5trangeE3

Discussion in 'Shock Brother's DIY Amps' started by shortcircuit, Jun 29, 2019.

  1. shortcircuit

    shortcircuit Tele-Meister

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    5trangeE3.jpg

    I didn't mean to follow the topology of the 5E3. My original intent was to revisit my failed attempt at a single tube triode-pentode input stage (http://www.tdpri.com/threads/single-ended-6bq5.869737/). I had used a 6X8, which had a common cathode between the triode and the pentode, which made biasing a real pain, and there was quite a bit of mismatch between the gain of the two sections. I scoured Frank's electron tube database and found the 6CM8 tube. It has separate cathodes. The triode section is pretty close to a 12AX7, with a gain of 100, transconductance of 2000 umhos, and plate resistance of 50K Ohms (12AX7 was 1600 umhos and 62.5K Ohms). The pentode section has a transconductance of 6200 umhos, which was not too high and usable in a pentode configuration. The 6EU7 is in there because I don't like wiring up the parallel heaters of a 12A_7 tube. As for the output section, I've been thinking about a push-pull 6G6G amp for a while. I can crank this sucker, and not get yelled at by the better half. So after I drew up the initial schematic, I looked at it and thought, "Wait, I've seen this before." Hence the name.

    I'd been experimenting with LFO circuits and thought it would be fun to incorporate tremolo into the amplifier. The 6AM4 tube also harked back to the aforementioned failed build. I borrowed the basic design from the Gibson GA-40 amp, but tweaked the values a bit. I ripped off the implementation of the circuit from the Silvertone 1482, and tested the circuit with my 6AK6 push-pull amp, and it worked great. The output of the LFO circuit was 38V, so the 1.6M resistor and the 500K pot will bring the max voltage down to 9V for the 6G6G.

    Layout:

    Layout.jpg

    I'll probably use a 220K mixing resistor coming off the triode section, and I'll adjust the resistor for the pentode section so that the voltage for the two sections coming into the first 6EU7 triode will be pretty close. The amp will fit into a 10" x 6" chassis. The shipment from Mouser will get here on Monday.
     
    Last edited: Jun 29, 2019
  2. shortcircuit

    shortcircuit Tele-Meister

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    5trangE3-schematic.png

    Refined the design a bit. Got rid of the original clunky master volume and switched to a Frondelli PPIMV. Also added @robrob's 3-way switched feedback. With the 6G6G tubes, I'm guessing the feedback resistor will be around 20Kish. I'll refine the value with a potentiometer.

    Here's the revised layout:

    5trangE3-layout.png

    Power supply built up, the UF5408 diodes are a bit overkill, but what the hey. Chassis laid out. Will drill it out tomorrow (or rather, later today)

    01 Prebuild.jpg
     
  3. tubeswell

    tubeswell Friend of Leo's

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    FWIW - Take a look at the pawnshop Excelsior schematic for another example of how to incorporate trem in a 2 x cathode-biased 6V6 amp. (It uses an IRF820 (mosfet) source follower to buffer the LFO waveform, but if you have a full dual triode handy, you could use a more conventional CF stage for that. Its also pretty simple to incorporate an intensity control for the Trem into the excelsior circuit)
     

    Attached Files:

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  4. shortcircuit

    shortcircuit Tele-Meister

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    That sounds very interesting. What would be the benefit of buffering the LFO waveform? The mosfet looks pretty easy to implement. I can mount it directly on the chassis and dead bug it.
     
    Last edited: Jul 7, 2019
  5. shortcircuit

    shortcircuit Tele-Meister

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    Aha! Just looked it up. So it appears that a buffer is good for output tubes that require a large voltage swing like a 6L6, which allow the LFO tube to develop the full voltage required. The 6G6G tubes only needed 9V of swing max to get the full spectrum of tremolo effect, so a source/cathode follower won't be needed. Thanks though, I'm gonna grab some IRF820's with my next Mouser order.
     
  6. tubeswell

    tubeswell Friend of Leo's

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    Correct, the buffer provides a low output impedance which is better for driving inputs that sink current. But, its not just the buffer that I was suggesting, its where the trem is inserted into the signal path in the Excelsior that is different.
     
  7. shortcircuit

    shortcircuit Tele-Meister

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    Cool, thanks. It looks like the tremolo voltage is injected via its own set of 470K resistors just before the power tube grid leak resistors. Seems a bit redundent, but am I missing anything? When I prototyped the tremolo circuit, I inserted the trem via the grid leak resistors only, and it worked great.

    The R3000 diodes on the plates of the power tubes of the Excelsior are interesting though. So my understanding is that they are there to shunt a flyback pulse to ground. What happens when a diode fail short? Yikes! What's more likely, a huge flyback pulse or diode failure? Ken Fischer talked about the subject in the Tranwreck Pages where he used 3 1N4007's in series.
     
  8. tubeswell

    tubeswell Friend of Leo's

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    Yes, the source follower is handy for this because when used as a trem buffer, it doesn’t carry any signal (which keeps tube purists happy), you don’t have to worry about high h-k voltages (there is no heater filament), and an IRF 820 is a depletion-mode device (and doesn’t require any bias setup), so it just drops straight in and is rated for 500V.

    And if you’re going to use flyback diodes, 3 in series is better than 1 (on each side).
     
  9. shortcircuit

    shortcircuit Tele-Meister

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    Finally got the chassis drilled and painted. I though it would be a good idea to put a layer of clear coat over the silver hammer tone. Doh! The chassis now looks like the surface of the moon. Oh well, too lazy to strip the paint.

    02 Chassis.jpg

    Got the heaters done.

    03 Heaters.jpg
     
  10. shortcircuit

    shortcircuit Tele-Meister

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    06 Almost done.jpg

    Almost done. I need to clip in the heater supply, the primaries of the output transformer, and the cathode resistor for the power tubes. Then I need to figure out if I need dropping resistors for the 6.3V rail as Hammond PT's run hot. After that, I need to sort out the mixing resistor for the pentode stage. I'll clip in a 1M pot, bust out the scope and figure what value resistor I need to get the outputs close to the triode stage. Then lastly, I'll determine what value NFB resistor I'll use by clipping in a 100K pot--and swap the OT leads as needed. Then finally, set the bias.

    07 LFO.jpg

    I stuffed the cathode bias LED in the socket hole. Kind of cheesy. I'm not sure if the blinking of the LED will be visible.

    08 External.jpg

    Plugged in the tubes to see what it looks like.

    09 Yikes.jpg

    Kind of close. Fortunately, the 6G6G tubes don't get very hot. The heaters only draw 0.15 A, and I'm going to bias the tubes cold at around 1.7 W (60% max) each due to the tremolo.
     
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  11. shortcircuit

    shortcircuit Tele-Meister

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    Wrapped everything up today. Here's the final schematic:

    5trangE3-schematic.png
    The 6CM8 tube sounds great, no microphonics. I was surprised that the triode section actually had more gain than the pentode. I must have looked at the load lines incorrectly. I was conservative with the pentode section, however because of it's highish transconductance. There was a bit of the pentode hiss. I love the 3-way switched feedback, and ended up with a 51K resistor. In my original schematic, I figured that the output tube cathode resistor will be around 1K. However, I forgot to double the current when I did the calculation. The 560R cathode resistor biased the 6G6G's to idle at 1.7W, and max clean output was 1.5W with an 8 ohm load at 440Hz. Because I messed up on the power tube cathode resistor, the tremolo was weak after I got the proper bias and I had to change the voltage divider resistor. The audio taper pot did not work well in this application with the majority of the changes coming at the end of the turn. I didn't have a 500K linear pot, so I went with a 250K linear pot. The voltages as measured were pretty close to my calculations.

    The completed innards, all wrapped up.

    08 Finished-1.jpg

    On its base:

    09 Finished-2.jpg

    Tremolo in action. I had to keep the ambient light low so the the flashing tube can be seen. I'm going to blame the poor playing on the low light too.

     
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  12. tubeswell

    tubeswell Friend of Leo's

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    Nice trem! (Link would be proud)

    Measure the HT, plate, screen and cathode voltages. We can figure this out
     
    Last edited: Jul 18, 2019
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  13. shortcircuit

    shortcircuit Tele-Meister

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    I had drawn the curves for a 75V screen, and with a 47K load line, I had set the idle point at 1.5mA. According to the datasheet, the screen current should have been 0.44mA, so that's where I got the screen resistor value.

    The actual measured voltage drop across the 240K resistor was only 85V instead of 115V, for a screen current of 0.35mA. The actual screen current is lower than that derived from the data sheet, so with a higher screen voltage, that where I'm losing the gain. A 300K screen resistor should bring the screen voltage down to about 75V. I'm happy with where the tube is at now though, and I don't want to increase the hiss from the pentode by increasing the gain.... Who am I kidding? Give me a couple of weeks and I'll have the first stage completely redone.:p
     
  14. tubeswell

    tubeswell Friend of Leo's

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    For more gain, you want the grid curves further apart (I.e. higher g2 voltage). The Ip:Ig2 ratio on the data sheet is your jumping off point. Then the plate load should be such to put the load line at the knee of the Vg1=0 curve. At 178v, it might be that you don’t have high enough HT to get the most out of your current stage setup. I’m away from my computer right now, but I’ll take a closer look at the chart later and post some ideas.
     
  15. shortcircuit

    shortcircuit Tele-Meister

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    Ok, now I'm confused... https://www.ampbooks.com/mobile/amp-technology/pentode-gain/
     
  16. tubeswell

    tubeswell Friend of Leo's

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    Look at this page under the heading 'Below:' where he explains the effect of screen voltage

    http://www.valvewizard.co.uk/EF86.html

    Gain is higher, as g2 voltage is increased, and vice versa
     
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  17. shortcircuit

    shortcircuit Tele-Meister

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    Awesome, thanks. Always look to Merlin. I think I get it now. I was mixing up input sensitivity and voltage gain.
     
  18. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Wow, that is a beautiful point-to-point amp and she sounds great too. Congratulations.
     
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  19. tubeswell

    tubeswell Friend of Leo's

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    The Ip:Ig2 ratio for a plate voltage of 200 is 3.4:1. With Ig2 at 0.35mA, you will only be getting around 1.2mA plate current.

    The 6CM8 Pmax = 2W, and Pg2max = 0.5W, so you can squeeze more out of it if you want.

    For the 6CM8, the datasheet suggests a Vg2 around 150V. Your one is sitting at about 90V, so you have got room to open it up a little.

    What plate and cathode voltages are you getting?
     
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  20. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Truly gorgeous work, and sounds grrreat. Thanks for letting us see and hear it.
     
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