• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Preamp suggestions - Help a hack have some fun

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

Member
2003-01-18 10:43 pm
England
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
 
7N7 said:
A "Cinderella" valve that I like is 6S4/B]


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



tubesmuggler said:
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

Member
2003-01-18 10:43 pm
England
tubesmuggler said:
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
 
tubesmuggler said:
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.
 

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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.
 
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. ;)