Author Topic: Wurlitzer 200 hum and power consumption  (Read 276 times)

Offline gacki

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Wurlitzer 200 hum and power consumption
« on: May 12, 2021, 04:33:43 AM »
Hello everyone; I'm currently working on a 200 that had some amp problems.

It uses a mid-production version of the amp. Originally the amp had a number of problems: when switching it on R10 was emitting "magic smoke" and there was huge ripple on the 44V rail (around 10V). TR1 was swapped for a BC168H (which was highly sensitive to any movement and vibration). R8 was open. There were some modifications (including an additional cap that was practically leaning against the transformer leads) and the aux out wasn't working. TR9 and TR10 seemed to be shot.
After swapping out TR9 and TR10 (and TR8) the power amp started to work and R10 wasn't smoking any longer (I swapped it anyway since it probably was stressed). I recapped the whole thing, installed new power resistors, removed most of the later mods and installed the resistors for the final revision of the aux out. TR1 first got moved to one of the other positions but meanwhile got swapped for an NOS part.

In general it seems to work rather well now but I'm still unhappy with hum and heat generation. From what I've read I know that there are limits to what can be done with this type of amplifier. I would like to know if I'm close to those limits.

The preamp seems to work rather well; it has some slight hiss and minimal hum (the unit has the reed bar shields). When I'm using the aux out through a DI box it's sounding quite good.
The power amp on the other hand still hums slightly.
I'm concerned about the ripple on the 44V rail. This is around 1.8V p-p before R73; less behind it. That ripple is also still slightly present at the base of TR8. In general TR8 (the driver) seems to cause or worsen this: without TR8 the ripple is around 300mV. Power through TR8 is around 125mA. Simultaneously the power resistors get VERY hot; I think R7 already shows some discoloration. This is happening with both the new TR8 and the old one. I'm actually somewhat surprised that the hum isn't dramatically worse given the (comparatively) high ripple.

(There also is the issue with the transformer leads; however I don't think the ripple problem is coming from that. Even worse, there are four transformer leads running next to the amp because its a 220V model.)

So is this as good as it gets? Or can this still be improved?

Offline theseacowexists

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Re: Wurlitzer 200 hum and power consumption
« Reply #1 on: May 25, 2021, 08:13:05 AM »
I've worked on a few 200 series amps now. At first I enjoyed the challenge of bringing them back to life. But the more of them I worked on, the more it seemed that the effort put into them outweighed the result. You can rebuild the entire amp and it will still be hissy and hummy. The next time someone brings me a 200 I'm going to strongly suggest just getting the RetroLinear amp. One client specifically requested that amp, and oh boy is it magical. I'm definitely in the camp of replacing the original 200 amps now.  The tone is spot on and the noise floor is practically nonexistent. Worth every cent if those things are important to you.

Yes, you are right about those power resistors. They get VERY hot.

Offline gacki

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Re: Wurlitzer 200 hum and power consumption
« Reply #2 on: May 30, 2021, 01:39:47 AM »
Thank you for your answer.

I'll probably leave the amp for now as it is. The owner's main Wurlitzer is a 200A and this 200 is mostly a backup at the moment. I am mostly concerned about the supply ripple since that seems to indicate that the power amp is either drawing too much current or that the power supply can't handle the regular load.