Magnatone Vibrato/Reverb build Project

[wsscott]

I just checked the filaments, and they're all reading 6.5 VAC.  So to be sure I'm measuring cathode voltage correctly, do I set the DMM to DCV, connect black lead to chassis, and then probe each cathode pin with red lead?

[sluckey]

Why are you wasting time checking filaments and cathodes??? You already said "AND IT WORKS!!!!!!  The Vibrato and the Reverb sound great!!".

Your next task is to try to get the B+ voltages to look more like the original. When you do that, then it's time to measure cathodes and record them on a schematic for future reference.

Any cathode voltages you measure now are subject to change when you increase the B+.

[wsscott]

Gotcha.

[wsscott]

Ok, here are the cap readings:  actual vs. schematic
A--328/410
B--327.9/338
C--310.7/316
D--278.8/256
E--252.2/212

Pretty close except for E.  Thoughts?

[sluckey]

How does it sound now? Is the vibrato stronger?

I'm not concerned about node E but it you want it to be closer to 212, then change the 10K resistor between D and E to a 22K 1W.

[wsscott]

Yes, the Vibrato sounds much stronger than before.  You don't really hear it though until the Vib Intensity pot gets up to around 2 o'clock position, with a max out at 5 o'clock where the pot stops.  I do notice that when the Reverb is active you hear oscillation of something even if the vibrato is not active.  If you turn off the Reverb then the oscillation goes away even if the vibrato is turned on.  If you turn the vibrato intensity to the max then the oscillation stops.

I did check the cathode VDC and their readings are as follows:
V1--1.034/1.490
V5--158.7/210.5 ****
V6--5.96
V7--1.234/1.950
V4--83.0/0.1  ****

So V5 and V4 really seem strange given the other readings.  Thoughts

[sluckey]

Compare these voltages from my M10A. You must disable the oscillator for voltage checks on V4 and V5 (see the notes below the voltage chart on page 3.

[wsscott]

Looks like my voltages are pretty similar to yours.  Any thoughts on the oscillating sound only when the reverb is active.  Its much more noticeable after jumpering the 2W R at Node A-B?  Does the range on the Vibrato Intensity pot seem typical, or should it the vibrato be more noticeable earlier in the pot range?

[sluckey]

Any thoughts on the oscillating sound only when the reverb is active.  Its much more noticeable after jumpering the 2W R at Node A-B?

No idea about the reverb. Could be related to layout, grounding, wire dressing, wrong value component, etc. It's more noticeable because you have higher voltages now. There ain't no 2W R between nodes A-B. It's a 2000Ω 10 Watt. The jumper was meant to be a quick test. Now replace that big resistor with a 100Ω 2 watt.

Does the range on the Vibrato Intensity pot seem typical, or should it the vibrato be more noticeable earlier in the pot range?

I bet you used an audio taper pot? Replace with a 1M-L (linear) pot and see if that spreads the intensity control a bit more evenly. BTW, you may notice that I didn't specify any pot taper on my schematic. That's because Magnatone didn't either.

[wsscott]

Thanks Sluckey.  I don't have the resistor, so I'll have to order it from Mouser probably (no electronics parts suppliers within 75 miles of Charlottesville where I am).  Yes, the oscillation really wasn't as noticeable before by-passing the resistor.  Yes all of the pots are audio taper (Log).  I'll get a Linear taper and see how that works.  What is the other type, a Reverse Linear/Audio, or something like that.  I've seen that used in some tremolo circuits.  Would that be appropriate for this design?  Should I leave the jumper in place and play it while I wait for the parts, or is it better to take it out now.  Thanks again.

[sluckey]

Should I leave the jumper in place and play it while I wait for the parts

Yes. What were the V4 and V5 voltages when you disabled the oscillator?

[wsscott]

I haven't disabled the oscillator yet, and was going to ask you how to do that.

[sluckey]

see Reply #356

[wsscott]

Sorry, I missed that. 

[wsscott]

So I jumpered the resistor by manually (not soldered) attaching a jumper wire to each side of the resistor, and then read the DCV at the cathodes and plates for V4 and V5.   

V4-Cathodes            Plates
P3-83.0                   240.4
P8-0.1                     Fluctuating-190-240

V5-Cathodes            Plates
P3--158.7                236.3
P8--210.5                243.8

Thoughts?

[sluckey]

Fluctuating plate voltage means you did not disable the oscillator.

[wsscott]

How would you disable it other than jumpering the leads on the R and soldering the jumper to the leads?

[sluckey]

I would use a gator clip test lead connected across the resistor leads. Or you could do it exactly as you jumpered the 2000Ω resistor. Once you do that the oscillator stops and the voltages will become steady.

Hard to say about V5. You need to post voltages for pins 1, 2, and 3 and also 6, 7, and 8. May be some clues there. Your voltages don't necessarily mean anything is wrong but the difference between yours and mine make me want to investigate.

Show me a hi-rez pic of V5 and the area of the board with the V5 components.

[wsscott]

Here are the DCV readings from V5, and the photo attached:
P1--236.8
P2--156.8
P3--160.6
P6---244.4
P7--121.7
P8--211.1
 
Thanks.

[sluckey]

There are several differences in my preamp circuit and yours and that probably accounts for our V5 voltage differences. Since you like the vibrato I'm not concerned about your V5 voltages.

We're at a point that I will probably be asking you several times to change resistor values. If you're gonna have to order a resistor everytime I ask, this is gonna get expensive and will drag on long past my attention span.

[wsscott]

Sluckey- sure, I understand the "attention span" issue.  You've certainly devoted a lot of your time to my project, and I appreciate that.  It may be that I have left over resistors to try, but I'll deal with that if I need to.  I guess my question is would your suggestion for my testing going forward be for the purpose of eliminating the oscillation sound when the reverb is on, or something else?  I'll try disconnecting the oscillator this morning to see if I can get some steady readings. Seems like taking the 2K resistor out of the circuit is the cause for the resulting effect, but what is the reason for it?  So, let me know what you were thinking I should try going forward, and I can make a decision if I want to pursue it.  Maybe you could give me the concepts and the methodology to use, and I could take it from there and not need to take up your time--when you probably have other projects to focus on.  Thanks.

[sluckey]

The attention span thing is not an issue unless if I get into a trouble shooting mode and ask you to try this resistor or that cap and then have to wait 10 days while you order the part. With that kind of delay in response I'm very likely to forget why I even asked to begin with. That becomes very frustrating. I lose continuity if too much time lapses. Turn it around and imagine if you asked a question but didn't get an answer for 10 days.

At this point you need to replace that 2000Ω/10W with a 100Ω/2W (or 3W). This gets the B+ up to a comparable level with the original and also provides a better filtered B+ than the wire jumper. And replace the INT pot with a 1M-L pot. Meanwhile play the unit. Get familiar with how it sounds. Try to describe the reverb issue as best you can. Post a sound clip that demonstrates the reverb issue. Take note if any knob positions will minimize or eliminate the reverb issue. Make sure your footswitch operates properly. Measure and post voltages for EVERY tube pin, even if it's a zero reading.

I'm still here with you.

[wsscott]

Sounds like a plan.  I'll try to work up a sound clip/video or something and hopefully the size of the file won't be too big to post.

[wsscott]

I used jumper clips and got the 820 R shorted and took DCV readings for all the pins on the tubes, and its attached.

[wsscott]

Sluckey-I compared my tube voltages with your chart, and they were all really close with yours except for V5, and on that tube all of the readings are around 2 times higher than your readings.  Thoughts?

[shooter]

2 times higher

tells me there's 2X the resistance 
maybe power OFF n re-verify with meter the plate n cathode R's for V5
Steve's V5 is an AU7, did you get an AX in there?

[wsscott]

P-1   21.88K / 22K
P-3   45.8K / 47K
P-6   21.66K/ 22K
P-8   40.6K  / 47K

So Cathode 8 is a little low, but is that enough to make a difference?

I've been looking at my wiring of the 2.2M resistor with the varistors.  Is the way I've wired it in the layout which I've attached OK in order to get the 2.2M R connected to the 47K R and the 0.047 cap going to ground?

Also, is the V-1 Pin 6 to V-5 Pin 2 correct?  This was how Sluckey's startup layout for me was done which I followed.  He just reversed the layout of Pins 1,2 & 3 with 6,7 & for a more convenient layout I believe.

[shooter]

is the V-1 Pin 6 to V-5 Pin 2 correct

guessing tubes got re-numbered?
neither schematic shows V1 anything to V5 

I'm not good enough to figure out the DC coupling from the 2 channels into V5-2, or the humidity is melting my brain 

[wsscott]

We modified the circuit by taking out the Tone Boost part of the circuit that leads from V1 on Pin 1 (switched by Sluckey to Pin 6 in his layout page 2 (attached) for convenience) and eliminating V3A and going straight into V5 at the Pin 2 Grid.

[wsscott]

Okay here's some analysis on the "hum" and "oscillating" issue.

Reverb On/Vib Off-Hum and Oscillation. Changing Vib. Intensity has no effect on oscillation.  Vibrato Speed adjustment changes the speed of the oscillation even though Vib is Off.

Reverb Off/Vib. Off-No Oscillation or Hum

Reverb Off/Vib. On--No Oscillation or Hum-Vibrato is fine.  Full range of Speed adjustment from very slow at 7 o'clock position to very fast at 5 o'clock position.

Reverb On/Vibrato On--No noticeable oscillation in background.  Hum even if Vib is On or Off.

Hum's loudness increases with increase in Reverb Intensity.

Seems like its in the Reverb circuit picking up oscillation even if Vib is turned off from the footswitch.

Remember I'm using a stereo jack for the reverb/vibrato on/off control

What if I jumper the 820K R and take the Vibrato out of the circuit like Sluckey had me do earlier?  Seems like it should eliminate the oscillation?

[wsscott]

Okay, so I just jumpered the 820K resistor, and as would be expected no oscillation--because its out of the circuit.  There's still hum when the Reverb is On, but no hum when its turned off--and of course no oscillation whether Vib  is on or off.

[wsscott]

One more tweak-
Start up with both Rev. and Vib. Off-Then

If Rev. is turned On and Vib. is turned On oscillation will begin after about 5 secs.
Then if you turn Vib. Off, oscillation will stop after about 5 secs.

If Rev. is On, and Vib. is Off, and then turn the Vib On, oscillation will start after about 5 sec.

Hum is present unless Rev. is Off.

[sluckey]

Let's tackle the oscillation problem first. The vib oscillator runs all the time, even when the footswitch is turned off. V4 and V5 are both fed from node B and apparently the oscillator is coupling to the reverb driver through the power supply. The easy way out is to kill the vib oscillator with the footswitch. Make the simple change in the attached pic. This will kill the V4 oscillator with the footswitch so it will have to be plugged in. This is how many popular Fender amps operate. If this is satisfactory then we are done. If not we may need to use separate B+ nodes for V4 and V5.

However, there is always a chance that your layout and/or wiring is the culprit for the oscillation problem.

Do this quick mod and test the operation of the vib and reverb. Let us know.

[wsscott]

Didn't work.  I disconnected the wire that runs from the tip on the foot switch jack to where it connects to the turret at the intersection of the 330K and the 47K, and moved that end to the turret where the 820K meets up with the 2 0.01uf caps. I left the connection at the jack alone. Still has oscillation when reverb is on, but now if I turn the Vib Off with the foot switch it emits a loud, static sound and still has some oscillation.

[wsscott]

Sluckey, V4 is served from B Node, but V5 is served from C Node.  B Node does also serve V6 for the reverb.  The schematic attached is what I went by.

[sluckey]

Didn't work.  I disconnected the wire that runs from the tip on the foot switch jack to where it connects to the turret at the intersection of the 330K and the 47K, and moved that end to the turret where the 820K meets up with the 2 0.01uf caps. I left the connection at the jack alone. Still has oscillation when reverb is on, but now if I turn the Vib Off with the foot switch it emits a loud, static sound and still has some oscillation.

Really! So how do you explain this earlier statement...

Okay, so I just jumpered the 820K resistor, and as would be expected no oscillation--because its out of the circuit.  There's still hum when the Reverb is On, but no hum when its turned off--and of course no oscillation whether Vib  is on or off.

Both of these puts a ground on the junction of the 820K and the two .01 caps. Both will kill the oscillator. Of course the footswitch should kill or enable the oscillator. I wonder if your footswitch jack is wired properly? Show me a pic of your FS jack.

Try this ... Completely remove the vibrato wire from the foot switch jack and the board. Do you still have oscillation in the reverb when the INT pot is turned down? Does the reverb footswitch operate as you expect? Did you use a proper shielded cable for the foot switch as we discussed a long time ago?

Sluckey, V4 is served from B Node, but V5 is served from C Node.  B Node does also serve V6 for the reverb.  The schematic attached is what I went by.

My error. I meant V4 and V6 share node B.

[wsscott]

Sluckey--Here's the photo.  Its the white wire that's connecting from the tip (nothing changed here and it turned on/off the vibrato fine) and then it now connects at the base of the turret where the 820K is. I have to believe the foot switch jack and foot pedal are fine because it was working fine before this change. Agreed, jumpering the 820K turned it off.  Now, it has loud static that wasn't before.  That's all that was done.  I've had an evening cocktail, so I'll wait until the morning to remove the Vibrato wire as you suggest.  Thanks for your time.  Best. Stephen

[wsscott]

Here's a thought.  If you look at the last layout that I uploaded, today I think, you can see that I grounded the footswitch jack, the 820K resistor, Pin 8 of V-4 tube, and the Reverb input and output jacks to the same Ground Lug screwed to the chassis. Could that be the problem?

[wsscott]

Sluckey-I failed to respond to your question about using the shielded cable.  I used it where you suggested.  The only instance where I grounded the shield was at the input jack, and I didn't ground the other end of the shield.  In the other instances where I used the shielded cable, I just cut back the shield and didn't ground either end, and just used the lead, after covering the wire with shrink tube.  Is that correct?

I haven't started working yet on your follow-up suggestions.  I thought I'd wait until you see my comments from last night about my grounding.  Since the static noise started after moving the wire coming from the Vib tip over to the 820K R, should I move it back to its original location before I start changing anything else?

[sluckey]

In the other instances where I used the shielded cable, I just cut back the shield and didn't ground either end, and just used the lead, after covering the wire with shrink tube.  Is that correct?

No. In order for the shield to be effective, you must ground one end.

Since the static noise started after moving the wire coming from the Vib tip over to the 820K R, should I move it back to its original location before I start changing anything else?

yes

I'm really concerned about your layout in the vib/reverb area. The wire to the reverb footswitch is very sensitive and there is a lot of other wires in close proximity. I'm beginning to think this may be how the vib is coupling into the reberb.Plus the FS jack is located in a noisy location. I'm gonna look at your layout this morning and make some suggestions that only involve rerouting some wires.

[wsscott]

Thanks.  Maybe the issue relates to my failure to use the ground on the shielded cable.  I'll re-work those wires, but will wait to see what you suggest before I do that.

[wsscott]

I used shielded cable without connecting the ground shield in 3 instances:
V-7 Pin 2 to Rev. Output
Rev. Jack Ring to 0.047uf cap
1uf cap. neg. to Rev. Input

The only other use of shielded cable was with the cable from input jack and I did ground the shield there.

[Williamblake]

Sorry if this misses the point but if you get tremolosignal into your reverb you probably already checked your tank (shield) is connected to the chassis (shield) like it should?

[wsscott]

Could you clarify for me how it should be wired?  I could have missed something.  I know that I didn't ground the shielding in the shielded cable connecting the jack coming from the tank's output jack that goes over to the V7 grid Pin 2.  So that's something that needs to be done.  That jack is grounded to the chassis though.

[wsscott]

Sluckey, Shooter, et al--Maybe I found something.  The .047 cap connects on V-7 P1 plate, and then I ran a wire from the cap directly to the Reverb tip jack connection.  Then I ran another wire from the Reverb tip jack on to the Reverb Intensity pot.  In looking at the schematic, it might have been better if I ran the wire from the .047 cap directly to the Reverb Intensity pot, and then picked up from there and ran the wire to the Reverb tip on the jack.  Thoughts?  If this is better, should either of those wires be shielded?

[sluckey]

That's one of the things I wanted you to do. Run a shielded wire directly from the reverb pot to the .047 cap on the board. Connect the shield to the ground lug on the reverb pot. But for now, don't connect a wire from the .047 cap to the FS jack. You can add that after we get the oscillation issue sorted.

Does this help?

[wsscott]

I removed the wire from the 820K R and returned it to as before, and now the static is gone.  I also re-wired as I described above without using shielded cable either when going from the 0.047 to the Reverb Intensity pot or from that pot to the foot switch Reverb jack connection, and the oscillation remains, but maybe not as strongly. So if you can tell me which connections need the shielding done correctly let me know and I'll give that a try. 

[wsscott]

Sluckey-Ok, I'll wire as you suggest.  So for the time being there will be no connection to the Reverb ring on the footswitch jack at all?

[wsscott]

Did the re-wiring but still has oscillation.  There is no wire connected from the pot to the foot switch jack.  So the reverb is on and can't be turned off.  Same symptoms as before.  The oscillation does vary directly with changes to the Vib. Speed pot.

[sluckey]

Read reply #395