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    Building, troubleshooting and testing of these amplifiers should only be
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300B SE with a DHT driver

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#341
You're welcome. The eTracer is really convenient to figure out "what's going on". Even with things like hacking together a version of the Silbatone NeoHyb, plugging it into the tracer and getting all info needed to turn it into a working amplifier. I use it a lot for these "what happens if" situations 😉


What was your operating point again? The 1k Ri was at 74mA.
 
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#343
801a driver

HI Andy,
I know you keep yourself away from fixed bias, but here is a good driver I'm currently working on which is performing extremely well. Pure iron and no bypass cap on the cathode as it's with fixed bias. Filament bias isn't an option as the bias is 25V.

I use an input SUT (LL7903) in 1:8 step up. Fixed bias in the secondary driving the 801a. The 801a is at 340V/20mA. Interstage 1:1 into output stage. The interstage is custom-made.

I'm getting amazing performance so far. 0.1% THD (mainly H2) at 250 Vpp. That is what I'm looking to use with a 845/813 output stage. For 300B, the performance will be much better as you will keep it below 200Vpp.

With one DHT (and one of your preferred ones) and 2 transformers you get a gain close to 35-35.5dB.

This will be my choice for the next amp....
Cheers
Ale
 
#344
Hi Ale - that's a really good idea. Seems obvious really, but you have to actually think of doing it. I'll keep that idea as a possibility. I don't have any LL7903 at present.

This thread has taken me in several unexpected directions. First one was using an op-amp on the input. I wouldn't reject the idea - the sound was quite good, and it's a usable option. The second unexpected direction was using an indirectly heated valve with a mu of around 18-20 as a driver. That way I discovered some really useful valves which sounded good, and the EL12n turned out to be the most useful.

The real breakthrough was dumping the 300b. I'd done that already 8-10 years ago in favour of PSE 4P1L because they had a mu of around 11 and they could be driven with another DHT (just about). So it then occurred to me to switch the indirectly heated tube and the DHT. And so was born what I call the "Inverted DHT Amplifier". The DHT is no longer the output, it's the driver. And bye-bye op-amps, no longer needed.

So now the EL12n lives in the output stage and I have a choice of DHTs to drive it. what I'm finding is that it's the DHT driver that has the most influence on the sound. If I use a 10Y the amp sounds like a 10Y. If I use an 01A the amp sounds like an 01A. I'm enjoying my "Inverted DHT Amplifier". It's not perfect and there's more experiments to do, but I've found a new solution to an old problem. This has been a very enlightening journey!
 
#345
Andy,
As you have moved on from a dht driving a dht, and have considered op amps, have you considered other hybrid topologies?
Check out the Silvercore blog
Der silvercore “Collectors Choice” | silvercore Blog
They write "JC Morrison, Dave Broskie and Frank Blöhbaum invented it at the same time, the principle of the “Supertriode” or the “NeoHybrid” circuit"
I'm sure Ale would be able to help with the design.
Stay safe
t
 

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#347
Someone help me out here. I thought one of the big gotchas with the 300b tube was that it draws some grid current. I know all triodes to some degree do draw a minuscule amount of grid current, but I seem to remember that the 300b draws more? Or that somehow it starts to draw a decent amount of grid current when it approaches the 0 line?

I know I read an article about it years ago, but I can't remember the specifics. But I do remember it saying that one of the Achilles heels of most 300b amps is that they don't take grid current into consideration.
 
#348
If your design is allowing for grid current (indeed when approaching the 0 line) you better incorporate a drive that is able to deliver power without getting upset. Most stay away from that point and accept 0,5W less output power or simply use an interstage transformer like in the old days. Slew rate limitation, blocking distortion and harmonic distortion are involved at this point as well.
 
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#350
EL3n, as far as I can see, is the same as EL33. I have some of those and they do indeed sound nice. I was using them as drivers with the op-amp stage in front. At some point I'll probably go back to them with a 1:2 step-up on the input. I have LL1554.

I also have some AC2 as drivers for the 300b I want to try, Seems they sound nice. Mu is 30. Just about enough.
 
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#353
suwaned said:
...the more I follow this thread and more confused I become.
Click to expand...

I fully sympathise with you! I started the thread with the belief that I would use a 300b as output. My challenge was to use a DHT as input in a 2 stage amp. Well, clearly the challenge was that there wasn't enough voltage gain for that to happen.

So part 2 of the exploration was adding gain. We talked about:
1 - a step-up transformer (SUT) on the input (Ale's choice)
2 - an op-amp on the input (my idea, worked OK but not perfect)
3 - a pentode driver (supported by several)
4 - dropping the idea of a DHT driver and using a conventional C3g etc.
5 - using an expensive boutique DHT with more gain (OK if you have the $$)

I then came to a crossroads. I decided to drop the 300b and keep the DHT driver. I quickly realised that for my own system I needed an output tube with a mu of at least 15. I just happened upon the EL12n and bought some fairly cheaply. This provided the gain I needed, so I could then use a DHT driver. I first used a 10Y which worked nicely. I switched to a 01A which probably needs a source follower or cathode follower, which is where I am now. I can go back to the 10Y which works fine, but I'm interested in some kind of source/cathode follower which also opens the door to a 26 for instance.

So that's where the thread is in my own terms. Which is confusing for those joining it and expecting the 300b to be the output tube. Outside of that, though, several distinguished audio minds have come in to the thread with pearls of wisdom.

So to sum up the thread, all are welcome to add their thoughts and experiences and for me personally this thread has completely changed my thinking and I've rebuilt my amps during the course of it. As outlined above, there are still a number of ways to drive a 300b including with a DHT. So some of the previous posts may turn out to be very helpful reading.
 
#357
Andy, AC2 sounds very nice.
But the operating point of best sound which I tried (160V, something like 0.5-1mA and -3.5V) is too low in current for driving a 300B.
Using ít with resistor load to drive 4654 in triode, which needs 40vpp of full power.

But some people regularly drive an AD1 (which is European 2A3) with it. 300B, I dont know...
 
#358
MikyK said:
Andy, AC2 sounds very nice.
But the operating point of best sound which I tried (160V, something like 0.5-1mA and -3.5V) is too low in current for driving a 300B.
Using ít with resistor load to drive 4654 in triode, which needs 40vpp of full power. But some people regularly drive an AD1 (which is European 2A3) with it. 300B, I dont know...
Click to expand...

I haven't tried it yet - I have 4 of them. It's on my to-do list. I hope to get a cathode follower sorted as well. Bit cold for working right now. I need to open the window and use a fan to blow out the solder fumes in my small workroom.
 
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