Lancer 35RVT

[dpm309]

Just finished changing filter caps and installing a 3-prong plug on a Gibson Lancer 35RVT and am now getting sound but a lot of hiss and weak volume.  Tubes tested out good and do not seem to be microphonic.  This amp has 2 6EU7, 2 12AU7 (have 12AT7s in the amp), and 1 12AX7 for the preamp.  The power section has 2 7591A and one 0A2 tube.  I am going to check the old CC resistors to see if they have drifted.  Is there anything else I should look at?  I have posted the voltages below to see if anything jumps out.  The voltages seem in line with what is on the schematic except for the 2nd half of V5.  There should be 136V on the plate (pin6) and 2.8 V on the cathode (pin8).  This looks like it connects to the tremolo section so that explains why I am not getting tremolo.  There is a component shown as Z1 on the schematic that I think produces the tremolo effect along with V6 (0A2).

   Tube   Pin Number                        
No.      1   2   3   4   5   6   7   8   9
V1   6EU7   H   H   NC   1.3    -   173   173    -   1.35
V2   12AU7   169    -   3   H   H   278    -   3.96   H
V3   6EU7   H   H   284    -   -0.5   75.5   175    -   1.24
V4   12AX7   202    -   1.9   H   H   214    -   2.1   H
V5   12AU7   281    -   4   H   H    -    -    -   H
V6   7591   NC   H   415   415    -   -20.4   H   415   X
V7   7591   NC   H   416   416    -   -20.4   H   416   X
V8   OA2   150      1.2      150   2.1    -   X   X

Any ideas or suggestions?

Thanks,

Dan
                              

[HotBluePlates]

Just finished changing filter caps and installing a 3-prong plug on a Gibson Lancer 35RVT and am now getting sound but a lot of hiss and weak volume. 

Silly question: did the amp work before the cord/caps were changed?

The voltages seem in line with what is on the schematic except for the 2nd half of V5.  There should be 136V on the plate (pin6) and 2.8 V on the cathode (pin8).  This looks like it connects to the tremolo section so that explains why I am not getting tremolo.  There is a component shown as Z1 on the schematic that I think produces the tremolo effect along with V6 (0A2).

Knowing Gibson, I may be assuming a lot if I think your amp matches this schematic.

The 0A2 (V8) has only 1 purpose in life: keep ~150v across the trem circuit.

Z1 is an optoisolator (trem roach), which is different than the Fender roach because it uses an incandescent bulb, not a neon bulb. The plate current for the second half of V5 must pass through the optoisolator. If you have a neon bulb (maybe replaced the original with a Fender-style roach), there's no path for current. Neon works by having a big voltage across the terminals which ionizes the gas inside, while an incandescent bulb passes current through the filament causing it to light up. Makes sure someone didn't swap the incandescent roach with a neon roach.

The trem circuit also needs the footswitch to activate. The trem oscillator (V3, pins 4, 5, 6) can't turn on until pins 4 and 5 of the footswitch are connected. That grounds the Speed pot ("Frequency") and provides a needed reference for the trem oscillator to pass signal.

You can test the circuit, even if you don't have the footswitch: only 2 lugs on the Frequency pot are used; connect the one that is not the wiper to ground with a jumper clip. That will turn the oscillator on.

Regardless, your lack of trem is probably a neon roach.

It kinda suck that Gibson split dual-triodes among each channel, preventing you from simply pulling tubes to isolate one channel and evaluate it for noise/gain.

It looks like R14 and R36 form a resistive mixer before coupling cap C9, which then feeds a T-filter tone network and the driver of the phase inverter transformer. Turn the amp off and verify B+ voltage has drained, then unsolder one leg of R36. That will leave on the Normal channel connected. It will throw off the voltages for V4A somewhat, but will allow you to test that channel by itself.

How's the gain/noise issue at this point?

[dpm309]

Yes, this is the schematic I am using.  Before I changed the filter caps and cord, the amp was making a horrible squealing/hum even on standby.  When I plugged the guitar into it, there was nothing coming out except the squeal.  It appears that there have been several mods made to this such as caps removed and different resistor values in the vicinity of V5.  Also, the optoisolator was missing and I could see where it was clipped off in a couple of locations.  I am wondering if these mods were made after the optoisolator was removed or failed.  C27 had a broken lead and I replaced it with a 1uf, 50V e-cap and it did not change the hiss and lowered the volume a tad.

Pulled one leg of R36 and am now getting tons of volume but still has the hiss.  I then pulled V8 (0A2) and the hiss goes away but I get a lot of hum.  I am wondering if I reinstall the optoisolator and restore the circuit to its original condition would this get rid of the hiss?  Mouser sells a few optoisolators but I am not sure of what value I need.  Any other suggestions to get rid of the hiss?

Dan

[HotBluePlates]

... It appears that there have been several mods made to this such as caps removed and different resistor values in the vicinity of V5.  Also, the optoisolator was missing and I could see where it was clipped off in a couple of locations.  I am wondering if these mods were made after the optoisolator was removed or failed. ...

Someone probably thought Fender mod recommendations applied to this amp. That is, the trem roach causes a loss of gain, so removing it will boost gain. Maybe, maybe not.

But then they cut out the original roach, try to replace it with a Fender one, nothing works and then they go crazy...

Pulled one leg of R36 and am now getting tons of volume but still has the hiss.  I then pulled V8 (0A2) and the hiss goes away but I get a lot of hum.

Leave the 0A2 in the amp.

We need to divide and conquer. We now know the effects channel is loading down the signal somehow, while the normal channel works but is hissy. With R36 disconnected, we should know the hiss is coming from the normal channel.

Now the fun part: You should go through, find all the "mods" and take them out; get the amp back to stock (maybe minus the trem roach). There's no way to figure out what misguided "improvement" somebody made that will lead us astray.

[dpm309]

The trem roach was bridged with a pair of twisted wires and a large shielded cable (grounded at the pot) was added from the tremolo gain to pin 7 of V5.  See pictures below.  I am wondering if this is causing the hiss.  With R36 disconnected, I am getting more gain on the reverb channel (same as the normal channel) and the reverb seems to be functioning properly.  For the heck of it I tried another 0A2 and am still getting the hiss.  Does the trem roach even need to be bridged or would it cause more problems?  Also, where can I get the proper optoisolator for this?  All the ones at Mouser are LED driven.  If I can't find one, I would be happy just to get the amp working without the hiss.  My next step is to determine exactly what else was modified and check the CC resistors for drift.

Thanks,

Dan

[HotBluePlates]

... Also, where can I get the proper optoisolator for this?  All the ones at Mouser are LED driven.  If I can't find one, I would be happy just to get the amp working without the hiss. ...

I'm not a good-enough resource, because I don't know anything about the rating of the original (meaning the incandescent bulb). If you knew what kind of bulb to use, you could cut open a neon roach, place the incandescent bulb in place of the neon one, and slide new heatshrink over it to make a proper unit.

If I had a string of christmas tree lights, I might steal a bulb out of it (one with wire leads coming out the end) and try it just to see if it worked.

I'm hoping someone who's faced this problem before will jump in (but Gibson amp experts seem few and far between).

With R36 disconnected, I am getting more gain on the reverb channel (same as the normal channel) and the reverb seems to be functioning properly. 

Sumthin ain't right. R36 is 100k mix resistor running from the reverb channel's V4b, pin 1, to the 750k resistor on V4B, pin 6. If you unsolder either side of this resistor, the reverb channel should not feed into the output stage at all. If you accidentally unsolder R35 (also 100k at V4B, pin 6), it should still kill the reverb channel because V4B will be disconnected from B+. That you get signal at all from the reverb channel is no bueno.

The troubleshooting logic was to kill one channel while evaluating the other channel's gain/noise, so that we can find/fix the problem efficiently.

I state upfront I've never been inside this particular amp. But all the new 225P orange drops, and the mix of cloth and plastic insulated wire tells me someone's swapped a LOT of parts, and may have made some other changes.

I think you might need to trace everything to figure out what's same-as the schematic and what's been changed. I'm having trouble figuring out what's going on just from the pictures, just because some parts are obscured by wiring or other parts. The two thick wires in place of the trem roach will only work on the bulb side; the light-dependent resistor side can't be effectively replaced with a wire. It should kill most of the reverb channel signal (or make it extremely muddy).

I'd probably remove those two thick wires, as well as C32 (the 0.022uF at the "top end" of those wires in the pic, by V3) and see what it sounds like.

[PRR]

Use an LED opto-resistor. Unlike many other circuits, in this one, it will "work". Current is not excessive and the tube will take-up the voltage slack. Trim R53 (perhaps a lot!) for smooth trem action. (Might have to go over 1K.)

But first-- follow HBP's advice to divide and conquer. I do NOT think the trem has anything to do with hiss or hum. Although you may need ~~100K where the roach's resistor side is to get a "proper" over-all gain.

Do NOT pull V8!! V5B will burn-up.

[dpm309]

It looks like I lifted the wrong resistor, R13 instead of R36 (it was getting late).  I found R36 but it was a 220k instead of 100k, pulled it and am now getting good volume and tone on channel one with no hiss. Hooked up my listening amp and was getting no hiss on V4 and hiss on V5 so the problem must lie in between somewhere.  Reinstalled R36 and pulled the thick twisted pair where the trem roach was and C32 and the hiss has disappeared.  I am now getting good volume and tone on both channels and reverb on channel 2.  My next step will be to get an trem roach and install it to see if I can get the tremolo working again.

PRR, I read on this forum that the LED opto-resistor would not work in this amp.  Here is a quote from Sluckey "Z1 is aincandescentnt optocoupler used as the trem roach. The Fender neon bulb roaches will not work. You may have to roll your own".  (Don't know how to include quotes from other topics) If you think it will work with some modifications, I am willing to try it.

Thanks for everyone's advice, at least I got this working without the tremolo.

Thanks,

Dan

[Willabe]

(Don't know how to include quotes from other topics)

Go to the post you want to quote, than right click and copy shortcut of the threads title of the reply # you want to quote. Then go back to your post, right click and paste it into your post. It will take them right to that #reply.

This goes to reply#4;                             
                                          
http://www.el34world.com/Forum/index.php?topic=13360.msg124981#msg124981                                       

                                                         Brad      

[dpm309]

Willabe, thanks I new it was something easy.  The owner decided to wait on the tremolo repair and just go with the amp the way it is.  Reinstalled the chassis, reverb, tubes and when I fired it up, I started getting a horrendous feedback, even with the volumes down.  Long story short I noticed that I mixed up one of my 12AU7s with one of his 12ATs (this amp should use 2 12AU7 but the owner bought 12AT7s) and I must of had this one in the V5 position while on the bench.  When I put the amp together, I put both of the ATs before firing it up.  I went back and replaced V5 with the 12AU7.  V5 does not appear to be in the reverb circuit but the problem disappeared once I put the right tube in.  Anyway the amp is working great, minus tremolo.

Thanks for all the help and advice.

Dan

[Willabe]

I goofed that up. That way only works when your quoting from the same thread. Go back and read what I changed it to.


                          Brad      

[HotBluePlates]

LEDs are not neon... So if PRR says the LED type will work in this amp, I'd go with it.

LEDs pass current to light up. Neon relies on almost no current, but a voltage difference between its terminals to ionize the neon gas and light up.

[PRR]

> So if PRR says

Well, check me.

V5B is shown with 2.8V at cathode and a 500K trimmer. Assume trim is 280 ohms, this gives 0.010A or 10mA on my abacus. The LED may idle at 40mA forever so this seems safe.

The B+ is 148V and the plate is at 136V. This suggests a 12V lamp.

What's worst-case, cathode grounded, grid grounded? Allowing 2V in LED there is 146V across 12AU7. A 12AU7 will pass 20mA in this condition. The LED is fine with that.

What is this opto-resistor shupposed to do? Reduce a signal. What is the signal source? A 220K+220K+68K from FX channel Loudness pot and a 470K from reverb recovery. Call it 250K. We need some idle loss so trem will boost as well as cut. Say the opto-R should be 250K.

Vactrol data for VTL5C3 says LED current should be about 0.3mA to get 250K (and VTL5C4 has to be starved to get anywhere near that high). We want Vactrol with around 1K lowest Ron (not ~~100 like the VTL5C4). VTL5C1 and VTL5C2 good also.

Hmmmm.... the 10mA for incandescent is still a lot for the LED. V5B cathode resistor may have to be upped to 30K (50K trim-pot). There's a more-parts way to fudge this and also have a front-panel trem blinking LED.

[dpm309]

The owner is still experiencing some feedback so I told him I would go back in one more time and install a Fender type optocoupler from AE:  http://www.tubesandmore.com/scripts/foxweb.dll/moreinfo@d:/dfs/elevclients/cemirror/ELEVATOR.FXP?item=R-VOP1020.  Will this particular optocoupler work?  I will also try to undo some of the mods the previous owner did to get rid of the feedback/squeal.

PRR I am assuming the R53 is the cathode resistor/trim pot which is currently rated at 500ohms.  Should I use a 30k trim pot?

Dan

[sluckey]

The Fender roach uses a neon NE2 bulb and will not work in your circuit.

FWIW, I used a Vactrol VTL5C1 to replace the incandescent opticoupler in my Sunn. I had to juggle a resistor value but it works quite well.

[mresistor]

Weber has some that use an LED , check it out

https://taweber.powweb.com/store/chansword.htm

[dpm309]

Thanks for the information.  I was't sure if the Fender one would work in this.  I will probably go with the Weber "roll your own".  The owner decided to use the amp without the reverb and tremolo for now since I was able to bypass the reverb circuit, thus eliminating the feedback.

Dan

[dpm309]

Looks like I am going back into this amp.  The owner now wants the tremolo reinstalled.  My question is which "roll your own" optoisolator I should use?  They sell 2 "roll your own", 1 incandescent and 1 LED.  They also have  an Optoisolator module that consists of an LED and photoresistor cell. The cell has an ON resistance of <1000 ohms and an OFF resistance of >20 Meg ohms.  Are there any other modifications I would need to do to the amp?

Thanks,

Dan

[HotBluePlates]

Since the original was incandescent, that's what I'd pick.

Looking back at the schematic, the 0A2 plate (and source for the opto) is at 148v, and the other side of the bulb portion of the opto is 136v. Sounds like you should pick Weber's 12v lamp.

[dpm309]

Thanks, that is what I needed to know.  Will post again when I get this installed.

Dan