Black Box preamp

[Murphyv310]

               
Hi.
A few years ago I ditched the Monarch deck in my second generation Black box for a Garrard AT6 & Sontone 9TA cartridge. Although adequate on older 45's & some 78's it would struggle for enough volume on LP's.
I considered building a preamp for some time but today after chatting to Radioman on Wednesday went ahead.
Andy said make a preamp with a FET, the thought of using these ultra modern devices in a 1955 record player made me shudder.
I also didn't fancy an ECC81 or EF86 as current is rather limited on these amps in the Black Box.
So I decided on a Rod Pentode 1J18B
The result is brilliant, I now have the benefit of PYE's compensated volume control and above half volume on an LP the the unit now distorts.
I have wired the valve as a triode, HT come from the main HT of the unit via a 1.8 meg resistor, the filament is fed from the live side of the 6.3 heaters, half wave rectified smoothed and clamped by two 1N4004 diodes that hold the voltage a little over 1.5 v. The filament is dropped by a resistor to 1.1v (Value on selection dependent on valve used) in my case a 33 & 100 ohm in parallel gave me the desired voltage. HT consumption is microscopic as can be seen by the the HT dropper, a whacking 1.8 megs.
The input impedance is very high and a 10 meg goes from G1 to the positive side of the filaments, taking it to ground or feeding it from a negative supply just turns the valve further off and it distorts, HT is around 33v.
So a good use for one of these valves, there is no hum even with half wave LT for the filament. The Zener (47v) across the HT feed to the valve would be a good idea to protect the 47uf 50v HT decoupler if the filament supply goes down, I didn't bother though.

[ppppenguin]

Nice neat idea for making a BB more usable. Have to admit that if I needed to do something like this I'd not be a purist and just use a FET. My own BB still has its original Monarch deck and cartridge. In practice I only use it for 78s as it would be a bit cruel to vinyl and destructive to anything stereo.

[Aerodyne]

Interesting choice of valve, Trevor. Very innovative of you. Neatly done modification, too.
Tony

[Ed_Dinning]

Hi Trevor, if you want to use a FET and still use vintage, use the Mullard TAA 320 amp chip.
Still available on e-bay, it uses a FET and a bipolar, all in a 3 pin can.
Power can be pinched from the HT rail and it is hum free. App note in the Mullard Audio and Radio circuits book.

Ed

[Murphyv310]

Hi Trevor, if you want to use a FET and still use vintage, use the Mullard TAA 320 amp chip.
Still available on e-bay, it uses a FET and a bipolar, all in a 3 pin can.
Power can be pinched from the HT rail and it is hum free. App note in the Mullard Audio and Radio circuits book.

Ed

Thanks Ed.
Could well use one in a new project with the rod valves. Crystal controlled FM superhet portable radio. Coming soon.

[`FRANK.C]

Hi Trevor
What an ideal use for these pencil valves and uses very little extra power. I know little about record players but I see many post looking for suitable preamps after replacing the cartridge, your amp looks like it would fit the bill for many.

Frank

[Refugee]

I can still remember getting my hands on an NOS BSR deck back in the late 1970s with no cartridge. I bought a new cartridge and found that it needed two 2N3819s to get the two channels up to volume.

[radioman]

I considered building a preamp for some time but today after chatting to Radioman on Wednesday went ahead.
Andy said make a preamp with a FET, the thought of using these ultra modern devices in a 1955 record player made me shudder.

Hi Trevor,
That's a good use for one of those Russian valves - well done.
I wouldn't call a FET an 'ultra modern device' though, I think it was developed before your rod pentodes !
A 2N3819 is probably contemporary with them.
https://en.wikipedia.org/wiki/Field-effe...or#History

Andy

[Murphyv310]

I considered building a preamp for some time but today after chatting to Radioman on Wednesday went ahead.
Andy said make a preamp with a FET, the thought of using these ultra modern devices in a 1955 record player made me shudder.

Hi Trevor,
That's a good use for one of those Russian valves - well done.
I wouldn't call a FET an 'ultra modern device' though, I think it was developed before your rod pentodes !
A 2N3819 is probably contemporary with them.
https://en.wikipedia.org/wiki/Field-effe...or#History

Andy

Hi Andy.
Thanks for the compliment   The rod pentodes I believe had their first use in 1957, used in Sputnik!! No doubt the development was done for many years beforehand but yes the FET would have been a neater way round, mind you I've a lot more rod valves than FET's.

I want to do a small mod to the circuit, the 1.8meg HT dropper tied in with the 47uf decoupler has a far too long time constant, it takes a couple of minutes for the rod valve anode/screen to get to 30v and once at that voltage it seems to slowly rise and fall no doubt due to the conduction of the valve. I will try dropping the value to 820k and fit a 47v zener across the decoupler. I did try a higher HT in the "development" stage when I was using PP3 batteries and the circuit works very well with 5 PP3= 45v.
Even though the HT seems to slightly rise and fall I had the Black Box in the house last night and we listened to a few albums, the result was very pleasing indeed.