Recommended PT for Vibro Champ

[Fretman_2]

I've got a vibro champ chassis that needs a PT. Can someone recommend a Hammond or other PT??

Tnx,

bill

[GR1950]

Weber W022772 ( 125P1B )

[TheTinMan]

Mojo sells quality transformers. Their MOJO760 puts out too high a voltage for a tweed Champ but should be fine for a Blackface. You may also be able to go direct to Heyboer for a high quality PT. (Mojo's trannies may be Heyboers BTW) Mercury Magnetics makes a fine product but IMHO the price is too high unless you can get the 10+ unit discount (or more).

HTH

Chip

[tedmich]

Edcor XPWR131 is a possible choice, if you are using the 330-0-330 taps, and at $61.89 its quite the bargain

EDCOR - XPWR131

[mlp]

Check Allen Amps. He has a great upgrade PT for Champs. I have used it to good success. Highly recommended.

[RichardM]

Hammond 290AX or for an "upgrade" Hammond 291AX.

Hammond Mfg. - Tube Guitar Amplifier Transformers & Chokes - INDEX

[TheTinMan]

That Hammond 270AX looks like a good, economical PT for this project. You will need a real (i.e. NOS) 5Y3-GT rectifier to get a B+ of about 350 out of a 325-0-325 PT. New 5Y3-GTs will put out roughly 20 volts higher than NOS.

I know that the 125P1B part number is batted around for single-ended Champs but it's not right.

Originally Posted by mlp

Check Allen Amps. He has a great upgrade PT for Champs. I have used it to good success. Highly recommended.

I should have mentioned Dave Allen's OT earlier. I've got the TO8C in a Tweed Princeton derivative. It is an improvement over Mojo's in terms of more clarity and bigger, tighter bass. Too much bass for an 8" speaker IMHO.

These are all unsolicited and random thoughts:
A 10" speaker and a few tweaks to reduce low end in the circuit would probably give you the best results (cathode bypass caps come to mind right away - maybe 4.7uf on the first stage and 10uf on the second?). I'd plan on tweaking the mid & bass caps in the tone stack as well. An extra filter cap and a Hammond 156L choke before the power tube plate would improve power rail filtering a lot. I would not recommend bigger filter caps - been there, done that. Touch response isn't the same.

Hope this helps,

Chip

[MWJB]

"I know that the 125P1B part number is batted around for single-ended Champs but it's not right." Tinman, why is it not right?

[TheTinMan]

Originally Posted by MWJB

"I know that the 125P1B part number is batted around for single-ended Champs but it's not right." Tinman, why is it not right?

It is the part number shown on Fender's BF Vibrochamp schematic. However, the same part number is on the Princeton Reverb schematic.

I don't understand how they could have been the same PT. The VibroChamp shows B+ of 355 with a 5Y3-GT rectifier tube and only one 12AX7 and one 6V6 drawing current. The Princeton Reverb shows B+ of 420 with a 5U4GB rectifier and three 12AX7s, one 12AT7, a reverb transformer, and two 6V6s drawing current.

Besides, Leo Fender spec'd the cheapest parts available that would do the job right. Why would he put a transformer capable of handling the Princeton Reverb's requirements in a single-ended amp with an 8" speaker?

Still, I may be wrong. I have read conflicting opinions on this topic and thought that the general consensus was that the Champ and the PR did not use the same PT.

Chip

[MWJB]

The 125P1B in a Princeton PT is just about adequate, the 2A of heater current (PR tubes only come to 2.1A) & 70mA B+ supply are borderline (the additional 12A#7 tubes in a PR don't draw appreciable B+ current) & they do occasionally fail (as do the OTs). The same PT in a champ (and it is the same) is pretty well bullet-proof.

BF & SF Champs don't run 355vdc on the plate, they are usually over 400vdc, can be as high as 430v (plate to ground)...then add to that cathode voltage and you get nearer 455vdc. So the numbers don't end up that far apart in reality, as PRs are often in the mid 400's.

The Princeton is fixed bias so can idle both its 6V6s at only a fraction more than the single, cathode biased 6V6 in the champ.

So, as you say, Leo was sometimes thrifty (in other cases not, the ss rectified brown tolex 2x6L6 amps all have 100W PTs) - in this case by asking a lot from the 125P1B in the PR & not bothering to commision a PT specifically for the job (which would have meant buying 2 types of PT in lower numbers than the original 125P1B, pushing up the cost for both parts).

[TheTinMan]

MWJB - your argument makes sense in the context of vintage power transformers. Not having an opportunity to compare a BF Champ PT with one from a BF Princeton, I can't disagree.

However, you also explain that the vintage 125P1B is barely up to the task of powering a Princeton Reverb. Perhaps the source of confusion is modern replacements for these transformers. Isn't there a chance that a new PT designed for a two-6V6 push pull circuit (Princeton Reverb) might be too much for the Champ circuit? (following the "Leo was cheap" theory)

I do know that Mercury Magnetics has different part numbers for a BF Champ and a BF Princeton. The unloaded voltages are very close, but other specs are different. Since they claim that their Tone Clone line of transformers are attempts to replicate vintage iron from specific sample amps, maybe the differences just reflect variations in the 125P1B PTs that Fender used.

Bottom line is that I've read a lot of threads from guys who built a Champ and now want to get their B+ voltage down to vintage territory. Haven't read much about that for Princeton Reverb builds. Makes me think the modern PTs designated for Champs may be over built.

Sorry for rambling too long on this subject.

Chip

[RichardM]

From an amateur like me, if the PT is "overbuilt" surely it would be only in the current (amps) it can supply not the voltage?

i.e. 650VCT at 80mA vs 650VCT at 100mA, not 650VCT at 80mA vs 750VCT at 100mA.

[RichardM]

Originally Posted by TheTinMan

............
Bottom line is that I've read a lot of threads from guys who built a Champ and now want to get their B+ voltage down to vintage territory......

Could be because of the brand/type/year of 5Y3GT rectifier used?

[TheTinMan]

Richard - could be the rectifier, could be higher wall voltages than in the '60s, could be PT design.

I believe, subject to correction, that you could have two power transformers with identical unloaded voltage ratings but with significantly different maximum current ratings. Your voltage goes down when you flip the Standby switch, right? My guess is that if all other things are equal, the loaded voltage would be lower for the PT with the lower current rating.

Unfortunately, I'm an amateur too!

Chip

[RichardM]

Originally Posted by TheTinMan

Richard - could be the rectifier, could be higher wall voltages than in the '60s, could be PT design.

I believe, subject to correction, that you could have two power transformers with identical unloaded voltage ratings but with significantly different maximum current ratings. Your voltage goes down when you flip the Standby switch, right? My guess is that if all other things are equal, the loaded voltage would be lower for the PT with the lower current rating.

Unfortunately, I'm an amateur too!

Chip

Time for an experiment I think.

Anyone?

[MWJB]

Hi Tinman,

Modern replacements will meet the original specs I quoted for currents, unless a PT supplier is building a tweaked version with greater current capacity (not a bad idea). Many factory 125P1B will run voltage equal to, or more than modern replacements. A PT will only be "too much" for a champ if the B+ results in dangerously high B+ for the tube at a reasonable current draw. Many custom amp builders use PTs that are capable of much greater current capacity than original Fender PTs, it's not a problem.

The confusion stems primarily from the fact that the voltages listed on tolex champ schems are low, compared to real life scenario.

FWIW my SF champ with fixed bias & original PT runs 450vdc at 30mA in fixed bias.

Richard M wrote: "From an amateur like me, if the PT is "overbuilt" surely it would be only in the current (amps) it can supply not the voltage?" One affects the other, draw less curent & voltage goes up, draw more current & voltage goes down. If the PT is "overbuilt" these deviations will be less at a reasonable current draw. Again, going back to my SF Champ, If I install a 6L6 and bias up to 50mA I get 390vdc on the plate due to the greater loads on B+ & heater windings...if the PT was substantially overbuilt then the voltage drop with the higher loads would be less.

[RichardM]

Originally Posted by MWJB

Hi Tinman,

Modern replacements will meet the original specs I quoted for currents, unless a PT supplier is building a tweaked version with greater current capacity (not a bad idea). Many factory 125P1B will run voltage equal to, or more than modern replacements. A PT will only be "too much" for a champ if the B+ results in dangerously high B+ for the tube at a reasonable current draw. Many custom amp builders use PTs that are capable of much greater current capacity than original Fender PTs, it's not a problem.

The confusion stems primarily from the fact that the voltages listed on tolex champ schems are low, compared to real life scenario.

FWIW my SF champ with fixed bias & original PT runs 450vdc at 30mA in fixed bias.

Richard M wrote: "From an amateur like me, if the PT is "overbuilt" surely it would be only in the current (amps) it can supply not the voltage?" One affects the other, draw less curent & voltage goes up, draw more current & voltage goes down. If the PT is "overbuilt" these deviations will be less at a reasonable current draw. Again, going back to my SF Champ, If I install a 6L6 and bias up to 50mA I get 390vdc on the plate due to the greater loads on B+ & heater windings...if the PT was substantially overbuilt then the voltage drop with the higher loads would be less.

Thanks for that.

All clear now.