GM70 Monoblocks’

[Paul Barker]

I was thinking 200v 25ma but I agree more would be better.

Hang on I’ll just have a look.

Its nearly there, but starting to veer off linear at first sight.(though I havent measured the 2nd, and often its a surprise when I do) But since the EL34 has more headroom within better looking grid lines (ditto, not measured) it is the better option for full power.

I guess 801a much better linearity and 211 ….nice, but I’m taking a break from dht heaters. Im not convinced dh valves are better, all things considered. Except in the final stage. Or is it not bettr there and I should mofo at the output. Lol. No!

[Nick]

Well I am not fighting for that valve . But a flat load line does wonders.

[Paul Barker]

Well I am not fighting for that valve . But a flat load line does wonders.

Yes I see 15v peak to peak and its beyond the requirement and looking again at the el34 there is no saying that is any more linear but requires almost double the input voltage. We cant see the grid lines outside the box, but we know the amp doesnt go into panic mode saying there is no more box I must stop all music.

Thanks for the point of view. There is nothing to lose.

Also I see the existing VA is enough as it stands to drive the el84 which in turn is enough to drive the GM70 and I dont have to fit a whole load of extra cathode resistors and caps in a smaller box. I can keep the cathode feedback of the VA that I like about it, and as the resistor load is big value most likely helps linearity. The current flow isnt beyond the hammond 150h chokes so I can put two of those per channel in the resistor chain. All valves in this smaller box are now small. And all known good sounding devices.

I was blinded by the one element of voltage output of the driver. I recon youre right, its got a good chance of simply working very well . The el34 amp is doing that in any case, so the VA is working well with that, it should do so with the el84. The el84 has more power than the 801a and wont hum. To coin a 2a3 quote “Ive never heard an el84 amp I didnt like.” That is genuin.

OK I got it at last.

It will be so much easier to build. 2 novel valves that I have and 3 bases. Only two cathode bypass caps which could mount on top and two recovered mcap silver gold coupling caps. Maybe the stepped attenuator wont be all that bad.

The project is shortened a great amount, and Im already bighting off more than I can chew.

Its a plan.

[Paul Barker]

Well I am not fighting for that valve . But a flat load line does wonders.

Yes I see 15v peak to peak and its beyond the requirement and looking again at the el34 there is no saying that is any more linear but requires almost double the input voltage. We cant see the grid lines outside the box, but we know the amp doesnt go into panic mode saying there is no more box I must stop all music.

Thanks for the point of view. There is nothing to lose.

Also I see the existing VA is enough as it stands to drive the el84 which in turn is enough to drive the GM70 and I dont have to fit a whole load of extra cathode resistors and caps in a smaller box. I can keep the cathode feedback of the VA that I like about it, and as the resistor load is big value most likely helps linearity. The current flow isnt beyond the hammond 150h chokes so I can put two of those per channel in the resistor chain. All valves in this smaller box are now small. And all known good sounding devices.

I was blinded by the one element of voltage output of the driver. I recon youre right, its got a good chance of simply working very well . The el34 amp is doing that in any case, so the VA is working well with that, it should do so with the el84. The el84 has more power than the 801a and wont hum. To coin a 2a3 quote “Ive never heard an el84 amp I didnt like.” That is genuin.

OK I got it at last.

It will be so much easier to build. 2 novel valves that I have and 3 bases. Only two cathode bypass caps which could mount on top and two recovered mcap silver gold coupling caps. Maybe the stepped attenuator wont be all that bad.

The project is shortened a great amount, and Im already bighting off more than I can chew.

Its a plan.

With a bit of luck the filament transformer provided is good to go.

[Paul Barker]

You are as usual, right to persist with me.. thank you? It is just much better for my situation, and it has every chance of being a great and powerful amp. A little less heavy. But not a lot less.

[Paul Barker]

So it should all fit to this chassis including its own power supply. The modern polypropylene caps are slimmer the ones pictured are cathode bypas in EL34 amp. I can mount caps in this amp likewise. The small transformer is 6.3 two windings will do all signal valves. Ill wind a small 5v torroid for the rectifier and mount it inside.

The el34 amp once this is built will then just have the output transformers and final valve stage. Using this same driver chassis which the gm70 project will get.

There will also be a transformer somewhere which will be on board power supply for the el34 power stage.

[Paul Barker]

That will then leave the gm70 power supply to build and output stage. Seperates I think for easier transport. Then watch out when I get a vehicle I’ll be setting off with it! Noone will be safe.

[Paul Barker]

If I use two of these

Instead of this

I have indirect heating 6 volt so dont need a dls.10 to delay b+ for the health of the E88cc and can get a cheap 6v transformer ready made. One of those cute little Chinese ones.

[Paul Barker]

Ordered a sheet of 1.7mm stainless steel, I’ll cut it to size glue it to the existing top with araldite. Meanwhile all iron to go on top will be cleaned and can sprayed maybe the blue colour one of the choke. I can listen to the present amp while Im building this so I can do a shoddy job!

[Paul Barker]

Ordered four 80 uF caps and two 4uF caps for this chassis. Two 80’s for power supply two for cathode bypas the two 4’s are the ultrapath power supply noise cancelling cap from cathode of el84’s to b+. Total cost £50 from same seller I last used who is a UK stockist . Shipping free this size order and discounted part cost too. Same caps as I put in the other amp.

I’ll also do the same noise cancelling two stage amp trick of both stages get same noise.

Both of these measures worked on the two stage EL34 amplifier. First I did the Jeremy Epstein Darling amp trick of same b+ no extra filtering for upstream valve. I was very happy no power supply noise. Then I added the ultrapath and that was night and day better yet. The el84 is mu 20 so the ratio is greater hence smaller ultrapath cap.

[Paul Barker]

How about this layout.
Imagine the new plain stainless top plate.Items mounted on top as seen. The capacitors same mounting bolt through side as pictured here on the EL34 amp. Two filament transformers and one more choke concealed inside. Rectifiers slow start because indirectly heated. Other than those parts two grid chokes and 4 very small hammond plate chokes. Mounted on hinged (for test and inspection) base panel on longish leeds.

Side panels with mounting bolts in covered with bamboo chopping boards from pound shop, routered to cover nuts and bolts. Side panels removable for maintenance.

Wiring point to point in usual Barker tradition.

That just leaves a few resistors to suspend from top plate.

[Paul Barker]

I might make a nickel core impedance matching pair of autoformers so this is a complete 1.9 wat SET amplifier aswell. The autoformers potted with in and out connections to inter connect from output of this driver to speaker wire for every day listening. Call it a energy saving hybrid like the cars.

[Nick]

Lots of progress today. I missed it till now as I was away driving in circles again all day.

[Paul Barker]

One birthday when I was long term sick my wife asked me if I wanted a track day. “ I said “not while I’m like this”! I never had that present.
My uncle says “if someone offers you something, take it; because they may not offer it again “!

[Paul Barker]

To get the 860VCT voltage to the B+ desired a 40v 50 watt transformer is connected in series with the 230v primary of the b+ transformer on my 243v mains supply.

Here is the signal section sketched out.



I miscounted anode chokes or left 2 in man shed.

Do you think I needs medicine I feel anxious when I haven’t bought enough for when shops not open. Im a bit like that with chokes.

The e88cc cathode resistors are two 220 ohm 1/2 watt in series by the way. In the el34 amp they measured slightly lower. Look at me Am I bothered?