Pre-amp driver

[Nick]

If you are putting the secondary of the added transformer in series with the primary of the existing transformer, then I could imagine that it would sort of work if the added secondary is open, as the transformer will start to saturate if the voltage across its secondary goes much above the rated voltage, and so will tend to stay about the expected voltage.

[Paul Barker]

Its not a hill worth talking about. I’ll carry on doing what works for me and not bother to talk about it.

[Nick]

Its not a hill worth talking about. I’ll carry on doing what works for me and not bother to talk about it.

Fair enough. I just like to know why something works rather than just "its does", but that's just me.

[Paul Barker]

Because the el38 amplifier uses ECC88 input valve, though that is resistor loaded, the bias for the el38 is fixed through 1/2 an EZ80. On that amp there are two ez80’s because the 60mA current would over work the ez80. This time I’ve set the current at 45mA so the other half of one EZ80 for the other output valve. If it were an output transformer the power out would be 2.7w due to the lower b+ and lower current. So this can stay for a single purpose. The reason I have adapted to copying my EL38 design is because it sounds just so good, so I am copying the method through to this project. I still have the more powerful version set up as an se transformer. Space allowing that could come to Owston. This should make it and hopefully the GM70 40 watt SE Amplifier, and perhaps one pair of bookshelf speakers.

I may make another with a phase splitting transformer as the output connected with balanced outputs to which ever PP output valve I wish to driver, which would include EL84, EL36, 300b, 6b4g.

Employing for example the QuadII output transformers cathode feedback included.

I also have some 300b PP output transformers, and some Stereo 20 output transformers.

Nick I tested the voltage dropping transformer which is 18v + 18v nominally 36v secondarey connected to neutral mains in then it passes the 0utput from it to the primary of the power transformer. It shall also heat the pl38 heaters in parallel, and that will be referenced to circuit ground through a capacitor. The 30v heaters are therefore 10% under rated. The b+ I require for the circuit is 298v. The transformer doing this dual role is 120 watt so not over watts capability.

[Paul Barker]

The intended purpose of this dedicated driver amp is a 40 watt se amplifier, which utilises just one of a pair of power transformers, since they are 250mA each one will keep it lighter and simpler and make more capacitance out of the finite number of 1500v dc polypropylene caps. Such that no further costly caps required. Every project-is compromised somewhere, the more voltage required the more size weight and component costs encroach. Better build this than nothing SE more powerful than 5 watts.

This is allegedly 40 watts, but as I havent allowed for grid current with a dc CF driver and grid current may begin before zero bias, it may crash and burn before 40 watts, but I don't think Ive made an amp more powerful than this in any case. Out of all my valves the GM70 seems to allow most power more simply. OTOH the most beautiful sound I have made was with a single oxide coated 211, If I had another I would defo build that amp. I paid for another but the US so called friend, never sent it. But that was when they only cost £100. Would cost much more today. Anyway have to let these things go, or youre BP will rise and you risk another haemorrhage if you have Cerebral Amyloid Angiopathy . Getting angry you are only harming yourself. One of my stroke consultant spent the entire telephone appointment telling me I must remain calm and not do things for angry people unless it benefits people. Walk away and maintain you’re calm. He said. Possible if you can avoid all human contact! But then you die of loneliness and sadness.

So the ratio of the 18+18 in series transformer takes in the event 27 volts of the house mains away from the primary of the power transformer and lights up two PL38’s in parallel at 90% which I think is enough. It works out exactly right for the b+ of the project.

[Paul Barker]

The output tube distortion is 1% at 12 watts, certainly at home that would not be exceeded.

[Paul Barker]

The first Watt

[Nick]

So the ratio of the 18+18 in series transformer takes in the event 27 volts of the house mains away from the primary of the power transformer and lights up two PL38’s in parallel at 90% which I think is enough. It works out exactly right for the b+ of the project.

Again, as I say, I have no dog in this race, if it works, great, but if I understand you are powering your heaters with a direct non isolated supply from your household mains. Now I know, back in the day, mains driven heater strings and big WW dropper resistors were the thing. But given you are the one that is a qualified electrician you can see the issue given current regs?

[Paul Barker]

OK then I’ll use a separate transformer for that.

[Paul Barker]

8.7% thd mostly 2nd on the EL38 made no sense to drive a dht. Also, realistically I may not achiever the GM70 40 watt option this time,

So I have gone 6b4g 5% 2hd, driving 9 watt 300b 5% 2hd.





The driver amp shall have its own power supply. The 300b shall have the bench supply. Likewise the existing EL38 4 to 5 watt se integrated amplifier shall also use the bench supply. So a few sounds to compare, should achieve this. The opt’s for the 9 watt 300b amp are 4k 10 watt R Cores. In an old Music Angel 300b chassis which sounded underwhelming so I pulled it apart, and as that might have been the OPT’s I’ll use my R Cores. Also the power supply may have contributed to the underwhelming nature. Likewise the volt amp and driver of the Music Angel. So I’m not using anything but the bare chassis.

[Paul Barker]

8.7% thd mostly 2nd on the EL38 made no sense to drive a dht. Also, realistically I may not achiever the GM70 40 watt option this time,

So I have gone 6b4g 5% 2hd, driving 9 watt 300b 5% 2hd.





The driver amp shall have its own power supply. The 300b shall have the bench supply. Likewise the existing EL38 4 to 5 watt se integrated amplifier shall also use the bench supply. So a few sounds to compare, should achieve this. The opt’s for the 9 watt 300b amp are 4k 10 watt R Cores. In an old Music Angel 300b chassis which sounded underwhelming so I pulled it apart, and as that might have been the OPT’s I’ll use my R Cores. Also the power supply may have contributed to the underwhelming nature. Likewise the volt amp and driver of the Music Angel. So I’m not using anything but the bare chassis.

My bench supply is valve rectified choke input Black Gates. Music Angel don’t do it that way.

[Paul Barker]

It is highly unlikely that I can do any such thing. So I have backed down and just use the el38 amp as an LCR driver for the 300b output stage + 1meg resistor for the moments foolish people undo the cables after amps are powerd. Even though its a single bench power supply.

Actually the opt on the el38 amp is primary 5k plus some, which is a mythical number anyway as its a simple choke for this use, the load depends on frequency. With speakers that do little lower than 50hz it shouldn't further audibly limit bass that the loudspeaker already limits.

So I only have to build a pair of opts and 300b’s. Surely I can do that in time.

Two amp to listen to though. The el38 about 5 watts the 300b 8 watts.

Ive had to set a realistic goal, and stop my mind exceeding my present ability.

[Paul Barker]

It is highly unlikely that I can do any such thing. So I have backed down and just use the el38 amp as an LCR driver for the 300b output stage + 1meg resistor for the moments foolish people undo the cables after amps are powerd. Even though its a single bench power supply.

Actually the opt on the el38 amp is primary 5k plus some, which is a mythical number anyway as its a simple choke for this use, the load depends on frequency. With speakers that do little lower than 50hz it shouldn't further audibly limit bass that the loudspeaker already limits.

So I only have to build a pair of opts and 300b’s. Surely I can do that in time.

Two amp to listen to though. The el38 about 5 watts the 300b 8 watts.

Ive had to set a realistic goal, and stop my mind exceeding my present ability.

Woops.