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gain and mid circuits

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emosms

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Joined 2010
#1
HI
any suggestions how to modify an amplifier and introduce a midtone and gain chains (knobs)?
The circuit itself, taken from old russian radio:

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


I am using it as electric guitar amp, has a pretty decent clean tone but want to overdrive it and add some crunch drive
 
#4
Here it comes the shematics (i hope):
View image: Rigonda 102 small
An externally hosted image should be here but it was not working when we last tested it.


I can implement the modification, but i don't know if I would manage to understand and calculate the chains, especially for the gain.
(I cannot even recognize which filter is for the low and which one for the highs now 🙂)

Looking for a brigth crunch, like SRV sound, little wing, e.t.c.
Possibly with a capability for a bit more gain, lets say like ibanez ts808
Overdriving the preamp tube? Or the EL84 twin?
 
#12
from what i can tell your not using the radio part and arent planning on using it, to be honest i think i would rip it all out and start a new build, to get it how you want it. follow the diagrams to get what you want back, and add to it. this way you havnt got huge unnecessary drains on the power supply or a large circuit thats doing nothing.
 
#13
Cannot take pictures now, but there are seporate plates for the supply, for the amp for the radio and for the UHF blocks in that order.
So I've just cut the frame left off the amp part .. 🙂
Power supply change is optional, it is just too big.
I concentrate on new tone chains and some gain (drive) control using the same valves. Reading "designing valve preamps for guitar and bass" now, but not sure if I can manage with the design 🙂
Also, introducing some "volume" knob between the preamp valve and the EL84 twin.
What do u think?
 
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#14
hmm you wont get alot of gain from a single stage, your better off having one volume/gain control unless you add another gain stage. by changing the tone controls you have, you can completely eliminate the NFB thats in there and ground the bias resistor of the EL84. its a strange power amp, ive never seen anything like that configuration as a single ended amp, maybe its more hi-fi? if you want a gain and volume, it might be an idea to have the gain at the input, and move the tone and volume controls right before the EL84 so you have nothing but a high pass filter between the 2 triodes
 
#15
emosms said:
the "gain" knob chain of this circuit - is that a positive feedback? :
View image: ibanez tubescreamer ts 9
Can I implement that somwhere in the preamp valve stages?

Or is it going to be better to set a cathode cap trimmer for the gain, at the second stage of the preamp valve? (R11)
Click to expand...

i think your confusing negative and positive feedback, its very rare to find positive feedback intentionally applied in an amp, and negative feedback will reduce gain, you can use negative feedback to control gain, but i think using a simple volume control would work better
 
#17
feedback is basically sending a signal from a later part of the path into an earlier stage, if the 2 signals mixed are in phase the feedback is positive, if they are out of phase (usually 180 degrees) then the feedback is negative. i think a rebuid is a better idea, and there are deffinately more efficient ways of using 2 EL84s
 
#18
Yess, I want to remove the tone chains and the "feedback" connected at the output transf. point 18.
in the original schema:
View image: Rigonda 102 small
- Is that a push-pull scheme and why it is not symetric?
I am particularly interested if I can use the original output transformer, but too many wires 🙂
* * *
Let's say I want to rework it like:
View image: ac15schem full
* * *
And the preamp part from this one, which seems to be excellent for my needs, but using 2 pieces of EL84 instead:
View image: ak01sc2 schematics
* * *
I am mostly concerned about the ouput transformer.
What if the power transformer provides less voltage?
 
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#19
it looks like some kind of SE UL kind of output, but im sure if you can find out the impedance of it youl be able to use it as a normal SE transformer, if its a 4w ish amp i doubt that transformer could cope with alot of power in EL84 PP if you look at the original schematic and look at what wire goes where im sure you could figure out what wire is one end and what wire is the other end, then you just need an impedance ratio, (probably best to use a low AC voltage input like 12v and measure the output on the secondary) other than that i would go for the simple design SE-5. however if you want to use both EL84s you might get away with a parallel single ended triode config.
 
#20
View image: Rigonda 102 small
- Is that a push-pull scheme and why it is not symetric?
I am particularly interested if I can use the original output transformer, but too many wires 🙂

The OP transformer is a standard UL output trannie. Pin 6 is the centre tap for the Supply in, pins 3 & 4 are the UL taps to feed the Screens.



The output is quite clever. Its like a LTP push-pull driver stage. The cathode resistor R14 is VERY important. This couples the signal to the second EL84 which is operating in grounded grid mode. C10 adds a little HF feedback to help stability. Both EL84s operate in UL mode. I've seen it before a few times. Very low distortion, apparently. :note:
 
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