4-65a EIMAC

4-65a EIMAC NOS DHT

Just got a couple of NOS EIMAC which I will be using in my SE design

So did some test on distortion, transconductance and driving them to +22.22dBu output to check the quality of these two ones.

I used similar test rig as before. At some point will be able to get a proper filament supply for this valve, but for the time being I will continue to use the hum pot and the big electrolytic cap across my old bench power supply which can gently provide the 3.5A for the hungry filaments!

I tested them at the limit of my CCS and bench HT supply which at the moment cannot provide more than 360V @ 100mA.

Transconductance is in the region of 3,800 – 4,000 μmhos.

So biasing the valve at -2.5V and over 90mA of anode current, the harmonic profile looks like this:

Both EIMAC measured about 0.12% THD.

4-65a glowing in the dark

These were taken at my lab in the attic during the summer evening twilight.

4-65a HT +300V supply

First breadboard of the 4-65a SE amplifier. Will start this long project with tuning the HT supplies. Will need a +600V, +300V and +100-150V supplies to be stacked.

Here is a simple breadboard using metal rails over an old wooden frame I had around. Oil caps will be glued temporarily and damper tubes will be fit later:

A snapshot of first test….

Don’t ever buy tubes from this eBay seller: toober

Failed 4-65a tubes — Failed 4-65a sold on eBay.

I was warned, I can’t say I wasn’t. But I think I learned my lesson. Morgan Jones clearly said it on his book: don’t buy on eBay from sellers who claim they don’t have the means of testing the valves. So if they don’t, why are in the business of selling valves?

Mr. William Donzelli (aka toober in eBay), sold me a pair of used 4-65a for the amount of $43.55. I took the risk, I’m not denying that, but after receiving the tubes on the post I did test them accordingly. One was completely flat despite filament being alive, and the other was completely gassy and filaments burned after 20 seconds of use. I do own many 4-65a and can test them accurately on my curve tracer and valve testers. If you don’t have one, you can always do some pretty basic tests anyway with basic lab equipment.

I contacted Mr Donzelli to try solving this out amicably. But his response was harsh and clear: no returns and items sold as-is. What kind of seller are you if you are not able to share the risk and compensate your buyer if you literally sold him a useless product and you got away with the money. Is that fair? Not even accepting the return of the useless product? Offering a similar one? a discount on the next purchase?

Not, that wasn’t offered. claim was raised on eBay and responses from seller were only made to protect himself:

“[…]I have certainly learned my lesson winning auctions the resulted in disappointment – I saw that those wins were my doing, not forced by anyone, and I had to adhere to the rules. So do you. Good buyers do this”. (Mr Donzelli responding to my claim on eBay case)

So Donzelli is telling me to learn the lesson: there are sterling buyers out there that the find a way or covering their back in the best way to screw their customers legally over eBay. What we had in the real world, clearly exists now in the virtual one 🙂

If you don’t want to make this man happy by selling you everything at your own risk, DON’T BUY FROM HIM! – you were also warned, so don’t be as stupid as I was next time 🙂

4-65a THD

Quick test on a EIMAC JAN-8165/4-65a NOS. Here is the THD response at output level of +22.22dBu:

Artefacts at 50Hz and above are all a result of a poor filament power supply 🙂

Nice to see this valve providing 0.14% THD at a low operating point: Ia=85mA, Va=246V. At least this is a nice test to show how linear this valve it is.

26 THD analysis

As I’m proud of my 26 DHT pre-amp and also looking to use this valve as the first stage of my 4-65a SE amplifier shortly, I looked at how linear this valve is.

Unfortunately I don’t have an extensive set of valves of same brands, so albeit I have about 40 valves if this type there is a big mix of different brands and many of them are used ones. Having said that, I think this may be the case of many of you out there, so I think that probably the results of this test may be relevant to you (if you are still reading this post).

So in summary, this is the sample set I tested:

39 valves
12 brands
ST (34 valves) and Globe (5 valves)
Type: 26, 226 and 326
State: used (80%) and NOS (20%)

All DUT were tested with the same test set and operating point:

Operating point: Ia=5.5mA and Vg=-10V
Anode load: Cascoded DN2540 CCS set to Ia
Test signal: 1kHz, Adjusted to obtain Vo=10Vrms (+22.22dBu)
THD analysis: Audiotester via Pete Millett’s interface:
- 32768 sample FFT (2.96Hz resolution)
- 256 averaged windows
- Van Hann window
- THD for H2+H3+H4

So in summary the results showed that you should expect a 26 to have around 0.08% (Std Dev = 0.00047). Here is an histogram showing a summary of the tests done:

Looking at brands, the following average THD ranking was produced:

26 THD by Manufacturer/Brand

Interesting to find National Union leading the chart. I found RCA and Sylvania to be my preferred ones in terms of sound. THD shown above are average of sample sets of 4 valves or more. Although I tested 12 brands in total, some of them were just a pair so they are not a representative subset.

Finally, here is a sample THD of a very nice 26 valve:

I wish I could have a bigger collection of 26 to improve the accuracy of this statistical analysis. Either way you can get a view of what you should expect from this great valve…

A beautiful thoriated-tungsten glow

Here are a couple of recent pictures of the 4-65a EIMAC under test. What an attractive glow!

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…