Futterman H3aa monoblocks restoration
- Thread starter Jfaustus
- Start date
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.
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?
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.
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.
right-o! yes def has the power up/down thump.
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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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...
I'd say copper. Reddish not silverish.
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.
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.
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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Thanks to @kcin for the additional info on these babies! Interesting to learn that "factory" (it was Julius Futterman's apartment) specs were 100 watts into 8Ω and 150 into 16Ω (adding this info to the top post). That's substantially more than I thought. Here's where we stand:
Following the setup instructions from kcin (attached here) I got some WOWZER readings.
I do actually have a few -- 12 altogether that came with the two channels and maybe 8 or 10 other singletons. I'd like to at least try to find a matching set among them. Even if I can only cobble together one channel I want to hear this thing in action!!
Any recommendations for a quick and cheap jig to match tubes? Need to be able to measure current at a range of voltages at operating conditions.
- Main speaker coupling cap (2500µf) left alone. Measured good out of circuit on a Sencre that tests at operating voltages.
- Most of the caps in the NFB section replaced with orange drops
- signal coupling caps replaced with solen polypropelene "fast" caps.
- Original paralleled group of three B+ filter caps replaced with single modern cap (720 µf total in original to JJ 800µf 385V).
- Out of spec resistors replaced.
- Confirmed that the original OC2 socket is now used as just a convenient wiring terminal; OC2 don't live here no more.
Following the setup instructions from kcin (attached here) I got some WOWZER readings.
- "Balance" isn't channel balance like I would normally think (duh in a monoblock). It's the balance between the upper and lower banks of tubes. Supposed to zero out with the pot. 63V was the lowest I could get it!
- Two readings that were supposed to be within 2 volts of each other were at 45V and 57V.
- Bias was supposed to be 95V (ish) and the lowest I could get it was 143V.
I do actually have a few -- 12 altogether that came with the two channels and maybe 8 or 10 other singletons. I'd like to at least try to find a matching set among them. Even if I can only cobble together one channel I want to hear this thing in action!!
Any recommendations for a quick and cheap jig to match tubes? Need to be able to measure current at a range of voltages at operating conditions.
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I wonder if you could cheat and do 2 tubes at a time to match them that way. Output impedance will be overly high but with a solid state power supply it shouldn't screw up the DC voltages too much.
I'd also wonder if there aren't other components at fault here. Looks like a zener diode in that balance circuit, if thats bad it will screw things up. May also want to check the zeners in the screen supply, if thats not right the rest of the voltages will end up whacked out.
I'd also wonder if there aren't other components at fault here. Looks like a zener diode in that balance circuit, if thats bad it will screw things up. May also want to check the zeners in the screen supply, if thats not right the rest of the voltages will end up whacked out.
I've been thinking about using the unit itself to test for matching -- and I did install sense resistors on the three lower bank tubes so I could measure current on first bootup. The sum of the lower bank currents was in spec but they were definitely different from each other. When you say solid state power supply do you mean NOT using the xformer from the amp and instead subbing in a regulated PS?
And good advice re the zeners. Pesky little buggers. I will pull them out and test.
Using the unit to test is a good method.... even with all tubes installed if there is an imbalance side to side you can swap tubes from side to side to get closer. In my experience bad tubes are the cause-- but if you have not fully restored it with new zeners then I would do that to get a base line.
If your unit has a grounding strap between negative and ground you can break the strap and measure current through the tubes in pairs easily. Even if it does not- you can rig it per the schematic - have a look.
If your unit has a grounding strap between negative and ground you can break the strap and measure current through the tubes in pairs easily. Even if it does not- you can rig it per the schematic - have a look.
Oooooh I just went through this. Please stop me if I garble this -- I'm still learning. In the schematic the lower bank tubes' cathodes were tied to a post at the speaker terminal which, yes, is strapped to a separate ground post. But the cathodes in my units (both channels) are each tied directly to the ground bus not the negative post. So I did break the strap and measure current but got bupkus. The led to a closer investigation and seeing the direct-to-ground wiring. So I've installed sense resistors between cathode and ground on each of the three lower bank tubes. I used 0.5Ω 2W 1%.
Zeners first, which no I haven't done. Maybe then I'll try swapping and/or add sense resistors to the upper bank tubes as well. Also I took the instructions' hint to heart that the wacky reading of the bias voltage in Step 4 pointed at the 6EJ7. So I swapped it with another one I had in the stash which tests meh on the Hickok but better than the one that was in. That's shifted the bias test point from the instructions from 143V at minimum rotation to 76V with the pot at maximum rotation.
ONWARD!
A resistor at the cathode of either tube will read basically the same current since they are in series. Probably a few milliamps difference to account for the second screen and it's current flow but nothing significant.
problem with all of this being direct coupled is that everything affects everything else. If any of the voltages in the driver section are way off, the output stage ends up whacky too.
problem with all of this being direct coupled is that everything affects everything else. If any of the voltages in the driver section are way off, the output stage ends up whacky too.