Hiwatt design question

[catalin gramada]

Man...I don.t know nothing about current distribution path into ground planes but looking at junk Sound City chassis, where any ground points are all over around everything seems to have a lot of sense. Signal ground path follow different traces from PS returns. It is far better than a bus where currents are forced to share common path. I have to reconsider all my grounding thoughts, this amplifier didn.t bear any audible noise but hiss, as thermal noise from junked components I believe...

[mhuss]

The important thing is to keep the heavy output stage currents away from the rest of the signal grounds. In a classic Hiwatt (for example), all the preamp grounds are connected to a single point near the input jacks. The presence circuit and output tubes are all grounded to the chassis at the sockets. The main power ground is at the end of the row of output tubes, on the opposite corner from the inputs. So, while a lot of ground current flows through the chassis, none of it goes anywhere near the preamp grounds.

Like real estate, it's location,location, location.

[catalin gramada]

Thanks.
One more question,please. My PT is a Partridge TG9232. My mains voltage is 230V around. With primary selector connected at 225V I get 500V dc under load, way too much for EL34 screens. Changing to 245V tap, for the same 230V mains I get 450V which is also over specs but more safe anyway. The problems comes from heaters secondary which under load drop to 5.8- 5.9V.
Thinking to wind two more turns without to open the transformer, is it enough space between iron and coil to pass the thick wire if I remove external isolation ? (I have to do it at the both ends in opposite directions )

[g1]

Don't you have room in there for a separate 6.3V heater transformer? They are cheap.
For me, 5.8V is close enough for the heaters. Less than 10% low.

[mhuss]

Theoretically, an EL34 should take 600v on the screens and 800v on the plates. (Ah, the good old days!) Most Hiwatts I've worked on have had 520v or more going to the OT and a bit less to the screens.

[Helmholtz]

Theoretically, an EL34 should take 600v on the screens and 800v on the plates. (Ah, the good old days!) Most Hiwatts I've worked on have had 520v or more going to the OT and a bit less to the screens.

I checked some original EL34 specs. The limiting value for the screen voltage varies between 425V (Philips, Valvo, Mullard) and 500V (Telefunken 1969).

[mhuss]

You are correct, the Mullard EL34 was rated for 425 screen volts as well. I was thinking of the KT88.

This is why the crazy high voltage amps like the Hiwatt DR201 and Fender PS300 have screen voltage more or less one half that of the plate voltage.

[g1]

You are correct, the Mullard EL34 was rated for 425 screen volts as well. I was thinking of the KT88.

This is why the crazy high voltage amps like the Hiwatt DR201 and Fender PS300 have screen voltage more or less one half that of the plate voltage.

SVT and Musicman amps also.

[Helmholtz]

You are correct, the Mullard EL34 was rated for 425 screen volts as well.

No wonder as the Mullard, Valvo, Mazda, Philips (and some more) brands all belonged to the Philips (of the Netherlands) group, so they all shared the same specs. I have even seen Valvo and Philips branded EL34s and ECC83s that were actually manufactured at the famous Mullard Blackburn (GB) factory.

[catalin gramada]

So, with what engineering argument those tubes was used at voltages around 500 V ? Looking arround some hand drawn schematics of 103 model I see the plate voltage 460V. Biasing pretty hot probably droping 10V arround over common 1K resistor. So maybe 450V arround for screens, more plausible. ...500V no way, it sucks screens like CocaCola