are in the signal path, they should go in favor of either 716P series Orange Drops or Soviet surplus K40s. Remember, when the amp was new, capacitance per unit volume was nowhere near as large as it is currently. is being drawn from the negative rail and the OEM filter values strike me as being inadequate. During the replacement process, increase the value of C3 somewhat and C4 substantially. in the unit should be replaced, as they literally dry out over time. If out of spec., the bias voltage should be somewhat high not low.Īll of the electrolytic caps. As you measure the bias voltage as being low, getting the noisy OEM diodes out and a pair of Schottkys in will increase the bias voltage slightly and eliminate a source of noise. However, you have observed what current higher average line voltages do to the B+ rail. The tubes I have are 6П14П-EP, the hardest ones I believe, a cryogenically treated matched quad set, and can't wait to put them to use.Īs far as the voltage you asked about, I measured just under -19 volts (which is not making it into the 6П14П-EP tube because there is no wire on pin 2) so in reality 0 volts, and about 434 V at the plates.Ĭraig, let's hope I do not have to worry about a biasing circuit, because space is at a premium. So later today, I will tie pins 1 and 2 inside the amp and see where we end up. The amp chassis calls for 7189A while the schematic calls for 7189, go figure. The 6П14П-EB apparently immediately and the 6BQ5 wait until you walk out of the room. As a result, the 6П14П-EB tube as well at the 6BQ5 will run without it in the Bell 2440 and tend to eventually self destruct. In the Bell 2440, pin 2 is blank and pin 1 is used to provide the power to the #1 control grid. In the 7189A, it does not matter since pin 1 and 2 are internally tied.
In the 6BQ5 and 6П14П-EB, the control grid 1 is energized by pin 2. It occurred to me late last night to compare pin configuration between all these tubes. They are the 2 tan Hershey kiss looking things to the right of the transformer I believe (in the pic attached).Ĭraig, you are on the right track I think as far as pin wiring differences-and some nay-sayer will always chime in and spoil the party when people talk about these tubes being identical pin configuration (they might have reason, see attached). Eli, I did miss those 2 diodes, they are silicon and are attached in a picture. Thanks Eli and Craig for the informative posts.
#Bias amp 2 amp list install
Do you have room for 2 more trim pots or even better 4 in a row? The existing balance pots are not configured like what you are accustom to in the Scott amps you have worked on in the past so they will have to be changed or if you can install 4 new pots in a row 1 for each tube then the existing balance pot would be disconnected and the cathodes tied directly to ground VIA a 10 ohm resistor for measuring the bias of each tube.
This circuit would be relatively easy to change to individual tube adjustable fix bias like on the Scott amps you have worked on. 19V should bias the Russian tubes low enough for them to not red plate. I wonder if Bell used some of the empty tube pins as solder lugs on what may have been no internal connection pins on vintage EL84's but not on the Russian mil spec. Could you put up a picture of all 4 output tube socket wiring. Do you actually have -19V at the control grids of the 7189 tubes? If so I suspect some other problem is causing the Russian tubes to red plate. I got your email and figured why not just reply here. rated for 250 mA., after the 1st filter capacitor. If you exercise this option, use a "potato masher" 5R4 and hold the 1st filter cap. If you are willing to do a rather complete rework, using a 5R4 as the B+ rectifier, instead of a 5AR4, will reduce the rail voltage by something around 50 V. Installation of Schottkys deals with both of the above matters. If the OEM diodes are silicon, they will be very noisy. If the OEM parts are selenium, replacement is mandatory, as selenium diodes are ticking, toxic, time bombs. Some resistance may have to be added between the diodes and the filter, due to reduced forward drop. The OEM negative rail rectifiers should be replaced with 100 PIV/1 A. Jim McShane is good source for the matched quad and lots of other stuff. That should stop the red plating, IF all 4 O/P tubes are properly matched.
and adjust the common bias voltage into the 23 or 24 V. A quick fix is to replace R82 with a 2500 Ω pot. Unfortunately, the amp really is fixed bias and requires that all 4 O/P tubes be very closely matched in both cathode current and gm. The Russian 6П14П-EB (6p14p-ev), AKA EL84M, is a genuine 7189 equivalent, which is quite tough.
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