Direct Coupled tube amp

[Nick]

I would have thought with a gz34 or similar you should get 400v out of 600v CT.

[Crazy Bill the Eel Killer]

Hello all,

thanks for your advice. It begs some questions.

pre65- I've seen that configuration before ( did a lot of studying on Andrea Ciuoffli's website ), and it sure looks interesting, as I understand the valve diode hides the audio characteristics of the sand diodes from the signal circuitry. But the question it raises to me is : wouldn't you still have the same forward voltage drop of the valve to deal with, or is there something going on I don't understand and you only have the SS diode drop to deal with ?

Nick- Thanks for the tip. It looks like there's only an approx. 10v drop w/ that tube, so I'll give it a try. Have any of you guys any thoughts as to the JJ or the Sovtek ?

I'll pick up some Cree silicon carbide diodes this week, and play with them and the GZ34 later. I've got a 10H and 5H choke in house for a pi filter, but I couldn't find them this morning to check the dcr. I'm sure I put them somewhere to keep them safe ( still in the box w/ the PT, all from Electra-print ). Initial thoughts and calcs show I need between 300-400 ohms total in the pi filter using the sand or GZ34 to get around 400 vdc.

Again, thanks to all for your help and advice.

Cheers, Crazy Bill

[Mike H]

I like to juice the EL84 quite a bit ( 290v @ 45 ma ),

That's 1 Watt over the maximum design rating for anode power dissipation. Just an observation

[Mike H]

pre65- I've seen that configuration before ( did a lot of studying on Andrea Ciuoffli's website ), and it sure looks interesting, as I understand the valve diode hides the audio characteristics of the sand diodes from the signal circuitry.

That's not the reason why I do it, it's usually 'cause I haven't got a CT transformer but I want to use a valve rectifier

But the question it raises to me is : wouldn't you still have the same forward voltage drop of the valve to deal with

Yes certainly.

If you've already got 300-0-300 then I agree with Nick that'll give you 400VDC, or close to.


BTW I often have a problem getting it up as well. Voltage that is.


 

[Crazy Bill the Eel Killer]

I like to juice the EL84 quite a bit ( 290v @ 45 ma ),

That's 1 Watt over the maximum design rating for anode power dissipation. Just an observation

You are of course correct. Since that figure is what I remembered from approx 5 years ago when I actually took measurements on my amp ( at the time an unmodified Decware SE84CSEX ), I dug out the notes I had taken to double check. It turned out to be one of those very rare moments when I remembered correctly. Mains voltage was 122 vrms.

Steve Deckart of Decware is known for hitting that tube hard in his amps, and Eddie Vaughn of the now defunct Vaughn Audio also favors that operating point. Both of their amps feature a shared 100 ohm bypassed cathode resistor. I've never had an issue so I never gave it a second thought. I tried going down to approx 260 b+ @ 35ma, but preferred the original op. point, so I never looked back.

Thanks for pointing that out. I have noticed that all of the EL84 designs that I have seen on the web seem to favor a gentler op. point. While I've never blown up a tube ( I use the 6P14P-ev ), I would not want to be the reason that someone lost one of their tubes based on my advice.

Cheers, Crazy Bill

[Mike H]

I only mentioned it (but without explaining more fully perhaps) 'cause it's a possibilty for premature failure of the EL84(s)

Sweet-spot versus reliability? I dunno


 

[Crazy Bill the Eel Killer]

Hello Mike,

I'm going by memory here again, but I'm pretty positive I clamped the black probe of my multimeter on ground, and referenced all measurements against that, meaning my 290 vdc b+ is from plate to ground, so subtract the -9v bias to leave 281 vdc across the tube. Still 12.5 watts dissipation, but I would guess that a 1/2 watt less when you're over the top is probably significant. I reread Eddie's original thread, and I think the way he stated it his 290 vdc b+ is also plate to ground. I'll try to get through to him soon for verification.

Thanks for all your help. Still can't find my box of Electra-print iron. Starting to get a little worried, as I'm running out of spots where I could have put it to keep it safe.

Cheers, Crazy Bill

[Paul Barker]

i have no idea what i did with my ae 1:1 amorphous interstage transformers.

it's tough isn't it?

[Crazy Bill the Eel Killer]

i have no idea what i did with my ae 1:1 amorphous interstage transformers.

it's tough isn't it?

I sure do hope your IT's turn up soon. It's is frustrating.

I'm not familiar with the initials " ae ". Is that the manufacturer ?

Also, have you used them yet. My present IT is an Electraprint from Jack Elliano, and I really do like what an IT does to the sound of an amp compared to standard RC coupling. When I finally get around to building a proper DHT amp, I'm thinking I would like to try one of Dave Slagles offerings for an IT using amorphous cores, but as you know, they aren't cheap. In your experience do you think the the use of the amorphous core is justified by a performance increase. Don't mean to put you on the spot, but I figure you've got a lot of experience ( anyone else also please feel free to chime in ).

I actually located the box in my basement last night. They were very well hidden which was why I couldn't find it. Can't for the life of me figure why I would have put it where it was in the first place.

Now if I could only remember where I put my DMM, I could actually measure the dcr of the chokes and get going. I'm sure that will turn up soon. Maybe I'll remember after a couple of these.

Cheers, Crazy Bill

[Paul Barker]

I think the manufacturer is AE Europe.

They are Amorphous.

I can't remember what they sounded like at all.

I do remember from past experience that either choke loading or grid choke with coupling cap was pretty well the same.

When adding inductive passives the key is to buy the best you can afford, and I think to better a good LC stage you need to go beyond Amorphous to as much Nickel percentage as you can afford. Nickel does sound considerably better than all the rest.

There are sometimes other practical reasons for using interstage transformers.

Plate chokes are usually more inductance as are grid chokes by a huge factor, so though the imperfectness of the coupling cap alters the sound the better inductive elements outweigh the interstage transformer which is usually a compromise which will create resonances at HF and a roll off a LF with a resonant peak and altered phase all of which will affect linear sound passage across the spectrum. You already have this going on in the output stage.

I think in summary if there has to be another transformer coupled stage (additional to the output) it had better be a bloomin good one.