Wiring st-35 clones as monoblocks

[Shannon Parks]

... wire one opt primary on each stereo amp in reverse, and each speaker between the hot speaker out terminals of each amp, and get a 32 wpc set of monoblocks.

Checkout the other thread. BTW, could you draw a sketch or refer to the assembly drawing? :)

Shannon

[Shannon Parks]

OK - I understand what you mean now! :idea:
I'll look into this - very interesting. BTW, you have the right idea - let's mod this thing and have fun with it. I'll see what I can come up with....

Shannon

[EWBrown]

Since the circuitry utilizes feedback from the OPT secondary, there could be some unexpected and unwanted effects from doing this.

On my simple SE amps, this is easy, as there is no feedback used, the OPT secondaries are floating, they are simple to "bridge" for monoblock use, tie the 2 inputs together, and connect the two secondaries in series
(not parallel) and feed the speaker with that. THe trannies are 9300 ohm pri / 6 ohm sec, which work well with 4 or 8 ohm speaker loads. In series
they are effectly 12 ohms (not mutually coupled, so they are 2X and not 4X the impeadance), and are happy with 8 ohms. THey can be paralleled, for 4 ohm speaker loads. Just watch the phasing in either case.

/ed B in NH

[Shannon Parks]

MG16,
I messed around with this tonight some. Combined the inputs. Paralleled the outputs. Got 27W easy before 1%THD (eyeball guess). I better read up on what I'm doing before I blow something up. :twisted:

Shannon

[EWBrown]

I was recently looking over the Dynaco ST-70 I have, they have a stereo/mono switch which ties the left and right inputs together.
They tied the two OPTs secondaries in parallel, according to which impedance tap you needed. This doubles the available current, but leaves the output voltage virtually unchanged.

On my simple SE amp, there is no feedback, and the OPT secondaries are floating, the method to monoblock that is to tie the two inputs together (or use a Y cable) and then wire the two OPT secondaries in series (the OPTs are 9300 ohm primary, 6 ohm secondary. This doubles the available output voltage, and works well with 8 or 16 ohm speakers.

My second SE amp, is the exact same basic circuit, but I use two of the HWT 10K primary (211 or 845) trannies. Using these 11 pound monsters with a single ended SV83 is like having the tail wag the dog, but they really do sound sweet.

The next step in the evolution of FrankenAmp III
is to use 3 SV 83s in parallel, and a One Electrom UBT-3 OPT.
(muahahahahahaha....) :twisted: :twisted: :twisted:

I'll try the first method with my DIY and see what happens.

/ed B in NH

[Geezer]

I'm the new kid on the block here. I built Heath and Knightkits as a kid and have always been into stereo--in recent years, I've been building and modifying speakers and crossovers. I know which end of a soldering iron to hold (knowledge acquired through several painful experiences) and have a decent grasp of general electronic theory, but I'm a little weak on the technical side. I've got an English degree, okay?

I picked up a blue board on Ebay a few months back and got a great deal on a partially-completed diy ST35--good original cloth-lead transformers, gold inputs and speaker outputs, some nice poly caps, new LCR electrolytics etc. --all point-to-point wiring.

Yesterday at a garage sale I picked up an SCA35 for $15. It doesn't work, but I powered it up on my Variac and the power transformer is good. The output transformers aren't getting any juice but appear physically fine.

I haven't gotten started on my original project yet, I still have to construct a chassis, so I'm naturally thinking "MORE POWER" and a second blueboard.

I dislike bridging because of the output impedance problems, and I do believe you pay a sonic sacrifice. Just for giggles, are there any stereo circuits using quad EL84's for each channel I might take a look at?

A completely separate question--I now have two PA774 xformers with cloth leads. The cloth is rotting and I can see copper in places. Should I just use heat-shrink to cover 'em up, or should I splice in close to the taps with Monster cable?

And what are the sonic advantages (if any) of a 35WPC EL34 design vs. EL84? Any ideas, suggestions, criticisms, reference material, explanations or insults are welcome.

[Shannon Parks]

Yesterday at a garage sale I picked up an SCA35 for $15. It doesn't work, but I powered it up on my Variac and the power transformer is good. The output transformers aren't getting any juice but appear physically fine.

Nice score. I bet a dollar to a doughnut the outputs are fine. I'll give you $20. :wink:

I dislike bridging because of the output impedance problems, and I do believe you pay a sonic sacrifice. Just for giggles, are there any stereo circuits using quad EL84's for each channel I might take a look at?

OK, ok - I plead guilty. I let this thread get away from me. Yes, I've done this on the bench twice. The first time flopped (bad hook ups?) but the second time I did this (June 4th post) went without a hitch. I combined the inputs (J1-1 and J1-3) and made the following connections (which leaves us with a parallel PP EL84 circuit at around 4k impedance):
-J2-2 to J5-2
-J2-1 to J5-1
-J3-1 to J4-1
-J3-2 to J4-2
-J3-3 to J4-3
You could probably solder wires on the bottom (err, top) of the board to 'neaten' this up.
Then hook up a ~4k OPT to either side and crank it up. I honestly forget if I hooked up the negative feedback, but the 1kHz sine wave looked real nice.
That's where my tests ended. I had planned to Spice it, but have refocused on getting the Eiclone done instead. I will look for a similar schematic and post it (ie PP parallel EL84).
Main question need answering: Should all the twin trode tube halves be completely jumpered to insure stability? I think a schematic or two will provide a lot of insight to the matter - I'll start Googling in a minute. If anyone would like to do a parallel PP, I would be more than happy to lend some of my brain, but my bench is spoken for. :wink:

A completely separate question--I now have two PA774 xformers with cloth leads. The cloth is rotting and I can see copper in places. Should I just use heat-shrink to cover 'em up, or should I splice in close to the taps with Monster cable?

Hmm, I've not done any tranny splicing before, so I'd kind of be interested in others ideas, too. Those leads get mighty stiff. I'd splice in some 18GA teflon coated stranded wire if I had too.

And what are the sonic advantages (if any) of a 35WPC EL34 design vs. EL84? Any ideas, suggestions, criticisms, reference material, explanations or insults are welcome.

Wait a second though, you've got four Z565s (wrong power, wrong impedance for a parallel circuit). I just botched this reply up as you couldn't parallel anyhow. :oops:

Build it stock and experiment with bridging if you like. Best of both worlds that way, right?

Shannon

[EWBrown]

Somewhere in my archives, I have a Russian schematic for a 4X EL84 amp circuit, I'll e-mail it to Shannon if I can find it here. The writing is in Russian Cyrillic, but it's still easily understandable. i'm not sure of the OPT primary impedances used. VTL also uses a similar lashup in their 25 watt "mini mono blocks".

/ed B in NH

[Shannon Parks]

Ed,
I would appreciate a copy of that schematic when you get the chance. I need to add a Files section to the website where I can post datasheets, schematics, etc., too.
Let me get my coffee first. :coffee:

Shannon

[dhuebert]

I was thinking of asking you to post the user manual somewhere that you need to login to get to, is this possible or desirable?

[Shannon Parks]

I've been working on the best method for this. What about one that just can't be printed? Either that, or one scanned in as jpgs. Will do!

Shannon

[Guest]

Files section sounds GREAT! :!: