Ampex 350 playback only question....

[Teeg]

  This is a tough one, but I'll ask anyway. I am putting together a 300 'playback only' rig with 350 (tube) electronics....headstack is original Ampex with one repro head and one dummy head. Does it matter to the tube electronics whether or not the record/erase heads are present?
  At one time someone mentioned that you can pull the unused tubes in the 350 record/bias sections if playback only, but apparently this is not a good idea, and looking at the schematic it makes sense to leave them in. But I am not sure what effect the absence of the heads would have in the same circuit.

Tj

[High and Outside]

It should be fine to leave the record head and erase head inputs/outputs unconnected.

Why would you think you shouldn't pull the tubes from the record and bias circuits?

I expect that this will be a good sounding rig...let us know how it comes out.

[Teeg]

  There was an un-finished discussion on one of the pro recording forums...probably Gearslutz if I recall...concerning the 350 preamps and whether or not all of the tubes in the circuit had to be present for proper operation. Many of those guys are ripping apart 350/351 units for mic preamps and the question was raised about needing all of it to work.
  Like many tube designs from days ago, the unregulated supply (B+) is intertwined amongst the various stages and removing some of it could change the operating point of the rest of the circuit. That's just a guess; I've recapped a few units but have never taken the time to fully understand how it is all laid out.

Rgs,
Tj

[docb]

All it takes is a meter and a few minutes to find out. Check both the plate and heater voltages of the repro circuit tubes, with and without the record circuit tubes in place. If the high voltage stays within 10% and the heater voltage is within 5% when the tubes are removed you are probably fine.

[cordcrazy]

I leave the record/erase cables unconnected as I use my machine for playback and this practice eliminates the possibility of accidental erasure.  Of course, if you reconnect the cables, it's best to make sure the unit is switched OFF.