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

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    The amp is derived from the Princeton Reverb so PR speakers should hold up. At least no one has blown one up yet.
     
  2. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    In reply to Wally: I don't think that link is too bad. And in your own opinion you give two (similar) reasonings where I read one should have a speaker with a higher power rating.

    In reply to Rob: I've looked at the original Princeton reverb and two reissues. All have a much higher power rating speaker:

    Original: 12-15W amp, 50W Jensen c10n
    65 RI: 22W amp, 100W Jensen c12k (200W musical power?)
    68 RI: 22W amp, 70W Celestion G12V-70

    I've little to no experience, but the information I have (been given) still gives me the feeling that one 20-25 W speaker is not enough. Am I missing something?
     
  3. Wally

    Wally Telefied Ad Free Member

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    1) the original BF/SF Princeton/Princeton Reverb amps rarely might have come with a C10N. Most of them had an Oxford 10J4 or perhaps a Jensen C10R. Those speakers would perhaps have been 20 watt speakers. There have been reports of a C0N having been used. I have never seen such. If that happened, I would think it would have been a case where there were no other options but to install the higher power...and more expensive...speaker because there were no low wattage speakers available that day.

    2) I believe in using at least twice the handling capability compared to output power. Ommv and...playing styles determine what a speaker undergoes. A 5 watt amp can take out a fairly high capability speaker if one goes about it a certain way.
     
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  4. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The double-the-amp-watt-rating speaker rule isn't hard and fast but it is conservative and good advice.

    From the Weber Speaker website on a Princeton Reverb replacement speaker:

    "The favorite for Princeton Reverb:
    (if you use no pedals for dirt and you want the speaker to break up a little):[10F150T, 8 ohm, 25w, light dope]
    (if you use pedals for dirt or you want the speaker to stay cleaner): [10F150T, 8 ohm, 50w, light dope]. "

    Since the JCM800 6V6 is a much higher gain amp than the Princeton Reverb it wouldn't hurt to go with a 40 or 50 watt rated speaker. I'll add some info on this to the JCM800 6V6 webpage.
     
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  5. NSB_Chris

    NSB_Chris Tele-Meister

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    My build has started.

    IMG_3462.jpg

    I have a gutted silver face twin reverb cabinet. I purchased an undrilled steel chassis with a right angle face. This will have two inputs with separate preamp circuits as I posted before in this thread, one trying to honor Rob's circuit and one tamed a bit. They mix in before the power section. This will be the "RR800V TWIN". The chassis has been drilled and painted. Front pannel was made with water-slide decals and clear coat. Most components have been mounted and eaters are wired. I will work on the front panel pots and main board tomorrow.

    Right now the cabinet has two 16 ohm greenbacks in it as I have been using it as an extension 2x12 speaker cabinet because it has the tilt back arms. I purchased two green baret speakers, but stupidly got 8 ohm, so the only way I can use them in this cabinet with this amp is with the 6V6 and the 16 ohm OT tap. Might leave in the greenbacks to see if I want to run it with 6V6 or EL34 power tubes. If it works of course!
     
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  6. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    I'm thinking about building this also. I will be following this thread closely, especially @NSB_Chris' build because I'm also very interested in a clean channel in the same amp.

    I like the Blencowe-correct heater wiring. You don't see it that often. I also didn't, because I couldn't get it done. Probably my wires were too thick. What thickness have you used? Any tips and tricks? Did you pre-tin the wires?

    What are the heater wires connected to on your AC/DC board?

    Can you give any more info about the chassis work: painting, water slide decals, clear coat, ...?
     
  7. NSB_Chris

    NSB_Chris Tele-Meister

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    This was the first time I wired the heaters straight back from the sockets to the corner. All my previous builds I flew them Fender style. Running them straight back was a lot easier in my opinion. I used 18ga for everything but the last 3 preamp sockets where I use 20ga.
    I never liked running the heater wires to the pilot light and then back across to the sockets and this amp I am using LED pilot anyway. I just terminated the heater wires from the transformer on the board and then put separate turrets for the wires going to the sockets. Hopefully this works out ok. I put the resistor and diode for the LED pilot on the board for easy change-out. The LEDs were uncomfortably bright on the last amp I built and it was not easy to change them out. This way I have easy access to switch if I need to.

    I added a lot of extra turrets on the AC/DC board and the main board to make connections easy. Little extra cost, more holes to drill and more turrets to swage, but I find it easier to wire up the components and hopefully easier to troubleshoot and change components in the future if needed.

    I used laser printer water slide decals for the graphics. I have used the inkjet printer water slides before and the ink can run a little on those if you end up messing with them too much. Laser worked much better!. Painting is simple. I used automobile wet sandable primer on the bare metal. Light wet sand with 400 grit sandpaper before the color coat. 4 or 5 very light coats of color to give enough thickness to sand smooth before stickers go on. Wet sanded with 400 grit until reasonably smooth where the stickers go. Waited 24 hours before sliding on the stickers. I let the stickers set for 12 to 24 hours and then clear coated with several coats of satin clear. I think this is the basic process that many are using on this forum. I should have used Crylon paint but used Rust-Oleum because it was convenient at my local store. Seems to work fine.
     
    Last edited: Aug 16, 2019
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  8. NSB_Chris

    NSB_Chris Tele-Meister

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    More progress, but still a long way to go!

    IMG_3465.jpg IMG_3466.jpg
     
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  9. NSB_Chris

    NSB_Chris Tele-Meister

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    IMG_3473.JPG IMG_3474.JPG She's alive!
    One little mishap with the wrong resistor value in the bias circuit. Had the correct value on hand. Finished last night and could not get her up to volume. Will post voltages soon to see what others think. I am going to purchase some EL34 tubes to A/B the difference.
     
  10. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    I'm really interested in you comparison with el34's. Robrob says on his website that pentodes distort more: "True pentodes such as the EL34 and EL84 flow more screen current and therefore offer up more distortion than beam tetrode power tubes like the 6V6 and 6L6." Could you please post some sound clips?

    Also, what is that green wire that is wrapped around the pot wires? Is just some sort of tie-wrap? Or is it grounded?
     
  11. NSB_Chris

    NSB_Chris Tele-Meister

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    I cannot say that I have put this amp through its paces yet, but with the first run through at home, the higher gain side is very different than the "pure" JCM800 (JMP2204) circuit that I built. The preamp circuit is almost identical, so the main difference must be the power amp side. Can't wait to get EL34 tubes in it to see the difference.

    The cleaner side is really interesting. I really like this amp even if it does not end up filling the role I expected it to. I put a bright cap switch in the lower gain side and that is really useful on that channel.

    The green wire that wraps the tone stack leads going from the board to the pots is something that I saw on Rob's website. Since this is a high gain circuit and I intend to run it that way, I wanted to do everything I could to limit noise. The green wire is grounded and is a "poor man's" shielding. I would have use real shielded wire if that were easily available in 3 conductor wire but I could not find any so I just wrapped some junk wire I originally bought for guitar wiring. I shielded all of the long run signal path wires in an attempt to keep it quiet. Some hum with the volume on 10 and the master low but I would say this amp is reasonably quite.

    I will try to get some sound clips when I get this amp in the studio.
     
  12. NSB_Chris

    NSB_Chris Tele-Meister

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    Oh, by the way, I would not call this a "micro"! This 20 or so watt version of the original is a very laud amp. For small rooms with the volume up high, the master volume is barely cracked to keep OSHA from breaking down the doors. The 1 watt version is probably a true micro, but I would say this is a "mini JCM800"!
     
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  13. NSB_Chris

    NSB_Chris Tele-Meister

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    OK, disregard what I said in the first paragraph of post #71!!!! This amp is awesome!!! I will never again do a first run of an amp in my back room. That room has too many reflections or something because it does not do a high gain amp justice. Smaller amps like a Gibson GA-1 RVT and Fender Princeton sounded great, but not this amp in that room.

    I tested this amp in my friend's studio at our weekly jam session tonight at volume. The room is a reasonable sized room with decent acoustics. Also had hotter pickups on the guitars that I keep at the studio so that hit the front end of the amp a little harder than the single coil Tele I had at home for the first test. I tested this amp side-by-side with the head that I built with "pure" JCM800 (JMP2204) circuit and they were remarkably close. Note that I don't get the amps to a volume level that would get the power section and speaker to the sweet spot of breakup. Most of what I get at 100db in the room is from the preamp, although I probably get the 6V6 power section of this 20W amp much closer than the 50W EL34 amp.

    I really like the clean channel with the 12AY7 in the first position also. Nice warm tone. Not as scooped and yummy as the Deluxe, but very nice. Makes the amp very versatile. The bright switch on this channel really makes a big difference, so I would not omit that.

    This amp is now my favorite amp and I left it at the studio. Sorry, but it may never see EL34 tubes! This amp is a working amp now!
     
    Last edited: Aug 23, 2019
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  14. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    Nice!

    Any sound clips from the studio?
     
  15. NSB_Chris

    NSB_Chris Tele-Meister

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    Not yet, but I will record some.

    One thing I might do if I am ever motivated to open this thing up again is to put a much larger resistor on the pilot light. I sized it for something like 15mA and it is still unnecessarily bright. When the amp was coming up on the variac the pilot started to glow when the heater was at about 2VAC. Next amp I will probably just run the unused 5VAC leads straight to the pilot and put the diode and resistor at the LED, with the resistor sized for much lower current.

    I could investigate some tone stack changes on the clean channel, but probably won't mess with it because it is fine as is. I made it so that I can check and set bias without removing the chassis from the cab, so I may never be inside this amp again.
     
    Last edited: Aug 23, 2019
  16. NSB_Chris

    NSB_Chris Tele-Meister

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    Ok, after the hurricane passed us by we managed to record a few seconds of this amp naked.

    Please keep the survivors in the northern islands of the Bahamas in your thoughts. Too few will have survived and those that have, certainly lost everything but their lives. Living on a coastal island, I am thankful to have come away unscathed, but humbled and shaken by the loss of those so close to our shores!

    We tried to capture the raw nature of this amplifier the Thursday after the storm spared us and passed us by. I will share the details, but I doubt that what comes through the internet will preserve sufficient fidelity to be able to distinguish any of the finer points.

    How do you share audio clips? I have mp3, but it won't take that. Will it take WAV files?
     
  17. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I'm glad you made it through the hurricane. What island do you live on?

    I don't know how to share a sound file through this forum. I put them on my website and then link to them.
     
  18. NSB_Chris

    NSB_Chris Tele-Meister

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    I am in Florida on the New Smyrna barrier island. We were threatened but completely spared any damage. Easy for us to evacuate and of course we have mainland infrastructure readily available if the bridges stay intact. 15 foot storm surge would have stripped our island clean like Galveston Texas.

    Looking around on the forum, it looks like I have to post the files somewhere like Youtube and then link to them. I will have to figure out how to set that up.
     
  19. NSB_Chris

    NSB_Chris Tele-Meister

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    I will try to share audio clips here. What a PITA to share audio! Hopefully this works.

    Guitar direct into the amp.

    Guitar: Mexican Tele with a Seymour Duncan JB trembucker in the bridge position used for the high gain channel take and a Seymour Duncan little ’59 mini-humbucker in the middle position used for the lower gain channel take. Basically, nothing special. Hot pickup in the bridge position for high gain channel and a medium gain and medium warm pickup in the middle position for the lower gain channel.

    Mic: SM57 on axis, on the grill cloth and centered just off the cone of one of the two 16 ohm greenbacks wired in parallel.

    Amp:
    - Bass, mid and treble set in the 12 o’clock position for both channels.
    - High gain channel, volume max and master just cracked to get to about 100db at about 10 feet away in a medium sized room
    - Lower gain channel, volume about 11 o’clock and master about 9 o’clock. About the same recording volume level as the high gain channel.

    Sample for channel 1 (High Gain):
    https://drive.google.com/open?id=1E681FLQ5QmGZpqZjIkvw7XeuBmv4joN2
    The track starts with the volume knob on the guitar backed off about a third. Open position G5, A5 & E5 chords and then some simple middle neck and 12th fret solo type licks. Then the volume knob is rolled up to max and the basic progression is repeated. I let the last section ring to get a feel for the sustain.

    Sample for channel 2 (Lower Gain):
    Just some simple chords and single note progression to give a feel for the warmth of the clean channel.
    https://drive.google.com/open?id=1ukIms6Nuk9ZfR47hAlqZfEEBV7k2mjC1
    I did not demo it here, but mixing the two channels adds a really cool third dimension. With an AB/Y pedal you can test the high gain alone compared to the high gain and the lower gain channel mixed together. You can really hear the width increase as the lower gain channel is mixed in.
     
    Last edited: Sep 8, 2019
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  20. jasonvilla

    jasonvilla Tele-Meister

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    I was looking at your schematic and compared it to the JCM800 schematic drawn by Mark Huss. Do you have an extra power filtering capacitor compared to a stock 2204? I only see 4 on his but 5 on yours. Just curious what the difference would be. Thanks!


    EDIT: Never mind. I see the 5th cap now. Also looked at Ceriatone's website and their 2204 has 5 caps as well. Sorry!
     
    Last edited: Feb 14, 2020
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