Express - Exploring 12AX7s

[Littlewyan]

I've been trying different modern 12AX7s in my Express amp to find one that gives me a quiet amp AND the tone I want. At the moment I have:

V1 - EH12AX7
V2 - JJ ECC83 MG
V3 - EH12AX7

The issue I have is my EH12AX7s hum like theres no tomorrow! And also sound just a tiny bit thin in the high end.

Anyway, one thing I have found when valve rolling is that every JJ I've got which includes the ECC83 MG, ECC83S and even a Harma ECC83-STR (Selected ECC83S) all buzz in the V1 position. This is without the guitar plugged in. Anyone else had this?

[Littlewyan]

No one have anything to add to this? Just tried the JJ Valves again to make sure I wasn't getting mixed up and each one buzzes, although the ECC803S doesn't buzz quite as much as the others.

Another thing I found is the Tung Sol 12AX7 (Also the Harma Retro) loves to hiss.

Anyway the EH12AX7, JJ ECC83S, JJ ECC83 MG and the Tung Sol 12AX7 all have nice qualities. I never thought any of them sounded bad, except for an older EH12AX7 that I stuck in V1, that sounded a bit dull, however I replaced it with a brand new EH12AX7 and that sounded great. Looks like I may have to buy some more to find quieter valves!

[mojo]

First post for me here in a while, and with the disclaimer that many others here know much more than I do. Having said that, I do have some advice for preamp tubes in an Express generally.

One thing to keep in mind is that in an Express, there is a progression of which gain stages distort when. Unlike some amps, the PI tends to go first and cause the most distortion. In most amps, I like to put a clean tube that provides good current - in new tubes the Sovtek LPS is a good choice. In an Express, my priorities change, and I pick a tube that sounds good distorted - the best being a vintage Mullard. I don't really care for much of the new production in this spot, but I'd probably pick the Mullard reissue or the Sovtek LPS. I'd stick with your EH tubes in V1. I kind of like the Tung Sol RI in V1 - you might try that again with the new PI tube. In V2 use a robust tube - the EH should be fine there - I like the Ruby for new production. V3 makes the most tonal difference in these amps IMHO.

Hope that helps.

[Littlewyan]

Well I just tried my Harma ECC83 Retro (Tung Sol) in V3 and compared to the EH 12AX7 it sounds so much fatter and has a much more pleasing distortion. The EH12AX7 sounded sterile compared. I didn't get a chance to finish my test though, I had a brand new balanced EH12AX7 that I wanted to try in V3 to see if it was just that one EH12AX7 that sounded bad. I shall try this another night.

[Gibsonman63]

vintage 12AX7 types can still be had at reasonable prices. Organ pulls can be cheaper than new production tubes on eBay and sometimes here. I ordered NOS Tunsgrams when I built my Express (because it was recommended in the build notes) and was happy from the begining. I fired it up initially with some new production tubes I had laying around. The NOS were night and day smoother.

[Littlewyan]

The problem with vintage valves now is whatever we buy are basically rejects. All the good valves were picked out of LOTs years ago and the LOTs have been sieved through many times. Not to mention they're going to run out in the not too distant future so my thinking is we may as well get used to the current production. But thats just my view on it, if you feel its worth the money buying vintage valves then go for it. I'm not criticizing anyone for it I just don't feel its worth it.

I do actually have a few vintage 12AX7s but I'm keeping them safe. I might try them in an amp for an experiment or for comparison every now and then but thats it.

Anyway I just had a bit of time to try my balanced EH12AX7 and it did sound better than the other EH12AX7 I had in there but still not as fat and harmonically rich sounding as the Harma ECC83 Retro (Tung Sol). The Tung Sol isn't as sharp on the notes as the EH which in most ways is good as its very pleasing to the ear but in other ways its nice to have good note definition. However I may get that definition back by swapping the valves in V1 and V2. O the beauty of valve rolling!

[strelok]

In regards to your noise problem, have you tried elevating the heaters? This is going to be my next try in attempting to get rid of some issues I'm having with my single ended wreck when I get the time. From what I understand it can eliminate a whole host of issues. It's quite simple, if you don't already know how, its just a voltage divider taken off of the B+, say from the preamp or screen rail. Something like 330k for the top of the divider 80k for the bottom, bypass the 80k with a 100v 10-100uf electrolytic cap. Then connect your center tap or artificial center tap of your heater winding to the middle of the divider.

[Colossal]

In regards to your noise problem, have you tried elevating the heaters? This is going to be my next try in attempting to get rid of some issues I'm having with my single ended wreck when I get the time. From what I understand it can eliminate a whole host of issues. It's quite simple, if you don't already know how, its just a voltage divider taken off of the B+, say from the preamp or screen rail. Something like 330k for the top of the divider 80k for the bottom, bypass the 80k with a 100v 10-100uf electrolytic cap. Then connect your center tap or artificial center tap of your heater winding to the middle of the divider.

Good call. I use 180-220k (top) and 47k (bottom) depending on where the divider gets placed in the B+ supply chain. Definitely a good idea for any amp with a cathode follower, but it's an easy and useful mod for any amp.

[Littlewyan]

That would normally get rid of hum wouldn't it? I was going to try a shielded wire from the volume pot to V2 to see if that lowered the amount of noise and microphonics. I know the Komet amps use shielded wire here and I heard that Ken designed those amps to work with modern valves.

[JoeCon]

Dave

So like a Rocket type amp with cathode follower the elevated heater voltage will help eliminate/reduce hum.
I've got no center tap and use two 100R to ground for my tap. So run those two resistors to that junction instead of to ground?

Thanks....

[Littlewyan]

In the Rocket you could run the two 100R resistors to the Power Valve Cathodes. That would reduce hum.

[Colossal]

Dave

So like a Rocket type amp with cathode follower the elevated heater voltage will help eliminate/reduce hum.
I've got no center tap and use two 100R to ground for my tap. So run those two resistors to that junction instead of to ground?

Thanks....

Hi Joe,

Yes, you would run the low B+ (50-60VDC is fine for an offset) to the junction of your 100R/100R virtual center tap. The heater winding then sees AC ground via the capacitor in your elevator circuit. Everyone gets happy and your heater-to-cathode voltage stays well within limits in your cathode follower. I do this in every Marshall I build where cathode follower voltages can run high and every one of them turns out dead quiet. Old production preamp tubes had higher heater-to-cathode limits and could handle the abuse. New tubes have about half the limit (about 100V). I add this to most scratch builds because it's three parts.

[Colossal]

In the Rocket you could run the two 100R resistors to the Power Valve Cathodes. That would reduce hum.

Yes, you can do that too and it requires no extra parts. But you are limited to the cathode voltage so if you want a bigger offset, you have to go the divider/cap route.

[Littlewyan]

I have been looking at photos of my build tonight and noticed that my V3 wires are a lot longer than Ken's. See how the Cathode and right hand grid wire (on the input to the PI) are in the air and made short as possible? Think I'll try shortening mine first to see if it lowers the noise floor at all and makes the amp slightly less prone to microphonics.

I haven't tried the shielded wire yet as I haven't had time. I wanted plenty of spare time so I can measure the hiss of the amp with a DB Meter before and after making changes. Also keep the valves in that make the amp ever so slightly microphonic at full tilt and make a note of if this improves at all. i.e. if it becomes microphonic later or not at all after changes. I know on V3 it won't make as much difference but I did move the NFB wire in IMG_4448 to the other side of the board screws and this made a difference to the amp feeding back with certain valves.

I don't doubt though that I will have to use shielded wire on the volume pot wire. The Komet Amps use this and I believe KF said those amps were designed with modern valves in mind. I'm sure he also said that the Express amps were outdated as they needed older valves but he still had a list of builds to complete so I'm not sure this is true!

[Littlewyan]

Unbelievable.

I redid the V3 cathode wide but this time I went straight from the turret to the wire between the 2 cathode pins instead of making the wire long and resting it on the chassis.

Before I'd have the volume on 10, presence, middle and bass on 5 and I wouldn't be able to turn the treble up past 9.5. Now I can turn everything on 10 except the presence which can't go above 9. I only took off 2cm of wire!

I'll shorten the grid wire now to see what difference that makes if any. The hiss hasn't decreased at all by the way, just the microphonics. Which is fine as howling feedback with no guitar plugged in is much more annoying than hiss.