Changing an antiquated design

[famousmockingbird]

I got an old console amp where there was one 12AX7 for the phono section and another 12AX7 as a paraphase driving push pull 7189's, cathode bias.

The amp will be used with a preamp so I was thinking of gutting it and changing the circuit around. I want to do away with the phono and equalization so that leaves one whole bottle to play with.

Four triodes seems like a lot to drive 7189's in pentode mode so any ideas what to do with the extra bottle would be warmly appreciated.

Here is what I came up with so far;

 

[Parafeed813]

Four triodes is too much indeed.
You could use the extra socket for a magic eye as a level indicator?
Or -if your PT allows it- an EZ81 rectifier?

 

[Eli Duttman]

KISS. A 12AX7 section voltage amplifier DC coupled to a 12AU7 section split load phase splitter. Wire 1 channel to use only system #1 and the 2nd channel to use only system #2. When the tubes show signs of wear, exchange them between the 2 channels.

Other than UL vs. pentode, the Dyna ST-35 is a good starting point.

 

[famousmockingbird]

Brilliant!!! I love the idea of swapping tubes between channels as they are using different triode sections per bottle.

I was figuring since I'm powering the heaters I'd do something with em. Maybe a 6BL8 where the pentode is a load for the triode section, feed that to a LTP.

Are there any noval voltage regulators?

 

[Eli Duttman]

Are there any noval voltage regulators?

AFAIK, gas discharge regulators come in either Octal or 7 pin mini packages. Fabricating rhombic shaped "hole shrinkers", from sheet metal, that adapt the Noval chassis openings to 7 pin mini should be easy enough.

 

[famousmockingbird]

That's what I thought. No biggie, I always like the looks of gas discharge tubes. I am thinking of adding them to my 6HJ5 amp, two 150v (OD3?) in series for 300v.

 

[Eli Duttman]

For 150 V., the Octal tube is the 0D3/VR150 and the 7 pin mini tube is the the 0A2.

 

[20to20]

Four triodes seems like a lot to drive 7189's in pentode mode so any ideas what to do with the extra bottle would be warmly appreciated.

Stereo, yes?

 

[famousmockingbird]

Yes it will be stereo. But for discussion purposes the 4 triodes would be for a single channel. Each channel originally has 4 triodes, two were for a phono stage. I am looking to use the two triodes per channel for something besides a phono section. My first schematic I drew up uses four medium mu triodes. The original design used about 16db feedback, my design uses 20db along with nested feedback. It should be cleaner at higher output.

 

[U130421]

Have a look at Harman Kardon A300