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Excelsior - reduced preamp bias!

Discussion in 'Amp Central Station' started by Dacious, Oct 17, 2017.

  1. Dacious

    Dacious Poster Extraordinaire

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    Reading from a few people that the Excelsior appears to based on a cathode-biased 6G2 brown Princeton I compared the schematic/s and indeed they're very similar, down to measured preamp voltages.

    The Excelsior has a sort of ratty tone the one and only Brown Princeton I've ever heard didn't. The biggest difference is the bias resistor on the two halves of the first preamp tube - 100k for the 6G2, 220K for the Excelsior. That's trying to drag a lot of gain. Mine squealed if I face it with my Tele above about half or click in too much gain. It also tends to saturate heavily as soon as you have decent volume. There was headroom but tricky to manage.

    I've temporarily jumpered 220k parallel resistors across R5 and R7 in this schematic - so they are 110kohm measured.

    images (3).png

    Far from losing gain or volume, it appears to have picked up some volume, or else the ratty sort of fuzzy distortion was due to too much gain saturating the 12AX7 halves.

    It's now fuller, smoother, feels not so hard/laggy to pull tones out of - more touch sensitive, more pick responsive. That hard garage-grungey tone is still there but more controllable - if anything, it's throatier and fuller but more transparent. The Tele sound is more prevalent. But Keef is a little more dig away!

    Now I know this works I'm going to pull the PCB and solder 100kohm resistors in properly, and I think I'll change the 1st bypass cap to a .022 as well as per the 6G2, as I think that might smooth it out a bit more.

    I don't know if it's louder or just its got better frequency response. I've wound volume back from 7+ to about 6. The bass is certainly more apparent. It reminds me of nothing so much as - a Marshall, crossed with a bigger, throatier 5E3!
     
    Last edited: Oct 17, 2017
  2. dqami

    dqami Tele-Holic

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    Would love to hear how it sounds when you are done. Are you using the stock speaker?
     
  3. CapnCrunch

    CapnCrunch Friend of Leo's

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    +1

    I picked up one of these amps locally for a little over $100 and while it has promise, I was thinking it needs a new speaker, and a new Output transformer to bring the low end to life. Maybe changing resistor values and a coupling cap would do the trick. I'd love to hear what you think after the coupling cap change. A 6G2 Princeton is on my build list. If I could mod this Excelsior and put it in a custom built cab, that might scratch that particular itch. Looking forward to hearing your review, and clips if you can post them!;)
     
  4. marshman

    marshman Poster Extraordinaire Silver Supporter

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    When you finish tweaking, please do drop by the Exy thread in the Amp Owners Forum and let us all know what you did and how it worked out!
     
  5. Dacious

    Dacious Poster Extraordinaire

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    This is the earlier clip posted. This is with 1958 Jensen P15R and a NOS longplate preamp tube. It's louder than the stock speaker and on single notes has a nice alnico 'thonk'.

    Once I've done the mods I'll repost. This also has a tone control and depth control on trem. This is before I modded the preamp values.

    [​IMG]
     
    Last edited: Oct 19, 2017
  6. Budda45ftw

    Budda45ftw Tele-Meister

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    Pics!?
     
  7. CapnCrunch

    CapnCrunch Friend of Leo's

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    I put a late 50's Jensen P15Q in mine. Big improvement over the original. It still needs help in the low end though.

    Your clip sounds good. What was the volume at and what type of pickups were you playing into the amp? I really like the alnico speaker with this amp.
     
  8. Dacious

    Dacious Poster Extraordinaire

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    The amp and tone were both bet set midway. I have a 1meg depth pot for trem off my MIM Thinline plus a 500k tone pot. The guitar is my 65 AVRI Strat with stock 57/62 pick-ups. It started in the neck-centre went to bridge then bridge centre for the three bits.

    I'm also going to try a grid-stopper resistor on the phase inverter, I think some of the braaang distortion when it's pushed up around 6-7 that comes off as gravelly harshness might be blocking distortion. That's right where I want it for gigging - if I could reduce it slightly it'd be good. With the speaker, 60's preamp and 6nPs 6V6s the basic tone is now very good.

    Checking schematics, this amp is actually close to a 6G3 brown Deluxe in terms of preamp voltages - very similar values of components to the 6G2 anyway.
     
    Last edited: Oct 23, 2017
  9. Dacious

    Dacious Poster Extraordinaire

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    OK, more progress. I've now undone the preamp board by undoing all the input jacks and controls. It's an easy job - plenty of room in the chassis.

    I also checked - you can drain the caps on this amp to virtually zero, by playing the amp powered up and pulling the power cord out of the socket. Measured volts immediately after = zero. I think they will bleed down fairly fast anyway but that's a reassuring thing.

    You have to undo the chassis screws, and get the top chassis out and undo the seven small screws holding the three-sided panel off the inside - don't lose the spring and flat washers, they're important to stop rattles. If yours doesn't have them, putting some rubber stick on strips around the main chassis is a good idea to stop future rattles.

    If you do this on a table, and undo the strain relief on the flex-cable on the cabinet side that holds the leads running down to the output section, you can sit the chassis out in front of you on your bench or table. You also have to unscrew the lamp bezel, top off, then big nut inside the chassis. that lets you pull the whole assembly inside. Three jacks poke back inside, the volume and tremolo/speed control knob will lever off. You can unscrew the small screws holding the bright/dark switch on, and any other controls or jacks you've fitted, plus one earth lead to the chassis. In the pic below, the top board is the input jacks flipped up out of the way.

    The good news about the PCB is, it's a nice thick one and not too wobbly. And the traces on the back are not only nice and thick, but showed no signs of lifting when I de-soldered and then resoldered them. Good work Mr Fender.

    The two 12AX7 preamp triode plate resistors have been replaced, and I've circled them. You can easily unsolder them from behind, then resolder the new ones in. They are 220Kohm standard. That is 'out there' from the reading I've done. On the Princeton 6G2 they are 100Kohm.

    Originally I just jumpered two 220K resistors in parallel to hear the effect. I was happy with the smoothness so now I've removed them and made the change permanent with 0.5 watt metal films. Excelsior preamp1.jpg

    That did a good job of smoothing the response, without losing volume. In fact, I think Fender copying the original Excelsiors, did too good a job of including the garage band rasp. Great if you are using it low volume or indoors, it makes for nice low-volume breakup. I think they have just plain a bit too much gain, given the rest of the tone circuit. Brown Fenders seem to have used plate load resistors in the 100-170K range. I notice this also seems to increase touch sensitivity - the amp 'feels' more responsive to play.

    The other change, I've been reading on RobRob's page and elsewhere about the reason cathodyne phase inverters using one gain stage of a triode and getting inverted signals from the cathode and plate like this one can sound raspy and unpleasant, cranked. Also makes them a little sensitive to overdrive/distortion units. This is the 'fart' everyone talks about with Tweed Deluxes and some similar amps.

    https://robrobinette.com/5e3_Modifications.htm#Phase_Inverter_Grid_Stopper_Resistor

    It is part of the character of a 5E3, and can be reduced by playing with individual channel volumes and or altering the preamp sensitivity with jumpering. It sounds like it's blocking distortion, and the Excelsior doesn't have it too bad - changing the speaker and better tubes does reduce but not eliminate the effect. On my amp, I don't mind it so much, but it meant just where I wanted the amp at gigging volume was where it tipped over and I found tended to get lost in the mix. You can hear it in the video above when I hit the strings harder.

    So to start with, I spliced in a 470kom grid stopper resistor as per the pic below. This slows the charging of the coupling cap, and calms the PI triode down

    The green wire coming off the phase inverter triode V2b (pin 7) is the grid connection, which is where signal from the first preamp tube enters the phase inverter. Trying it out, it certainly smoothed out the response. The amp became very, very clean and lost a lot of volume. The tremolo also became very deep. Obviously, it was affecting signal a little too much.

    I've since replaced it with a 100Kohm metal film. Not had the opportunity to crank it yet (late at night here and warm so windows open) but it sounds crisp and clean and a couple of experimental strums with amp/guitar dimed showed a sizzling snap/crunch and pleasant gruffness and it seems like there's still healthy volume. Tomorrow I'll crank it and take another vid. This value may need more tweaking, but it 'feels' right and sounds like cross between a gritty Tweed and middy, crunchy Marshall, which is exactly where I want it to be.


    Excelsior preamp.jpg
     
    Last edited: Oct 27, 2017
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  10. Dacious

    Dacious Poster Extraordinaire

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    Ok, more progress - I wasn't happy with the 470k resistor on pin 7 of the phase inverter.

    I soldered a 100kohm in it's place - Wow! What a transformation! This amp opens out and blooms, there's just no other word for it.

    I will.post a video soon, but at rehearsal last night with the Rock Fossils I found myself turning down, from about 3o'clock to about 1pm or just over half on volume. With the guitar backed off in the rythym position, I could strum softly and make myself heard. There's still a nice 'edge' to the clean, but it's lost a fair bit of the harshness that could be present. I might just try 220k as I think that might be an incremental improvement, but I am really pleased with it. Putting my Tele in the series-in-phase position pushed it straight into lovely Marshally crunch - and now I can engage an overdrive without an instant squeal in this position.

    So with 100k plate load resistors I think drops the gain from outrageous to merely 'hot' - and the blocking distortion is nearly all knocked out by the PI resistor.

    It now has a bit more Fender character, probably closer to a Brown than Tweed or Blackface.

    Just for a giggle, I put in a set of Ruby 6L6s. Trying these out on my bandmate's HRDx we couldn't get the bias up past 26-27ma so I knew they were a pretty low-draw pair and seemed a good match.

    I read about a guy in New Zealand who's been doing this for a while in an Excelsior. Now - I don't say there will be no bad consequences so be warned. But both transformers after ten minutes of thrashing showed no signs of heating. Both were cool to the touch. I know there would be an impedance mismatch, with the 6L6s it'd rather a 3-4k primary for the 8ohm speaker. I might work out the turns ratio, it might be closer to 6K than 8k.

    But it sounded marvellous. Bigger, rounder, brighter. I could hear some speaker breakup. I don't think it was much if any louder. There was a bit less fizzy distortion.

    I think I prefer the 6V6s, but I gotta say - it sounds more Bassman like, that punch and big-glass rounded bottom!

    What an amp Fender has provided! It's solid, seems easy and tolerant to mod.
     
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  11. CapnCrunch

    CapnCrunch Friend of Leo's

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    Dacious,

    I hadn't checked on this thread in a while. Thanks for the pics and the write up. So, as I understand it, the only things you've modded are the plate resistors from 220K to 100K and you added a 100K grid stopper on pin 7 of the PI? Did you give up on the idea of swapping the cathode bypass cap to .022? Having done lots of mods to my 5e3 before selling it, I figured the bypass cap swap might net pretty noticeable changes.

    There are two things about this amp that I personally find lacking. The anemic low end, and the ratty, non-musical distortion that I think you are calling blocking distortion. It is similar to the nasty PI distortion I used to experience with my 5e3, so I think you are right on with your diagnosis of the PI as the problem.

    These mods you've done are pretty simple, and easily reversible, so I'm just going to go through my parts bin and find the right value resistors and try these. I don't think I've got any resistors smaller than 1/2 watt and they'll probably be vintage carbon comps, so they'll look funny, but they're what I've got.

    I don't know what the Power Transformer is capable of in terms of current draw, but I do know that BillM sells a PT that is designed for 6l6's. He and David Allen designed an output transformer also that is designed like a HiFi transformer that is supposed to "boost" the low end. I've been toying with popping one those OT's into this amp to see if that fixes the perceived problem.

    I'd love to hear your amp with the changes, if you get a chance to post a clip. Again, thanks for the write up!
     
  12. Dacious

    Dacious Poster Extraordinaire

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    I've been busy in the last week or so with gigs and then copped an awful cold with a blocked ear that actually hurts when I play guitar so been a bit slow.

    I tried some alternative values in the PI and it appears any resistance is good. 470k totally smoothed it out but resulted in a big volume drop too. 57-100-200k resistors no big change but all tame that fizzy rough distortion IMO. There's still some brrrrrr there but

    I jumpered the trem triode bias resistor to 110k and it has brought the tremolo back into usable range and depth adjustment works. Will make that permanent.

    I am pretty happy with the amp tone now. It's middy/Marshally in that the clean isn't really, but there's good note definition and the trem is nice and swampy. It's reduced the annoying squeal when you hit a dirt pedal too.

    I haven't done the bypass cap but will try that I think. I am really happy with the volume jump - it needs 'less' on the volume to sit in the band mix with drummer, which means it's cleaner. I don't want to turn the amp into something it's not.

    I think the amp sounds great with the Ruby 6L6's and seems pretty stable thermally. I have a rewound output trans on the way with 6500 primary and 4-8-16 ohm taps. I might try that just to see. By my reckoning 6L6s on the 16 ohm tap should be a rough match for 6V6s on the 8 in terms of impedance.

    100k resistors on preamp triode plate resistors (all three including trem). 100k blocking grid resistor.
     
    Last edited: Nov 7, 2017
  13. CapnCrunch

    CapnCrunch Friend of Leo's

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    Hey Dacious,

    I'm getting ready to solder some 220K 1/2 watt resistors in parallel with the plate resistors at R5, R7 and R20 and am going to attach a 150K 1/2 watt grid stopper on the grid of V2, I also have a 220K, and may try that value. You have stated that the grid blocking resistor should be soldered to pin 7, but I think it actually goes on pin 2 right? Maybe I'm thinking of the tube backwards but I think it is pin 2.
     
  14. Dacious

    Dacious Poster Extraordinaire

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    Pin 2 is the grid of V1A according to the schematic which is the triode driving the trem.

    V2B is the half driving the pi.

    I'm going to try replacing C15 with .022 today.
     
    Last edited: Nov 8, 2017
  15. CapnCrunch

    CapnCrunch Friend of Leo's

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    I must be visualizing the tube socket upside down. I'll have to have a close look when I get home. Thanks for the help!
     
  16. Dacious

    Dacious Poster Extraordinaire

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    Yep I had to look several time to be sure....

    It's the green wire that runs from the left as you're looking at the socket from where the chassis cover would go.
     
  17. CapnCrunch

    CapnCrunch Friend of Leo's

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    Am I correct that you also reduced the value to 100K on the Plate resistor for V2A? It is R20 and is 220K.
     
  18. Dacious

    Dacious Poster Extraordinaire

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    Correct. Otherwise the trem can't be modulated down enough, IMO with my 1meg pot it's still a bit too deep. Now with the one meg pot, I can take it from a light shimmer to a deep 'wap-wap-wap'. Alternatively you could increase the value of R15 or use a 2 meg or 3 meg pot to replace it.
     
  19. Dacious

    Dacious Poster Extraordinaire

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    Ok, quick video. I get more nervous doing these more than playing live for some reason! Apologies for crap playing.

    Amp is on 5 volume and tone, trem depth on 1/3.

    Guitar is my MIM Thinline with stock pickups and minor tweaks, vol 8, tone 8. Eagle Rock on bridge, What's My Scene on middle, tremolo on middle,. Jumpin Jack Flash on series-in-phase.

    Apart from kicking in trem, no controls touched. Now, the clean tone is quite sweet, although there's that Tweedy sort of overtone going on. I can get the distortion going with the series/in-phase pickup setting, only when I want it. Compared to previous, that's hitting with aggressive, but not hard picking.

    I think the sound is not quite as full here as in person due to no really good microphone, but what the video also doesn't capture is how touch sensitive the amp is now. Before, I felt like I was struggling to get notes out - now, it's far more 'singing' and sustaining.

    This was on five - to me, that's loud enough for gigging with a drummer for rythym. I recorded this with an iCast mic on my Pixel set to low. It's definitely got more headroom - five used to be OK but not really loud. It reminds me of a 5F1 on it's Incredible Hulk pills.
     
    Last edited: Nov 8, 2017
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  20. CapnCrunch

    CapnCrunch Friend of Leo's

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    Thanks for that!

    I rushed home from work and soldered in the plate resistors and the grid stopper. I used the same values as you did. Put it all together, turned it on to make sure it worked and then had to pack everything up in a rush and went to band practice. The amp sounded great, and mysteriously was quite a bit quieter than it used to be. Not quieter volume wise but electronically dead silent, no hiss, no hum. I hit it with a boost and a Timmy and played my hollow Tele with TV jones filtertrons. It sounded great with the boost and the OD. It handled them much better than it did before. It also sounded stellar mostly clean with some reverb and delay.

    I've only played this amp once before with this band, and that was months ago. The band leader remembered the amp because it is pretty unique looking, but he told me after practice that he didn't remember the amp sounding as great as it did tonight. He described it as big, smooth and singing.

    We did a song in drop D tonight, and it just killed it. Pretty happy with the sound and feel of it. I've got a high dollar hand wired boutique AC 15 variant that is going to be lonely at home for a while, I think. I've got a vintage 50's p15q in it right now, but I think I'm going to spring for a Weber 15A150 O which is basically a smooth cone P15N. It was a cheap amp, but with these tweaks, it is totally worthy of a great speaker.
     
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