• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Any Ideas

Does anyone know what bias levels would be required to run in screen drive mode for the popular sweep tubes?

Would there be any benefit from running a positive bias on the control grid rather than just attaching it to the cathode? Space-charge grid tubes were run exactly like this in car radios where the tubes ran off 12 volt plate voltages directly.

Instead of connecting the control grid to the cathode, what would the characteristics be if the control grid was connected to the screen grid with both used for drive?

I realize this is a lot of questions, but I am curious about what configuration would be best with typical sweep tubes (because I have a basket of them).
 
I have to do some more TX digging. The PA060 I was thinking about would make to many volts I think. Crap, back under the house with the spiders.:para:
 
Some folks over at diyaudio.com did some experimentation with positive voltage on the G1. While some said it made a small improvement in some cases, IIRC, the general consensus was that it wasn't a big deal....

As for connecting control to screen... that was also tried. I think George (Tubelab) over at diyaudio blew up a number of tubes before deciding it was too unstable, with sweep tubes...

For bias... SY over at diyaudio.com confirmed that the bias values used by Berning (only 3ma per tube, idle!) actually worked very well, for low distortion. You can almost run them "pure class B" (zero bias) without significant crossover distortion, in some cases!

I like the idea of low bias like that... the transformers just can't get enough DC offset to saturate, unless something goes HORRIBLY wrong. Not just tube variation with temperature/age and such... something in the amp would have to be BROKEN for there to be enough offset for a problem to exist there...

Regards,
Gordon.
 
jaymanaa said:
I have to do some more TX digging. The PA060 I was thinking about would make to many volts I think. Crap, back under the house with the spiders.:para:
Click to expand...

Remember, though, Jay... these sweep tubes were made for some pretty insane voltages in their TV applications. Like 770 to 990 volts, depending on tube!

With the extreme low bias, you don't have issues with red-plating under high B+ either... even with a 600v B+, the idle dissipation is like only 1.8 watts per tube, with 3ma current... :thmbsp:

The limiting factor is only how much you current you force through them, at what voltage, and how much voltage your OPT can handle before it saturates, from pure voltage effects...

Regards,
Gordon.
 
GordonW said:
Remember, though, Jay... these sweep tubes were made for some pretty insane voltages in their TV applications. Like 770 to 990 volts, depending on tube!

With the extreme low bias, you don't have issues with red-plating under high B+ either... even with a 600v B+, the idle dissipation is like only 1.8 watts per tube, with 3ma current... :thmbsp:

The limiting factor is only how much you current you force through them, at what voltage, and how much voltage your OPT can handle before it saturates, from pure voltage effects...

Regards,
Gordon.
Click to expand...

Good points Gordon. It looks like the old Dynaco 70 TX is making right at 485vdc into a 150mA load, connected like the power supply in the Berning schematic.:thmbsp: The only trouble I see with that, is it's mighty close to the 500v rating on the only 500uf cap I have handy.:scratch2: However, looking at the schematic again, I see he used a 450v cap at 430 volts, so...................
 
Maybe I should check the voltage with no load though, since it won't actually see one in the amp untill the tubes warm up. I wonder how critical that 500uf size is on that first cap?
 
You can always series two 350v or 400v caps (with 100K balancing/bleeder resistors)... yeah, they'd be big, but it would definitely work.

I'd think that, since this amp is close to Class B, having a VERY stable power supply would be pretty critical to good bass control and power, since the power supply will see, pretty much, the output signal variations (the load on the PS will be proportional to power output). I wouldn't scrimp on cap values.

Regards,
Gordon.
 
:scratch2: :scratch2: What Micah said... :scratch2: :scratch2:

It makes sense... 440V is the AC RMS value... the peak value would be 1.414 times that, or 622V. It'd obviously have to handle the peak voltage, each cycle...

Bet you could get some real big boys in that type, huh, Jay?

Regards,
Gordon.
 
Yep, I have some options. I actually have some 4700uf units that have a 500vdc surge rating (450vdc cont.). I could use two of those in series with bleeders to make 2350uf. I'll have to check the in rush though, to make sure the diodes can handle it (and the TX I guess). I hate to use a limiting resistor, but maybe a CL90 in front of the TX would do the trick. That setup would definately give me plenty of reserve oomph for the bass notes.:D
 
2350uf!! :yikes:

Yep, you'll DEFINITELY need an in-rush limiter there!

Hey, one other thing- if you use an in-rush limiter, it'll drop the voltage just a tad... might be able to kill two birds with one stone (get the B+ down a few volts where you can use a 500v cap, and protect the PT)...

Regards,
Gordon.
 
Looks very cool! Some day I'd love to try my hand at building an amp from scratch. Ya'll do some cool stuff...

I gotta ask though. Why make a single stereo amp instead of a pair of mono's? Does it mostly have to do with the added cost of needing two power supplies instead of a single?
 
similost said:
Looks very cool! Some day I'd love to try my hand at building an amp from scratch. Ya'll do some cool stuff...

I gotta ask though. Why make a single stereo amp instead of a pair of mono's? Does it mostly have to do with the added cost of needing two power supplies instead of a single?
Click to expand...

Yep, single iron is lots cheaper to come by. My next one will be monos, and the iron was nearly 1K (2 PTs, 2 chokes, two rectifier tubes, etc...). Monos are my favorite though, without a doubt.:yes:
 
Once I get the three amps I have left to recap and restore, I think I'll eventually try my hand at a scratch build, but I'm gonna have to learn a lot like how to figure out what kind of trannys you need, or how to work the ones you have into a design... but I have a feeling there is more math involved there than I understand when it comes to electronics... *sigh*
 
similost said:
Once I get the three amps I have left to recap and restore, I think I'll eventually try my hand at a scratch build, but I'm gonna have to learn a lot like how to figure out what kind of trannys you need, or how to work the ones you have into a design... but I have a feeling there is more math involved there than I understand when it comes to electronics... *sigh*
Click to expand...

Don't be fooled mang, scratch building is "to me" easier than re-building commercial stuff. You can make the chassis nice and roomy, and you are starting with everything new and clean. Once you master some simple metal working technics (hole cutting and such) and get a decent tool set together, it really is quite simple. I was really surprised at how much easier scratch is than "re".:yes:
 
and if you are worried about the math (which I know you could handle), just follow a proven schematic to the letter.
 
Just sent the Mouser order in for all the passives.:thmbsp: Can anyone recomend a place to get the meter ports for the chassis?:scratch2:
 
You must log in or register to reply here.
Back
Top Bottom