Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Hebert on April 11, 2020, 06:30:09 pm

Title: Understand if my scribble is too much to deal with but please take a look.
Post by: Hebert on April 11, 2020, 06:30:09 pm

I have three questions. 1. do I need to connect ground between the two input jacks and then to the preamp ground bus? 2. does how I handled .02, 100k and 1m (preamp side) work? and unlike a champ where the volume pot goes completely to the preamp tube and is grounded to the preamp ground this volume pot goes both to the power tube and the preamp tube. so where do I ground it? as you can see I chose power ground. Oh, also as per uncle Doug's direction I put two voltage dropping resistors on the filter caps so I can use a magnet speaker. thanks!

Title: Re: Understand if my scribble is too much to deal with but please take a look.
Post by: ac427v on April 12, 2020, 08:37:20 am

Nice drawing. Seriously. Components are spaced well and I can easily compare to the schematic.
The grounding looks fine. The volume pot is the resistor to ground that is needed by the power tube. So I think it grounds in the right spot. I don't like power switches on pots but if this is an existing amp you may be stuck with that.
If your build hums, you could try connecting the transformer heater ground to the power tube cathode. That would elevate the filament string.
The power supply on the original schematic doesn't make sense to me. I would not choose to have both filters in parallel and both tubes powered from the same node of the high voltage supply. Your layout makes more sense. But I don't understand the 1k/7watt resistor before the caps. It will drop the supply voltage to the whole circuit and get really hot. Maybe that is a necessity due to overly high Power Transformer voltage? Other viewers with field coil experience may have better advice on that. Interesting little amp. I look forward to the rest of the story.

Title: Re: Understand if my scribble is too much to deal with but please take a look.
Post by: Hebert on April 13, 2020, 07:44:04 am

thank you!   this my first attempt to turn a schematic into a layout. I am planning to build a clone that looks like the original amp. I will take your suggestions as I build it. going to build the cab, chassis, grill cloth, all of that first.

Title: Re: Understand if my scribble is too much to deal with but please take a look.
Post by: yorgle on April 13, 2020, 10:12:02 am

The two 100k resistors off the input jacks need to be separated on the input jack side.

Title: Re: Understand if my scribble is too much to deal with but please take a look.
Post by: Hebert on April 14, 2020, 09:01:24 am

Quote from: yorgle on April 13, 2020, 10:12:02 am

The two 100k resistors off the input jacks need to be separated on the input jack side.

Thank you, I see what you mean

Title: Re: Understand if my scribble is too much to deal with but please take a look.
Post by: Pietro on April 15, 2020, 08:55:24 am

increase the first filtercap to 50Uf if you have one..

-- from rectifier to first node needs no resistor. if HV is to low then drop the 10K between node 1 and 2.

-- bypascap on 6v6 cathode is to small. increase to 50uf mimimal. 20uf makes for a very stiff dynamic response.
-- same for cathode bypass cap on v1. 10uf needs to be 33uf minimum

- do not ground volumepot and 6.3 v winding at same point. its going to cause hum. physical separation needed.

nice drawing b.t.w.. work of art !