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69.71.1.1
In Reply to: RE: Lafayette Two Tube Preamp posted by jec@afo.net on December 08, 2014 at 06:50:09
Sell it! Start over, or virtually replace every part in it. The bias is grid leaked because each cathode is grounded. So it is necessary to have those caps on the input grids (.05 and .005 uf shunted with 3M resistors) They are probably ceramic (boo-hiss). We are listening to bad DF capacitors. The power supply is laughable (half wave-only one diode and it is probably selenium which drifts with age. Replace with 4 UF4007 diodes. Using a Shure M3D might be problematic because this was designed for the GE variable reluctance cartridge and the EQ may be off. I could recommend other designs that use the same tube. Dr. Torsten Lousch(AKA Que Yang Wang) has a design on his THUNDERSTONE AUDIO website that I would investigate if I were you. YOU MUST use DC on the filaments if you don't want boo-koo oodles of HUM and the versatility of differing brands of tubes and the Lafayette is using AC filaments (another BOO-HISS). There are many DC filament designs on the web that are simple. One big capacitor (4000uf, 25v) and four UF4007 diodes is all it would take to convert to DC filaments. M.B.
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
Follow Ups:
FWIW, it has been said that the "acid test" of a phono preamp builder is achieving sufficiently low hum levels with AC heated filaments. Frankly, I'll never "sit the exam". However, it's not accidental that the "classic" passive EQ setup found in RCA tube manuals calls for 7025s. Hum bucking heaters give AC heating some chance of being satisfactory. Back in the day, DC heater supplies were not the simple thing they currently are. Look at the numerous "integrated" amp and receiver designs that used the "Cheap Charley" method of DC heating, in which the phono section filaments were used as part of the O/P tube bias network. I LOATHE "Cheap Charley". :> (( Consider that phono section heater current can be modulated by the power O/P signal.
I would not attempt to rectify the filament winding of the OEM power trafo in question. It takes approx. 2X the AC RMS current compared with the DC draw, when building a DC supply. Given that unit's "Bargain Basement" nature, there is no reason to believe the current handling capability necessary is present.
Eli D.
the cheap charlie method, can you go further to explain the deficiences in that arrangement, and how one could go about making changes to the cheap charlie that would make for a better phono stage... Thanks!
Sorry, I missed this post until now.
IMO, 2 obvious defects of "Cheap Charlie" DC heaters are the possible modulation of phono stage heater current by the power O/P signal and the use of a single bias network for all 4 O/P tubes.
The solution to the problems is a separate, true, RC bias network in each channel's O/P stage and a regulated 12 VDC phono section heater supply.
A good 12 VDC heater supply for 2X 12AX7s can be made from an AnTek AN-0112 toroidal transformer, a MBR20100CT common cathode twin Schottky diode, a large valued 25 WVDC 'lytic, and a 7812 3 terminal regulator IC. Wire the trafo's secondaries in series, FWCT rectify, and cap. filter. Refinements include a 1 muF. Panasonic ECQ-V cap. physically adjacent to/electrically across the regulator IC I/P and a 15 muF. 'lytic physically adjacent to/electrically across the regulator IC O/P. Don't forget a clip on heatsink for the regulator IC. OTOH, the (sic) 10 A. diodes in a MBR20100CT will be loafing.
Eli D.
Eli constantly finds faults but might not have even heard the potential within. AC filaments can sound fine in these chrome preamps. Half wave rectifiers can be found in more than a few classic sounders.
Marantz 7, Lafayette KT-600, Shure M65 are great sounding examples. Have you heard these ?
Power supply, input R and EQ upgrades can make these cute chrome preamps sound incredibly sweet...
Also the power transformer is probably too close to the 12ax7s if it is what I think it is so it's inductive coupling (Lenz lines) is gona cause hum no matter what he does even if the tubes have shields over them. He would be better off to power it from the Magnavox power amp power supply.
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
These sound awesome when upgraded. The power tranny is fine. Grid Leak bias is found on many classic phono stages which also sound nice. The AC filaments can sound hum free; or, at least the hum is non-obtrusive. These preamps simply need a little TLC...
Looks like the "sell it" idea might be the way to go......unfortunately. I want to thank you all very much for your input. It has been greatly appreciated.
All the best,
Jim
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