New 2 watt Express: MINI CAJONES

[Decko]

Hi All,

Sorry this is not the "Dana build".
I just finished up a "new" 2 watt build for the Trainwreck Express (for bedroom use). The circuit is based on Cliff's 440 2 watt build but follows more closely Ken Fishers layout of separate power board and preamp board. The main difference between the 2 watter and the true Express is this circuit uses a 6SN7 dual triodes as the main power tube. The power board biases each triode separately. The circuit captures the "clean to mean" characteristics of the 50 watt Express but it has a tighter low end response and a more saturated distortion tone when dimed. (Probably due to overdriving the 6SN7) All the harmonics are there including touch response.

The sound is great but I have an issue with HUM. It gets louder as I increase the volume. I have isolated the hum to the 6.3 VAC filament wires. That said I applied two 40uf caps to each filament lead and referenced them to ground. The hum is gone. In addition I have decided to redo the power section rewiring the heater wires and overall improving the lead dress to the heaters on each tube to further reduce any possiblility of hum.

Question: To apply Electrolytic Caps to the filaments...is this an uncommon practice? How will this affect the tone? What is best practice to elimate hum at the heater leads?

Thanks!
Danny

[Colossal]

I just finished up a "new" 2 watt build for the Trainwreck Express (for bedroom use). The circuit is based on Cliff's 440 2 watt build but follows more closely Ken Fishers layout of separate power board and preamp board. The main difference between the 2 watter and the true Express is this circuit uses a 6SN7 dual triodes as the main power tube. The power board biases each triode separately. The circuit captures the "clean to mean" characteristics of the 50 watt Express but it has a tighter low end response and a more saturated distortion tone we dimed. (Probably due to overdriving the 6SN7) All the harmonics are there including touch response.

The sounds is great but I have an issue with HUM. It gets louder as I increase the volume. I have isolated the hum to the 6.3 VAC filament wires. That said I applied two 40uf caps to each filament lead and referenced them to ground. The hum is gone. In addition I have decided to redo the power section rewiring the heater wires and overall improving the lead dress to the heaters on each tube to further reduce any possiblility of hum.

Question: To apply Electrolytic Caps to the filaments...is this an uncommon practice? How will this affect the tone? What is best practice to elimate hum at the heater leads?

Thanks!
Danny

Danny,

Very nice build. I'd like to see a few more gut shots and a shot of the top if you can post them. I actually have the iron for a 2W build (one of Cliff's custom Heyboer OTs) and some nice RCA NOS 6SN7s that I planned to originally use for a build similar to yours. Then I thought to make it into a 2W Concorde instead. I need to get back on that.

Anyway, in regards to your humming issue, you should have a very, very quiet amp so something is odd as you have noted. FWIW, I have played Cliff's 4-4-0 and you can barely tell that it is on. He wires a mean amp and yes, the 4-4-0 is very smooth, chimey and has great distortion.

In regards to your problem, something that comes to mind from a discussion I had with Cliff. One of the reasons for the dual bias is that the 6SN7 triodes need to be balanced very closely to one another. Since the bias point is 12mA, a 10% mismatch is significant. If they are not very closely matched, you will get hum. So I'm just wondering what your bias matching is like. With the dual bias you should be able to dial it right in if you haven't already done so.

I may be totally missing the mark as you already said that you have isolated the hum to the heaters but wanted to mention the bias, just in case.

[Decko]

Thanks Colossal for the kind words. I will post more gut shots when I get the amp back together. I am go to get meticulous with the lead dress and improve the star grounding.

I did buy Cliff's Heyboer transformer...much better than the Hammond version. I believe it will be awesome in a 2 watt Concorde. The goal for this 2 watt amp was to lay it out as close as possible to the "Real" Express layout. I understand that Ken was into the details with his builds so I wanted to emulate his style even if I could not logically make sense of his methodology. Perhaps I will get more creative on future builds.

I did bias each triode so that they are matching. So no 'hum" is coming from the 6SN7. And yes I did isolate the hum to the heater leads by attaching some extra Mallory caps that I had in stock. As soon as I attached the caps to the leads and grounded them to the chassis the hum went away. Hopefully my new lead dress will eliminate all the hum.

This amp is fiesty and for a 2 watt. It still has that "tone" but it has a tight low end chunk. Sounds awesome through an open back 2 X 12 cabinet with greenbacks.

[RJ Guitars]

Decko,

Are you using two 6SN7 (dual triode) tubes and essentially getting a quad in the output section? This seems like the easy path if you wanted to follow the layout of the Express. I would expect that you wouldn't radically change the tone but probably add a little fullness and complexity to it.

I have an amp (non-Express design) that uses a single 6SN7 tube in push pull output. http://www.boozhoundlabs.com/12sn7-geetar/ To me these tubes sound kinda dark but have a very thick and full distortion. I don't get very much clean head room on this amp even with a fairly tame preamp configuration. I've never measured the output of this but if its a watt, that seems pretty generous to me. I could play this thing at church (conservative Baptist) and often had it pegged with a minimum of raised eyebrows.

thanks,

rj

[M Fowler]

Colossal,

As I was reading the opener post I was think man Dave should be working on his 2w Concorde and the next post is you brother.

Both the TW and Concorde would be great projects. Rj I had looked your build a long time ago and forgot about it, thanks for reminding us about it.

I just got my cage chassis to start on a Fender 5B6 octal amp, trying to decide if PTP or Board.

Mark

[Decko]

Hi Rj,

This build is all Express with little cajones (balls).

I am using one 6SN7 and biasing each triode separately.

Here are some pics for you to see the layout.

This amp uses one 6SN7 in the power section. It is a nice fat sounding tube. Depending on the PI tube (12AX7 or 12AY7) will net more gain and less headroom or vice versa. Clean and chimey to chunky and gainy.

I will post sound bites so you can hear it later.

Still trying to squelch the hum on the heaters so you will notice those leads are loose.

Cheers!
Decko

[RJ Guitars]

very cool.... I would be curious to hear the differences if you went with another 6SN7 and made a quad output since you already have that hole in there. What output tranny and power tranny are you using, would they support another 6SN7?

I expect you have captured most of the real sound with the single 6SN7. There is some theory and rumor floating about that says you can get a little more harmonic complexity using a four output arrangement in any given amp... begs the question, what if we make one with 6 or 8 or 10... etc., output sections

rj