Collins 30L-1 help question - please

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Peter Chadwick:
As far as I can make out, the 'funny length' is, as Tom says, for stability. It is interesting that the design in the RCA tube handbook of the same period for a four 811A amp shows 220pF grid capacitors and neutralisation, while several commercial 572B amps used neutralising, even back in the 1960s. I haven't seen an 811/572B amp where they grounded the grids directly, but a number where 0.005 microfarad disc ceramics (allowing for bias) were used with neutralisation, and those were much better in terms of stability. Several people over here have also found that the stock 30L1 is only marginally stable on 10, and some claim even 15 is not as good as it might be.

I still maintain the correct length of lead from exciter to amplifier is one that reaches with a couple of feet or so to spare so you can pull the units forward on the desk for servicing....

Tom Rauch:
You will get a lot of answers, but the fact remains 4 811 tubes in parallel are NOT stable on upper HF without neutralizing. This has nothing to do with the "design impedance" or anything else being off.

The cathode to anode capacitance of the four tubes provides enough regeneration to allow the amp to oscillate at upper HF under some repeatable tuning conditions, and even when that point is not reached the positive feedback enhances distortion.

By adding a broad-bandwidth loss to the input system, through use of 20 feet of RG 58 size cable, the input is loaded down at upper HF and this tends to stabilize the amp.

Other 811A amps using four tubes from the 50's and 60's, including the Heathkit Warrior, neutralized the tubes. As G3RZP points out, articles by tube manufacturers even showed the tubes neutralized. Collins is the only manufacturer I'm aware of who did not neutralize four parallel tubes.

The quality of coax shielding or phono plugs has nothing to do with this feedback is a root problem inside the amplifier caused by poor circuit design.

Grids in grounded grid amps belong grounded, and not though capacitors. Sometimes capacitors are used, but it is less and less and good idea as tube quality deteriorates.

73 Tom


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