diytube.com

I have had a half completed Tubezone version of this amp for years. I decided to pick it up again and finally complete it. Yes, I am aware of the "search" function.

The kit is obviously designed around the z565 transformer which has two secondaries. The unused secondary is used for the negative feedback. If you use an OPT with a single secondary, does the feedback use the same secondary as the speaker output?

The instructions states "connect the screen power supply to all the connectors labeled "UL Tap" " when using an OPT without screen taps. So, where does one find this screen power supply source on the board?

I was originally going to use the transformer set from a scrap SCA35 but I have determined that one of the OPTs is bad. I don't know the condition of the power supply transformer. I really don't want to try to score another vintage transformer from eBay to match my "good" one.

I have other transformers available for use. Some have multiple outputs and some have UL taps. None have both. My inclination is to use a pair of Edcor CXPP60-8-10K outputs with an Edcor XPWR005-120 power transformer all of which I have in my stash. The Edcor OPTs have the UL tap but a single secondary.

Thanks in advance

John

electrovice wrote:I have had a half completed Tubezone version of this amp for years. I decided to pick it up again and finally complete it. Yes, I am aware of the "search" function.

The kit is obviously designed around the z565 transformer which has two secondaries. The unused secondary is used for the negative feedback. If you use an OPT with a single secondary, does the feedback use the same secondary as the speaker output?

The instructions states "connect the screen power supply to all the connectors labeled "UL Tap" " when using an OPT without screen taps. So, where does one find this screen power supply source on the board?

I was originally going to use the transformer set from a scrap SCA35 but I have determined that one of the OPTs is bad. I don't know the condition of the power supply transformer. I really don't want to try to score another vintage transformer from eBay to match my "good" one.

I have other transformers available for use. Some have multiple outputs and some have UL taps. None have both. My inclination is to use a pair of Edcor CXPP60-8-10K outputs with an Edcor XPWR005-120 power transformer all of which I have in my stash. The Edcor OPTs have the UL tap but a single secondary.

Thanks in advance

John

Is this a rubber biscuit post? Sounds like a "wish sandwich" amp from "Tube zone." It's comforting to know you're aware of the search function.

And yes, I read the entire post...

The Z565 do not strictly have multiple secondary windings per se, but rather a single secondary with multiple taps for various impedance loads. Regardless, I suppose the end result is similar for you - you've got instructions that tell you to connect the NFB loop to a particular terminal on the OT, and you don't see it there.

The simple answer is to connect the load and the feedback both to the same leads off the secondary. You will need to adjust the value of the NFB resistor accordingly. There's a thread around here that describes how to calculate the value of the NFB resistor. The best way is to connect a distortion meter to the amp, and adjust the NFB resistance with the amp running.

If you've got an OT without screen taps on the primary winding, you'll need to connect a screen power supply as the instructions say. There is no screen power supply on the board. You will have to build your own. You can probably just tap off the B+ supply with a cemented power resistor and end with a big capacitor (a hundred microfarad or so). This will change the amp from a UL output stage to a true pentode, so again the tuning of the NFB may be impacted.

I think your idea of the using the Edcor CXPP is a good one. It has the requisite screen taps, which will simplify your work. You just need to determine the proper value of the NFB resistor.

If you're determined to use a n OT with screen taps, and don't mind building a screen power supply for pentode operation, try checking out this thread:
http://www.diytube.com/phpBB2/viewtopic.php?t=755

20to20 wrote:Is this a rubber biscuit post? Sounds like a "wish sandwich" amp from "Tube zone."

Huh? There's no red herrings here. His questions are legit. Tubezone is Ned Carlson, who used to be active around here. Ned has since taken a different direction and doesn't seem to participate any more, but he was a good guy. My Poseidon amps were built from a Tubezone kit.

Thanks Ty for the quick response. You confirmed what I suspected to be the case.

Now my only remaining question is what is a "rubber biscuit " post.

John

Rubber Biscuit: (Blues Brothers version)

http://www.youtube.com/watch?v=gSF9QT4jmsQ

/ed B

Ty_Bower wrote:

20to20 wrote:Is this a rubber biscuit post? Sounds like a "wish sandwich" amp from "Tube zone."

Huh? There's no red herrings here. His questions are legit. Tubezone is Ned Carlson, who used to be active around here. Ned has since taken a different direction and doesn't seem to participate any more, but he was a good guy. My Poseidon amps were built from a Tubezone kit.

Ty,

My post wasn't a comment on Ned's board. I'm very aware of his influence and the respect he's gained here. My post may have been a bit too cryptic.

I used the search function... (cryptic subliminal message)... and found Shannon's "Ned" board instructions... If it's one in the same "build" as what all the questions are about, uses the 8 ohm tap for FB for non-Dynaco 8 ohm trannys. Didn't look deeply at it but I assume the FB resistor is matched accordingly, if necessary.

My search... "Tubezone ST-35" put the PDF link at the top of the list.

Why waste a 60W 10K tranny on a 17W 8K channel?

With that Edcor you would get more bandwidth running it close to 20W and actually you could probably get a little more power too! 10K would give lower distortion and a higher dampening factor also. Heathkit used close to 10K on their EL84 PP OPT's.

If you have them why not try them and report the comparison! Maybe Run the good Z-565 in one channel to compare to.

Maybe open up the bad one to see if a wire maybe burned off on the outside windings. Triode Electronics clone should be really close except for the 4 & 8 ohm secondaries if you want to buy a replacement according to their posted tests.

If you want to buy an exact replacement Mike @ Magnequest makes exact replacements with the same secondaries of 8 & 16 ohms for a little more $.


Randy