Anyone use the Mojo PT for Tweed Champ??

[Ringwraith]

I built a 5F1 & used the PT from CE Dist. I think my voltages are too high.
According to the old schematics, B+ should be closer to 340. Mines at 392. The PT I got from CE (P-TF22772) says 325-0-325 @70ma on the spec sheet. The Mojo759 says 290-0-290 @70ma on the spec sheet. I'm just wondering if anyone has used this PT & got better B+ voltages?

Thanks
Sean

[MWJB]

392v at the B+ is fine...340vdc is very, very unlikely for a tweed champ in reality.

A lower B+ is only "better" if it sounds better to you (I'm not saying it definitely won't) so how does the amp sound?

[Ringwraith]

It's ok but there's tons of really trebley fizz on top when pushed. Reminds me of solid state distortion. Not sure if the voltage would change this but I think the PT I have is more for BF/SF era champs which had higher voltages. I've tried a few different speakers/caps/tubes but I still have the thin fizz. Maybe this is normal for a 5F1 from about 8 up??

Cheers
Sean

[Ringwraith]

I should also mention that there's sometimes a weird buzzing coming from the innards when not played. I think it's the PT & was thinking I should replace it anyways. Not sure if it's about to fault or short out. This buzzing comes & goes.

Cheers
Sean

[MWJB]

Doesn't sound typical. What preamp voltage do you have at pins 1 & 6 of the 12AX7?

SF Champs run more B+ voltage than you have, up to 430vdc. Let's say your 5F1 champ really did put out 340v in 1950 whatever (& probably rose somewhat before going to tolex), add another 15% and you get 391vdc. Voltages can be 10-15% higher than the schems, so you are in the ball-park.

Any pics of the build?

[pentatone]

Hi Sean -

it's only an idea but if you haven't already you should consider installing a NOS 5Y3 rectifier tube. These 5Y3s produced nowadays (mostly Sovtek) don't have this much voltage drop like a NOS one.
Let's do some math. Given 325-0-325 VAC on the secondary of your PT should give you about 325*1,13=365 VDC (1,13 ist the conversion factor of a "real" 5Y3) after the rectifier. If you have 392 VDC the conversion factor must be 1,2 which principially correspondends to a 5U4 rectifier tube and to the modern Sovtek 5Y3s (Sovteks are acting pretty similar to NOS 5U4s).

Again - it's only an idea.


Kind Greetings!

[Ringwraith]

Ok it's been awhile but I'm back with more voltages & pics! ;-)

These voltages & the ones I listed above are with a NOS Ken-Rad 5Y3.
I've tried a modern day Mesa 5Y3 & get even higher voltages! LOL

Ok...
118-119AC off the wall

348 VAC off each PT secondary lead no tubes in
Isn't this reading supposed to be 325-0-325???

388 VDC B+
351 VDC Second Filter cap
304 VDC Third Filter cap

381 VDC Raytheon 6v6 Plate
24 VDC Cathode
30.7ma plate current

198 VDC EH 12AX7 Plates
1.5 VDC Cathode











Cheers
Sean

[Ringwraith]

FWIW, here's the CE Dist PT Specs (P-TF22772)

Primary: 117 volt, 60 Hz
Secondary: 325-0-325 V, 375VDC@70 mA(DC)
Filament winding 1: 6.3 V, 2 A
Rectifier Filament: 5 V, 2 A
Mounting Centers: 2" x 2.5 horizontal
Weight: 2.9 lbs

Also for interests sake...
A while back, someone posted unloaded voltages from a
real '56 champ PT, using a Variac to adjust the PT for different line voltages:

110 VAC -> 240-0-240
117 -> 255-0-255
120 -> 262-0-262
123 -> 268-0-268

Cheers
Sean

[MWJB]

Your voltages are not the problem, 348-0-348 loaded sounds spot on to me. Written specs are nominal and should be taken as just a guide, wall voltages, manufacturing tolerances, bias etc all affect what you get in reality.

Those overly long OT secondary wires could do with shortening and not flying over the preamp tube.

Plate wires (pin 1 & 6 12AX7) should be pressed down to the chassis floor, grid wires (pins 2 & 7 of 12AX7 and pin 5 of 6V6) should be relatively short & direct (leave some slack for when/if the board warps) & should not be running parallel & close to any plate wires or heater wires.

Are pins 1 & 8 of the 6V6 joined? Could be optical ilusion but exceswires at pins 1& 2 of the 6V6 look like they could be shorting.

How did you come up with 30.7mA plate current, at 381v with a 470ohm cathode resistor, I'd be a little surprised if this was so? 30.7mA is a little low, if all checks out try a different 6V6.

Those real '56 champ voltages? Hmmmm, I'm not convinced. Besides, yours isn't a '56.

Most folks would have buit this amp with a two wire twisted pair filament circuit, with 2x100ohm resistors to ground (one rom each leg) as a a virttual centere tap.

[Ringwraith]

Thanks for having a look MWJB.

This is/was my first build. I sourced the parts & went off the Fender schematic/layout & old pics.

I'll adjust my wires as you mentioned. I'm assuming these suggestions are to reduce noise/hum?? I've since noticed folks do the heater wires different but at the time I just did them like the originals. The amp is pretty quiet but I'm sure it would be even better once the lead dress is improved.

Regarding the OT wires, should these be as short as possible on the other side of the chassis as well?? Or should I just push them back through to get the excess out of the inside?

Pins 1 & 8 on the 6V6 are not joined.

I'm confused about what loaded & unloaded means. Isn't loaded with tubes in, drawing current?? I'm getting 348 VAC with no tubes in.

I got 30.7ma measuring from the 6v6 plate to the other leg of the OT which is attached at the first filter cap.

I know my amp isn't a 56 but I'm just trying to get it a s close as possible to an original.

Thanks again for your time!
Sean

[MWJB]

I'd cut the OT wires short, long enough to reach the jack with a little slack. Also solder a jumper from the ground tab to the switch on the jack plug, so in the unlikely event that you pull the speaker cord out & power up the amp, the jack will be grounded rather than open & less likely to kill the OT/tube immediately.

The layout tweaks can reduce noise but also improve tone & reduce the risk of parasitic oscillations.

Loaded means with a full tube compliment, drawing reasonable current. It's normal to measure voltages with the tubes installed, otherwise everything reads artificially high due to lack of current draw. Your champ is unlikely to draw 70mA so you may never see 325-0-325, but don't sweat it, your voltages are close to an original.

Try another 6V6.

[MWJB]

"Also solder a jumper from the ground tab to the switch on the jack plug," I mean jack socket, just so there's no conusion.

[Ringwraith]

Try another 6V6.

What should the current be closer to??

I've tried a bunch & the highest I got was with a JJ 6V6 at 33.6ma.

I get higher current draw out of these same tubes in my 5E3 build.

Thanks
Sean

[Ringwraith]

A little off topic here but seeing how you've got my 5F1 info above...
What would I need to change to run a 6L6 in this amp?

Cheers
Sean

[MWJB]

OK, current should be bout the same as your 5E3 build if that has similar plate voltageand a common 250-270ohm cathode resistor.

You say that pins 1& 8 are not joined, but assuming your grounded heater wire goes from pin 2 to the ground tag near the 12AX7, what is that connected to pin 1? Looks like the cathode resistor. Triple check that you have the correct pins wired up on the 6V6. Pin 8 goes to the cathode resistor, 2 & 7 are the heaters, pin 3 gets the blue PT wire, pin 4 goes to the junction of the 10K & 22K power supply resistors, pin 5 gets the green wire that exits the board between the preamp filter cap (RH most) and the 470ohm cathode resistor.