low input gain on UA 176

[jhaible]

[quote="soundguyJH- Im not sure how the circuit is wired, but those results always worked for me. It would be nice to figure a passive way out of that thing to just use a real amp on it in the first place.

dave[/quote]

Haven't looked it up yet. I should know, as I have re-built the whole preamp when I have bought it (PCB was mechanically broken). My Wurlie is a 200A, BTW.

What I said about "passive" is just what someone told me. Never tried it.
IMO, the best way to amplify a Wurlitzer would be an internal tube amplifier. And the first Wurlitzers were just built that way, but I don't have these schemos.

Apart from that, IMO the best way to amplify a Wurlitzer would be to get rid of the electrostatic pickup at all. (;->) The idea to just apply a high polarising voltage to the mechanical construction, and using that as a capacitive pickup, is certainly brilliant at fist glance, but in practice its tendency to attrackt dust, which in turn causes terrible noise, makes it a rather bad solution.

But on a day with relatively low noise, it makes the most wonderful sounds, and it has this excellent keyboard action.

If I could get rid of that permanent noise problem (SNR, if measured in peak voltage, is actually *negative* on my Wurlitzer - the noise peaks are louder than the signal on a bad day ...), I would build a tube preamp for that instrument immediately.

JH.

 

[soundguy]

brad has a schematic on his site which is worth a look-

http://www.avensonaudio.com/tech/Wurlitzer/schematic%20rev-A.jpg


dave

 

[jhaible]

[quote author="soundguy"]brad has a schematic on his site which is worth a look-

http://www.avensonaudio.com/tech/Wurlitzer/schematic%20rev-A.jpg


dave[/quote]


Interesting.
Is this the original circuit?
Looks more like a modern clone ...
I would be very surprised if they had LM317-based DC heating in these old Wurlies.

JH.

 

[jhaible]

On the Wurlitzer sub-topic:

Looked thru my schematics at home.

The 200A has its level pot after the preamp - so reducing the volume will not change the signal level in the preamp at all.

Tremolo is performed with a LDR in the feedback path of the preamp. Tremolo *will* change the signal level in the preamp. (On my 200A, the preamp clips with high tremolo settings. I guess I have to reduce the gain, or the pickup DC voltage. Of really build a tube preamp.)

The 140 (tube model) has a very different architecture. Volume is controlled by a shunt potentiometer *before* the grid of the first tube.
Also, the low level output is derived from that point with a 1Meg resistor. So apart from the internally generated pickup bias voltage, using this output means running the Wurlie passive. And very high impedance.

In that tube model, Tremolo is done with a vary mu methiod in the power amp. (No miking the speakers, no tremolo.)

Tubes are 9k11 triple triode for preamp, phase splitter and tremolo LFO. Plus 2 power tubes in pp.

I guess there are other models that even have other circuit architectures.
(For instance, I know there were at least 4 different ways to make the tremolo VCA: Vari Mu, Diode, Vactrol, and Varistor pair.)

Maybe I should really build a tube amp into my Wurlie.

Maybe get rid of that transistor preamp (it doesn't have a power amp anymore) and just build a single tube stage + transformer as a kind of internal DI box. Or clone that 140 circuit, complete with vari mu tremolo.

No idea if keeping the original 9k11 is a good idea or not. I've started another thread about that.

JH.

 

[SSLtech]

[quote author="blackbox"]ssl tech,
thanks but i'm not following you very well. ignorance on my part im sure.....
so.... the 176 is fine? I'm coming out of the preamp (1073, api, whatever) via the patchbay and going into the 176. I have to crank the Input a lot. Seems like more than the 6db of difference that Bjorn mentioned. If it is just a matter of 6 db, is this really acceptable. If the Elan is unbalanced everywhere, then is all of the rack gear dealing with a gain structure nightmare? I only use the board for monitoring while tracking and avoid the inserts while mixing by patching gear in after the Tape Machine Outputs and then into the Line Inputs. Does this theoretically avoid the problem?[/quote]
It depends. It depends how you wired it and it depends on what the output of each device is. Servo driven outputs behave differently to 'dumb' electronically balanced outputs, which behave differently to transformer outputs.

"Losing 6dB" is a vast over-simplification ... I direct the Jury to disregard that suggestion and ask that it be stricken from the records.

The fact is that if you have a transforemr output, you *MUST* make sure that both pins 2 and 3 are connected to the unbalanced device. Otherwise you'll lose a LOT more than 6dB, and the sound will be thinner than Tony Iommi's top string! So many people think that you connect an unbalanced signal to hot and ground on a balanced piece of gear, that this crops up a lot. -Connect to hot and cold. Ignore ground more often than not, and only go back and connect it if you have a buzz. (*NOTE* the difference between a buzz and a hum... a hum is most often too many grounds, a buzz is usually too few grounds.)

If that don't get it, it's something else... (Hmmmm.. that sounded so much 'wiser' in hy head! :wink: !!!)

Keith

 

[gyraf]

[quote author="SSLtech"]NOTE* the difference between a buzz and a hum... a hum is most often too many grounds, a buzz is usually too few grounds.)[/quote]

That's a good one!

 

[Guest]

On the Wurlitzer Subtopic, Can you post a schematic of the 140 with vari-mu tremolo?

 

[jhaible]

[quote author="thomasholley"]On the Wurlitzer Subtopic, Can you post a schematic of the 140 with vari-mu tremolo?[/quote]

I only have it in paper form, but I can scan it and put it on the web.
Not tonight, though. (I'm looking forward to a nice evening with MAM's Stefan Schmidt and Lintronics' Rudi Linhard in my little studio tonight. ).

The tremolo is just adding a common mode LFO signal to the differential mode audio signal on th epower tubes' grids. I guess this counts as variable Mu, but it's quite common in guitar amps AFAIK.

JH.

 

[CJ]

blackbox, PM jh for some 175/6 info.
cj

 

[jhaible]

[quote author="thomasholley"]On the Wurlitzer Subtopic, Can you post a schematic of the 140 with vari-mu tremolo?[/quote]

http://www.oldcrows.net/~jhaible/wurlie/jh_wurlitzer_145.pdf

The model number is 145, not 140 - my mistake.

The scan isn't beautiful, but it should be readable.

JH.

 

[Guest]

Thanks a lot for the schematic.