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#1 2008-04-26 03:40:00

Diablo
Member
Registered: 2007-12-01
Posts: 139

Tube Bias V4 with 7027A

The V4 has a fixed bias which is not adjustable. Since the bias is not adjustable, I was wondering if anyone has gone to the trouble of adding a bias adjusting pot to the circuit to set the bias for the 7027 power tubes. Also wondering if anyone has gone to the trouble of measuring the mA of cathode current on the 7027A tubes when using the standard Ampeg non-adjustable bias voltage. I'd like to know how hot the tubes are biased as stock. I'm thinking that the tubes are probably biased fairly cold so that the amp tolerates sticking in a new tube without being able to adjust the bias. Any info on this would be greatly appreciated.

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#2 2008-06-14 02:47:51

Diablo
Member
Registered: 2007-12-01
Posts: 139

Re: Tube Bias V4 with 7027A

Here's some data if anyone else is interested in tube bias. After refreshing the bias circuit with new caps and resistors, I measured the bias voltage at -62 volts, which agrees dead on with the schematic for my 1972 V4. I changed the output tube plate resistors to 10 ohm - 5 W and measured the resistance across each one at 10.2 ohms. Then I measured the voltage drop across the plate resistors for each output tube. I've been running only two 7027 output tubes to cut down on the power, and the two I picked were not very well matched. One was idling at 0.0315 amps (x 543 volts = 17.13 watts), and the other was idling at 0.0277 amps (x 543 volts = 15.06 watts). If I assume the tubes are rated for 35 watts max, then the stock Ampeg bias setting is a little less than 50% idle. This probably explains why you can get away with sticking in any 7027 tube and not worry about the bias setting - it's set mighty cold.

Step two was to add a 20K bias pot and swap R49 from 75K to 60K. The sweep is from 60K to 80K, and allows good bias adjustment. I selected another tube that matched better to the opposite side and adjusted the bias to get 0.039 amps of idle current. Both tubes matched up perfectly. This setting is 70% idle watts if I assume the tube maxes out at 30 watts - for a little safety margin. I know the 7027A tubes are rated for 35 watts, but some claim they are really no different than a 6L6GC (30 watt tube). I watched the tubes for a while to make sure they didn't redplate and checked plate current to make sure the bias was holding steady over time. Plugged in a guitar and let her rip and it sounds fine to me.

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#3 2008-06-14 18:55:13

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: Tube Bias V4 with 7027A

Great stuff Diablo!    let us know how it performs in the long run too.   I wonder if this will have any effect on the tube life expectancy?

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