• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

300B SE with a DHT driver

My experiments with the 300b have so far fallen short of my preferred topology of 26 with plate choke into PSE 4P1L in filament bias. I've been using indirectly heated drivers to achieve a 2 stage 300b amp, and it's not quite doing it for me. I'm going to have another shot at this - the goal is a 2-stage amp with some kind of DHT driving the 300b. I don't want 3 stages, just 2 stages after my 2v DAC.

One possibility is using a DH pentode. I'll put that on the shelf for now and consider whether it can be done with a triode. Contenders I have in stock are:
- 26 (my go-to driver)
- 4P1L (lower Ra)
- 10Y (a touch less gain but excellent)
- 46 (probably not enough gain here)
- 2P29L (prefer 26, but it's OK)
- 12A (another OK driver)
- 3a5 (wild card..... mu of 15 but Vmax 135v)

This is going to need a transformer step-up on the input. I have a couple of LL1515, which if they are the same as the LL1530 should be specced for +10dbu. They can work in 1:7 or 1:3.5. I have 2v out of my DAC so not sure if there's enough headroom there. I also have a couple of LL1540, +40dbu, but they would only be 1:2. I figure I need at least 1:3 or more depending on the driver. I have a couple of LL1689/PP but they're for headphones right now, and they'd be overkill. I also have a couple of LL1635/PP but those would only give me 1:2. The transformer of choice would be something like the LL7903, but that's an expensive experiment.

It may be possible to steal a bit of gain from an interstage after the driver, going into the 300b. I prefer plate chokes, but would consider this if it didn't compromise the 300b. I have LL1660/18mA

There would be some sort of solid state solution, but I don't build with solid state. I'd consider it if I had a schematic I could work from.

So there we are. How to make a 2-stage 300b amp with a DHT driver? I kind of think it's possible, but it needs all the combined brains on this forum - a very special resource - to help me figure out how to do this. Even better if anyone has tried and succeeded, or tried and failed!
 
Consider EML20/30 series. Yes, they are expensive, but the quality is better than most old DHTs, IME.

Also, you may save a fortune on transformers by using it, as the 1:2 at the input you already have is probably enough, and a choke load for the driver should work, too.

The EML30A promises a gain of x38 for 480V 10mA and effective load of 9.3KΩ, if you choke can satisfy that.

EML-20A-20B-30A Data sheet. By Emission Labs.

300B can sound really marvellous - but the driver, and all of the power supplies must be first rate, and blocking-distortion must be attended to.
 
Member
Joined 2004
Paid Member
My measurement: EML20A cCCS loaded, 150Vpp.
 

Attachments

  • EML20A 150Vpp_jobb.jpg
    EML20A 150Vpp_jobb.jpg
    64.8 KB · Views: 2,333
looks very good , but at 600€ for a pair i will play with your vintage DHT just for the fun
- 26 (my go-to driver)
- 4P1L (lower Ra)
- 10Y (a touch less gain but excellent)
- 46 (probably not enough gain here)
- 2P29L (prefer 26, but it's OK)
- 12A (another OK driver)
- 3a5 (wild card..... mu of 15 but Vmax 135v)

i have tried the 46 in driver and ouptut tube mixed with a 2A3 as driver
and did'nt like them in all position , perhaps the fault was on the small 5k hashimoto OT .... or it 's just that i don't like 45 type of sound. and prefer more rounded one like 300B or AD1 .
you could try your favorite 26 as i do now if you can live with 1W :)
 
Hello Rod!

Aha....... that's an expensive solution. Wouldn't transformers be cheaper? They don't wear out either - my audio is on 24/7 since everything goes through it, so in terms of longevity I'd have to give it to the transformer solution. Nice idea, though, and it would work!

How about some suggestions for a step-up input transformer? I always think of Lundahl here, but there are plenty of other manufacturers that make nice input transformers. Also, any suggestions for which DHT would work better with a step-up before it? 4P1L and 26 are the short list, 12A maybe. 10Y is possible, but I'm basically hoarding the ones I have as an investment.

Audio transformers • Line input transformers • Lundahl Transformers

Audio Transformers • Microphone transformers • Lundahl Transformers

Line input and line output audio transformers • Lundahl Transformers
 
Member
Joined 2004
Paid Member
As I wrote https://www.diyaudio.com/forums/tub...or-transformer-interestage-4.html#post6159334 in post #33, using two stage DHT amp without step-up transformer is very difficult, because high/er/ gain DHT tube list is very short to drive -for example- 150Vpp.

20, 30 very expensive (now only EML design these tubes), 841 is very rare.

I tried 20a, usually use 841 (I bought these, when price wasn't such horrible), but -as Rod wrote- without proper designed driver structure these VAS tubes can de used limited.
 
300b requires a non linear driver as 310a conceived specially for it. It can drive with +80v -120v.

got a marvellous sound with a WE301a in pentode mode two days ago , best tone you can have with the 300B . but more noisy and soft sounding with the 91k plate and load on the driver. i love it but i understand why others hate it.
running now the 26 the sound seems more direct more 3D, less colored than with the pentode .
what a pity to not have a distorsion analyser on hand :D
 
Would this article on "mixed bias" using 300B tubes be of any use to eke out as much performance as possible? Split Williamson Amplifier & Class-G
Quote: "In a power amplifier that holds low-mu tubes, such as 2A3, 6AS7, 300B tubes, in the output stage, mixed bias works well, as we are able to preserve more of the vital B+ voltage. In other words, we waste less voltage across the cathode resistor."
 
Last edited:
I've been exploring the 4P1L as a driver into my 4P1L PSE outputs. Replaced the 26 to see what it sounds like. It can be a trifle sharp in the treble and using 2 amplifies that, which is why I preferred using 26 as a driver. But going back to the 4P1L (which I tried a few times before as a driver) it is certainly very detailed, more so than the 26, and that's a bit of a revelation. It's also at its best with filament bias. And should sound good into a 300b if i can get the step-up right.

In response to audiowize suggesting that the 300b isn't a good choice for an amp on 24/7, I would agree that 4P1Ls are considerably cheaper and I have a lifetime supply of them. I just happen to have around a dozen 300b in various states of disrepair, so they are welcome to live out their last days giving me music.

I think the next step is setting up the 4P1L as a driver into the 300b so I can start experimenting with step-up transformers. I use a modular system so I can easily put together the top plates required. Hope to have some results soon.
 
I think my fall-back option could be using my LL1540 in 1:2 and then using 2C22 as a driver valve. Yes - that funny 6J5 with 2 top caps. It was the best indirectly heated driver valve I had ever used before going all-DHT.

Hope to make some progress today. Assembling the pieces on a new chassis.

you change your mind to fast ! :D what about the 2P29L as driver a little more gain than the 26 but much more power to drive the 300B grid .

how about his sound ? :)
 
I have built this:
- Sowter TVC wired for 0/6/12dB gain, switchable
- 4P1L
- LL1692A wired SE-PP, 1:1.15 gain as I remember
- PP KT88, fixed bias -35V

The gain structure is barely enough to drive KT88's to full swing when Sowter is switched for +12db
My thinking is that when I don't need that power I switch the TVC for lower gain and hence better bandwidth

300b's will need twice the drive, looks like too much for 4P1L. What impedance would transformer present at that much step-up ratio?
 
Im toying around with this schematic. I haven't built it yet, and im not sure I ever will considering that its still a potential fire hazard if something goes wrong.

The idea is that the current through the 10y along with the DCR of the plate chokes will create the correct bias voltage for the 300b. This should allow you to direct couple the 10y to the 300b and eliminate blocking distortion without using really high voltage.

The catch is that if the 10y is removed, then the cathode and the grid are effectively shorted together and the 300b along with the plate choke will probably melt. Adding a CCS to the PSU solves some of the risk so long as the CCS can handle the dissipation in case of a fault, but it is FARRRRRR from ideal. Im still searching for a better way to make this circuit less of a fire hazard. If successful, I could see this being a really neat 300b amp that may solve your sonic issues.
 

Attachments

  • 300b totem pole diy audio.jpg
    300b totem pole diy audio.jpg
    313.8 KB · Views: 1,084