Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: goldstache on January 03, 2019, 09:25:21 pm
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I built up this amp to get to know the EF86 a little better.
I really like how it's all sounding. Though it certainly needs some dialing.
The Preamp is AC30 inspired as far as topology, and allows you to put the "top boost" Gain/CF/EQ stage after the parallel 12ax7 or the EF86.
Overkill as far as drive voltage to the PP EL34's, but the chassis is nice and open and it's easy to swap stuff out and try different preamps, etc..
Got some questions:
Lug 3 of both channels volume controls have pretty nice looking sine waves on em with a 200mv @ 1k test sig input. Not a hint of clipping, as to be expected from the single stage. But if I go to the grids of my PI, pre input cap or post, I get some clipping to the positive side at max. Which is also present on the wiper of the pot feeding it. But not on lug 3 of the volume.
Is my coupling cap presenting some sort of phenomenon? Also, the output of my PI just looks like a mess. Too much clipping. I know I have a ton of available gain here, especially for the cathode biased operation. So any pointers would be appreciated.
I know sine wave linearity doesn't make a killer amp, HA! That being said, what I'm seeing from the PI grids to the Power tube grids, Looks excessively distorted. Which translates to a cruddy sounding amp when driven.
Two things I struggled with here:
Inter-stage filters and impedance.
Cap selection, Filter calculations, series resistance, etc..
Time to get a better grasp on this stuff. Any links or explanations would be greatly appreciated.
For instance, if I use a voltage divider, how do I maintain a similar frequency response as the signal pre attenuation. Or does the frequency response stay pretty linear, the amplitude reduction is just perceived as an EQ shift?
Vox style PI.
First time I've used the 2nd input of a LTP. It appealed to me because I could use an A/B box and select which input like a two channel amp kinda vibe and phase seemed to jive.
When I look at the plate outputs post coupling cap (grid side of Power tubes), on the scope, they aren't out of phase. And this is using just one channel. Aren't they supposed to be inverted from each other???
Also, they are drastically different in clipping characteristic and gain. The 6SL7 PI tube doesn't have matched sections, but these seem strange.
So from the PI to the Power tube grids, it needs tweaked.
Sounds good though. Both in fixed bias and Cathode biased "modes"
Thanks for having a look and in advance for your thoughts.
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When I look at the plate outputs post coupling cap (grid side of Power tubes), on the scope, they aren't out of phase. And this is using just one channel. Aren't they supposed to be inverted from each other???
In order to see a phase relationship you must use two scope channels and trigger the scope from one or the other. DON'T USE ALTERNATE TRIGGER MODE. Or, use one vertical input to view either plate output, ***BUT*** trigger the scope externally with the test signal. Either of these methods will show the phase difference.
When you only use one channel to view either signal they will always appear in phase. This is due to the trigger selection. If you use internal trigger with a positive slope, the display will always start with a positive going waveform. Or if the trigger slope is set to negative, the display will always start with a negative going waveform.
This all applies when you are tracing a single signal through an amp and want to see the phase shifts. Best to trigger the scope externally with the test signal for this.
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Sluckey!
Fantastic. That looks better!!!
PI plate signals are not very linear, even before clipping onset. Still wondering why the input caps to the PI grids are showing a slight clip, then when the feeds are lifted from the caps they look superb??
I remember reading that you want a target AC drive voltage of around your Power tube bias voltage, as a fudge target. I'm trying to stick with that and I get a bout 18Vac at max volume on the EF86 Channel heading to the PI, so that makes me think that the drive voltage of my PI may be a bit extreme. Probably should rebias the PI and have a listen/look?
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For instance, if I use a voltage divider, how do I maintain a similar frequency response as the signal pre attenuation. Or does the frequency response stay pretty linear, the amplitude reduction is just perceived as an EQ shift?
The voltage divider itself doesn't change the frequency response at the junction of the resistors. Adding a capacitor to that junction will, though. Your 'scope probe has a capacitance to ground that will change the response. The probe also has a resistance to ground that will change your voltage divider. So what you read with the probe connected is not what is going on when the probe isn't connected.
Your volt meter also has a resistance to ground, so your reading of 26V on the PI grids is not what it is when the meter is not connected. Left unmolested, the grids are actually at 42V or so.