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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Sun Dec 18, 2016 10:08 am
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Strat, the source impedance of your generator is often found under the output jacks. Otherwise it is speced in the manual. They are seldom less than 50 ohms.
Yes, your impedance readings can be thrown off if the generator is loaded too much.
The only time I don't use an amp with the generator is when probing input stages. I have an 80 watt SS bench amp hooked up to it. I usually test at about 1 volt. Or however loud you can stand it.
Mostly I do it to find the resonant point of a speaker, where the impedance jumps.
Its also useful to run a speaker at 10-20Hz at full power to check for speaker noises. At that low frequency, you can only hear mechanical noises

Newdyne,
Tube manuals spec the impedance requirements for a pair of 6L6.
With 3 pairs in a super 6, you can divide that impedance by 3 to obtain the expected OT impedance. The factories do not spec their parts, its proprietary info. They have also been known to push specs.

But I continue to insist that there is nothing wrong with your transformer.
Transformers do not go a little bad. The full power of the amp goes through it and when things go wrong, they go wrong in a big way.
Blown fuses or no sound are the usual result.
In a live circuit of a shorting tranny, you can hear the bad transformer arcing and sizzling through the burned carbon. It smells like burned varnish where the insulation has melted and the acrid smell of ozone fills the air.
The OT in a big Fender is almost indestructable as well. There are very few failures.


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Sun Dec 18, 2016 2:20 pm
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Thanks very much for the information.

I will test the audio output transformer this week and post my results.

Best Regards

CE


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Wed Dec 21, 2016 8:10 am
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Tested the audio output transformer in the Fender Super Twin yesterday.

Tested impedance by two methods:

First Method as described by stratele52 on this forum., using HP 200 AB signal generator at 1000 HZ. Coupled signal into the secondary using a .1 cap. Measured the input signal and the output signal with a Tektronic TDS 3032D scope. Did the calculations as described in his earlier post. The result I got is 1607 ohms primary impedance at 1000 HZ.

Second method as described by ElPaso TubeAmps at his YouTube channel. His method uses 60 HZ, a variac, and a small step down transformer as the signal source. Input is to the transformer primary direct. Input is set to a low voltage by adjusting the variac (5.11 volts RMS) in this case. Levels at the input and output were measured with the same scope. The math is the same as described by stratele52. The result I got with this method is 1648 ohms at 60 HZ.

So, I think the input impedance spec is probably 1700 ohms +/- 10 % or so. Very close to the numbers suggested by several folks.

This transformer appears to be good.

Also did the following:

1) measured the screen resistors as suggested on this forum. They are all OK.

2) inspected between pin 2 and 3 of each socket. They are all OK

3) measured the grid resistors. They are all OK

Will update this post when more information is available. Will not work on this amp again till after Christmas.

Thanks to everyone that posted about this subject. I appreciate it very much.

CE


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Wed Dec 21, 2016 8:17 am
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1600-ohm impedance at 1000Hz seems fine.

You wouldn't happen to have a photo of that output tranny? I've never seen a Super Twin OPT. Must be huge.

Thanks!


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Wed Dec 21, 2016 10:15 am
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BMW2002Ti wrote:
I've never seen a Super Twin OPT. Must be huge.


+1

It resembles something you might see in a Sunn 2000S. Just monstrous.

Arjay

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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Wed Dec 21, 2016 10:43 am
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OK, lets back up a bit.
Your original problem is weak output.
What makes you think its weak?
Image
This schematic shows 1KHz AC signal levels in circles along the signal path. Be sure to only use that frequency.
Down in the lower left is the Test Data needed to set up the controls to replicate the results.
These voltages depend on recreating the original test conditions. Unfortunately, AC line voltage varies. You need to use a variac to adjust your line voltage down to get the 500 v B+ voltage at the standby switch to match the drawings.
Then you should get a pretty close match to the AC gain voltages along the sugnal path.
You should only get 10 watts output at these settings, so its going to be a low gain amp anyway you look at it.

The volume boost jack removes negative feedback to increase volume. Have you inspected that circuit?

As Retroverbial pointed out in a post from years ago about the Super Twin, it uses two tubes that are very rare and unobtainable.
I would check these circuits very closely to determine whether the amp has any future at all.


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Wed Dec 21, 2016 11:01 am
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TimsAudio wrote:
As Retroverbial pointed out in a post from years ago about the Super Twin, it uses two tubes that are very rare and unobtainable.


Oh they're obtainable......contingent upon the heft of one's wallet.

The 6C10 triple triode is the more common of the two but in either case, there are no re-issues and the originals haven't been made in decades. Hence, whatever's out there is all there's ever gonna be.

Arjay

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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Wed Dec 21, 2016 12:46 pm
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TimsAudio wrote:
OK, lets back up a bit.
Your original problem is weak output.
What makes you think its weak?
l.



+1000

The drummer is louder :lol:


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Wed Dec 21, 2016 6:28 pm
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What was the status of the 6L6GC tubes? These ultra-linear amps tend to be hard on their output tubes. Running mil spec 7581 or even 7581A maybe in order with this amp.


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Wed Dec 21, 2016 6:59 pm
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BMW2002Ti wrote:
What was the status of the 6L6GC tubes? These ultra-linear amps tend to be hard on their output tubes. Running mil spec 7581 or even 7581A maybe in order with this amp.


+1

The B+ is pushing in excess of 500 VDC. Personally, I'd recommend the Tung-Sol STR re-issues as the most economical alternative.

Arjay

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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Thu Dec 22, 2016 10:53 am
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Thanks for all the info so far.

This amp belongs to a friend of mine. He is a working musician in the Atlanta area. I'm trying to help him figure out what is wrong with the amp and restore it to full function.

Regarding the symptom (weak);

Reason for describing it as weak is it cannot hold it's own on stage next to a 30 Watt amp (not as loud).

So, upon examining it myself and comparing to other amps, it sounds weak and somewhat distorted to me as well (distorted means not clean in this case).

As I have mentioned in earlier posts I have moderate experience with guitar amps. But quite a lot of experience with other types of equipment.

This Super Twin is without reverb. Also, it does not have the function to turn off the negative feedback.

It has 2 foot switch jacks (one for distortion on/off only , the other for equalizer control).

Has a rocker switch on the front panel for the bright switch.

Output tubes seem ok at this point but I've only tested them on my tube tester (Eico 667). So, not too sure about them.

Reason for focusing on the AOT first was we thought it may have been abused since it had only four output tubes plugged in when purchased. This amp was recently purchased, so our knowledge about it is sketchy.

My plan is go through it with my scope looking at signal levels compared to the schematic.

I have a schematic of it (I think it's the right one). Schematic is labeled "Part No 012380". Rev block on the schematic I have is empty (no revisions). So, this is an early version schematic and seems to match the amp. Although my copy is not too good.

Anyway, only questions I have at this time are about the input level and method to supply the signal.

Questions:

As I understand it, the correct method is to insert a plug to the input jack to open the ground connection on the tip and apply 1000 Hz 2.5 mv (2.0 mv on my revision of the schematic) to the 33 K grid resistor through a .1 capacitor. Is this correct?

Also, what signal level should be needed to produce full output power?


Thanks in advance for all your help.

CE


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Thu Dec 22, 2016 11:10 am
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Newdyne wrote:
As I understand it, the correct method is to insert a plug to the input jack to open the ground connection on the tip and apply 1000 Hz 2.5 mv (2.0 mv on my revision of the schematic) to the 33 K grid resistor through a .1 capacitor. Is this correct?

Also, what signal level should be needed to produce full output power?


You should be able to inject that 1kHz test signal directly into the input jack on the front panel. An amplitude of around 500 mv should do it.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Thu Dec 22, 2016 11:19 am
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Ok I repeat;

Tubes tester are nor reliable to test weak tubes.
Ok if tubes are dead or short. Not between .

Only new tubes will tell.

I check 12XX7 tubes on amps with Audio signal generator;
With the suspected 12XX7 tube plug in V1, I check the output at pin 1 and 6 with scope . I compare with a brand new 12XX7

Power tubes; with the bias reading, I can see if they are weak or not.

A very low heater supply voltage or a very low B+ supply = weak output


Tubes amps issue; always check tubes first. Forget the output transformer.


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Fri Dec 23, 2016 11:36 am
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Your experience with a 30 watt amp being louder than a 185 watt amp is not uncommon. What you are perceiving is the differences in dynamic response and one of the ptrimary distinctions between amplifiers. It is critical for selecting an amp to use for lead, rhythm, or bass applications.
While you perceive that the amp is not as loud, it also sheds light on its dynamic characteristics
Its not as important how much power an amp has as what it does with the power. Its very difficult to determine the dynamics of many amps unless its a comparative A/B test, such as you were doing.

While an organ produces a even volume, guitars make percussive gain envelopes with each string drop. The peak decays to a sustained level
While the pluck of your fingers determines how the string responds, the total internal gain of the amplifier.determines how the amp handles the percussive envelope.
The sum of the gain of each stage of an amp determines the total gain for the whole amp. In a tube amp, you can turn the amp up and down and you won't change the relationship of peak to decay..
With the same signal and volume levels, A lead amp with high gain may produce 10 times the level of a rhythm amp. And that may produce 10 times more than a bass amp.
Your choice of speaker impedance also affects dynamic placement.
This is one of the requirement of a band to set the dynamics of the performance.
A lead amp with 16 ohm speakers will slap you in the face and say, Hey, this is a lead note. The high gain is like driving with power steering. Fingering is easier if your arms aren't strong. Mistakes , however, pop out,
The rhythm guitar amp with 8 ohm speakers can keep a strong loud pallette that doesn't overwhelm the lead with percussion. The lower gain makes it easier to hold a sustained level and blooms into distortion when pushed further. If you have a strong right arm and like to use it, lower gain compensates.
The right bass amp can deliver tons of volume at 4 ohms without the plucked string overwhelming the whole band. A lot of bass power is needed to compensate for our reduced ear sensitivity at low frequencies. If that power is misplaced dynamically, it masks vocalists and other low dynamic instruments such as keys and acoustic guitars.
The main trade-off is gain for sustain. A high gain amp won't sustain. A low gain amp won't cut through a band.
The ratio of peak to decay is exaggerated by a high gain amp, leaving little for sustain volume. A bass amp cuts dynamics to remain in the background.
If you have the right amps, its easy to get a live sound right. if not, there's nothing you can really do to balance the amps other than turn or muffle the loud one.
So your 30 watt amp should play lead and the Fender should naturally fit in rhythm behind it. It's not an unusual situation.
The distortion you hear is also probably normal.
Fenders are designed to distort through the midrange. The tone stack sends the signal through two different size capacitors. The difference in phase can be seen by hooking a 2 channel scope to the treble conrol pins 1 & 3. Make sure your scope input is rated for 250volts input or more as there is plate voltage lurking around..
Inject a sine wave and sweep the midrange audio spectrum, 100Hz-1KHz. Observe the different phase of the two signals. The treble signal comes out leading and starts raising in the 200-300Hz range.
The bass signal starts dropping over 600-800Hz.
Where these signals overlap, it causes phase distortion when they are mixed at the treble wiper. This effect is called the Fender shimmer. Guitar signals create overlapping and harmonic interweaving phase distortion to richen the complexity of the tone.
It was a historic discovery by Leo Fender. He had been experimenting with various tone controls through the 1950s and this was a breakthrough in guitar sound. It was promptly stolen by the whole world.
You can also evaluate the phase distortion with your scope in X-Y mode. This is my favorite scope setup. The probes are connected to each channel. . When there is no phase difference between the channels, there is a diagonal line. The phase difference is displayed as a circular lissajous pattern.
You can use this to fine-tune the bass balance resistor for even interaction of bass and treble levels for maximum or minimum distortion. You can also use it to fine tune the interaction point for max distortion at lower or higher frequencies with different tone stack capacitors. The hand over point is denoted by a dip in output voltage at the treble wiper caused by the losses induced by the distortion.

For those of you without a scope, you can hear the phase distortion go away when you turn the treble coltrol to either extreme and hear it clean up as you strum.


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Wed Dec 28, 2016 7:42 pm
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Bump up! :mrgreen:
We ever find the idle bias of each 6L6GC? And status of 6C10 compactron tubes? I'd check and confirm the strength of your tubes before doing anymore diagnostics. Do you have a tube tester that can test those 6C10s?

I think I have some 6C10 or equivalent NOS tubes lying about, if you should need some.


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