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A question about preamp stages
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Author:  kitchi [ 06 Jul 2019, 20:16 ]
Post subject:  A question about preamp stages

Hi peeps
I was just wondering what are the pluses and minuses on a pre-amp using a srpp or a single or 2 stage design in a vacuum tube amp. I am thinking of using a 12a_7 pre-amp circuit driving a 6l6 single ended amplifier. These are the tubes I have on hand and would like to use them.
I have the output transformers from edcor and a power transformer along with choke coils.So I am ready (finally) to start assembly as soon as i make a decision on what preamp design to use.

Author:  Woodo [ 06 Jul 2019, 21:00 ]
Post subject:  Re: A question about preamp stages

Not sure if this is of interest but this very promising project proposes to use a 6L6 at least although a different pre-amp section to what you propose. See @suncalc viewtopic.php?f=9&t=6526

Author:  gofar99 [ 06 Jul 2019, 21:41 ]
Post subject:  Re: A question about preamp stages

Hi, IMO all have pluses and all minuses. For simplicity a SE single tube stage is the easiest and there are lots of plans for them. Depending on how much gain you need. For 7-20 a 12AU7 is fine. up to about 50 a 12AT7 and about up to 75 with a 12AX7 (practical values not book ones). Multi stage SE sections are good for lots of gain unless the final stage is a cathode follower. Then the previous values are close as the CF will have a "gain" of about 0.9 or so. I don't recommend that for a driver stage. It is fine in a line stage as it provides a low impedance output impedance. SRPP stages that I favor have roughly half the gain of the SE ones. In a line stage this is really not an issue as anything over about 3X is just fine normally. They do require more tube sections and unless you know what you are doing can be tricky to get right. They were originally designed as TV high power drivers and can lend themselves to audio fine. But IMO not as high power devices as the linearity in that mode is not all that wonderful (think along the lines of awful). Used as voltage amplifiers however they can be extremely linear and low distortion. I find they make great voltage drivers for tube amps and very nice line stage preamps as well. You do need to pay attention to the idle current vs tube (anode to cathode for each section) voltage to get the best swings. Often design charts will be misleading as the lower section that provides the gain is actually operating at about 1/2 the total B+ applied to the stage. With an anode B+ on the upper section of about 275 V I can easily get a 60 volt drive to a power stage. I use SRPPs to drive KT120 as well as smaller tubes. The down side is that you need to use a non-grounded heater supply and preferably one with a DC reference voltage applied to it at about 1/3 the upper anode B+. Otherwise it is likely to have the heater to cathode voltage exceeded and then tube failure. Such failures can damage other equipment as they usually cause a huge spike that is amplified by all the following stages. Think along the lines of a full power spike with a very fast rise time. Often repetitive. I find that as tubes age the spec value for the H-C is invalid. It really applies to new tubes only. A side benefit of the reference voltage is that it tends to scavenge stray electrons from the cathodes that if left alone increase the noise level by between 1 and 3 db. This is not an issue in line stages, but a huge deal in circuits like phono preamps.

Good listening

Author:  kitchi [ 07 Jul 2019, 11:05 ]
Post subject:  Re: A question about preamp stages

Thank You for the link to a different thread Wodoo it was a great source of info.

And Thank You Gofar99 for the clear write up you made on this thread.

Both of your comments gave me a bit more to consider. I am leaning toward not using the srpp and using a single stage or a 2 stage preamp..

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