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In Reply to: RE: Bell 6060 rectifier posted by SteveBrown on October 10, 2014 at 07:40:35
The "Cheap Charlie" method of DC heaters used in the Bell 6060 SUCKS. IMO, the OP should build a proper, regulated, 12 VDC supply for the preamp tubes. Also, a single bias network for all 4 O/P tubes imposes a matching burden that should not be endured in today's climate. :>( A separate RC bias network in each channel reduces matching to pairs, not quads.
While some work to get the B+ rail voltage right is required, Mike Samra's suggestion of SS rectified B+ has merit. The 5 VAC winding used to energize the 5V3 rectifier's filament could be voltage multiplied (3X) to provide the "raw" DC needed by a regulated preamp tube heater supply. Plenty of current is available, given the (sic) 3.8 A. draw of a 5V3.
BTW, replacing the incandescent pilot lamps with LEDs provides some additional 6.3 VAC current capability that might be taken advantage of.
Eli D.
Follow Ups:
The only thing I may do here is replace the lamps with led. I just want to know the best way to run the rectifier as SS instead of tube. Will the WZ34 (450ma) be a good choice?
"I just want to know the best way to run the rectifier as SS instead of tube. Will the WZ34 (450ma) be a good choice?"
I don't think so! Aside from the switching noise issue already mentioned, the forward drop characteristic is dead wrong. Look at the 5V3 data sheet. Notice the 47 V. drop. As vacuum rectifiers go, the 5AR4/GZ34 exhibits a relatively small forward drop.
Mike Samra gave you good advice about high PIV Schottky diodes. They don't generate reverse recovery spike noise. To use SS rectified B+ successfully, you will have to rework the PSU filter. IMO, a reasonable approach follows. Use a smallish 1st filter capacitor. Follow with a choke whose inductance is substantial. Then use a large reservoir. capacitor. The filter is cLC. You adjust the value of the smallish 1st cap. so that the minimum value which keeps the B+ rail voltage up is used.
Eli D.
ebay??
Mouser. You run a LED, a UF4007, and a resistor in series. Size the current limiting resistor to the AC RMS voltage present. Therefore, you can put the pilot lamp stuff across the power trafo's primary. ;> )
Eli D.
nt
As is often the case in units of this vintage, my advice is to rework the source selection. Tape head I/Ps are passé in today's environment. A single set of RCA females should be hard wired to the low level mag. preamp and that section should be configured for RIAA EQ only. Switches in lines carrying mV. level signals is asking for trouble. All other I/Ps should be at "line" level.
Less than $5 buys a Lorlin brand rotary switch from Mouser (stock # 105-14572) to finish the source selection upgrade off.
Eli D.
All good suggestions, Eli. The only consideration is impact on the value of the amp should one wish to resell it. Another modification which is pretty easy to do is to bypass the tone controls and filters (which I did on mine and it made an excellent sonic improvement). This is pretty easy to reverse if the amp is later sold to a collector. I suppose an even more radical update would be to replace the PECs with individual components, though the space around where they are is pretty tight.
Steve,
Every now and then, tone controls can be of use. To avoid fouling the vast majority of recordings up, tone control circuitry needs to be 100% defeatable and (preferably) "unity" gain.
The attached Max Robinson design, a Baxandall implementation, is "unity" gain. As shown, it's not practical. 4X 12AX7s are needed for a stereo setup. :> ( Fortunately, it's not particularly difficult to replace the cap. coupled 'X7 cathode followers with DC coupled ZVN0545A source followers. Now, the bottle count is 1/channel and the number of caps. in the signal path is reduced. :> )
BTW, AC heating should be fine, if the Sovtek 12AX7LPS is used in Max's setup. The 'LPS is a genuine 7025 equivalent that contains a hum bucking, spiral wound, heater.
Eli D.
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