GM70 PP

After the success of the 813 PP amplifier, I needed to future proof the design and that meant changing the 813s as they had become expensive and hard to get. The GM70 had been an option from the beginning and now seemed the best option.

The journey is documented here.

GM70 PSU HT

GM70 PSU LT

GM70 Audio

The architecture of this amplifier is reminiscent of the theatre/PA amplifiers of the 30s and 40s by Western Electric and Altec. I love the simplicity of these circuits bought up to date with modern iron.

Starting with the power supply, there is a seperate supply for each stage. This proved to be a major improvement over the decoupled version off the main HT. There is also a lot less heat as the dropping resistors were dissipating about 40W. Each are choke input for best regulation and smooth operation of the rectifiers. I’ve used 6CL3 damper diodes now in place of the previous MV types as there was no future for them being hard to get and expensive. Similarly, I was using mercury for the first stage but had no spares going forward so used a direct heated 4V rectifier, DW4 which could use the same filament transformer. I prefer the sound to the MV rectifiers so suspect I had some RF interference now the chassis was a lot more compact. The supplies are connected to the audio chassis via 1000V rated shrouded 2mm test plugs and sockets. The double switches are now not required as there’s no need to preheat the MVs; the damper diodes and DHT rectifier ensure a slow ramp up of the HT supplies.

The LT supplies for the filaments are a voltage controlled current source (VCCS) design from the Audio-Talk forum which were suitable for the majority of lower current and voltage DHTs. They were modified to run the GM70 at higher voltage and current. The Meanwell SMPS were used to save space, weight and cost. There might be a downside but I’m not sure what that would be over conventional supplies. I’m not aware of high frequency artifacts using them like this. They had been used to run the GM70 direct via a CMC choke to good effect however the VCCS does improve performance. Rod Coleman and Guido Tent have similar DHT supplies. The VCCS supplies have all the advantages of DC, quiet, without the disadvantages that many DC supplies have compared to AC. The combination of well smoothed HT and these filament supplies means the amplifier is very quiet. Only when I stick my ear right in the speaker can I tell it’s on.

The bias supply is more complicated than many PP amplifier bias supplies. I think there is an assumption that they don’t need to be that good as it’s low current and the PP stage cancels any noise. As the grid resistance needs to be kept under 30k, the bias supply draws 8mA so not that low in current hence the more sophisticated design. I’ve found the better I made it, the better the sound. I may not have finished in this area yet.

The audio circuit need only be two stage in my system as I have reasonably efficient speakers. The Lundahl 1676 is a superb performer as a phase splitter at the input. I used the 4P1L based on it’s reputation and I preferred it to my first choice 46. However the 4P1L is quite microphonic but that doesn’t worry me. Selection can reduce that and better mounting/isolation should improve things on the rebuild.

The 4P1L, configured as triodes are transformer coupled to the GM70s. 4P1L is a small transmitter valve and can be used as finals so in effect the GM70s are being driven by a small power amp. It is probably not an easy amplifier to drive and should be driven by a relatively low impedance line stage or similar.

I used fixed bias to again, reduce the heat, have easy adjustment of the DC balance and gauge the effect of bias point on the sound quality. The DC resistance in the GM70 grid should be kept under 30k to ensure stability and prevent the GM70s from runaway. The GM70 has the advantage of no top cap keeping the high voltages inside. It is relatively easy to drive, fantastic value for money and sounds great. Apparently the copper versions are a step up in some areas but come at too great a cost for me.

I used a Sowter output transformer that was in use for the 813 amplifier. It has configurable secondaries so the a-a primary impedance could be 7k or 14k. I’m running 7k at present but can try 14k which could be a lower distortion option.

In a rough test using an 8R dummy load, I saw about 21 to 22V out before visible distortion of the sine wave so approximately 55 to 60W out. That required about 7V input so this is hardly a standard gain power amp but that’s not needed in my system to go very loud even without gain in the line stage. The point of this amp was to provide effortless sound with huge headroom which I think it achieves.

Top and side views, audio chassis and power supply base.

This chassis has been in use for about 10 years for the various builds and it has sides now for safety reasons as it is my daily driver in a shared living space. Exposed 1000V could be a killer. I intend to make it considerably narrower but similar height and depth.

Planned Layout

Although the layout means the rectifiers are hidden, I need the chassis to be narrow and split in two otherwise it’s too heavy and given the packaging, seperation allows it to be worked on. It has casters so it can be moved easily. Size is about 400mm from lower deck to upper, 430mm deep and 220mm wide (currently 350mm).