V76s build - work in progress

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Ilya

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I'm moving on with the PCB design, and I'm currently finalizing the PSU section. I've settled on the switcher for heaters, and a basic LM317 linear reg for auxiliaries. HT will be a neat circuit from Merlin Blencowe's book (not sure I can post this circuit here - haven't seen it published anywhere on the net).
With the SMPS section, I have 2 questions. Since this is a buck topology, it's main noise is on the input. I had successfully used PI filters and CM chokes to combat the noise, but those were all SMD components. I aim at the TH components for this section though, and I've stumbled upon these TDK hybrid chokes that have a rather large stray inductance that allows to build an LC filter "integrated" in the CM choke.
I've never used those, but given the numbers from the datasheet (200 uH stray approx.) this should be enough.

Another question is the output filter. Currently, it's a simple LC filter. Do I need to use cap multiplier in that position instead?

1652465395710.png
 

Ilya

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Tubes.
EF804s are hard to source and very expensive. Reading through various sources and datasheets it looks like EF806s are identical (but with a different pin-out). I'm going to settle on 806 tubes. There seems to be a current production of those by JJ. Besides, this pin-out will allow me to use Russian version (6Ж32П) directly in case I can't source the 806.

The E83F, however, seems to have no recent production versions, only NOS. Are there any equivalents for this tube?

Also, what are recommended procedures for screening 804/806 tubes?
 

rock soderstrom

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Tubes.
EF804s are hard to source and very expensive. Reading through various sources and datasheets it looks like EF806s are identical (but with a different pin-out). I'm going to settle on 806 tubes. There seems to be a current production of those by JJ. Besides, this pin-out will allow me to use Russian version (6Ж32П) directly in case I can't source the 806.

The E83F, however, seems to have no recent production versions, only NOS. Are there any equivalents for this tube?

Also, what are recommended procedures for screening 804/806 tubes?
Good EF86 (Telefunken, Valvo, etc) are also suitable for the EF806 position.

For the E83F there are no well available alternatives, but still some NOS suppliers for relatively little money. EF83 is a different tube.




 

Ilya

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Costs.

I must say that this is not a cheapest preamp in the world. While it looks like it will take Mouser forever to ship my order (177 lines, mind you), I thought that it's time to sum up the costs for a single channel. Here we go (most prices include shipping in the US).

Iron, Sowter (Input, Output, 2 chokes) - 770$ (probably a bit more now, looks like the prices went up)
Bulk of the components, Mouser - 350$
Tubes, shields, sockets and jewel lamp holder - 240$
Panel meter, T-Pad - 85$
PCBs - 200$ (estimate)
Enclosure - 80$ (estimate)
Front panel - 100$ (estimate)
Power transformer - 100$ (estimate)

TOTAL: 1925$

Almost 2 grands for a single channel. Builders, beware. Cash hog)
 

leadbreath

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Woaw!! Super expensive!!?
I managed to build one at less than half your cost, transformers and tubes made up the bulk of it but the rest was but just using bits I had lying around.
Definitley woth it though, its one amazing sounding pre, probably the best one I own!
 

Ilya

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Cool! I can't see how it's possible to build this for less than 1k$, given that tubes and transformers alone cost that much. I agree that a stripped down version can be built cheaper. If you're willing to DIY a front panel and have most of the bits around. That's not my case though)
 

leadbreath

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Yes the transformers are crazy expensive...
I built mine into an old upcycled bbc voltmeter case and this ghad all the tube sockets, tutretboards etc...saved me a pile.
As for the tubes ebay.de is your friend even though tube prices on there are starting to get a bit stupid now too..
 

Ilya

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While waiting for parts to arrive, I'm slowly working on PCBs. Here's how the PSU will look like:
1653475012559.png
And this is the main board:
1653475080832.png
The round board at the top is IC simulated inductor.

Still a lot of work to do...
 

Ilya

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I've redone the PCB layout. Mostly to keep the sensitive input networks further away from the power transformer.
I'm still waiting for parts to arrive. Don't want to commit the final PCB design before I get my hands on all parts.

The mechanical design is finished. I've got the 3U enclosures manufactured and front panels are being manufactured right now. I will get them by the end of this month. Meanwhile, here's how the device will look like:

i76 V1_5_exploded.jpg
 

My3gger

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Ambitious!


I think FET DIs sound inferior. I would never use a FET option with a tube preamp. A better alternative IMO (if you really want a second option) would be a DI step down transformer (i.e. JT-DB) into the mic transformer.
You cannot do a direct input to the tube grid because if the v76 bias scheme - needs a blocking cap.


Unregulated B+ with AC heaters worked great in my build and the measured performance was very good. (EIN < -120dBu). AC heaters when implemented correctly can match the performance of DC.
If you build the PSU in the 3U case, be prepared for some hum/buzz battles. Sowter offers the chokes in mumetal cans, for a pretty reasonable price increase (I built mu metal shields for the CJ wound chokes AND for the power toroid). External PSU, you may avoid some headaches.
Going direct into tube is my preference too, i see no reason to include IC's and then having to power them, etc. Do you pad the signal or just cap couple it?

This circuit can benefit from heater regulation (haven't tried AC at high gains), current draw isn't high so using two regs is easy. Mine are normally LDO with low drop diodes easing choice of mains transformer. Heat is very low with properly chosen voltages.

Some time ago a friend was unhappy with mumetal shields, he cut and bent them to use for home made mic input transformers. Did you have to heat treat mumetal later, or it was not cut and bent which ruins magn. properties? Who knows what he got, if material was even properly heat treated.
 

dmp

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Going direct into tube is my preference too, i see no reason to include IC's and then having to power them, etc. Do you pad the signal or just cap couple it?
Just cap couple. For a passive bass guitar you wouldn't want to pad. If you were going to use higher level you may want an optional pad.
This circuit can benefit from heater regulation (haven't tried AC at high gains), current draw isn't high so using two regs is easy. Mine are normally LDO with low drop diodes easing choice of mains transformer. Heat is very low with properly chosen voltages.
What is the benefit?
Some time ago a friend was unhappy with mumetal shields, he cut and bent them to use for home made mic input transformers. Did you have to heat treat mumetal later, or it was not cut and bent which ruins magn. properties? Who knows what he got, if material was even properly heat treated.
The mumetal I used is very flexible. I did not heat treat but bent it into shape and taped with metal adhesive tape. It should be as close to continuous as possible.
 

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