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#1 2014-04-26 16:57:58

doug_olitsky
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From: Gold Coast, Long Island, NY
Registered: 2009-02-19
Posts: 45
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Gemini II volume pot... audio or linear

OK I'm saving a Gemini II and broke a volume pot while trying to remove the chickenhead that was stuck.

Schematic calls for 1meg ohm, but does not spec if its
audio / log / linear

does anybody know?

thanks

Doug

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#2 2014-04-26 22:41:46

hangman
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From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: Gemini II volume pot... audio or linear

use an audio log,  even if ampeg used a linear,  using an audio log pot will give it a smoother seeming travel. 

you bummer about the pot.   can you reuse any part of it?

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#3 2014-04-27 06:33:13

doug_olitsky
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From: Gold Coast, Long Island, NY
Registered: 2009-02-19
Posts: 45
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Re: Gemini II volume pot... audio or linear

Thanks Hamgman

I will go w/ the audio taper, the G-12 uses linear so do my v4's so I assume its also linear. But I do like the taper on the audio pot so I will go that way.


How do you bias this thing? I see what looks like -2.2v on the schem. I'm guessing neg of DMM to gnd, pos of dmm to where pot meets the 100k resistors, amp out of standby.

Sound correct????

I look all over for a user manual and could not find one.

Funny... doing a set of mods on a Blues Junior for a friend and I'm replacing the linear pots for audio on that amp as well.

No this pot is really FUBAR'd, I pride myself on not breaking things when I disassemble... This time not so much. once the shaft broke free of the pot I had to put a box wrench in a vise to hold the chicken head back while I use vice-grips and a hammer to extract the shaft. These days replica knobs cost more than a pot does so I guess I broke the right thing, If you can look at the positive.

Last edited by doug_olitsky (2014-04-27 06:35:06)

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#4 2014-04-27 17:05:15

hangman
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From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: Gemini II volume pot... audio or linear

the schematic says that the bias is factory adjusted to give *0ma of plate current for both v1 and v2. 

I can't seem to find a high res scan of the schematic,  it looks like either a 3 or an 8.  but 30ma seems dreadfully underbiased and 80ma seems extremely hot.   
I can only assume that they mean 30ma per tube.   meaning the plate current is set for each tube at 30ma.

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#5 2014-04-28 11:02:56

doug_olitsky
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From: Gold Coast, Long Island, NY
Registered: 2009-02-19
Posts: 45
Website

Re: Gemini II volume pot... audio or linear

Here is a nice hi res of the schematic... my unit is 7868 based and I'm modding the pin out of the 7199 to use a 6U8A.

It denotes 80ma for V1 & V2

In the schematic it shows what looks like -2.2v.


What do you recommend as a good bias setting so I don't beat the heck out of the output tubes.

-@ what points should I insert my DMM probes to make the adjustment?

-what voltage should I dial in to get a sane ma setting??


http://i1306.photobucket.com/albums/s57 … g~original

http://i1306.photobucket.com/albums/s577/2wheelsteel/gemini_2_schematicBIAS_zps2798ff4a.jpg~original

Last edited by doug_olitsky (2014-04-28 11:15:00)

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#6 2014-04-28 12:03:02

hangman
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From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: Gemini II volume pot... audio or linear

interesting,  well the 7868 is basically a 7591 in a different package.  still 19 watts max plate dissipation... 80ma still seems quite high for both tubes.   But who am i to contradict ampeg?

the bias proceedure is going to be a little annoying.   basically you have a couple options.   you could put a 1 ohm 1% resistor between the center tap of the power transformer and power supply point A.   and then measure the voltage across that resistor.   because it will be a 1 watt resistor,  you can translate the mv directly to ma.   v = IR where R= 1. 

so in that case you would set the bias for 80mv across R1.   
then,   you could adjust the balance pot on the cathodes of the power tubes.   either with an oscilloscope (make the amp clip symmetrically)  or a less exact way would be to listen to the output with no signal going through and set the pot for least hum through the speaker.

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#7 2014-04-28 16:48:01

doug_olitsky
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From: Gold Coast, Long Island, NY
Registered: 2009-02-19
Posts: 45
Website

Re: Gemini II volume pot... audio or linear

How does this look... using a 1/2 watt metal film resistor.

Looking over the schematic it looks like I'm connecting the CT of the PT ( "H" ) to the CT of the OT ("A"). True???

How about knocking off 10% of the bias to 72ma to cool things off a bit?
The 7868 data shows 90ma max for cathode current... 80ma is 90%

80% would be 72ma & 70% would be 63ma

http://i1306.photobucket.com/albums/s57 … e50e51.jpg

http://i1306.photobucket.com/albums/s577/2wheelsteel/gemini_2_schematicwithbiasmeasureresistor_zps33e50e51.jpg

http://i1306.photobucket.com/albums/s57 … 801cf3.jpg

http://i1306.photobucket.com/albums/s577/2wheelsteel/Addbiasmeasurementresistor_zps20801cf3.jpg

Last edited by doug_olitsky (2014-04-28 19:33:06)

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#8 2014-05-03 21:17:37

doug_olitsky
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From: Gold Coast, Long Island, NY
Registered: 2009-02-19
Posts: 45
Website

Re: Gemini II volume pot... audio or linear

Did the rebuild today and the new audio taper pots are in for the volumes.

Nice and smooth and all works on the amp and nice and quite.

Did not install resistor to check bias current draw as I'm not sure whit I outlined above is correct. But the bias is set for -22v

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#9 2014-05-05 20:06:18

hangman
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From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: Gemini II volume pot... audio or linear

so,  resistor you put in above isn't going to measure just plate current.  it has to be between the output transformer center tap and power supply point A.   so that the only current going through the resistor is that which is going to the output transformer.

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#10 2014-05-05 21:26:35

doug_olitsky
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From: Gold Coast, Long Island, NY
Registered: 2009-02-19
Posts: 45
Website

Re: Gemini II volume pot... audio or linear

OK I think I have it now.

What wattage on the resistor?

My revised diagram...
http://i1306.photobucket.com/albums/s57 … 08060e.jpg

http://i1306.photobucket.com/albums/s577/2wheelsteel/gemini_2_schematicBIAS_zps8e08060e.jpg

http://i1306.photobucket.com/albums/s577/2wheelsteel/gemini_2_schematicBIAS_zps8e08060e.jpg

Last edited by doug_olitsky (2014-05-05 21:42:00)

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#11 2014-05-24 17:42:36

doug_olitsky
Member
From: Gold Coast, Long Island, NY
Registered: 2009-02-19
Posts: 45
Website

Re: Gemini II volume pot... audio or linear

Build of the 7868 based is all done... After first phase one of the EH7868's blew in 10 minutes. Took it to Richie Johnson of NY Repair & Guitar.

Said my work was fine but w/ modern voltages new stock 7868's get beat to hell. He changed the 470ohm screen resistor to 1k ohm 5watt. On his suggestion I put in a 10k ohm 50watt resistor for the screen voltage... to bleed off the excess voltage and get it down to 430v again.

using NOS 7868's

Re rewired the 7199 for 6U8A

Pwr Spply filtering beefed up a bit...
80 uf upped to 110uf (2x 220 in series w/ bleeder resistors)
40 uf's upped to 47uf
20uf's upped to 30uf

http://i1306.photobucket.com/albums/s57 … 34ffef.jpg
http://i1306.photobucket.com/albums/s577/2wheelsteel/GeminiIIUpdatedSchematic_zpsfc34ffef.jpg

http://i1306.photobucket.com/albums/s57 … e06339.jpg
http://i1306.photobucket.com/albums/s577/2wheelsteel/IMG_0012_zps96e06339.jpg

http://i1306.photobucket.com/albums/s57 … ea9594.jpg
http://i1306.photobucket.com/albums/s577/2wheelsteel/IMG_0011_zps83ea9594.jpg

http://i1306.photobucket.com/albums/s57 … adc529.jpg
http://i1306.photobucket.com/albums/s577/2wheelsteel/IMG_0010_zps3dadc529.jpg

Last edited by doug_olitsky (2014-05-24 17:56:21)

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