Valve Tracing & Testing Service

If you are looking to test your precious valves and you don’t have the equipment, I can now offer this service for you which includes:

  1. Valve health check
  2. Valve bias point measurement (Ia, Is @ Va, Vs)
  3. Transfer curves
  4. Characteristic curves
  5. Microphony (optional)
  6. Distortion – THD (optional)
  7. SPICE model for triodes (optional)

20130929-202754.jpg

 

Some examples of a lovely EL34 Mullard tested on request:

EL34 02 EL34 02 Vs=250V

My fees are very competitive and this service is worth if you are looking to sell your valves or match them.

Terms and conditions:

  1. Price varies on level of testing and amount of valves
  2. Postage is not included and depend on your location. I’m in London, UK so you can easily estimate the costs beforehand.

Update – August 2021

My apologies but due to pandemic and lack of free time, I’m not offering this service anymore.

New HT bench supply

600V HT Bench Power Supply

After being out of action for over a month due to visits, holidays and business travel, I finally got the opportunity to get my hands back on the 600V bench supply. I need to repair my valve tracer, but firstly need the bench supply back again. Otherwise won’t be able to do all tests I want to around my 814 DC-coupled SE amplifier and many driver stages I want to try on my workbench before moving to the next stage.

An improved design to avoid fireworks

Tired of my old HT (+600V) variable bench power supply to suffer collateral damage when accidentally shorted whilst testing transmitting valves for output stage (i.e. FQP3n80c MOSFET passive regulator blowing out), I decided firstly to decide a simple and yet effective valve stabiliser. As nothing comes for free, these were my design constrain factors:

  1. Input raw supply is +620V @ 100mA
  2. Filament secondary winding is 15V @1.5A
  3. No additional secondary winding is available for a floating screen supply (e.g. pass valve is pentode)
  4. Output voltage ideally should be 0-600V

So with the restriction of not using a pentode as pass valve, I looked out for candidates to match my requirements and instantly thought about GU-50 in triode-strapped mode. Yes, I know that UG2 limit is 250V, not 1,000V as anode max voltage. But, in triode strapped specs are not shown. As recently checked this with the 814 triode strapped, and seems to be ok UG2=Ua in triode mode. 7N7 also said this was ok and Morgan Jones previously tested this as well with similar valves.

So, question here initially was: could the GU-50 withstand 600V in triode mode or should I needed to look out for other options?

After asking for some help in DIYaudio forum to see what was the best option on this topology and  the recommendation was to use the GU-50 in “right-handed mode”:

GU50 right-handed

In this mode GU-50 was able to provide the regulation required (or close to it) with minimum driving requirements (i.e. 0V to +15V)

First version of the passive regulator:

The MOSFET is in source follower mode to provide the necessary grid current, albeit not sure how much grid current the GU-50 needed in right-handed mode. So the value of R6 must be adjusted on test. In order to survive an output short, R6 needs to be in the order of 100K and 3 or 4W to avoid blowing up the 18V zener protecting the grid.

The circuit above is very interesting as the anode dissipation of the GU-50 is very handy for this setup and requirements. Is important to highlight that the circuit has no regulation, so if this is a requirement a different topology must be explored.

If we look at the sandy option here, then the following equivalent is comparable:

R2 and R3 are required to protect silicon from output short. Q1 provides current limitation with R6. This topology suffers from same issues related to regulation as previous circuit.

A big issue on the two circuits presented before is that the potentiometer is stressed at full raw HT supply. This is far from ideal and despite the specific power requirements of the pot,  we also need to ensure that the part can withstand the voltages used.

A slight modification (requiring an additional LT supply) can solve this problem:

Now P1 is connected to 15V. The gain of the M2 stage is significant so stability of circuit above is an issue now.

So if we have already introduced an LT supply in the circuit, then is a better choice to look at a feedback regulator.

A more complex circuit indeed, but a more effective one. The op-amp provides regulation by sensing output from R7 and R8 divider and comparing it to the stable reference from the output of P1. R10, R9 and C1 limit the HF gain response of the op-amp. R12/C3 and the 10pF FKP2 Wilma cap across R7 optimise the HF response of the overall regulator. D1 will protect the op-amp input (specially if 10pF cap is fitted). R1 is an additional protection for M1. M1 requires a good sink if wider regulation is needed. When output voltage is low and high current is drawn, then M1 is bearing all the effort and will dissipate a lot of heat (just do the maths).

The raw supply stays the same, however the gyrator stage was optimised as shown below:

The MOSFET was changed for am 1000V  TO220FP plastic package one which is better from an insulation perspective given voltages used in this circuit. R5 and R6 changed to 150K 3W ones to provide protection to the zener and M1 in case output is shorted.

I will build this and report results shortly.

4П1Л/4P1L triode curves 2

Previously I publish my 4P1L/4П1Л triode-strapped curves using my curve tracer. A forum friend who helped me in building my tracer as he was building his own, suggested using two tools for capturing curves (“Graph Grabber“) and plotting curves, loadlines, etc. (“Graph“).

Here is an example of the 4P1L/4П1Л triode-strapped curves using both applications to produce the safe operating area based on 7.5W anode dissipation. This can be extended to 9W if we consider the screen dissipation as well.

 

4-65a SE driver

46 DHT in triode mode as a driver

46 driver THD analysis

Continuing with the design of the 4-65A SE amplifier based on M. Koster design, I’m in the process of tweaking the 46 driver to optimise the operating point and provide maximum distortion to drive the demanding 4-65a. Here is the circuit I’m currently working on.

The current 46 driver will be biased at around 25-30mA using filament bias, so Vgk will be around -16 to -17.5V using a 10Ω filament bias resistor array. This will set the 46 at around 185-210V which will give sufficient headroom (i.e. need about 200Vpp max) to drive the 4-65a.

So today I look at varying slightly both anode currents and Vgk to see impact on THD.

So minimum THD is around -17V and Ia=30mA. So if setting the Rod Coleman filament regulator to ensure that Vgk=-17V and the anode gyrator to set anode voltage to ensure Ia is close to bias current would provide the minimum distortion (which is 0.04% in this example). Pa is close to 7W, but looking at the datasheet we can see that maximum Pa is 10W (as the latter 45 version).

46 Super Silvertone THD

 

So next I need to build a prototype of this driver with filament regulator and gyrator load.

4-65a EIMAC

4-65a EIMAC NOS DHT

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

4-65a EIMAC NOS

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:

4-65A NOS EIMAC

Both EIMAC measured about 0.12% THD.

 

VT-25 / 10-Y triode curves

VT-25 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 🙂

 

 

 

RCA10Y VT25 spice model

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

 

12P17L curves and Spice model

A great russian pentode valve similar to 4P1L, but with indirectly heated cathode. You can check the valve specifications here.

I tested transconductance in left-handed triode mode: Gm=9.6mA/V @ Va=150V, Ia=50mA , Vg=-5.6V.

Here are the curves for this valve:

12P17L triode curves

For the ones who want to test the SPICE model here is my take on it:

You can try the model and please let me know your results! You can always email me

Valve THD analysis

Measuring triode linearity

Today decided to do a quick distortion test of on a sample of a variety of different valves. All either triodes or triode-strapped pentodes/tetrodes. As per my previous tests, distortion was measured at +22.22dBu (10 Vrms) at the output of the valve in common-cathode mode. Valves were loaded with the CCS I use in my curve tracer. The operating points were quickly optimised at hand, so I’m sure there may be some better operating points for some of the valves below which may improve their overall THD. If you have any suggestions, please let me know!

Tested valves for THD
THD analysis

Need to retake these measures as the soundcard interface got damaged and results are showing significant distortion

Interesting to see in the chart above, that 6e5p and 6C45p are the best ones. This is in line with their reputation as drivers as they are capable of swinging many volts and producing very low distortion. In terms of harmonics I noticed that 6e5P provides a richer H3 and H5 as being a triode-strapped valve, whereas the 6c45p provide a dominant H2.

Also good to see that my favourite 46, 4P1L and 6CB5A (all triode-strapped) are very linear with anode currents of 40mA (with the exception of 46 as I measured THD on a previous operating point used for transconductance measurement). I should retake the 46 and drive it harder, I’m sure it will perform better at higher current.

Surprised with the results of the 6N6P-I. Was expecting this one a bit better, but perhaps it’s the pulse version distortion, so may need to get hold of an 6N6P and compare the results.

Update:
It looks like I blew up Pete Millett’s interface after measuring THD in float mode and exceeding the 10Vrms limit in this mode. Therefore measures such as 26, CX301a and others are not accurate. When testing 26 with my Ferrograph test set it came out to be 0.05%…
Stay tune until I repair the unit!

Measuring Transconductance (Gm)

GM tester jig circuit

After a bit of work, got the transconductance jig working fine. Made an obvious omission which was not bypassing the CCS. The CCS present a very high impedance in AC to the circuit, therefore not developing the current variation on the measuring resistor. Bypassed by an electrolytic presents a path to ground.

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!)