• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

OTL designed by Tim Mellow with 4 6C33C?

The rectifiers need to be good for several amperes. In addition, the B+ is low as far as tube amps go, so the usual voltage drop that tube rectifiers have is unacceptable. :headbash: Now if you had enough of them (more of them than power tubes) you might be able to get away with it. That seems like it would be a prodigious project!

OK, so you are saying that tube rectification has a voltage drop that SS does not? So, excuse my ignance, but what is a voltage drop? Thanks for your help.
 
djn,

Simply, when converting AC to pulsating DC there is an internal voltage loss absorbed by the rectifier. In SS, that number is a fraction of a volt, depending on the specific rectifier chosen. In tubes, it can be as little as 11 volts or so, for a damper diode type, or even 30 volts for a more conventional type.
That's part of the problem, but for you, the bigger issue is current. OTL output stages use a lot of current, several amps. Most tube rectifiers can only deliver fractions of an amp. Damper diodes, can deliver up to 2.5 amps or so. Maybe a little more.

Your best bet is a Hexfred type of SS rectifier. Really durable and can deliver about 15 amps of current.

One more word about the current: when charging up a big bank of capacitors, there are big inrush current pulses, until the cap is charged. These pulses can far exceed the rating of the rectifier. I've blown up a few Schottky type SS rectifiers because of the big cap banks I tend towards. The advantage of the Schottkys is that they don't generate RF noise like other SS diodes do, so they are the next best thing to a tube diode from that standpoint. However, they blow up where my Stealth Hexfreds have lived for years.

Stuart
 
I would have though 200-250mA would have been a better choice for each power tube. If a 6080 is pulling 100mA or so and the bias goes off, then it will shoot up over 200mA or more.

How would a 1.5Amp fuse protect it?

Please correct me if I'm wrong or misunderstood.

1.5amp may be for the 6C33?

6AS7s will put out about 750mA when grid driven a little positive.

I have never bothered with fuses other than mains, and never had problems. I figure I built it, I can fix it, even if it blows up:)
 
Just checked out the Hexfred diodes....man they sure do look serious.

Q: if a PS needs a certain voltage and amp rating for the diodes, can you go WAY bigger for protection? Or will over sizing them be a problem?

The reason I ask is a friend of mine told me last night that he has a stash of very big ones that I could rob. He didn't know off hand their rating, but was sure it was much higher than what I needed.
 
I would have though 200-250mA would have been a better choice for each power tube. If a 6080 is pulling 100mA or so and the bias goes off, then it will shoot up over 200mA or more.

How would a 1.5Amp fuse protect it?

Please correct me if I'm wrong or misunderstood.
You will blow out a 250mA fuse just about right away.

1.5 Amps is used because one of the failure modes of the tube is arcing. You want a fuse that is large enough to allow the amp to clip, but small enough to prevent damage.

BTW, if you get the 6C33s new: they come from the factory untested. You will want to run them for at least 72 hours continuously, **filaments only**, no B+ or bias voltages, to precondition them. This will double their life expectancy, and will substantially reduce the possibility of arcing!
 
BTW, if you get the 6C33s new: they come from the factory untested. You will want to run them for at least 72 hours continuously, **filaments only**, no B+ or bias voltages, to precondition them. This will double their life expectancy, and will substantially reduce the possibility of arcing!

Good to know, thanks.
 
you guys use a fuse on the secondary and primary? Secondary makes sense to me if the bias fails.

But I'm looking into a bias B+ relay protection also.

The fuse on the secondary side, if you are using 6C33s, is set up so that there is *one for each tube*.

For the protection relay, a system that detects 0V on the grid with respect to its cathode of either tube is what is needed. I would set it up so that musical transients don't set it off.
 
Just checked out the Hexfred diodes....man they sure do look serious.

Q: if a PS needs a certain voltage and amp rating for the diodes, can you go WAY bigger for protection? Or will over sizing them be a problem?

The reason I ask is a friend of mine told me last night that he has a stash of very big ones that I could rob. He didn't know off hand their rating, but was sure it was much higher than what I needed.

The voltage rating you see on a diode is PIV or Peak Inverse Voltage. Basically, this is the maximum voltage the diode is asked to block in the reversed direction. The actual number is much larger than the DC output of your rectifier, and varies by type of rectifier connection. Using 1000V rectifiers for the 150V supplies will be just fine.

As far as the current goes, as far as I know, the more the merrier. SS devices are nowhere near as forgiving as tubes, as I may have mentioned before. Exceed their ratings and you'll be sniffing up magic smoke. Use the ratings as "never equal or exceed" numbers. If you need a 1.5 amp rectifier, choose at least a 3 amp unit. Remember also that inrush of charging the capacitors will exceed the 1.5 amp steady state rating. I think the Stealth Hexfreds are 1200V 15A or so.:D

Stuart
 
I think the Stealth Hexfreds are 1200V 15A or so.

That is what I am going to get I think. This is all good to know. I talked to the friend who has the machine shop and he can't make me chassis until mid Dec. so I have some time to collect parts.
 
Thanks Chris, nothing special but it is all that is needed :). Working on a BOM, so far almost all the parts will be coming from Mouser.

Brad

the one i have playing is using all fast diodes and the bridge is a 1200v 17A fast diode bridge from ixys out of digikey. i use these as replacements on my futterman's/nyal's. part number vbe17-12no7-nd.

well i have the 6c33 amp playing side by side with one of my nyal otl3's and she is opening up and sounding nicer every day. Sound nice and clean with a rich tone. I'll give her a few more weeks and see where she ends up.

Enjoy
 
Hi Des, are you hearing any preference with the TM OTL? Like is it better at highs, mid, bass? that kind of thing?

What does "fast" mean when you are talkiing about diodes?

On the amp i think in general it pretty even so far across the baord. Its controls Quad 63's very well even in the bass area where Quads have a specific sound.

Fast on diodes means recovery time
 
Thanks D. I normally tri amp because I like the flexability to match amps with drivers. But things are changing around here and I might need to drive a passive speakers with it.

Edit: I just looked up the Quad as I'd never heard of them and see that they are ESL speakers. Don't ESLs need a gob of power? That is what I've always heard at least.
 
Thanks D. I normally tri amp because I like the flexability to match amps with drivers. But things are changing around here and I might need to drive a passive speakers with it.

Edit: I just looked up the Quad as I'd never heard of them and see that they are ESL speakers. Don't ESLs need a gob of power? That is what I've always heard at least.

The discussion on Quads could easily be another thread:D.

I have had success with many diferent power schemes on 57's and 63's. I think it really depends on the amp. If you read back on Tim article loads above 16 ohms realiy help in pulling more power out of the amp. I think I remember hearing 40 or 50 watts and Quads very often will range in those areas under active load.
 
The only reason I ask is that a friend of mine offered me a pair of Sound Lab ESL panels without the power box. I have the scheem for those, but thought that even with my Mellow OTL, (which will be my highest powered amp), it would not be enough wpc.