Tweed champ 5f1 PT question

[cubby01]

I built a 5F1 circuit using some parts I had on hand. It works but not quite right. I'm beginning to realize the PT I used (pulled from a console) may be the wrong fit for this amp. I belive its rated 325-0-325. Somehow I got it in my head that this was was was called for in this amp but now come to find it should be maybe more like 260-0-260 PT that should be there.

What's the proper way to verify what I have in this current PT?

What do I actually need or want for this amp circuit?

Assuming I need lower voltage on B+, can I lower it using what I have by adding a resistor somewhere in circuit?

Thanks,

 

[ThermionicScott]

No sweat, just replace the 6V6 with a 6L6GC.

Welcome to TDPRI!

- Scott

P.S. Alternately, Weber has a Champ transformer with a 300-0-300 winding on it. That should get you close enough.

 

[mojo2001]

That trans should work. Voltages will be on the high side for tweed spec perhaps but workable, especially with a late model NOS 6v6 tube.

Some replacement champ transformers are 325-0-325, eg. Triode Electronics version. I'd rather see lower voltages like 525 vct but I like my tweed champs running off of a low B+.

If you want to drop the voltage, I'd say add an extra filter cap and dropping resistor before the filter cap that goes to the output transformer, but build the amp first and do some voltage measurements.

 

[cubby01]

... but build the amp first and do some voltage measurements.

The amp is built. It's +375 out of the 5Y3GT at the first cap. (+315 at the second, and +269 at the third) I put a meter on it there at the fist cap when I power on it runs up to about +450 or +460 then starts to back down to +375. As it crest this voltage curve on the way down I begin to hear the amp through the speaker.

Another 'problem' I seem to have is caps aren't holding a charge after shut down. There is no bleeder resistor yet voltage drops down to +2 within about 5-10 seconds. I thought maybe bad caps but I replaced the 25-8-8 filter caps with same result. I've traced and haven't found any missed connections or shorts (... yet).

Cracks up badly after about 8 on the volume. Where the sputterinng starts depends on whether I'm using the normal or hi-gain input and which various guitar pickup settings, tone and volume controls on the guitar, etc. (It's a Lead III by the way) I do get some nice sounds out of it when all the settings seem to align, so I hear the potential.

Thanks,

 

[ThermionicScott]

That's normal. The still-hot tubes bleed down the caps once you turn off the amp. Bleeders are just added insurance.

375 V sounds close enough, too. Does the amp still sound bad if you sub a different set of tubes?

- Scott

 

[mojo2001]

A Champ should be pretty ragged at "8+"... something like a buzzsaw.

At 3-4 should be fat, chesty, singing, and rich.

A Champ is jekyll and hyde with the volume setting.

Sounds like it is working. What speaker are you using?

Did you check the heater voltage? If that is high, it seems the amp flips out earlier.

You could put a series resistor between the rectifier and first filter to see if you like lower voltage better. I do but it is not a night and day change, at lower volume settings particularly.

What exactly don't you like about what you're getting?

 

[JohnnyCrash]

The amp is built. It's +375 out of the 5Y3GT at the first cap. (+315 at the second, and +269 at the third) I put a meter on it there at the fist cap when I power on it runs up to about +450 or +460 then starts to back down to +375. As it crest this voltage curve on the way down I begin to hear the amp through the speaker.

A stock 5F1 is "supposed" to be at about 340 off of the rectifier.

BUT as mentioned in an earlier thread, 5Y3's these days are not exactly up to old specs and spit out 10 to 25 volts more than NOS or old 5Y3's.

375 is not too bad. It'll be a tad "hot" in tone, but that's what Champs are for anyway.

Add to this modern wall AC is generally a tad higher than in the '50s... all of this adds up to higher running voltages... only by a smidge

Another 'problem' I seem to have is caps aren't holding a charge after shut down. There is no bleeder resistor yet voltage drops down to +2 within about 5-10 seconds. I thought maybe bad caps but I replaced the 25-8-8 filter caps with same result. I've traced and haven't found any missed connections or shorts (... yet).

This has happened to 3 or 4 of my Champ-ish builds. New components, still drops to about 5 to 12 volts after power off. This is pretty normal - BUT if it is major flucuations in voltage it may be part of your crackling problem.

Cracks up badly after about 8 on the volume. Where the sputterinng starts depends on whether I'm using the normal or hi-gain input and which various guitar pickup settings, tone and volume controls on the guitar, etc. (It's a Lead III by the way) I do get some nice sounds out of it when all the settings seem to align, so I hear the potential.

I would check all of your solders. Cold solders are likely the culprit.

If not, check all of your grounds, your input jack wiring, your OT wiring, and all other solders.

Depending on if it is high freqs that do it or low freqs, it can help you track down the problem. High freq induced crackles have usually been bad solders in the preamp for me (but not always) and low freq caused crackles have generally been poor solders vibrating loose in the build -or- a poor speaker (or OT) connection.

Check every single solder again... Reflow more in when in doubt.