Multi-purpose tube amp (line and guitar inputs)

[smsmonster]

Ok so I've hooked up my preamp to a scope and a function generator and this is what happens:

http://www.smsmonster.nl/files/IMG_3884.JPG
(957KB, 1600x1200)

Trigger turned all the way down:
http://www.smsmonster.nl/files/IMG_3885.JPG
(914KB, 1600x1200)

Input signal: 1KHz, 1Vpp. B+=120V, Ra=10k, Rp=1k2, Ia=2.65mA

Is this supposed to happen? What is it?

 

[lndm]

A slow varying DC component seems to be riding under your output. This appears to be motorboating and in small amounts is no big deal. It can usually be fixed by increasing the supply rail decoupling resistances.

 

[smsmonster]

In this particular case i've hooked up one tube onto a professional 120VDC power supply, not the power supply from my schematic.. But I think i know what's causing this, the ground of the heater wasn't connected to the ground of the power supply.. Silly me.
I'm going to add a grid stopper (1k) as well and decrease the grid resistance from 1M to 100K... Good idea?

 

[lndm]

Aha

Lowering a grid resistor is almost always a good idea. There is the issue of noise, and HF response and overload recovery that a small resistor fixes. However, pre stages don't typically overload, and line level stages are not overly critical about noise. In a pre stage, a major consideration is the load a grid resistor puts on the previous stage, and lighter loading can mean lower distortion.

 

[SHiFTY]

I saw this the other day, might be of interest:

http://www.the-planet.org/Shishido.html

 

[smsmonster]

Work in progress pic:

An externally hosted image should be here but it was not working when we last tested it.

Is there some way of measuring if the supply voltages and resistors are correct without an output transformer?


SHiFTY: Is the 12BH7A similar to the 6350? Too bad there aren't any specs on the OPT..

 

[lndm]

Ah! the fun part!

You can build from the left of your schematic up to C9,10 plus the power supply if you wish. Without loading from the output, the supply rail may be sligbtly higher than it will be. Consider physical mounting, followed by heaters and then HT.

 

[smsmonster]

Small update:

Finally I've found a way of building (ie. finding something to mount on).. I'm using an aluminum panel (2mm) on top of a wooden box which contains the transformers and capacitors.

An externally hosted image should be here but it was not working when we last tested it.

^Test fitting the components^

I've sand-blasted the aluminium panel to cover up some minor scratches, the matte appearance looks cool. All I have to do now is mounting every thing in place, wiring the whole bit up and install two switches (filament and B+). How about earthing? I'm using a star-ground, one earth point that is.. Should I connect it to the PE of my mains connector (and the aluminium as well)?

 

[lndm]

This is looking good

There are many schools of thought re grounding, YMMV. Here are some w/p's I have found interesting in the past.
http://www.sonic.net/~ktstrain/Groundtutor.htm
http://www.aikenamps.com/StarGround.html

I think you should use heavy wire for collective ground runs. I have found that whilst using 2.5mm solid copper, putting my cro leads across this wire at two points 150mm apart will show a tiny but noticeable current.