DR One-channel - No Reverb

[Lectroid]

This is my DR 1-channel amp.  Everything is now working pretty well in early testing.  Except for the reverb circuit—no reverb at all.  This is my third amp but my first attempt at reverb.  Schematic below, also a pic of the RCA sockets.  I’m using a ¼” jack for the footswitch.

I tried Uncle Doug’s speaker trick and the reverb transformer (RT) will drive an 8 ohm speaker no problem.  On the scope, with a 200mV going to V2 grid, I see a 50mV coming out of the RT and going to the reverb tank SEND jack.  The tube should be trading voltage for higher current to drive the tank so I think that’s okay.

I can’t detect anything coming out of the tank.  I realize it will be a small signal but at 20mV on my scope, I can’t see any coherent signal, just a fuzzy flatline that may or may not be noise.  But if there’s a sine wave there, it’s too small for my scope to detect.

I’ve checked the cables for continuity.
I’ve tried three different 12AT7s for the V2 reverb driver.
I’ve tried the foot-switch jack grounded to the chassis and not grounded to chassis.
Both RCA jacks are grounded to the chassis.

The tank is a new MOD 4AB3C1B, with 1.2 ohm resistance on the IN socket and 218 ohm resistance on the OUT socket, right in line with what they should be.  The tank looks pristine, suspension springs and reverb springs look nominal.

What else could I test to narrow down what’s wrong?   

Thanks in advance.

[sluckey]

I tried Uncle Doug’s speaker trick and the reverb transformer (RT) will drive an 8 ohm speaker no problem.

OK. That means the driver is good. Quit messing with V2.

Unplug the footswitch to eliminate it. The reverb will work without the FS.

So, the problem is the tank or the V3A recovery circuit. First suspect... wiring error. Check closely the recovery circuit wiring. Measure plate, grid, and cathode DC voltages. What have you?

Now plug your guitar into the reverb return jack. Use a phone to phono adapter or gator clips. The guitar should be heard through the speaker and the reverb pot will control the volume. Any luck?

If you still have not found the problem post some hi rez pics of the amp, especially around V3 and circuitry. Post your layout too.

[Lectroid]

@sluckey,

Thanks, exactly what I was hoping for.  I"ll post it all tomorrow.  I'll also try some other 12AX7s in V3.

[Lectroid]

1.  V3a (reverb recovery):  Plate - 207V, Cathode = 1.65V, Grid = -1.66
     
2.  Guitar plugged into Return RCA jack was inconclusive, see below. 

Here are pictures.  Yes, it's warthog-ugly but it actually works pretty well.  I did have a layout  sketch at one point but it's not worth posting since the actual layout is fairly different from it.  The pic shows a 1/4" shorting plug in the jack but I did test with it out of the jack.

I seemed to get a signal from the jack that was twitchy--sometimes there, sometimes drowned out in noise.  This got me looking hard at connections and reflowing solder in that area, but nothing has cleared things up so far.  At best, signal was ca. 10mV on the scope.  The good news is that at least I could see it and begin tracing it using the scope

I have to go out now.  Tomorrow I'll resume tracing the signal and report back.

[pdf64]

Grid -1.6V?
That’s weird, should be 0V, can you recheck / investigate?

[Lectroid]

I will re-check tomorrow.  To be clear, measure DC volts from grid to ground?

[Lectroid]

pdf64 was right.  I re-measured the V3a grid to ground and got 0 V.  I'm still trying to verify my wiring and to trace the reverb return signal with the scope but it's tough.  Does anyone know what range of voltage that should be coming out of the reverb transformer?  Now I'm not sure if I'm chasing a real signal or an artifact.

[sluckey]

Just repeat this test...

Now plug your guitar into the reverb return jack. Use a phone to phono adapter or gator clips. The guitar should be heard through the speaker and the reverb pot will control the volume. Any luck?

If you don't get a good guitar sound through the speakers then troubleshoot the recovery tube circuit. If you do get a good guitar signal to the speaker then look at the tank and cables.

Another simple test is to connect the send and return jacks together. Gator clip lead is easy. If the recovery circuit is working you will get a boost when you turn up the reverb.

Your cables are good, right? And you do have the cables connected properly, right?

[Lectroid]

Now plug your guitar into the reverb return jack. Use a phone to phono adapter or gator clips. The guitar should be heard through the speaker and the reverb pot will control the volume. Any luck?

Yes, some luck.  I put a .1V 440Hz signal into the reverb jack and saw a nice signal on the scope, but I got nothing out of the speaker. Tracing with the scope, a nice strong signal comes out of the recovery tube and goes to the 100K Reverb pot.  With the pot, I could vary the signal on the scope from ca. 2V down to nothing. Perfect.  When the reverb path signal gets to the far side of the 470K mixing(?) resistor, heading for V3b, is where it dies away to nothing. 

I've so far measured the 220K, 470K and 3.3M resistors and they are nominal.  The Normal side signal moves through this area fine to my ears.  My cables have continuity.  I'm looking at connections right in the mixing area now.

Does that make sense, or should I be looking elsewhere?

[sluckey]

You're on the right track. Replace that 470K. Any joy?

[sluckey]

I'm looking at your latest pic and wondering... Is that a 10pF cap across the 3.3M, or, is that a 10µF @25V tantalum cap? Take it out of the circuit temporarily. Any joy?

[Lectroid]

@sluckey,

You nailed it, bro!  It was, in fact, a 10uF cap instead of a 10pF.  I ordered that part special so that's double the mistake on my part.  I found a 10pF cap and installed it, and presto: Rendezvous Ballroom, 1963.     Wow. 

Thanks for your help drilling down into this.  I may have found it myself eventually but no doubt you've saved me a lot of frustrated confusion.

The amp is strong and loud.  The master volume lets me add dirt with the Volume knob (or even the guitar volume knob) and still get the right volume.  I'm really pleased with how that turned out.

There's still some icepick going on when the Strat's on neck pickup.  I have to turn Treble completely down to get a decent Strat "treble" sound.  If you have an suggestions for that, I'll try them out, but I'll start searching here.  I sure someone's already had this problem.

Many, many thanks again.   

[sluckey]

There's still some icepick going on when the Strat's on neck pickup.  I have to turn Treble completely down to get a decent Strat "treble" sound.  If you have an suggestions for that, I'll try them out, but I'll start searching here.  I sure someone's already had this problem.

Your schematic shows a .022µF bright cap on the volume pot. Replace that with a 47pf like the real DR uses. Should be a lot more Strat friendly. I used a 120pf on mine because that's what the SR uses.

[Lectroid]

Actually, that bright cap never made it into the build...  ( You:    ) 

I'm going to sub in some values for the tone stack caps and see what I can learn that way.   Thanks again for all your help.

[mresistor]

If the neck pup is ice-pickey.. one can only imagne what the bridge pup sounds like...

[Lectroid]

I played around with some small-value caps here and there to try to kill any oscillation but the I got the best results from two adds:

1.  A 120pF cap across the Master volume terminal and wiper really killed the icepick buzzy harsh quality.
2.  Changed the 0.1uF caps from the PI with 0.047uF, as many here have done.   Tamed everything right down to a tight, recognizably 'Fender'-ish sound.  I'm still playing with it but overall, it sounds pretty good to me.

Everyone, thanks again!!