Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Wurli Dad on January 22, 2019, 03:21:05 pm
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My son has a Wurlitzer Model 145 Electric Piano with a tube-type amplifier. The piano played fine, but had a persistent loud buzzing sound coming through the amplifier. In 2017 I helped him troubleshoot via FaceTime as I am in Canada and he is in NYC. We eventually took the following steps to reduce the buzz to somewhat bearable levels, based on advice from a number of online forums:
1.) Removed the additional ground wire that someone had added after market (IEC). This YouTube video from 2017 demonstrates the very negative effect of the additional ground wire.
2.) Checked all grounds throughout the piano, and the bonding wire to the piano cover in particular.
3.) Removed the so-called “death cap”.
So, we got the Wurli working but still had some annoying buzz. But now the tremolo has quit working. Our first troubleshooting step (again via Facetime) was to clean the two pots and check continuity to all wiring between the volume and tremolo pots and the connector on the amp. Everything metered out fine. A posting on this forum (from DummyLoad in July 2010) recommended removing the line voltage wiring from the switch on the back of volume pot as that had solved a buzz issue he had with a Model 145. This did not fix the issue for us.
I did once have an electronics background in the Canadian Air Force, but my study of vacuum tube theory is dated back to 1973! Does anyone have any thoughts about what might be going on here? A copy of the Model 145 schematic is attached for info.
And in terms of the tremolo problem, I’m not sure what the next step might be except maybe to get the 6K11 tube tested.
Any help from the experts on this forum would be greatly appreciated!
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What are the voltages on pins 5, 6, and 7? That tremolo circuit requires a strong tube. Try a new one.
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Measured with a digital multimeter, tremolo and volume pots in mid range, 6K11:
Pin 5, 116.5 VDC
Pin 6, 1.012 VDC
Pin 7, 20.9 mVDC
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Those voltages are a bit low. What are the voltages on all four terminals of the cap can?
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Clockwise from the bottom:
274 VDC, 305 VDC, 266 VDC, 330 VDC
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Those are a bit low too, but not enough to hurt for now. All those filter caps will need to eventually be replaced though.
You're really gonna need a known good 6K11 to substitute. Meanwhile, some things to do...
Measure the resistance of R21 and R22. You can do this in circuit. Replace C10. Any trem yet?
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Sorry for the delay - the piano had to temporarily go back together for use at some rehearsals (without trem!).
Ordered a new/tested good 6K11. Installed it and still no tremolo.
R21 (470 K) measured 498 K in circuit.
R22 (2.7 K) measured 2.75 K in circuit.
Have ordered a 10 mfd electrolytic capacitor to replace C10. Will advise once received and installed.
Will post some pictures of the cap can.
Cheers,
Wurli Dad
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Cap Can looks like this. Markings as follows:
Y-40M
X-20M
U-20M
O-20M
10%+30%
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Sorry for the long time since there has been an update on this Wurli problem. During the course of troubleshooting another unrelated problem emerged that had to be addressed (turned out to be a broken ground wire in an unrelated area) to get the piano back up and running . Also my son is a busy working musician in NYC so hooking up times to work on this can be a challenge. We are back at it now...
C10 has now been replaced. Still no tremolo, including with the replacement 6K11. We remeasured the voltages on 6K11 as follows: pin 5 115.2 V, Pin 6 1.081 V, Pin 7 11.6 mV.
Ideas for next steps?
Wurli Dad