A few questions ,maybe more

[veg1515]

I need some help I am building a sca-35 rev B ,it has been collecting dust after a few giant roadblocks I have decided to cross the finish line. I am going with a tube rec version. Here are my questions.

1. I need a selector switch I have setup my amp with 4 pairs of inputs on the back. I am confused on what type is good and poles/positions .
2. I would prefer to have one of the rca's on the back setup as a output is this possible? and how?
3. I picked up a can cap for C9 and it is a 50uf + 50uf 500V , there are three colors on the can silver (i assume is negative) red and yellow , (i assume are the positive sides) now do I only hook up half of this badboy? or can I use the whole cap?
4. for tube rec it says to replace r34 and 35 with a two terminal connector. Rev B does not support this so which holes on the board do I hook up the choke wires to ? or does it matter?
5. somehow I forgot to purchase R43(50ohm 10w), I assume I still need this guy for tube rec. What is a good quality I have not cut corners so far so I would like to keep it that way if it matters.
6.I picked up a 166ms for the tube rec . Is this just for the heater filament ? I am not sure what wires go where.

I am sorry for all the newb questions. If i had no chose to go tube rec this guy would be finished.

Thanks for your help!

[Ty_Bower]

I can address 4 and 5, and part of 3. You're reading instructions on adding a tube rectifier to a Rev D, but you are building a Rev B. Remember that some of the part numbers (R34, R35, etc.) have changed between revisions.

3) On a Rev D board, you must replace C9 (the first cap in the power supply) with a smaller part if you are going with tube rectification. A tube rectifier cannot supply enough current to charge a larger cap, and will eventually be damaged. You would not want to use both sections in parallel on a Rev D board. 50 uF paralleled with 50 uF would give you a combined total 100 uF, or essentially the same as the stock C9.

On a Rev B board, the function of C9 is accomplished with four parts: C9, C10, R34, and R35. The two caps are in series to achive a higher voltage rating, as the caps in the Rev B board are only rated for 250V. The resistors around the caps form a voltage divider, to ensure the voltage drop across each cap is balanced. You could use one half of your can cap in place of C9, and the other half in place of C10. The two halves of the can cap in series would still give you 50 uF, but at an even higher voltage rating (1000 V). Be sure to keep R34 and R35 as specified in the Rev B manual.

Another option might be to remove C9, C10, R34, and R35 entirely. Replace C9 with one half of the can cap, making sure to connect the negative lead of the can directly to ground (not to C10). The other half of the can cap can remain unused, or maybe you could consider using it to "upgrade" C17.

4) On a Rev D, R34 and R35 is the resistance in the CRC filter. It is a pair of 100 ohm resistors in parallel, for a total resistance of 50 ohms. On a Rev B, the equivalent part is R43. You would connect the choke to the two holes where R43 mounts on a Rev B board.

5) If you are going to connect a choke, you will not need R43 on the Rev B. You will have a CLC filter, instead of a CRC.

[TomMcNally]

1) You want to look for a rotary switch with 2 "poles" and 4 "positions"
one pole will be for left, the other for right. In other words, it will
have 8 inputs and 2 outputs.

2) I assume you want an output for a recorder ? You can simply
bridge the output of the rotary switch with some 1k or 2.2k
1/4 watt resistors for some isolation, to your output jacks.
It will load the input slightly, but work well.

6)The Hammond 166MS uses standard color codes ...
the blacks are your 115VAC primary, and the greens are
the 5VAC 3 amp secondary ... the center tap is green/yellow
The blacks go to your switched 115VAC (across the main power
transformer) and the greens to the GZ-34 filament. You don't
need the centertap, cut it off or tape it up.

[veg1515]

ok ,

1. so I would assume that this is just the positive side. Looks like a common ground setup would work. So i will just daisy chain the grounds and connect them to the pcb.

2. the more I think about it , I am going to just do 4 inputs.

3. ok , so I am going to take the positive side of the can and connect to positive on c9. I will leave nothing connected to negative side of c9. the can i will take straight to the earth of the amp. You mentioned the size of the cap and that it was smaller , this cap is much bigger than the stock caps is this just due to the voltage increase? I removed r34 and r35 would it hurt anything by keeping them out? (i destroyed them getting them off the board) I know you mention this in your post.

4.That's why I did not order r43(is been awhile since i ordered this stuff) . thanks for the heads up!

5.Whats clc filter and crc filter and where do they go?

6. thanks for the instructions!


Thanks again for all of your help and sorry for the silly questions!

[K-MAN]

I'm a bit confused, are you building a ST35 or SCA-35? I'd be interested in any pics you have of your project.

[veg1515]

Maybe I am the one confused . What is the difference?

[Ty_Bower]

An "SCA-35" is a Dynaco integrated amp (volume, tone, balance controls, phono preamp, power amp - all in one). A Dynaco ST-35 is a power amp only, with the same iron as the SCA-35, but different circuitry. I think veg1515 made a typo in his original post when he said he was building an SCA-35. It is far more likely that he is building a diytube ST35, revision B.

3) When you say you will connect to this or that side of C9, I'll assume that C9 isn't actually installed. You're going to connect to where C9 would go, if it were actually there. It isn't, 'cuz the can cap is replacing it.

The original C9 has a larger capacitance value (uF), hence I refer to it as a "larger" cap. You are correct - physically, it is quite a bit smaller than your can cap. Of course, the can cap is physically bigger due to its higher voltage rating.

If you have removed R34 and R35, do not attempt to connect the other side of your can cap in place of C10 (as I suggested you could do, before I knew R34 and R35 were gone).

Study the power supply schematic in the DIY manual, and make sure you understand what changes you are making before you start soldering stuff. Photocopy the page and use a red pen to mark up your changes if it helps. If you don't feel comfortable that you understand how the power supply works, remember that it contains voltages close to 400 VDC. Think about damage to expensive components, smoke, fire, and electrocution.

5) CRC filter is capacitor / resistor / capacitor. In Rev B's case, C9 & C10 form the first "C", R43 is the "R", and C11 through C14 form the second "C". "L" is the shorthand for inductor (a.k.a., a choke). After you install your choke where R43 was supposed to go, you'll have a CLC filter instead of a CRC filter.

[veg1515]

So the secondary voltage goes to pins 6 and 4 on the rec tube . Where does pins 8 and 2 go to? pin 1 is present I assume i do not need to connect this one. thanks for all of your help.

[TomMcNally]

Follow this link to the ST-35 doc section of this forum and
take a look at the tube rectifier mod ...

http://www.diytube.com/phpBB2/viewtopic.php?t=837

it explains it all ...

[EWBrown]

The rectifier tube (5AR4/GZ34), pins 2 and 8 are the 5V filament. These tubes have a cathode, which is connected to pin 8, and the High Voltage DC should be taken from this pin.

4 and 6 go to the (normally red wires on the) HV secondary. Pins 1, 3, 5 and 7 are not used, but may have internal connections, which are not related to normal tube operation. Best to NOT use these as tie points.

HTH

/ed B in NH

[veg1515]

I am using neds trannys . The brown wire on the signal tranny is a 16 OHM. What is the purpose of this and I dont think I need to hook it up to anything correct me if i am wrong. Things are coming along I will post pics when I am done. Thanks again!!

[erichayes]

Back in the '50s and '60s, quite a few speakers were 16Ω and thus used the 16Ω tap. Tape them off but don't cut them. In the future you might want to bridge the amp and you'll need them for operating an 8Ω speaker.

[veg1515]

it shows the 16 ohm taps connected to J3-3 and J4-3 on a rev B board so don't hookem up? I have no plans on bridging the amp. Would it hurt anything to hook them up?

[veg1515]

When you go with a tube rec. does the test measurements change? Mine are way off and I am curious if this is normal.

[Shannon Parks]

it shows the 16 ohm taps connected to J3-3 and J4-3 on a rev B board so don't hookem up? I have no plans on bridging the amp. Would it hurt anything to hook them up?

That would be fine.