Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BlackCrowe604 on December 29, 2019, 08:52:31 pm
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Hello All, I hope everyone had a great holiday, and had some extra time to spend at the workbench!
I've got a 70s SF Princeton Reverb from a friend who asked if I could take a look at the health of it. Just had a couple of questions before I jump in.
Now this one is a bit of a time capsule. Everything is original, even the original Fender tubes. It has a factory-installed 3-conductor power cord, and is the Canadian version of the amplifier.
Because of the CDN-config, it has a thermal red-wire fuse in the chassis, and a rubber stamp on the top of chassis reading "replace circuit breaker with same type 010872". I am guessing this is the wire code for this type of fusing? I have not seen this before, but seems to come from a unique Canadian electrical code requirement?
I have seen some additional fuse-boards added to 70s Marshalls for the Canadian market as well.
As this amp does not have a Standby-switch, I am going to install a GZ34. Since this will increase voltage, I am going to upgrade the B+ drop resistors wattages. Will this additional voltage have any affect to the additional wire-type fusing?
I would like to use a GZ34 to tighten it up, and to reduce wear on the power tubes.
I have play-tested the amp and it just sounds noisy and a little tired.
I plan to replace all of the electrolytic caps, test the coupling caps for leakage and re-tube it, but I noticed this amp does not have screen, or grid-stop resistors on the powertube sockets, nor the phase inverter tube.
Would adding these tame the noise a bit?
I can also hear some mechanical noise from the tubes as well, so will add retainers.
Does anyone else have experience tuning one of these up? Any additional info would be appreciated, especially with the thermal fuse -> (it doesn't really seem like it's necessary?.... can add some photos later).
Thanks for reading! Cheers!
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> Everything is original, even the original Fender tubes.
Then Fender did it right. Don't second-guess choices which worked fine for 40+ years.
Carefully replace the power and cathode e-caps, that may be your "tired". 100k plate resistors may be tired (carbon-comp is crap). Tubes can be noisy.
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Got any Pics?
silverfox.