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20 lb Marshall (or Bassman) Inspired Amp

Discussion in 'Shock Brother's DIY Amps' started by printer2, Oct 29, 2017.

  1. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    I gave it more thought and doodled up two different schematics in response to the other thread. Just a cut and paste,

    OK dis some more figurin'. The first, second and Phase Inverter stages of the BF amp and the 5E3 run of of 250V so no having to change the supply voltage to them.

    The outputs and screen voltages do change as well as the bias voltage. If you were so inclined you could have some solid state diodes in series with the 5Y3 and in BF mode two switch poles shunt the current around the 5Y3 giving you a boost in voltage of 50-60V putting the amp in BF territory. The BF amp uses a 6.6k output transformer while the 5E3 uses a 8k. I would stick with the 8k. With the voltage on the output tubes higher you need more voltage on the grid so an additional cathode resistor in series with the normal cathode resistor is needed. One pole of a switch can be used to short this resistor out when in 5E3 mode. In BF mode the other contact can connect another bias cap, 470uF, to ground which stiffens up the bias and makes it more like fixed bias. Another pole is used to switch the NFB on for BF and off for 5E3. The additional voltage will increase the voltage on the preamp sections making them a little more clean. I am not including a schematic for this version, don't want to freak people out. Anybody capable of making the amp with the voltage boost could draw up a schematic from the description. Now moving on to the simpler version.

    Well two versions. There are always options and compromises. The first is the power supply does not change so the BF version has the same power as the 5E3, not a big difference but it is there. In these two versions the stiffening capacitor on the outputs cathode is increased. The difference is the first schematic you need a four pole switch to change the preamp version from 5E3 to BF but in BF you still get a Tweed channel. So in both modes you can change the NFB and bias stiffness, might find it useful.

    [​IMG]

    The next version, one four pole switch. There is a catch, when changing to BF mode the other channel is disconnected. The negative feedback and bias capacitor gets switched at the same time as the input configuration. You could easily enough have the Tweed channel hooked up also in BF mode, rather than where it is now connect it to the second stage input. The thing to remember here is that it will interact with the BF channel.

    [​IMG]

    So there you have it, I think I exhausted the realistic options. The layout and the schematics should be able to give an electronically competent person enough to put together an amp with the features/complexity they want. Not for the first time builder, you have to consider wire routing when going off the beaten path. I am going to try the second last schematic, I like what it has to offer the most and I have a chassis that can accommodate it.
     

  2. awasson

    awasson Friend of Leo's Gold Supporter

    Age:
    54
    Nov 18, 2010
    Vancouver
    So, I find this idea of a switching power supply such a compelling idea. I’ve got some old school HiFi mono blocks and the separate power supplies with their massive transformers and massive chokes, easily weigh twice the weight of the amps. Are there any examples in the Wild of reasonably powerful tube amps with switch mode power supplies?

    I had a conversation many years ago on this subject with a guy who designs power supplies and he couldn’t find any reason it couldn’t work but he was doing industrial electrical engineering and has zero interest in audio.
     

  3. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    Well that sounds boring.
     

  4. awasson

    awasson Friend of Leo's Gold Supporter

    Age:
    54
    Nov 18, 2010
    Vancouver
    Was a bit uninspiring but it’s kept me wondering if there was a downside to switch mode power supplies for tube amps.
     

  5. grim83

    grim83 Tele-Meister

    462
    Jul 14, 2014
    alabamy
    I admit this is well above my head (usually the case in this section of the forum) but this looks so cool. Man i wish i had the knowledge to try something like this....
     

  6. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    Not too hard to learn. All I am doing is using classic circuits and just swapping things around with switches. You just follow the signal flow and see where it takes you. If you have a few hours (ok a few evenings) a document that started many a tweaker on the road to ruin,

    http://ax84.rru.com/media/ax84_m35.pdf
     
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  7. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    The first step is admitting you have a problem. My wall of shame. I made some shelf room and stuck the guitar projects that I have to finish. No more starting new guitars. There are about a half dozen amp projects in there and parts for future consideration.

    [​IMG]

    Dresser full of tubes, cigar boxes with them, switching power supplies to build into amps and a mess of speakers, ten's in the boxes also.

    [​IMG]
     
    Last edited: Apr 26, 2018

  8. supersonicobr

    supersonicobr TDPRI Member

    74
    Jun 6, 2011
    Brazil
    Pretty certain you've already seen it but this guy here



    tested the boost converter and found it does what it says on the box as long as that diode at the back doesn't give out halfway through...

    With 40 watts, 70 peak, worth of B+ out the booster, what sort of voltages are you looking at pumping the 6V6s with? Also, what sort of and how much HV filtering would you use, same as the original amps?

    I've got all the required gear to do a 2x 16A8 push-pull power amp using a boost converter like yours (along with a Yamaha keyboard power brick, 16v@3.2A) but it uses a floating paraphase PI whose schematic I'm too much of a newbie to translate into a usable turret board layout, even working with the 5C3/5D3 schems as a basis. Looking forward to seeing what comes out of your projects and your boost convo use.
     

  9. bftfender

    bftfender Tele-Afflicted

    Dec 21, 2017
    York PA
    under 20 lbs, el34 pushing 7 watts , boost switch,vari pot, wgs vet 10" speaker, this is what started me on real power tube OD, now have a 1987 plexi clone, all other amps are in the closet
     

    Attached Files:


  10. SSL9000J

    SSL9000J Tele-Meister

    Age:
    46
    105
    Dec 18, 2016
    Atlanta, GA
    Printer2, what software are you using to plot your schematics? I'm trying to make the transition from wireless analog integrated schematic composition hardware.



    OK, I use graph paper and a pencil.
     
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  11. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    I am using ExpressSCH for the schematics. I used to use DIY Layout Creator software for the layouts.

    Nice looking tube the, 16A8. I tend to derate China specs, I think the booster should be fine for some 6V6's, maybe even 6L6's 25-35W circuit. I have a 6.6k OT that would do nicely for a 30W, some day. I did rewire one of my amps to have a 5E3 input or a BF base and treble with a tweed volume and tone channel. I still need to put in a switch to jumper the channels, I only have one set of hi/low inputs in this amp. Then I have to check off I did the wiring right. I did it a couple of weeks ago but other things took priority and I haven't confirmed I got it right yet.

    I am torn between doing the amps or the guitars. I am waiting for fret wire and tuners to finish a few guitars. I just got everything I need to finish my pine Tele. I think I will check out the 5E3/BF amp today and get the tuners on the Tele. I am in the process of organizing all my crap and making the place livable also. That is where the pictures came from, finally getting my stuff together. A lot more to go though.


    Just finished watching the video, well worth the time. I think his original load of 3k might be pushing it, I'll have to keep in mind the heat when I lay out the amp.
     
    Last edited: Apr 27, 2018
    SSL9000J likes this.

  12. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    I have moved over to the dark side, I have gone SS. I have been playing with some Fets and Class D amp modules. Just a couple of Mu-amps in series, nothing too exciting but just wanted to try both. Running 12V it can really kick the snot out of an 8" speaker. I will have to delve a little deeper. And then go sidetracked with Marshalls. Somebody was trying to get a good Plexi and JCM800 sound. So I drew up what I thought would be a good compromise between the two types of amps. And only using one four pole switch. The two pole switch on the tone stack gives you two flavors of Marshal tone with the center position (three position, center off switch) giving a Blackface curve. Ignoring the tone stack the switch position is in the 2204 position, switch the position and it goes to Plexi.

    Had an extra set of eyes look it over at the Marshall forum and everything seems right. The one thing that it does not do is add the 0.68 uF cap to the second stage for the Plexi mode. I needed to use one pole of the switch to ground out the normal channel when it is not in use.

    [​IMG]
     
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  13. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    I won't say I kicked the wood carving thing, just too busy with summertime stuff too get back into it for now. But since I did start plugging some parts into a circuit and drew up the above I thought I would take it a little further. One guy on the Marshall forum said it was a good compromise but it is too bad you had to change jacks. Well you didn't if you wanted to stay in the Plexi bright channel and then go into the Hi channel of the 2204 circuit. So given that you only want to use one hole and no changing jacks I started rearranging. I thought it would be good to have a normal volume and a drive level and dirty volume, pretty much standard on that type of amp. I was thinking in terms of a pedal sized amp using a Class D chip in the back. Figured you get most of the power amp flavor with the addition of the PI. It only needs a standard foot switch to switch from normal to dirt.

    [​IMG]

    It is only the first iteration, someone might say, 'You can't do that because of... '.
     
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  14. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    A little more conventional. A pre and post volume for the clean and lead channels. Then to an effects/reverb loop then the amp section.

    [​IMG]
     
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  15. jonrpick

    jonrpick Friend of Leo's

    Mar 17, 2014
    Marietta, GA
    Unrelated but... kinda...

    I may be getting my first Marshall this year. It'll definitely be a Mosfet Lead 100. After watching hours of demos I've concluded that I don't need an actual 2204 in my life.
     

  16. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    I had a suggestion that I should put in a small output tube after the PI. Then it will be a regular amp, sort of. I will have to look at my iron, I do have a P-P that used a pair of 35L6's, kind of small and maybe a couple of watts. I don't want to buy any more tubes, I only have the pair of 35L6's. Maybe a quad of 6AK6's? I don't know. I have some 12BK5's which might work. But the point was to make a Class D amp with a tube front end to see what it will sound like. A 19V supply with three tubes work (6.3V heaters in series), or a pair in series at 12V for a 24V supply although a little short of voltage. A TPA3118D2 will put out 20W@19V and 30W@24V. Maybe should go for 30W. So two 12AX7's with a little short of heater current or run them at 19V and reduce the 24V by 5V. 150 mA x 5 = 0.75 W. It is decided, I can loose the difference.

    Now what I was really going to do today. The schematic at the top of the page has a 5E3 converted to BF and Tweed channel with a four pole switch. I drew up the wiring diagram, of course it is upside down as compared to the amp I want to use it in. I really should check these things first. No matter, I have the 5E3 bright channel converted already, I need to do the BF side. I am putting it in a 6K6GT P-P amp I built before. I had it running BF or Tweed but the extra triode of the BF amp in parallel with the first triode. Think of the Princeton Reverb but without the reverb and its signal drop of the 3.3M resistor. It had a fair bit of gain, more than I wanted in the amp. I am fine with the BF channel being mainly Clean. Besides, it has the Tweed channel if needed.

    [​IMG]

    So the little blue lines represent the closed contacts (switch toggle down) and circuit in 5E3 mode. The other way BF and Tweed voltage divider Volume and Tone.
     

  17. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    OK I give up. Well at least building amps this summer. Projects just keep popping up and less and less time to get anything done, done in the finished stage. The guitars are even on hold and I really want my little acoustic to play. Woe is me. I managed to buy a Hammond tone cabinet although I got it in two pieces, the cabinet and then the chassis (no tubes). The output transformer for the nine 10" speakers is bigger than a Bassman transformer, probably could put out 100W. No real need for it. I basically bought it for the treble output transformer, it was fed by four 6V6's, looks to be a 15-20W size. There was a reverb transformer with it, what it will do in frequency response we will see some day. Also about as much iron in it as the treble transformer. Because of the multitude of 10" speakers I decided to slap together a Bassman cabinet, not done yet but got the major woodwork done while it still is summer.

    I also picked up an old Lowry organ from about 1962, not your high end instrument but I did not care as it had tubes and most likely had a P-P output transformer. Got lucky as it was fed by a pair of 6V6's. I plan on making a little grab and go amp for my neice's husband. He has a 5E3 amp I built that has 6AK6's for output tubes putting out maybe 3-4W with a 10" speaker. He took it to a solo gig and found he had to get it more distorted than he wanted. He took a chance on it rather than take his Peavey 30W. I was going to make him an amp with a SE OT that might do 10W but given the new old P-P OT I thought to use it. I also had a couple of Frontman 25R speakers and thought to use it in the amp. So since I was set up to do box joints for the Bassman I made a Frontman sized cabinet also.

    [​IMG]

    Back to the circuit for the amp. I have used a fixed bias Mosfet PI before and it worked well so I thought to use it again, none of the bad habits of the Cathodyne circuit, pretty much neutral. That leaves me with two triodes for the first and the second position. With a regular BF tone stack the gain would be limited, not that it was to be a gain monster but I want a little more on tap just because. Rather than the gain sucking regular BF stack I picked the supporting parts to give a Bassman/Marshall response. More midrange and less bass and treble range but that is ok. Added a Mosfet follower before the tone stack to give another couple of dB gain. Fairly simple with only a three position switch to modify the negative feedback.

    [​IMG]

    I am going to go with a laptop power supply rather than a power transformer to reduce weight and to put less stress on the chassis which then can be less substantial. All in the quest to make a lightweight amp. The wood was selected to find the lightest pieces I could find for both the cabinets. I just do not know if I should go 6V6 for the outputs or 6AQ5's or EL84's. He does have EL84's in his Peavey and it would save him on another extra set of spare tubes but a different tube would give him a little different sound. Not a big hurry as I am now putting them aside in order to do work on the house before winter starts creeping up on me.
     

  18. awasson

    awasson Friend of Leo's Gold Supporter

    Age:
    54
    Nov 18, 2010
    Vancouver
    Well, we’ll be waiting for the next instalment whenever you get around to it.

    This does seem like a very busy summer to me too. I realized a little earlier this evening that I still have a 12” cabinet to finish off as well as a couple of pedals and two guitars in partial build. I’m building a deck out back, landscaping the whole back years, then come the sidewalks and eventually I have to do the front yard/garden, sidewalks and the driveway. And I have to finish up the McIntosh MC275 my dad started building before he passed away and deal with all of his “stuff”.

    Anyway, lots to do so good luck with all of your projects and I’ll be waiting to hear more about the amp and especially that little parlour guitar your making in the pine tele thread you started last year.
     

  19. radiocaster

    radiocaster Friend of Leo's

    Aug 18, 2015
    europe
    I was surfing the internet a few weeks ago and I did find a couple of tubes for car radio which work at 6 or 12 volts for the plate voltages.
     

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