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Post subject: 5F1 voltages question
Posted: Tue Oct 30, 2018 10:18 pm
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I recently finished building a 5F1 champ. It was sounded great until it started a loud buzzing. I turned it off within 2 or 3 seconds to investigate, and found that the ground from the output transformer that would normally be connected to the speaker had come loose, making that an open circuit. I resoldered it better and went back to playing. It still sounded great, although there was now a quiet but noticeable transformer hum when it is on. Much quieter than even the 60 Hz hum from the pickups.

Within an hour of playing I had destroyed two different power tubes.

I opened it up to check power amp voltages and this is what I found: (all voltages with respect to ground)

Cathode: 21 V
Plate: 366 V
B+: 381 V

With a 470 ohm cathode resistor, that means there’s a plate current of 47 mA. With a plate to cathode voltage of 345 V, that makes a plate dissipation of 16 W. Explains the dead tubes. This also means the output transformer was shorted, as a voltage drop of 16 V from 47 mA means it has a resistance of a little over 300 ohms. The spec I seem to find is anywhere from 5-15 kOhms, for champ output transformers on the classic tone website. The 5F1 schematic I have labels the cathode voltage at 19 V, which gives a plate current of 40 mA. This current with a transformer resistance that high seems like it would cause a tiny plate voltage.

What am I missing here, and does anyone have good recommendations for a replacement transformer?


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Post subject: Re: 5F1 voltages question
Posted: Wed Oct 31, 2018 7:08 am
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I will not say your output transformer are short. Are you sure the 6V6 are good before ?
Are you sure about your wiring ? Did you plug the output transformer reverse ?

Post pictures

Replacement transformer ;

https://www.tubesandmore.com/products/t ... r%20Fender

A champ transformer will work fine;
https://www.tubesandmore.com/products/t ... 8-w-32-ohm

Or this one;
https://www.tubesandmore.com/products/t ... nder-5w-7k


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Post subject: Re: 5F1 voltages question
Posted: Wed Oct 31, 2018 7:37 am
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The transformer resistance is normal. There is a difference between dc resistance and ac impedance.
Normal bias is 19 volts at the cathode. 21 is in the ballpark. The other voltages you describe are normal.
You have to select the proper gain power tube to get the right bias. In your case, a low gain version is warranted. Most nos tubes are worn out and won't bias properly.
Unlike a push-pull amp where the tubes are biased on just a little, the Class A Champ biases the power tube to 50% conduction. So they are harder on tubes.
You also have to include the dc resistance of the power tranny with the 470 in your calculations.
While I think the output tranny is ok, why would you use the Classic tone? Its only 5 watt and the stock Fender is 8 watt. Also, the extra windings take up room internally and compromise performance.


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Post subject: Re: 5F1 voltages question
Posted: Wed Oct 31, 2018 8:43 am
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If those voltages are normal, that means my plate dissipation is sitting somewhere between 15 and 16 W. The max spec for a 6V6GT is 12 W. I've destroyed multiple tubes in a matter of minutes, both new production and NOS. Can see them redplate within 30 seconds of turning the amp on.

This is the OT I have in it:
http://www.classictone.net/40-18110.html

My speaker measures 4 ohms DC resistance. The OT secondary is labeled at 4 Ohms in the schematic. That's at DC I'm pretty sure. So are you saying the 17 kOhm primary impedance is an AC measurement? If so, at what frequency?


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Post subject: Re: 5F1 voltages question
Posted: Wed Oct 31, 2018 10:43 am
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Pants187 wrote:
. The OT secondary is labeled at 4 Ohms in the schematic. That's at DC I'm pretty sure.


No, it is impedance. Check with your Ohmeter you will not read 4 ohms.


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Post subject: Re: 5F1 voltages question
Posted: Wed Oct 31, 2018 12:12 pm
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Alright so after some more research it looks like the output transformer is working, and I also wasn't taking screen current into consideration. The nominal DC resistance of the primary of that OT is 376 ohms, so with the voltages listed above, that gives me a plate current of just under 40 mA and a power dissipation of 13.5 W.

Time to find a 6V6GTA or JJ's.

Anybody know what would cause the transformer hum I'm hearing?

Thanks for the replies


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Post subject: Re: 5F1 voltages question
Posted: Wed Oct 31, 2018 12:34 pm
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Pants187 wrote:
Alright so after some more research it looks like the output transformer is working, and I also wasn't taking screen current into consideration. The nominal DC resistance of the primary of that OT is 376 ohms, so with the voltages listed above, that gives me a plate current of just under 40 mA and a power dissipation of 13.5 W.

Time to find a 6V6GTA or JJ's.

Anybody know what would cause the transformer hum I'm hearing?

Thanks for the replies


A single end amplifier alway hum.
And could be worse with how you dress wiring.

Picture is needed as I ask before, we must see the amp to help you.


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Post subject: Re: 5F1 voltages question
Posted: Sat Nov 03, 2018 7:26 am
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There are 2 or 3 sources of hum on a champ.
1. Pull all the tubes. If it still hums, the problem is the output transformer is too close to the power transformer. The laminations should be at right angles and as far away from each other as possible. This is not usually a problem on stock units, but a kit...?
2. Pull the preamp tube. If the hum goes away, you should consider using a hum balance pot in the filament circuit.
3. All class A amps hum more than push-pull amps. The push-pull ot cancels hum out. Class A doesn't. A common cure is a 100-200 ohm 5watt resistor added to the rectifier and a 100uf 450 volt cap added to the b+string. This lowers the b+ a bit and removes a lot of line hum
Good luck


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Post subject: Re: 5F1 voltages question
Posted: Sat Nov 03, 2018 9:19 am
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TimsAudio wrote:
3. All class A amps hum more than push-pull amps. The push-pull ot cancels hum out. Class A doesn't. A common cure is a 100-200 ohm 5watt resistor added to the rectifier and a 100uf 450 volt cap added to the b+string. This lowers the b+ a bit and removes a lot of line hum
Good luck


Tim, one slight correction. Single-ended amps (in Champ's case = single-ended pentode) hum more than comparable push-pull amps (whether Class "A" or AB).

I agree, careful wire layout and running two twisted-pair heater lines (pulling the one grounded line off the chassis) --- helps to reduce hum.

:mrgreen:


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Post subject: Re: 5F1 voltages question
Posted: Sun Nov 04, 2018 8:40 am
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Ok, if you want to argue that one...
Class A : conducts for 360 °
Class B : Conducts for 180°
Class C : Conducts for less than 180°
Class D : PWM
Class AB : 2 Class B coupled for Class A operation.

Class A amps are single ended. While Class AB amps can be biased into class A operation, it is just a trick of bias voltage and can't really be called Class A, regardless of all the ads.


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Post subject: Re: 5F1 voltages question
Posted: Sun Nov 04, 2018 1:23 pm
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Tim, true single-ended amp are in Class "A." However, you can have a push-pull output stage in Class "A." My Matchless is a push-pull EL84 in Class "A." Usually, these output stages have cathode-bias arrangement.

https://www.vintageguitar.com/21904/matchless-dc-30/


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Post subject: Re: 5F1 voltages question
Posted: Sun Nov 04, 2018 1:47 pm
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Old vintage Vox AC30 is a push pull Class A in my opinion.


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Post subject: Re: 5F1 voltages question
Posted: Mon Nov 05, 2018 7:58 am
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To quote wikipedia,
"Though widely believed to be a class A amplifier, the AC30 is in fact class AB."

Changing the bias doesn't magically change the configuration when you go past an arbitrary level.
Every push-pull amp could be considered Class A if you define bias as conduction.


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Post subject: Re: 5F1 voltages question
Posted: Mon Nov 05, 2018 8:51 am
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Thank You TimsAudio


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Post subject: Re: 5F1 voltages question
Posted: Tue Nov 06, 2018 11:01 am
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TimsAudio wrote:
Ok, if you want to argue that one...
Class A : conducts for 360 °
Class B : Conducts for 180°
Class C : Conducts for less than 180°
Class D : PWM
Class AB : 2 Class B coupled for Class A operation.

Class A amps are single ended. While Class AB amps can be biased into class A operation, it is just a trick of bias voltage and can't really be called Class A, regardless of all the ads.


Class A/B is not that. Two class B amps coupled together would still just be a class B amp.

Class A/B is the solution to crossover distortion caused by push/pull class B amplifiers, and by your standards above, would be defined as "conducts between 180° and 360°". This way one side is always conducting and there is no crossover distortion.

A push/pull amp can easily be class A, if both sides are biased so they conduct for the entire waveform. But it is rarely seen, because if you go to the trouble of having a push/pull amp, biasing in class A/B gives a higher power output.

Basically if you bias a class A/B push/pull hot enough, you will get a class A push/pull.

Also, I replaced the PT on my champ with one that outputs 290 RMS instead of the 325 RMS I had in there, and my melting tube problem has gone away.


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