Listening to the 45 Single Ended (SE) Amplifier

A perfect and warm sound

45 SE amplifier

Hard to describe in words what I felt when I played first time “Goodbye Pork Pie Hat” from Charles Mingus. Perhaps it was the due to the fact that I play saxophone, but inevitably for me this song is one of the best ones to test the response of any amplifier. Sax duets, solos, a great bass line and varied moods along the way can take the amplifier to their best of their abilities. Being a jazz fan, couldn’t help myself when listening this amplifier for the first time. I used to play an EL34 push-pull, and liked its sound and power – specially for rock. But when it comes to the first watt, this amplifier is breathless. Pure and a warm tone. Playing this song made me understand all the fuzz about single ended amps. But, far from opening the debate about that, I’m sure that a 45/46 in push-pull has its own merits and would probably will one of those amps I’d like to build in the future.

Having a FE167E full-range driver that have a relatively high efficiency (94dB/W), the 45 SE doesn’t sound low. It has its own personality. Brilliant for jazz, it may need a tad of power perhaps when playing some rock and classic stuff.

7193 valve

It took me some time to optimise the driver stage. Started with an SRPP, then a paralleled 6SL7 to an 6J5 loaded with a gyrator in mu-follower achieving less than 0.20% THD at 150 Vpp. A great driver in my opinion. Now need to replace the 6J5 for the lovely 7193.

It looks like a Frankenstein valve, but it’s a lovely thing to me. Many claim that it’s a superior valve compared to the 6SN7/6J5 or even CV1988.

Adding the 7193 will require opening my 45SE and not having music at home for some time so at the moment is not what I have in mind.

Perhaps when I get the 4-65A up and running? It will take some time for that…

 

DHT preamp evolution

My 01a DHT preamplifier has performed flawlessly over the last 4 months or so. I do enjoy its warm sound and clear tone. I do prefer it to my 26 OT preamp, despite everyone says the contrary. I personally feel that the thoriated tungsten filament gives some sonic unique mark to the sound here.

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I want to do the following test and compare differences in sound:

1) Gyrator load
2) Antitriode load
3) Choke
4) OT

Have my LL1660s which can take out temporarily from the 26 preamp, and also got a couple of chokes to use as well, oil caps, etc?

What do you think?

Updated 6e5P SPICE model

6e5p under test

Having fixed the bias offset problem in my tester (actually my oscilloscope). I took again the curves this evening to get a better model….

If you want to read how did I manage to get here, please read this post

 

6e5p triode strapped curves (offset fixed so grid voltages are fine now!)

Measuring valve transconductance

Today I breadboarded the CCS I will use for the transconductance tester jig which is an addition to my curve tracer:

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Bias circuit is a classic from fixed bias amplifiers. I had the 80V available from the curve tracer circuit. The meter is an external panel AC voltmeter which is a trueRMS meter that will measure accurately the 1kHz signal.

The MOSFET CCS is a simple cascoded which can help setting the valve current and operating point. I source it with my bench variable HT power supply, which also helps in setting out the operating point.

We know that we need to have a small AC signal in the grid to increase the accuracy of the Gm test as the transconductance is given by:

G_{m}=\frac{\Delta I_{a}}{\Delta V_{g}} _{\Delta V_{a}=0}

So can’t feed with 1Vrms, so will use 100mVrms. If a valve has a transconductance of 1mA/V, then the variance in the anode current will be of 100uA (rms). This represents a challenge to measure accurately using an anode resistor of 10 ohms for example since the developed AC voltage across the resistor will be 1mV (rms). Therefore we will use an anode resistor of 100 ohms which will help capturing small transconductance values as this one.

 

Edit:

Found that the CCS bypass was omitted in my first circuit. Also the sensing resistor was reduced to 10 ohms to accommodate the AC true-rms meter I have. See updates on this post here

6e5p triode-strapped

6e5p valve

The 6e5p is high-frequency indirectly-heated tetrode from our friends in Russia. The specifications can be found here.  Anode can easily dissipate 8W and screen can take up to 2W and has a high transconductance of around 30 mA/V

Wired as triode this chap becomes very attractive. The anode resistance drops to around 900Ω – 1KΩ and effective mu is about 30-35. This turns this valve into a low anode, medium mu and high transconductance fellow which is highly regarded as a driver in SE amplifiers. Check out there in the jungle and you will find many good examples of how this valve is being used effectively.

When testing this valve on my curve tracer I found that it probes to be a challenging device. You need to leave this guy running on its own for a while (Lars recommended 30 min to 1 hour). I found that indeed after 20-30 min it stabilise.

First run on my tracer

Dmitry came up with a very good model. When I created a model based on my curves found a mismatch between my notes and simulation. Checking my notes I think I set up the tester to start plotting curves at 0V with a step of -0.5V, however looking at the model produced by Dmitry’s tool, I got this:

SPICE model to fit 6e5p in triode mode

 

It looks like the curves starts at -2V. Need to re-check and probably trace this valve again. Either way it does match very well and not far off from Dmitry’s model from above.

Here is my model.

 

I’m planning to use this valve in my OTL (cap-less) headphone amp. Stay tuned…

Continue reading “6e5p triode-strapped”

4-65a triode curves

Tracing curves of a power transmitting DHT

Finding triode curves for the 4-65a valve has been a challenging task. There are some available from a Spice simulation which I couldn’t get hold of the model, so when I finished the curve tracer, it was the right time to take on this challenge. My curve tracer is not capable of handling this valve as I don’t have the appropriate socket and also the anode and grid drivers are limited to:

  • Anode voltage sweep range: 0-330V
  • Anode maximum current: 100mA
  • Grid voltage sweep range: -80V – +15V

With this constraining factors in mind, I decided to build a test jig for the 4-65a. The jig had only a grid stopper resistor (10kΩ), a screen stopper (100Ω) and ferrite beads in the anode and grid as well. When traced first set of curves got very disappointed with a double tracing for each anode curve which made me suspect that the valve was oscillating somehow due to long cables,etc. Its transconductance is below 3 mA/V, so shouldn’t be that problematic. I remember tracing 6e5p,6C45 and E180F being a real challenge for the tester due to its high transconductance.

Here is a sample of the double tracing from the first test:

I tried many things with cables, stoppers and ferrite beads with no success. Suddenly the penny dropped and looked at the old DC raw supply I was using. I had only one capable of providing 6.3V @ 3.5A. And it’s regulation was appalling. The ripple was clearly a potential candidate for this image distortion. If ripple was high enough, it will modulate the cathode and therefore Vgk. Ripple frequency is same as refresh frequency of the curves (i.e. 100Hz).

 

My test jig was modified to include a hum cancellling pot. as shown in the following diagram.  I added a 100Ω and a 22mF electrolytic.

Tracing curves again then was a success. I had to trim the pot to cancel the hum and alas, the curves were very neat.

 

The addition of the grid stopper limited the grid current closer to 0V or above. Therefore the 0V curve gets packed closer to the following one (i.e. -5V). This can be clearly seen when the Spice model is generated

My tracer has not been designed to trace positive grid curves, so the current capability of the grid driver at positive grid voltages is limited. I need to modify the circuit, but it will have to wait as I have already spent too much time in this tracer so far!

 

 

After playing a while with Dmitry’s tool, I came up with a very reasonable model for the 4-65a. I’m sure it can be optimised, but for a couple of hours work, I’m very happy with the results…