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Futterman H3aa monoblocks restoration

have to have a sense of humor when dealing with dangerous stuff.

that said if you wanted to add in a speaker protection relay it probably wouldn't be the worst idea, though as long as the output coupling cap is good its really not that serious a risk. Lots of old solid state things ran for decades with output coupling caps without frying speakers.
 
Yeah, all in good fun! Anyway these aren't going anywhere near my good speakers until they're both thoroughly tested on the scope. Including and especially for oscillation which they are prone to.

My nice speakers are individually fused. Is there any advantage of a relay over a fuse?
 
A relay on a properly designed circuit would open with any DC over whatever the trigger amount is, fuses take a while to open depending on how far over their current rating they are. Fair chance the fuse won't open before something bad happens to the speaker if the output cap did start leaking enough current to be a problem.


They kinda do different things though. A relay won't prevent damage from being over-driven from just being cranked up too loud, but a fuse might.


one other thing a relay may do for you, if this makes funky noises at startup or shut-down like a lot of cap coupled things do, one that has an on-delay and drops out as soon as power is shut off will take care of that.
 
gadget73 said:
A relay on a properly designed circuit would open with any DC over whatever the trigger amount is, fuses take a while to open depending on how far over their current rating they are. Fair chance the fuse won't open before something bad happens to the speaker if the output cap did start leaking enough current to be a problem.


They kinda do different things though. A relay won't prevent damage from being over-driven from just being cranked up too loud, but a fuse might.


one other thing a relay may do for you, if this makes funky noises at startup or shut-down like a lot of cap coupled things do, one that has an on-delay and drops out as soon as power is shut off will take care of that.
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right-o! yes def has the power up/down thump.
 
Jfaustus said:
After some hiccups, we have full, stable power-on with all tubes and 8Ω dummy load on one channel. Woooooooooo!

Updates
  • Recapped and out of spec transistors replaced on one channel.
  • I bought a B+ filter cap thinking I just wanted to get it working and that the aesthetics wouldn't bother me. Now that it's here and replacing the can I hate it. Maybe it will fit inside the can? It does indeed slide in nicely -- for the first 1/4". Then the can narrows just a tiny bit and... no. GRRRRR. This will wait for later but I may go the Heyboer route (depending on cost).
  • Slowly brought up on a variac and.... one of the banks of the output tubes redplated. Ok. Shut down. Reexamine. Found a mistake in the wiring that I made when reattaching some leads. (Thank goodness for a second reference channel.) That's now fixed.
  • Also found two places in the reference channel where connections were made with the tiniest wisps of wire that were missing in the one I'm working on. Added those in.
  • Tightened loose receptacles on one of the tube sockets. Only one out of 12 was even a little loose. Not bad for 51 years!
  • Checked the bias pot which I should have done in the first round. There are 3 or 4 spots where it shows no reading or open. No change after DeOxIt. It was stable and smooth outside those spots so I just left it close to the highest resistance value and didn't touch it when powering up.
Questions

Disclaimer: I don't have a schematic for this and I'm not sure there's one to be found. The one gadget posted earlier in this thread is more like the inspiration for what I have in front of me.
  • Should I suck it up and try to write out the full schematic? I've never done that before and suspect my drafting would be, well, bad. But I suppose I should for posterity if nothing else. Please someone help and tell me that no no no there's no reason at all to do that. Someone...?
  • Replace bias pot altogether? I'd like to keep these as original as possible but not to an irrational degree. If there's something sensible to do to the existing pot I'd prefer to try but if not I'll replace.
  • What's with the wisps? I mean one small copper filament, smaller than a single strand of speaker wire. I did replace with a single strand of speaker wire since that was the smallest diameter I had on hand. Is that potentially a problem?

View attachment 3800985View attachment 3800986
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The copper wisps as you call them are fuseable links of small guage wire that were used in the cathode circuit of each tube. because of the high turn on current of this design - futterman found fusing each individual tube safer than some large slow blow fuse.

I can find the correct gauge for you if you need it. Futterman provided a bundle of the correct gauge "wisps" under a cable clamp bolted to the transformer. Written on the black chassis in pencil was some thing like "spare fuseable links" - really hard to see!
 
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kcin said:
The copper wisps as you call them are fuseable links of small guage wire that were used in the filament circuit of each tube. because of the high turn on current of this design - futterman found fusing each individual tube safer than some large slow blow fuse.

I can find the correct gauge for you if you need it. Futterman provided a bundle of the correct gauge "wisps" under a cable clamp bolted to the transformer. Written on the black chassis in pencil was some thing like "spare fuseable links" - really hard to see!
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That's amazing! Yes please on the gague. The strands I have used are slightly larger than stock so I would appreciate getting it right. Interestingly the fusible links do appear in two places that are not tube sockets.

And good timing. I've just returned to these after letting the project sit for awhile. I got slightly obsessed with stripping the paint off the built-in that serves as my workbench.

In the interim, the gremlins absconded with a pair of 33µf caps needed for the second channel. Poof! At least they're cheap but now I have to wait...
 
Was this just copper wire of a particular gauge, or was it actually fuse wire? I have seen rolls of fuse wire meant to rebuild old style fuses. It looked more like solder than copper.
 
gadget73 said:
Was this just copper wire of a particular gauge, or was it actually fuse wire? I have seen rolls of fuse wire meant to rebuild old style fuses. It looked more like solder than copper.
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I'd say copper. Reddish not silverish.
 
interesting. It works, I mean fundamentally a fuse is just a piece of wire that burns up at a predictable current level. The actual element could be anything that fits the need. Just have to mind gauge and length since both will change how it behaves.
 
Here is a link to one of the instruction pages for the futterman:


It details the gauge of the wire.


The other two pages may be of interest to you as well:



the bias instructions are not here I have two versions of them one for the h3 and the other for the h3aa -- similar but different--- if you get stuck I will dig them out. They are buried in my files.
 
kcin said:
Here is a link to one of the instruction pages for the futterman:


It details the gauge of the wire.


The other two pages may be of interest to you as well:



the bias instructions are not here I have two versions of them one for the h3 and the other for the h3aa -- similar but different--- if you get stuck I will dig them out. They are buried in my files.
Click to expand...
SUPER HELPFUL. Thanks!

I’ll reach out when I’m closer to full power up and testing for the bias instructions. I was going to go by the H3 instructions which are online. But better to have the real ones.
 
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