Dynaco ST-70: Base Line Testing
- Thread starter dcgillespie
- Start date
That’s who I bought from. I bought a matched quad first and liked them so well I bought four more but he didn’t have quads. Unfortunately, I didn’t mark them so now I have eight that may or may not be matched. My amp has individual bias pots and had no trouble setting each tube at 45ma. These tubes are very affordable, and despite the lower output sound very good.
I bias them to a range similar to 5881s. 23 Watts/max dissipation.
Lavane
Lunatic Member
I wouldn't mind snagging an extra quad myself to try in my ST-70.
I knew someone in the 90's that ran them in a Dynaco ST-70 without issue.
I bought one quad for my son's Russian made guitar amp. All I have now is a single pair. It would be nice to find a new trusted seller.
How did you wire the screen grids being the Eico transformers do not have a screen tap? Off the PS or just running it as a triode with the screen tied to the plate. Just curious because I have an Eico ST-70 I started restoring years ago and quit and have thought of just using the iron on a different build.
Never heard of EFB so I AI it and found info. For those interested.
AI Overview
In a vacuum tube amplifier circuit, EFB stands for Enhanced Fixed Bias. It is a smart regulation circuit designed by audio engineer Dave Gillespie that supplies and dynamically tracks both the control grid bias voltage and the screen grid voltage relative to changes in the main high-voltage (B+) power supply. [1, 2, 3]
When a pentode power tube is operated with an EFB circuit, the supply mechanics work through the following behavior:
1. It acts as a "Tracking" Bias Supply
Traditional power supplies can "sag" under a heavy audio load, causing the plate voltage (B+) to drop. Instead of leaving the tube's grid voltage static, the EFB circuit samples the fluctuating plate supply voltage. It then automatically scales the negative grid bias voltage up or down in exact proportion to the plate voltage. [1, 2, 3, 4]
2. It Regulates Screen Grid Voltage
Pentode tubes are incredibly sensitive to fluctuations in screen grid bias. The EFB circuit uses solid-state components (typically a small adjustable regulator like an LM317 or a transistor pair) to supply a rock-steady, regulated voltage to the screen grids. This stops transient music peaks from modulating the screen grid and causing unwanted distortion. [1, 2, 3]
Why This Is Done
By actively adjusting and supplying these precise voltages, EFB provides several major benefits over classic cathode-biased configurations (like those found in vintage Dynaco ST-35 or SCA-35 amplifiers): [1, 2]
- Lower Distortion: It keeps the pentode's ideal operating point perfectly locked in, significantly dropping Total Harmonic Distortion (THD). [1, 2]
- More Power Output: It allows the pentodes to clip cleanly and deliver closer to their maximum theoretical power output. [1, 2]
- Extended Tube Life: Because the tracking is so precise, the tubes can safely idle at a much cooler bias current without degrading sound quality. [1]
right concept, wrong execution but yes both the screens and grids are run that way. It lets me drop ~100v to the screens without it being a mushy mess of terrible regulation. Possibly I could wind the screen voltage up some more but its currently within datasheet limits. It makes about 30 watts, and I'm fine with that.