All Triode Express - Standard Wreck Sized Chassis

[johnnyreece]

Yeah, they are running them way over max spec, but I guess that seems to be pretty common. I guess data sheets are more like guidelines than rules, right?

Of course, they're running cathode bias, which lets you run them a bit hotter than fixed. I suppose we'll see what happens!

[romberg]

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I have a couple questions for the guys who have built an Express circuit using a triode output. Does the Express keep the edge and attitude that is prevalent in the pentode version of these amps? Big triodes are known for their smoothness and I am curious how this plays out in the Express circuit.

I have two express based amps. One with el34s and the other a 4-4-0 with a 6SN7. I can tell the difference between them. But they are using different speakers. And of course the full sized one is as loud as Satan. You can tell you are playing an express with either. So, the little 4-4-0 mimics that part well.

On a scope my little express (with the split plate loads on the PI) puts out very similar wave forms to this full size one:

http://home.polstra.com/amps/wreck1/scope/

I strongly suspect the remaining small differences in dynamic behavior may be due to how the power supply scales from 30 watts to 2 watts. I don't know how much this was or was not looked into when the 4-4-0 was developed.

Mike

[RJ Guitars]

Thanks Mike, those are great scope shots. The 6SN7 is definitely the best documented of the triode output tubes for guitar amps. It's great to see the comparison with the full sized Express, especially the PI stuff. I'll try to remain inspired to put the scope on my build once I get it fired up.

I'm guessing you're going to have around -40v for your bias? That might get you into range for a more robust tube I'm wanting to try: 6AH4. It's so hard to find a load resistance for these guys, though, making it hard to determine OT primary impedance. I'll be following this closely!

That 6AH4 might be an easy fit for a larger octal triode. It's at least got a 6.3v heater

I am starting to wonder if we'll see a big movement on these low power triode amps. Although, some claim to not care for the triode sound, so I guess we'll see. I'm kind of wanting to mix things up and try a pentode preamp with a triode power, but that's for another thread...

Express amps are loved by most who have them but the massive amount of sound is always a tricky issue to overcome. I've tried attenuators, master volumes, power scaling, and even iso cabinets, and all of them work to some degree, but none of them seem to keep the magic when you really dial things down or else the hassle factor is too great - especially the iso-cab. I think so far may favorite method of dropping some volume has been using smaller tubes... this will be the extreme on the lower power end.

My intention here is to keep the TW chassis and boards in a familiar layout and also use a lot of my other standard hardware like cabinets faceplates, etc..

I did make a little progress tonight. I like to wire up the AC power first and verify that the PT voltages.

Also I forgot to mention, I did some homework and for the purposes of filament heaters, AC RMS and DC voltage are fundamentally the same.

[johnnyreece]

I see it looks like you settled on dual bias. I think you'll be much happier! One thing to note, if you're looking at my layout: I have the pots wired backwards. So, when you turn them fully clockwise, it's at the minimum bias setting, and vice-versa. It works, but it's counter-intuitive. I've not updated my layout yet to fix it. Now that I know someone's looking at it, maybe I'll make my final revisions and repost.

[cbass]

Sorry I don't want to beat the heater voltage thing to death but I'm not understanding. If you do a voltage doubled circuit with 6.3v taps 12.6 *1.4=17.6 Minus the diode drop so you would end up around 16vdc.
Maybe I'm just not understanding something or my math isn't right.

[johnnyreece]

cbass, I don't necessarily understand it myself, but here's a page on it with some handy info: http://www.fourwater.com/files/heaters.txt

They seem to think it will end up being around 14v, without using r26 from the voltage doubler schematic. I still don't see what r25 is for, though...

On a side note, from my Google research, it appears triodes aren't super-sensitive to OT mismatches, so it may be worth experimenting with different taps to see what you like best. One of those little Hammond jobbies with the multiple taps might be fun to play around with.

[cbass]

I was thinking like a full size amp with 4 amps of heater current this one will only be less than one amp so it should be easy enough to adjust the voltage with a 10 watt resistor.
If you needed to drop 4volts a 4ohm resistor would be dissipating 4 watts
I think your pt will need to be at least 3 amps though.
Super cool project didn't mean to sidetrack it.

[RJ Guitars]

No apology needed and no sidetrack issue, I think you were right on target. I started the thread because there are plenty of answers I don't have and lots of cool stuff for us to discover along the road.

I had to do the "Missouri thing" on the heater supply so I could keep moving forward on this. I know all those HiFi guys are good at this stuff and was sure they were right.... but I had to see it for myself.

I built a temporary voltage doubler circuit and yup it read ~18 volts open circuit. I then ran a couple wires to a tube socket, pushed in a tube and viola, 14 volts. I still have several more tubes to light up so I sorta expect it to come in pretty close...

[cbass]

What is your 6.3v taps rated at?current wise?

[johnnyreece]

Excellent! Well, that's one part sorted out. Cbass does have another good point; that site I linked said to make sure you count the current draw double for those hooked up to the voltage doubler. Again, I'm no expert, but that's what some guys on the internet said.

[cbass]

From what I'm reading the ac current rating is divede by 3.6 on a voltage doubler circuit http://tubes.nekhbet.com/power.shtml
I'm really interested to see how it turns out

[RJ Guitars]

Many of the Darling Audio amps use a Hammond 269EX Power tranny with a 6.3v Heater tap rated at 2.5 Amps of current. They use this to drive two of the 1626 tubes and a 12SL7 driver. Apparently this works very well...

My PT is an Edcor with 3 amps of heater current available and I'll be driving the two 1626 power tubes and three 12AX7's, so hopefully this will work just fine.

I need to find some bigger diodes and I'm also waiting on the 2200uF filter caps to arrive then I can populate my PS board. Meanwhile I am working on a layout drawing.

[johnnyreece]

If my math is right (and the assumption of doubling current requirements is right), you'll need 1.9A, so you should be well within range. Merlin's power supply book also says double the current. I only know enough to be dangerous, most of the time, so take my advice with a grain of salt.

Edit: After reviewing Merlin's power supply book, I didn't look closely enough. He states, "For a given load current the transformers need the same VA rating, or, in other words, the transformer used for the voltage doubler must be rated for half the AC voltage but twice the current as the equivalent bridge rectifier circuit." I was derating the transformer based only on the AC rating; not the bridge rectifier current. I believe this page might be helpful for calculating the current rating of the bridge rectifier (of which you would then reduce the rating by half): http://www.hammondmfg.com/pdf/5c007.pdf

I've not compared it to the data cbass provided, but I'm betting it's a lot closer to reality than my original guess. I suppose we'll see what happens! I'll shut up about this part now and enjoy the build.

[rp]

As for the 14V thing I'd say go for it and hope for the best, you can always retrofit a small 12.6V tranny in there if it doesn't work out.

[RJ Guitars]

Thanks guys, I was sorta trusting the HiFi guys on the engineering, starting with their declaration that this is an awesome sounding output tube. It's all starting to settle in and I'm waiting on some parts but have plenty to do in the meantime.

I see it looks like you settled on dual bias. I think you'll be much happier! One thing to note, if you're looking at my layout: I have the pots wired backwards. So, when you turn them fully clockwise, it's at the minimum bias setting, and vice-versa. It works, but it's counter-intuitive. I've not updated my layout yet to fix it. Now that I know someone's looking at it, maybe I'll make my final revisions and repost.

Yeah, it got kinda obvious as I realized that my OT only had one output tap and I was going to have some extra holes back there... I knew then that you had made good use of the extra holes which happen to be right where you need them for tube biasing.