Gibson GA-95 RVT

[Travst]

The problem that I'm having with this amp is that it's about as loud as a 2-watt amp. I started off replacing all the electrolytics, as the original Astron caps were still in place. I also replaced the bad midrange pot in the reverb channel, the 470k resistors on each 6L6, the resistor at R15, and the cap at C11. I'm gradually going through and testing each component right now.

The plate voltages at V4 (12ax7) are double what they are supposed to be. I performed some basic tests of the OT by measuring resistances and the results looked ok. All the tubes or new or verified good. I've attached a voltage chart.

So far, the changes have cleared up a lot of static, some hum, and improved the tone a few hundred percent. However, I'm still in the low-output range. I'm looking for any ideas from the experts here. I've built a couple of amps and worked on a few more, but am very much the intermediate student. Thanks for your help!

[Phil_S]

I'm incredulous. This amp is nearly identical to my GA20-RVT of the same era, the biggest difference being 4x 6L6 finals vs. 2x 6BQ5, and the output transformer. Even the rats next board looks the same. The reverb and tremolo circuits appear to be identical. I've been through mine with a fine tooth comb and feel as if I know quite a bit.

How do the 6L6's do with the pop test (probe with DMM)? If you get a good pop, I'd guess the power section is OK and I'm expecting it is.

The weak point in this amp is the interstage PP driver transformer (T3). While these are made of unobtanium, Mercury Magnetics makes one for which they charge an arm and a leg (about $90 last I looked). Your part no is TF1001D and mine is TF1001D-1, so that makes some sense as the turns ratio is probably different for a quad of 6L6.

I suggest you cut to the chase by lifiting the leads on the interstage tranny and checking it for the turns ratio. Mine is about 1:2.4. If you find the ohms on each side of the secondary to the CT are significantly uneven, then you know your culprit, though mine went bad on the primary. The symptom that I did not recognize was intermittent static-like poping.

Also, the grounding scheme in mine introduced gobs of hum. I moved the ground leg on the interstage tranny to a dedicated ground near the PT. I used the end lug on the big terninal strip... maybe you don't have that.

Cathode bypass caps were either missing or didn't do much at the spec'd values. There is every reason to put in 10u or 20u bypass caps on V1 and V4.

Your plate voltages mostly look reasonable and I'd expect them to be on the high side, given that line voltage is higher than it was 50 years ago.

The one tube plate voltages that look "wrong" are V4, which are both 2x what it should be. I might be inclined to try a divide and conquer strategy here. Try lifting the appropriate parts to remove sections of the circuit from operation. I think I'd start with R44, whose purpose escapes me, and I'd follow with R42 (without reconnecting R44). The next lif is R32. I'd work those to see if I could get the dry channel to work like you think it should. If lifting the reverb channel out of circuit cures the problem then you have a good idea of where to be looking.

I'd like to know what is the B+ voltage at C28A (the node that feeds V4), and on both sides of R47 and R49. I'm fishing here...

The sorry truth is that these amps are typically nothing special. The wierd notch filters at c3 and c12 are examples of what not to do in an amp. I have often toyed with the notion of putting another circuit in mine. What keeps stopping me is that I see nice ones (and mine is cosmetically nice) sell for $450 on eBay. I remain like a deer in the headlights. I don't play mine. I recently loaned it to the daughter of a friend who plays fingerstyle hollow body electric and she loves it for the clean tone it produces. I suppose these were jazz amps in their day.

The problems on this amp can be fixed. You should be prepared to spend a great deal of time doing it.

Phil

[Cliff Schecht]

The GA15RVT I spruced up for a friend sounded frickin fantastic after a few circuit tweaks. The big changes needed for these amps are decent tubes, usually an increase in the capacitor size (from .0022u to.022u for coupling caps, IIRC) and the obvious safety fixes. A better speaker also helps quite a bit. The one I worked on had a great jazz sound, almost no breakup but took distortions like a champ.

[labb]

see if any of this helps:

Low Power or Loss of Volume

Back to the top of the Amp Debugging Page Back to the GEO Home Page
Faulty Preamp Tube
Faulty Power Tube(s)
Bad preamp cathode resistor
An unbypassed cathode resistor has drifted upwards (to 5K-10K or over)
Faulty phase inverter
If for some reason the phase inverter input side is good but the inverted side is bad, the power amp will still work, but power will be very low. This can be a bad 1/2 tube, a faulty socket contact, a broken or open plate resistor or coupling capacitor to the output tube, or a bad solder joint on any of these.
Open cathode bypass capacitors in preamp
if they go open, the stage they're in loses gain, but does not otherwise fail. If they short, it dramatically shifts the bias point, and may cause distortion as well as low volume.
Faulty vibrato circuit on neon/LDR vibrato Fenders
If there is a dummy plug in the footswitch hole, or a bad footswitch so the vibrato is always active, sometimes the vibrato tube turns on and stays on, not oscillating. This keeps the neon bulb on all the time, shunting lots of signal away. Same thing can happen if there is a shorted vibrato tube (rare) or a bit of wire or solder shorting the vibrato tube. Check the plate voltage on the vibrato tube to be sure it's oscillating.
High voltage isn't high enough for some reason
Failing rectifier tube - try swapping in another one
Failing power filter capacitors
Failing or open series dropping resistor in the bypass networks leading to preamp stages
Failing bypass capacitor - treat as in power filter caps.
Open screen resistors on power tubes
Amp cuts out or "goes dead" when the volume control is turned up higher than "X" or when you hit a specific note
You have a parasitic oscillation above hearing range. This can overheat an output transformer, and really needs to get fixed fast. It can often be fixed by tube swapping, but you often need an oscilloscope to see what's happening in the electronics.

[Travst]

Thanks for the great ideas! I'd read a post here about the T3 replacement. I'll try the tests described here and post my results. I'm also aware that these amps are usually considered mediocre. I'm using this as a mule to learn and tweak on. Fortunately, I traded a cheap used guitar for it and don't have a lot in it even with the parts that I've put into it. Cosmetically, the amp is decent enough, but the original speakers are gone. Suprisingly, the odd match of replacement speakers sounds fairly good. I'll know more when the amp is pushing some power to them. So far, the amp sounds ok, but is a bit thin and sterile.

[Phil_S]

I would expect thin and sterile, but your main symptom is lack of volume. 4x 6L6 should be pretty frickin' loud, even if it turns out only 40W. I'm not sure what this amp should put out for final wattage, but I'd expect about 60W -- just guessing.

Cliff gives good advice for circuit mods. FWIW, I'm in full agreement. That GA15-RVT shares quite a bit with this circuit, too.

First, you should address the low volume.

BTW, that LDR for the tremolo is done with an incandescent and the only way I know of to replace it is to roll your own with a bulb, LDR and heat shrink and guessing at the correct values. Regardless of what you think that symbol is on the schematic, I'm 100% certain of what I say on the matter.

[Travst]

I would expect thin and sterile, but your main symptom is lack of volume. 4x 6L6 should be pretty frickin' loud, even if it turns out only 40W. I'm not sure what this amp should put out for final wattage, but I'd expect about 60W -- just guessing.

Cliff gives good advice for circuit mods. FWIW, I'm in full agreement. That GA15-RVT shares quite a bit with this circuit, too.

First, you should address the low volume.

BTW, that LDR for the tremolo is done with an incandescent and the only way I know of to replace it is to roll your own with a bulb, LDR and heat shrink and guessing at the correct values. Regardless of what you think that symbol is on the schematic, I'm 100% certain of what I say on the matter.

The amp is rated at 90 watts according to the published specs. I'll look into the vibrato circuit. I did not receive a footswitch with the amp, and it's a DIN type plug arrangement... not something that I have the parts for laying around.

[Phil_S]

I think we can agree that 90W should make you deaf if you're not careful. Work on that aspect first. Well, you already know that.

The foot switch is a simple pair of on/off stomp buttons. I never looked, but I imagine there is a shield (to ground) for the reverb and 2 pairs of wires inside the shield. If you need to modify or replace the connector, do it. Otherwise, you'll need to build yourself a pair of jumpers you can push in or out of the correct holes in the connector. I don't recall, but I guess the default is "on" and off is when the circuit is shorted? Sorry, I don't have mine here to look at.

[Travst]

I think we can agree that 90W should make you deaf if you're not careful. Work on that aspect first. Well, you already know that.

The foot switch is a simple pair of on/off stomp buttons. I never looked, but I imagine there is a shield (to ground) for the reverb and 2 pairs of wires inside the shield. If you need to modify or replace the connector, do it. Otherwise, you'll need to build yourself a pair of jumpers you can push in or out of the correct holes in the connector. I don't recall, but I guess the default is "on" and off is when the circuit is shorted? Sorry, I don't have mine here to look at.

You are correct. I was planning to make up a footswitch or mod one. I'd replace the DIN plug on the chassis with matching male/female 5-pin DIN 180 plugs. Simple and effective. I didn't have much time tonight, but I looked over some things and spotted the LDR on the board. I'm not clear on how to test the LDR.

[Phil_S]

Hmmm....I think to test the optoisolator (LDR + incandescent lamp)...

With the power on, and the tremolo switched on, pots on 10, you should be able to hear it oscillate. If there is any oscillation at all, chances are the part is working fine. Put your voltage meter on the LDR side and you will see the voltage fluctuate. Either of these is an indicator of it being in working order. Also, there is a pot on the board for tweaking the tremolo. You can adjust this while the circuit is live and it might bring it to life. Keep your meter on the amp, looking at the spec'd voltages of 148 and 136 to make sure you aren't too far off. I don't see the internal pot on your schematic, but it is there in the picture. I think it controls the cathode of V3B, so keep your eyes on cathode voltage, too -- I think your schematic says 1.65V (not 0.65V as it might seem).

If you need to go further, with power off, check the LDR side with your meter. It has very high resistance and is probably too high for your meter to give a reading, so you meter will go off the chart, so to speak. If it reads open, there is a problem. The LDR side is not likely to go bad.

The lamp is another matter. It is the lamp that blows, ruining the device. Once again, with power off, a bulb/lamp will show very high resistance when cold. If it is open, then yours is bad and you'll need to craft a replacement.

If you find this, post here and I'll dig up my notes on this item -- mostly educated guesses and possible lamp numbers that might work OK. It is possible, with care, to dig out the lamp and relamp it. I'll note that I failed at this task miserably and ruined the LDR, too. It is possible to cut it in half, retaining the LDR and putting a lamp on it with heat shrink, but you must guess at where to cut (and not to cut).

[Phil_S]

BTW, silly me, your foot switch is diagrammed on the schematic. Open is off, and closed is on. I suppose the default, then, with nothing plugged in is off. A pair of jumpers at the DIN plug should turn on the reverb and tremolo. Just make sure you jump the right pins

[Travst]

Phil, I was planning to start with this tip from you:

The one tube plate voltages that look "wrong" are V4, which are both 2x what it should be. I might be inclined to try a divide and conquer strategy here. Try lifting the appropriate parts to remove sections of the circuit from operation. I think I'd start with R44, whose purpose escapes me, and I'd follow with R42 (without reconnecting R44). The next lif is R32. I'd work those to see if I could get the dry channel to work like you think it should. If lifting the reverb channel out of circuit cures the problem then you have a good idea of where to be looking.

Then I can work with the reverb channel if that's the issue. It's pretty easy to follow the wiring diagram for the footswitch and fab something up. This is turning out to be an interesting project and quite the learning experience for me.

[Phil_S]

I've been trying to think this through, the high plate voltage on V4. You likely have the same problem affecting both plates. The high plate voltage is causing lower cathode voltage -- that's how it works, so I think the cathode is not where the problem exists. That leaves the grid and the plate.

The last 20u filter cap is connected to 4 100k plate load resistors, V1 and V4. V1 and V5 have high plate voltage, too, but not unreasonably high.

I suspect, if you lift the 100K plate load resistor and 68K resistor tied to V4B pin 6. you will see a change. I would lift the end closer to the 20U cap, not the end tied to the plate. If the plate voltage on V4A (pin 1) gets into the normal range, then I would suspect one of the two resistors you lifted is way out of spec. That goes for plate load resistors, too.

I would also lift one end of R45 (4.7K dropping resistor) to see what it meters. That is the common dropping resistor in the B+ supply ladder. Because it is common to the problem, I'm liking it for the cause. When/if you replace it, use a metal film or metal oxide if you have it, as no tone is running on the B+ supply.

The other possibility, as I see it, is the grid feed, but I don't think that is it because something would have to be wrong between both of the volume pots and the grid. That seems too unlikely to me. We are looking for something that is common to both channels.

Unrelated to the comments above, on your power tubes, I think the plate votage (pin 3) is reasonable, the grid voltage at -43 is right on target, but the screen voltages running above the plate voltages don't make sense to me. If i read the schematic correctly, the screens each have a 470 ohm screen stopper and are fed from the same B+ node as the plates. This suggests your screen readings may not be correct, however, I don't think this is where your problem lies and I'd hold this for later.

[Structo]

Phil,
What do you think of the T3 interstage transformer as a suspect in the low output?
I suppose if an earlier stage is not working right it could be the cause but I think if it were me I would be pulling one or both sides of that transformer and measure the resistance.

[Travst]

Thanks for the tips. I'm unable to work on it this weekend, but will get back on it next week.