Wurlitzer 720 back panel?

[velo-hobo]

Greetings all, for a long time I've been thinking about fabricating a back panel for my 720 (non-A/B - it has a 145/720 tube amp and the nameplate says only "720")

However I have never been able to find any image of any model 720 variant with a back panel in place.

Of course it's an assumption that it was even provided with a rear panel to begin with, but there are screw holes in places that make me suspect it was the case. As well, in my ancient digital copy of a xerox of the 700 service manual, there are a couple of grainy B&W photos which show there is indeed a back panel on that one. Those can be found on pages 700-4 and 700-5, where the procedure for removing the top cover is explained.

In all the digital copies of the 140/720 service manual I have seen, there is no mention of this component nor any photos.

My interest in this is both for the sake of keeping dust out of the action and reedbar area, possible enhancement of EMI shielding, as well as the possible changes in frequency response that come from having a more fully-enclosed speaker cabinet. My 720 has a 12" speaker, FWIW. Of course, port size and geometry can have an effect, and I'm not sure whether or not Wurlitzer would have concerned themselves with this or not.

I am guessing any full-size rear panel has to have some cutouts for hand-holds underneath each end of the dividing panel between the speaker/amp area and the action/reedbar area. As well as a cutaway for access to the amplifier. These together would provide some airflow for cooling of the amplifier, and I'm wondering if there were other features such as additional ventilation or access cutouts.

I could design something from scratch using these common sense notions but I am interested in seeing what the original design actually looked like. I also assume it was probably made from masonite paneling, 3-6mm thick, in keeping with the typical conventions of the era for back panels of audio electronics cabinetry, but being able to confirm any of this would be great.

[pianotuner steveo]

Yes, they had a panel, both the models 700 and the 720,made from Masonite. Yes, there were oval cutouts just below the back of the keybed to put hands in (really just your fingers) for moving. I do not know what happened to my 700 back panel, so I can't take a picture to show you, sorry.

[velo-hobo]

Thanks for the input!

I'll add that if I'm able to get enough intel to reproduce the original design, I'll gladly produce and share out a technical drawing and CAD assets for others to utilize.

[velo-hobo]

Had some time this morning to hunt around for images and found a few in places I hadn't seen before. None of these are of a 720 (or 720A) with the tube amp, and I'm not sure whether or not Wurlitzer would have designed the panel any differently for the solid state variants.

The 726B that is pictured was shown on the Tropical Fish Vintage site and later sold on Reverb which included a lot more photos, which is where I ultimately found that image. Note the extra cutouts for the student model multi-pin plugs:

https://reverb.com/item/5692439-wurlitzer-720-electronic-piano-with-vibrato-in-walnut

I also found a 720B listing on Reverb that shows the back panel in full:

https://reverb.com/item/12335086-wurlitzer-720b-vintage-electric-piano

Also found back panel images from a couple examples of the model 700, which seems to indicate Wurlitzer may not have added any extra ventilation for the tube amp variant, but I can't say with certainty:

https://reverb.com/item/80893036-wurlitzer-700-64-key-electric-piano
https://reverb.com/item/84021807-excellent-wurlitzer-700-serviced-and-tuned-includes-unobtanium-bench-and-music-stand-extremely-rare (image is actually from a CL ad for the same item)

This is probably enough for me to go on, though if anyone has an actual 720 or 720A with tube amp and the original back panel, it would be nice to confirm whether there was any design variation such as additional ventilation etc.

[pianotuner steveo]

You reminded me why I originally removed my back panel. My 700 overheated and died when a friend of mine accidentally left it on. The next morning, it was no longer working. I was a teen at the time, so I wasn't able to repair it myself even though I took electronics classes in high school. I personally recommend drilling ventilation holes near the amp for tube models.

[velo-hobo]

I've read a couple of Doc's posts here about the 145 tube amp being susceptible to transformer failure, especially (according to them) if left powered on for long periods of time:

https://ep-forum.com/smf/index.php?topic=10145.msg56515#msg56515
https://ep-forum.com/smf/index.php?topic=9541.msg52622#msg52622

My 145/720 amplifier has a replacement output transformer but the power tx appears original. I suspect it's just as likely the output was blown due to operating the amp without a speaker load as it is to have failed due to overheating under ordinary use. The transformer mortality rate is a bit of a tangential topic but worth citing when thinking about this. From your story it strikes me as possible and likely that a number of back panels were removed (and then lost) due to a perceived need for more ventilation. It is also likely that many back panels were removed and lost for less intentional reasons.

In pursuing this idea I'd apply the same principles I use when designing equipment enclosures at my day job. Since the amp is in the bottom of the cabinet and there are already vents/hand-holds at the top, passive convection is the existing and preferred solution here (else it needs fans, etc). I use a formula that relates open area of the ventilation affordances to the amount of power that needs dissipating, in relation to ambient temps and a target operating max temperature. I am considering the surface area of the wood enclosure to be a non-factor in terms of radiant cooling.

So it may well be a good idea to increase the ventilation affordances, and I'd have to do some measurements and calculations to assess the design needs.

FWIW, adding more/enlarging the outlet vents doesn't matter unless the inlet open area is correspondingly large enough to avoid restricting the convective air flow. As an analogy, if you're inhaling air through a straw it doesn't matter if you exhale it normally, you're going to notice something feels a bit unusual...

[velo-hobo]

I was thinking about this some more and it makes sense there are reports 145 amps can fail in the portable models if left on for extended periods of time - there's barely any ventilation affordance in that cabinet! There's the one array of slots on the bottom face (below the speaker, on the other side of the cabinet brace/bulkhead from the amp) and then the cutouts for the power cable, fuse holder, extension speaker and phono jacks.

There's no upper vent to encourage passive convective cooling, and I doubt the painted steel lid is efficient enough at radiant cooling to do much.

In the 720, at least the hand-hold cutouts are above the amp, encouraging convection flow. I'm thinking to test the potential difference in sonics, I could just shove a piece of plywood up against the back face of the console. Anyhow, that didn't stop me from whipping up a quick model, screenshots attached.

I can lasercut this out of butcher paper or cardboard to check the fit before bothering to cut some masonite (even though that stuff is cheap, we don't generally keep a lot of it around the shop space that I have access to)

If it fits well enough, I'll be happy to post a drawing and a dxf so folks can more easily make their own if they like. This layout hews close to the original examples I've seen photos of, in that the amplifier is mostly covered up except for the rear panel connections. I thought about making the cutout taller so more air could flow over the tubes etc but then I'd also want to enlarge the upper slots to match the area of the lower opening.

[pianotuner steveo]

Yes, that looks a lot like my old back panel, but I would personally drill some ventilation holes near the amp.

[velo-hobo]

Adding bigger/more holes near the amp is probably not optimal with this design. The warm air rising off the amplifier then exits the enclosure through the two upper slots due to convection. This establishes an airflow current which draws fresh air in through the bottom vent by the amplifier. It's a passive ventilation scheme that behaves somewhat like a fan blowing air over the amp.

If the lower hole is enlarged without a corresponding increase in the open area of the upper vents, then it probably won't make much of a difference in the amount of cooling, and could potentially reduce the cooling factor if for example it creates turbulence that disturbs the convective flow.

Think about how a wood stove/fireplace works - you need enough chimney height so that it "draws" else if it's too short then you might end up with a room full of smoke/hot gases.

[velo-hobo]

No updates on the panel design/tests yet but I did want to correct what I wrote at the top of this thread. It turns out I do have a 720A - I was mistaken because the nameplate has "720" printed upon it in the typical fashion, and then the letter "A" stamped into the metal. I didn't notice the "A" before crawling around down there with a flashlight at some point recently.

For reference, the serial is 40690, so that seems to put it on the earlier end of "Series 1" 720A production, according to Doc's model rundown. That would jive with a possibility that they were using up old "720" nameplates and just stamping the "A" into them, unless that's how all those were done across the entire period of production prior to the debut of the 720B.

I can look at the transformer and speaker date codes at some point to add some data. Sorry my thread is drifting slightly, topic-wise, but I figure it's an OK place to add some model data.

[pianotuner steveo]

If more ventilation holes are added near the bottom, wouldn't that help with the air flow? I don't mean just randomly adding holes in the back, but towards the amp on the bottom.