GM-70 mono block amplifier

Made in Poland by a small local company which preferred to have their name withheld.

Lampized by Fikus in 19th april 2009

OK, so there is the story. I felt that I need a new amp.
I asked myself some questions about the criteria.
I figured that I want:

So there I was - my choice is really limited to four supertriodes: 845, 805, 211 and the Russian copy - GM70.
Of course - I can not afford the Western made amp like that. A SET Mono costs minimum 10 000 Euro per pair or much more.
In China at  the moment only Meixing Mingda offers a mono 805 amp pair for 1490  USD with 500 USD shipping, plus EU customs and VAT. So we are talking in the end 2000 Euros. Not bad, but still they probably require substantial reworking.

By lucky coincident I found an auction for the Polish made "garage industry" amplifier pair - using the mighty GM70.
The pair sold for 1000 Eu with two separate monstrous power supplies.
I expected something fishy,  So cheap for the real amp look-alikes ? But hey, what the hell, if worse comes to worst I will have to RE-DO the whole amps but EVENTUALLY I will have them singing. So I took the risk and the plunge. In my imagination - I bought the case and tubes with sockets, I did not expect HIGH-END for one grand.

GM70 Single Ended Directly Heated Triode mono amplifiers

The black wiener-dog thingies are power transformers for dual 250 and separately for dual 620 V AC in two boxes for L and R channel,  before these voltages get rectified inside the amps.

gm70 amplifier

I brought them home and admired for a moment - just what I needed - a Russkij  Gakuon for the poor. In my very own living room.

But after first listening - my face looked less and less happy.
They played something, no, not MUTE. But the music was poor. It was like from a very cheap stock music angel or worse.
So what I had - was not a complete melon, but not a gem either.

I started scratching my head and sniffing around.
The amps internally look okay. Hand made - that's for sure, but made well.
All supplies are separately tube rectified. (including the big GM70)
All supplies have chokes - GOOOOD.
All supplies have CLC topology with good caps.
All supplies have separate transformers.
So far so good. So why such poor sound?
I tested every point and I found that one transformer had mis-wired secondary and produced only 1/4 of voltage for GM70 Anode.
BINGO. This amp circulated around Poland like a hot potato and nobody seemed to like it. Nobody was able to fix it either. Hence the price.

After assembling it again - ooooo - much better. Now it plays like a nice stereo kit. Sound is even, stable, image focused - all is OK.
I expected a sonic blowjob, not mere Okay sound. I had to do better. This amp was still missing somehing compared to the Music Angel 845 with Dietmar front end.
But potentially it should be mopping the floor with the Angel because of the whole construction being so much superior.

I spent many weekends experimenting - with different options of the input and driver.

Now is the tricky part:
1. There is ONLY ONE tube at the front end. Normally - there should be at least two.
2. This tube is 6H6P which has small amplification ratio.
3. GM70 likes strong drivers, maybe a DHT like 300B or something.
4. Dietmar schematics to the rescue ! But - it  is calling for 4 triodes, I have 2 in one tube. No good, no good.

A possible way out is adding one more tube (2 triodes) and that would do. I could for example add a 6H2P for input amplification and the 6H6P would become a powerful driver.
But that requires drilling one hole for the socket and getting a "parasitic" power supply and heaters.
So I desperately tried to keep it at one tube and finally I arrived at the scheme that works.
It is not Gaku-On but it plays like my best Angels and Liungs used to play. It plays VERY VERY well. And the amp is so simple, so pure, without interstage transformers, complicated circuits - nothing like that.

On good lampized CD the music really makes me shiver in excitement the way a SET DHT should.

The tube circuit for CD lampizator is a very easy job, because the triode - capable of 200 V signal swing - is swinging only 2 V so it is loaded at 1% of its capability so to speak.
In this amp - the operating point must be DEAD  SPOT-ON. And each and every parameter is utilized to the maximum - voltage, current, power, heat and so on.
For the lampizator - this tube can work at 50 V, 100, 150, 200 or 250. It can draw 2 mA or 20, does not matter. But in the amp - every ohm matters etc. This one tube must provide a circa 50 X amplification and do it with very low output impedance.  Any smallest error and goodbye good sound. The GM70 will distort like hell.

Anyway - below is the schematics.

GM70 triode mono

ERRATA: There is an obvious error in my drawing - the GM70 triode is DC heated at 20 V. Not AC.
The +820V DC supply has one more 100uF capacitor by 1200 V DC after the rectifier and before the choke.

In red area - the section invented by me. Changed from the original which I did not like too much. It sounded far less like SET.  Now it sounds already equal to Liung in Dietmar's version, or Music Angel. Quite mature, maybe even more refined sound. It can be attributed to tube rectifiers in power supply of both tubes and to better output transformers, or both factors combined.

This setup I came up with for the input/driver is simple, cheap, easy to build, works stable, not too hot, the 6n6P does not get abused, and sounds very very good. I will keep this solution. There is no noise, no hum, no pop, no click.  But there is BASS, soo goood, mmm. The voices are like ghosts. The space is a huuge 3D thing. Oh man, triodes are the way to go.

I like the fact that I was able to keep one tube only in input section, there is no need for interstage transformer, no need to use 300B as a driver, but to keep the signal high enough I had to use in the first triode the cathode resistor 1K with  bypass electrolyte (Oscon SEPC series 470uF/25V), and no need to use exotic tubes.
My beloved good old 6n6P is just a great versatile tube.
Besides - I did not have to mix russian GM-70 with non-russian tubes, making this amp less kosher. This way it is a true monument of Russkaja Tiechnika at its best. And it is deployed for the MUSIC and not for the nukes ! Thank God the guards of the Russian National Defense warehouses and stock depos are easy to bribe with vodka to remove some "unnecessary" parts from their war machine.

The reason why I did not try to play GM70 in class A2 with 1200 VDC power supply and high grid current was the necessity to build triple series caps for anode to reach 1300 V ratings and order custom chokes and build a special DHT driver section.
My GM70 is great as starting kit for the DHT supertriode territory exploration.
So far - the GM70 NOS tubes come cheap and plenty so I built up myself a stock for the future. Mr. Putin will not threaten me with an embargo. I have supply for 20 years !

After one year I confirm everything that I wrote. GM70 is a great triode - cheap, stable, nothing goes wrong with it. I installed a 24 position attenuator in the front plate and I have nice adjusted volume monos.
NOTHING is broken during the first year, except that all screws are loose (everything was open too many times).

Copper plate version of GM-70

Recently I bought the COPPER plate triodes. about 5 x the price of graphite ones. (USD 200 a pair, new and mint, made in 1982. The year of "almost nearly" Soviet intervention in Poland)
The first thing that stroke me was (no, not a 900 V shock)  the light. These triodes probably emit the brightest filiament light of all tubes in the world. Like 40 watt bulbs, but much warmer, reddish from the copper plate refflection. Looking into this light is magical, like into a fireplace.

The sound is good, maybe 10 % better in mids, equal in highs, and 50 % weaker in the bass.
It may be my amp operating points being not optimal for this tube, but "internet people" agree - weaker bass and better mids. This is closer to 300B sound. For me - overall net change is negative. I absolutely love the graphite GM70. So I can have a spare pair of copper for the hell of it, but for every day operation I am gonna stick with the graphite.
Maybe on lesser speakers - like minimonitors - the bass issue would not matter, but mids would be better enough with copper. But on my no compromise Tesla bass setup - I miss the THUNDER.

Three and a half years later - GM70 Filus Lampizator amp is born.

Three and a half years later -

first of all, the wooden Polish amps served me flawlessly all the years and I never had to change a single tube.
After setting up my business as a DAC manufacturer, I used the amps to evaluate my DACS prior to sending. But I also felt that my DAC customers are struggling with their amps. Either too weak (300B) or too brutal (Push Pull KT88) or simply - transistor (not musical sound).  I saw the opportunity to offer a good amp to those customers.

So far, I havent seen any serious competition to "big triode" amps - and only GM70, 845, 805, 211 and 813 tubes make me interested at all.

I had the following design criteria: Triode mode on all stages, Zero feedback global or local, zero silicon in the amplification circuit,  no signal cable inside,  no sound altering tricks anywhere, no interstage transformers, no PCB,

Since I have a reputation as a Soviet technology advocate - I decided to skip the Chinese Suguang imitations of 845 and 211 and settle on the real deal NOS GM70 triode. Especially that it is: A) cheaper and B) much more powerful than the 845. The supplies of the NOC triodes should last our lifetime.

I chose monoblock approach to reduce overall unit weight to be more manageable
I chose tube rectification for all stages.
I chose one box approach over the separate supply box for simplicity and short distance from power source to the consumer.

The design is a result of my DAC experience - the first two tubes - input and driver - are configured and powered just like in the Lampizator Level4.

I designed the main box to be large enough so I never have to take space-related compromises. EVERYTHING I need - will fit in.

I designed the top plate that holds everything. The amp can be operated without the box - the top plate holds all transformers (4 of them) and all parts and tube sockets and RCA sockets and speaker terminals - EVERYTHING on one thick aluminum plate.

The choice of the box is optional - as wooden, metal or black neutral.

The choice of anode voltage is a practical 1000 V, not over 1KV,  to be able to build 1000 V supplies. Going over 1000 V requires very hard gymnastics and redesigning everything and probably more space and weight that we can imagine in anyone's living room.

The small tubes must be powered with around half of the main anode voltage, meaning - 450 V DC
The  driver tube must be as current-capable as possible. Hence the choice of 6N6P doubled which can swing 300 V peak to peak at 30 mA current.
The input tube must be linear and pure. The QUALITY is of paramount importance here. 

Summing up: I built the Lampizator analog stage for the small tube section, with EI power transformer, toroidal powered AC heaters, choke loaded tube rectifier, CRC separate sections for every tube anode (three in total).
The big power section is a single ended pure class A zero feedback design with 1000 VDC anode supply, CLC power filter, 0,3 mm steel output transformer (laminated, annealed, multi section).
The bias is manual (with negative supplies and a pot for adjustment and the needle meter for verification). The choice of excellent Ogonowski Output Transformer was preceeded by othert transformer testing and the Ogonowski was a winner by a good margin. No, it doesnt have silver wiring but otherwise it is there with the best.

There are no resistor in cathode section.

The power tube heaters are Soviet designed DC scheme from dual power windings of +/- 20 V.
Each power tube is precisely matched and manually de-hummed.

One of the design criteria was ZERO SIGNAL CABLE INSIDE. Meaning - there is ONLY your own precious cable between source and amplifier's input tube. I don't run 2 feet of no name cable inside to drive the signal from behind of the amp to the front. Whatever is your choice of interconnect - it wil be the only one we will have in the chain. Thats why the RCA socket is so close to the front where the input tube has it's grid. The pot (stepped attenuator) is the only part between RCA and tube grid. As a necesstity we use 2 inches of solid core silver wire from RCA to the pot but no more than that. So if your favorite flavor of cable is say Van den Hul Carbon - we will not contaminate the system sound by adding 2 feet of mickey mopuse cable. Or even a good Siltech for that matter. Only your cable will play.

GM-70 Monoblock pictures

Above: the silver front version of amps with the Lampizator DAC in the middle

Mr. Liang and Lampizator

Above: the Lampizator mono amp (left) compared to Mr. Liang stereo integrated from China (based on 845).


Above: the pair of prototype mono amps with black front.


4 years later:

Time flies, it is 2013 now. It 's been 4 years since I devoted myself to the audio business, no longer a hobby. Looking back - here is what I think about the article written above:

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