Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jackplug1 on January 09, 2015, 03:08:36 pm
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Hi
just an update on the problem I am having with too much gain from the hoffman stereo pre-amp project.
Having checked the circuit using the highlighter technique twice without finding any wiring errors I checked all the grid connections using a signal tracer which is a small tube amp with a probe and a little speaker
I haven't done this before because i bought the tracer second hand and I needed to repair it first.
I discovered there is no significant overdrive from V1 A pin2 and B pin7 and that the overdrive is apparent at pin 7 V2 with massive gain there.
I'm wondering if I should change the cathode resistor on V2 - A at pin 3 (820R) to a larger value to reduce the gain.
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... I checked all the grid connections using a signal tracer which is a small tube amp with a probe and a little speaker ... there is no significant overdrive from V1 A pin2 and B pin7 and that the overdrive is apparent at pin 7 V2 with massive gain there. ...
Turning down the Volume control doesn't reduce the distortion there?
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It's kind of clean-ish at position "1 to 2" on the volume control at very little volume if "10" is maximum.
Also at this volume position, If the tone control on my strat neck PU is turned to "0" I can hear a mild fuzzy bass distortion which is not noticeable with the treble control on the guitar at maximum.
So don't really have usable clean tone
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What is the output of the stereo preamp connected to?
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Marshall EL34 50/50 dual monoblock
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I still think something is wrong somewhere
If I read your statement up above correct, the distortion happens at the input grid of V2?
Massive gain at V2 & V4 input grid could be some sort of distortion, not gain, right?
Did you build a stereo pre amp?
If so, is the distortion the same at the input grid of V2 and V4?
It was suggested before that you can adjust the gain at the points I circled in red
Did you try that?
I updated the schematic on my project page
I noticed that I had V3 and V4 labelled wrong
Hit refresh to get the newest image
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Gain causes distortion so:
Using the "listing amplifier/probe" V1 and V3 are clean
V2 pin 2 and V4 pin 2 sound "overdriven" when turned up
there is high gain and overloaded distortion at V2 and V4 pin 7 will little or no clean there
However as the listing amp can be easily overdriven also, it just tells me that there is a signal there, so i will need to check the waveform with a scope
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addendum to previous post
(V2 pin 2 and V4 pin 2 sound "overdriven" when turned up)
The peaks are starting to flatten off if the volume control is turned less than halfway V2 pin 2 and V4 pin 2 as seen on the scope
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If you are certain the entire circuit is assembled correctly, then try this:
See the two 470kΩ resistors in red, in the schematic EL34 posted? Rather than have 1 resistor to ground and the other to the grid of V2/V4, place the 2 resistors in series after the 0.022uF cap (so 0.022uF cap, then 470kΩ, then 470kΩ then ground). Connect the junction of the 2 resistors to V2/V4's grid.
Now you have a voltage divider that's almost 1MΩ total load to the previous stage, and cuts signal strength by half.
... the overdrive is apparent at pin 7 V2 with massive gain there.
I'm wondering if I should change the cathode resistor on V2 - A at pin 3 (820R) to a larger value to reduce the gain.
Gain causes distortion so:
... V2 pin 2 and V4 pin 2 sound "overdriven" when turned up ... there is high gain and overloaded distortion at V2 and V4 pin 7 will little or no clean there
"Gain" is not the only thing that causes distortion.
If you added a diode to ground at any preamp tube plate, you'd get almost no clean output; it would all be half-wave rectified and sound very heavily distorted.
Similarly, if you had an 82Ω resistor at the cathode of V2/V4, bias would be insufficient to accept almost any signal before saturation, and would lead to distortion. In this case, the trouble really is the small range of allowable input signal. If you have a 100kΩ plate load at V2/V4, but accidentally used an 82kΩ or 820kΩ resistor, V2/V4 would distort but now it would be because they are biased to cutoff so strongly the signal gets clipped on the other side of the waveform.
There are other things which could be wrong, but hopefully you get the idea from the above.
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Thanks - that is very helpful
I'm sure my wiring is correct, there is a cathode V2/4 820 ohm on both channels .
However, maybe there should be voltage dividers after the coupling capacitors at grids V1 pin7 and V2 pin2 (V3 and V4)
I might modify it similar to this:
http://ax84.com/static/corepreamps/Lead/AX84_Lead_Preamp_Schematic.pdf (http://ax84.com/static/corepreamps/Lead/AX84_Lead_Preamp_Schematic.pdf)