Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on August 04, 2019, 01:24:52 pm
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I have a discontinued Ampeg J-20 amp made around 2005-ish. (it's been discontinued, but for $300 new and an all tube, turret bd, made in Vietnam a deal) it's base on a Fender 6G3 but shares the tone and volume pots for bright and normal. The Ampeg isn't bad sounding but turning vol up a little pass the start of distortion around 5, the amp gets raspy and maybe a little too bassy. I'm sure changing the pre-amp cathode caps and R's would help with this issue. I was thinking of changing the Ampeg J-20 to a true 6G3 build, the face frame is basically the same as a 6G3 except one vol and tone, has bright and normal inputs. I could easily take out the bright vol and tone in the 6G3 and share.
My question is I never heard a 6G3 and was hoping I could get some feedback on how they sound. I attached the two schematics, there are a few differences but the tremolo circuit is exactly the same. The pre-amp plate R's in the J-20 are 100K, 220K in the 6G3 and a few caps here and there added. Would be easy to do this change, but any feedback on the difference in sound would be appreciated between the two circuits.
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FWIW my Son is the musical one, and he really liked his, was his "just throw n go" weekend gig'n amp, his twin ( ~'65) was for the bigger shows.
as a tech It didn't matter which of the 2 I had to work on they were about the same :icon_biggrin:
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In the j-20 schematic there is a 10k from the single vol pot going to 2nd stage, the 6G3 has two mixing R’s, (220k’s) from the two volumes going to second stage. To keep close to the 6G3 would l not use any resistor going to 2nd stage, like the 6G2, as l’ll have one vol? Or?
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Or?
I'd try and fit 2 gain pots like G3, you have more tonal control when jumping channels
if you don't plan on jumpin, embrace the limits :icon_biggrin:
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Trying to keep pots to the Ampeg's face plate, basically same as Fender 6G3 except one channel. The face plate has volume, tone, speed and intensity, not two channels with bright, normal with each having there own vol and tone.
But the Ampeg has two inputs, normal and bright, so I can just set up the the bright with a 500pf across the mixing resistor and run the volume to the 2nd stage. I'm not clear on whether to place a resistor before the 2nd stage or not, the Ampeg has a 10K but the Fender 6G2 (basically same as the 6G3 but with one channel) has no resistor before 2nd stage. I have three 12Ax7s in the Ampeg so I want use all. I'm pretty sure a single channel Fender 6G3, sharing the vol and tone with the bright and normal inputs can be done in this conversion, just need a little advice.
Any input would be greatly appreciated, I planned on using two 470K mixing R's from both 1st stage plates after the coupling caps (.02) to the one volume, as in the J20 schematic above.
Sorry I can't seem to get this amp right side up
Hopefully, I'll get some advice,
Sincerely,
al
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I've been looking for a reason to do one of these.
tubeswell did us a favor with this one.
https://el34world.com/Forum/index.php?topic=21535.msg228568#msg228568
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Do you think C10 (where a presence pot would be) is contributing to harshness?
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Do you think C10 (where a presence pot would be) is contributing to harshness?
I took that .1 cap out good while ago, not really that harsh but breaks up a little too bassy, I did mess with the circuit, lowered pre-amp cathode caps , etc but I hear the 6G3 is a nice Brown Deluxe with Marshall overdrive and ZZ top use to use these amps. So I wanted to keep with the original face plate and copy exactly or close as possible to the 6G3 with a shared tone stack.
Just need a little direction, on changing the dual channel tone stack to single tone stack, then I'm good from there.
al
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Would you consider getting rid of the bright input and just using that spot on the faceplate for a bright switch?
The only difference between the bright and normal channel on the 6G3 is one cap (.02 vs .01).
You could make the bright switch switch between those 2 cap values.
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Would you consider getting rid of the bright input and just using that spot on the faceplate for a bright switch?
The only difference between the bright and normal channel on the 6G3 is one cap (.02 vs .01).
You could make the bright switch switch between those 2 cap values.
If you look at the front of the Ampeg , I did have a toggle switch in the bright side, but it switch the 470pf that was across the bright input's mixing R. Now that I think about it. the switch was on/on/on, up for bright, down for normal, middle for both. Good idea, as I could switch that .02 cap instead, just like the 6G3, thanks.
Also, that EC Tremolux looks interesting, thanks for that too.
As far as the resistor to the 2nd stage, I'd just leave it out, like the 6G2, can always add some resistance if needed.
al
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The schematic posted, EC Tremolux, only half of the the 12Ay7 is used, I am reading this correctly...?
There is no intensity pot either, and if the switch on the speed pot is turned off the trem is off? also can be turn off by the foot switch, do I have this right? so what is the purpose of the switch on the speed pot?
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The schematic posted, EC Tremolux, only half of the the 12Ay7 is used, I am reading this correctly...?
Yes
There is no intensity pot either, and if the switch on the speed pot is turned off the trem is off? also can be turn off by the foot switch, do I have this right? so what is the purpose of the switch on the speed pot?
Yes, if the switch on the speed pot is off then the trem is off...the footswitch will only work if that switch is closed.
It's just there as a manual way to shut off the trem if you don't have a footswitch plugged in.
If you'd like to add a depth/intensity control take a look at the 5E9A for clues
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Changing the Ampeg J-20 to Brown Deluxe 6G3, the amp was modeled after the 6G3, close but not the same. Pick it up at the Philly Guitar Show new for $300 about ten years ago, iron is huge, has a nice 12" alnico spk too. There often on ebay cheap.I did make some changes years ago but not true 6G3 specs. The carbon comps have drifted up to 20%, should I replace them with 5% MF or film? The big cap on the stdby I added as it popped, the cap stopped that but maybe it's just a band-aid? leave it or could it be something that doesn't need that cap?
This amp originally had the stdby on the CT, I put it after the rectifier, solved the static noises going to stdby but then the pop..?
Thanks in advance about the CC.
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should I
You can answer ALL your questions by 1st asking yourself this one;
"what am I doing this for"?
a keeper in your collection
something to get working n flip
I'm bored
.......
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Probably keep it a while, but I couldn't sell an amp unless it was good to go. I'll change out anything over 10%, 5% in critical areas.
Thanks
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Take a look at this and consider moving your standby switch so that the rectifier is connected to the reservoir cap with the switch open:
http://www.valvewizard.co.uk/standby.html
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Interesting article from Valve Wizard, in the post about 4 up, I mentioned I put an extra reserve cap on the standby switch to stop the speaker pop, it worked as the Wizard mentions. I just checked to make sure the stdby cap was on the lug before the current hit the B+, it was so when the amp is switched (on), current flows to rectifier tube, heater filaments and that 47/500v cap. Not to be too thick, I think I've had this cap as the Wizard suggests...? Not sure what interface resistance is but can't hurt to add that 47R either to let a little current to the tubes.
Can't remember why I added this cap in the first place, I think David Allen of Allen amps had that reserve cap on an amp I got from him. I must have communicated with him, I bought his first Old Flame kit, way back.
Thanks for the tip. I assume the pre-amp plate R's in the 6G3 (220K) over the usual 100K would contribute to more gain, but I want to stay close as possible to the original Brown Deluxe as I like the Marshall like crunch turned up, ZZ Top like.
I have to make a choice to use that bright input to switch the .01 and .02 caps off the tone pot as you suggested or leave the on/on/on toggle there, it gives me the options of normal, bright or both but to get the bright I'd have to use a 500pf across the mixing R's on the bright channel side rather than that .02 cap off the tone...? (can't figure how to have that .02 cap switch on the bright channel without losing the on/on/on as I mentioned). Wonder if there is much difference anyway. Kind of cool having the choice to use both channels pre-amps together..?
Thanks again for all your input,
al
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What effect would changing the PI, (6G3) from 6K8, 820ohm to 10K, 470ohm on tone?
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Build it stock. Play it for a while. Then make the change so you can hear it with your own ears.
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Yeah, sounds like the right way to go, the Ampeg is so close to a true 6G3 wouldn't really take more than an hour to change completely to 6G3 specs. Even the voltages are almost dead on to a 6G3 with a 5V4. The Ampeg's first two filters are 47uf, way too much, I feel I definitely need to change them to at least 22uf. I've been reading old posts over at gear page, (seven pgs of changes to the J20) that's influencing me to not go completely 6G3. But tone is in the eyes of the beholder. I got out of the switch this, switch that thing a while ago, now I need to use my own ears. BTW, I'm still building those 6V6 plexi's. Thanks, for the advice.
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Changed the Ampeg to a true Deluxe 6G3, except Ampeg had only one tone stack for both channels, used two 470K ohm dividers from coupling caps, with a 500pf parallel on the bright channel's 470K. Changing the cap to ground off the "one tone pot" like the 6G3 was not possible, as I used an on/on/on mini toggle in the bright input. That toggle let me choose bright, normal or both channels. Other than these changes , same values as 6G3. Took Sluckey's advice, played amp for a few days, different speakers, etc came to conclusion that the normal channel was too dark for me, bright channel was very bright, too bright.
Made a few small changes in the dropping R's to get the 6G3's voltages, put a 47uf filter before the standby, per Silvergun, no pop any more, saves my rectifier tube, changed the 500pf cap to 250pf on the 470K divider to bright channel, gave the 1st stage their own bias cathode caps, 10uf, 1K5 on the normal and 4.7uf, 1K5 on bright. Just for more flexibility, add a presence control and switch to turn off NB, on the back of the amp.
I'm very happy with both channels now, bright channel is not overly bright and normal is not too dark. With both channels together, the best of both worlds, gives a louder boost. Lowered the FB resistor to 27K for a better range on the presence control which was a savor, enables the normal channel's treble a much better range.
Ended up with a Red Fang and all used old stock USA tubes, added a bias pot too.
This amp turned out to be one of the best conversions, I can get 5E3 tones to clean Princeton and dissipation is at 20 watts.
Thanks for all the advice here, the pic's show a little sloppy soldering, but this amp has been modded several times, now it will stay as is.