wurlitzer electronic piano amplifier

[alchemy]

I just picked up an old Wurlitzer Model 700 (actually says 600 on it, but it's the same as a 700). Nice little 1x 12AX7, 1x 5Y3, 2x 6V6 amp inside. I got it to tear apart and turn into a guitar amp (complete with the P12r Jensen) but upon getting it home my daughter convinced me to give it to her. So I figured if it's gonna take up space, I might as well make it more versatile. I rewired the piano signal input on the amp and the plug to 1/4" jacks so that I could run the signal through my pedal board. BUT, it doesn't want to work.

My initial thought was that the signal was just too small to travel through all that circuitry, but then I can't imagine it being any less than a guitar signal. The piano sounds fine through the amp on it's own, and through just a big muff it works as well. But through the whole pedal board, as well as an MXR delay and a homemade Big Muff (seperately) there is no signal getting through. And it's not like it's a quite signal, there's nothing.

I'm a bit confused. Could it be that the signal from the piano pickup is simply to small, and that it will work to a certain point of impedance and then just cut off completely? Is it something much simpler that I am overlooking? Just run it through a preamp maybe, but shouldn't a Muff boost the signal? Any suggestions on how to proceed?

Thanks for any help....

Here's a link to the service manual, which has a schematic on page 26 I believe...

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[Chuck H]

Try running your pedal board (exactly as it is now) with your guitar and another amp. Doesn't matter if it worked fine the last time you used it, do it now to test full continuity through the board.

Chuck

[alchemy]

yeah thanks, did that.

[52 Bill]

Are you saying that you just changed the input jack to a 1/4" and then are trying to take the signal directly from the harp/reeds to the pedal board?

If this is what you did, you have killed the pickup output by disconnecting the power supply from the harp. The pickup is charged to 300-400 volts and is capacitor coupled to the input of the amp. If you try and hook the harp up without the charging voltage there will be no output.

Be very careful working around the harp/reed assembly when it's turned on.

[alchemy]

Ahah! Somehow I overlooked the B+ coming through the 22meg resistor from the 8mf cap. I guess I was thinking that since it worked with the one pedal, it shouldn't make a difference, but apparently I was wrong (not uncommon!). I have never worked on one of these. I am fully aware of safety precautions however.

So it looks like my only options are to get the B+ further up the towards the pickup so that it is charged before the pedals/jack, or to move the jack to after the two .001 coupling caps and 470k resistors. Any other ideas?

Why did it work with the big muff is whats really confusing?

[Chuck H]

Ahah! Somehow I overlooked the B+ coming through the 22meg resistor from the 8mf cap.

Where the heck are you inserting signal??? It should be at a tube grid. And there should be no B+ at a tube grid.

Why did it work with the big muff is whats really confusing?

Perhaps the Big Muff has a capacitor at the input. This would block the DCV from that circuit.

Not sure how you "move the B+ further up towards the pickup". The B+ is right where it needs to be if the amp is amplifying... Hmmm.?.

Chuck

[alchemy]

Take a look at the schematic I attached (page 26 I think, of the pdf). There is a node coming off the B+ that goes to the piano pickup. According to the last poster, the pickup needs to be energized (i am new to EP pickups,and did not know this!), and this makes sense judging by the schematic. There are coupling caps in between this and the grid, so no B+ on the grid. I swapped out the jack before the coupling caps, thus running the B+ through the pedal board and not to the pick up. So, no sound. It seems i can either take a node off the B+ and run it up into the piano and hook it to the pickup, or reposition the jack to after the coupling caps (on the grid of V1). Make sense?

[52 Bill]

I would try adding essentially an FX loop between the caps at the input of the amp. The problem that I see here is that the pickup and input to the tube are both fairly high impedances, so the FX that you insert will need to deal with this.

The pickup does need to be charged in order to work. Think of it like phantom powering a mike.

[J M Fahey]

1) No disrespect intended, but I find it a crime to destroy this beautiful piano just to cannibalize its power amp.
If I had found it in a damp basement, eaten by thermites or some mess like that, well, it would be different.
Your Daughter deserves full credit for her action, good for her !!!.
(*My* Daughter would have done the same )
2) I don't see where you can put an effects loop there, because it has no preamp, the electrostatic high voltage high impedance goes straight to the Phase Inverter and the Volume control is actually a PPIMV.
The signal is probably in the tens (not tenths) of Volts, too high for any pedal effect.
3) If you want to have your cake and eat it at the same time, I may suggest a new FET input SS amplifier, *with* effects loop to replace the original one, and then you can pull that amp chassis without guilt feelings.
I think your Daughter will love the new 60W amp and matching 10" (or even 12") speaker Daddy is adding to her piano.
Or you might just build a preamp, and let her plug into Daddy's amp, PA system, whatever !!!

[Rhodesplyr]

As a vintage keyboard collector and tech, I'm with JM & your daughter. I wish I'd bought a Wurlie back around 1990 when people were giving them away for $150. These days, you're lucky to find one in any kind of decent playing condition for less than $600.

I've rebuilt the electronics of the 140 types, and for one a tube amp of this era, I would recommend a thorough rebuild since these, according to a quick web search, were last made in 1962.

Wurlies do use a capacitive pickup that's charged to 180-200V, but, as you saw, it's charged through such a high value resistor (22M) that it can't deliver much current at all: .009mA via a direct short to ground. In fact, you can't directly measure the pickup voltage with a multimeter. The meter's input impedance will load it down, giving you a false reading.

[alchemy]

Thanks for the replies. I came to the same conclusion that adding an effects loop would probably not work, at least with out another gain stage or preamp. So I guess for now it will stay stock.

I did go through the amp. New caps, power resistors, power tubes (robbed the rca 6v6's for my tweed deluxe clone), and got all the voltages back to spec. It sounds great now, though the volume seems like it could be louder for 2 6l6's. What else is involved in a complete rebuild as far as the rest of the piano?

I am really happy with this thing, especially for $40 from the Habitat for Humanity thrift shop...

Any other suggestions for making effects possible would be most welcome. I like the SS idea, but would prefer to keep the tube amp in it....

[kurtdaniel]

I have an opportunity to get a Wurlitzer 700 for $100. you can always use an external amp with vibrato, but you can not bypass the internal amp..