JCM800 6V6/6L6/EL34 in a Princeton Reverb Chassis

Discussion in 'Shock Brother's DIY Amps' started by robrob, May 1, 2019.

  1. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    7,519
    Joined:
    Dec 29, 2012
    Location:
    United States
    See https://robrobinette.com/RR2104_Master_Volume_Micro.htm for more info.

    The JCM800 6V6 is a slightly modified JCM800 designed to fit into an easily sourced Princeton Reverb chassis. By design it can run 6V6, 5881, 6L6 and EL34 power tubes. Many people gig with Princeton Reverbs so here's a shredding machine in a Princeton Reverb package.

    For the JCM800 6V6 I spec the AllenAmps.com TP25 power transformer and TO26 output transformer which also support 6L6 and EL34 power tubes.
    Both of these transformers are Princeton Reverb upgrade transformers with a lot more oomph than stock transformers. They fit perfectly in the Princeton Reverb chassis. The preferred choke is the Classictone 40-18032 5H 120ma. The bias circuit uses the power transformer's 50v tap and features bias balance and bias adjustment for 6V6, 6L6 and EL34 tubes. The power tube sockets are wired for EL34 compatibility.

    Power output is around 22 watts with 6V6 power tubes and somewhere around 25 to 30 watts with 6L6 and EL34 power tubes. Output is limited due to output transformer saturation. A pair of 23 watt 5881 power tubes is also a great match for this amp. We compensate for the different power tube plate load requirements by adjusting speaker load. For 6V6 power tubes we use an 8 ohm speaker with the 8 ohm speaker jack. For 6L6, 5881 and EL34 power tubes we connect an 8 ohm speaker to the 16 ohm speaker jack. The TO26 output transformer has both 8 and 16 ohm speaker outputs and I connected each to a separate speaker jack to make switching between tube types easy. Note that neither speaker jack has a shorting switch. Also note the NFB is tapped off the 8 ohm speaker jack.

    If you don't need the 6L6/EL34 option then standard Princeton Reverb power and output transformers will work fine.

    [​IMG]
    [​IMG]

    The cap can is a JJ 40/20/20/20uF 500v can. I sourced it and its clamp from Mojotone.com. The can's terminals are marked with Y, X, U, O and "-" stamped in the terminals. The Y terminal is 40uF and the X, U and O have 20uF. The "-" is the negative or ground terminal.

    You can use a standard Princeton Reverb size cab with either a 10" or 12" speaker. Mojotone sells a nice Princeton Reverb head cab too.

    Circuit Tweaks
    I reduced the V1A (first preamp stage) .68uF cathode bypass cap to .47uF to deemphasize lows and low-mids. I replaced the 470k treble peaker attenuator resistor in front of the cold clipper with a 150k resistor for added preamp gain and less shrillness. I reduced the bright cap on the Preamp volume pot from 1000pf to 330pF to keep from thinning out the tone too much at low volume. Of course you can use the original amp values instead of my tweaks but these circuit changes are time proven mild tweaks that really do improve the amp's character.

    The pre-phase inverter master volume is dropped in favor of a Trainwreck Type-3 post phase inverter master volume (PPIMV). This allows the phase inverter gain to contribute to the pre master volume distortion and makes the control more useful.

    The NFB resistor stays at 100k because the drop in output voltage from 6V6 power tubes is perfectly offset by moving from a 4 ohm secondary tap to an 8 ohm.

    A JJ Princeton Reverb 40/20/20/20uF 500v cap can is used for B+1 through B+4 and a 22uF 500v cap is placed on the circuit board for B+5.

    The 0.47uF V1A cathode bypass cap does not need to be an electrolytic. Any type of cap will work fine. I'm a fan of tantalum caps for .47uF and .68uF bypass caps. The bypass cap should be rated at 10 volts or higher.

    The 6.3v heater center tap is elevated by connecting it to a Bleeder/Divider circuit at 65 volts. This heater elevation makes life much easier on the cathode follower and reduces heater hum (see layout above).

    You can upload the JCM800 6V6 Hoffman circuit board DIYLC file to Hoffmanamps.com and Doug will make an eyelet or turret board for you. He can also populate the board for you if you'd like. The empty board with eyelets or turrets installed is $20 + shipping. I used to make my own turret boards but with HoffmanAmps.com accepting DIYLC files to make custom boards it's just too easy and inexpensive to bother with making them myself.

    If your amp has stability and oscillation issues try chopsticking the amp's leads around. Try to separate the preamp plate and grid wires as much as you can. If you still have stability and oscillation issues, placing a 1000pf 600v+ ceramic disc cap across V1 (first preamp stage) pins 1 & 3 will help. This stability disc cap is present in many factory Master Volume/JCM800 amps.

    Speaker Suggestions
    An 8 ohm speaker is preferred because it makes it easy to switch between 6V6 and 6L6/EL34 tubes. For 6V6 tubes connect the 8 ohm speaker to the 8 ohm speaker jack. For 6L6, 5881 or EL34 tubes connect the 8 ohm speaker to the 16 ohm speaker jack.

    I'm not a fan of the Celestion G12T-75. I prefer the standard Celestion G12M Greenback for Marshall closed back cabs but for a JCM800 6V6 combo cab my recommendation is the Warehouse Guitar Speakers Green Beret in 10 or 12 inch. I prefer the 10 inch speaker for Princeton Reverb cabs. If you want a 12" speaker in a Princeton Reverb cab consider enlarging the cab. Adding two inches to the cab's height will allow a 12 inch speaker to breathe and not sound so boxy. Most cab makers will customize their cabs if you ask. The Celestion Classic Lead is also a good speaker choice. I recommend the Celestion Vintage 30 if you like a slightly darker tone.

    To summarize the parts sources I used: I purchased the Princeton Reverb chassis, cab and most parts from Mojotone.com. The power and output transformers came direct from AllenAmps.com. I purchased the Classictone choke from AmplifiedPartsDirect.com. Doug Hoffman at HoffmanAmps.comcan supply the turret or eyelet board.

    See https://robrobinette.com/RR2104_Master_Volume_Micro.htm for more info.

    I also have a 1 watt JCM800 Micro that is also designed for the Princeton Reverb chassis and cab.
     
    Last edited: May 1, 2019
  2. jsnwhite619

    jsnwhite619 Friend of Leo's

    Age:
    35
    Posts:
    2,384
    Joined:
    Sep 10, 2013
    Location:
    Georgia
    Please post links to any sound clips as they come along. This looks to be an awesome case of "dynamite in a small package.". I would love to see someone's face to think they were looking at a Princeton Reverb and hear a JCM800.

    Sent from my XT1585 using Tapatalk
     
    robrob and Snfoilhat like this.
  3. arielyitus

    arielyitus Tele-Meister

    Age:
    35
    Posts:
    355
    Joined:
    May 4, 2016
    Location:
    Clarkston, Mi
    Wow, great job Rob!
     
    Jerry J and robrob like this.
  4. micadoo

    micadoo TDPRI Member

    Age:
    54
    Posts:
    18
    Joined:
    Apr 28, 2019
    Location:
    Melbourne Australia
    That looks like an awesome build. I’ve heard Boogie got started hot rodding Princeton’s to be gigging machines?
     
    robrob likes this.
  5. Luthier Vandros

    Luthier Vandros Tele-Holic Silver Supporter

    Posts:
    577
    Joined:
    Aug 19, 2015
    Location:
    Down by ya mom an them's
    Damnit! Great job, Rob! I remember when I was getting into building - Hoffman’s website was so instrumental. Such a good resource.
     
    Last edited: May 1, 2019
    robrob likes this.
  6. 24 track

    24 track Doctor of Teleocity Silver Supporter

    Posts:
    13,403
    Joined:
    Nov 6, 2014
    Location:
    kamloops bc
    nice stuff Rob, beautifull design!, I have an 83 JCM800 4104 with the North American 5881's is it hard to convert this to the EL34's? ( the distributer thought that it would appeal to th the N/A clientel appearently) this one is still original ( the celections are 3 days older than the amp )

    P1010005b .JPG
     
    bftfender and robrob like this.
  7. chris m.

    chris m. Poster Extraordinaire

    Posts:
    6,509
    Joined:
    Mar 25, 2003
    Location:
    Santa Barbara, California
    I miss my jcm800 combo...
     
    robrob and 24 track like this.
  8. Greg_L

    Greg_L Tele-Holic

    Posts:
    969
    Joined:
    Jun 27, 2018
    Location:
    Texas
    This is really cool. Thanks for sharing.
     
  9. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    7,519
    Joined:
    Dec 29, 2012
    Location:
    United States
    Many North American JCM800s came with 6550 power tubes. I've never heard of any coming from the factory with 5881s. If your JCM800 has 5.6k power tube grid stoppers (preferably 2 watt) and 1K 5 watt screen resistors you should be good to go with EL34s.
     
  10. Wally

    Wally Telefied Ad Free Member

    Posts:
    32,176
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Subscribed....
     
  11. 24 track

    24 track Doctor of Teleocity Silver Supporter

    Posts:
    13,403
    Joined:
    Nov 6, 2014
    Location:
    kamloops bc

    thanks for getting back to me....
    you're correct , 6550's, I just went off of your post , but when i bought the amp I made a spec sheet and tube lay out sheet plus dates and serials and placed them in the amp.....whoops my bad
     
  12. FenderLover

    FenderLover Friend of Leo's

    Posts:
    4,729
    Joined:
    Jun 11, 2009
    Location:
    Minnesota
    Rob, have you built one of these?
    Just to note my observations - the TP25 will do 22W with 6V6's and about 25W with 6L6's (measured). That's also what Mr. Allen says. Minor point, but I doubt anyone will see 30W with any tube set supplied by the TP25. I haven't tried EL34's, but it stands to reason their output power is also limited by the PT current, as it is with the 6L6's.

    Another minor point to throw out for discussion - since the output power with either tube set is essentially the same, the applied B+ is not being altered, and the current is limited by the transformer, I found the same output tap works with 6V6 or 6L6 equally well. The drawing shows 7K OT primary, but also noting a 50% reduction with 6L6/EL34. Of course it will also work, just curious what people like to hear when changing these loads, or if it is an automatic response that we change the load with the tube sets.

    If the PT were capable of higher current for the power that 6L6/EL34 can use (at least 40W), then I'd suggest changing taps. But then we'd also be swapping tubes and re-biasing. At that point I'd just make a decision on what I wanted and build it/use it that way.

    I love Princeton and Champ chassis. They are great sizes for a million single channel designs. Just design custom face plates and you're golden.
     
    jsnwhite619 and Mr Ridesglide like this.
  13. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    7,519
    Joined:
    Dec 29, 2012
    Location:
    United States
    A JCM800 Micro has been built but not this 6V6 version.

    A lower plate load typically has more even order harmonics and less 3rd order so it usually sounds "better" so I recommend the speaker jack switch when changing tube types. It's usually better to run low plate load than high.

    You can bias the amp so that you don't exceed 70% bias with either tube type so re-bias isn't necessary.
     
  14. NSB_Chris

    NSB_Chris Tele-Meister

    Age:
    54
    Posts:
    123
    Joined:
    Sep 2, 2017
    Location:
    New Smyrna Beach Florida
    I am going to build the 6V6 version but will add a second channel also. Will post questions on that later when I finish my draft circuit layout. One question regarding grounding scheme: I like the space saving of the cap can. Using the cap can forces using the power ground for all but the B+5 on this circuit. Does it matter that for the cathode follower, the B+ supply filtering is grounded to the power amp ground but everything else on this stage is grounded to the input star ground?
    In my previous amps I have tried to have all grounds associated with each stage use a common ground. Maybe that was not necessary. I know the original Marshall amps grounded all over the place.
     
    Last edited: May 7, 2019
    robrob likes this.
  15. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    7,519
    Joined:
    Dec 29, 2012
    Location:
    United States
    The cap can grounding scheme works for the Princeton reverb and JCM800 Micro so it should work for this amp too.
     
    NSB_Chris likes this.
  16. jsnwhite619

    jsnwhite619 Friend of Leo's

    Age:
    35
    Posts:
    2,384
    Joined:
    Sep 10, 2013
    Location:
    Georgia
    I have a recent thread with a Princeton Reverb I just built. I skipped the can and fit a cap board inside between the PT and board. Split the grounding and it was pretty near silent in the end. Anyway, if concerned, a regular cap board can fit inside.

    Sent from my XT1585 using Tapatalk
     
    robrob likes this.
  17. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    7,519
    Joined:
    Dec 29, 2012
    Location:
    United States
    The JCM800 and JCM800 6V6 are 2 channel amps. The High input runs through an extra gain stage compared to the Low input.
     
  18. Paul G.

    Paul G. Friend of Leo's

    Posts:
    2,644
    Joined:
    Mar 17, 2003
    Location:
    Rhode Island
    Looking around for a trashed PR first, to do on the cheap.

    If no go, I'll start sourcing parts a little at a time. This is right up my alley.

    Thanks for this.
     
    robrob likes this.
  19. NSB_Chris

    NSB_Chris Tele-Meister

    Age:
    54
    Posts:
    123
    Joined:
    Sep 2, 2017
    Location:
    New Smyrna Beach Florida
    Yes, and the channel switching with a relay would have been a lot simpler. The big bottle JMP2204 I just built is wonderful when set for a healthy amount of preamp distortion. I made it so I could relay switch between two Volume and MV settings, but I thought running it down on the clean side was a little thin sounding. The circuit is designed to get unwanted bass out with the bright caps for distortion.

    For this one, I wanted to be able to have a true distortion channel and a true clean channel and to have it be a little more manageable on the power amp side (6V6). My thought was to get the lower gain side to be more of a pedal friendly fender circuit and keep the dirty channel true to the Marshall high gain. Rather than using this to go between rhythm and lead within a song, I would be using this almost like switching amps for different songs that need different amps without having to have separate amps. I thought having the clean channel be relatively true to the Bassman circuit, the dirty channel be the JCM800, and everything on a 6V6 power section could be a really cool and versatile amp. Just have to add one 12AX7, a relay and some miscellaneous components to get both.

    Or, like other amps I have built, I end up with features built in that I never use. :)
     
    robrob likes this.
  20. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    7,519
    Joined:
    Dec 29, 2012
    Location:
    United States
    Sounds like an interesting amp. Please post details of the build.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.