Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bruno on May 02, 2012, 05:47:08 am
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Anyone here tried building an amp with a direct coupled gain stage?
I'm interested in trying one, to check the results.
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Do you mean in cases other than d.c. coupling from a plate-loaded stage into a cathode follower?
5F6-A Bassman (http://www.el34world.com/charts/Schematics/files/fender/BASSMAN_5F6A.pdf)
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Hi HBP,
I'm refering to DC coupling as shown in Merlin's book... coupling two gain stages directly, no coupling cap, the previous stage affecting the bias of the following!
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(http://www.freewebs.com/valvewizard1/DCCF2.jpg)
If that is the one you're referring to, Geezer's HoSo amp (which I also built) does exactly that. It sounds great. :smiley:
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That is exactly what HBP indicated in the 5f6-a schematic... as anyone tried doing the DC coupling to a regular gain stage, rather than a cathode follower?
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Probably no, without a negative supply handy (the bias supply may or may not be suitable).
Why? You can't use the coupling cap to block the first stage's plate voltage from the second stage's grid. Since the grid will probably be (in a preamp stage) near 1-3v more negative than the cathode, that means the 2nd stage's cathode voltage has to be a couple volts higher than the 1st stage's plate voltage.
Same problem applies to the 3rd stage, but now it has to be that much higher than the 2nd stage, and twice as high compared to the 1st stage's plate.
The cathode follower naturally will have it's cathode close to the prior stage's plate voltage, due to the large cathode load. The direct coupling actually causes the tube to settle to an operating point where the cathode is slightly higher than the grid voltage.
Design of a 3-stage amplifier using dc coupling throughout is a major feat of engineering; you can pretty much forget most hobbyists going for 4 or more stages, without copying a known good plan. There are also few of those known-good plans, because the hassle usually outweighs any benefit obtained from eliminating the coupling caps.
Note also that you lose the single biggest tone-shaping mechanism found in your amp.
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As usual the HBP, thank you. I got curious, because other then the obvious DC coupled cathode follower, I had never thought about dc coupling stages until reading Merlin's book. A whole preamp with dc coupled stages would be a tough job for sure, thanks for the interesting reply!
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Take a look on the AX84 site. There has been a lot of work done with DC (resistive coupling). Search DC Drive, DC-Drive, Melissa, Raffler. Most of the activity on dc drive was several years ago.
http://www.ax84.com/bbs/index.php?area=-1 (http://www.ax84.com/bbs/index.php?area=-1)