I had no idea this 20.5 foot cable was so controversial.
Indeed. It was with no small amount of trepidation that I raised it, HH75
Pete,
The first text in the bibliography “SSB Principles & Circuits” 1964 – Pappenfus, Bruene & Schoenike" is probably the most comprehensive description of the design principals of the 30S1 and 30L1. What I have not found in any written material including the authors of the Signal and other Collins documents are the design intimacies of the 30L1. The above text describes the ALC "Automatic Load Control" and has an unfinished schematic of the negative feedback/stabilizing system in the 30L1. Also missing is the design review paper for the 30L1. All three of the authors of the above text were in that review.
Regards Jim
Jim,
I know of no in-depth exposition of the 30L-1 design, apart from the material you reference and the much later CCA articles. My own suspicion is that the design was never, a priori, subject to the same high-level engineering expertise or scrutiny as was, e.g., the 30S-1. I'm very attached to all my Collins gear and use it regularly but an objective radio engineer would find it hard to claim that the 30L-1 was the best-engineered box the company ever produced. Notwithstanding, it was a pace setter in many ways and mine gets a near-daily workout, driven by a variety of old and new exciters across 80-10m. Whatever reservations one may have about the design, it's hard to be too critical of an amplifier that still looks and works great after 50 years of regular service.
When I tested my 30L-1 after an electrical restoration, I noticed two things. First, a low-level mains-triggered medium frequency oscillation (~ 1MHz) and, second, a tendency to signal-triggered instability on 10m, even when operated into a matched antenna (but not a dummy load). The MFO was easily cured by some of the CCA suggestions, based on a comprehensive circuit analysis. The oscillation, by the way, had remarkably little effect on the signal output and, I suspect, would easily pass un-noticed. There are several ways of picking up the MFO but I just happened to have my old SM-220 monitor scope sitting too close to the 30L-1's transformer, giving a tiny amount of hum on the trace, upon which the small MFO was clearly visible as a stationary burst. In addition to the standard MFO fixes, I also noticed it was beneficial to keep the keying line impedance low (use a relay or MOSFETs) and, for good measure, to externally choke the keying line via a few cable windings on a small toroid.
Turning my attention to the 10m signal instability, I concluded that my problems were not as bad as some commentators reported and, after a few more "standard" partial fixes and purely out of interest, I inserted a 20.5 section of RG58 between the TS590S exciter and the amplifer. The residual instability vanished convincingly on both the monitor scope and spectrum analyser. The length of the cable was not critical. At this point, having a stable amplifier, I called it quits.
I suspect that, with the acknowledged conditionally stable design, the changes in tubes over the years (my 30L-1 has two RCA and two Chinese 811As), and a host of other factors that can impact stability (including grid/bias circuit component changes), it's likely pointless looking for very simple explanations. The circuit should have included some neutralization but it's perfectly possible to have a functional workhorse amplifier in 2018, providing a few basic tests are done. This is not a plug and play amplifier and is no doubt best avoided by the set who can melt 811As with a sideways glance.
Incidentally, I run my 30L-1 via a bucking transformer to give a 220V supply and 1.8 kV on the plates, via a Harbach HT supply upgrade. It makes over 600W PEP into a dummy load with 55W drive, and displays excellent IMD characteristics. I even have the "Tune" system working again, but actually never use it. Hard to complain.
73, Peter.
