.

[gtrgeek1]

I am also preoccupied with hum in my builds. Have you tried swapping the heater wires on one of the power tubes so pin 2 goes to pin 2 and pin 7 goes to pin 7? My understanding is that these heater wires should be "in phase" with each other so the hum gets cancelled in the output tranny. It may not change things much since the hum goes away with V1 & 2 removed, but it looks like you have enough length on the wires to give it a shot.