Modding gain stage on amp

[splintertheory]

If you change the resistor value after a gain stage in an amp, would you have to bias the tubes again?

[gwell1]

re-bias the pre amp tubes or output tubes?
On a pre-amp stage, it depends what resistor and how far you deviate from the original resistor. I wouldn't mess with the resistors right away. If you want more pre amp gain, maybe adding some bypass caps in parallel with the cathode resistor on a 12ax7 would give you the gain you need. If the circuit already has a bypass cap, say a 25uf, maybe bumping it up to a 33uf. Also, if the grid of a 12AX7 has a bleeder resistor (resistor from grid to ground), you could alter that for gain. The higher the resistor, the less the signal will be shorted to ground thus giving more of the original signal into the 12AX7. The smaller the resistor, the more you shunt the signal to ground.
The pre-amp section is can be tricky if you haven't had much experience. Be careful.
What exactly are you trying to acheive and what type of amp?

[splintertheory]

Its a Bugera 333XL. I'm trying to get a little more gain out of the lead channel. I saw on here that someone had achieved this through a few resistor and cap changes but he didn't go into detail

[gwell1]

try swapping C69 (0.47uf) to a 25uf. If still not enough maybe increasing R94 from a 100k to a 120k

[J M Fahey]

Mirror mirror on the wall
have we become "My Les Paul"?

[splintertheory]

If I swap the cap or resistor, would I have to bias the preamp tubes again?

[Chuck H]

If I swap the cap or resistor, would I have to bias the preamp tubes again?

The short answer is no.

The longer answer is that you should not mod that amp unless you:

1) Understand circuits and safety.

2) Follow exact instruction for a mod known to give the results you want AND research safety procedures for working on tube amps.

Since you don't understand circuits you should NOT just start changing parts to see what happens. Well, you CAN do that, but that amp you have isn't a good choice for an experiment platform. The build style and board aren't up to the abuse and it would be very easy for a novice to damage it. That's one reason some of the old hand wired amps and kits are popular. They're almost indestructible WRT poking around.

I say don't "experiment". If you have a known course of action (a known mod) and are patient and careful then go for it.

Do a *oogle search for 'bugera 333xl mods' and look for something that offers what you want. It should also be a mod that has actually been done successfully. More than once if possible. Use your best judgment to pick through the crap and find a mod that is clearly designed by someone that knows circuits. There are dozens of mods for that amp.

[olddawg]

The short answer is no.

The longer answer is that you should not mod that amp unless you:

1) Understand circuits and safety.

2) Follow exact instruction for a mod known to give the results you want AND research safety procedures for working on tube amps.

Since you don't understand circuits you should NOT just start changing parts to see what happens. Well, you CAN do that, but that amp you have isn't a good choice for an experiment platform. The build style and board aren't up to the abuse and it would be very easy for a novice to damage it. That's one reason some of the old hand wired amps and kits are popular. They're almost indestructible WRT poking around.

I say don't "experiment". If you have a known course of action (a known mod) and are patient and careful then go for it.

Do a *oogle search for 'bugera 333xl mods' and look for something that offers what you want. It should also be a mod that has actually been done successfully. More than once if possible. Use your best judgment to pick through the crap and find a mod that is clearly designed by someone that knows circuits. There are dozens of mods for that amp.

Or just use a boost pedal.....

[Jazz P Bass]

Yeah.
Not the amp to be nutsin' around with.

[gwell1]

The short answer is no.

The longer answer is that you should not mod that amp unless you:

1) Understand circuits and safety.

2) Follow exact instruction for a mod known to give the results you want AND research safety procedures for working on tube amps.

Since you don't understand circuits you should NOT just start changing parts to see what happens. Well, you CAN do that, but that amp you have isn't a good choice for an experiment platform. The build style and board aren't up to the abuse and it would be very easy for a novice to damage it. That's one reason some of the old hand wired amps and kits are popular. They're almost indestructible WRT poking around.

I say don't "experiment". If you have a known course of action (a known mod) and are patient and careful then go for it.

Do a *oogle search for 'bugera 333xl mods' and look for something that offers what you want. It should also be a mod that has actually been done successfully. More than once if possible. Use your best judgment to pick through the crap and find a mod that is clearly designed by someone that knows circuits. There are dozens of mods for that amp.

I have to agree with this as well. Don't screw up your PCB! I've been soldering for years and actually received training courses in soldering when I worked at Motorola. I just worked on a Vox ac15c1 and I have to say the PCB's on that thing are made cheap! Those soldering pads are only made to be soldered once.

[TimmyP1955]

Increasing the bypass cap will give more low end, but much of an increase in cap value will just cause a lot of power to be wasted on low end. This is why cranked Marshalls have a nice grind, while cranked Fenders sound kinda sick. When Marshall 'copied' the Tweed Bassman, they fixed that issue.

[Danglin' Fury]

I would agree with you guys but I have a 333xl and I've soldered on those pads more than 7 times... You just have to use plenty of liquid flux and be patient... And now my 333xl has enough useable gain that it sounds like a cross between a Mesa Mark and an Engl.... Seriously... Not kidding... It has slightly less distortion than a Peavey triple xxx but way more than a 333xl.... The low end growl and note separation is phenomenal and the initial pick attack has more crunch than a bag of corn chips! Lol! Palm mutes are focused and hit you in the gut... Of course, I've been soldering for 12 years so....

[splintertheory]

Lol that's what I'm going for. And for soldering and safety, I have plenty of years of experience soldering

[splintertheory]

So danglin fury, what all did you change to achieve it? If you don't mind me asking

[Danglin' Fury]

I'm not gonna tell ya... But... What I am gonna do is point you in the right direction.... Reason is, I've done waaaaaay too much investigation logging countless hours in trying to find what to do... In said inquiry I have gleaned some information that I would say I could not place a value on... I'm not being a dick, I genuinely want to help you, so... That being said go to the great Google and type in Regis Coyne... Go to his tech page.... You will see what I'm talking about... That should give you enough info to get where you are going... And dude... Listen to these guys on this forum... It may seem like they are arrogant but they have logged countless hours as well.. More than me....and they for sure know what they are talking about... Also... Resistor 112 in your 333xl is not the same value the schematic says it is.... That will help you as well... It says it is 470 K but its really 330K... That will make a difference.... That is 1 part of what I did to achieve the tone... Good luck... Read what I asked you to read.... When you understand it, come back and lets talk shop... And I MIGHT give you the values I used.... Thanks and good luck and God Speed...