Deluxe Reverb Build

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strelok
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Deluxe Reverb Build

Post by strelok »

I built this kit a few weeks ago at the request of a friend of mine. Figured I'd share it here. He got it from marsh amps, and from what I understand they're mojo kits with some directions included. All in all it went quite well aside from a few minor mechanical things. It took a lot longer to complete than I anticipated, but hey it always does.

Here's a link to the pics:
http://imgur.com/a/S2XjY

A few things I encountered:
-The faceplate was a bit too large for the cabinet. Once it was installed there was just no getting the chassis to fit into the slot in the cabinet for the control panel. It was rather unfortunate as once I realized it was stuck, pulling it back out caused the ends of the faceplate to bend and chip off some of the paint. I was rather dissapointed by this as it's a beautiful faceplate, but once I got it out I was able to bend them back a bit and touch it up with a black paint pen. I offered to buy my friend a new one but he wasn't bothered by it so we just left it. In order to get it to fit I used the dremel tool with some sort of milling bit (I think, not sure what its called) with the faceplate installed on the chassis and just run it across the top using the lip of the chassis as a guide. It looks fine now and the lip of the cabinet hides it very well.
-Mounting holes where not drilled in any of the boards, not a big deal, just takes a bit of time to line them up properly with the holes drilled in the chassis. I used a white paint pen and taped the boards in place to mark through the holes where the boards should get drilled.
-Chassis mounting holes in the cabinet where not drilled. Again no big deal, just something you want to be damn sure to catch before you start installing parts in the chassis, as I think the best way to do it is to lay the chassis flat on top of the cabinet (with the handle removed) line everything up, tape it in place then use white paint pen to dot through the holes in the chassis where the cabinet needs to be drilled. This method worked rather well.
-Missing parts. With a kit that has as many parts as this one did it doesn't surprise me that a few would get forgotten. Marsh amps was very quick to respond and send the parts directly out to me even though I wasn't the one who purchased the kit.

The electronics went together very smoothly thanks to the nice big printout of the schematic and layout that was included. That and the divided parts bins that kept all the component values neatly organized instead of sifting through a giant bag of parts. Upon firing it up it worked right from the get go, well not right away, I forgot a power supply wire but that was quickly fixed. Biased it up. Normal channel seemed good, vibrato had a weird oscillation. I started pulling preamp tubes to see if the issue would go away and the reverb driver did in fact do this. The only thing I had done different was use buss wire and teflon to parellel all three elements of the reverb driver instead of having the anode connection loop over the socket. I figured that might cause a problem and it turns out it did. Clipped that piece out, looped a wire between the two anodes and the problem went away.

Also had an issue with the tremelo circuit making the popping noise at high speeds. It always occured at the same volume level and was affected some by the intensity and especially the reverb control. Probing the control side with a pencil didn't yield any results. However when I moved the cathode wire of the oscillator half of the tremelo tube up and over the socket the problem dissappeared. Guess it must have been coupling with the reverb return. Once all that was done the amp worked perfectly. The only other issue I had was some of the knobs had stripped set screw holes, but again Marsh sent replacements out to me very quickly no questions asked.

The only things that weren't included with the kit were the traditional reverb tank strap so I tacked the bag in with some #8 wood screws and finishing washers. They also didn't include the cable clamps that most fenders had but we had some laying around the shop. I also got some #6 finishing washers to go on the screws from the back panels to keep them from digging into the wood. Plus they look nice and are cheap and easy to get. I also ended up using my color coded wire instead of the traditional yellow included with the kit. I like to make everything color coded, makes it easy to keep track of what's going where. Plus it makes the inside of the amp look like a fiesta :P

The amp sounds really good. I've never been much of a Fender guy but this thing started to change my mind about that. Its just the right volume, loud enough to hear over a drummer, but not so loud it will piss off the neighbors in the house next door. Of course an apartment would be another story. We ended up going with the Weber 12F150 50w. Its their recommended for Deluxe style amps. A bit too scooped in the mids for me (I've always been more of a marshall guy myself) but I can definitely see why people like it and of course the cleans are sparkly and fantastic. He got many compliments in the show he used it on even compared to the Deluxe re-issue the other guitarist in the band uses.

I also did a couple mods:
-Switched intensity pot. Really nice if you don't use the tremelo much or you want the gain of the normal channel plus reverb. This breaks the connection to ground for the opto output and gives you back the gain that part of the circuit sucks out, even when its disabled. Put a 1M resistor across the switch terminals so it doesn't make a horrible pop everytime the switch is engaged.
-1R bias resistors. The fairly common treatment. They're the huge 10w dales above the powertube sockts. Totally overkill but they were the only 1R's we had and we have a shitload of them lying around here. I hand picked a pair from the pile of old pulls using a high precision ohmmeter to get the closest readings.

Edit: Plate Voltages where also awfully high. Around 450-460 for the 6v6's. However the JJ's should be able to stand up to it from what I understand. Got them biased at just a little under 70%. All the preamp voltages checked out normal.
joeboo88
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Re: Deluxe Reverb Build

Post by joeboo88 »

I thought I was looking at a store bought Deluxe....nice job
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martin manning
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Re: Deluxe Reverb Build

Post by martin manning »

Very nice amp and thanks for posting this detailed report. I believe the faceplate is a Marsh product, and for the $40 up-charge it ought to fit correctly. Mojo seems to have various fit issues with their kits too. The power transformer's HT voltage should be reduced by about 10%, unless your line voltage is unusually high.

I'm not a fan of monochrome wire either, and I detest fake cotton-covered push-back wire.

I see Marsh has an upgrade option for Malory 150's, which is curious considering they aren't any more expensive than the CDE 715-series coupling caps.

You mentioned that the preamp voltages are normal, are the dropping resistors the same values as in the AB763 circuit?
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dkochan
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Re: Deluxe Reverb Build

Post by dkochan »

Really nice work! I'll be building one, one of these days, and this is going on the reference list!

Dave
strelok
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Re: Deluxe Reverb Build

Post by strelok »

Thanks for the compliments!

Yeah the faceplate was definitely the most frustrating part. However after working in small batch electronics manufacturing for several years I've kinda just come to expect that there's going to have to be some finagling with the metalwork due to things being just a little off. But you're right it should just fit, especially for $40. But my friend was happy with it, so I'm happy. Though I should let marsh know about the issue so that other customers don't have the same problem.

As far as the B+ goes, when I say the preamp voltages where normal, what I really meant was that they weren't a mile off. I don't remember the exact values but they where in the 170-180 range if I remember right. Which is what the schematic says they should be. The dropping resistors where the stock 10k's. I'll have to double check the mains voltage when I get back on monday and see if the transformer diagram shows maybe a 115 or 125 volt primary tap. I'm pretty sure it was just 120, 220, and 240 though.

The nice thing about the tube rectifier though is that if it turns out the 6v6s are getting killed by the high voltage I can always sub in a 5u4gb and that should bring them into more sane levels as it has greater voltage drop. Unless you where thinking we got the wrong PT?

I'm with you on that pushback wire as well. I hate working with that stuff. The stuff I use is easy to strip and work with. You just have to be quick on the joints or the wire will melt through the insulation haha. Also something I've gotten used to after working with OFC for a while.
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martin manning
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Re: Deluxe Reverb Build

Post by martin manning »

Nice detailed pictures too, BTW. On the voltages, Fender's AB763 '64 layout shows 420 B+, and the '65 DR reissue shows 396. I don't know if maybe the PT is vintage spec (so around 10% too high for modern voltages), but I don't see the appeal. If that's the case, it should have two sets of primary leads. A 5U4 is a good idea if you have 3A for the filament.
strelok
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Re: Deluxe Reverb Build

Post by strelok »

Thanks! Yeah I'll have to double check the diagrams when I get back on Monday. I just can't remember the transformer configuration. Though I'm 99% certain I selected the 120v primary tap. Anyways, I will keep you guys posted when I find out.
strelok
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Re: Deluxe Reverb Build

Post by strelok »

Found the specs for the PT today, here's a link:
http://www.mojotone.com/transformers/Fe ... O761EX.pdf

For whatever reason the B+ winding is much higher compared to a couple other PT's for the Deluxe Reverb that I've found:

http://www.classictone.net/40-18016.pdf
https://www.tubesandmore.com/products/P-TF41316

330-0-330 vs. 354-0-354 for the Mojo transformer. I'm not sure why they spec'd it out to such a high voltage. I checked the curves for the 5AR4:

http://www.drtube.com/datasheets/5ar4-ge1959.pdf

They confrim the voltage I was getting at the output of the rectifier. At about 50-60ma (just ballparking the idle current, each 6v6 is biased at about 18ma). The output is at about 450VDC for 350VAC on each plate.

Also remembered that I had plugged the amp into the Variac when I checked the voltages and I had it set right at 120v according to its meter. The pictures I took also indicate the correct primaries are hooked up (white and black). This along with the above specs indicate the line voltage was correct when I had measured the B+.

Again I'm not sure why they spec'd this transformer for such a high voltage. The JJ's should be good though, unless you guys have experience that says otherwise, please let me know. I have a spare Deluxe reverb transformer I got from Hoffman amps a while back for a project that never came about that should have the correct voltages I could trade out of the amp, assuming it would fit, that or just use the 5u4. I'd have to call mojo and make sure the 5v winding could handle it though as they don't list the current ratings on the datasheet... :roll:
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martin manning
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Re: Deluxe Reverb Build

Post by martin manning »

Before doing anything rash I would pull the rectifier and measure the no-load voltage on the PT secondary. Something around 350 unloaded would be about right for 330 loaded, and that may be what is specified on the Mojo diagram. The other windings are specified at their nominal 6.3 and 5.0, and that would surely mean they are loaded voltages. I'm not at all impressed with Mojo's documentation.
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martin manning
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Re: Deluxe Reverb Build

Post by martin manning »

Something you could try is connecting your 120V mains to the 100V and the 230V primary leads. That would reduce all of the voltages by 120/130 or about 8%. I'm betting the filament voltages will still be within tolerance and your B+ will come down accordingly.
strelok
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Re: Deluxe Reverb Build

Post by strelok »

Don't worry I'm not about to start pulling transformers haha. Sorry if that's how it came across, I was just thinking of options.

I thought of trying exactly what you suggested with the 100v and 230v primary, but I figured that it would drop the filament voltage too much. Though I believe what you said that they would be okay. I'll have to get the amp back from my friend run some tests, assuming I can pry it away from him, he really seems to like it 8)
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jjman
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Re: Deluxe Reverb Build

Post by jjman »

At 18ma per OP tube that's probably only about 40-45ma total draw. More draw brings down the voltage somewhat. I run my '71 DR op tubes around 25ma on the plates and the plates are at 440v. (11 watts) If you increase the idle the voltage will come down somewhat. Of course you want to stay within acceptable idle wattage.

So although your voltages are high on paper, I don't think they're high compared to an actual vintage amp in today's world.
If it says "Vintage" on it, -it isn't.
Firestorm
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Re: Deluxe Reverb Build

Post by Firestorm »

My 1966 DR runs 465V B+ with the GZ34. At first I tried the zener diode thing, then I just went with high voltage modern tubes. 6V6s are weird. They can take very high plate volts as long as the duty cycle is short. And in MI amps, even when run past the rated dissipation, I don't think I've ever seen one redplate (provided you can see the plates), but the screens light up like Christmas.
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martin manning
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Re: Deluxe Reverb Build

Post by martin manning »

An actual vintage amp running at today's line voltages will see higher plate voltages as noted above. In the case of the DR, a 6k6 primary impedance in combination with voltages in the mid-400's puts the 6V6's in a high plate dissipation zone. An 8k primary would be a better match for the higher voltages. It is true that higher idle current brings the plate voltage down, but at the expense of higher idle plate dissipation. Depending on the power supply regulation there is the possibility of going too far with that strategy.
Firestorm
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Re: Deluxe Reverb Build

Post by Firestorm »

Plate voltage doesn't make much difference; screen voltage matters. The plate voltage will swing from B+ down to 50-75 volts with signal. In a triode, the inverse relationship between plate current and plate voltage makes plate dissipation self-limiting, more so with cathode bias. But in tetrodes and pentodes, the screen voltage is semi-fixed, so it becomes the "anode" during positive current swings, making plate current independent of plate voltage.

The published plate voltage limits for tubes are mostly about flashover, an arc between tube elements. The plate is so far away from everything else that plate shorts occur at the bottom of the tube, where the pins are connected. Screens though, which track the B+2, are very close to the other elements, so if limiting voltage is exceeded, they can arc in any number of ways. Most catastrophic output tube failures are screen shorts.

So the fix is (or might be) limit the screen voltage. Big screen resistors will do this, but at the expense of significantly altering tube behavior and sound. Better is to rig a voltage divider with high-watt resistors feeding the screen supply, post-choke. Andy LeBlanc has posted a lot on this.
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