A big thank you, a couple of pics and a circuit question or two.

C

chicagozer

Active Member
I wanted to say thank you to everyone who's been posting technical info on this forum. I mostly lurk while I'm trying to learn as much as I can, but the information here has been invaluable.

Due in no small part to help I've gotten directly and indirectly here, I've managed to bring this Radio Craftsmen 500 amp back from the dead.



Aside from two dead chokes and a dead power transformer, this is what I started with underneath.



The resistors didn't test anywhere near spec and not trusting old caps, I decided to go ahead and haul out the passive components. Knowing my limited soldering skills, I was cringing at the point to point wiring so I decided to use a turret board instead.

I was hoping that it won't introduce any newfound interference. Worst case, I could just start again the old way.

A couple dozen hours later, everything wired up, checked and rechecked and ready for launch.



Started up on the variac; no smoke, go to 100% and it (surprisingly) started playing on the first try. Very nice feeling indeed.

The sound was very nice; just the faintest of hiss out of the speaker with no input. Couple days of goofing with it. No observable change and figure for a 60 year old amp I can live with a hiss I can't hear from a foot a away.

Anyway, this morning I was noodling with it again, and discovered I had not wired the negative feedback correctly and shunted it out. So got that resolved, and whoa that fixed the hiss. Dead silent.

So on to my questions. The circuit uses a LCLC circuit in the rectifier stage. Big choke on this guy, but because there is no leading capacitor the DC voltage doesn't get the normal bump. The 550-0-550 power transformer was cooked and the biggest replacement I could find was 500-0-500.



So my tube voltages are a bit underspeced. I was goofing around with the Duncan PSU II software and it appears I can add a small capacitor in front of the first choke to get the necessary bump in the voltage. I tried a .560uf cap but I think it may have been too much because the balance pot started smoking :(

Thankfully, it didn't seem to cook anything else once I removed the leading cap. Should I try a smaller cap here? I was wondering if it's safe to test the B+ voltages with only the rectifier tube installed? I tested with the power tubes out but wanted to know if it was save to run the rectifier with the 6SN7s removed until I can confirm the voltages.

Also, the negative feedback 5.1k/680pf really did the trick on the limited amount of hiss I was hearing. But it cut the gain noticeably. Is there a way to "lessen" the impact? Or should I not bother about this and just inch up the preamp volume?

If you got this far, thanks for reading :)

Jim
 
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No you don't want to test voltages without ALL the tubes installed. (it'll be to high)
And yes you were on the right track with the small cap to raise voltage as well here.

BUT, if you were pretty close to start with I wouldn't worry about it, also I would go
with 5AR4 rectifier instead of the 5U4, as it's a slow start rectifier AND has less drop.
 
Gotta agree on the 5AR4. That should get you another 30 volts or so, and draw an amp less current from the power transformer. At the end of the day, it'll likely be close enough to not worry about.
 
I agree with Tom, per my Sam's it spec's 5V4G... I also agree with Kegger because he has suggested the same with mine and I believe supplied me with the tubes..

Unfortunately I lost a power tx from one amp, the mps are well known for this. They are side lined until I procure till I procure some, Gonna do both amps at same time. A sub is readily available from Triode, money not the issue but time is for me right now
 
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Very nice job, Jim! Regarding the negative feedback, the reduction in gain is exactly what is supposed to happen. As much as the gain is reduced, so is noise, and so is the distortion. And, the damping factor is driven higher as well -- all good traits of a quality power amplifier.

This unit is nearly a direct knock off of the original Williamson amplifier, which is considered to be the first truly high fidelity audio amplifier design. It employes 20 db of negative feedback (providing a 10X reduction), so the gain should have decreased substantially when the loop was closed. As a result, it was always intended that this unit be operated with an external preamp unit to develop normal input sensitivities for various input sources.

Be advised however that the cathode resistor for the input AF amplifier stage is not a 470K resistor as shown, but a 470 ohm resistor. As that resistor is made higher in value from the specified value, gain will drop even further than that which 20 db NFB creates. I mention this to make sure your unit is not trying to operate with greater than the original NFB level.

Good luck with your amp!

Dave
 
dcgillespie said:
...Regarding the negative feedback, the reduction in gain is exactly what is supposed to happen...It employes 20 db of negative feedback (providing a 10X reduction), so the gain should have decreased substantially when the loop was closed.

Be advised however that the cathode resistor for the input AF amplifier stage is not a 470K resistor as shown, but a 470 ohm resistor....
Good luck with your amp!

Dave
Click to expand...

Thanks Dave! I was wondering how much negative feedback was going through. I need to study the maths some more to learn how to do this calculation.

Thanks for confirming the cathode resistor. Thankfully the parts list in SAMS was correct, just not the schematic.

Jim
 
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