Gibson Lancer...help?

[bainesamps]

Gibson Lancer amp....small, simple tweedish 5Y3, 2 6V6 and 2 12ax7 design and its still not right!! Crazy! As in driving me.

...replaced filter caps to start, double checked that work, hit every solder joint thru out the amp with fresh solder, replaced all tubes with known good tubes (twice), replaced several resistors that were 20%+ out of spec or dry rotted with good new CFs, replaced several suspect coupling caps with M150s, checked all pots, jacks, the speaker (reconed, good), tube sockets and installed qaulity AC cord and replaced fuse etc. Also subbed a known good OPT as part of the trouble shooting process and bypassed the on/off switch to eliminate it. VDC looks good on all pre and output points.

Results, with the trem/vibrato fully ON, it achieves full volume, only with extremely choppy results, but the overall output volume is there!

With the trem/vibrato OFF...it loses 90% of its volume and distorts, fuzzy and weak. As you turn up the intensity pot, the volume comes back, the distortion cleans up and it sounds like it should, only extremely choppy as mentioned, as if the mix is all trem/vibrato with no dry sound mixed in with the oscillation?

Hope thats clear as mud? What am I missing!? Not seeing?

Power tube sockets could be suspect? Checked all the socket connections and resoldered?

Any and all help appreciated!

[Steve Conner]

It's probably a bad coupling cap in the trem circuit, causing DC to appear on the intensity pot. Wouldn't hurt to replace every single capacitor in the amp: if some are bad, the rest will go sooner or later.

[Rob Mercure]

Well, while Gibson is notorius for identifying amplifiers with varying circuits using the same name - "Lark" for example - none of the schematics I can find under the name "Lancer" have a tremelo. Are you sure that is the name? Actually checking three sources I can only find one schematic with that name - and this amp had the same tube compliment using 1/2 12AX7 as an input for each channel and the other 12AX7 as an anode follower (the "see-saw" type) phase inverter (also designated as the GA-6). In other words there isn't a tube available to drive the tremelo. Is it possible that someone added a tremelo (perhaps with an FET circuit)? And does your amp have two channels with two volume controls?

If it is an "Lancer" with two volume controls I suspect that it is a variant I don't have a schematic of (and I have a PDF copy of the Gibson factory master schematic manual which has, to date, happened to have every factory variant that's come up) and is probably using a split load phase inverter or PI transformer with a single section tremelo oscillator that varies the output grid bias such as in Gibson's GA-19RVT circuit. If so then a defective intensity pot or, as Steve pointed out, the DC blocking capacitor connected to it is probably the culprit. You might want to see if the intensity pot's wiper (center terminal) is connected to two identical resistors around 220 k-ohms (red, red, yellow - or any value with a yellow multiplier band). If so short the wiper to ground and see if that fixes your problem. If so it's either the pot or the cap (Steve's suggestion).

Rob

[bainesamps]

Thx!

I can make out what might be an "L" and what might be an "er" on the chassis top and was sorta reaching for the name Lancer hoping to make a solid connection to a model and/or a schemo.

The chassis top is fairly pitted and rusted. Its a 3 input amp with one vol and tone and the two knob trem circuit looks factory stock, footswitch jack etc all look factory. Looking at it again the "L" might be an "E" and that could make it an Explorer?

It resembles a GA-16T in circuit, except it houses a 12" speaker (aftermarket mod maybe?....but the baffle looks factory cut?), the cathode bias resistor also looked factory stock, but was a 330 ohm sandcast instead of the more typical 250 ohm and and there are no 1.5k swamper resistors in the circuit and no signs that there ever were. It could very well be a GA-16T circuit with some minor factory changes.

The trem cap values differ from the GA19RVT (cool amp!) .01, .01, .033 vs. three .047s and resistor values differ slightly.

Ill take up both suggestions re: shorting the wiper on the intensity pot (will need to order a RA if its the problem), though I think I did replace the coupling cap Steve mentions, will double check and see if I did... thx!!... will check and get back....

[Rob Mercure]

OK - I'll look for Explorer variants when I get a chance. But what the "H" is a "swamper" resistor?

Rob

[bainesamps]

Swamper resistors

Slang for resistors strapped on the grid of output tubes to make amps less prone to power loss thru high frequency parasitics (inaudibles above the presence range)....

Leo F used the word when discussing the tweeds amps that were "improved" by using grid "stoppers" or "swampers" in the late 50s. High tech!

.....still need to check out the Explorer (?) trem circuit, will be back to it Mon...Thx for looking for schematics..!!

Teagle and Sprung use the term in the Fender book....

http://books.google.com/books?id=N_8...um=3&ct=result

[R.G.]

I don't have the schemo at hand, but as I remember some of the Gibson amps used 6EU7 duotriodes instead of 12AX7. Is it possible the Lancer was such, and someone in the past converted it to 12AX7, with more or less success?

[Rob Mercure]

BA.

Well I finally got a chance to stroll through my Gibson master service manual - check out my posts in the "theory" section - and I've got a couple of updates. First I found another amp under the "Lancer" name - the GA-35RVT - but this isn't related to yours in any way.

But how about the "Viscount" - GA-16T on page 129 of the service manual? Seems to have the same tube line up and control layout you suggest and the same tremelo circuitry that we've already suggested. So if you continue along the same path that Steve and I recommended you should at least be able to get some idea as to what's not functioning. Enzo replied to my post on the "theory" section and noted that the service manual was available from Schematic Heaven so you'd probably do well to download it.

Let us know what's happening we're waiting with "baited breathe." (yeah, I've been eating worms again <grin>)

Rob

[TD_Madden]

Sounds a LOT like my Explorer....except for that pesky 12-inch speaker!

[Rob Mercure]

Well, typical Gibson - the GA-18T in the master manual is 6EU7s and 6V6s and missing the nfb loop - instead there's a padded down 1/4" jack on the output marked "monitor."

Figures

Rob

[Enzo]

LOTS of old Gibsons have 6EU7 sockets rewired for 12AX7s.

[TD_Madden]

Well, typical Gibson - the GA-18T in the master manual is 6EU7s and 6V6s and missing the nfb loop - instead there's a padded down 1/4" jack on the output marked "monitor."

Figures

Rob

I have a schematic that was sent to me by Gibson years ago....iit's hand-marked "GA-18" and "Explorer". It's the same as the Maestro Viscount (GA-16T), and matches my Explorer.

In the same way, the schematics for the Gibson GA-19RVT, Epi EA-28RVT, and Maestro M-216RVT all match

[bainesamps]

More....

First off....thanks for the responses and help, sincerely!!

This circuit IS identical to the GA-16T schematic, with the exception of the speaker....which could be a very well done aftermarket switch (and done LONG ago since there are no recent telltale signs of hacking on the baffle) and the values in the trem, which were all .047s. vs. the two .03s and the .01 on the schematic. I just changed them out Tues morn. as well as substituted a known good .1 cap for the bigboy .33/600vdc that jumpers the pots.

Still have a problem with low volume and distortion, after changing out all the coupling and lytics in the entire amp, shorting the depth pot wiper to ground doesnt clear it up and it is connected to the 220ks in the PI, sooooo.

I did notice Tues that the 6V6s are running cool (to the touch) and one much more so than the other. Installed more good 6V6s with same results. Also noticed that the vol pot pops right as it first brings up the volume in the amp (about 9 oclock on th pot) and then the mud and weak vol follow. Bad vol pot... bad power tube sockets?

Its still louder and cleaner with the trem on...

Ill sit and stare at the GA-16T and recheck voltages and see what I can see there...hmm? Also hooked up another speaker just to rule that out a second time...thx!

[Rob Mercure]

Grounding the trem pot wiper eliminates the tremelo as being the problem - but this trem works by, for all practical purposes, increasing and decreasing the bias on the output tubes. So since when the trem is firing the amp gets louder that means that the bias voltage on the outputs control grid is too negative when the trem isn't firing. Can't remember if you've checked/replaced the output tubes cathode resistor/cap but if not that's where I'd look (and if you've already checked them -then replace them 'cuz some faults just don't show up when measured up only during performance). After all, you noted that the tubes were "too cold!"

Rob

[bainesamps]

The tubes ARE cold, Im on it.

I did switch out the cathode resistor bypass cap with a qaulity 25/100v, but Ill try a new 250^ res there with a diff bypass cap.

Im ready for something to happen with amp, Im also thinking change out vol pot (and the first input jack and temp bypass the others) and then one or more pre/power tube sockets if the problem doesnt resolve with running the 6V6s hotter...theres something in the front end making the vol pot pop loudly as its turned up early in the rotation? Thx again........!