ENGL E350 circuit and layout anyone?

[Mark]

Hi

I’m still working on a ENGL E350 classic valve 50 amp. I do have the attached circuit but it’s not particularly accurate.

The amp has a strange noise issue, where there is a swooshing sound which is louder in the hi gain channel, that would be normal, except I have the gain controls all the way down and the master volumes all the way up. I then pulled the first two preamp valves and the amp still was noisier in the hi gain channel. This is unusual as the amp has no preamp valves only the effects loop valve and the P.I. valve. If I ground the grid of the effects send triode, there isn’t any change in volume and the amp is suitably quiet apart from a click when changing channels as the mute circuit isn’t working.
What I find strange is with the first two 12AX7’s removed the voltage divider to reduce the gain of the gain channel R28 (470K) and R30 (47K) should make the noise quieter.

I have been looking at the boards and sketching out the real circuit, but it is laborious and timely. If anyone had the real circuit diagram and board layouts that would greatly assist me.

Thanks for your assistance. If anyone wants voltages of the circuit let me know and I can post them.

[Mark]

Okay, got it, no one has a E350 diagram and layout. Thanks for looking at my post anyway.

I would like to post some information to assist others fixing the E350 amp.

The amp mentioned a 230Vac primary and when taking the measurements mains voltage was 239.7Vac

Rail voltages.

B+1 (o/p txer primary ctr tap) is 451VDC and 420VDC when the amp is delivering full clean o/p.

B+2 (screen supply) is 446Vdc and 405Vdc when the amp is delivering full clean o/p.

B+3 (P.I. stage and effects loop) is 392Vdc and 360Vdc when the amp is delivering full clean o/p.

B+4 (preamp supply) is 370Vdc and 337Vdc when the amp is delivering full clean o/p.

V1a plate voltage 230.8Vdc, grid voltage 0Vdc, cathode voltage 1.39Vdc.

V1b plate voltage 223Vdc, grid voltage 0Vdc, cathode voltage 1.95Vdc

V2a plate voltage 231Vdc, grid voltage 0Vdc, cathode voltage 1.94Vdc

V2b plate voltage 233Vdc, grid voltage 9Vdc, cathode voltage 1.94Vdc

V3a plate voltage 250.3Vdc, grid voltage 0Vdc, cathode voltage 2.05Vdc

V3b plate voltage 219.6Vdc, grid voltage 0Vdc, cathode voltage 1.68Vdc

V4a plate voltage 266.8Vdc, grid voltage 67.7Vdc, cathode voltage 75.2Vdc

V4b plate voltage 258.6Vdc, grid voltage 65.6Vdc, cathode voltage 75.2Vdc

The bias voltage was a bit of a problem as I could only get-48.2Vdc, I had to modify the bias supply to deliver more negative voltage.

IMG_2220.jpeg

[Tone Lover]

Sloclone forum would probably have the info your looking for Mark.

[Mark]

Sloclone forum would probably have the info your looking for Mark.

Thanks for your recommendation, I will look into it.

I drew out a rough circuit that shows the real circuit and values as opposed to the ENGL E350 circuits online. I may do a better version later.

IMG_2221.jpeg

[Purgasound]

I reached out to Engl not too long ago and they provided me with a full schematic of one of their old power amps. I was surprised at how courteous and prompt they were to hand it over.

Is the sound you are describing rising a falling or is it a steady pink noise type of sound? The circuit in the schematic you linked above in the first post has a bunch of TL074's for the line out which are tapped off the PI and the OT secondary. If you can disconnect this wire or alternatively lift R88 and R56 you can disconnect this and they should have no other ties to the signal path. I've had TL072's go bad in some amps before and they make a nasty sound like a distorted waterfall. I always described it as "wooshing" so maybe your "swooshing" could be the same thing. I'd look at those opamps.

[Mark]

Sorry that I didn’t see your message sooner.

If I ground the input to the effects loop the amp is dead quiet. So I thought the issue is in the preamp somewhere. I haven’t given up on the idea that it’s a design fault, as I haven’t heard another E350 amp and nothing seems wrong on the amp.

ENGL are courteous but their diagrams are inaccurate, there are a few circuits online with the corrections.