Checked eBay for the 7687. $40 for one. $149 for five. All Dumonts. How is that less than a 7199? I'm going with 12DW7. Still in production all triode.
I got mine for about $ 8.00/pce on the same site some time ago, but in auction format not inflated dealer B.I.N price. $40.00 is way too expensive, the 7687 is currently listed at $23.35 in the A.E.S catalog, which is still cheaper than a genuine (RCA) 7199.
Dumont branded 7687's are actually made by SYLVANIA.
N.O.S 12DW7/7247's are about $20.00/pce but usually requires more extensive circuit modifications/redesign. For a new design I would certainly go for the 12DW7 because it is still in production. For restoration/upgrade purposes the 7687 is the best choice to replace the 7199.
Tublogic,
I want this amp to be low maintenance. So I'm going to use new ST35 boards with new 12DW7's.
I want this amp to be low maintenance. So I'm going to use new ST35 boards with new 12DW7's.
Back in 2011, Tom Bavis said "In most cases, the 6GH8 has worked well with the original resistor values, 6BL8 has required a little adjustment."
Can someone patiently explain what the adjustment criteria would be in, say adjusting a circuit using a 7199 amp/phase splitter (like the old RCA amps) to use a 6BL8?
A few years ago, I accidentally threw out a tray of NOS 7199s #@$%&!!, but at least I still have a few 6GH8s.
I also have some used Toshiba 6GB8s -- anyone done anything interesting with them?
Can someone patiently explain what the adjustment criteria would be in, say adjusting a circuit using a 7199 amp/phase splitter (like the old RCA amps) to use a 6BL8?
A few years ago, I accidentally threw out a tray of NOS 7199s #@$%&!!, but at least I still have a few 6GH8s.
I also have some used Toshiba 6GB8s -- anyone done anything interesting with them?
Rich, thanks for the circuit. What is the adjustment criterion for the 1M pot in the screen grid resistor chain -- is it for 35V at the screen, or is it for 110V at the anode?
cheers
Ritchie
cheers
Ritchie
I used a 6AU8A in my Dynaco Mark IV,and it did a really nice job,and it is a very inexpensive tube as well. With no curcuit changes at all,except for pin re-assignment.
110V or so on the pentode plate. It's a direct coupled circuit, and we're trying to find the best balance between (1) maximum output swing and minimum distortion in the pentode and (2) maximum voltage swing and minimum distortion in the triode phase inverter. When the power supply is too low or you need more output swing, it's best to revert to AC coupling and bias each stage separately - three more components to fit around the socket. It's also possible to bias the phase inverter with a resistor divider for "offset" DC coupling.
Thanks. I'm trying to learn how to set up pentodes -- could it be done also by having the screen resistor fixed and varying the cathode resistor?
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