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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: May Fri 13, 2016 6:38 am 
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Location: Vancouver, WA
Hmmm... I found a bit more info concerning the compensation cap [what I was calling the bypass cap]. From what I've gathered, you first need to measure the gain and phase shift of the open loop gain stage. If it crosses 180° then you pick a cap that will shift it to under 180°. All the equations shown make it look quite complicated, but I've found that this kind of thing is actually pretty simple to setup in Excel then use goal seek to work it backwards.

Use the COMPLEX function to convert reactive components to a complex value, then you can use the same basic equations that you would use for pure resistors. You need to use the IMSUM, IMSUB, IMPRODUCT, and IMDIVIDE functions instead of +,-,*, and / in the formulas. In the end, use the IMABS function to get the amplitude from the complex number and the IMARGUMENT function to get the phase angle.


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Jun Thu 02, 2016 10:13 pm 
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I have yet to pick up a soldering iron, life really gets in the way sometimes. I did manage to fully geek-out on the circuit analysis. What started all this was a concern that the 6N2P tubes would have too much gain compared to the 12AX7 tubes that the Fender circuit uses. Both tubes have the same amplification factor of 100 but the 6N2P has a lower internal plate resistance so the actual gain will be higher, especially for the first pre-amp tube that feeds the tone stack.

In order to check this, I had to build a Thevenin equivalent circuit for the tone stack which took me the better part of a week. The rest of the math was pretty straight forward and includes the low-frequency roll-off from the cathode bypass capacitors and the high-frequency roll-off from the Miller capacitance of the tone stack recovery tube.

The following graph shows the pre-amp gain from simply swapping the various tubes.

Image

The 6N2P is basically the same as the 12AX7 except it doesn't roll off as fast on the high end due to it's lower capacitance. The 12BZ7 and 5751 are also very similar but the 12BZ7 has higher capacitance and rolls off sooner. The 6AQ8, 12AY7 and 12AT7 are all grouped at the bottom, with the 12AT7 the lowest of the bunch.

Things change pretty drastically if you re-bias the tubes by changing cathode resistors.

Image

The 12AX7 and 12AY7 stay pretty much the same. The 12BZ7 gain skyrockets. The 6N2P also jumps considerably above the 12AX7. The 6AQ8 is about where the 5751 started. The 5751 is right at the heels of the 12AX7, and the 12AT7 jumps from the bottom to almost overlapping the 12AX7.


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Jul Tue 12, 2016 9:24 pm 
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Joined: Jun Mon 01, 2015 7:58 pm
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Location: Vancouver, WA
I finished drawing the schematic over a month ago and have collected up most of the parts. Unfortunately, since then, I have filed for divorce and am now a single father of three. I was planning to post the schematic after I had a chance to experiment with it and make improvements, but at this point I have no idea when/if that is going to happen. So, here is the unfinished schematic.

http://www.gofastforless.com/junk/po_reverb_schem_2016_06_10.gif

I started with a Fender Pro Reverb schematic and removed the Tremolo and Normal channel, realizing after the fact that it now looks more like a Princeton Reverb and it would have been a whole lot less work starting with the Princeton. The preamp, reverb, and phase inverter are basically direct copies of Fender circuits. The stuff to the right of the phase inverter is mostly ideas I've had floating around in my head and decided to try out here. It may be the bee's knees or just a recipe for smoke. The plan was to "breadboard" all the experimental stuff in the old Bogen chassis then transfer anything that worked into the new amp chassis and add the tried and true Fender preamp and reverb. Hopefully, I will be able to get back to this project one day.


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Jan Sat 20, 2018 7:02 am 
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I'm happy to report that the dust has settled and I'm able to pick this up again. The plan has changed a bit. Rather than converting the speaker cab into a "Blackface" combo, I'll convert the Bogen to a "Brownface" head that I can use with the cab.

I've never been a fan of Tremolo but after hearing the Brownface "harmonic tremolo" in some youtube videos I just had to make one. In a nut shell, it's like a modulated tone control instead of a modulated volume control like most tremolos. The circuit looks insanely complicated at first, using two and a half 12AX7 tubes, but when you break it down it's an ingenious collection of simple circuits. A cathode follower buffer on an oscillator, driving a cathodyne phase inverter which modulates the bias on two common cathode gain stages. The preamp signal is split and run through two different tone shaping circuits each going to one of the gain stages. The outputs of the gain stages are mixed then sent on to the phase inverter for the power tubes. Since the two gain stages are modulated opposite each other, the volume stays fairly constant but the tone moves around as well as having some interesting phasing due to slightly different phase shifts of the tone shaping circuits. There isn't anything that sounds quite like it. Although there were some Vox amps with similar tremolo circuits.

If I can scare up the unique tapped tone pot then I'll copy the Brownface preamp and tone stack. The power supply and output will be tweaked to work with the Bogen transformers and a pair of 6GT5 power tubes.

I've just about finished gutting the Bogen chassis. It needs some straightening because it was dropped pretty hard at some point. It'll get a thorough washing but all the surface rust and flaking paint will remain [for now]. I think the only parts I need to buy are the odd ball pots.


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