Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on December 02, 2023, 03:39:27 pm

Title: How's the schematic?
Post by: Diverted on December 02, 2023, 03:39:27 pm

I just drew this from an EL84 amp a friend of mine brought over. A lot of the connections were buried and I wasn't able to get accurate readings of some of the resistors in circuit (they're all heat-shrinked and I don't want to remove it to check colors).
Anyway, how does this look? Any glaring, smoke-releasing errors that you see?

Thanks,

Ted

Title: Re: How's the schematic?
Post by: shooter on December 02, 2023, 04:10:37 pm

just drafting comments;
input jack might need a ground
note a + symbol for Ck


the 2M R's on the PA grids seem overkill?

Title: Re: How's the schematic?
Post by: PRR on December 02, 2023, 04:17:40 pm

Filter cap at A switched to ground makes no sense.

The 2Megs are safety caps for when the wiper gets dirty and loses contact.

Title: Re: How's the schematic?
Post by: AlNewman on December 02, 2023, 05:22:59 pm

You could possibly remove the 2M grid leaks, and put the 220k at the ground leg of your master volume?  And probably the B node would run your screens, rather than the C.  But not sure, just spit balling.

Title: Re: How's the schematic?
Post by: sluckey on December 02, 2023, 07:25:12 pm

Quote from: PRR on December 02, 2023, 04:17:40 pm

Filter cap at A switched to ground makes no sense.

I suspect the HT winding CT is also connected to the switch. The switch appears to be a DPDT (ON-OFF-ON) that's being used as an ON-STBY-OFF switch.

Title: Re: How's the schematic?
Post by: Diverted on December 03, 2023, 09:49:51 am

Thanks. Yes Sluckey you are correct, HV CT is also switched. I will update the schematic at some point.
One thing I could use info on, if anyone can help. There is a small LED indicator light tied into the on/off/standby switch. It is two components in parallel tied to either side of the PT primary via the switch. I didn't know how to show this on the schematic ... the program didn't appear to have symbols accurate for it.
If I were to build this circuit, what would I use here?
See photo attached of the array. Thanks!

Title: Re: How's the schematic?
Post by: sluckey on December 03, 2023, 10:07:05 am

There's a high value resistor in the white heatshrink. One end is connected to one primary lead. The other end is connected to the LED which has a parallel diode and then connected to the other side of other primary lead. It's just a simple power on indicator. You may prefer to just use a more traditional power indicator that's connected to the filament circuit.