GM70 SE — a summer idea for a winter amplifier
ADDENDUM: Dariusz’s GM70 SE build from Poland
ADDENDUM 2: another GM70 SE version from Poland
As its name suggests, GM70 SE is a single-ended amplifier based on the Russian GM70 output triode. This large, powerful and very linear triode can dissipate as much as 125 W. Exploiting even part of its impressive capabilities requires a literally lethal supply of around 1,000 V, 60 W of heater power and a serious driver stage.
How do you decide to build such an amplifier? Simple: let someone talk you into it.
Besides being a diving expert, Toni Plančić is a great DIY audio enthusiast. His first love is tube equipment, which he builds with such precision and craftsmanship that every creation looks like a high-end product from an expensive specialist manufacturer.
The summer before last, cooling ourselves with wine spritzers and enjoying our leisure on a local café terrace, discussing the world and dreaming of future projects, we struck a bargain. I would make a working prototype; Toni would handle the mechanical construction, always my least favourite part. The amplifier would be a GM70 SE.
The tubes had been sitting on my shelf for several years. After seeing them a few days earlier, Toni began imagining a large triode amplifier unlike anything he had yet built. A few spritzers too many, a little persuasion from Toni, and the deal was done. His unused pair of suitable output transformers helped the decision considerably. At that moment it seemed a simple summer diversion.
A few months later the amplifier played music. After several more weeks of adjusting operating points and trying coupling capacitors, resistors and the other small details of amplifier development, the result exceeded expectations. It sounded as powerful as a strong push-pull amplifier and as refined as the best single-ended triode designs. Measurements fully supported the listening impressions. Its only drawback becomes obvious in summer: the GM70 runs very hot. This is an ideal winter amplifier.
The topology is very simple: three successive common-cathode triode stages. The first uses LED bias; the other two conventional fixed bias. All three tubes are Russian. The 6S3P is a high-gain single triode, the 6S19P a very-low-gain triode with low internal resistance and high current capability, and the final triode is the GM70. Coupling capacitors are Russian PTFE film types; anode resistors are Russian 2 W metal-film. The first power-supply filter capacitor is electrolytic; the remaining capacitors are Italian ICAR film types and Russian MBGO 100 µF/1,000 V paper-in-oil units. Power transformers and chokes were made by Emat of Novo Čiče. Toni’s amplifier and the prototype use Serbian Trafomatic output transformers; mine uses Italian F.I.A.T. srl units. Both are multilayer-wound on substantial double-C cores and measure and sound excellent. Interestingly, Rome’s F.I.A.T. was considerably cheaper than Trafomatic, even allowing for a supposed ‘special price for you’.
Toni devised a layered mechanical construction, packing all components into a relatively small space on a thick aluminium bottom plate, with separate iron enclosures or covers for individual sections. I have never seen or tried lifting an amplifier of these dimensions with such weight. Despite the difficulty of handling its roughly 40 kg, the effort was worthwhile.
The lesson is clear: wine spritzers help you make the right decisions.
1 W / 8 Ω: < 0.03% THD
10 W / 8 Ω: < 0.08% THD
20 W / 8 Ω: < 10% THD
Frequency response: 20 Hz–20 kHz, −0.5 dB
Sound: 😀
ADDENDUM: Dariusz’s GM70 SE build from Poland
A year or two after this project appeared, I received an email from Dariusz in Poland. He wanted to build a GM70 SE and asked about details of the project and my general impressions and experience. At first I doubted that his plans would become reality. After a while, however, emails began arriving at almost regular intervals, reporting his progress.
Dariusz is not the fastest builder, but appears extremely thorough, meticulous and skilful — a true perfectionist. The amplifier was finished in a little under a year. Unlike my project, his uses three enclosures: one for the power supply and one for each channel. Another difference is the output tube’s grid choke.
The workmanship is impeccable. Although he praised the sound, Dariusz was not completely satisfied. When sourcing parts, availability and positive experience had led him to choose a Polish manufacturer’s output transformers instead of F.I.A.T. or Trafomatic. The resulting measurements were somewhat poorer than the original project’s.
To improve matters, new high-quality output transformers were designed and built. With these the amplifier finally breathed freely and delighted its hardworking builder.
Dariusz’s comments and photographs of his project follow.
‘I am presenting my tube amplifier project, built around three triodes with a semiconductor-rectified power supply of approximately 970 V. When assembled correctly, this configuration causes no operating problems. The main power supply is common to both channels; heater and negative supplies are separate for each channel.
The transformers have wound and cut cores made from annealed, very-low-loss, 0.23 mm silicon-steel sheet. They were made to my dimensional specification. Their unusual dimensions meant that suitable bobbins were unavailable, so I designed and made them myself.
Split bobbins were used, not simply because of the unusual dimensions but because the winding arrangement is especially suitable for higher operating voltages.
The highest-quality insulation materials were used to ensure safety, performance and durability.
New transformer parameters:
Ra: 8 kΩ
Load: 4 Ω, 8 Ω
P = 30 W at 30 Hz, THD < 5%
I₀ = 90–120 mA
L = 100 H around the operating point
Rp = 165 Ω
New transformer frequency range:
6 Hz–53 kHz, −3 dB, 1 W, 8 Ω load
6 Hz–51 kHz, −3 dB, 1 W, 4 Ω load
14 Hz–28 kHz, −0.5 dB, 1 W, 8 Ω load
14 Hz–25 kHz, −0.5 dB, 1 W, 4 Ω load
18 Hz–38 kHz, −3 dB, 30 W, 8 Ω load
18 Hz–37 kHz, −3 dB, 30 W, 4 Ω load
25 Hz–25 kHz, −0.5 dB, 30 W, 8 Ω load
25 Hz–22 kHz, −0.5 dB, 30 W, 4 Ω load
Build cost: €3,000.
The sound is beyond the scale. At the next stage, the amplifier will drive a GM100 tube.’
Dariusz Lesner
R1 — 1 kΩ
R2 — 330 kΩ
R3 — 250 Ω
R4 — 33 kΩ, 10 W
R5 — 4 kΩ, 10 W
R6 — 11 kΩ, 20 W
R7 — 1 kΩ, 5 W
R8 — 380 kΩ, 5 W
R9 — 100 mA milliammeter
R10 — 470 Ω, 5 W
R11 — 160 kΩ, 2 W
R12 — 51 Ω, 10 W
R13 — 51 Ω, 10 W
R14 — 200 mA milliammeter
P1 — 100 Ω potentiometer, 25 W
C1 — 0.1 µF, 600 V
C2 — 50 µF, 600 V
C3 — 0.22 µF, 1,000 V
C4 — 10 µF, at least 160 V
C5 — 10 µF, at least 160 V
C6–C14 — 680 nF, 1,200 V
L2 — MagneQuest grid choke, model BCP-13 GC NI
ADDENDUM 2: another GM70 SE version from Poland
Dariusz wrote to say that his friend had also built the amplifier, and sent photographs of this impressive work.
drzlab.com/