That great Express and...hiss

[mojotom]

First I want to thank you all for this great forum. I build an Express some month ago and I love it, all the posts here and the pics of francesca were the missing part I was waiting since I've heard about the trainwreck on the Ampage forum long time ago.

I build this one using Nik, Moose and Dana parts along with Allen Bradley resistors, Sozo caps an PEC pots. The tone is just great ! I had some issues about brightness and hiss so I switched carbon comp to tantalum resistors and it was better defined with less hiss. As an attemp to reduce volume in order to be able to play it outside an isolated room I use a split load switch type of thing on the third stage. it works really great to create another less brutal sound without messing too much with the amp but even if it really quiet in "low mode", it's even more hissy too in "normal mode" .
The bright switch is almost unusable at low volumes. It's ok when played trainwreck loud but at lower volumes it's annoying.

I followed francesca layout as much as possible :

- Filaments CT, bias ground, PT CT and Ground from the mains
near the IEC socket

- Filter caps grounds to the lug of the bleeding resitors

- Board, pots and jacks grounds to a bus running from the input jack to
the presence pot (as the pots are PEC I can't solder to them).

- Output tubes grounds just like Francesca (without the 100R heaters
fake ground because I already have a CT)

- OT CT to output jacks

- I was aware of a potential hiss issue if the Output tranny primaries to
the plates of the power tubes are switched, so I wired them just like
the pics I've found where the blue wire is tied to the power tube closer
to the power board

I've read all amazing posts from Paul Ruby but I really wonder about rewire again the amp as I don't want it to look like crap because of too much desoldering.

Please tell me if you find something wrong here. The amp is so great I have to find a way to make it quieter.


Thanks.
Tom

[keithrick]

Glad your happy with the amp. I am as well.

With regard to some of your concerns regarding hiss. The Express is like an amp on 10 you only have a gain control. Even though hiss is present in my build it is comparable less then my Fender Bassman on 10.

The amp is not designed to be played at low volume. It is basically on full out and you control the volume from the guitar knob. In my humble opinion based upon what I have read and learned on my personal build is that it is not a bedroom amp but an amp that is on the verge of being out of control. In a band setting the hiss in not a factor.

I would suggest: (in no particular order)

1. Looking at your split load circuit (lead dress). Shouldn't it be on the second stage? Look at Mr. Ruby's wiring for his Rocket split load.

2. Consider shielded cable on the input wire and the grid wire to the volume control.

3. Use carbon film resisters like Mr Fisher did.

4. Is the bottom of the chassis shielded? If not do so.

5. Change the location of your rig and see if the hiss level changes. (lights on Dimmer switched can produce hiss)

6. Rule out your guitar as a culprit of hiss.

7. Change your bright cap values.

8. Check for cold solder joints and examine your ground connections.

[mojotom]

Thanks for answering !

I finished the amp in June so I had time to test it with lots of guitars and cabs. I know it's meant to be played loud and I have a soundproof room so I can use it as loud as I need but sometime I just like to play it between 9 and 11 when I try to save my hears a little. The fact that the 100p bright cap introduce some noise is a bit annoying but 500p is worse. I don't have a cab for the chassis but a wooden plate recovered by shielding aluminium foil help a lot. Good "quiet" tubes is a must.
I've just asked myself if I made something wrong about the wiring.

Shielded wire sure and I checked everything (It's not my first amp).


I had almost no noises when I changed the resistors but when I wired the split-load switch it was humming even more than before in normal mode. Maybe I should remove it. I use the third tube for this as Paul Ruby advise to do, I was a bit worried to mess the preamp structure if I done it on the second stage. The wiring is just like the Komet by the way with the same shielding

[mojotom]

I solve the issue mainly with a decent mains ground, some JJ preamp tubes and a quieter guitar. I still got some hiss when the bright switch is on but it's ok.
The split load switch is nice too because it's quieter and less in your face but still nice.

I found the amp to be a bit uncontrollable and a bit too bright so I switch to 47k feedback resistor and 680n presence cap so the amp is fatter sounding and more tight.

Just my two cents.

Tom

[Tube_S_Cream]

Has anyone got experiences with low µ tubes like 12AU7 in Pos. V2 and a 12AT7 or 12AU7as PI? The overall gain of my TW is enormous, so i can hear a significant hiss when the volume is turned down. Currently i'm using 12AX7EH tubes in all stages and a selected low noise 12AX7EH in V1.
Output tubes are a pair of "winged C" Svetlana EL34, because they shall be very close to the Siemens / Mullard design.

To describe the noise: Its really a hiss (like Tape hiss), no buzz, hum or crackling.
My amp is based on a Ceriatone Kit (but with vintage Dynacord trannies).
It has got a hell of gain and volume 9 is enough for a Strat to drive the amp into clipping. Especially for V2 i've heard, a hard driven low µ tube may have more character than a high gain one.
Simulating the preamp with LTSpice using the tube models from DuncanAmps shows V2 producing a very hard clipping of positive amplitudes while a 12AU7 looks much smoother.

Edit: .... first result: I've replaced the PI with an old Telefunken ECC81 .
Much better !!! With the EH 12AX7 the overdrive was extremely brittle and harsh. The gain is still ooompph but imho the overdrive is creamy have a lttle more clean area. If i want it sounds like a Blackface with JBL but it turns into a fat warm lead when i turn on th volume. Yeah that's the right way i want to go. Crsip and brilliant a low volumes and fat and meaty at high gain .
What are you favorite preamp valve combinations?



Greetz

Tube_S_Cream

[Ron]

If you like 12AX7EH tubes and a 12AT7 (ECCC81) as a phase inverter, I'm guessing that your amp sounds too harsh and a bit uncontrolled without them.

If you are using polystyrene-film coupling caps, you might want to change them for polyester-film types. The standard "orange drop" capacitors come in both varieties, but the polystyrene-film type sounds harsh. If you have access to orange drop capacitors, use the "PS" type instead of the 715 or 716 types.

[Tube_S_Cream]

If you like 12AX7EH tubes and a 12AT7 (ECCC81) as a phase inverter, I'm guessing that your amp sounds too harsh and a bit uncontrolled without them.

If you are using polystyrene-film coupling caps, you might want to change them for polyester-film types. The standard "orange drop" capacitors come in both varieties, but the polystyrene-film type sounds harsh. If you have access to orange drop capacitors, use the "PS" type instead of the 715 or 716 types.

The amp is fitted with Mallory 150 CAPs and Silva Micas for the values below 2,2nF. I think , a major problem are the 12AX7EH. Perhaps one new tube was crappy and causing a lot of rumble and noise.
I'm thinking about replacing the tubes with JJs or something else with some warmer character. What would you recommend for a very low noise preamp tube?
I'm deciding to change from carbon composite to metal film resistors around V1. Will there be a significant noise reduction or is it just "mojo".

[Ron]

It's funny, but I would have recommended 12AX7EH tubes for low noise in general. With today's tubes, you have to investigate each one for noise and microphonics and select the few that have the best characteristics.

I just bought a quantity of 12AX7EH, 12AX7LPS and 12AX7WB tubes and tested each one. I returned the smallest percentage of 12AX7EH tubes for noise, microphonics and oscillation.

[keithrick]

I found the amp to be a bit uncontrollable and a bit too bright so I switch to 47k feedback resistor and 680n presence cap so the amp is fatter sounding and more tight.

Just my two cents.

Tom

Hi guys,

Can I partially derail the thread....with regards to tuning the amp and possible reduce hiss by getting an exlpanation of the feedback resistor and precense cap?

I have seen the 680n presence cap discussed on the "ahem" Dumble side of the board as well. Does this switch from 1uf to .68uf make that big of a difference?

Also, with regards to changing the feedback resistor to 47k. Is this a significant means to tune the amp, or reduce hiss? I have never really heard it mentioned. Just curious....

Keith

[mojotom]

Both are ways to tune the amp to taste.

The presence cap shut down more high so the amp seems less bright and the feedback cap make the amp more controllable, more tight.

As the presence cap is increased and feedback is added, the amp had less
hiss but those values were choosen to tune the amp, not the hiss.


As far as hiss is concerned, lots of NOS preamp tubes are unusable in this amp and after some tests the best brand was JJ when these are not microphonic.


I also added some kind of Fast/Gradual on the last preamp stage and it's
really great to have this "way less loud" option when you don't want to use the amp at full blast.



I find the amp full of mids, maybe a bit too much. I'm gonna try Fender values for the tonestack caps to see if I can have a more scooped kind of sound.


Hope this help.

Tom