Building the GM70 tube amplifier

GM70 Dual mono block amplifier.

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Buying information

In case you are interested to build this, please note this:

This is a very large amplifier, we recommend to put the power supply in a separate housing, so the result will be as hum-free as can be. Please look at the pictures we have on the website, to see how large this amplifier is. It is all designed in the style of Mr. Sakuma, and t has been build by several people before.
We do not sell this as a kit in a box, complete and ready packed.
What we do have, is tested schematics, and most of the parts as loose products.
We sell all critical parts the way you choose them. This means yoou could choose the better Yamamoto sockets, but you could as well use lower cost Chinese sockets. The same is for the capacitors, we recommend Mundorf, though this is not specifically a "Mundorf" design. They are used in the power supply at some critical places, you may not even find a good replacement easily. Furthermore Mundorf is used at some places were this influences sound, and is is possible to downgrade to lower costs parts here, by your own choice.
For this reason, the costs for the components is hard to say in advance, but it will be somewhere in between 3000..5000 Euro, depending how far you go with first class components or a downgrade to lower priced components.
All Lundahl signal transformers should not be replaced by other products. This includes the anode chokes of GM70.
For powering the heaters of the GM70, we have chosen the method with heater chokes, in order to decouple the power supply via chokes completely from the amplifier. Yet this can be done at lower cost with an electronic power supply, of your own design, or switched power supply from China.
In case you want to build this, please make your materials list, and we can make a price offer for you.
The mains transformer we don't have for you, but such are actually not expensive, and can be build by any transformer winder company. The specifications are not overly critical.

Building information

Note, there are more methods to build an amplifier. For legal reasons, we do can not advise you here, how to do this. We do not take responsibility for what your work. So the following text is only how I build this amplifier myself.

I prefer to take some more time, and test things step by step. Like this, any error is detected right away.

First it needs to be sure, the high voltage and low voltage transformers are wired correctly, so the "stand by" and "on" and "off" function work properly. For this, wire the switch circuit for the both transformers, attach the low voltage transformer primary winding(s). Do not connect the secondary windings yet. Instead of the high voltage transformer, you connect a light bulb. Connect a 6V light bulb to the connections of the transformer for the rectifier heaters. After you have done so, you can test the transformer switch, and you understand how "stand by" an "on" and "off" is working, because you see the light bulbs go on an off. After this works, go to the next step.
Wire the heaters of the two rectifier diodes, and no other wiring at this moment. Insert the diodes, and exercise now how to switch on the amplifier. First go to "stand by". You can observe how the Mercury makes the cold glass opaque at first, and then in becomes clear when the glass warms up. After the glass becomes clear, you can switch to to "on" and the high voltage lamp will burn, this simulates the high voltage is switched on.
Construct the GM70 heater circuits, including bias and cathode resistors, both for power supply and amplifier. These circuits are fully passive. (Passive means, no active components such as transistors or IC's are used for regulation, and no trouble related to this). So you can indeed build first a heater circuit for just one GM70 only. Check if the GM70 heater burns at 10 Volts DC. Also AC voltage on the heater should be close to zero, or you have wired the heater chokes wrong.
Once the heater circuits and transformer switching is working, you could construct the passive amplifier circuit around the first GM70 tube (the first stage). Check and test if the gain function and potentiometer is working, so you should have a gain factor of 2.5 x indeed all the way to the socket connection of the GM70 grid. Then, without connecting yet coupling capacitor C12, build and test the second LL1660 gain stage the same way. Then, without coupling capacitor C16 installed, wire the output transformer circuit completely. If done correct, you can attach a 1kHz, 1 Volt signal to pin 11 of Transformer T13, and already now, a (very) soft tone should appear from the loudspeaker. If you have no tone generator, you should buy one, but indeed it is possible to make a direct connection from the RCA input connector to pin 11 of Transformer T13, and play some music. Though sound will be very silent, it will be just audible, when you listen directly into the speaker, because the output transformer is high impedance at it's input.
Wire both bias potentiometers such that the minimum resistance is achieved while turning them fully clockwise.
Now with the mains plug and GM70 tubes removed, measure the resistance to ground via one the GM70 heater connections, directly inside the socket. Check the functioning of the bias pot meter. You should measure from 600 Ohms to 1600 Ohms for the left GM70, and 800 Ohms to 1800 Ohms for the right GM70, and check if this is so on BOTH heater pins.
Turn the bias pot meters all to the right, and connect a + 12Volt DC power supply to Test points TP1 and TP2. This gives a current of 21.4mA through R11 or R19, and so the Yamamoto meters should then indicate 21.4mA, proving they work good.
These tests above here prove functionality of a large part of the circuitry, before you connect any high voltage wiring.
Wire to wire the amplifier + power supply fully, but keep the mains plug out.
Connect a voltmeter to test point TP1, and realize a value measured of 200mV corresponds to 200mA output of the power supply. This is the maximum current, the power supply can and should deliver.
Tightly connect an 8 ohms (10Watt) load resistor to the speaker output terminals. Set hum pot meters initially to the center position. Turn bias pot meters all to the left.
Now comes first time high voltage testing.
After switch off, the capacitors need some time to discharge. Please measure this yoruself, so you know what you have. At the power supply, we have chosen to use TubeCaps from Mundorf, because of their superior Q-factor. Some words about those: These are no capacitors of the regular kind. These are fully new products. Dimensions are smaller than foil caps, and larger than electrolytics of the same value. Internally they are none of those two. TubeCaps have most of all incredible low internal resistance, much lower than an electrolytic, and most of all, the internal resistance will no go up as they age. Such wonderful caps. Yet, do NOT short them to discharge. Technically they probably survive, but the energy peak is surprisingly high, much higher as with an electrolystic cap.
If you have an appropriate load resistor (4700 Ohms, 100 Watt) you could test the power supply without amplifier connected. . Since most people won't have such a resistor at hand, the power supply gets tested directly with the amplifier as a load. Do not test the power supply without a load, because output voltage may go up to almost 1200 Volts, risking the capacitors. Though the capacitors can take that, 1200 Volts (unloaded) makes a big difference to 920 Volts (loaded) , also t for the user. For this reason, at first operation, monitor the current through the Yamomoto meters, and stop when the current is too high. Also monitor the voltage of TP2, it's function is to monitor the total current out of the power supply.
Before you begin, verify of T9, T11, and T12 are connected the right way, so pin9 to pin12. With T3 check the wiring also. This is important, if you reverse by mistake one of the two coils, the choke becomes a copper wire only. This mistake is often made. So check once more.
Under miscellaneous parts, you find the NTC resistor for soft start of the mains voltage. Such NTC are standard devices, all large shops like Farnell or RS components sell those for the the required AC "on" current. They are reliable and low cost. For a second soft start, you do need to wait a few minutes of course, so the NTS gets cold again.
Connect the mains plug, and switch the pre-heater function on. All tubes should glow, and the rectifiers get opaque. High Voltage is still off.
After the rectifiers glass has become clear, do a prayer, and switch the High voltage on for the first time. Observe the Yamamoto meters, and the voltage of TP1. TP1 should be lower than 200mV, so less than 200mA current is flowing. The current on the Yamamoto meters should each be lower than 100mA. Check if the power supply current is exactly the sum of the Yamamoto meters. Check of the bias pot meters do their function. If yes: CONGRATULATIONS, the amplifier is biasing as it should.
Now comes final testing and adjustment. Switch off again, and connect loudspeakers and set the volume to zero. Set the right meter (output tube) for 100mA or a bit lower, to your own choice. Then, set the driver tube initially to a medium level plate current. Now adjust the hum potmeters, for minimum audible hum. If there is no hum, just set them to the center. If there is hum, set the output hum pot meter to the center, and first adjust the driver tube for minimum hum, and then the output tube. Then again adjust each tube by itself until a zero hum adjustment is found.
Now finally test the whole amplifier, it should be working. The distortion cancellation can be done by ear (setting of pot meter R10) or by measurement (see below here). Yet it is a matter of taste to allow some minimum level of harmonics if you prefer, so an electrical measurement or test by hearing will likely result in different setting, depending also on loudspeaker efficiency, as inefficient loudspeakers force the amplifier to work at higher harmonics level. Wheras very sensitive loudspeakers may sound better at somewhat lower plate current of the output tube, and higher plate current of the input tube. However this is very individual, and the possibility to find your an personal operating point, to your own speakers, is the merit of this amplifier.