Hi will this work

[dark1stu]

as above will this work or am i going to blow stuff up , if thears any changes i should make i would love to no ,

hear is a hi res of the image attatched its 700k , thanks

http://i81.photobucket.com/albums/j228/dark1stu/amp.jpg

[Herald1360]

Not sure about the right hand valve in the top row.... looks like the circuit it's in started life as part of a phase splitter. As it stands, it will have a gain of only about x2.

[dark1stu]

is this better thanks

it did power a push pull amp . i need a pre out from it , im going to connect it balanced with a pot on the output to trim it

Im wondring it i will need the second stage after the volume controll , thanks all

[neon indicator]

pin 7 of V1 has no grid resistor. Need 1M or 2M2 to ground.

I'm not too sure about the Tremelo i/ps. Have to go for tea.

[dark1stu]

thanks ill do that , the Tremelo circut has been copyed as is from a working diagram

[neon indicator]

The input sockets should be isolated with 250V 0.1 micro farad capacitors
1) Avoid lethal shock if a valve fault
2) Avoid damage or overheating (of cathode or anode circuit or valve) if there is DC voltage on input.


Not all working schematics actually are safe.
Not all working schematics work as well as possible.

[Herald1360]

Looking again, I'd be inclined to make the input circuit for V1A the same as for V3A. As it stands with the pencilled in link, if there is nothing plugged into the HI input, the grid will just be shorted to ground.

For the grid leak on the V1B a 1M to ground from pin 7 would do dc wise, but you would be left still with the tremelo volume control shorting out most of the straight through signal if it was set to minimum. I don't really know... maybe the original design intended this

Any chance of an unmodified copy of the original circuit you are working from?

[Michael Maurice]

I would increase the value of C5 to about 0.1µF otherwise thew volume control will act as a tone control

[peter_sol]

I think you don't need c1 but you do need 500k after each volume control otherwise they will interact too much.

[dark1stu]

hi neather of the amps will be used together so will intrection be a problem , im going to make some mods today and ill re post the circut thaks

[dark1stu]

hi all hears whear i got my idias from thanks

http://www.dreamtone.org/pdf/18watt_...c_08-28-03.pdf

mods if i should not of put this link in please erase it

I have updated the digram , taking in to account your comments thanks

[neon indicator]

You need to move the 2 x 1Meg resistors on inputs to the grid side of the valves, pin 2 V1A and 7 V3A I think. They are wrong side of capacitors.

V11B pin 7 still missing grid bias resistor (1M or 2.2M Ohms)

A grid AWAYS needs a 1M to 2.2M resistor to ground. (Signal input grids on normal amplifiers. Control Grids, Screen grids and specialist circuits are different).

The coupling capacitors need to be 100nf = 0.1 uF not 0.01 and rated 250V, not 20V transistor circuit parts.

The pdf you linked is a very old less than optimal design in safety and performance.

[Herald1360]

Well that explains a few things!

The original is quite interesting- the straight input LO/HI gains come from using just one or both in parallel of the halves of V1. With both in parallel the effective gain is doubled since you get twice the anode current swing on the common load resistor.

For the tremelo input, the LO/HI 2:1 comes from the arrangement of the 68K resistors and the input jack switching.

Why do it that way? I've no idea.

Presumably you are trying to save on a valve in your version.......?

Anyway, I think the mods below should work, but bear in mind that the original was (I believe) a guitar amp, hence the low value coupling capacitors and oddish tone controls. If you're looking for fidelity (as opposed to an integral part of a musical instrument) you may be disappointed with the results.

[dark1stu]

Hi yes it is a guitar amp im aming to make a unit that behaves like one without the power amp so we can connect it up to a mixing console , and yes im trying to keep it down to two valves thanks for the diagram

[Herald1360]

Right, understood....

How good it will sound I guess will depend on how much of the original sound was generated by the interaction and limiting imposed by the output stages in the source amp. Not all (much?) of the colouration and overall effect will have come from the preamp alone.

Try it and see- nothing to lose and possibly a worthwhile result available.

[dark1stu]

Ye i will do im just waiting for the valves to come from Russia my main consern was that it wasnet going to go BANG and the magic bule smoke exscape

[neon indicator]

You do know you need the PA and speaker as at the high volume used you got acoustic coupling between PA speaker and guitar strings? Recording direct will sound "flat" as it will lack the "sustain" and slight reverb created by the slight acoustic feedback.

Also a huge factor in the sound is the acoustics of the loudspeaker driver design and cabinet design.

That's why at the time they used a a studio mic to "pick up" the sound from from the guitar pa and didn't patch direct.

[Kat Manton]

Hi,

Originally Posted by dark1stu

im aming to make a unit that behaves like one without the power amp so we can connect it up to a mixing console

Ah.

Sorry to disappoint, but it won't behave like one. You might like the sound if you're aiming for a very clean, dry sound which doesn't sound like a miked-up guitar amp and speaker.

If you're after a bit of overdrive, you might get some, but watch the output level if you try. Solid-state mixers may use input coupling caps rated 16V or 25V and won't take too kindly to high-voltage input signals (besides clipping in a way you may or may not like.)

Devices which do what you want are available; generally employing modelling an entire amplifier, speaker(s), cabinet and microphone in the digital domain. Alternatively, there are units which include a "power sink"; allowing one to run a valve amp at high level while producing a line-level output which includes selectable speaker and microphone emulations.

None are exactly simple and cheap as producing something which sounds like a miked-up guitar amplifier and speaker without actually being one isn't all that simple...

But, experimentation is fun, build it and see what happens

(I'll stick with bunging a knackered Unidyne in front of one of the 'greenbacks' in my 4 x 12" while hoping the neighbours don't complain...)

Regards, Kat

[dark1stu]

Makes sense. It's got to work better than a £20 DI box. It's for a mate and he has been using a DI, but wants something a little warmer. I'll take it on board the comments about overdrive. We have a old valve limiter that we can pop in the output to protect the console. I suppose the thing to do is build it up and see how it works. I dont play unfortunately, I just fix amps etc. I have a old Vox with a blue back on due in today. New output stage needed.