Anyone use the Mojo PT for Tweed Champ??

Ringwraith · 03-30-2009, 04:31 PM

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 · 03-30-2009, 04:49 PM

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 · 03-30-2009, 05:03 PM

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 · 03-30-2009, 05:07 PM

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 · 03-30-2009, 05:11 PM

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 · 03-31-2009, 07:17 PM

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 · 04-03-2009, 07:33 AM

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 · 04-03-2009, 07:39 AM

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 · 04-03-2009, 09:30 AM

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 · 04-03-2009, 02:39 PM

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 · 04-03-2009, 03:02 PM

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 · 04-03-2009, 03:09 PM

"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 · 04-03-2009, 06:31 PM

Quote:

Originally Posted by MWJB

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 · 04-04-2009, 12:16 AM

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 · 04-06-2009, 09:23 AM

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.

Ringwraith · 04-06-2009, 03:04 PM

Pin 1 & 2 on the 6V6 are joined & grounded to the lug near the 12AX7.
There was a bit of wire sticking out the back of pin 1 so it looks like it's part of the cathode resistor wire which is connected to pin 8 only.

I just measured the 470 Ohm cathode resistor & it's measuring 750ohms!!
Not sure how it could be that far out of whack but I'll order some 470's &
check the current again. With 750 ohm, this would mean the amp is under biased right??

Sean

MWJB · 04-06-2009, 03:13 PM

Yes, a 750ohm resistor would account for the low bias. It would also raise your B+, this will come down when you fit a 470ohm. Don't muck about with 1W resistors, use a 5W, use a 50v rated cap and keep the cap physically away from the resistor.

Ringwraith · 04-07-2009, 05:57 AM

So I'm digging through my supplies looking for a 470 ohm & of course I don't have one. I'm about to make a couple orders from the suppliers & I'm thinking this sucks because I'm going to have to wait to get this champ going again! I rigged up another resistor in parallel to get me about 490ohms & noticed a huge improvement!!! 2 seconds later the post guy's at my door with a package from CE Dist. ;-) I don't remember ordering 470 ohm resistors last week but there they were, a 5 pack of 2W Metal Oxide sitting in the box!! So I popped it in & again, the amp is sounding much better! Fuller, rounder with more guts maybe a tad louder. Basically all the things a hotter, or in this case properly biased amp brings! lol The original resistor must have been out since day one or pretty soon after because the amp never sounded this good! I actually built it about 2 years ago & have never really been impressed with it until today! Only got to play it for a bit but am looking forward to tomorrow when I can really wined it out!

Here are some measurements & as you said, the voltages did change a lot...

EH 6V6
B+ from 391 to 367
Plate 385 to 361
Cathode 25 to 19
Current 30ma to 39.6ma
12AX7 Plates 198 to 165

Thanks for all you help MWJB!!

Cheers
Sean

black_labb · 04-08-2009, 07:27 PM

not sure about a champ, but most tweed era amps tend to have the plate voltage on the 12ax7's closer to 150v or so from memory. 198v could be part of the reason for the fizzy top end. lower voltages should give a "browner" sound (whatever that means

)

MWJB · 04-09-2009, 09:24 AM

Most tweed amps (late 50's) might have more like 180-235vdc on the plates of a 12AX7 in V1. The bigger amps typically used 12AY7 which have a higher current draw and return less plate voltage than a 12AX7 in the same circuit.

As a rule of thumb, in a Fender preamp configuration (100K plate/1.5K per cathode) a 12AX7 shows around 2/3 the voltage applied to the plate resistor, a 12AY shows around half the voltage.

black_labb · 04-10-2009, 08:26 AM

Quote:

Originally Posted by MWJB

Most tweed amps (late 50's) might have more like 180-235vdc on the plates of a 12AX7 in V1. The bigger amps typically used 12AY7 which have a higher current draw and return less plate voltage than a 12AX7 in the same circuit.

As a rule of thumb, in a Fender preamp configuration (100K plate/1.5K per cathode) a 12AX7 shows around 2/3 the voltage applied to the plate resistor, a 12AY shows around half the voltage.

good to know, thanks.
to the op, sorry for the confusion

03-30-2009, 04:31 PM	#1
Ringwraith Member Join Date: Apr 2008 Posts: 46	Anyone use the Mojo PT for Tweed Champ?? 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

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03-30-2009, 04:49 PM	#2
MWJB Old Timer Join Date: Jun 2006 Posts: 1,924	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?

03-30-2009, 05:03 PM	#3
Ringwraith Member Join Date: Apr 2008 Posts: 46	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

03-30-2009, 05:07 PM	#4
Ringwraith Member Join Date: Apr 2008 Posts: 46	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

03-30-2009, 05:11 PM	#5
MWJB Old Timer Join Date: Jun 2006 Posts: 1,924	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?

03-31-2009, 07:17 PM	#6
pentatone Junior Member Join Date: Nov 2008 Posts: 20	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!

04-03-2009, 07:33 AM	#7
Ringwraith Member Join Date: Apr 2008 Posts: 46	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

04-03-2009, 07:39 AM	#8
Ringwraith Member Join Date: Apr 2008 Posts: 46	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

04-03-2009, 09:30 AM	#9
MWJB Old Timer Join Date: Jun 2006 Posts: 1,924	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.

04-03-2009, 02:39 PM	#10
Ringwraith Member Join Date: Apr 2008 Posts: 46	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

04-03-2009, 03:02 PM	#11
MWJB Old Timer Join Date: Jun 2006 Posts: 1,924	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.

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04-03-2009, 03:09 PM	#12
MWJB Old Timer Join Date: Jun 2006 Posts: 1,924	"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.

04-04-2009, 12:16 AM	#14
Ringwraith Member Join Date: Apr 2008 Posts: 46	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

04-06-2009, 09:23 AM	#15
MWJB Old Timer Join Date: Jun 2006 Posts: 1,924	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.

04-06-2009, 03:04 PM	#16
Ringwraith Member Join Date: Apr 2008 Posts: 46	Pin 1 & 2 on the 6V6 are joined & grounded to the lug near the 12AX7. There was a bit of wire sticking out the back of pin 1 so it looks like it's part of the cathode resistor wire which is connected to pin 8 only. I just measured the 470 Ohm cathode resistor & it's measuring 750ohms!! Not sure how it could be that far out of whack but I'll order some 470's & check the current again. With 750 ohm, this would mean the amp is under biased right?? Sean

04-06-2009, 03:13 PM	#17
MWJB Old Timer Join Date: Jun 2006 Posts: 1,924	Yes, a 750ohm resistor would account for the low bias. It would also raise your B+, this will come down when you fit a 470ohm. Don't muck about with 1W resistors, use a 5W, use a 50v rated cap and keep the cap physically away from the resistor.

04-07-2009, 05:57 AM	#18
Ringwraith Member Join Date: Apr 2008 Posts: 46	So I'm digging through my supplies looking for a 470 ohm & of course I don't have one. I'm about to make a couple orders from the suppliers & I'm thinking this sucks because I'm going to have to wait to get this champ going again! I rigged up another resistor in parallel to get me about 490ohms & noticed a huge improvement!!! 2 seconds later the post guy's at my door with a package from CE Dist. ;-) I don't remember ordering 470 ohm resistors last week but there they were, a 5 pack of 2W Metal Oxide sitting in the box!! So I popped it in & again, the amp is sounding much better! Fuller, rounder with more guts maybe a tad louder. Basically all the things a hotter, or in this case properly biased amp brings! lol The original resistor must have been out since day one or pretty soon after because the amp never sounded this good! I actually built it about 2 years ago & have never really been impressed with it until today! Only got to play it for a bit but am looking forward to tomorrow when I can really wined it out! Here are some measurements & as you said, the voltages did change a lot... EH 6V6 B+ from 391 to 367 Plate 385 to 361 Cathode 25 to 19 Current 30ma to 39.6ma 12AX7 Plates 198 to 165 Thanks for all you help MWJB!! Cheers Sean

04-08-2009, 07:27 PM	#19
black_labb Senior Member Join Date: Apr 2007 Location: sydney australia Posts: 777	not sure about a champ, but most tweed era amps tend to have the plate voltage on the 12ax7's closer to 150v or so from memory. 198v could be part of the reason for the fizzy top end. lower voltages should give a "browner" sound (whatever that means )

04-09-2009, 09:24 AM	#20
MWJB Old Timer Join Date: Jun 2006 Posts: 1,924	Most tweed amps (late 50's) might have more like 180-235vdc on the plates of a 12AX7 in V1. The bigger amps typically used 12AY7 which have a higher current draw and return less plate voltage than a 12AX7 in the same circuit. As a rule of thumb, in a Fender preamp configuration (100K plate/1.5K per cathode) a 12AX7 shows around 2/3 the voltage applied to the plate resistor, a 12AY shows around half the voltage.