Preamp suggestions - Help a hack have some fun

[tubesmuggler]

I just picked up a Jolida JD50A "passive preamp with remote control". I installed it last night and discovered that unfortunately it won't cut it. At max output it's barely loud enough for my requirements. It's fine for tv, but for serious listening I need just a touch of gain to make things happen. I previously used a 12SN7 SRPP preamp that sounds good and has lots of gain to spare.

The nice thing is that now I have a huge, nearly empty, chassis. Why let that go to waste? I immediately thought of converting it to a *simple* tube preamp so I could still use the remote but get a little gain.

Here's what I have in my inventory:

• One huge Jolida JD50A chassis with all gold RCA's, etc. (already has vents so tubes shouldn't over heat)
  toroidal PT- 500VCT/100mA - 6.3VCT/6A - 14VCT/1A
  Lots of PS caps, Solens, Auricaps, tube sockets.

The chassis is 2" high. My PT will fit no problem. The tubes can be mounted horizontially or I could punch holes through the top cover.

My thoughts were a 12B4, 2C51, or 6922 circuit. I prefer to keep it simple and use a CRC filter. Either Tube or SS rect would be fine with me.

Thanks for your comments.

 

[7N7]

There is great difference between the µ of 12B4 and 2C51/6922!

From what you say it does not seem likely that you should need µ of 30+; perhaps 12B4 is a good answer, but be careful as they are notorious for poor heater/cathode insulation which can make them noisy. If you use them, elevate the heater supply to say +40V, always a good policy anyway.

Another factor is the length of the cable you intend to drive. Any more than a couple of metres (6') and you should be thinking of a cathode follower to drive the capacitance of the cable - your 12SN7 SRPP would manage this nicely as would 12B4 if you run decent current through it.

A "Cinderella" valve that I like is 6S4; this has µ of 16, about half that of 2C51/6922

7N7

 

[astouffer]

Go with the 12B4, the gain should be just right for your application. Theres plenty of people who have built the 12B4 line stage.

 

[tubesmuggler]

7N7: My cables 3ft.

How about the 6H30p? I have a bunch of those. One tube would be fine then. Or how about 6h30 srpp?

 

[dsavitsk]

A "Cinderella" valve that I like is 6S4/B]

Try the 12A4 -- similar mu, lower rp, good valve.

How about the 6H30p? I have a bunch of those. One tube would be fine then. Or how about 6h30 srpp?

The gain will be high, but even from the plate it will drive cables. Not a bad choice at all other than the high mu. You can get very nice plate curves for this valve that you can play with from http://www.glass-ware.com/Vacuum_Tube_Live_Curves.html

 

[7N7]

7N7: My cables 3ft.

How about the 6H30p? I have a bunch of those. One tube would be fine then. Or how about 6h30 srpp?

As Dsavitsk says, 6H30 is very capable, I like them, but they do need a bit of current for best results. µ is about 15 from memory - about the same as 6S4 but 6S4 has higher anode resistance of course. I was thinking that a 6S4 SRPP would be fun, cheap and (possibly) quite original!

7N7

 

[Eli Duttman]

I just picked up a Jolida JD50A "passive preamp with remote control". I installed it last night and discovered that unfortunately it won't cut it. At max output it's barely loud enough for my requirements. It's fine for tv, but for serious listening I need just a touch of gain to make things happen. I previously used a 12SN7 SRPP preamp that sounds good and has lots of gain to spare.

The nice thing is that now I have a huge, nearly empty, chassis. Why let that go to waste? I immediately thought of converting it to a *simple* tube preamp so I could still use the remote but get a little gain.

Here's what I have in my inventory:

• One huge Jolida JD50A chassis with all gold RCA's, etc. (already has vents so tubes shouldn't over heat)
  toroidal PT- 500VCT/100mA - 6.3VCT/6A - 14VCT/1A
  Lots of PS caps, Solens, Auricaps, tube sockets.

The chassis is 2" high. My PT will fit no problem. The tubes can be mounted horizontially or I could punch holes through the top cover.

My thoughts were a 12B4, 2C51, or 6922 circuit. I prefer to keep it simple and use a CRC filter. Either Tube or SS rect would be fine with me.

Thanks for your comments.

Dude,

I AGREE with the 12B4 idea. The low mu seals the deal. I'm attaching a schematic for your consideration. Ditch the PSU shown, but the signal circuitry will serve you well.

Voltage double the 6.3 VAC winding of your power trafo with a pair of Schottky diodes. Use 2X 10000 muF. 'lytics in the doubler stack. Feed the resulting DC into well heatsinked (1 per 12B4) 7812 3 terminal regulator ICs.

That 1 A./14 VAC winding can, with the addition of a small series resistance be used to power the heater of a Locktal based 14Y4 rectifier. Hold the value of the 1st filter cap. down, as you have more Volts than are necessary. A gyrator is a good substitute for a choke and has (in this case) the added benefit of dropping a few of the extra Volts.

Oh yeah, since you are using a motorized volume control, eliminate the 100 KOhm pots. and install 1 MegOhm safety grid leak resistors.

 

[tubesmuggler]

Hi Eli,

I have two DN2450 units. Could I use one for each plate load? I read somewhere they aren't good on the plate unless cascoded. Does that mean they don't sound good, or they don't measure as "as good" as cascode?

Also, what's the B+ for your circuit? I'm guessing ~250V?? With the PT I have would I be close with a 6X4 or 6CA4? I don't have 14Y4's but I have the others.

Thanks.

 

[SY]

Without cascoding, the impedance is mediocre and the bandwidth is limited. They're cheap. Do it right.

 

[Eli Duttman]

Also, what's the B+ for your circuit? I'm guessing ~250V?? With the PT I have would I be close with a 6X4 or 6CA4? I don't have 14Y4's but I have the others.

TS,

The pseudo choke I/P filter shown has the "fudge factor" cap. (initially 47 nF.) tweaked at the bench to make the B+ rail voltage, under load, come in at 120 V. 30 V. is enough compliance for a Bottlehead C4S. Why make the "sand" work hard and get hot?

I suggested a LOW cost 14Y4 to keep the rectifier's heater supply separate from the signal tubes' DC heater supply. A 12X4 will do, but the 14Y4 is dirt cheap. Heck, the Locktal socket costs more than the tube.