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Post subject: Tech help need on "the twin"
Posted: Mon Feb 06, 2012 1:21 pm
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I have a "the twin" - one like the red knob model with the similar black knobs.

It sounds good - nice and clean on the high power setting but when it is set on the low 25w seting it has a lot of distortion.

I have it apart right now cannot see any thing burnt, but I tried several things before taking it apart:

1)Moved the output tubes to different positions.

2)Took out the two center output tubes. Tried each pair separately in the out side positions.
3)Plugged the guitar directly into the "power amp in" jack to bypass the preamp completely.

Through all the above it still has distortion on the low power setting and still sounds good on high setting. I am looking for ideas. As I understand it the low power setting just runs the output tubes at half the voltage - is that right? What should I try next?


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Post subject: Re: Tech help need on "the twin"
Posted: Mon Feb 06, 2012 4:49 pm
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I just look a the schematic and right it use half voltage for 6L6 on 25 watts seting. IMO better to check your 6L6s.

If you replace them a bias adjust is a must


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Post subject: Re: Tech help need on "the twin"
Posted: Mon Feb 06, 2012 5:29 pm
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I wil lsee if Ican find a place to test them - but I thought if it was just one tube was bad it would be OK on low power with either one pair or the other. Maybe two bad tubes?


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Post subject: Re: Tech help need on "the twin"
Posted: Tue Feb 07, 2012 3:09 am
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The only way tro test output's tube is to try new one with bias chek. Tubes tester are no good for that.

You could have more than one tube bad. Tubes have many hours of playing ?


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Post subject: Re: Tech help need on "the twin"
Posted: Tue Feb 07, 2012 9:17 pm
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Well the tubes tested OK - might mean nothing. I have tried all all combinations of two in the setup described in the manual and the problem persists. Maybe the low power setting has too low a voltage? As I understand the voltage on the tube plates should drop to 236v from the 400+ volts. I don't know how to check this out.


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Post subject: Re: Tech help need on "the twin"
Posted: Wed Feb 08, 2012 2:53 am
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jlp4951 wrote:
Well the tubes tested OK - might mean nothing. I have tried all all combinations of two in the setup described in the manual and the problem persists. Maybe the low power setting has too low a voltage? As I understand the voltage on the tube plates should drop to 236v from the 400+ volts. I don't know how to check this out.


How do check tubes ? sustitution with bias check or tubes tester ?
Tubes tested OK ; this do not mean nothing for me.

Voltage . do you have a voltmeter ? Voltage in tubes amp are lethal


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Post subject: Re: Tech help need on "the twin"
Posted: Wed Feb 08, 2012 3:57 am
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You can have to replace the tube for the phase inverter, the tube closer to the 6L6s. Or the Phase inverter circuit may have some bad resistor


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Post subject: Re: Tech help need on "the twin"
Posted: Thu Feb 09, 2012 10:40 pm
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I had the 6L6s tested on a tube tester. Didn't really think that would turn up anything but I don't have any more of those around and was going into town anyway.

I have tried the four 6L6s in all combinations of two (in the outer two sockets)
as described in the manual to reduce power to 60w high and 15w low.The distortion is always there when the switch is on low, with either four or two output tubes in. The amp sounds very good on high power (100w or 60w) settings.

Well I just tested putting a different 12AT7 in to replace the original PI tube next to the 6L6s and that didn't change any thing.

The amp has 401vdc when the amp power switch is set on high and when it is on low it is only 168vdc on pin 3 of the output tubes. So as I understand it the plate voltage is kind low. It shoud be +453vdc on high and about 1/2 that for low. I don't know that much about these things but am learning it now.

According to the manual the bias setting should be set at 0.04vdc on high but will not go above 0.019vdc on high when I turn the pot all the way up and drops to 0.004vdc (yes that is what Imeasured) when on low power setting. This measurement was taken on the back of the amp at the ports made for this.

I also measured the voltage on the bias wiper contact and get 52.9 on high and about 1/2 that on low power.

Are there any clues in this info? Any ideas what to check next?

thanks for any ideas!


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Post subject: Re: Tech help need on "the twin"
Posted: Fri Feb 10, 2012 3:28 am
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First of all ; you are not sure tubes are good .
Like I write before : Forget tube tester for testing tubes working a high voltage AND working hard .

if you are not able to adjust proper bias could mean tubes are NO GOOD OR you have an other problem in the amp.

Sorry but unless we are not sure that power tubes are good , you loose your time working on the amp.

401 volts could mean your wall outlet little bit low but a 10% error reading is not bad. This is not a real problem . Or it is because your power tubes need too much current , voltage drop . Another reason to suspect power tubes .
What do you use for voltmeter ?

You did not answer how old are output tubes ?

You can try to replace the Phase Inverter tube , the tube close to the 6L6's , a 12AT7 or 12AX7 this tube working hard too.

You 'll understand I answer the best I can with the information you give without testing and seeing the amp myself.


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Post subject: Re: Tech help need on "the twin"
Posted: Fri Feb 10, 2012 11:08 am
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By using the 6L6GC two at a time I had hope to eliminate any two bad ones - but I guess I could have three bad ones.

I did try the 12AT7 PI swap from another amp that works. No change in the pin 3 output tube voltage.

The meter is from Ideal, cost about $65 has:
vdc ranges 0-400m/4/40/400/1000V
resistance 0-400/4k/40k,/400k/4M/40M ohms
capacitance 0-40n/400n/4u/40u/4000uF

The voltage in at the power cord inside the amp is 120vac.

I ordered somes new tubes two days ago but they will not be here until the middle of next week. The only other output tubes I have are two 6L6GTA tubes in an older Super Champ - They look very different from the 6L6GC tubes and I haven't found any interchange info on them.


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Post subject: Re: Tech help need on "the twin"
Posted: Fri Feb 10, 2012 5:45 pm
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jlp4951 wrote:
...The amp has 401vdc when the amp power switch is set on high and when it is on low it is only 168vdc on pin 3 of the output tubes. So as I understand it the plate voltage is kind low. It shoud be +453vdc on high and about 1/2 that for low. I don't know that much about these things but am learning it now...


That is too low, indicating a problem, either with the output tubes, or more likely the power supply itself has a problem. Turn off the amp, pull all four output tubes and then power up the amp and recheck the DC supplies. The DC supplies should be much higher than the schematic shows when the output tubes are removed.

Using only two output tubes is fine, either pair, inner or outer, with the power switch in either position also. It is hard to believe that two or three tubes are bad in the same way. Maybe you have an open diode, resistor or other problem in the DC supply. Maybe you have capacitor problems. :idea:

The Super Champ output tubes are 6V6, not 6L6. :!:

There are only a couple of types of 6V6 that will work in your amp when the amp is on high power. A JJ 6V6 and GT 6V6 S will work, but most other 6V6 will not.

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Post subject: Re: Tech help need on "the twin"
Posted: Fri Feb 10, 2012 6:22 pm
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I Just pulled out only the four output tubes, powered it up and measured 175vdc on low power and 419vdc on high at pin3 on an output tube socket. So it's not the output tubes drawing too much power?

Already put the super champ away when the tubes looked wrong - nowI know why they looked wrong.

Thanks


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Post subject: Re: Tech help need on "the twin"
Posted: Fri Feb 10, 2012 7:31 pm
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Yep, I'd say that the problem is not caused by the output tubes. Sounds like a power supply issue. You should have well above 470 VDC with the tubes removed, and close to 470 VDC with the tubes installed.

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Post subject: Re: Tech help need on "the twin"
Posted: Sat Feb 11, 2012 3:34 am
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+100 Shimmilou


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