2-stage DHT: high-mu vs. interstage stepup

[Rocky]

I have a question applying to design of 2-stage low-power DHT amplifiers:

Getting enough gain from only two stages to me means one of two things: 1) using a high-mu driver tube (like Telefunken KC3), or 2) stepping up the signal by means of a transformer.

Now, high-mu DHTs, KC3 in particular, are hard to come by and you must be ready to bring $$$ if you find them, while "typical pre-amp tubes" like the 26 or 12A with mu around 8 but higher current handling, come relatively cheaply and can be stepped up in an 1:2 interstage for instance, and thus provide decent gain while still being able to deliver enough current for a suitable output tube, the 71A is on my mind.

Does anyone have experience on the matter and wish to share their POV? pros and cons of using high-mu tubes vs. transformer stepup?

Thanks

EDIT: I should mention that with high-mu DHTs one also must deal with high Rp and thus choke/transformer loading is out of the question as inductance requirements raises ridiculusly high, so RC-loading would be a con for the high-mu tubes I think..

[kevinkr]

I have used both approaches driving 2A3 and 45 type output tubes.
I have so far only used 1:1 transformers because of my concern for source impedance.

Say for example you used a 27 which has a plate resistance of about 8K, a 1:2 stepup transformer would give you a 32K source impedance (remember the impedance transformation ratio is the square of the turns ratio or voltage transformation ratio) which is not going to result in very good high frequency response to put it mildly when you consider the miller capacitance of the output tube and likely strays on secondary side of that transformer.

Consider a 5842 or 5687 driving a 1:1 transformer to your output tube, source impedance will be low and both have moderately high mu so sensitivity will be reasonable in both cases.

Hey I love the 26, 27, 30, 56 and others, but their low mu and relatively high rp can be a design liability in driver stages and asking them to swing large voltages on their grids in order to get the required plate swing can result in poor linearity as well. (Which might be an asset if the resulting spectra matches the output tube's well aka distortion cancellation.)

Just my thoughts..

Kevin

[kevinkr]

Another thought, the 26 can be very microphonic as well. I use them in my pre-amplifier design where the 5:1 stepdown transformer reduces the source impedance to roughly a few hundred ohms, and gain is very limited as well. If you are curious see my website at www.kta-hifi.net, go to pre-amplifiers and click on the topmost link (26dht). The design was published not long ago in the positive feedback online magazine.
This will give you some idea of the problems involved with transformer coupled dht circuitry.

Kevin

[tubelab.com]

Finding a high Mu DHT is not easy. Finding a high Mu DHT that is not microphonic is harder.

Kevin mentioned the 5842. It is not a DHT, but I have used it successfully in several 2 stage amplifiers including 45, 2A3, 300B, and several triode wired pentodes. The key to getting good gain is to use a very high impedance load. I use a CCS and a mosfet buffer to isolate the driver from its load.

I know that many will want to build a pure tube circuit, which is possible, but this works very good. If you are interested, the schematics are on my web site.

http://www.tubelab.com/TubelabSE.htm
http://www.tubelab.com/powerdrive.htm

[Rocky]

Thanks for pointing out the source impedance issue, Kevin. It completely slipped my attention. Using IDHT drivers are not of interrest at the moment, as I've been playing with high-mu IDHTs in spuds for quite a while. I can drive my speakers with 6C45Pi or similar and still have headroom like a fart in free air so to say, so if I am to build myself a DHT amplifier it only makes sense to me if I can get rid of the IDHT, not just adding a DHT stage for the sake of it.

I've gone over the numbers again and again, and in the end I don't think I'll need that much gain from the driver tube afterall, so IT-loaded (1:1) '26 (mu=8.3) into a 5K-loaded 71A (mu=3) output tube would work. I need some time to decide on the '26 though. I'm not sure it is my kind of tube, and I know I am not going to power those 1.5V/1.05A filaments with batteries.

I have heard rumors about the 26 and the difficulties of implementing it due to it's notorious microphonics and strange heater requirement.. All senses is telling me I really shouldn't be messing with that tube, but I think I'll give it a shot regardless, knowing that desperate attempts to battle the hum will probably cause a bad headache If all attempts to quiet the hum fails, I can always substitute the 12A if I keep a spare heater winding for it.

[Rocky]

Kevin,

I am curious about your experience with the '26. You use rectified filaments wich probably introduces hum to the '26, but you are also stepping down that hum on the output with the rest of the signal..

I would want to IT couple 1:1 the '26 to a 71A output tube, and would like rectified filaments on both tubes. My worry is hum and microphonics. Regarding microphonics, I'll be using teflon sockets to provide mechanical damping, and with a low amplification factor of only 3x following the '26, would you think microphonics will be a problem provided I do some tweaking to get it right?

Hum is the second and probably largest issue, but not for me I think. The amp will do duty from 350Hz upwards, so I believe I can tolerate quite some hum as I doubt it will be audible on my horns. What do you think about that issue? Here, the low amplification factor following the '26 also comes into play.. the hum will only be amplified 3x before being stepped down in a 5K:16R OPT.

EDIT: typos

[kevinkr]

Hi Rocky,
Actually with decently filtered/regulated dc filament supplies hum is not going to much of a problem at all from this source - the problems I had related entirely to coupling into the output transformer, and this will be possibly be an issue with your driver interstage transformer if the power transformer(s) are in close proximity.

Incidentally the 26 is a great sounding tube and well worth the effort to get running properly.

The overall broadband noise coming out of my transformer coupled pre-amp is < 100uVrms over a 22kHz measurement bandwidth, and there is no measurable or observable hum.

I expect that sensitivity will be quite low in this configuration, as a rough guess sensitivity for full output might be around 3.75Vrms, which may also exceed the linear swing possible at the grid of the 26. (too close to cut off at one end, grid current at other assuming -9V grid bias ?)

An interstage with a ratio of 1:1.5 might be a reasonable compromise, raising the source impedance to 20K, then again...

I still recommend the 5842/417A with 1:1 interstage tranny - you will get great linearity, very low source Z and lots of current capability.


Kevin

[Miles Prower]

I have a question applying to design of 2-stage low-power DHT amplifiers:

Getting enough gain from only two stages to me means one of two things: 1) using a high-mu driver tube (like Telefunken KC3), or 2) stepping up the signal by means of a transformer.

There are a couple of other solutions as well. You could use a pentode stage to drive the DHT. This is the Western Electric solution. Suitable pentodes would include: 12BY7A, 6AG7, or the 6BQ5. All have enough "moxie" to drive the DHT. You could also include a common drain MOSFET driver stage. That, too, can certainly drive a DHT with ease.

[Rocky]

Kevin,

PSU/mains will not be anywhere nearby. I've just made a nice solidworks layout for dual mono with external PSU, and I think that'll be the way to go. You are right about the sensitivity, it will be low, but then again my power requirements will not lay anywhere near the 600mW-700mW this beast can put out at 4Vrms swing on the grid of the 26. I'll most likely never pull more than 100mW out of it, and that is in extreme cases only, the speaker sensitivity is above 114dB/W/m, so even with my 2.5Vrms output DAC, I think passive preamplification will be an option no matter how crazy it sounds..


I think I'll make a deal on AE's 50H 1:1 ITs at 98eur/pair (special offer). 50Hy is low for the 26, but given required bandwidth it'll work great for my horns. My existing amp is likely to get scavenged for input and outrput iron, Lundahl and Tango.


Sorry guys, both IDHTs and pentodes are probably great drivers, but they are not an element in this experiment. I do not wish to use them even if they perform great, I'm just stubborn on this matter. This will be a DHT experiment or no experiment at all.

[tubelab.com]

Everyone is entitled to their own opinions. Just let us know how it works out so that we can all learn something. Very few of us have speakers with this sensitivity level, so you are in a unique situation.