Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: silverfox on January 27, 2016, 10:14:08 pm
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So I'm adding a Preamp section to the PA I've completed. Here is a basic overview shot. It's a Plexi style preamp. The circuit is constructed in a prototype style so I can either add an overdrive circuit or easily change component values. After listening to a huge number of amps over the past 2 weeks I finally settled on the Plexi style.
I've also shortened the original PA wires and tried to make them a little neater. Still working on all that layout stuff,
silverfox.
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For those of us that did not see the schematic of the PA's power amp, could you post it.
Thanks.
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I've got the board reinstalled and tested the PA tonight. Still have to finish wiring the pots and adding a couple dropping resistors to the supply. I'll be posting more shots and likely some questions in the morning. I'll be sure to post all the tech details then. I used Sluckeys redrawn "Huss 6V6 Plexi" as the pre.
I've still got to edit a schematic together and I'll won't be able to do that until Sunday at the soonest but here's a couple pictures of the rewire and added preamp circuit.
silverfox.
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Well I'm nearly done on this project with the exception of matching the preamp to the power amp, signal level wise. I'll try a 220K resistor in series with the Master Volume due to the fact that, if I barely move the MV control the amp gets too loud. The signal from the preamp is too big for the PA. It isn't as neat wiring wise as I wanted it to be but a vast improvement over my previous builds.
My question on this is: By adding a resistor to the input to the PA is that going to attenuate the highs? (Turns out that didn't change the tone much if at all)
edited..
As requested I've attached the schematics for the amp. The voltages do not match what is on the sch. I haven't had a chance to go back through and measure everything all over again. (taken down to edit).
Since the power here is low I run the amp off a Variac to get the line voltage up to 120 VAC, and that has boosted the B+ to 445 at the Plates. The bias is currently set at: -40 ma.
silverfox.
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Well every thing is not okay. I tried the 220k resistor and that really didn't bring the volume down much at all. It went between the input to the EF86 and the output from the treble pot.
Another problem likely related to changing the bias level from -34 ma to -40 ma is there is now a sort of crackling sound associated with two events: Low frequencies and even just turning up the volume level up with no input signal, causes a static-crackling sound. I do not see any arcing though... I'm going to have to revise the schematics as they were for illustrative purposes only so I'm taking those back down until I go through and measure all the voltages again- which are much higher than they should be according to the Huss version this build. It did sound good until I added the resistor now that I think of it. But still a faulty MV control scheme.
silverfox.