Need help with Gibson GA-5T

[vortex]

Hi all,
WHAT'S UP:I just bought a Gibson GA-5T that needs a bit of TLC. It has had a "cap job" and three prong power cord added but I am not sure if it was done competently so I have a couple of questions about that. It also seems to have very low output so I hope to get a bit of direction on what may be the culprit.

THE AMP: Circa. 1965 GA-5T, 6EU7, 6CA, 2xEL84. Per factory schem this little guy puts out 12.5 watts. Right now I think it just might be putting out 1 watt or less. It doesn't sound bad, nice trem, nice tubey chimey highs and decent bass. A little bit of hum but not too bad on that either.

MY EXPERIENCE: I have been building pedals for 7 years or so and am getting into amps. I can work on an amp safely but have limited knowledge on what really makes them tick.

FACTORY SCHEM WITH VOLTAGE READINGS:
http://www.informatik.uni-bremen.de/~dace/gg/ga-5t_skylark_white-panel__back_qual6.jpg
http://usera.imagecave.com/vortex/ga-5t_factoryschem.jpg

I've spent time making a LAYOUT WITH THE ACTUAL VOLTAGE READINGS to compare...

POWER SUPPLY:

http://usera.imagecave.com/vortex/GA-5T-PowerFW.jpg

TUBES AND BOARD:
http://usera.imagecave.com/vortex/GA-5T-Layout--VoltagesFW.jpg

POWER SUPPLY QUESTIONS:
1) The two rectifier diodes have been replaced with 1N4001's. Can they handle this much voltage? If not, what are suitable replacements?

2) There are 4 -20uF filter caps in the schematic but only three were used in the "cap job". The 20uF cap for the preamp stage (C13A)was omitted. Is this an OK way do do things or was the cap job a hack job? BTW the work looks cleanly done, but I noticed the "death cap" was still in place after the grounded power cord was installed?

3)The resistors after the B+ have been modified (1K down to 22 ohms and 3.3K down to 330 ohms) is that kosher?

TUBE AND CIRCUIT VOLTAGES:
How do these look? (See Tubes and board image) To my untrained eye the specified voltages seem to be pretty close to the factory schem. I measured what I could, where I could, with as much precision as my digital meter allowed. I am sure I must have some redundant data there but I don't know how to filter this information, so I included it all. I was surprised to see so many AC readings in the circuit but mostly everything I have built is DC. If there is something I have missed let me know and I will measure it.

WHAT I HAVE DONE: I plugged the GA-5T into a 1x12 -8ohm cab and it did provide a bit more volume and bottom end, I think that the original speaker is in OK shape. I just ordered some new tubes and a 20UF 500V cap. I tried substituting the EL84's with some spares I had kicking around and the amp sounded the same. I don't have these preamp tubes on hand to substitute... yet. I wonder if the volume problem isn't with the original OT wearing out, but I thought if an expert here spotted something in the data I have gathered I would follow that up first. What would be a good replacement OT in that event?

Well, I think that's it (probably not...). Sorry for so many questions! I feel like a "gopher", able to collect the data but not able to apply it. I really would love to know how an amp tech would approach this situation. Any insights appreciated.
Many thanks in advance!

[Chuck H]

O.K. Thats a pretty old amp. So who knows when that cap job was done. That amp could easily have been capped twice in it's lifetime. So if that's cap job number one, it may be due again. But I don't think the caps are to blame for the low volume issue. Not directly anyhow. Perhaps the guy that rewired that amp has made other mistakes as well.

Your schematic image is impossible to read and your interpetation drawings are unclear. If the amp were wired as the drawings suggest it would not be working at all. So be sure you understand the schematic, at least well enough to duplicate it, before doing any work to this amp. Also, be sure to drain the caps before you start.

"The two rectifier diodes have been replaced with 1N4001's. Can they handle this much voltage? If not, what are suitable replacements?"

No. If those diodes ARE 1n4001s they should have failed long ago. Replace them with 1n4007s

"The resistors after the B+ have been modified (1K down to 22 ohms and 3.3K down to 330 ohms) is that kosher?"

No. Some hairbrained attemt to raise the preamp voltages or some guy working with an analog meter didn't know how to read it.

"There are 4 -20uF filter caps in the schematic but only three were used in the "cap job". The 20uF cap for the preamp stage (C13A)was omitted. Is this an OK way do do things or was the cap job a hack job?"

I can't be sure what was going on when this "work" was done. Hack job doesn't begin to explain some of what your finding. You should rewire the amp to match the schematic. Check the entire amp. Anything you find that is not stock should be changed to match the schematic. Only because, 'who knows whats going on in there now?' The only exception to the schem should be the three prong power cord. If you want to mod the amp after that, you can get good advice here.

"but I noticed the "death cap" was still in place after the grounded power cord was installed?"

Clip it out. It can cause problems if it shorts.

Chuck

[vortex]

Thanks very much for the recommendations Chuck.

Your schematic image is impossible to read and your interpetation drawings are unclear.
I changed the image links, it seems that Imageshack isn't reliable, sorry about that. Things should be easier to read now.

If the amp were wired as the drawings suggest it would not be working at all. So be sure you understand the schematic, at least well enough to duplicate it, before doing any work to this amp. Also, be sure to drain the caps before you start.[/I]
I have gone through the schematic many times and made these layout files. The amp does work, and the wiring matches the schematic with the exceptions noted. Is there something you see in the layouts that looks way off? Or is there a better way I could represent my findings?. (note: i did skip the off board wiring).

Also, be sure to drain the caps before you start.
Yes, I certainly do, thanks for mentioning.

No. If those diodes ARE 1n4001s they should have failed long ago. Replace them with 1n4007s
I rechecked the rectifier diodes and indeed they are 1N4007. They were very difficult to read in circuit.

I will get the parts to return the amp to stock. I just needed to know that whoever had done the resistor changes and cap omissions wasn't operating with some "greater wisdom".

Death Cap has been clipped out.

Thanks again for the guidance.

[Chuck H]

Woah...I can read it now. Ha Ha

Looks like a nifty little amp. Unfortunately I don't think the volume issue is because of the cap job shinanigans (how do you spell that?). That should just cause noisy operation the way it was wired. So you may have a bad or loose componant/s or some other wiring mishap that you overlooked.

If you have a signal generator (or anything that can make a consistent signal) to put into the input of the amp, you can take AC voltage readings along the signal chain and hunt down the problem pretty fast.

Chuck

[vortex]

I don't have a signal generator but the my old Micromoog on "drone" fitted the bill!
Here's my readings along the signal chain. Let me know how it looks and if you need more info.
Thanks again!
http://usera.imagecave.com/vortex/Signal-Chain-AC-voltagesFW.jpg

[rf7]

Those readings at the speaker make me think it's a bad output transformer or speaker. Have you looked into that? Have you tried a different speaker? Go to www.geofex.com and look at the tube amp debug section for info on how to check out the OT.

The place where it goes down to 0.6mV is questionable too. It seems a bit low to me there, although you're certainly getting some signal into the OT.

[Chuck H]

Wow...That was fast.

O.K, I see what could be a couple of problem areas. There are real geeky formulas for figuring the amplification factor of a gain stage but I don't know them. With that preface, I'll continue.

You should probably have at least 5.5 volts downstream (signal wise) of c3. Could just be a weak tube, or a problem further downstream is robbing signal. I don't think the problem is here. So taking the voltage we do have as a staring point, I'll move downstream.

Your dropping alot of voltage through the bass control circuit, but thats normal. But your dropping way too much past that 470k resistor (r8). Something is bleeding signal to ground downstream of that resistor. Check r10 (250k) to be sure it's the correct value. You may aslo want to check c6 to be sure it's not shorted. less than 1millivolt is a truly tiny amount. Your guitar puts out generously more. So why the extra gain stages??? Right?

Moving downstream.

At the grid of the 6c4 you have .6mv. But at the plate you have 326.5mv. Thats a voltage gain of over 500, which is impossible. So something went wrong here. Either you had your meter set funny, read it wrong or some other AC is getting into the chain here...But, let's take that for what it is and move downstream.

Going into t3 primary you have 326.5mv. But at the secondary you only have 17.5mv. I'll admit that I've never worked with an interstage coupling transformer as a phase inverter (though I've wanted to try building with one) but this seems like alot of loss to me. I would have expected it to be just shy of unity gain. At the end of the chain so far you would normally want to be driving the grids of those el84s with about 8 to 12 volts at full tilt.

Last thing. You have indicated .1mv at the + speaker lead and 32mv at the - which is gounded. In a normal world there is no possible way that can happen. Ground as it is indicated should be 0 volts AC or DC. And .1mv of drive at the speaker should be barely audible.

You may have missed something that has been changed inside the amp. If it were mine I would start by fixing the power supply and B+ rail. When all my DC was as it should be I'd move onto the signal path.

Chuck

[vortex]

Chuck: Wow...That was fast.
You are pretty fast yourself! For me to be able to get this kind of info in a relatively remote part of Canada (Whitehorse, YT) is quite amazing. I owe you dinner at least...

rf7:
I've tried the amp with a very efficient 8 ohm speaker and the results were about what you would expect, a bit more power but not a mind blowing leap in performance. I think that I have ruled out the speaker as a problem.

I'll check out Geo for the OT testing. RG Keen deserves a medal! I can't believe how helpful he has been to me and everyone else in the pedal world.

I have an old Tweed Deluxe that I have cherished for years and the OT was wearing out through many of those years. It still is the best sounding amp I've heard even though it was only putting out about 2 watts near the end. I had the feeling that this might be the problem with this Gibson GA-5T amp, it still sounds pretty good but it doesn't have nearly the amount of poop it should. I had my Deluxe repaired by Dave Vidal in Vancouver, BC and it is still my main amp to this day. I can't say enough good things about Dave, we've never met but I can say that he is a first class act all the way.

Ok, so there's lots of things for me to check out. I have ordered all the necessary parts and they should be here for the weekend. In the meantime I will scope out what I can.
Thanks again for everything!

[Chuck H]

FWIW I don't think it's the OT. There are some funny readings there, but not the kind of thing I'd expect if the OT was going. Lots of stuff to check first. Heres a neat test technique that has worked well for me. Once you get the power supply dealt with you should try it. Use you "signal generator" (moog) set to "drone" and measure the output AC voltge with your dmm. Set it for .5 or 1 volt if you can just to make math easier. Now, starting at the grid of the 6c4, inject the signal there. Probably going to surprise you how loud it is. Then move back to the signal path input of the 6eu7. I think this will show the problem is somewhere in front of the 6eu7 plate and behind the 6c4 grid. But thats just a guess. You can inject signal anywhere that doesn't have B+ in the circuit (meaning, in front of decoupling caps). For example, you should not try to inject signal at the t3 (coupling tranny) input. There is B+ there. But you can do it at any point in the signal path that is downstream of a decoupling cap up to the grid it is feeding. This is a fasr way to locate some problems.

Chuck

[Enzo]

If your readings are correct, it looks to me like R8 is open.

[Chuck H]

OOOh. Good call. I didn't consider it because I've never seen a resistor go open without being burned to a visible crisp. But I know it happens. Could be a loose solder joint or a lead short too.

Chuck

[Enzo]

Those PI plate resistors in Fender Hot Rod Devilles and similar open often enough without appearances.

[vortex]

Hey, a small success today!

I found a splash of solder under the Volume pot (R10) lugs that was connecting pins 2 & 3. I removed the solder splash, tested the pot and put her back in. I tested the amp and it has a bit more volume but nothing near 12 watts. So that's helped a bit. I did the signal generator tests and am now getting much more AC after R8 (470K), unfortunately the readings further downstream didn't change. In fact when I tested the plate and grid of the 6C4 I couldn't get a reading. I still am getting AC readings on the speaker leads.

"You may also want to check C6 to be sure it's not shorted."
Checked it: seems OK

OT Tests:
OK, so the plot thickens:
I started going over the offboard wiring and checking the OT. The first thing that I discovered is that the OT model # doesn't match the schematic. I missed that. The schematic specifies a TF-18-01 but the OT in this amp is TF 504-0. It actually appears to be stock, but I only say that because it is riveted to the chassis and there is no evidence that it had been modified when installed.


I performed the "simple tests" for OT's outlined at Geofex.com.I pulled the tubes but I didn't pull the rectifier tube as there isn't one, so this might skew the results.

The measurements from the B+ to the primary leads (output tube plates) were encouraging (137 & 153 ohms). I checked the resistance from the chassis to the leads and things seemed weird. My digital multi meter was having a hard time getting a reading, (this has never been a problem before). The readings from the chassis to the B+ and primary leads (all around 8M) would read for a few seconds and then would not provide a reading. The speaker leads showed 1.5 and 1.8 ohms. So either the OT is toast, the tranny is a poor match or something else is creating a problem? I pulled the tubes but I didn't pull the rectifier tube as there isn't one, so this might skew the results. Should I have disconnected somewhere else?

[vortex]

If your readings are correct, it looks to me like R8 is open.

Hi Enzo, I just checked R8 "in circuit" and got a 500Kish reading.

[Enzo]

Well, the shorted R10 would have the same effect: killing the signal at the righthand end of R8.

Your transformer is probably fine. Your meter is reading the B+ caps. Resistance to ground from the OT promary leads has the B+ caps across it. The little voltage from your meter is trying to charge those caps. So what usually happens is at first you get some resistance, and then it goes higher and hiogher until it shows open. You could disconnect the center tap wire from the B+ to check for fframe shorts. If the frame were shorted to the windings, it would still measure short on the meter. I suspect it is fine. COnnect your meter to measure resistance from the transdformer leads to ground as you already did, and leave them connected. Your resistance would rise up and then no reading. Leave it that way. now with a wire, short from B+ to ground a moment. This will discharge the caps. When you pull the wire away, watch the reading ramp up from something to ever higher and then open. That is what you are seeing.

This is a Gibson amp, they usually do not match the schematics, I am surprised when they do even. I am sure the transformer is suited to the amp.


If you get no signal at the 6C4, it is pointless to look for any output, if it is not there, it will not be anywhere after there either.

YOu now have signal at R8? Fine. Is there signal now at the top of R10? Turn R10 all the way up. Is there now signal on the wiper? Better be the same on the wiper all the way up as there is on the right end of R8. And since the wiper is wired direct to the grid of C4, there should be the same signal there. R10 could be faulty.

The 6C4 lights up, but are the DC voltages OK? Is there 8 or 9 volts ( or 7 or 12) on the cathode there? If there is zero, then the tube is not working.

If the tube is indeed working, then you need to get signal to its grid. In fact, just touching its grid with a screwdriver should inject some hum into the amp and you would hear it coming out the speaker. If the power amp is working. Monitor signal at the plate and touch the grid with a screwdriver. Can you measure an increase in signal at the plate then? You should.

You show 300mv at the plate of the 6C4 - that could be signal or ripple. Measure for signal at the other end of that transformer - the end that goes to C13B. There would be no signal there, so any "signal" you see would be ripple.

Turn the intensity control to zero. Now with power off, measure resistance to ground from each power tube grid. Is there continuity? We are now checking for the phase splitter transformer secondary. I don't know what resistance to find there, but it should not be open.

[Chuck H]

Enzo is right. If r8 is fine and soldered in place proper then you should have AC at the grid of the 6c4 unless r10 is bad or wrong somehow.

Chuck

[vortex]

I ran the signal generator test again and I am still getting a large voltage drop on R8. I ran the test using 0.5V input. I measured 10.8V just before R8 and 1.46 directly after. I made a new chart, see below.

"You now have signal at R8? Fine. Is there signal now at the top of R10? Turn R10 all the way up. Is there now signal on the wiper? Better be the same on the wiper all the way up as there is on the right end of R8. And since the wiper is wired direct to the grid of C4, there should be the same signal there. R10 could be faulty."

I am reading the same voltage at the R10 wiper (pin 2) and the grid of the 6C4 when R10 is turned "up to 10" .

"The 6C4 lights up, but are the DC voltages OK? Is there 8 or 9 volts ( or 7 or 12) on the cathode there? If there is zero, then the tube is not working."

I am getting 8.94 volts DC on the 6C4 cathode

"If the tube is indeed working, then you need to get signal to its grid. In fact, just touching its grid with a screwdriver should inject some hum into the amp and you would hear it coming out the speaker."

I tried this and I hear buzzing through the speaker when contact is made.
I can't get a reading on the 6C4 plate, it seems to read 325mv and then I get no reading. There is a pretty good crackle when I touch the 6C4 plate lug, though it crackles on the initial contact.

"You show 300mv at the plate of the 6C4 - that could be signal or ripple.

Ok, the voltage reading I got for the plate is another one of those fleeting readings I see the value for a second and then I cant get a reading, i do get the similar value if I try the reading again.

"Measure for signal at the other end of that transformer - the end that goes to C13B. There would be no signal there, so any "signal" you see would be ripple."

Another fleeting reading, I get 360mv for a second. Seems close to the plate reading. I measured the resistance on T3's coils and put them an the chart.

"Turn the intensity control to zero. Now with power off, measure resistance to ground from each power tube grid. Is there continuity? We are now checking for the phase splitter transformer secondary. I don't know what resistance to find there, but it should not be open."

No continuity, no resistance reading either.

Thanks again for all your help!

http://usera.imagecave.com/vortex/sig-gen-AC-voltages2FW.jpg

[Enzo]

OK, you have 1.3v signal at R10 wiper, but next to nothing at the grid of V2 pin 6. They aer connected by a wire, they should be the same. Either your wire is broken, the solder has failed, the wire is not connected properly, or the 6C4 is loading it down somehow. No if that last were the case, it would load down R10 also.

Pull V2 6C4 and don't stick it back in until you can get a signal at pin 6 of its socket.

The secondary of phase splitter T3 is wired to the output tube grids. You show it to have 3.7k across and close to 2k on each half. Good. But does not the center tap follow that wire down off the image to the intensity pot? And if that pot is all the way down, shouldn't its wiper be at ground or very close to it? So then shouldn't you get a resistance reading from T3 to ground? Specifically close to 2k from the grid of either 6BQ5 to ground. A bad intensity pot could leave your power tube grids floating. Not much output that way.

If your voltage readings are intermittent, then use clip wires to connect the meter probes to the circuit and keep hands off.

Turn the tone control all the way up. No point in having it roll off your test signal.

I don't think your T3 is bad, but the fact you get the same reading on both ends of its primary - even though it comes and goes - says to me it isn't your signal you are seeing.

[Chuck H]

Enzo, Even though it isn't indicated in the diagram, Vortex did mention above that with r10 up full he now has the same voltage reading across that circuit.

"I am reading the same voltage at the R10 wiper (pin 2) and the grid of the 6C4 when R10 is turned "up to 10""

Vortex, Have you repaired the power supply yet??? Don't just change the resistor values and stick a cap in there. Actually remove the whole mess, wires and all, and rebuild it to match the schematic. That way you know that everything is going where it should be.

Something that helps trace AC is to place a largish (1uf @ 600 volts) film cap in series with the red probe. Just build the thing with tape on a chopstick or something. The capacitor makes sure that only AC can reach your probe.

Chuck

[vortex]

Good evening gentlemen. Finally, my parts have arrived and I am currently drafting out how to lay out the power supply and filter caps in a clean fashion. I am working on the premise that I can assume nothing and started with the basic AC wiring from the wall plug. I have done a fair amount of searching on the net about converting a two prong plug to a three prong plug and it appears that the wiring in this amp is different than the best example I have found.
http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/Mods/Images/kalm1cord.gif
I traced what I have going on in this amp, do you see any problems that this "original" wiring would create? As always, thanks!
http://usera.imagecave.com/vortex/ac-WIRING-fw.jpg

[vortex]

Re: preceeding post about AC wiring.
I rewired to match http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/Mods/safe.html#ground

[Chuck H]

Well in your initial drawing the pilot would remain on in the event that the amp fuse blew. In the second drawing it looks to me like the lamp is wired into the circuit after the fuse. So if the amp fuse blows you will have no lamp. Either drawing is fine as far as I can tell. But I do prefere to have my pilot lamp remain lit even when the amp fuse blows. That way I know it's the amp fuse and not the cord or the wall socket or the house fuse.

Chuck

[vortex]

"Vortex, Have you repaired the power supply yet???"
Yes, The PS is rebuilt with new parts. Tested and working.

"Something that helps trace AC is to place a largish (1uf @ 600 volts) film cap in series with the red probe."
Thanks for the tip! I built one of these and have used it for all subsequent AC testing.

"Pull V2 6C4 and don't stick it back in until you can get a signal at pin 6 of its socket."
OK, the 6C4 is out and the test results you see below are the results.

"The secondary of phase splitter T3 is wired to the output tube grids. You show it to have 3.7k across and close to 2k on each half. Good. But does not the center tap follow that wire down off the image to the intensity pot? And if that pot is all the way down, shouldn't its wiper be at ground or very close to it? So then shouldn't you get a resistance reading from T3 to ground? Specifically close to 2k from the grid of either 6BQ5 to ground."
I tested this and am getting 1.97 K.

When you have a chance please have a look at the voltage readings I have collected. The new PS is putting more voltage on the preamp tubes and quite a bit more voltage on the tremolo section. I am still getting AC readings on ground points. These ground readings jump around on my digital meter but hopefully they are telling us something. Er, I mean they will tell you something and you can tell me. Ha ha.

http://usera.imagecave.com/vortex/ps-REBUILT-ac-VOLTAGES-FW.jpg

[Chuck H]

O.K. The most glaring inconsistancy is that you show 3.5v on the wiper of R10 but .4mv on pin 6 of the 6c4 socket. The schem shows a hard connection between these two points so there should be 3.5 volts on pin 6 of the 6c4 socket. Something has to be amiss here. Turn off the amp and drain the filter caps for this next test. Use your dmm to test resistance between the R10 wiper and pin 6 of that socket. If it doesn't show 0 ohms or very damn close to it then you have a break in the circuit, could be in the lead or a solder joint. It could also be that R10 only works when you apply pressure to the wiper pin with the probe and is not making contact when nothing is applying pressure to it. With R10 on "10" and looking at the back of that pot, test resistance between the right hand leg of R10 and pin 6 of the 6c4 socket. Just for a lark test resistance between pin 6 of the 6c4 socket and ground.

Chuck

[Enzo]

Exactly.

Once you get signal to that grid pin, you can stick the 6C4 back in.

No matter what you work on, ther is ALWAYS some little voltage anywhere you care to measure. On AC for example, your meter leads act like little antennas picking up ambient noise signal. Half a millivolt is close eenough to zero to be called zero.

And when you are measuring voltages at "grounds," where then is your meter black probe connected? if you are usuing the chassis, then it is not rare for there to be tiny differences at various ground points. The chassis is very low in resistance, but not zero, so the little currents flowing through it cause a voltage potential difference. \

If readings jump around, two things come to mind. One is to use a clip wire, a test lead. CLip it to chassis or whatver ground reference you are using, and clip the other end to your black probe tip. Now you have a steady stable connection to ground. Moving probes makes readings vary. You can also use a clip on the red probe. Needle sharp probe tips help.

The other thing is what is really happening on the meter. If you are reading 6v and it reads 6.02, 6.09, 6.03, 6.06, etc, step back a pace and look at the big picture. You are getting 6 volts, just the last digit is not stable. But the variation is only a couple hundredths of a volt! Could be because your probe is moving, but most likely than not it is the power mains bumping around. COnnect your meter on AC to the wall outlet and watch that 120v move around.

Now if the reading is 6v, 8v, 3v, etc, then something is up.

[vortex]

"Use your dmm to test resistance between the R10 wiper and pin 6 of that socket. If it doesn't show 0 ohms or very damn close to it then you have a break in the circuit, could be in the lead or a solder joint. It could also be that R10 only works when you apply pressure to the wiper pin with the probe and is not making contact when nothing is applying pressure to it."

I am reading 1.6 ohms.

"With R10 on "10" and looking at the back of that pot, test resistance between the right hand leg of R10 and pin 6 of the 6c4 socket."

Assuming you mean Pin 3, I am reading 81 ohms. (Pin 1- no reading)

"Just for a lark test resistance between pin 6 of the 6c4 socket and ground. "
I am reading 275K.

"And when you are measuring voltages at "grounds," where then is your meter black probe connected? if you are usuing the chassis, then it is not rare for there to be tiny differences at various ground points. The chassis is very low in resistance, but not zero, so the little currents flowing through it cause a voltage potential difference."

I have the black probe clipped to the chassis. Is there a better place to connect to ground? I do have a set of clip on leads for my probes but they are a pain as they are rather short. I usually use the black clip on for ground and probe around with the red "needle" probe.

"The other thing is what is really happening on the meter. If you are reading 6v and it reads 6.02, 6.09, 6.03, 6.06, etc, step back a pace and look at the big picture. You are getting 6 volts, just the last digit is not stable. But the variation is only a couple hundredths of a volt! Could be because your probe is moving, but most likely than not it is the power mains bumping around. Connect your meter on AC to the wall outlet and watch that 120v move around."

Great !, this "big picture" info is exactly what I need to be familiar with.

[Enzo]

Clipping to chassis as ground is fine, I wanted you to be CLIPPING it there instead of holding it, and it sounds like you are. Just ignore readings like 1/10mv, they are irrelevant.

[vortex]

OK, unsoldered the wire connecting pin 6 (6C4) to the wiper of R10 at the wiper. No tube in the 6c4 socket. Measured 1.6 ohms accross the wire to pin 6. measured Pin 6 to ground, no reading.
Pin 3 of Vol to ground (Vol on 10-275K, Vol on zero -no reading).
Pin 2 of Vol to ground (Vol on 10-275K, Vol on zero -19 ohm)
Pin 1 of Vol to ground - 1.5 ohm

So, does this mean that the Vol pot is the problem?


I have a 250K audio pot I can substitute. It is a guitar pot (Profile brand) I have no specs on the pot. Is there any danger that it will not be able to handle the wattage?
Thanks!

[Enzo]

There is not wattage to handle, any pot type would work there.

If you do not get a reading of somewhere in the 250k-300k range from one end terminal to the other of the volume control, it is bad. Disconnect the wires from it to make the test reliable. The center terminal is the wiper of the control, and the two end terminals are at the ends of the resistive strip.

Your guitar pot would work fine. It may not fit the knobs or something, but to get the amp working, try it. You can always get a more suited control later.

[vortex]

Good news! I tested both the wire connecting R10 to Pin 6 of the 6C4 and R10 (Vol) itself and both appeared to be in good shape. Despite this finding I replaced both R10 and the wire connecting R10 to Pin 6 of the 6C4.
Now I am getting the full 3.5 watts voltage from the wiper of R10 to the grid of the 6C4.
I have been testing the amp and it definitely has more volume. Not Blues Jr. type of volume but much more than it had originally. I swapped in the new tubes I purchased. The Sovtek 6eu7 replacement I bought has some issues so I am limited to using the NOS 6EU7 that came with the amp. The new JAN Phillips 6C4 seems good. The amp came with a set of JJEL84's, these tubes are decent and have a nicer high end than the Sovtek EL84's I bought to replace them.

I do notice a bit of crappy distortion on the transients. It sounds like tube noise to me. Do you think that there is something else up with the voltages I have posted?

Now I am getting 45volts on the plate of the 6C4 and 45/53volts on the grid of the EL84's. I can't get a reading on the plates of the EL84's.

Thanks!

[Chuck H]

"Now I am getting the full 3.5 watts voltage from the wiper of R10 to the grid of the 6C4."

Good, perfect.

"it definitely has more volume. Not Blues Jr. type of volume but much more than it had originally."

Have you tried both amps through the same speaker? Speaker eficiency can play a big role in overall volume. That amp should be about 3/4 as loud as the Junior through the same speaker. Otherwise there is still work to do.

"I do notice a bit of crappy distortion on the transients. It sounds like tube noise to me." AND "The Sovtek 6eu7 replacement I bought has some issues so I am limited to using the NOS 6EU7 that came with the amp."

Can you be more specific?


"Do you think that there is something else up with the voltages I have posted?" AND "Now I am getting 45volts on the plate of the 6C4 and 45/53volts on the grid of the EL84's."

Now that you've repaired the power supply and there is some signal getting through the amp it's probably time test the DC voltages. But 45 and 53 volts AC on the grids of the EL84s would be too much and seems odd to me. We may get back to that.

"I can't get a reading on the plates of the EL84's."

Thats because there shouldn't be any AC there. The voltages indicated on the schem are DC volts. They are marked for location. Test and compare.

Chuck

[vortex]

Now I am getting the full 3.5 watts voltage from the wiper of R10 to the grid of the 6C4."

Good, perfect.
Yes , many thanks to both you and Enzo!

"it definitely has more volume. Not Blues Jr. type of volume but much more than it had originally."

Have you tried both amps through the same speaker? Speaker eficiency can play a big role in overall volume. That amp should be about 3/4 as loud as the Junior through the same speaker. Otherwise there is still work to do.


This is a subjective opinion on my part. I've cranked a friends Blues Jr. in a small studio room and it was "I should get some earplugs" loud. I used the Blues Jr. as a comparison due to the similar output tube configuration. I am realistic and realize that there are going to be differences in the circuit design and components between the two. If this Gibson is rated at 12.5 watts I would say that it is still lacking in poop. Right now I would compare it to a Vibro champ in volume. I have to say that the amp sounds good, nice chime and depth. It doesn't get any kind of wound up overdrive on 10, perhaps this was Gibsons intention...
I did test the amp with it's original 10" speaker, which is a good sounding alnico but I switched up to a 1x12 cab with a 8 ohm Celestion 80 (fairly efficient) that I am more familiar with for a better impression of the amps range.

"I do notice a bit of crappy distortion on the transients. It sounds like tube noise to me." AND "The Sovtek 6eu7 replacement I bought has some issues so I am limited to using the NOS 6EU7 that came with the amp."

Can you be more specific?

I tested the amp with a different guitar today and wasn't able to duplicate the transient distortion. What I was hearing last night sounded like some gain stage was being hit too hard and there was a bit of "kacking" on the transients. Bad distortion, not good distortion...I've had this happen with other amps before and it was fixed by swapping out the preamp tubes. Good results today but I will keep on top of this. The test guitars were both decent strats with single coil pu's.

Regarding the "new" Sovtek 6EU7, when I swapped it in, it was immediately noisy. Also a bit disturbing was that the Sovtek's pins seemed to be slightly wider spaced and was not a smooth fit for the socket. I've seen this before with an old Ampeg Reverborocket where the Sovtek 7591 tubes fit poorly and were prone to falling out! A NOS 7591 fit like a glove...

"Do you think that there is something else up with the voltages I have posted?" AND "Now I am getting 45volts on the plate of the 6C4 and 45/53volts on the grid of the EL84's."

Now that you've repaired the power supply and there is some signal getting through the amp it's probably time test the DC voltages. But 45 and 53 volts AC on the grids of the EL84s would be too much and seems odd to me. We may get back to that.

I've gone through and documented the DC voltages , see below. DC indicated in Red, AC in blue. The 45 and 53 volt readings on the EL84 grids are with the 0.5 volt signal input and the Vol maxed. In general the DC voltage readings look promising with the exception of the tremolo section. Please have a look and let me know what you think and if there is any other information I can provide.
http://usera.imagecave.com/vortex/voltages-post-vol-pot-repla.jpg

[Chuck H]

"If this Gibson is rated at 12.5 watts I would say that it is still lacking in poop. Right now I would compare it to a Vibro champ in volume."

Many Gibsons are not very loud. Not sure why. But I built an amp not long ago using salvaged parts. The PT puts about 300 volts on the plates of a pair of 6V6s. The OT is quite small. About the size of three nine volt batteries placed beside each other. I'm pretty sure that it's straight wound too (= no interleaves). My point is this. The amp I built is similar enough. Probably around 12 watts, kinda cheesy OT and lowish plate voltage. That amp is about twice as loud as a VibroChamp. By the schem I would think yours would be too. I just worked on a VC for a friend and as usual it was about as loud as a shout.

One thing still looks funny (to me anyway). The high AC on the power tube grids. At full clip I would expect to see less than 25 volts on each of of those grids. From a design perspective you can really pound on power tube grids when using transformer as a phase inverter. But in that era most designers didn't want power tube distortion. My guess is that in your circuit those power tubes should start to distort with about 10 volts or less on those grids. So, considering the amp, 50 volts seems insanely high. But I've never designed with an interstage transformer before or ran a trem into one for bias modulation. So I could be missing something here. Perhaps Enzo will see this and help out.

By the way, How well does the trem work? Could be key. Also, What is the DC on the ungrounded end of R19 with the amp idling and the trem off?

Chuck

[vortex]

By the way, How well does the trem work? Could be key.

Chuck

The trem works well and sounds quite beautiful. A nice warm pulse.

Also, What is the DC on the ungrounded end of R19 with the amp idling and the trem off?

I measure 10.5 volts.

[Chuck H]

Well...Hmmm.

Everything looks fine with the DC voltage. The bias point is near dead on, but we can't let that fool us. so:

Are there any circuits built into the amp that arent on the schem. I'm looking for a negative feedback loop that would be connected to the speaker output via a resistor.

And:

Check C12 to make sure it's in place and in good working order. You would need a meter that can read over 20uf capacitance to do this. Otherwise you could just replace C12 with a new cap to be sure. I might use a 47 or 100uf for that instead of 20uf. If that cap is missing or open it would reduce gain. It's a polarized cap so you need to make sure the + is on the tube socket end and the - is to ground.

And:

How old are the EL84s? Pull one of the power tubes and remeasure the DC on the ungrounded end of R19 with the amp idling and so signal input. Then swap the power tubes so the other one is out and measure again. If it turns out that one tube is doing most of the work you could get much louder with good matched tubes.

Thats all I have for now.

Chuck

[vortex]

Are there any circuits built into the amp that arent on the schem. I'm looking for a negative feedback loop that would be connected to the speaker output via a resistor.

No extra circuits, the amp matches the schem.

Check C12 to make sure it's in place and in good working order. You would need a meter that can read over 20uf capacitance to do this. Otherwise you could just replace C12 with a new cap to be sure. I might use a 47 or 100uf for that instead of 20uf. If that cap is missing or open it would reduce gain. It's a polarized cap so you need to make sure the + is on the tube socket end and the - is to ground.


I did replace the C12 when I redid the power supply. Orientation is correct. If I replaced the 20uF with a higher value what would be the result?

How old are the EL84s? Pull one of the power tubes and remeasure the DC on the ungrounded end of R19 with the amp idling and so signal input. Then swap the power tubes so the other one is out and measure again. If it turns out that one tube is doing most of the work you could get much louder with good matched tubes.

I did this test and measured 8.5 volts with one tube and 9 volts with the other. I did swap in a new set of matched EL84s and there wasn't any appreciable volume increase.

Maybe this is all the volume I am going to get out of this amp. The amp sounds good.

Once again, thanks for all your help, much appreciated!