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Amp schematic questions

post #1 of 5
6/11/11 at 8:12pm
Thread Starter 
First off, this isn't a headphone amp, but considering it is an amp, I figure somebody can figure it out. I was looking for an amp to build, and came across the Oddwatt 225 which looks pretty interesting, but the schematic is confusing me a little.

6SL7-KT77-Tube-Amplifier-Power-Supply.png
6SL7-SRPP-KT77-Push-Pull-Tube-Amp-CCS.png

My first question about this thing is where does the 225V connection come from in the amp section?

Second, what is the difference between the two rectifiers in the power section? Is one full wave and the other not or something? I'm kinda stupid when it comes to them.

Third, by "are contained inside a line filter" is he saying he used a pre-built filter, or is he saying they must be isolated if you are to build one yourself?

And finally, is he is rather vague in his description of what to do with R* as he mentions a diode chain or a resistor but not much on how to determine what is necessary for each option.

I tried emailing him awhile back, with no reply, so I don't see any reason to ask again. If anybody can help, I'd appreciate it.
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post #2 of 5
6/11/11 at 8:31pm
Quote:
Originally Posted by sml1226 View Post
 I was looking for an amp to build, and came across the Oddwatt 225 which looks pretty interesting, but the schematic is confusing me a little.


I have to ask - why build something from a schematic if you clearly do not have the understanding to attempt such a thing? Not criticizing or chastizing just curious why anyone would want to play with lethal high voltages without a decent amount of know how. 

 


Quote:
Originally Posted by sml1226 View Post
 
My first question about this thing is where does the 225V connection come from in the amp section?
 

 

It is derived from the 250V HV - 4K7 resistor drops the voltage to 225V for the 6SL7 B+



Quote:
Originally Posted by sml1226 View Post
Second, what is the difference between the two rectifiers in the power section? Is one full wave and the other not or something? I'm kinda stupid when it comes to them.

They are both full wave - one seems to be "integrated" but both will rectify AC to DC - one for HV and one for the heater supply

 

 



Quote:
Originally Posted by sml1226 View Post
Third, by "are contained inside a line filter" is he saying he used a pre-built filter, or is he saying they must be isolated if you are to build one yourself?

And finally, is he is rather vague in his description of what to do with R* as he mentions a diode chain or a resistor but not much on how to determine what is necessary for each option.

 

He is using a filtered IEC plug , the *R is either a voltage dropping resistor to get the heater supply to the correct voltage ( high wattage ) or use a diode or 2 to set the voltage. There are many ways to do this , he is just suggesting a simple option. I would use a high current LDO voltage reg on a heatsink. 

 

be safe ..dB

 

 

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post #3 of 5
6/11/11 at 8:59pm
Thread Starter 
Well, the reasoning behind the desire to build this is the desire to learn the amp topology and the ability to learn better first hand. Figuring it all out (what each part is to as I'm better at seeing it all there than I am at just looking at a schematic and knowing exactly what I'm doing immediately) prior to actually running any voltage through it would be the preferred way of doing so.

But anyway, it's more that I am new to tube schematics that I am worrying about clarifying than anything. The rectifier differences seemed unnecessary as I though they were the same, but again, wanted to clarify, and same with the filter, seemed unnecessary to include what he did. The 225V appeared to be a connection (basically coming from nowhere which was confusing me) rather than a probe point showing the voltage drop.

The R* was the only thing really troubling me as to what he was doing. It seemed like a regulator would be simpler than using a 20W resistor or venting high wattage diodes anyway so I didn't see his logic anyway. It seemed like he was just avoiding putting a big heatsink in the thing.

Anyway, it's going to take some more time and consideration before I get around to building this or anything else anyway. And it may not happen anyway if this job I'm supposed to be doing for a guy falls through.

EDIT: After seeing that the prices in tubes and transformers alone have risen considerably, that really makes the price of this thing climb higher than expected, so it may be better to just buy something comparable, used, in the price range I read this should cost.

Anyway, nice information from this, but I still want experience, so I'm not sure where I'm going yet.
Edited by sml1226 - 6/11/11 at 10:10pm
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post #4 of 5
6/11/11 at 10:18pm

Get the books:

"Building Valve Amplifiers" and "Valve amplifiers" by Morgan Jones.

Read them. A few times. 

 

Quote:
Originally Posted by sml1226 View Post

The R* was the only thing really troubling me as to what he was doing. It seemed like a regulator would be simpler than using a 20W resistor or venting high wattage diodes anyway so I didn't see his logic anyway. It seemed like he was just avoiding putting a big heatsink in the thing.
 


A regulator has 3 pins and must be hooked up properly where a resistor has only 2 pins which can be interchanged. How could a regulator possibly be less than twice as complicated?

 

The regulator is about guaranteed to need a heatsink, even for a laughable 1W where a cheap resistor will laugh 3W off and ask for more... Three times as complicated for the regulator. 

 

A regulated heater supply will require a power transformer with a non-standard heater winding voltage to accommodate regulator dropout voltages. One could do it with 2 off-the-shelf transformers or 1 custom wound unit but this is an additional expense. The regulator starts to look about 4 times as hard with a dmerit for hidden expenses.

 

There are applications that require regulated DC heater supplies, but this isnt one of them. at all.

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post #5 of 5
6/11/11 at 10:35pm
Thread Starter 
I don't know about 4 times as hard, but if I do build it, I'd likely go with a diode chain here anyway.

And I know it doesn't need regulated DC, in fact it doesn't need DC at all, but it is an option nonetheless.

I'll check out the books, thanks for the recommendation.
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