We had a terrific day at George’s on Saturday. Weather played in our favour and barbecue was on the agenda. I’m not looking to review a fantastic day with the circle members on this post, but just to capture a moment of this day with Susan Parker fitting her new cartridge on George’s Audiomods arm. I contributed with my Phono RIAA stage which was very well received (I think) despite the hum picked up by George’s system unfortunately. Great records played though…
Thanks George for the hospitality and a great day with friends.
Susan’s cartridge grabbed all the attention this day. I wish I could afford it!
Recently I acquired a second-hand and untested TF2700 LCR bridge from Marconi. This is a fantastic piece of engineering and a superb LCR bridge that can only be beaten by an expensive impedance analyser, something I can clearly not afford.
Matching Russian NOS capacitors
This bridge is perfect for selecting accurately capacitors for a RIAA network with extreme accuracy. In fact, the 1% precision can be improved down to 0.25% by replacing the main bridge 0.5% resistors with 0.1% ones as suggested by Morgan Jones in his “Building Valve Amplifiers” (Second Edition) page 330 and 331.
I was looking to repair my LCR bridge and found that a 2k5/25k ganged potentiometer was needed. I found this great supplier of potentiometer that can provide you with a customised ganged potentiometer at a very competitive cost. Worth considering this great Welsh company!
Last Sunday had the great opportunity to spend a day out in New York prior to a hectic work week ahead. Obviously record hunting was on my list of things to do. It’s been a long time since I last went to NYC and can say that the record scene has changed a lot. I’d like to highlight this great place owned by Jonathan Sklute. A passionate guy that loves vinyl records so much that has this cool small shop located in 218 East 5th Street, right in the East Village. The record store is called “Good Records NYC” and the owner proudly curates a selection of Jazz, Rock, Blues and other genres. Each record is carefully clean using a nice RCM (which I didn’t manage to capture model as it was placed on the back).
I came out with four gems from the Rolling Stones, The Kinks, Eric Dolphy and Little Sonny.
Last week I did some preliminar tests with the LL2746 in 1:2 step-up mode. Despite having measured good results with it, it will be a challenge to drive grid current given that the output impedance of the 4P1L will be multiplied by 4 so about 5KΩ.
Before looking at the LL1671/20mA which is suitable for multiple driver valves, let’s see how the LL2746 driver performs with the addition of the input step-up microphone transformer LL7903. I’m currently using the LL7903 in my 814 SE A2 amplifier and sounds really nice. The LL7903 was wired up in 1:4 setup so gain can get about 63:
Thanks to the great work that Derk put into his great Extract Model tool, I helped him to refine and debug the application by tracing the challenging 6e5p beam tetrode. After 4 versions we managed to optimise the model:
The model fits really well including the kink section but given the saturation of the 6e5p tetrode at currents above 50mA there is a slight divergence as the valve cannot reach the same anode current at higher voltages.
After a recent discussion in the DYI Audio forum about the 4P1L drivers, I decided to do some quick tests on an idea I had around to use a step up transformer (1:4) – 4P1L and step up interstage transformer (1:2) to drive a 300B or similar using the 4P1L in filament bias.
First suspicion is on whether the 4P1L has the grunt to drive a capacitive load which would be a real challenge in a 1:2 step up as load capacitance is multiplied by 4 when impedance is reduced by a factor of N^2=4.
I built a test rig with the 4P1L in filament bias using a 15Ω wire-wound filament resistor and connected the filaments in parallel to obtain easily a nice bias voltage with 650mA of filament current. Also lower Rf will improve the low frequency response as helps keeping low the output impedance:
4P1L rat nest
The valve was biased at Ia=30mA / Va=160V and grid bias is about -10.2V. A 10KΩ resistor was added as a primary Zobel as per recommendation of the datasheet. Then it was replaced by a 25kΩ potentiometer (P1) and the right value was found by looking at the frequency response.
Initial tests showed a very good response at 1kHz with only 0.24% THD @200Vpp output. The gain is approximately 16. The mu of the 4P1L with paralleled filaments is around 8 and lower than when used in series which is approximately 9-10. Albeit the results were promising initially, the real test of this stage is by looking at high frequency response where the capacitance will makes it real pain.