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Futterman H-3 restoration

135.180.105.29

Posted on March 4, 2024 at 17:51:38
33dot33rpm
Audiophile

Posts: 79
Location: Nor Cal
Joined: June 8, 2006



Hi All, I've completed the restoration of a friend's Futterman H-3 amp. It was essentially non-functional, now in it's restored / repaired condition the amp sounds very good indeed.

All the operation point adjustments are spot on to the manufacture's specifications.

All the tubes where tested on a Hickok 752A and replace with new ones where necessary.

DC voltages at various points in the circuit seems reasonable.

From the available documentation the idle current for the output stage is 200ma per channel, 400ma in total which is a lot!!!

My question for the H-3 owners: During operation, is it normal for the power transformer to become hot to the touch?

Please let me know, I don't want to inadvertently destroy the transformer.

Enjoy the posted photos. I included a before and after of the "guts".

 

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RE: Futterman H-3 restoration, posted on March 5, 2024 at 07:35:52
rogerh113
Audiophile

Posts: 513
Location: San Francisco South Bay
Joined: October 20, 2014



Just starting to bring my H3 up after a full rebuild. Had to have the transformer rebuilt due to internal shorts. Pic of what they found on the inside....

In the next week or two, I should get some time in on it, possibly enough to get a sense of where the heat of the transformer is.

 

RE: Futterman H-3 restoration, posted on March 5, 2024 at 09:32:13
33dot33rpm
Audiophile

Posts: 79
Location: Nor Cal
Joined: June 8, 2006
Wow! That is a real bummer!!!

Just yesterday my transformer began an annoying buzz when powered up. I immediately shut down and inspected the internals for loose parts or anything out of the ordinary - nothing. Subsequently the amp powered up with no problems. Later that evening same thing and this morning no problem.

On review of the circuit diagram and reflection of prior experience I suspect a problem with the PS bridge rectifier network. The original parts are four 1N2484 600V 750ma and pretty old tech. Also in my amp the main filter cap was bad. I believe that the diodes where going bad but not quite failed due to the old cap. The diode recovery time may have been compromised whereby loading the transformer. However because of the intermittent nature the problem still has me puzzled.

I'm going to replace the old diodes with new 1N4007 and if I have the proper capacitors on hand, add snubbers to the bridge.

Also if you haven't done so replace the single diode on the secondary power supply located before C15 with a 1N4007.

Can you provide me with company that's rewinding your PT along with the specifications?

Thanks 33dot33rpm

 

RE: Futterman H-3 restoration, posted on March 5, 2024 at 09:52:52
rogerh113
Audiophile

Posts: 513
Location: San Francisco South Bay
Joined: October 20, 2014



Attached is the spec sheet for my H3 transformer. They measured everything when taking it apart so they could accurately rebuild it. Not cheap.

I did gut the Futterman when I rebuilt it. The old parts were not useful and the wiring was pretty poorly done. Everything was replaced aside from tube sockets. I was hoping to keep the transformer, but when I started to bring up the amp on a variac it was obvious that was not to be. If you keep a bunch of old parts in there, you will likely experience death of a thousand cuts as they fail (hard or soft) one after another.

regards -- Roger

 

RE: Futterman H-3 restoration, posted on March 5, 2024 at 12:04:53
33dot33rpm
Audiophile

Posts: 79
Location: Nor Cal
Joined: June 8, 2006
Thanks for the info Roger.

Good luck with your H-3!

I agree with you that the original build quality left a lot to be desired. My amp had been worked over by someone else and that added to the poor wiring job. In my case I decided to leave the CC resistors in place if they tested within reason. I replace all the electrolytic, paper and "plastic" caps but retained the small "dog bones". In the process of replacing the remaining diodes.

Please keep us posted on your progress as I will mine. If you have any photos of your rebuild it would be much appreciated.

Cheers Ed

 

RE: Futterman H-3 restoration, posted on March 5, 2024 at 14:05:21
rogerh113
Audiophile

Posts: 513
Location: San Francisco South Bay
Joined: October 20, 2014



Hi Ed,

Here are a couple of pics. Wired in on the bench, so access not ideal, but should give a sense of things. I have passed signal through it, so it does seem to work. Need to go through the power tubes (some bad ones in place based on voltage setting level on one side). Just have to spend some time. Can't see it, but I did install an IEC power socket.

 

RE: Futterman H-3 restoration, posted on March 5, 2024 at 14:09:18
rogerh113
Audiophile

Posts: 513
Location: San Francisco South Bay
Joined: October 20, 2014



The top

 

RE: Futterman H-3 restoration, posted on March 5, 2024 at 14:19:07
rogerh113
Audiophile

Posts: 513
Location: San Francisco South Bay
Joined: October 20, 2014



Some in progress detail under the wires...

 

RE: Futterman H-3 restoration, posted on March 6, 2024 at 05:49:52
AJ
Audiophile

Posts: 533
Joined: February 24, 2001






Great work.

The transformer is definitely a weak spot of this amp. I am fortunate that mine is ok after all these years. I did re build mine and made a new wire harness from teflon wire. I have some KEmet poly 600uf output caps that I need to install and get rid of the electrolytic cans that I replaced years ago. They will fit with some creativity.
I actually restuffed the cardboard shells for sprague atoms.

Your transformer information could help others - that is helpful.

 

May I suggest a NTC thermistor on the primary?, posted on March 6, 2024 at 09:40:48
garymuffley
Dealer

Posts: 448
Location: Maryland
Joined: March 29, 2006
A very simple way to reduce the sudden surge when you switch the amp on.

 

RE: May I suggest a NTC thermistor on the primary?, posted on March 6, 2024 at 17:30:25
33dot33rpm
Audiophile

Posts: 79
Location: Nor Cal
Joined: June 8, 2006
Hi Gary, thanks for your suggestion. The thermistor is certainly a good idea. It will lessen the initial stress on the rectifier and good for the large electrolytic cap on power up. Since the diode replacement there has not been any transformer buzz issue.

With a restoration project I'm hesitant to modify the circuit in any way but that said another change I would make to the PS is to do like the H-3aa and put a bleeder resistor after C16. In the event that the down stream 1.5 fuse trips, C16 will remain charged and yes I got shocked by that cap!

 

RE: Futterman H-3 restoration, posted on March 6, 2024 at 17:46:40
33dot33rpm
Audiophile

Posts: 79
Location: Nor Cal
Joined: June 8, 2006
Hi AJ, you did some fine work restoring your H-3! Yes C12 is a sore spot in the amp's circuit! I was thinking of paralleling it with something like a 10uf PIO, but it's not my amp so best to leave well enough alone.

BTW how hot does your PT get after driving it for a while? With mine just idling it gets warm. After playing music for say 1/2 hour it get hot to the touch.

Thanks 33

 

RE: Futterman H-3 restoration, posted on March 6, 2024 at 18:42:41
AJ
Audiophile

Posts: 533
Joined: February 24, 2001
Hi,

That sounds just about right to me regarding how the temperature of the transformer behaves.

 

RE: Futterman H-3 restoration, posted on March 7, 2024 at 08:39:48
33dot33rpm
Audiophile

Posts: 79
Location: Nor Cal
Joined: June 8, 2006
Thanks AJ, now I have confidence returning the H-3 to it's owner!

 

RE: Futterman H-3 restoration, posted on March 7, 2024 at 08:43:29
33dot33rpm
Audiophile

Posts: 79
Location: Nor Cal
Joined: June 8, 2006
Kudos to you, excellent work and quality parts!

 

RE: Futterman H-3 restoration, posted on March 7, 2024 at 09:14:12
rogerh113
Audiophile

Posts: 513
Location: San Francisco South Bay
Joined: October 20, 2014
Thanks. My objective was to hear what a respected, period OTL would sound like if brought up to optimal (in my opinion) standards. I build a SET amp before, under the same objective, and am very pleased with the result.

Hopefully I can hear the OTL soon, if a bunch of other high-priority projects stop coming up around the house. Given that I am listening to the SET, and it is unclear how that can be bettered, I don't feel under any pressure to complete the OTL.

 

Futterman H-3 restoration frequency response, posted on March 7, 2024 at 16:58:18
33dot33rpm
Audiophile

Posts: 79
Location: Nor Cal
Joined: June 8, 2006



Just for fun I ran some test on the amp. Here is the frequency response.

 

Futterman H-3 square wave, posted on March 7, 2024 at 17:01:25
33dot33rpm
Audiophile

Posts: 79
Location: Nor Cal
Joined: June 8, 2006



Just for fun I ran some test on the amp. Here is the square wave.

 

RE: Futterman H-3 hum level, posted on March 7, 2024 at 17:03:43
33dot33rpm
Audiophile

Posts: 79
Location: Nor Cal
Joined: June 8, 2006



Just for fun I ran some test on the amp. Here is the hum level.

 

Futterman H-3 check for oscillation, posted on March 7, 2024 at 17:07:07
33dot33rpm
Audiophile

Posts: 79
Location: Nor Cal
Joined: June 8, 2006



Just for fun I ran some test on the amp. Here is the check for oscillation at idle, nothing detected.

 

RE: Futterman H-3 restoration frequency response, posted on March 8, 2024 at 07:16:30
rogerh113
Audiophile

Posts: 513
Location: San Francisco South Bay
Joined: October 20, 2014
Thanks for posting the response curves. Most surprising to me is the roll off at 8kHz. Wonder what is causing that? The signal caps should all be presenting less impedance the higher the frequency.

 

RE: Futterman H-3 restoration frequency response, posted on March 8, 2024 at 08:05:29
33dot33rpm
Audiophile

Posts: 79
Location: Nor Cal
Joined: June 8, 2006
IMO the roll off is minor at less than 1db. It's the same for both channels so I think it's in the circuit design. What's more interesting is the square wave response. There is practically no ringing but rolled off shoulder. The cause is most likely the compensating network of C6-R12 & C8-13. I would try reducing the value of C6/C8 to mitigate the roll off and maybe live with a little ringing. When you get your H-3 running and you find the sound too warn, try disconnecting the network or drop C6/C8 to 0.047uf. That should brighten the tone. If you look the the H-3aa diagram that network has been deleted.

 

RE: Futterman H-3 restoration frequency response, posted on March 8, 2024 at 08:12:58
33dot33rpm
Audiophile

Posts: 79
Location: Nor Cal
Joined: June 8, 2006
Oops I mean to say rounded shoulder not rolled off. That network may also be responsible for the 7k ramp down of the frequency response! If I find time this weekend I'll try disconnecting the network and report back.

 

RE: Futterman H-3 restoration frequency response, posted on March 9, 2024 at 06:55:03
rogerh113
Audiophile

Posts: 513
Location: San Francisco South Bay
Joined: October 20, 2014
I agree that they are likely related. You need all the speed you can get to climb the leading edge of the square wave and then flatten off cleanly.

Probably more representative of musical transitions or blended frequencies of multiple instruments and voices. There is a lot more demand with those transitions than a couple of sine waves.

 

If they are going to oscillate, it will be load dependent. nt, posted on March 13, 2024 at 14:10:53
Ralph
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Location: Minnesota
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