What are terminal 1 & 4 of 300B tube

[Kofi Annan]

Forums were down fir a couple of days, so I couldn't respond.

A pentode has five elements (excluding the heater): the cathode, the control grid (g1), the screen grid (g2), the suppressor grid (g3) and the anode. Pentodes are notciably noisier than triodes, and their high plate resistance means that most of the power supply hum is coupled to the output. I would stay away from pentodes (in pentode mode) for most preamp applications. Of course you can triode-strap them (connect g2 to plate via small resistor), then you just basically have a weird triode (with three grids).

I won't ask any more questions about pentodes. Actually, a am re-re-re-reading Bruce Rozenblit's Beginner's Guide to Tube Audio Design and its starting to make sense. Its still a pretty tough read for me, but I'm making headway. He makes mention of pentodes and how to work with them in a preamp design, so I'll work on my understanding that way.

No, you'd wire 5V across the heater pins (pins 2 and 8), and connect pin 8 to the input of your power supply filter (i.e. the caps, chokes and stuff).

Got it. Makes sense.

Yep, V=IR, the great truth. You'll be using PSUD for the rest of the power supply, so just add that extra RC leg to your simulation.

Great!

OK-- no more easy-questions-for-me-to-ask-but-hard-for-you-to answer questions.

I built a Bottlhead Paramour / Foreplay kit for my first attempt at tubes. I was afraid of the high voltage and knew nothing about them, so I went with the kits to solve some problems I was having in the knowledge department. They're 2A3 SETs and while I like them, I think I would like to try a push pull.

I am not asking for further guidance on push-pull topologies-- I know that this is markedly tougher stuff and I don't want to press my luck by asking more questions, so I will be heads down in books for this one (of course, if you want to...). I am, however, asking why some feel that PP sounds better than SET.

Also, are there any design tools that are worth buying? The software on the Glass Audio website is pretty cheap and intriguing at that (TubeCAD, Push Pull Calculator, etc.).

Also, also, I bought an old Pilot mono tube amp from a TV repair shop and I'm thinking I can use the iron. Is there anything I should check to make sure this is safe to use?

Thanks again,
Kofi

 

[audiousername]

Forums were down fir a couple of days, so I couldn't respond.

Yeah. It's good that diyAudio is back online.

I won't ask any more questions about pentodes. Actually, a am re-re-re-reading Bruce Rozenblit's Beginner's Guide to Tube Audio Design and its starting to make sense. Its still a pretty tough read for me, but I'm making headway. He makes mention of pentodes and how to work with them in a preamp design, so I'll work on my understanding that way.

I don't have that book, so I can't comment.

Got it. Makes sense.

Great!

OK-- no more easy-questions-for-me-to-ask-but-hard-for-you-to answer questions.

All good

I built a Bottlhead Paramour / Foreplay kit for my first attempt at tubes. I was afraid of the high voltage and knew nothing about them, so I went with the kits to solve some problems I was having in the knowledge department. They're 2A3 SETs and while I like them, I think I would like to try a push pull.

I am not asking for further guidance on push-pull topologies-- I know that this is markedly tougher stuff and I don't want to press my luck by asking more questions, so I will be heads down in books for this one (of course, if you want to...).

Think of push-pull as two single ended stages working in opposing phase (well that's what PP amps are...) The major new thing you'll run into with PP amps is the phase splitter. They look weird at first, but you'll get used to them after a while. Basically the most common ones you'll run into are the cathodyne, or split-load phase splitter (they're the same thing), and various circuits based on the differential, or long-tailed pair.

Claudio Bonavolta has a webpage up with a basic rundown of the most common phase splitter topologies here. Actually, his is whole site worth a good look, especially the schematics of a few PP amps so you get a feel of how they are put together. Of course, since PP amps are more complex, there are a fair few ways of putting them together, as opposed to SE amps, where you don't really have all that much choice.

I am, however, asking why some feel that PP sounds better than SET.

PP is actually more linear - the nonlinearities in one part tend to be the opposite of the other part (which is working in opposing phase), so the nonlinearities tend to cancel - in class A anyway. Have a look at the picture below of composite characteristics for a PP Class A triode amp, and compare that with the ordinary anode characteristics of the average triode.... Things aren't so rosy when you head into Class B, so don't go there

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

Also, PP amps may have a different distortion profile to that of an SE amp, and many have argued that PP is bad because it cancels the 'benign' second harmonic. But that is only the theoretical case, with perfect phase splitting, perfectly matched valves, and perfect summing in the transformer. Unfortunately we don't live in a perfect world. Below is the distortion characteristic of VSE's PP-1C amp (EL34 triode-strapped PP). The fundamental was 1KHz, so second harmonic is the peak at 2KHz, third is at 3KHz and so on... You can find out more about that amp and schematics here. I think I recall Allen Wright posting somewhere that it was taken at 1W output, but I'm not sure.

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

Actually, I should have reccomended this before. Steve Bench's Of Loadlines, Power Output and Distortion series (you'll have to look for it on his site - it spans five pages) is an excellent introduction. And he explains things much better than I have - and there are formulas too. You really have to be able to understand a loadline to design something competently.

Also, are there any design tools that are worth buying? The software on the Glass Audio website is pretty cheap and intriguing at that (TubeCAD, Push Pull Calculator, etc.).

I really don't know. Sorry, can't help you there. I don't have any designing software apart from PSUD, and LTSpice - which I don't know how to use. Both of them are available free.

Also, also, I bought an old Pilot mono tube amp from a TV repair shop and I'm thinking I can use the iron. Is there anything I should check to make sure this is safe to use?

If you think that the transformers are of good quality, I suppose you could try. Sometimes things have detreriorated to the point that they're not worth using, but you never know...

On safety issues, I suppose one thing you should look out for is the state of the insulation of the windings. You really don't want your B+ appearing at the output terminals There are probably other things which I don't know about and maybe others could help you out there. I really don't have much experience recycling old stuff.

Thanks again

Glad to be able to help

 

[pingfloid]

Is it a valid alternative to the hum pot?

I've made my 300B amp based on the JeLabs and, since I don't have a 50 Ohm/2W hum pot, I have connected the cathode resistor from the centre tap of the 2,5-0-2,5 V ac transformer to ground. (as they show at the schematic from the "SE Amp CAD" software).

I still have a very low 50Hz hum, so I am wondering if it's really worth to buy the 50 Ohm/2W pot. ¿Do you know some supplier in Europe?

perhaps the best I've come across for the basics, and not so basic, is Morgan Jones' Valve Amplifiers

It's a good book, but this forum have an amazing didactic value. Thank you very much

 

[audiousername]

Re: Is it a valid alternative to the hum pot?

I still have a very low 50Hz hum, so I am wondering if it's really worth to buy the 50 Ohm/2W pot. ¿Do you know some supplier in Europe?

Sorry, I don't know of any suppliers in Europe. Actually, I don't know of any suppliers for these in Australia

Using the CT of the filament transformer as the cathode connection has been done before, it's just that it doesn't allow fine adjustment. Oh, and the signal has to go through the windings of the filament transformer, which some may object to.

Thank you very much

You're welcome

 

[pingfloid]

Thank you Jason. I will probably order the pot from Angela and don't let my signal pass through the supply transformer anymore. Too many turns may make it feel dizzy

BTW: is it normal to have a blue light at both sides of the 300B? It's beautiful, but I don't know if it means there's something wrong. Rebel electrons want to escape from the bulb?

Cheers

Manuel

 

[audiousername]

BTW: is it normal to have a blue light at both sides of the 300B? It's beautiful, but I don't know if it means there's something wrong. Rebel electrons want to escape from the bulb?

Hi Manuel,

I don't know.... I actually have never worked with the 300B personally! As long as it isn't the soft purplish glow associated with a gassy valve I don't think it should be anything to worry about.