VT-25 DHT Preamplifer

VT-25 in action

Now I’m back from our long trip, I found some time to play with the “Mule“. I wanted to revisit my old VT-25 preamplifier. Many years ago I had my first VT-25/10 preamplifier which was based on a gyrator load. Then it morphed to a transformer coupled (LL1660/40mA) version to drive my TVC before I settled into the 4P1L for some long time.

The circuit design

The VT-25 has always been on my list of favourite DHTs. It’s gone ridiculously expensive these days and is hard to get. I have a couple of pairs in very good shape luckily.

Continue reading “VT-25 DHT Preamplifer”

4P1L: pump up the current!

Background

I’m a firm believer than sharing knowledge and experience is the best way forward to continue learning yourself. It always pay pack at some point. This time Paul Prinz, a fellow implementer of the 3B7 DHT Preamp using the gyrator PCB, came back with a great suggestion. He found a MOSFET which could do high drain currents, it has high transconductance and most importantly the parasitic capacitances were low even close to the BF862. Hooray, I thought. We may have a great solution here to use the gyrator load for currents above 25mA and with similar performance to the great BF862. There are some other depletion MOSFETs that can do high currents, however they all have relatively high capacitances and low transconductances when VDS is low, like in the cascoded gyrator circuit.

The BSH111BK is an enhancement MOSFET, so doesn’t have a “depletion” behaviour like the jFETs. This isn’t a problem as the bias voltage can be set by the reference CCS.

For comparison, here is a brief summary of the key characteristics of these three devices:

	BF862	BSH111BK	MMBFJ310L
Ptot (W)	0.3	0.3	0.225
VDSmax (V)	20	55	25
VGS off (V)	-1.2		-4
IDSS (mA)	25	210	60
Gfs (mS)	45	640	18
Ciss (pF)	10	19.1	5
Crss (pF)	1.9	1.5	2.5
Coss (pF)		2.7

Continue reading “4P1L: pump up the current!”

801a DHT Triode Spice Model

My friend Bela, who is an experienced DHT user and very fond of the 801a, asked me if I could help in developing an LTSpice model for the venerable 801a. I have several 10/10Y but unfortunately no 801a at hand to trace so I used the data sheet curves which are a bit challenging due to resolution/scaling of the characteristic curves:

I had to compromise the model fit as if you want accuracy in A1 region the matching is less than perfect in A2. Still, the model is not perfect but is very good for simulation purposes.

Continue reading “801a DHT Triode Spice Model”

VT-25 / 10-Y triode curves

Everyone loves this thoriated-tungsten DHT valve. I’ve only used it in a preamp and was hooked with its sound. Really warm and nice. Downside is, it’s very pricey these days and also is quite demanding from a filament perspective. You can check the characteristics here.

For those who like testing their designs with LT SPICE, I produced a model which matches really well the traced curves. Would like anyone to use this one, to drop me a note with any feedback 🙂

Here is the VT-25 spice model. Let me know how it works for you!

THD benchmark

I’m still in the process of testing valves, here is how the ranking is coming up so far. This is a mix of driver and output valves. All tested at Vo=+22.22dBu:

Looking at the chart above a couple of interesting points to highlight:

4P1L is the most linear valve I’ve found so far.
6e5P and 6C45 are expected to be on the top five anyway.
12P17L despite of having similar characteristics as 4P1L is not that linear
6N6P and 6N6P-I disappointed me. I thought the would be more linear..
46 and 47 in triode mode are superb drivers

Have so many other ones to test, but limited time….

Expect this chart to be updated in the future, so stay tuned 🙂

Finishing the curve tracer

Today I did a bit of extra work on the curve tracer with a view of finishing it. It has been a long and painful journey, but I’m reaching the end of it.

The transconductance tester is working perfect. I need to use the following ranges in my true RMS AC voltmeter:

0-2,000 μmho: 100mVrms scale
2,000μmho-50,0000μmho: 1Vrms scale

It’s probably the DC bias which affects the low scale. As an example when testing a 46 in triode mode (see datasheet for details), I tried the following operating point: Vg=-33V, Ia=22mA and the measure should be around 2.35 mVrms over 220mVdc. But in my bench voltmeter, above 17mA in the 46 doesn’t like it and cannot measure it, so need to change scale. I tested low transconductance valves in the lower AC scale such as CX301a, 26, 4P1L, 71a and then using the high AC scale, used 6e5P, 6C45, 6N6P amongst others.

The tracer now has a common-mode mains filter. This was required as at certain times during the day, specially in the evenings when the mains is really noise or my wife is using the microwave oven!, when tracing curves with the 1Ω sensing resistor and low anode currents (e.g. CX301a) then the noise level was sufficient to impact and distort the traced image. With the common-mode mains filter it works brilliantly.

Now need to place bottom plate and standing feet. Job done then and will move to some proper audio work!

Testing the circuit today, I measured 29 46 valves. Ended up discarding two which measured low and then when tested with the tracer found that curves weren’t good at all. Probably electrode misalignment as they weren’t just with low transconductance. Will upload some examples as it’s very interesting to see the difference