Recommend Black Face Deluxe Reverb Kit

[MWJB]

Diablo, next job is to make sure your fibreboards match each other & the chassis holes. You don't want to have loaded up the board to find that nothing lines up.

[TD_Madden]

I agree that it would be nice in a perfect world...
Some people buy the cab to use with a different chassis and the cab is made in a factory. Stocking two sets of cabs, one with holes and one without, would be a pain. Having some guy/girl in the warehouse drilling holes in cabs just before packaging them would cost more and risk quality control.

Chip

I can see that....I suppose only the higher-quality kit-makers (like Mission) take the time to do that....

[Diablo]

Diablo, next job is to make sure your fibreboards match each other & the chassis holes. You don't want to have loaded up the board to find that nothing lines up.

OK, I've got 6 fibreboards, for each pair there is one with eyelets and one with no eyelets and no holes. The instructions don't say what to do with the blank fibreboards. I assume the blanks go under the loaded boards and insulate it from the chassis?

[TheTinMan]

IIRC you wire up the board(s) with the eyelets, including all of the under-board and off-board wires. Then you use the blank board as insulation between the chassis and the eyelet board with components & wires.

Hope this helps,

Chip

[Diablo]

OK, thanks for verifying what the blank boards are for. I drilled the cabinet last night for the chassis straps. I took off the handle first so the chassis would sit flat on the top of the cabinet when drilling. I also figured the job would be easier if I didn't flip the chassis over because the bottom holes in the chassis are smaller diameter than the top ones - you get a more precise drill location into the cabinet using the smaller bottom holes. When I mounted the chassis I found out that the cabinet hole locations aren't super critical because you can push the chassis forward and back, left and right as you tighten the bolts to center the chassis.

[Diablo]

I stuffed and soldered the main circuit board. I want to solder the hook up wires to the board, but the kit only includes 30 ft of yellow (22 gage solid), 10 ft of green (18 gage solid), 3 ft of black (18 gage stranded), and 3 ft of white (18 gage stranded). The layout shows red, white, blue, yellow, black, green, brown. Oh, and Marsh Amps is closed for vacation or I'd be bugging them right now. I don't understand why they don't include the proper colored wires. I checked their website and they do sell all those colored wires, but for very high prices ($50/spool...wow!).

I guess the only thing to do is to order some wire from antique electronic supply, or if anyone has any better ideas let me know.

[TheTinMan]

Diablo - you can order true vintage style cloth covered wire from Hoffman Amps by the foot. Figure out if you need any nuts, bolts, washers, shielded cable, etc. while you're at it. Doug Hoffman is amazingly fast at turning orders around and his shipping costs are very reasonable IMHO.

BTW the cloth covered wire AES sells is "modern wire" with a cloth covering... not my first choice but YMMV.

Chip

[Diablo]

Thanks Chip for the link to Hoffman. I was a little quick on the trigger and ordered the wire from AES.

Reading ahead in the instructions, they mention shielded wire to connect the input jacks and volume control wiper to the preamp tube grids to reduce noise. There is no shielded wire included in the kit although the instructions say there is!

So, can you recommend a shielded wire? Do I connect the shield to chassis ground on one end, both ends or?

[TheTinMan]

Personally, I use shielded wire for the run(s) from the input jacks to the grid of the first triode(s) and from the volume pot back to the grid to the second triode. In your build, there are two sets of inputs and two volume pots. Common theme - grids are sensitive!

You only ground the "quiet" end of the shield - at the input jacks and at the volume pot, respectively, in these cases.

I've been trying to find the web reference that shows how to separate the shield from the inner conductor and how to attach a lead to the shield so that you can ground it. No luck so far... sorry!

Chip

[Diablo]

Thanks again Chip. I found someone who sells RG174 coax cable for this application.

[Diablo]

I finished putting together the amp last night, and had some problems of my own doing. Did about 3 hours of trouble shooting this morning and fixed 2 stupid mistakes and found a bad board connection. Biased the power tubes to 20mA each and they match well. The normal channel is just perfect now, sounds great. The amp is very quiet in regards to hum. The vibrato channel has one issue that I need help on. Vibrato and reverb work fine as does the volume and tone controls. However, the guitar breaks up a lot earlier on the vibrato channel with a kind of buzzing distortion.

Any suggestions for trouble shooting would be greatly appreciated. I have a voltmeter, but no scope.

[TheTinMan]

Troubleshooting is the most difficult part of our hobby/passion IMHO.

What are the plate and cathode voltages on each triode of V2, V3 and V4? If they're not close to spec, check all component values.

My first guess is that there is an issue with one of the Vibrato channel tubes. V1 is a "known good tube", right? Try swapping it for V2 and/or V4. I've had trouble recently on two different Princeton Reverb builds with NOS 12AT7s - neither really likes the JAN Phillips 1980s production 12AT7s I got.

Are you hearing reverb and/or tremolo fine on the Vibrato channel?

Hope this helps,

Chip

[Diablo]

Thanks Chip for the info. After I posted the question, I figured I ought to try tube swapping. I have lots of new old tubes and good used tubes in my stash. I changed out all the preamp tubes on the vibrato channel from JJ to RCA and that didn't seem to make a difference. Then I desoldered and lifted one leg of the brightness cap on the vibrato channel. That fixed it....I think. I was originally hearing a ringing metal tone with an open E string. I don't know how the brightness cap was interacting, but it seemed like the cap went microphonic when the volume was louder than 5. I am hearing reverb and tremelo just fine now, and I was before I clipped the cap out.

I'm going to check all the voltages tomorrow to see if I find anything else peculiar.

[TheTinMan]

Diablo - you might try biasing the power tubes a bit hotter, depending on your plate voltage. If you're dead on the vintage 410, then 25 ma is about right for me. Sounds like you know this already, but just in case:

.025 ma at cathode - .001 ma for screen grid = .024 ma
.024 ma * 410 = 9.84 watts
9.84 watts / 14 watts "Design Maximum" = 70% dissipation

Here's a spec sheet for 6V6-GTA showing the 14 watts. I know that earlier specs show 12 watts for "Design Center Max" plate dissipation. Even so, 24ma plate dissipation is "only" 80% of the 12 watt max.

All I'm suggesting is that you try it - you may like the tone better or you may go back to 20 ma after a couple of minutes.

Chip

[Diablo]

Today I went in and swapped the power tubes. I tried some used vintage tubes, but couldn't put together a pair that matched well, so I put in some NOS RCA 6V6 tubes and biased them to 18.6 mA and 22 mA - that's close enough for me. The plate voltage in this amp is a womping 460V, so I'm a little over 70% dissipation on the hotter tube. I wonder why Mojo uses a power transformer that puts out that kind of voltage? Consequently, every voltage I measured was 10-15% higher than the schematic. I also put in an RCA 5U4GB rectifier to complete the package. The amp was stable and behaving well on the bench for a half hour, so I installed it back in the cabinet for some guitar tests. Both channels sound great and all controls work well.

My only criticism of the Marsh kit is some ambiguous and missing instructions - one example is no mention of installing the ground for the bias supply board! That was one of my stupid mistakes for not checking the layout schematic. They also tell you to ground the wrong (right hand) side of one of the pots - that one I caught. They also don't mention a thing about measuring and adjusting the bias. Fortunately, I already understood how to do that. The best advice I can give is to check the schematic and layout to compare to the instructions on every connection. My last complaint is that they don't include a metal Fender logo....my amp looks incomplete.