Ampeg Gemini VI--Anybody PLEASE have a schematic with voltages?

[Mr Johnny Birchwood]

Howdy folks. Long-time forum browser, first time poster here. My current project is an Ampeg Gemini VI rebuild. I have the widely available Joe Piazza schematic, but it does not have voltages at any of the filter caps/supplies and no voltages at the tube pins. My Gemini VI was pretty thrashed when i got it--shorted power tube, bad PT full of carbon conducting 119vac intermittently to the chassis, hack-job ac filter 'repair' from a previous owner that cooked when the power tube shorted(I'm sure the 25amp fuse they installed didn't help the situation.) diodes were cooked, some resistors were cooked. thing was just a mess. I have new matched pair of EH 7591's, good old 12ax7's and 6cg7s, and a good selection of 6u8, 6gh8 type tubes(I rewired the 7199 socket for the cheaper tubes, the 7199 that was in it showed substantial grid leakage). New diodes and filter caps. Replaced the coupling caps and all of the other old electrolytic caps with new orange drops. New bias diode and bias cap. Bolted a power transformer from a Hammond A-100 to the chassis and wired it up. I am still getting a weak and grainy sound out of it. Power tubes are matched and the bias set to the spec -50v. If anybody has a schematic with proper voltages on it, or if anybody has a good working Gemini VI and can take voltage readings at the filter caps and tube pins I would very much appreciate it. I've had this amp on the bench for 2 months now and I just want it to work.

[CornElvis]

What I normally do is : write the factory an email, so in your case Ampeg.
Normally they will reply to you within one or two days and in all my cases they have sent me the service documents, just like that.
Just tell them you bought one and it needs repair or something like that.

Greetzzz,

CornElvis

[T9c]

There is one in The Tube Amp Book by Aspen Pittman, but it doesn't exactly match the Piazza schematic...close, though.

[Mr Johnny Birchwood]

Thanks guys! I just got the schematic from Ampeg Tech Support. It doesn't have all the voltages but it has enough to make some progress I hope. The hammond A-100(that's a 6V6/12AX7/12AU7 chassis) power tranny is nearly spot on for this application. Filter cap voltages are within 1-2% of spec. I have 318v on the plate of the pentode section of the drive tube(the 6u8/6gh8 I swapped for the 7199) and the schematic calls for 125v. Fixing that should help. The power tube plate voltage is 426vdc with a -50vdc bias. The schematic doesn't give plate voltages for the power tubes, but the value I show is within the range I would expect for a 7591A(360-420VDC) I will post an update when I have the amp closer to spec V's.

PS--anybody have experience with the 6u8/6gh8 swap for the 7199? I have access to a good supply of these tubes. Got a 6an8 too that I can plug in. I want to get this thing working good before I drop in my pair of vintage RCA 7591's

[Mr Johnny Birchwood]

The power tube plate voltage is 426vdc with a -50vdc bias. The schematic doesn't give plate voltages for the power tubes, but the value I show is within the range I would expect for a 7591A(360-420VDC) I will post an update when I have the amp closer to spec V's.

So..I got the voltages in line with spec and fired it up. It was still grainy and weak sounding. The Piazza schematic lists the grid bias as -50VDC but I've got it dialed in at -22VDC now with 436VDC on the plates and it sounds great, like God's Own guitar amp. I'm only pulling .556VDC/.020A at the cathodes of the 7591's, and the Ampeg schematic says I should be pulling .080A. The amp sounds amazing. Any thoughts on the difference?

btw.. somebody sawed off the speaker cab from the chassis of the Gemini VI, which was found in the parking lot of a strip-club in LA before it came to me(reverb tank still attached) so I'm running it through a homemade cab(picture below) with a 1960's Oxford AlNiCo 6X9 I pulled from a PA cab found by the side of the road and a Russell AlNiCo 12-incher I got for free(buddy pulled it from a mono-console unit). Puts big bucks 'vintage spec' speakers TO SHAME! HA!! Including the plywood, glue, and screws the cab cost me about 5 bux. I've got a 1959 Jensen P15R ribbed cone gold-frame organ-pull speaker I'm gonna use when I build a new enclosure to turn this thing back into a combo amp.

Further note on the tubes(picture below): On the left is my matched pair of old-stock RCA 7591 power tubes(labeled: 3M Revere Wollensack) and on the right are the new-manufactured Electro-Harmonix 7591A's. Note the size difference. The EH tubes are a little louder and brighter, with an earlier breakup. With the same tone and volume settings on the amp and guitar, the old-stock RCA's were a touch quieter, and had a darker, more bass-heavy tone with less distortion. 436VDC on the plates, -22VDC grid bias, and .020A on the cathodes. The cathode reading was basically the same with the both the new and old tubes.

tubes: http://www.facebook.com/photo.php?fb...type=1&theater
cab: http://www.facebook.com/photo.php?fb...type=1&theater

[Enzo]

The amp sounds amazing? Then stop fixing it. DOn't worry that some voltage doesn't match a schematic.

[Mr Johnny Birchwood]

The amp sounds amazing? Then stop fixing it. DOn't worry that some voltage doesn't match a schematic.

I am gonna stop fixing it, now that it works. I'm just curious if anybody with more experience and knowledge than myself might have some insight on why the current draw through the cathodes is so much lower than spec, and why the -biasVDC is so much lower than spec, so that I might improve my understanding for future projects and problems.

[52 Bill]

...why the current draw through the cathodes is so much lower than spec, and why the -biasVDC is so much lower than spec, so that I might improve my understanding for future projects and problems.

Don't worry that something doesn't match what was printed on a schematic, especially an old one. Use the schematic as a guide, but not as a bible. Remember that when the schematic was drawn up the voltages were taken from a working amp. That amp could have had average voltages or higher than average or lower than average voltages.

When the original readings were taken they were probably taken with a VTVM that had an analog meter and a 20% tolerance factor. And modern line voltage may have increased since the time the amp was designed, throwing another variable into the mix.

Often an amp could go through a number of circuit modifications and when the schematic was revised there could have been mistakes made in the printing.

And then there are the deliberate mistakes or omissions that some companies used to keep the real circuitry a trade secret.

[Mr Johnny Birchwood]

This is all quite true. If i get a reading within 10-20% of spec I usually don't give it a second thought. If the dmm reading and spec are off by a factor of 2x or 4x and the amp works, then it makes me stop and think about it. I'm aware of the difference between the 117VAC line voltage in use when the amp was made and the 122VAC I read off my wall outlets. The thing works great, so no complaints. I just wonder if I made some other error somewhere and that the low grid bias and low cathode current draw mean that I am putting extra wear and tear on the tubes. The new EH 7591A's are only $40 a pair, but I don't want to trash my pair of old-stock RCA 7591's, that's all. Thanx for the insight.

[davohilts]

Hey Mr. Johnny,
I've got a Gemini6 I'm working on and have the same issue, no schematic with voltages. I'm wondering how you biased the amp. I'm thinking of adding a couple of 1 ohm resistors off the power tubes to ground to allow an easy bias method. Also I'm wondering if you used 500v caps for the filters, Joe's schematic calls for 600V 80mfd (i'm assuming 2-40's tied together). Do you have the info from ampeg in a form you could email me?? davo@clear.net
Thanks
Dave

Thanks guys! I just got the schematic from Ampeg Tech Support. It doesn't have all the voltages but it has enough to make some progress I hope. The hammond A-100(that's a 6V6/12AX7/12AU7 chassis) power tranny is nearly spot on for this application. Filter cap voltages are within 1-2% of spec. I have 318v on the plate of the pentode section of the drive tube(the 6u8/6gh8 I swapped for the 7199) and the schematic calls for 125v. Fixing that should help. The power tube plate voltage is 426vdc with a -50vdc bias. The schematic doesn't give plate voltages for the power tubes, but the value I show is within the range I would expect for a 7591A(360-420VDC) I will post an update when I have the amp closer to spec V's.

PS--anybody have experience with the 6u8/6gh8 swap for the 7199? I have access to a good supply of these tubes. Got a 6an8 too that I can plug in. I want to get this thing working good before I drop in my pair of vintage RCA 7591's

[stokes]

davohilts,if you are looking for filter caps try fliptops.net they have "cap kits" that closely match the can and axial caps in the original.You can fit axial replacements for the can,but it takes some squeezing to get them to fit under the chassis.

[davohilts]

thanks for the tip!!!

[Mr Johnny Birchwood]

thanks for the tip!!!

sorry it took me so long to respond. I emailed ampeg tech support with the model and serial number and they sent me a schematic with voltages on it in about 15 minutes. They told me not to distribute it and I won't, but you can get one just as easy. The 7591a's didn't sound good until -22 g1 bias and sound best @ -20. The hum balance pot can be used to determine idle current. My power transformer was a giant brick of carbon on the inside, so I repaired it with a hammond organ tranny. The 2ry AC is lower but it does not sag one bit. The original power tranny was higher voltage but saaaaaaged like a mofo, which is why it went up in smoke. Probly engineered for a price point. I used a replacement multi-section cap 40-40-20-20 450v or so from Antique Electronic Supply that cost $35. The first cap was 80uf but I replaced it with 2 350v 220uf caps in series for a 700v 110uf. Good luck finding 600v electrolytic caps, I scoured the interwebs and couldn't find anything close to that size for less than a million bux. Another option is oil filled motor run caps, which can be salvaged out of industrial freezers and such. It helps to know a qualified industrial refrigeration mechanic with a well stocked junk bin. 40uf uf caps are easy to come by, but are physically larger(twice the size or more) than electrolytics. THey are freakin' indestructible though. A 450vac run cap can easily handle 700dc. New you might pay 10 bux each or so. Btw the hammond tranny has a 5v winding so I'm toying with the idea of a 5y3 or 5r4 vacuum rectifier instead of the si diodes. Also, I rewired the 7199 socket for the cheaper 6u8a/6gh8a triode/pentode driver tube and EVERY ONE I have tried is microphonic. I will probably end up rewiring it again for either a 6an8 or 6gn8(i have a few of each.) Great sounding amp, the Gemini VI. Great reverb, great tremolo, dark and smoky sounding. I blew a couple vintage low power jensen 15's so now it's sporting a PAS 15 with a 6 lb magnet, 4 inch vented voice coil, and SPL of close to 100. It is LOUD with lotsa headroom. The vintage speakers were brighter and much less efficient, letting the amp overdrive without being overly loud. Good luck with yer Gemini VI

[davohilts]

Thanks for the progress report! I will contact Ampeg and get the schematic!
I've got a big square magnet eminence in my cab and I know it will sound great as soon as I can get the amp biased properly.
Dave