417A-300B Plate Choke or Interstage Coupled?

[sgerus]

After reading 100’s of posts on the topic of input/drivers for a 300B I have come to the conclusion that there may be a better valve to drive a 300B than the 6sl7 or 6sn7.

So, with that being said, I’m thinking of replacing my 5691 input/driver stage of my 300B amp with a 417A.

Question: Which configuration do you recommend?

1. 417A interstage coupled to the 300B
2. 417A with a plate choke cap coupled to the 300B
3. 417A with a constant current source cap coupled to the 300B

Thank You

[i_should_coco]

417A coupled by your favourite method to a 46, transformer coupled to a 300B.

Just my 2p.

[kevinkr]

Depending on who made your 417A/5842 mu is between 43-50, unfortunately the 417A is not going to swing enough voltage to drive a 300B to full output directly. Here an IT is your only recourse and to do it a ratio of at least 2.25 seems indicated - higher values resulting in better linearity at the expense of HF extension. (Higher reflected source Z driving the 300B grid.. IMO anything much > 10K is going to hurt the HF extension with 1:2.25 it's already around 8.6K) The 417A/5842 is an almost ideal IT driver for an application like the 2A3 or 45 where the drive voltage requirements are more modest.

Consider a 6CK4 driving an IT, it will swing at least 400Vpp clean on a 300V supply with a 1:1 IT.. Mu of 10, so it will not be hard to drive..

[arnoldc]

Hi,

I have experience with #2 and #3.

My 417A-45 uses plate choke (custom made) on the driver, cap coupled to the 45.

My 417A-300B/10Y/50 uses diyAudio CCS on the plate at 10mA, driving any of the three output tubes I mentioned. The output tube has a grid choke (for the 50 actually). In 300B mode, I run it at a modest 6W.

I like them both.

ps.

My 45 amp uses WE 417A, and my 300B/10Y/50 amp uses Amperex gold pins 5842.

[tubelab.com]

Depending on who made your 417A/5842 mu is between 43-50, unfortunately the 417A is not going to swing enough voltage to drive a 300B to full output directly.

That is news to me. I use a CCS loaded 5842 to drive a 300B (or 45 or 2A3). The later amplifiers (all of the TubelabSE boards) include a mosfet buffer to allow A2 and eliminate transient overloads. Early in the design phase I tried conventional resistor loading of the 5842, and I could not drive a 300B into clipping.
Schematic:

http://www.tubelab.com/TubelabSE.htm

I run the CCS directly off of the main power supply and set the plate voltage on the 5842 to about 175 volts. I can drive a 300B into clipping directly from my CD player on some CD's. On other CD's there is not enough gain to reach clipping without a line stage, but there is plenty of drive voltage available.

[kevinkr]

Hi George,
I'll admit to being a perhaps little more conservative in their use than you are.. I generally run them at 150V and below (125V typical) based on the specs I have and what I had to pay for the last few I bought (kinda expensive for such a little chunk of glass ) - at these voltages the drive versus distortion seemed a little marginal in the samples I had on hand. (Raytheon) Unfortunately details no longer to hand..

[sgerus]

Thank you all for the replies…

Last night (before reading these replied) I did a load line plot of the 5842.
Looks like best case is about 1V=27V based on using a 12mA ccs.

With a coupling cap or 1:1 interstage I would need 2.1V to drive the 300B at -61V
This might work in my case (running off my pre-amp output), but I would prefers an input sensitivity of around 1V in order to run the amp right off an mp3 player.

I guess the answer to my original question really depends on what input sensitivity you need.

Lesson Learned: Don’t trust every schematic you find in the net…. I asked on question based on a schematic using a 1:1 it driving a 300b at -74V

I’ll check into the 6ck4 (and the 6aq8 as in the electra-print directive reactance drive 300b)

In general, would you guys agree that moving to a high current tube is a step in the right direction?

[tubelab.com]

Mine are all Raytheon 5842's. With 175 volts on the plate, the voltage swings to about 300 volts on the positive peaks. I have not seen an issue with this in about 20 amplifiers.

A few years ago I was given a truck load of "worthless TV tubes", and another truck load of military surplus tubes. Most were indeed junk, with about half physically broken. There was a box with about 100 Raytheon 5842's. I have been using them in amplifiers ever since. A CCS loaded 5842 is one of the most linear tubes that I have found. Since the audiophile community has discovered this tube the price has risen sharply, and my box is getting close to empty.

I have tried dozens of other tubes to find another tube with similar characteristics. A few have come close, but none have matched the 5842.

[kevinkr]

I'm totally envious, I have just six 5842 and while they are still cheaper than a 417A they ain't cheap..

[protos]

Hi tubelab,
Your powerdrive circuit is going around in my head for a year now but other projects have taken precedence..I tried the ccs loading and I liked it.But ccs loading is somewhat more acceptable from a purist point of view.I wonder how running the signal thru a mosfet follower impacts the sound compared to normal tube driving.OK I understand your point about most tubes not having enough current to drive A2, but what about the rest of the time?
Do you only listen to rock music/live guitar for evaluation?
By the way another question:when is a tube suitable for A2 drive?I guess certain grid parameters are involved.Has anyone tried 6c33 with A2 and is this ok?
An alternative to 417 and 5842 would be the 6c45p probably.I tried this as a (ccs loaded) driver for 6c33 but it just misses swinging enough voltage in practice to drive a grid of around 90vpk.