300B schematic recommendations

[Ketje]

I would go with the first schematic, it's a bit simpler. You could build a stereo amp. The power transformer should be capable of at least 250mA. However, if you are going to use a GZ34 rectifier, you should reduce the HV secondaries to 375-0-375, otherwise your HV will be much too high.

If he want it simple, that first schematic can made even more simple.
Mona

 

[Wavebourn]

No reason other than that I've seen your posts mentioning local/nested feedback and I wondered what you'd do if starting this circuit for yourself.

For myself I prefer TV or military pentodes.

300B is already pretty linear, it has low internal resistance, and needs already huge voltage swing to drive. If you remember, I presented here an amp without any feedbacks (well, except servo in a gyrator). There are many ways to skin the same cat. Either to use textbook schematics with awfully expensive tubes, or design schematics using modern inexpensive tubes.

Is that the 500pF?

Yes.

 

[45]

If he want it simple, that first schematic can made even more simple.
Mona

Much better results IMHO can be obtained with RC coupling between the first and second stage. The idea of DC coupling these two is not bad in general but doesn't work well with 300B. The reason is limited swing and limited (undistorted) power you can get out of it. I made the "same" three stage amp, on budget, RC-coupling the 1st and 2nd stage and using a PCL 82 instead of the 6SN7 + fixed bias with 420V plate voltage. I got 12W clean. Also used 6 dB overall feedback to get really low distortion for such kind of an amp.

 

[Bigun]

I agree with Waveborn that there are many tubes that can be good when triode-strapped to give very linear results so I would be thinking that the purpose of using a 300 B is indeed to get the flavour of that tube which means not using Feedback because that tends to linearize the flavour away ??

as an alternative idea why not a 6SL7 as the input and voltage gain tube which is RC coupled to a source follower, in turn DC coupled to the grid of the 300 B. The source follower is loaded on a choke so that it doesn’t need a negative supply rail. I was looking at toapology like this in my Krakatoa thread.

 

[Hearinspace]

For myself I prefer TV or military pentodes.

Well, that brings me to a question I've wanted to ask you for a while. Somewhere you mentioned that your Gubernator-71 was a really good amp but that you preferred one of the smaller amps you did. (I forget now, Was it the Edelweiss?) I wondered why. The file you posted of the GK-71 amp sounded impressive, and I wondered if you liked the other because of lower voltages.

If you remember, I presented here an amp without any feedbacks (well, except servo in a gyrator). .

I don't remember. Can you remember where you posted it or what you called it ?(so I can search it?)

Thanks . . . . . again.

 

[U130421]

The circuit you posted on post 18 is the one approaches the original W Electric version. The bootstrap capacitor requires separate pair of resistors 100 ohm each across the filament . The rectifier tube effects on the high frequencies character. Shuguang has the replica of WE274 .

 

[Wavebourn]

Well, that brings me to a question I've wanted to ask you for a while. Somewhere you mentioned that your Gubernator-71 was a really good amp but that you preferred one of the smaller amps you did. (I forget now, Was it the Edelweiss?) I wondered why. The file you posted of the GK-71 amp sounded impressive, and I wondered if you liked the other because of lower voltages.

It was a great amp, actually, but dissipated constantly more than half of a kilowatt, and used GK-71 military pentodes that are not in current production. Downsizing it to Edelweiss - 3 I covered more of available tubes, since speakers with sensitivity 93 dB and higher are available in abundance. It is a commercial project, I am going to manufacture Edelweiss-3 line, not just a power amp, but also an integrated amp, planar headphone amp, preamp. Probably even a preamp with phono input for MM and MC pickups.

I don't remember. Can you remember where you posted it or what you called it ?(so I can search it?)

Here you go:
One more 4P1L SE

Thanks . . . . . again.

You are very welcome.

 

[Sunwontrise]

OK

so the biggest problem is, that the 300b need a potent driving stage.

The mentioned 6SN7 circuit from diyaudioprojects isn't enough?

A more complicated circuit would't be a problem - for example the one mr. Wavebourn mentioned (ef86 into 12bh7)

@Wavebourn - Am I right, that the 5V 300b heater wiring is directly connected trough the 300b's filament? No hum pot, no 880ohm or 1k 50w resistor?

I found a website with a lot of schematics:
http://www.simpletube.com/databank/circuit_map.htm

Thanks, and have a nice day!

Felix

 

[U130421]

It is only according to Rangon that 6SN7 double triode is not enough. If you are looking for high gain low output impedance here is mine . Direct driving a 300b . With pentode, the output impedance is just enough for 20khz, but the sound is much warmer .

 

[euro21]

I've been using this for years ... but this is not a beginner project.

 

[waltube]

Also this one run well. The E88C is a single triode and you can find some Siemens gold pin at reasonable costs. In fig. 1 the response with 450 mV in and 20 Vout of the input stage. The Zout is around 1,3 kohm The OT is 2400 ohm prim and 6 ohm sec, made by Fiat in Roma. Fig. 11 is the response at 8 w - 8 ohm load

Walter

 

[RajkoM]

Bela,
did you try your amplifier with 3k or 3k5 OPT?
Just asking.

 

[OldHector]

FYI the information written on the transformer should read 18-17-16 for 0 ohm, not 17-16-15, but you probably spotted that.

 

[euro21]

did you try your amplifier with 3k or 3k5 OPT?

No, I prefer higher impedance OPTs (4k4...5k) in this amp.
Power is adequate (over 10W -clipping- in A2), distortion is lower (0.35% at 1W, below 2% at 10W).

 

[Sunwontrise]

I already have two output transformers: 2,5k or 3,4k

Cheers,
Felix

 

[RajkoM]

Thanks Bela

 

[Ketje]

It is only according to Rangon that 6SN7 double triode is not enough. If you are looking for high gain low output impedance here is mine . Direct driving a 300b . With pentode, the output impedance is just enough for 20khz, but the sound is much warmer .

That's a gain of 115x thanks positive feedback.But with only ½mA in the output not much drive capability.
Mona

 

[U130421]

You can listen the sound about 7wmax .
YouTube
The positive feedback just cancels the negative to get 0v ac on 1.2k , by this no need for bypass capacitor. Note the output is not symmetrical providing more negative than positive about 30%, increasing the output excursion hence more power less distortion.

 

[Ketje]

The positive feedback just cancels the negative to get 0v ac on 1.2k , by this no need for bypass capacitor.

No, it's over compensating.
Simpel bypassing the Rk gives gain 80x and with the feedback you get about 115x.
Mona

 

[Sunwontrise]

I've found another two schematics, c3m or c3g should be a good driver for 300b?

An externally hosted image should be here but it was not working when we last tested it.

what do you think?