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Old June 3rd, 2009, 10:56 PM   #1 (permalink)
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Harvard (5F10)

Thought I'd share the latest pics on my current personal project. A Tweed Harvard 5F10 made from a Weber 5F2A chassis and cab.

Got a bunch of parts including the transformers recently. All that's really left is to make the circuit board (I have all the electronics), a couple of gromets, and the speaker (still shopping). Here's the latest round of pics:









A few more pics to follow...

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Old June 3rd, 2009, 10:57 PM   #2 (permalink)
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That Allen/Heyboer TP25 is TALL! There's, maybe, 1/8" clearance from the bell to the front of the cab. It fits though. The OT needed one hole drilled in the chassis, too.

I'm thinking about a pot with SPST switch in it for the volume pot to wire up as a standby. Not sure if I'm going that way yet.

I'll update this thread as the wiring gets going.
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Old June 3rd, 2009, 11:19 PM   #3 (permalink)
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Hi

beware of a pot mounted switch passing B+ as it may not be stout enough insulationwise. Also, I'm not sure I'd want B+ so close to the signal path. Some may disagree.
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Old June 3rd, 2009, 11:40 PM   #4 (permalink)
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It looks like he'll be using a 3 way Off-Standby-On switch instead of the Volume pot On/Off switch.

I like Harvards. Primitive, but with a bit more oomph than a Champ.

This will be a fun thread to watch...
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Old June 3rd, 2009, 11:46 PM   #5 (permalink)
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Hmmm...that's not something I thought of, Johnny. That's just an On-Off DPST that I planned on for all the PT leads.

Also good points, Shadow. Again, I'm not decided if I even need a standby, but still entertaining ideas.

Thanks, fellas.
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Old June 5th, 2009, 03:54 PM   #6 (permalink)
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That's always looked like a great in-betweener to me. With a few lower-valued coupling caps than the Deluxe, it might be a little brighter. I've always wondered how that 6AT6 preamp sounds. Cool build. I'll be watching this thread!
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Old June 5th, 2009, 11:53 PM   #7 (permalink)
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Hey, FiddlinJim!

I actually scored five NOS RCA 6AV6s for this for 99˘ I'm as anxious as you are to hear them. They're apparenly just one half of your standard 12AX7 type with a diode section that's unused. The 6AV6 is slightly higher gain than the 6AT6, from what I have read/seen. Kind of like a 12AT7 to 12AX7.

I have the tube retainer clips on and plan to make the circuit board this weekend!
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Old June 6th, 2009, 12:15 AM   #8 (permalink)
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Quote:
Originally Posted by MeanGreenBlues View Post
Hmmm...that's not something I thought of, Johnny. That's just an On-Off DPST that I planned on for all the PT leads.

Also good points, Shadow. Again, I'm not decided if I even need a standby, but still entertaining ideas.

Thanks, fellas.


I don;t believe the cathode stripping thing. On-Off is plenty good :)

Unless you want a "mute" switch... er I mean a Standby :)

I really hope you make sound clips!
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Old June 6th, 2009, 12:50 AM   #9 (permalink)
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I hope to get some sound clips...if you can tolerate my playing. I'm hoping to capture the Steve Cropper Stax type sounds, but it's really just that I love the split-load/cathodyne tweed PI, but wanted a low watt, grab'n'go in the stable.

I'm thinking this is goint to be a 10-12 watt deal. Not sure if it needs a standby, personally, but I always keep that option in the back of my mind just in case.

I'm with you, though, just turn the volume down all the way if needed, eh?
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Old June 7th, 2009, 10:20 PM   #10 (permalink)
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Got the board built and populated. Mounted it in the chassis and grabbed a few pics:







Wiring begins next! I always forget how tight it is in the tweed chassis until I get inside one again.
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Old June 8th, 2009, 02:07 AM   #11 (permalink)
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Nice work.

Very cool going with a trimpot in the bias circuit - old fixed-bias always benefit from adjustable trimpots.

Weird how the new Mal's use white for the smaller caps these days, huh? I guess they're not called "Mallorys" anymore though, are they? I still use em all the time :)

Is the switched output jack just temporary, or are you planning on putting a big-watt resistor in there for protection?
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Old June 8th, 2009, 10:33 AM   #12 (permalink)
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Yeah, Johnny, I noticed that Mallory switched to the white on the latest few batches of .022µF caps that I got. I wonder if the rest will follow? I hear the white film isn't as warm sounding as the yellow film.

The switched jack is pretty much what I had on hand. Which resistor are you referring to? I assumed having the jack shorted was safer for the OT than it being open in the case that a speaker wasn't plugged in or something like that. I was going to wire it modern style with the switch for shorting.

Maybe I'm just confused though...happens often.
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Old June 10th, 2009, 12:25 PM   #13 (permalink)
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You DON'T want to short the output! (probably worse than an open circuit for the OT); open circuit will blow the primary, short will blow both primary and secondary.
For safety you really need an 8ohm 20W resistor; problem is finding one (you could use 2 10W@16ohms OR 4 5W@32ohms --or closest values--resistors in parallel).
Another solution is to hardwire the speaker to the jack in such a way as to disconnect it when you plug in an external cab.
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Old June 10th, 2009, 04:22 PM   #14 (permalink)
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Interesting, Shadow.

I just re-read some transformer FAQs written by a transformer designer R.G. Keen, and he states, "It's almost never low impedance that kills an OT, it's too high an impedance." Following this, wouldn't an open load be worse since it's a maximum impedance rather a shorted load being minimum impedance?

My understanding is that an OT can handle a shorted secondary but an open secondary (or too high and impedance) can blow the OT.

Of course, this is a combo amp, so it's less of an issue to mistakenly forget to plug in the speaker.
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Old June 10th, 2009, 04:40 PM   #15 (permalink)
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Fender went with the shorting option.
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Old June 10th, 2009, 05:18 PM   #16 (permalink)
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"Too low impedance, for instance a 2 ohm speaker cabinet hooked up to an amp expecting 8 ohms, will overload the output tubes and the output transformer. The lower impedance will force lots of amperage through the tubes and out through the transformer. You would get a very loud amp for a short period of time. Something would blow. The tubes probably would go first, but you could also blow the output transformer or the speakers. It is possible that a small power transformer would not take the high amperage and blow up.

Too much impedance, for instance an 8 ohm speaker cabinet hooked up to a 2 ohm Bassman head can be damaging. Mostly you get crappy sound, but there is less danger to the amp. The tubes will operate at higher voltage because the load is not letting the voltage bleed off into output and you might get shorting in the tubes or the output transformer. This is another way to fry an output transformer and it is why you shouldn’t run your amp without a speaker load".


This came from here: http://www.harpamps.com/transformers.html

A dead short is by definition too low an impedeance, in fact it's 0 ohms. It stands to reason that in a dead short in the secondary, the current x voltage (=watts) have nowhere to dissipate except as heat and therefore causing self destruction of the secondary. This is why Fender cautions about turning the amp without a speaker plugged in. If there is an open circuit, essentially an infinite impedance, then the primary does not dissipate its energy which can take its toll on the output tubes as well. All the above points to the fact that a suitable resistor is a better substitute to either an open circuit or a dead short.

As to why Fender use (and I know they do) a shorting jack is a mystery to me unless the feedback loop plays a greater role than I ever thought it could in dissipating the bottled up energy.
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Old June 10th, 2009, 06:43 PM   #17 (permalink)
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I think Fender went with the shorting thing to be cheap!
I was always assuming that if you really stepped on a fender with no speaker connected and one had the proper fuse in, the fuse would go before anything else smoked- with luck.
If it were open, the fuse would never blow, or not until the insulation broke down somewhere in a winding.. Maybe the feedback would save the day then in an open load situation?
Some modern things seem to have an r-c load if left open, the little crate v5 for example.. something like a two watt ten ohm in series with a .1uf cap or similar.. I believe that is an artifact of solid state training though.
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Old June 10th, 2009, 09:03 PM   #18 (permalink)
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Hmm...great discussion.

TO expand on my source from earlier:

If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, and acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer and short the windings. I would not go above double the rated load on any tap. And NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways.

It's almost never low impedance that kills an OT, it's too high an impedance.

The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all.

If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.

There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike. This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punch through was one time, it wouldn't be a problem, but the burning residues inside the transformer make punch through easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer."


I'm far from an EE, but I've read a lot. Still, I'm always open to new information.
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Old June 10th, 2009, 10:23 PM   #19 (permalink)
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My understanding (very very limited) is that open load on an OT is very bad very fast. This is very similar to the flyback effect in tv's
No load (dead short) is also very bad but you can get away with it longer. Shorting jacks are the lesser of the two evils.
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Old June 10th, 2009, 11:01 PM   #20 (permalink)
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On a lighter note some updated shots of the wiring:












:D
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