New tubes -- is it safe?

[bluefoxicy]

I just got my gold pin JJ 12AX7 and my matched and balanced EL84. I still haven't rebiased the amp, or added a 1K 1W screen resistor, or moved R1 and made it 1M 1K.

 

Is it safe for the tubes to replace the crap Sovteks, or should I hold off until making the above 3 modifications? It's a V1. Also it's not using a 5.2k OT; the OT has 7.5k impedance, so it's probably pushing more power towards the tubes (electricity takes the path of least resistance), which is bad (and will require bias adjustment once changed).

[m-theory]

Yes, it's "safe," but what's the point? A V1 with no mods is going to sound pretty mushy, blatty, farty, and inarticulate regardless of the tubes.

 

Btw, the screen grid resistor isn't a make or break mod. It's there to protect the amp in case of a tube shortout. I believe the power rating should be a minimum 3 watts, as well, not 1.

 

As someone who's marshall modded a v2, I can tell you from experience that nothing made a greater improvement than swapping the output transformer. I did the circuit mod changes, including bias, prior to the OT swap, and nothing made nearly as dramatic a change as that iron. Imo, the rest of the changes are pointless without that single critical piece.

[bluefoxicy]

Ah thanks. I'll still hold off for now but at least I know I won't damage the tubes throwing them into an over-gained imbalanced circuit No use starting on them if there's a critical change that's going to massively affect the tone.

 

I've got a lot of mods I'm prepping for; looking at the "basic mod" sheet, with R6 1M and R7 1M going to 100k, and thinking about dropping in 1W pots to replace these. The mod sheet says to raise R7 and lower R6 to increase gain, so I'm thinking 100k resistor into 150k (250k 500k 1M is there a "you should never be here" value?) pot across R7 for a "gain knob" and 50k resistor into a 50k pot across R6 for a "tone knob" (though I'm also adding a 3-band EQ with a bypass switch). Also going to add a 4 amp rectifier to T5 and T6 (filament transformer leads) with a DPDT bypass switch (exposed in the bottom of the head, inside the cabinet) to switch AC/DC modes.

 

As someone who's marshall modded a v2' date=' I can tell you from experience that nothing made a greater improvement than swapping the output transformer.[/quote']

 

Yeah. Ebay has 1 Valve Jr V3 OT and someone seems to have bid $20 or so on it; might as well pay $32 for a Hammond 125DSE or $36 for a Hammond 125ESE. It's $10 to ship either way.

[m-theory]

at least I know I won't damage the tubes throwing them into an over-gained imbalanced circuit

 

I didn't actually say that. The stock set up on the early versions will definitely pound the living crap out of the output tube. Whether you'd damage a new one or not is unknown. It might be fine for a while, it might get cooked right away. No way to tell. "Safe" is a relative term here. It's "safe" in that it won't burst into flames. It's still not necessary good for new tubes.

 

R6 1M and R7 1M going to 100k, and thinking about dropping in 1W pots to replace these.

 

I'd suggest you keep the holes out of the chassis until you're absolutely certain you really want these there.

 

The mod sheet says to raise R7 and lower R6 to increase gain

 

I think you misread. You can jumper R6 and remove R7 entirely, if you want stupid amounts of gain. An increase at either position reduces gain.

 

Also going to add a 4 amp rectifier to T5 and T6 (filament transformer leads) with a DPDT bypass switch (exposed in the bottom of the head, inside the cabinet) to switch AC/DC modes.

 

There's absolutely no reason on earth to do this, unless you have extra money and time, and want the option of introducting hum at the flip of a switch.

 

Ebay has 1 Valve Jr V3 OT and someone seems to have bid $20 or so on it; might as well pay $32 for a Hammond 125DSE or $36 for a Hammond 125ESE. It's $10 to ship either way.

 

Any of them would be a dramatic improvement.

[bluefoxicy]

I'd suggest you keep the holes out of the chassis until you're absolutely certain you really want these there.

 

I think you misread. You can jumper R6 and remove R7 entirely' date=' if you want stupid amounts of gain. An increase at either position reduces gain.

[/quote']

 

The basic mod sheet at http://www.sewatt.com/files/sewatt/Basic.pdf says something cryptic about gain/brightness.

 

There's absolutely no reason on earth to do this' date=' unless you have extra money and time, and want the option of introducting hum at the flip of a switch.[/quote']

 

I read somewhere the tone's a bit brighter with AC, though I can't imagine how. Unfortunately I also can't explain it away.

[TWANG]

you're not going to need any extra brightness over what the mods will produce. this thing with a tele will bite your head off.

not your hat head, either.

I'll second the don't drill holes comment, it's better to set up so you can make a mod and try it out by ear.

I run R6 and 7 at 100K. plenty o'gain.

I have R7 on a pot, I think it's 250K, and I like that as well.

(two heads is funner than one)

R5 on a pot was useless. I had R8 R9 and R6 on switches but I'm redoing the faceplate because I was better off without them.

(holes!)

 

My final choices were tweed tone, gain, vol., loop d jr for at least the power amp break in portion, nfb, C3in/lifted, and a fuse on the back panel instead of inside, and standby.

I wound up doing different components but basically the same panel options.

the two amps dont' sound the same, and boy do they sound good together.

[bluefoxicy]

I run R6 and 7 at 100K. plenty o'gain.

I have R7 on a pot' date=' I think it's 250K, and I like that as well.[/quote']

 

What's the wattage on R6 and R7? Can I just buy a Fender/AllParts/DiMarzio guitar pot for $5 or do I need a 1W pot or what?

 

Also I'm thinking I'm gonna grab a pot for R2, 100k. Not sure what I can use there either.

 

I'm going to be dropping a 3-band EQ in there that'll drop gain by 18dB, so boosting gain a little might actually be helpful to me. Or not. Who knows.

[m-theory]

The basic mod sheet

 

Never seen it, and it runs counter to everything that I've previously seen. Just do the Marshall mods and you're golden. Forget about mystical gibberish.

 

What's the wattage on R6 and R7?

 

Normal pot. Rat Shack will work.

 

Also I'm thinking I'm gonna grab a pot for R2, 100k.

 

Pointless.

 

I'm going to be dropping a 3-band EQ in there

 

Again, do the output transformer and the marshall mods, make sure your speaker is right, play around with it for a while, and THEN decide if you need to go further. You keep trying to shotgun this, and that's really not the way to go about it. There are some really bright minds who've already braved this path. There's no need for you to re-invent their wheels. You'll waste a lot of time and money, and muck up a perfectly fine chassis with extra holes, if you dive in with a hood over your head like this. Trust me, the mods that you want to begin with are the OT and the Marshall mods. If that doesn't suit you, move on from there.

[TWANG]

I'm using 1 watt on R6 7 , 2, 1, etc.

 

flameproof everywhere needed, like R10, and at least 3 watts, sometimes 5.. you can see where in the stock vj, even epi makes some bigger.

 

pot on R2 .. I agree with M.. no reason.

 

you put a TMB in there and you'll suck tone.

He's right you know.. new OT and marshall mods.. no holes.. no big thinky thoughts..

get your voltages right.. then judge the tone and ask about how to go from there if you're not happy.

 

If you have a 90db senstivity speaker, you'll be killing a lot of tone and volume right there so a good 100db speaker will do a lot by itself.

 

If you avoid carbon film resistors and use metalized, you'll be happier, too.