Ampeg Gemini Vl Recap repair and troubleshoot

[Toxophilite]

Just crazily bought a Gemini 6 for Approx $250 US
Maybe overspent
has a replacement realistic speaker and strangely someone has added a tweeter?? with NO crossover?? lunacy!


Decent tubes but needs cleaning and probably a recap
has a couple of amperex bugleboys in so if they're good I can recoup some $$

[Toxophilite]

This sort of thing is always reassuring!
Good old Red Green-esque Canadian know-how!
Actually duct taping the main cap top secure it!
wow!
Amp works but lowish volume and early distortion
preamp tubes test good so far
Haven't tested the power tubes or opened it up
Anybody have a schematic with voltages?

[Toxophilite]

Hilarious!! The head wasn't held into the cab with ANY screws, Ye old gravity mount! No wonder the can cap was busted. However I was reassured that it wasn't just duct tape holding it in they'd also slipped a bit of old cardboard in there for safety!
However the inside of the amp looks about right. No signs of horrific fires etc. Outside looks good to. Nice Skookum transformers


tremelo works and the reverb works but squeals evilly when turned up


I think some cap replacing is in order though!

[DummyLoad]

usually there's a schematic pasted to one of the backboards.


--pete

[Toxophilite]

Oh thanks I think I found a good one here on the site that seems to be right.
Had a look and the one on my amp only has the bottom half remaining

[NewYorker]

A diamond in the rough.  It can only get better.

Ed

[sluckey]

I love that old blue diamond look.

[Toxophilite]

The tolex on these amps is very pretty. beats plain black fenders!


I picked up a 50 50 /500VDC JJ can cap, I'm going to tag a 47 across one of the 50s
That's make 97 in the first filter position and 50 in the next. That should be okay shouldn't it
More capacitance but not huge amounts more
Thanks
(schematic below)

[silverfox]

97 uf sounds high. It really depends on the rectifier type and size. If it's ss rectification most likely no problem. If it's a tube rectifier, you have to check the max input capacitor size for the rectifier in the amp. Too large a value could lead to failure of the rectifier tube. I suspect, a too large filter cap can also change the response characteristics of the amp as will also too small an input capacitor value.

silverfox.

[Toxophilite]

HI
and thanks
As per the schematic posted the first filter cap on this amp is supposed to be 80 then 40 and 40
It's SS rectified
That's why I figured going 97 and 50 for the first two wasn't too outrageous
I couldn't get an 80 that was 500 volts so it would've been at least 94 or the other option would've been a 47 and a 22 for 79 which is less but very close.
As I was removing the old broken can cap it seemed like a nice idea to have something in it's place


I will say if you have an old blue check Ampeg that's  looking dirty, a bit of quality time with warm soapy water and a nail brush makes a huge improvement. shiny and pretty again!

[sluckey]

97 uf sounds high.

97 will be fine.

[PRR]

Needs more duck-tape than that.

[Toxophilite]

That's what I thought, and this guy calls himself a fellow Canadian!!?? I went out and bought another roll.

[Toxophilite]

So I have a 3 prong plug installed. instinct has me running the hot side (black wire) to the fuse and switch
Is this right?

 I also replaced the electrolytic caps.


The amp has the .047 'death' cap disconnected (was like that alreadt)
and it looks like a 4 diode (bridge?) rectifier was installed instead of the 2 diode rectifier shown on the schematic


The amp is working. Very quiet with nothing plugged in and the volume down
I get some buzz at the top of  the volume control with nothing plugged in between 7-10, at 10 it's pretty loud.


One thing that's worrisome is that when I flip the standby switch off I get a really loud hum buzz that fades pretty quickly.


B+ voltages all around seem to be high , up to 80 volts high, makes me worry about the 500 E caps (per the schematic)
UPDATE- I tried another pair of 7591s that I had and B+ dropped about 40 volts to 465v on the power tubes Still about 40 volts too high but not as nutty


Other than that the biggest discrepancy would seem to be pin 9 of V3 (7199 PI) which should measure 18V and measures 41
All the others (other than the B+ ) seems to be relatively in the ballpark


Any ideas about the loud noise on the standbyswitch?
-the uber high B+ ?
-and the high voltage pin pin 9 of the PI ??




V1 -3 505        V2 -3  505
      4  505             4  505
      6 -21.96          6  21.96


V3              V4                  V5                 V6
1 -304          1- 241           1 - 124          1 - 113
2 -162          3 - 2.13         3  - 4.7          3 - .79
3  -67           6 - 219          6 - 267          6 - 227
6  -2            8 - 2.14          8  - 5            8 - 4.23
8  -64
9  - 41

[sluckey]

All the voltage readings on that schematic were taken with a 20,000Ω/volt meter such as Simpson 260 or Triplett 630. If you are using a DMM most of your readings will be high, especially when checking voltage across a 1.5M resistor such as on V3 -9.

[2deaf]

has a replacement realistic speaker and strangely someone has added a tweeter?? with NO crossover?? lunacy!

Probably a Realistic piezo tweeter.  They don't require a crossover.  They had the horn model and the potato-masher model.  I tried them with a 4-12 cabinet back in the '70s and quickly concluded that electric guitar and tweeters are not compatible.

[2deaf]

and it looks like a 4 diode (bridge?) rectifier was installed instead of the 2 diode rectifier shown on the schematic

It looks like it is just two diodes in series with a conventional center-tapped power supply.

_ _ _ _ _ _ _

What kind of reverb pan does it have?

[Toxophilite]

Looks like the type of reverb pan used in Hammond organs, much like ampeg stereo echo twin I had.
in fact there's a lot of construction similarities between these amps and hammond organ amps.
The stereo Echo twin had Hammond written right on the onboard schematic.


The rusty tweeter was yanked almost immediately


Any idea as to why I'm getting such a loud noise our of my standby switch ?

[davidwpack]

Both of my Ampeg B25-Bs have a little noise when switching off the standby. I also discovered that half the B+ is still going to the tubes in standby. Both of them do that. Do you have voltage on the tubes in standby? Just curious.

[PRR]

> ...hot side (black wire) to the fuse and switch...

Treat white and black as BOTH HOT. You never know who wired the outlet. ASS-ume either or both wires may be live to floor or radiator. Unplug before you stick your hand inside.

> when I flip the standby switch off I get a really loud hum buzz that fades pretty quickly

The standby breaks both B+ and C- (bias). The bias supply takes longer to come up. In that time the amp is running max-hot, and buzzy because the B+ is working double-hard. Probably no harm is done. It does confirm "I'm alive!"

If you play church weddings and must standby for Moment Of Silence then come into Hava Nagila, a "BUZZZzzz" may be disruptive. I would move the standby switch to the B+ like a Fender, so the bias is always live.

[Toxophilite]

True
I wasn't making assumptions regarding which is live when i stick my hands in the amp, just what I figured was good practice and confirmed by my electrical engineer brother-in-law.


I like the idea of switching the standby to B+ like Fender as teh noise is pretty loud and doesn't necessarily sound good.


This is a pretty sounding amp, interestingly, more so tahn my other amps it really grabs and amplifies the hiss from my tape echo


S.Luckey, What incarnation of the gemini do you have?


I'm contemplating pulling the whole tone stack volume control deal, putting it aside for safe keeping and putting in a new slightly tweaked Baxandall bthat gives me a little higher scoop (frequency -wise) and witha push pull bright switch instead of just at the top of the treble control.


I quite like the mandolin-esque percussion/stutter you get when you activate the switch at the top of the tremelo intensity control, Very cool!

[sluckey]

I had a new Model G15 Gemini II back in the '60s and '70s. Mine had 7591 tubes. It's basically a two channel version of your Gemini VI.

[Toxophilite]

Did yours have the switch at the top of the tremelo intensity?

[sluckey]

yes. I never bonded with it.

[Toxophilite]

Hey PRR


On changing the standby switch. Can I just put it right after the rectifier diodes? Or should I try to put it after the first filter caps ala fender?

[Toxophilite]

Okay that was probably a dumb question


So hook the PT centertap straight to ground (instead of to standby switch) and insert switch between first filter caps and the OT/


I'm getting some strong hum off this amp
not sure where it's originating

[Toxophilite]

Hmm so I wired the standby up like a fender and grounded the center tap of the PT


All seemed to be working well when the 470 ohm 2 watt resistor between the 80 and 50 went up in magic smoke!


Then I realized that as everything isn't wired up in a nice line like the fender style I'm used to, my standby switch was JUST cutting B+ to the OT and not to everything else...which I figure made made the old 470 ohm 2W resistor unhappy.
Well time to put it back to normal and wait till monday when I can pickup a new 470 ohm resistor

[sluckey]

Let's hope the 7591s survived.

[davidwpack]

I was asking about that in reply 18.

[Toxophilite]

Hey David you were asking if I had voltage on my standby tubes in the original configuration when the the amp was on standby. I messed with my standby and didn't test the voltage prior so I didn't have an answer for you. Apologies.

S. Luckey I didn't have the standby on for 5 min. I just flipped it a few times too see if it was working.
The tubes I have in it aren't the tubes it came with but an essentially free pair out of and AO-63. It would be a shame to roast them due to idiocy but not happily ,the end of the world.


Anyway I'll stop bugging you guys about this amp

[sluckey]

If it was on long enough to burn up that resistor it was on long enough to probably damage the screens in those tubes. The screens were acting kinda like plates and were drawing excessive current. That's what burned up the resistor.

[Toxophilite]

Thanks
They do still test good on my tester
I'll see when I get a new resistor what gives
As my father says, 'speculation is useless'

[Toxophilite]

So I need some help! PLEASE! 


Not surprising S.Luckey was right
I cooked a 7591 tube. A shame but it was essentially free and it had been sitting unused for a long time


Anywho!
I put a new 2 watt 470 ohm resistor in the amp
i seem to have good voltages everywhere
I am now trying it with the ampeg labelled 7591s the amp came with
the amp powers up and isn't noisy and the B+ voltages seem to be good


However , trying the amp in rehearsal it seems to be quite quiet and distorts far too early


-there are some variables


-the original 7591s test good..but I only have an emissions tester


I did some simple tests on the OT ala this page
http://www.geofex.com/ampdbug/outtrans.htm
It tests good on those tests 145 ohms on each plate winding and testing it's turns ratio gives me approx 27-1
No shorts to chassis ground


Anywho I guess it could be the tubes but I don't remember them being bad before I did the recap etc
i will go measure all  the voltages again


Could my mishap have cooked my OT?
As soon as i heard the resistor go and saw the first bit of smoke(the amp was out of the chassis and I was watching it0 I turned it right off

[Toxophilite]

Nobody??

[sluckey]

First thing to always try is new tubes.

[Toxophilite]

Okay
normally I'd try the easy stuff first

 But I have to buy some, which have to be ordered in, two weeks, alas.


There may be a guy up the street who'll lend me a pair. I'll check on that.


As I can't do that immediately i there anything else I could look into and or test.

[sluckey]

speaker

[shooter]

The one I fixed, had bad tubes, and a bad? OT, but neither were obvious, arcing in the tubes wasn't instant, the OT wasn't open with ohm meter, but after 20 min whatever tubes I had in started arcing.  The owner had a 6V6 Hammond chassis, so I used that OT, new 6V6's on one side.  I believe the cathode R n possible the screen R were also bad, or became bad, whiffing smoke when tubes arc'd.  Once I rebuilt the bad side, I used the good side and confirmed 1 of the 4 original tubes was bad.  Owner didn't want to spend more money so I picked the "best 2" of 3 tubes and shipped it out

which came 1st, couldn't say, didn't have my normal 90% confidence the OT was bad, but I believe there was thermal breakdown

[Toxophilite]

I compared it plugged into the two 10's in one of my vibroluxes (2 x 16 ohms m= 8 ohms) and it was much clearer and louder and those were not the most efficient warehouse speakers (veterans)


Also subbing one of the ampeg (sylvania?) 7591s with the one still good hammond branded 7591 also helped a little
So I think it's mostly crappy speaker with weaker power tubes not helping..which is a relief really.
there's a guy locally that has a lot of 15' full range rola etc tone cabinet speaker pulls for $30 a pop so I'll go grab one of those and ditch the worn out radio shack speaker.
Have some tubes on order too.


Thanks for help and suggestions!!!