6SJ7 upgrade for old "Champ"?

[Aaron]

Hey yall,

i've got an old amp that's unlabeled. can't tell who manufactured it, but it's an old one from late 40's or early 50's. inside it's basically an old champ following this schematic:

http://www.ampwares.com/ffg/schem/champ_5c1_schem.gif

the amp has a decent sound, but i'd really like to beef it up a bit. Even dimed, it barely breaks up. my guitar is a semihollow guild with humbuckers and breaks up amps pretty easily, so that's not the issue. i'm guessing it's just the preamp tube. does anyone know of a higher gain substitution for the 6SJ7 preamp tube? thanks for helP!

-Aaron

[zaphod_phil]

That 6SJ7 tube's got a gain of 1650 already. I don't know how much higher gain you will get in an octal pentode, and in any case the stage gain is more likely being limited by the value of plate resistor being used. So you will still get around the same stage gain with a higher-gain (and possibly more microphonic) pentode. The tube is also being grid leaked biased, which will tend to make it more noisy and limit the bias adjustment range it has. You need to try doubling the anode load resistor, and then mod-ing the circuit to a more conventional cathode-biased arrangement, so you can adjust the bias point for your plate voltage to sit at around half of preamp B+.

[stevesuk]

Interesting topic. I have built two of these with tweed covering and old 8" speakers. The first one lacks volume, using a costly Torres OT, the second one I thought I would experiment with I calculated the required turns ratio and used a modern RS 240V to 12V transformer as the OT. It screams (not literally) like an overdriven Marshall instant EVH. Unfortunately that's all it can do, it doesn't want to know about clean.
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Anyway, back to the thread, I think it has a lot to do with the OT.
Photo under stevesuk.

[zaphod_phil]

That's very interesting. Somehow it seems the RS OT got the 6V6 operating in a different part of it's characteristics, where it was both louder and more distorted. Still amazes me that the difference was so profound. Another thing is that those grid-leak-baised preamp stages can't handle much input drive, and typically have poor headroom. Changing the preamp's load resistor and rebiasing will definately provide more gain - hence drive into the 6V6. There *may* also be a need to reduce the 22k dropper resistor a little, to get the preamp B+ somewhat higher and correct the operating point, if the very high load resistor brings it too far below half of B+.

[Plexi]

looks like if you were going to take the time to re-design the small circut for a higher signal..also re-design it to use maybe a different octal..like a 6SL7 or something. OTs can make or break an amp..
Richie

[zaphod_phil]

Yeah, the two cascaded triodes will generally provide a higher overall gain than a single pentode, and hence a btter level of drive to the 6V6 grid.

[zaphod_phil]

OTOH some people prefer the bite of a pentode....

[Aaron]

thanks for the input guys.

i think the easiest thing by far for me to do right now is swap the grid resistor for a lower value. i think there's plenty of room here cuz i've used a rat and fuzz face together full on in front of this amp without any blocking distortion (something which has actually happened on my 18watter).

i'll try 50k first. also, i'll scrounge around for a bigger input/coupling cap. i might get more bass out of it (?).

-Aaron

[zaphod_phil]

Well try those mods, but I doubt they'll significantly improve the sound of your amp. The caps you have should be fine for guitar, and going much bigger you run the risk of getting blocking distortion.

BTW, some 18watters are known to suffer from blocking distortion when cranking hard. Paul Ruby has analyzed the problem and come up with a simple fix - see http://www.ax84.com/pdr/18watterbuzz.ht ... Distortion

[zaphod_phil]

he second one I thought I would experiment with I calculated the required turns ratio and used a modern RS 240V to 12V transformer as the OT. It screams (not literally) like an overdriven Marshall instant EVH. Unfortunately that's all it can do, it doesn't want to know about clean.

Have a look at this comment on the subject, after I mentioned your amps on the AX84 board - http://www.ax84.com/bbs/dm.code?id=76375 James Marchant has had quite a lot of experience playing with transformers.

[Aaron]

alright. last night i put in a 100k resistor in series with the 75k grid bringing it down to about 45k. already there was noticably more bite to it and i could get a nice crunchy sound with the volume dimed. i could even get feedback - something i've never been able to do with this guitar. as a test, i ran the rat full on into the amp and got no blocking distortion. so far so good.

i'm not quite sure where to start on modding the bias type of that 6SJ7. The only biasing i'm familiar with is cathode (from the 18W and 5e3 circuits). grid screen leak biasing is something new to me.

is at as simple as:
1. remove 0.05uF from screen grid
2. connect other side to ground
3. disconnect grid input from cathode
4. add appropriate R in parallel to 0.05uF cap, possibly biasing a bit hot since i want to push this thing. (or would it be better to keep the preamp medium and bias the power tube hot? i'll have to see).

not really sure what's going on here. would this basically be making it act as a tetrode seeing as the grid is connected to suppressor grid (and if this is the case, what's the point of using the screen grid at all???)

i hope i aint getting in too deep

-Aaron

[Aaron]

looking things over a bit more and reading up a bit on tube theory, i'm seeing i was wrong. thats how those pentodes are supposed to act. still, im not quite sure how to disconnect what i've got there right now and set it up for cathod-biasing.

[zaphod_phil]

Change the resisitor between grid and ground to the more usual 1M value. Place a suitable bias resistor (say 1.5k) between cathode and ground, bypassed by an electrolytic cap of roughly around 25uF (for a Fendery sound). For more gain increase your plate load resistor to 470k, then maybe reduce the cathode bias resistor value to keep the anode voltage near around half of your preamp B+.

[Aaron]

wow. awesome. thanks zaphod!