I have two complete amps, of this model. And, one completely trashed. I don't mind trashed so much, if the amp chassis/faceplate is intact. This one was.
I used the trashed one as my test bed for experimenting, as well as troubleshooting. My first opinion of this amp was that it was kinda cheezy in appearance. You know, the way 60's amps were trying to make the transition into a more modern look, as compared to the 50's and before. But, looking past the cheezy, I could see that there was a lot of thought put into the build.
So, I approached it as a good amp that could be made better. I added an FX loop. Plus, because it was a fixed Bias amp, I modified the Bias Supply to have a Level Pot. That way, I could dial in the correct Bias current for measured plate voltage. I've done this on a number of old amps, of various origins.
These mods were a success, but one mod that was the best of all was the grounded power cord. Not for just safety reasons alone. If an old amp has the original ungrounded cord, I connect an external earth ground to the amp chassis, for safety, while troubleshooting. In the case of this amp, the chassis ground eliminated virtually all hum. I already had it down to a subtle hum. This took that away. A must on this model amp.
Another mod was to install a thermistor in series with the 'hot' conductor of the power cord, and the fuse. My line voltage typically runs 123vAC. These old amps are buillt around a supply of 117vAC. With the thermistor installed, I measure 118v-119vAC after it has heated up. But, the primary use of the themistor is to limit the inrush current.
I monitor plate voltage and Bias current, when powering up an amp. The plate voltage on these amps can approach 600vDC, dropping only when the tubes begin conducting. The de-coupling caps in the power supply are only rated for 500vDC. After the tubes begin conducting, the plate voltage drops to somewhere between 470vDC and 490vDC.
With the thermistor in place, the plate voltage never went above 485vDC, and dropped to 470vDC, when the Bias current began increasing. So now, I'm running the rated line voltage to the primary side of the power transformer. And, I'm preventing the voltages from going excessively high, while everything is warming up. In other words, I've added a soft-start feature to the amp, which should extend it's life greatly.
I made a schematic of this amp, only because I wanted to document my FX loop and my Bias Level pot:
http://www.mediafire.com/view/dlvv63...ss_-_rev_D.pdf
And, before this essay gets way too long, let me say one more thing about this amp. It has a really wonderful tonal range. Even though it is a Bass amp, the Tone controls are capable of making it a very bright Guitar amp, really nice anywhere in between, and deep Bass on the other end.
So, if you don't mind a bit of cheezy 60's look, this amp is a great one to have. Not super loud, even though it has 6CA7/EL34 power tubes. The pre-amp is a bit tame. Probably did that to keep it clean. Really, really good sounding amp, any way you want to use it.
I will say that for you tech-types who do your own troubleshooting and repair, the chassis is a royal pain to remove and re-install. I'm good at it now, after doing three of them in a row.
Jack
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