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Custom Tube Amp Build Thread

72LML

72LML

Dec 25, 2009
639
39
Burnsville, MN
#1
Well I've finally been able to get things together in order to start a build thread on my custom tube amp as requested by a few others. I first posted about it here:

http://www.talkbass.com/forum/showthread.php?t=710734

Scott (our guitarist, aka RoctalAmps) is building the amp so any technical details will have to be answered by him.

Here are a few details about the build and what we're shooting for.

-Tube compliment: 6sj7, 6sl7, 6sn7, in the pre-amp

-Traynor style tone stack with mid-shift and parallel/serial mixing effects loop.

-EL34 driver stage through an Fender PS-400 interstage transformer, with clipping feature to get the 'svt' cranked sound in the driver stage

-4CX250B RF tetrode in the output stage running around 900 volts on the plates.

-Antek toroid power transformer

-300-watt Heyboer output transformer with home audio bandwidth of 20hz to 20khz

-2/4/8 ohm taps

-Shooting for 200 to 250 watts, possibly more.


Here are a few shots of the chassis:
5402462936_f51f260b2d_b.jpg

5402461742_00873f6b79_b.jpg


And a quick shot of a partial mock up:
5402460376_78fee0f8c1.jpg

The big transformer the right is the output transformer, power transformer on the left. Ceramic RF tube are in the middle with a pair of 600v capacitors flanking them. The tubes run off of 1000v supply.

I'll try to answer any questions that I can. Otherwise I'll direct them to Scott.

I can't wait to try this thing out! :hyper:
 
#7
MooneyDriver said:
You're gonna need a lot of air to cool those puppies!!!! :hiding:
Click to expand...

Actually, it's not as bad as one might think... The ceramic tubes cool fairly efficiently without radiating the surrounding components. We will be pressurizing the chassis as the air escapes through the ceramic tube towers through the heatsinks. I think about 5 cfm per tube is needed. I am guessing I will end up with a push/pull fan setup. They can definitely get hot when running at max plate dissipation of 250w per tube, but we will not be running them that hard.
 
#8
As long as you can get good airtight sealing (save for the air intake) under the chassis, you'll get plenty of air for the two tubes up the chimneys. That's quite common in using the 3cx800a7, which needs a lot more air for a pair of them. A single small squirrelcage fan should be enough.
 
#10
Yes, a tone stack is equalization, but each tone network has an effect on the rest of the stack when changed. A true EQ (non-active) has the ability to attenuate the band around it's center frequency with limited affect on the neighboring band. They are different animals.
 
#11
That's hilarious. Of course Fender-type tone stacks have interactive bands, much more so than Baxandall/James. But the fact remains, they are there for the EQ. Even saying "non-EQ tone stacks" in a whisper when nobody's around sounds patently ridiculous. Unless you want to take the bass mid and treble knobs off the menu and silk screen "tone stack" above the pre-gain and post-gain controls ; }
 
#12
greenboy said:
That's hilarious. Of course Fender-type tone stacks have interactive bands, much more so than Baxandall/James. But the fact remains, they are there for the EQ. Even saying "non-EQ tone stacks" in a whisper when nobody's around sounds patently ridiculous. Unless you want to take the bass mid and treble knobs off the menu and silk screen "tone stack" above the pre-gain and post-gain controls ; }
Click to expand...

OK, Dude, I concede to your prowess.
 
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#13
Roctal-Amps said:
Actually, it's not as bad as one might think... The ceramic tubes cool fairly efficiently without radiating the surrounding components. We will be pressurizing the chassis as the air escapes through the ceramic tube towers through the heatsinks. I think about 5 cfm per tube is needed. I am guessing I will end up with a push/pull fan setup. They can definitely get hot when running at max plate dissipation of 250w per tube, but we will not be running them that hard.
Click to expand...

I think it's a fantastic idea! :hyper:
 
#14
okcrum said:
As long as you can get good airtight sealing (save for the air intake) under the chassis, you'll get plenty of air for the two tubes up the chimneys. That's quite common in using the 3cx800a7, which needs a lot more air for a pair of them. A single small squirrelcage fan should be enough.
Click to expand...

It's common for RF amps, I know, but there's nothing common about a 3cx800a7 in a bass amp!:cool:
 
#16
okcrum said:
Sign, the comparison was thermal, not electronic.
Click to expand...

You are correct sir, RF guys usually used the squirrell cage fan to pressurize the chassis as they can produce more pressure than the average axial fans but I have my reservations about the squirrel cage fans' noise/vibration. So I was leaning towards the standard brushless axial fans in a push/pull arrangement. I need some box/cover over the tubes anyhow to protect from the anode voltage, why not have a second set of 'pull fans' on top of it.
 
#20
Roctal-Amps said:
You are correct sir, RF guys usually used the squirrell cage fan to pressurize the chassis as they can produce more pressure than the average axial fans but I have my reservations about the squirrel cage fans' noise/vibration. So I was leaning towards the standard brushless axial fans in a push/pull arrangement. I need some box/cover over the tubes anyhow to protect from the anode voltage, why not have a second set of 'pull fans' on top of it.
Click to expand...

That makes sense. You'll get more CFM (running the fans in series) as long as each of them can make decent CFM into the system back pressure. If either can make the required CFM, that's even better, as you can survive a single fan failure.
 

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