Ale Moglia

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 🙂

Gyrator

A simple PMOS gyrator for driver stage anode load

This is a very simple PMOS based gyrator with only one transistor. You can use a Zetex ZVP0545a which is easy to obtain around here and is not a surface mount type (in case you don’t feel confident in working with SMD components).

Steps to complete the design on this gyrator are as follows:

First you need to ensure you can bias correctly the Mosfet. Knowing your anode quiescent current (Ia) you can set R4 to ensure bias is stabilised as shown in the diagram (point 1)
You then have to ensure you keep the MOSFET Vds at least 25V to 50V to ensure output capacitance is minimised and expected frequency response performance is achieved. Then R1 can be calculated as well knowing your target anode voltage (Va). (point 2)
R2 and R3 will set the anode voltage (Va) which is a design parameter in the gyrator, right?. And also you know the gate voltage (Vg) based on R4 and Vgs (i.e. +B-VR4-Vgs). You can calculate this resistor divider given these two definitions. Also R2 plays with the capacitor to define the rolloff frequency of your gyrator.

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…

Listening to 01a

A warm and subtle tone, CX301a…
Superb sound!

Not in the office mood

When your mind is on some other place (not at work really)

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 🙂

6CB5a output valve in triode mode

6CB5a, as many TV power pentodes, is a great valve for a single ended amplifier. Specially now that popular valves of similar anode dissipation are getting really expensive.

Thomas Mayer came up with a great design for this superb TV pentode that wired in triode mode is very linear and can sustain at least 25W of anode dissipation. If you want to check Thomas’ design, have a look at his website which is very interesting.

If you want to look at the specifications, you can download the datasheet here.

When testing this valve on the curve tracer it was evident the fantastic features and linearity at every point.

As a power valve, testing a 6CB5a for harmonic distortion showed THD=0.07% @ Vo=+22.22dBu (10Vrms) at the following operating point:

Ia=40mA
Vg=-50V
Va=253V

Transconductance in triode is high: 6.4 mA/V @ Ia=40mA, Vg=14.6V, Va=100V. In summary:

Gm = 6.4 mA/V
μ = 4.5
ra = 700Ω

Here is a set of triode-strapped curves that you may find very useful if you’re planning to use this valve:

Also you may want to try the following spice model. It can be improved, but I think is an accurate representation of the curves shown above. Please try it and let me know what you think about it…

6n7 as a driver

I have tested more than 10 different ST and metal 6N7. Some GT, other simply old ST G ones and metal as well. Both triodes in parallel as usually this is the configuration used as an amplifier driver. Found a good operating point from a distortion perspective around Ia=6mA, Vg=-5.6V. As you can see you should expect getting around 0.09% THD. With some good valves reaching as low as 0.04%, but will have to be hand-selected.

Great driver from a sound perspective, with low distortion close to a 26 and on average slightly better than the 6J5. Need to review famous 6SN7, but there are lots of measurements for this one out there.

Author: Ale Moglia