300b lovers

@charles1dad - Thank you. 

@atmasphere - One of the reasons I was thinking of using single section tubes was to improve matching. I've noticed this section matching problem in LTPs I've been playing with, particularly with some of the soviet tubes (e.g. 6N6Ps) where the quality control seems to be lacking. 

Are there any tubes that, in a PP IT loaded design, would be reasonably linear and provide enough drive for a single 300B without a cathode follower stage? 

I'm confused why a bipolar supply would help. I'm assuming I'd need to have some kind of negative supply for the fixed bias circuit which I would also use for the LTP CCS connection, but I would expect this to be around negative 100V. 

Alternatively, I could use a balanced input transformer with the secondary center-tap tied to a positive voltage to allow enough voltage across the CCS tied to ground.

Are there any tubes that, in a PP IT loaded design, would be reasonably linear and provide enough drive for a single 300B without a cathode follower stage?

@jaytor Sure! We are assuming that you plan to use a high efficiency speaker so you’re not also looking for a lot of gain. But I have to get some clarification- do you want the ability to have enough drive for the power tube, along with enough voltage gain at the same time? They are not always the same.

Tubes that can swing the voltage needed for the 300b include the 12AT7, 12AU7, 6CG7, 6SN7 and so on. I would stay away from the 6DJ8 and similar frame grid tubes; while they are very linear and can handle the current you need, they aren’t very good with microphonics as they are not meant for audio.

A bipolar supply supply is helpful because there is always a bit of differential performance left on the table by any CCS. The more negative the supply, the higher the impedance of the CCS can be (which is a good thing). You can, with modern semiconductors, easily built a good CCS that will work on -100V, but If you are already building a B+ supply, its really not all that hard to build a B- supply from the same transformer. I like to use a separate power transformer for the driver and voltage amplifier, so as to minimize noise appearing in the power supply of the output section from messing with the rest of the amplifier. This helps reduce IMD.

Here’s the latest: Don is training the folks in Salt Lake City as we near production.

The Raven preamp is the same preamp you heard at the show, while the Blackbird now has KT88 drivers, at twice the power of the 6V6 drivers, with third-generation Cinemag interstage transformers and Monolith output transformers, If you’re wondering who Monolith is, you heard them in the Songer room on the Whammerdyne amplifier.

Merry Christmas, Happy Hannukah, and a Happy New Year, to all!

Hi @lynn_olson ,

I have a couple of technical questions:

1. Is it a good idea to feed indirect heated tubes AC filament with a separate dedicated power transformer? In my integrated SET 300B amp input tubes receive relatively low level signal after the input potentiometer.   

2. I use interstage transformers and separate PS for driver and input tubes. As a result, I have two separate GNDs: 1st - for output circuit and PS  2nd - for input/driver  circuit and PS. Should I connect both these GND together to the amplifier chassis at one point? Or can I leave one of these GND floating?

1. If the heater windings are not shielded from the high-voltage B+ windings, switch noise from the B+ section can be capacitively coupled to the low-voltage heater windings. Although indirectly heated tubes have significant isolation from line noise, it is not 100%, so some of that switch-buzz gets through. If you listen closely, you will probably hear it.

There are several solutions: A) get a power transformer with electrostatic shielding (copper foil) between HV and LV sections B) go to DC heating with a regulator C) use a separate heater transformer.

When none of these are done, yeah, that’s cost-cutting, no matter how fancy the name brand. If swapping the rectifier tubes makes a notable difference in the sound of the amp, there’s something wrong, or not optimal, with the power supply design.

2. Yes, the two grounds must connect together at one point, and only one point, preferably the chassis. There’s an old joke: when two engineers get together, they will come up with three grounding solutions, and all of them will be right.

Grounds (and shields) should never float, ever. That’s bad practice and a possible safety hazard. Determining the best connection for the ground or shield usually takes a bit of thought (which is why it’s called engineering).

There are whole books on grounding. It is not a trivial subject. Part of the problem is language, because one word covers several things:

A) Safety ground 

B) Current return path for the DC currents that power the tubes or transistors

C) Current return path for the AC audio signals going through the circuit

D) RF and noise shielding