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Well, summer is ending, fishing is about over so it's time to start designing a preamp to match my GM70.
My GM70 goes full power with about 1.7v in.
The preamp will drive the grid of a 6E5P on the GM70 amp.
Preamp needs to have 2-3 inputs.
Cable length between amp and preamp will be 1 meter.
#1 Looking for some suggestions on your favorite preamp tube that's transparent.
#2 would you do cap or transformer output?
#3 looking for a good switcher that won't introduce crosstalk
I'll use LTSpice in the coming weeks and post the progress on the build.
Follow Ups:
Using Lundahl LL-1680-8ma in single ended mode or C3g triode wired.
Lets look at 6J5 single ended using LL-1680, 8ma gap
HT - 250V, Rk - 1.2k, Ra - 7.5ma, Rkc - 70uF, Gain 2, output z less than 200 ohm ideal for preamp, Freq 20hz-50kHz
The Lundahl LL-1680 is a modern day equivalent to legend UTC LS27
Very very good at noise immunity, also compact size and simple build, what is not to like ?
Johno
Yesterday finished an active/passive parafeed 6n6p line stage with about 10dB of gain active, output impedance ~ 80 ohms active, with 4 inputs. Remote (2nd chassis) choke input critical inductance PDM power supply (LCLC) using TV damper diodes, and PIO filtering caps. Simple parafeed 6n6p triode gain stage, diode biased, CCS loaded, Magnequest high nickle 15K B7 transformers. TVC attenuators, used secondaries only as autoformers. I implemented a DPDT switch, and two sets of output RCAs, so I can go back and forth from passive to active.
Active sounds much better. Sounds amazing, I'll leave it at that. The 6n6p is a very linear tube, cheap and very available. The very best versions of these tubes are the mid 60's vintage Reflector (old diamond shaped logo) tubes with the square getter.
twystd
Similar to this design?
I have been considering an "iron heavy" design to match my boat anchor...
Here's my schematic very simple. I actually made a couple mistakes, on the schematic (getting late). The voltage on the plate is actually 165V, and ma is 25, dissipation is ~ 4 watts. Operating points are 160V plate, -5 bias, 25 ma. As you can see the curves look good, sounds good too. Running the tube right at max when running both triode sections, but they are pretty tough tubes. I had a bunch of those tubes, so found one that the two sections matched, and were very close to the curves.
twystd
Whats the secondary impedance? 600?
I'll run some spice calcs when I get back from vacation
600 ohm secondaries are correct.
twystd
nt
Sorry Vinnie, but I didn't photograph the two chassis, weren't much to look at as I built them for a friend of mine that has eclectic tastes. He had me put the power supply in a black Hammond cage, and supplied an ancient battleship gray Western Electric rack mounted case. If it was up to me I'd build it more like the 300B amps I built him a while back. So I'll give you a little eye candy of those.
A pair of mirror image monoblocks, loosely based on the schematic of highly modified Welborne Laurels, with my own custom chassis. I originally built them years ago, but did a complete rebuild, refinish, rewire, new tube sockets, and final round of modifications about a year ago. He'd been after me to sell them to him for years and I finally relented.
The sale of the amps will help me finance the parts for my final statement amp, that I'll start on in about 6 months.
.
Nice work. Thanks for the pics.
"My GM70 goes full power with about 1.7v in."
Is that derived from a RMS measurement? If so, you might not need a preamp with gain. All your sources will probably drive the amplifier to full output, particularly when the dynamic range of the program material is taken into account. I came to that conclusion recently for my own preamp project. Although much-maligned by some, the cathode follower is the most linear of all VT topologies. It neatly supports the concept of a unity-gain preamp, and dozens of tube types that would be less suitable in common cathode will work well.
--------------------------
Buy Chinese. Bury freedom.
Most my sources are OK, vinyl source is slightly on the low side but I can live with that since I don't usually crank it up with that source.
I've been following the progress on your design as well. Very interested in how you're going to handle switching as well.
The 6SN7 based hybrid I've posted should easily deal with your signal requirements. Another fellow ran a distortion simulation of the design and found it to be extremely linear. A 1 M. cable is a light load for the MOSFET to lift.
You can reduce the O/P coupling capacitance value, if the downstream load is comparatively light. What's shown neatly deals with the IHF "standard" value of 10 Kohms.
Mercury wetted reed relays controlled by a rotary switch will ensure cross talk between sources does not occur.
Eli D.
Something like this relay?
Would you use single pole to keep the channels separated? or will the 2 pole provide enough isolation?Not sure about the Mosfet but I do have a CCS in my amp....
Just for fun, I'll run a simulationAttached schematic of the amp (it may not be accurate after tweaks)
Edits: 08/16/17
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