Harvard (Futterman) OTL H3 w/ 6 6FW5 Tubes / Channel

chinacave

Its Like a Jungle Sometimes; It makes me Wonder
I am servicing some nice tube gear and tone arms for a pal of mine, retired from the Pro Mastering / recording studio supply / importing / consulting business -

Today I delivered 3 restored Marantz 7c's to him and he put this unit in my car to check out. He MAY have the service info somewhere... the photo below is not mine but is the same version as the one on my bench - cribbed the photo from the web...

1) Can anyone share a schematic of this unit? I have found some similar ones around the interwebs, but it seems there were several / many versions, 6 tubes, 4 tubes per side - different tubes etc.

2) there is a hair-thin wire on each tube from pin #3 to Pin #4 - and also in at least one place on the terminal strips- Acting as a fusible link for safety? Funny thing is that the tube date sheet I have seen for the 6FW5 shows NO connection - nuthin' at pin #4 (?!) - Strike that - #4 IS n/c - but used as a tie point for one of the ferrite-shielded connections to the tube - I noticed in another photo of the underside of a different model the maker included a short length of spare fusible wire for servicing... Interesting - have not seen any indication of it on the other H3 schematics found so far for other versions -

3) 3 Big computer caps here - the speaker outputs are capacitor-coupled through them - 800mfd @200 vdc - and another 400mfd @375 vdc - all are Sprague "computer-lytic" I have seen the warnings on other threads on these amps to FIRST, before anything - (IE especially w/ the OTL amps, ) replace the capacitors before testing. I have repaired / restored many PP tube amps, never an OTL one. So I have the equipment and experience, I think, to slowly bring this up on a variac to A) see if it works and B) possibly 'reform' the caps. This would be for testing only, I have NO intention of playing 40-50 year old gear with old time-bomb capacitors - Is there something unique to the OTL / Futterman circuit that argues against that course of action?

4) Any advice / clues / hints always welcome!




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There are schematics on the web for this- if you don't find them -I have everything including the bias instructions.

I have this exact amp refurbished. It is a gem. Don't let all those that claim the Futterman is a fire breathing dragon fool you. It will eat most push-pull for lunch.

What a great piece. I planned on Unlylitcs for the output couplers but never got around to it bypassed it with something like a 80mf solens.

Have fun and don't be afraid!!
 
Thanks, gents!

Kcin, i'll pm you - could really use the bias data and voltages to look for if you have them- and any advice on service / restoration.

Jazbo8- many thanks for pointing that out- I had seen that, was not sure with the different tubes if it was the same circuit. Most helpful.

Very neatly laid out, but TIGHT as a...... it's tight. several caps hidden under 12 other bits. But looks untouched or serviced, and the fuses are intact. Will try it cautiously on the variac in perhaps a week or so due to other time needs. Unless there is some reason to replace caps 1st.

On a closer look, two 1-2 watt 130k (?) looking resistors are burnt & cracked... haven't spotted them on that schematic but will print & look closer. One in each channel @ the same location. It's great w/ tube gear you can often simply LOOK for parts that have departed this mortal coil.
 
Hi

If you scroll throught the pics you will get to the bias instructions sooner than I can scan them to you.

http://home.arcor.de/nor.mei/futterman/futterman.htm

I have schematics and a breakdown of parts descriptions etc..

Remember the 6HB5 = 6FW5 one is compactron the other is octal. I have not seen a schematic of the 6FW5 ... 6LF6 can be used here equivalently as well.

If you look for some of my posts on audio asylum (my moniker there is AJ) you will need to look at screen regulators; often the chassis was labeld with one tube in mind (e.g. 7044) and then substituted with another 6EM7 I believe.. with no change to the chassis notation silk screen.

The fusible links for the outputs were provided for under the nut of the transformer held by a cable clamp for spares. They were simply a bundle of cut #44 or such AWG wire .. I forget the gauge but I have that info too.

The transformer 120v is not fused so that is a concern with this amp. I have reviews and comments from futterman that claims the turn on surge would blow any fuse that he would reasonably use and he preferred the fusible links.



I have about 100 of the 6FW5 tubes NOS as spares.

It has a couple of hermetically sealed oil caps as main couplers and the rest goodall brand in most that I have seen ... they should all be changed along with the screen and bias supply caps electrolytics. The main power supply and speaker output caps I changed with Mallory computer grade electrolytics. I will change them to unlytics as it will be an improvement.

Make no mistake this is a killer amp, especially matched to the appropriate load.. I use it on ESLs .. let me know how you make out.

If you want some fun reading look up futterman authored by Joe Rosen at audio asylum tubes or vintage.. contraversial and outspoken but he knew his stuff and I share much of his thoughts on tubes, speakers etc...

Have fun!
 
I think I have that Rosen book if it's the one I'm thinking of. Very entertaining read.

I would love to hear one of these. If they're so awesome (and I totally believe they are, no reason to doubt you or Rosen) I have to wonder how come there aren't more DIY versions being built? Seems like the tube jockeys around here would be all up in that. :D

How old you reckon this beast is? Didn't he build these in the 70s-80s?
 
Hi ,you are getting Rozenblit and Rosen mixed up.. Rosen writing a book would be hilarious.

To answer your question -the art of a OTL is difficult to master -few have. A child can practically put together a SET amp , for example and have it work.

Ask Karsten, Berning, Modjedski or Graaf amps how long it took them to perfect their designs which are far more elegant than the published Mullard circuits recircuilating since WW2.

Alot more circuit knowledge, nested feedback loops or in the case of Berning high frequency carriers and separating the audio from that.. Hard to get right and limited in the speaker selection. Most commercial amps try to be the most they can for everyone.

OTLs are like a Bugatti... the right application and it pulls ahead of every one.

This amp is about 1963 vintage.. Futterman worked his whole career and died perfecting his circuit. I doubt anyone would put that effort into a commercial unit today - cost prohibitive. There is some great history and reading to interactions with Futterman on the web by folks that understood and lusted after the sound un-fettered by magnetics and their potentially dulling affects.

Havey Rosenberg developed a company trying to perfect the Futterman just after his death. He realized its "greatness"

YMMV and all that!
 
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Yes, Harvey Rosenberg! Thanks for clearing that up.

OK so they are not easy to design, but once designed and built, they can be copied...unless they are so finicky that adjusting bias becomes a black art.
 
You cannot completely trust the schematic on the web. The splitter tube is label as a typo and the small 0.47 caps in the upfront feedback networks are actually 0.047 caps in the actual amps, or at least in the 4 or 5 I have seen and owned. You really need to change every cap in the amp as these are at least 40 years old now. I also change all the pots to Spectrol multi-turn pots using the same values in the original amp. I add small trimmer pots in these adjusting circuits as well so I can fine tune the voltages since getting well matched tubes now is so tough from this era. I added a 3 amp turn on switch protected by a CL30 in series where the single inlet plug was originally. I changed all SS diodes. I use a bridge instead of individual diodes as well. Hope this helps future folks.
 
:lurk:
Curiosity makes me watch
I guess the irony is I watch a bunch of threads on restores and mods,
some I try or think I might try
some are pipe dream projects I'd consider if I had a lot more time and a little more money (very not likely to happen).
Then there are these, I'm fascinated (not limited to these) If I won the lottery tomorrow and you gave me back 25 years of my youth there is little chance I'd be crawling inside one (well maybe if those two things happened)
I'll go for the ride.........Carry on gentlemen
 
What a find! I'll be watching this one.

If this one was built by Julius Futterman, rumor has it that the parts varied somewhat based upon he had in the parts bin, so that could make things interesting.
Is it point to point wired or on a PCB?

Slightly OT, but Harvey Rosenberg's NYAL ad copy was very funny stuff.
 
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I see this thread is back. The picture found on line is my Harvard amp. I have not restored mine, but I did power it up as the Ham radio guy I got it from had been playing it. I do not plan on powering it up again until restored
 
Pictures of Harvard Amp inside

Here are some pictures inside amp and the back
 

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I'll get some detail photos of the underside of the amp shortly. I have not powered it up, and won't 'till I replace the caps and double-check the circuit.

Appreciate the input.
 
Those red caps underneath look like cardboard jobs, or am I seeing things? Yikes.

The computer caps, I don't know that you'd have to replace them before powering up. I am no expert on this here beast so I'm probably wrong, but it seems to me they are there at least partially to prevent DC from getting to the speakers, correct? If you used a dummy load on the outputs you could check for DC w/o risking speakers. But don't take my word for it, I'm an amateur. And a restoration thread watcher. :D
 
"Those red caps underneath look like cardboard jobs, or am I seeing things? Yikes." Yes they are cardboard
 
Thanks to all for the replies and info - before I start on this I wonder if anyone has any close-up photos of the input / output section of the underside - that is where I suspect mine differs from the schematics I have seen, I suspect there is a extra resistor in there. and a burnt one.

I'll post photos shortly.
 
Here are some more pictures of my Harvard Amp
 

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Wow - thanks -

That DOES look like mine, I will have to get in (back) on the bench and check for sure - from memory, its that new resistor in your photos that is burnt / cracked on mine, and does not appear to be on any schematic I have yet seen -

BTW - does yours have the hair-thing wires I mentioned in post #1? fusible links? Also not on any schematic I have seen.

Many thanks!
 
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