Adding extra gain?

[Kramer]

So far, I'm enjoying my 50 watt plexi, but I'm most interested in tweaking the circuit to get more gain out of it. So what have some of you all tried and found sounds good? What are some key values I should look for changing, and how can I manipulate them to add extra gain? Would cascading the preamp like a 2204 be worth it? If I am looking into modding for extra gain, should I change the preamp filter cap to a 50x50uF like in metal panel plexis to keep the low end firmer? I got 6 NOS LCR 50x50’s I can use, so I’m mostly just wondering if it would pay off to use one in the preamp versus the 68' spec 32x32uF and what I could expect if I did put the 50x50 in.

I don’t want to change the amp to where it’s some over the top high gain at bedroom volume beast, but when I crank it up loud I want to dish out plenty of heavy tube saturation.

Thanks,

--Alex

[neikeel]

First thng would be to try cranking the amp up to 7 or 8 out of 10 on the dial, plenty of gain there for lots of things pretty loud at this setting so a Larmar PPIMV would help.

Do you have a 0.68uF on V2a? Will give mids gain boost.

Next you could swap out your 47k NFB for 100k hooked up to 4ohm tap, makes the amp hairier.

If that does not do it then you will need to cascade V1 triodes, either simply with the 1 wire mod:

http://forum.metroamp.com/viewtopic.php ... it=cascade

or the RR mod:

http://forum.metroamp.com/viewtopic.php ... it=cascade

but I prefer this one (yes I know I am going on about it but it is brilliant and can see no drawbacks):

http://home.comcast.net/%7Ejbjdav26/2in1version2.GIF

http://home.comcast.net/%7Ejbjdav26/195 ... 20grnd.jpg

My version uses a push-pull 1meg pot on normal 1 input as a gain knob with PPIMV in presence pot position, pre-mv in normal pot position (presence goes on the back or with fixed setting). The low normal input is the lo 2204 input (active when pot out).
You get full 2204 with Larmar dimed and push-pull pot out or full 1987 with predimed and pushpull pot in.

[Kramer]

Thanks Neil, lots of good info in those posts.

[rjgtr]

Another idea is to do the "one wire" mod, although I would do it on a switch. The one wire mode cascades the channels for more gain. You essentially drive the normal channel with the bright channel, which is similar to the two input Master Volumes of the 70s but with it on a switch you can still get the original tones as well.

When I converted by 1987x to Plexi spec with a new board and OT, I added both the one wire mod on a switch and rewired in the inputs so one of the Lo jacks is now a Y jack.

[Kramer]

The NFB resistor is the 47k connected to the secondary/output of the impedance selector? If that's so, should I just change the resistor to 100k, or should I also disconnect it from the output and connect it to the 4 ohm terminal of the switch? What can I expect from changing values and possibly terminals on the impedance selector?

I've also read in another post that changing the mix resistors can have a different effect on your amp, how/why I'm not sure.

Also, I was wondering about this for my next build, what kind of effect will having a power transformer with a lower B+ yield? I'm guessing it will give a "variac" like feel, but I was wondering about the specifics. George does not produce a 50 watt PT with a low voltage B+ option, however, his MM upgrade, the P4550JT-G2, comes in several variants ranging from a 250V to 515V B+ winding. The P4550JT-G2 has a 420V B+.

[neikeel]

The NFB resistor is the 47k connected to the secondary/output of the impedance selector? If that's so, should I just change the resistor to 100k, or should I also disconnect it from the output and connect it to the 4 ohm terminal of the switch? What can I expect from changing values and possibly terminals on the impedance selector?

I've also read in another post that changing the mix resistors can have a different effect on your amp, how/why I'm not sure.

Also, I was wondering about this for my next build, what kind of effect will having a power transformer with a lower B+ yield? I'm guessing it will give a "variac" like feel, but I was wondering about the specifics. George does not produce a 50 watt PT with a low voltage B+ option, however, his MM upgrade, the P4550JT-G2, comes in several variants ranging from a 250V to 515V B+ winding. The P4550JT-G2 has a 420V B+.

Wouldn't bother just cascade the front end if you are starting out. I have 50watters with 390v and 450v and makes little difference to the gain but I find higher B+ gives greater punch and clarity

[wavey63]

Totally new-b question but the cascade mod is NOT the same as jumpering the channels with a cord, correct? I am looking for the Blackmore-esque tone of the early Purple era and almost have it with the amp at bloodletting volume, although a bit bright. Any recommended mods for that type of tone?

[neikeel]

No jumpering the inputs or y cord just gives you both V1 triodes in parallel. It gives a little more gain and you can shape the tone with how much of each channel you add in. Output of this goes into V2a (the second gain stage).

Cascading is when you have V1a triode having an input. You then take that input via a pot (gain pot) and add it in series to the other half of V1 (V1b). This signal then goes into the first half of V2 which in this set up is the third gain stage. The signal then goes into the second half of V2 as a cathode follower, controlled by the tone stack and its controls. To help regulate the volume the output from the tone stack is fed into a pot as a pre-phase inverter master volume.

If you have an amp you like the tone of when you max it but don't like the volume I suggest you go for the Lar Mar PPIMV (lots of threads )

[wavey63]

Kinda lost me there but no worries. I have the Lar Mar kit but was waiting to get the tone dialed in first. Need to maybe convert it from lead to bass specs since it is a bit too bright for me.

[toner]

The NFB resistor is the 47k connected to the secondary/output of the impedance selector? If that's so, should I just change the resistor to 100k, or should I also disconnect it from the output and connect it to the 4 ohm terminal of the switch? What can I expect from changing values and possibly terminals on the impedance selector?

Yes, the NFB resistor is between the presence pot and the impedance switch. Decreasing the NFB adds some gain and makes the amp sound/feel more "raw" like '70's amps.

larger NFB resistor = less NFB for more gain and "raw" feel
lower impedance connection = same as above

Increasing the NFB resistor value is similar to connecting the wire to a lower impedance. Doubling the resistor has roughly the same effect as moving the connection down one step on the imp switch. i.e. - changing 47k to 100k on the 8 ohm tap is equivalent to leaving the 47k but connecting it to the 4 ohm tap.

There are lots of little things you can do to add preamp gain such as the V2 bypass cap Neil mentioned. You can play with the V1 cathode and plate resistor values also. None of these will sound exactly the same as a cascaded preamp (2204) but it gives you some options. Search for "V1 gain" or similar and you'll find a lot of info.

[rjgtr]

The good news is that you can do any or all of these mods and easily turn the amp back stock if you don't like them. If you're looking for a lot more gain, the cascade will give it to you. If you're looking for a small amount more gain, then try y-ing the channels and using a 25uf/25V bybass cap on the cathode resistor of V2a.

As far as Blackmore goes, it sounds to me like he's either using the cascade (on-wire) mod or he's pushing the front end of the amp with a boost pedal.

[Kramer]

Has anybody used a 4th tube and found success? I know a lot of gain can be added with just these mods here, but I've seen a few plexi's with a 4th tube that absolutely screamed. I'm not looking for anything that will be tone degrading, which can sometimes be hard with a extra gain stage mods, but just something that will add that extra gain while attempting to keep that plexi vibe in tact. It would also be nice to have it clean up with just the turn of guitar volume and be able to have classic plexi tones before going into heavier territory.

I'm just wondering. I have not even began modding yet, and at any rate, adding an extra tube is the last thing to try on my list. What turned me onto this was listening to Curt Mitchell's Tone Junkie after seeing this post

[rjgtr]

I've seen amps modded with a 4th tube and even done it myself. At that point you're in Soldano/Mesa/5150 territory. The sound and feel of the amp will change.

If you're wanting more gain with a plexi vibe the one-wire cascade mod on a switch will give you what you want. The other option is to simply use a boost pedal in front of the amp.

[JTyson]

Has anybody used a 4th tube and found success? I know a lot of gain can be added with just these mods here, but I've seen a few plexi's with a 4th tube that absolutely screamed. I'm not looking for anything that will be tone degrading, which can sometimes be hard with a extra gain stage mods, but just something that will add that extra gain while attempting to keep that plexi vibe in tact. It would also be nice to have it clean up with just the turn of guitar volume and be able to have classic plexi tones before going into heavier territory.

I'm just wondering. I have not even began modding yet, and at any rate, adding an extra tube is the last thing to try on my list. What turned me onto this was listening to Curt Mitchell's Tone Junkie after seeing this post

I have a Metro that Dave Friedman did a "Brown Eye" mod to one input (3 tubes), and a "Hairy Brown Eye" mod on the other input, which uses a 4th tube, it has a lot more gain for sure, but is slightly more compressed than the 3 tube channel. It stays tight when pushed hard, and the added compression is minimal, but it sounds great. Curt Mitchells Langner has a 4th tube, but retains the raw growl Marshalls are known for. Todd was amazing at what he did

[uiovbged332]

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