Tube Amps¶
OTL Tube Amplifier for Headphones¶
I started with a Bottlehead Crack circuit with the C4S upgrade (which is just replacing the tube resistor loads with active [CCS] loads), then a Crackatwoa (added basic pentode shunt regulators). From there I began upgrading the circuit, making my own mods and eventually arriving at a solution.
Here's an overview of the amp:
- Chassis is 1/4in 17-4 stainless steel.
- Dual-mono power (one power rail for each channel)
- 6336/6528 tube draws ~50ma per plate/channel
- mu-follower takes output from anode point instead of mu-out point
- volume is 75k 24 position stepped attenuator. vintage ITT mil-spec silver rotary switch.
- output capacitors are Audyn 100uf PP, 1u 160v PIO (magic sauce), 0.22u soviet teflon.
- solder is cardas quad eutectic
- flows very nicely
- figured the silver content would make good bonds to socket tabs (since everything is silver plated)
- resistors are Vishay MELF for PCB's, Dale wirewound for "criticals", Dale RN60 for various P2P, Dale RN55C for volume and input.
- transformer is Astek 1t150, potted with oil-based polyurethane via heated vacuum impregnation so nobody can hear it scream.
- all sockets are mil-spec cinch sockets. ceramic bases. what my opinion of audio is
- driver tube heater is DC rectified (not shown in schematic). Uses a 12.6VCT transformer and DPDT switch into a "tuned" linear psu for 6.3/12.6v DC selectable.
- amp has an integrated AC noise filter not shown (inductorless).
- amp has a balance control not shown. 4PDT switch for complete cutout if not used, and uses a 100k 10-turn wirewound pot with 20k wirewound resistor to maintain impedance to ground.
This amp uses an extremely common topology. Here are other amps that use the same topology:
Wheatfield HA-2 link schematic file
Apex Teton (upgraded HA-2)
Harman Kardon A700 Restoration¶
Vacuum Tube Circuitry Reference¶
Active Loads¶
CCS¶
Active Load: uses active components, i.e. transistors, to create a current-stable non-linear resistor.
CCS: constant current source/sink
The Bottlehead Crack uses CCS loads pretty much straight out of Morgan Jones' tube amp book: Morgan Jones 4e
The best CCS I've come across is the "bias multiplier" CCS by Walt Jung here: https://www.bartola.co.uk/valves/2015/08/31/ccs-not-everything-that-glitters-is-gold-part-i/
Walt Jung's original article pdf
I use cascoded DN2540's for high current, and for bougie stuff I use cascoded IXTP01N100D's for ultimate performance.
Mu Follower¶
What I use: https://www.bartola.co.uk/valves/wp-content/uploads/2018/12/DHT-Preamps-ETF2018-final-notes.pdf
This thing is great because it lets you set a voltage rather than a current. Its comprised of a voltage reference generated via LND150 CCS, and an active load with a JFET as the bottom device, which controls the current going through it based off what it gets from vref and the signal going through the tube (see the coupling cap).
Read the slides (slide 13 mostly) for instructions.
Tube Biasing Methods (Cathode)¶
These are the methods I know of for cathode biasing. Each has a sound signature and various pros and cons.
- Resistor
- One of the simplest. The problem is that any anode voltage instability will change the bias voltage.
- This method is great if you have an active load on your anode (CCS / mu follower)
- Resistor + Capacitor
- Same as above but bypassed with a capacitor to stabilize the voltage. Unnecessary if actively loaded.
- LED
- Great method if you're careful about LED choices.
- You won't have a lot of options for choosing a forward voltage (largely color dependent).
- You want ones with low ESR and a very stable forward voltage (current dependent).
- HLMP-6000 is the most popular LED.
- LEDs tend to sound crisp and some would say harsh.
- Schottky Diode Stack
- Fantastic method, again be careful about component selection
- Low forward voltage means you'll typically be stacking these in series to get a desired voltage
- C3D08065A seems to be the best I've found that gets around 1.5-1.6v stacked.
- Battery
- Phenomenal method. incredibly stable, very low esr.
- NiMH batteries are what you want to use.
- Batteries discharge, so you need enough current flowing to keep them trickle charged.
- Not running the amp for a while can cause the batteries to go flat (bad!).
Capacitor Rolling¶
I am not a huge fan of spending a million dollars on esoteric bougie capacitors. What I do like is different capacitor media.
- Polypropylene
- My go-to for film capacitors. I don't bother with inferior mylar or anything
- WIMA are fantastic. For my tube amp's output caps I use Audyn. Great value.
- Paper in Oil
- I do not know why, but the paper in oil capacitors I've used dramatically change sounds for the better when used as couplers.
- I have tried Sprague Black Beauties which are great
- I use some unknown 160v rated PIO caps in my tube amp
- Teflon
- Extremely good characteristics, incredible responsivness.
- Soviet ones are the only "cheap" ones.
- They are massive for their capacitance.
Tube Rolling¶
6SN7¶
The driver tube my amp uses, along with the following variations / equivalencies:
B65, 12SN7/B36, VT-231, 2C22/7193, 6F8G, 6J5G, 1578
I own every great American 6SN7 ever made, along with a B36 from Osram. The tube that works the best in my amp is a 1941-1945 RCA VT-231 (grey glass).
Other tubes I have liked especially:
- Sylvania 6SN7w tall bottle
- Tung Sol BGRP
- Raytheon tall bottle
- Sylvania 1951-1953 "Bad Boy"
- Osram B65/B36
- MELZ 1578 (real ones)
Tube Shielding and Vibration Dampening¶
Shielding input/driver tubes is very wise. They are extremely sensitive and pick up EMI easily. Tube shields for 7 and 9 pin miniatures is exceptionally common and come as tin/alu/steel cylinderical caps. These have an actually primary function to help the tube maintain heat, and to have a spring press down on the tube from the top of the shield to maintain socket seating.
Power tubes do not need electrical shielding because they're operating with an already-amplified signal, and are thus quite insensitive to any emi. However, power tubes can exhibit oscillations and vibrations. On many many amps, if you disconnect the load (so you can hear in silence) and crank the volume, you can hear the music coming off the tubes themselves.
6336 and especially 6528 tubes have a colossal output and transconductance, with huge inter electrode capacitances. They are very susceptible to oscillating and producing vibrations. I have mine under two 100R grid stoppers, and in this picture you can see a silicone ring on it. This ring technique is common. However this still was not enough to completely stop it, so I placed a crucible I had lying around on as a little hat and its been happy that way ever since.
Heater Rectification and Elevation¶
I always recommend DC rectifying input/driver/preamp stage tubes to get rid of hum. This article is the bible for wiring heaters: https://www.valvewizard.co.uk/heater.html
I twist my wires by clamping one end in a vise, and the other in a drill chuck. Apply tension while spinning, don't overspin, preheat with a heat gun for best results.
Grid Stoppers¶
I need these on my amp due to the transconductance of the 6336 or 6528 tubes. Grid stoppers are resistors placed just before the grids of a tube to mitigate parasitic oscillation, which is a possible source of hum. Gospel is to place the resistors as physically close to the socket pins as possible.
When taking apart a vintage tube tester I've found that this was practiced via iron "bracelet beads" on all the wires, one per wire per end, always close up to the socket. These beads act as one-turn inductors.
Tube Datasheets and Loadlines¶
How to read transconductance units¶
- g = conductance
- gm = conductance mutual = transconductance
- mho = opposite of ohm (resistance) i.e. conductanc
- mS = millisiemen
- S = Siemen = gm = mho
- example: 2.2mS = 2.2 m℧ = 2.2 mA/V
Loadline Calculator¶
Best loadline calculator I've found yet:
https://www.vtadiy.com/loadline-calculators/loadline-calculator/
Tip: active load like a CCS is a reactive load of practically infinity
Power Supplies and Regulation¶
Wave Rectification¶
Reservoir Capacitors¶
Maida Regulators¶
Precision high voltage regulator. Will do pretty much whatever you want. You need at least 15v to drop across it, but don't go crazy high because that power mosfet has to bleed it all off. I have had interesting experiences with how hot these get.
Schematic and BOM Configurator Tool
The person who made this decided to start selling it and went back and removed the schematics and configurator so I had to dig them up. Here is what should be rev2.0 based off what I read on the forum.
Shunt Regulators¶
Design notes and trickery¶
Layout should be carefully constructed to minimize "runs" i.e. wire runs. They should be as short as possible.
Use star ground method. Use a 12-16AWG solid copper conductor as a "spine" or ground bar. This works really well. Ground loops will result in hum.
Layout should be constructed such that sensitive components are as far as possible from power components. Transformers are especially bad.
Things with magnetic fields (transformers, inductors) should be placed ideally away from each other, and at right angles so the fields minimally interact with each other.
Wirewound resistors these days are not inductive. They should not affect signals going through them.
Silver-plated ptfe wire is very common in aerospace and industrial applications, therefore ebay is swarming with it for cheap. Thin jacketing, high quality, slippery. Stripping this stuff sucks.
Carbon film resistors are a popular audiophile choice for "smoothness". KOA SPEER SPR series resistors are "Kiwame" but available on Mouser for cheap, and they're awesome.
Shielding is often overlooked and underdone. Different materials also exhibit different shielding properties. For audio frequencies my research points towards stainless steel as being the best.
"Audio taper" pots are just two linear tracks stitched together. Shunt pots like TKD pots are horrible as they screw with impedance. For this reason as well, shunt topology stepped attenuators are annoying too. Series based stepped attenuators are awesome and with precision resistors have no balancing problems. Relay driven binary stepped attenuators are the best (most steps or resolution).
Switchcraft 13B (or similar) jacks are what you want. Switchcraft uses a very nice alloy with good spring tension characteristics in their connectors. Nickel plating is what you're going to get for these. They're used by professionals so they're made to stand up to hard abuse.
Lead-free solder is dendritic, so over time joints can pop apart. It also is more damaging to components since more heat is required to solder with it.
Gold is an embrittling agent in solder. Therefore, gold plated connectors are bad with solder.
Silver oxide is more conductive than gold.
Shielding tips:
- use stainless plates to separate transformer from circuitry (if using a toroid inside a chassis)
- slip a braided shield around heater wires for extra emi absorbtion
- use copper foil tape to wrap sensitive components like output capacitors and attenuators in a faraday cage
- use copper mesh to make a faraday cage for input/driver tubes if you don't want to use a classic tube shield
synergy snake oil
headphone cable
tube amps harman student crackatwoa custom