Paramour 76 driver

[kuramura]

I've changed 12at7 to 6cg7 as a driver (thank you Paul for help) and I love the sound. The amps are connected to Foerplay also 6cg7 instead 12au7, DC heaters on both. All have c4s boards. I would like to try type 76 or other DHT as a driver for 2a3 ( EML 2a3-s ) so if someone could help me what changes in circuit need to be made I would greatly appreciate.

[Paul Birkeland]

The big change that has to be made is the hole in the chassis plate to accommodate the 5 pin socket.

9V of bias at the 3.2mA of current for the stock C4S configuration (check for 237 Ohm current set resistor on the C4S board) would mean you need a cathode resistor of around 2.8K (any wattage will do).  I suspect that you will prefer to bypass the 2.8K resistor, with maybe a 470uF/10V capacitor.

-PB

[Paul Joppa]

Paramours had full-size holes, so that's no problem. The oldest C4S had two paralleled 499 ohm resistors, more recent ones 237 ohms.

Grainger did this long ago; he may have some numbers and values.

[Grainger49]

I also posted a how to for Paramour and Paramour II.  Those were probably lost somewhere.  I did some tweaking to the 76 cathode resistor the combination measures 1988 ohms and the voltage at the plate is 197V.

If he likes I have it in Word.  I don't think the PM will let me send anything that big.  But I'm willing to try.  I can always attach it to an email.

[kuramura]

I should probably start at the beginning. When I asked about going from 12au7 to 6cg7 Paul suggested cathode resistor of about 1200 ohm to get 175-200v on the plate of the driver. I've installed 1200 ohm but my plates are 160v and 285v going to c4s boards . I have changed 15k resistor to 12.5k in hope to rise the voltage and I wonder if smaller resistor would help.
Plates on  2a3 run at 340v instead 365v as per manual. The amp has schottky bridge, choke from paraffed in the power supply
full nickel iron upgrade and whole lot of pio capacitors. I left it alone with lower 6cg7 plate because it never sounded so good but now I wonder if these lower voltages should be adjusted before I start tinkering with 76 tube  ??

[Grainger49]

The instructions were sent, twice.  Yahoo locked up while the first one was sending.  So I sent it again.

No, you can do the conversion then work on the cathode resistor.  I did the same and will post some guidelines.

[Paul Birkeland]

I should probably start at the beginning. When I asked about going from 12au7 to 6cg7 Paul suggested cathode resistor of about 1200 ohm to get 175-200v on the plate of the driver. I've installed 1200 ohm but my plates are 160v and 285v going to c4s boards . I have changed 15k resistor to 12.5k in hope to rise the voltage and I wonder if smaller resistor would help.

The C4S regulates current, so you have a fixed current passing over the 1.2K resistor setting your grid bias, and the plate current is set by the CCS, so that determines the operating point.  To bump the plate voltage, go to 1.4K cathode resistors.

If you have 285V coming out of one tube with a 1.2K cathode resistor and the same R1 as the other channel, then you have a dead tube.  (There may also be a wiring error)  One way to be sure would be to measure the cathode voltage on both sides across the 1200 Ohm resistor to ensure that those are the same.

I personally wouldn't sweat having 160V on the driver plate at all.

-PB

[kuramura]

The instructions were sent, twice.  Yahoo locked up while the first one was sending.  So I sent it again.

No, you can do the conversion then work on the cathode resistor.  I did the same and will post some guidelines.

Thank you I got the email. It's great write up and I should have all the parts ready in few weeks.

[kuramura]

The C4S regulates current, so you have a fixed current passing over the 1.2K resistor setting your grid bias, and the plate current is set by the CCS, so that determines the operating point.  To bump the plate voltage, go to 1.4K cathode resistors.

If you have 285V coming out of one tube with a 1.2K cathode resistor and the same R1 as the other channel, then you have a dead tube.  (There may also be a wiring error)  One way to be sure would be to measure the cathode voltage on both sides across the 1200 Ohm resistor to ensure that those are the same.

I personally wouldn't sweat having 160V on the driver plate at all.

-PB

I have 285V going into C4S and 160V coming out and into plate with 1.2K , Ill get the 2.5 pot and make fine adjustments.
My concern was that after all my mods C4S  boards were not getting 295V as manual recommends.
Thanks for help.

[Paul Birkeland]

I have 285V going into C4S and 160V coming out and into plate with 1.2K

Oh, that's quite alright, I wouldn't make any further adjustments.

[kuramura]

Up and running, will work on cosmetics later. I have 180vdc on plates 1.65 rk, I hope its not too much for 76.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi190.photobucket.com%2Falbums%2Fz134%2Fkuramura%2F76-driver-1.jpg&hash=aa98a77b4a0c2d6d6ece1c9e36cfaadef9856e57)

[Natural Sound]

Yes but how does it sound?