The Amp Garage

xtian wrote:Question: can I pull one set of power tubes and run as 50w? Need to rebias?

Yes, but remember to set the amp one impedance below the cab (set amp to 8 ohm when using a 16 ohm cab for example). No need to re-bias.

It probably won't sound that good; tends to lose bass and treble.

What is the resistor and cap pair there that has been added on? It looks like the stock attenuator/bypass cap after the first gain stage. Normally it's mounted directly between the low input and the preamp gain pot but it looks like someone has messed with the input jacks and wiring to/from V1. A l...

One popular PPIMV is the "Lar/Mar" (based on Fischer's), which I have in a couple of Marshalls: http://metroamp.com/wiki/index.php/Lar/Mar_PPI-MV It's similar to how you wired yours but usually has shielded cables and the resistors Colossal mentioned (1.5M to 2.2M is fine). If you don't use shielded...

Hi. I'm very familiar with Marshalls and would be happy to help. Depending on what you consider "high gain", you may not get there without cascading V1 similar to a 2204. However, there are several easy mods you can do to increase gain in a stock 1987. I recommend a PPIMV for a master volume, which ...

Not exactly "rudimentary" but this explains it very well:
http://www.freewebs.com/valvewizard1/Co ... _Stage.pdf

Start on page 24.

...

Merlinb wrote:Highly likely!

Thanks for your help Merlin. I have your preamp book and I'm still learning.

I added a 100k to one side of the DPST AC standby on my 100w Marshalls and it seems to work okay. It allows the filter caps to partially charge up to around 60 VDC while in standby and sends some voltage to all the plates. It's completely muted when in standby. Maybe it will save some wear and tear.?

Thanks for the info everyone. I've decided to avoid the standby in tube rectified amps.

If I still wanted to use standby in a solid state rectified amp (100w Marshall for example), simply adding a 100k 2w resistor across one side of the AC standby switch (DPST) should work, right?

Thanks again.

I've read a lot about this today, including searching this forum. I've come to these general conclusions: It's better to NOT use standby for warm up in a tube rectified amp. Warming up in standby doesn't really extend tube life (regardless of rectifier type). Anyone disagree with these? I dare ya! :...

I've read Merlin's (Valve Wizard) page about standby switches and would like some input from experienced builders/techs, please. http://www.freewebs.com/valvewizard1/standby.html I have a Blockhead 18w, which is a Marshall 18w clone. The standby switch is positioned: EZ81 cathode > standby > first f...

On the topic of 18watt power sections, I noticed a on a couple schematics the coupling caps to the power tubes are .01uf. I brought it up to .047 to match with a rocket (which has twice the power tube compliment). I got high static distortion when I hit anything hard, so I reduced the caps to .022 ...

Yes, it makes a difference. If you're using the Mojo supplied OT (part #7021300) there's a good chance that the colors of the primary (blue and brown) are backwards. Heyboer makes those and has been known to mess up the colors. I helped someone with a Mojo JTM 45 kit recently and his needed to be op...

Nice job. One minor suggestion: Lift the green grid wires up away from the chassis and heater wires to help reduce hum.

Here's a quick drawing that shows how to connect the missing cap. You should be able to fit one under the board and solder the positive lead (red in the image) to the bottom of the turret so it won't be visible. It will be an electrolytic cap that looks similar to the two black caps on the left end ...