Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: blown240 on September 15, 2011, 09:42:50 am
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I picked up a vintage Philco radio from a Garage sale a few days ago. It doesnt work, all the tubes are gone and the the speaker is shot. Originally my wife just wanted to use it as decoration, but I figure why not do something cool with it.
SO, I want to build it into a simple guitar tube amp. I was leaning towards a champ, but I am open to suggestions. There are a few specific things I want from this build.
1. I dont want to modify the original radio cabinet.
2. Since its got 4 knobs originally, I can only have 4 knobs/switches, inputs total. So probably a power switch, input jack, volume, and either gain or tone.
3. It will only take a 6" speaker, so power will have to be fairly low. Maybe 5-10 watts.
4. I would like to use the original radio output transformer, but I realize that I am asking alot with this one.
I hope to not spend too much $$$, and I have a box of misc amp parts from a build that I did about 10 years ago.
Does anyone have a suggestion/schematic for what I should build? I am a real newbie with amp building and have only ever built a preamp, so it needs to be pretty straight forward.
Thanks!
Here is a pic of the radio. I will get pics of the rear later.
(http://i268.photobucket.com/albums/jj26/thebattens/2011-09-14102504.jpg)
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I would just restore the radio.
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The radio isn't worth much restored. And it takes 7 tubes that are hard to find. It wouldn't be cost effective to restore it. Plus I have a couple other vintage radios that already work.
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>>IF<< you wish to go the restore route and need/want tubes, PM me, I probably have them, yours for the shipping.
The obvious choice is to build a Champ-type affair and not to get too fancy. A 6" speaker imposes quite the limitation on how good the thing might sound, IMO, unless you buy a high quality one.
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>>IF<< you wish to go the restore route and need/want tubes, PM me, I probably have them, yours for the shipping.
The obvious choice is to build a Champ-type affair and not to get too fancy. A 6" speaker imposes quite the limitation on how good the thing might sound, IMO, unless you buy a high quality one.
Thanks for the tube offer. I will get a list to see if you have them, though I will probably make it an amp.
I agree on the speaker limitations. I have a 12' Jensen I would like to use, but its just TOO big. I also need to find the specs on the stock output transformer to see if it will at all work. I have a Bassman preamp that I built, I may see if I can mate it to the power stage of the original radio. That would either be awesome, or a total pile of junk.
The Champ is definatly the way to go for a ground up build, I am not aware of one with 2 knobs though. Just the single volume knob. Is there one with a tone or gain knob as well?
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18Watt Lite II may be a candidate. It's a small, simple 2 knob amp. May sound kinda cheap and tinny with a 6" speaker, but that's probably gonna be an issue with any amp.
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I love that radio cabinet! Beautiful wood. Craftsmanship back then was incredible. It should have something nice done with it. Looks like maybe a 8" speaker would work or a 10"?
With respect, Tubenit
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I have an 8" car sub and a 10" car sub, and a 12" jensen guitar speaker, they will at least let me experiment with speaker size. Once I figure out if the output transformer is good and how big it is, that will help me decide on what to build. Its pretty heavy, so the transformer may have some grunt behind it, or its just REALLY old.
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Vibro champ has vol. and tone, just leave out the vibro part.
Brad :icon_biggrin:
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I believe a Vibro Champ has volume, treble, and bass.
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If you can get a 10" in there you might consider a Fender Pro Junior- two controls and a switch. You could use switched pot as well. Simple circuit with push-pull EL 84 power amp. Or go way back to a Champion 600/800- I think they only had volume and tone controls. You can also try one or more Denelectro guitar case amps, no power tranformer. Weird tubes there, though. Looks like a fun project, good luck.
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I believe a Vibro Champ has volume, treble, and bass.
Doh! :BangHead:
Well how about a 5E3 tone control? Just 1 knob.
Brad
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Must be a perspective thing, but that looks bigger than a 6" speaker.
what is the tube compliment?
does it have a 6.3 and 5V tap? or will you not salvage the PT?
That sure is some pretty veneer!
It was likely finished in shellac or varnish but nitro is not out of the question either.
Do you plan a refinish? or leave the patina?
Cool project!
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The Fender Princeton 5F2 and 5F2-A are two knob. Volume and Tone. Basically a Champ with tone control.
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Thanks for the replies and suggestions guys!!
I am definatly going to keep the patina. the only thing I will replace is the grille cloth since it ripped. I am going to get some pics of the inside of it and the other parts I have and will post them when I have a chance.
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If you want the original type grille cloth, grillecloth.com may have that color & pattern.
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Recall that tweed Champs and Princetons used a 1M volume pot which had a switch on the back, which is used to switch on the line cord power. The switch engages when you turn the volume above zero.
As a result, the part is still available. Doug has them. That will save you a control position.
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OK, Here are some pics of exactly what I got. Some good, some bad:
The Front:
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5569.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5569.jpg)
The Back:
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5570.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5570.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5571.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5571.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5572.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5572.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5573.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5573.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5574.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5574.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5575.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5575.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5576.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5576.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5577.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5577.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5578.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5578.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5579.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5579.jpg)
Here are some parts I have that I can use:
Caps, resistors, tubes, sockets, turrets, diodes, etc:
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5580.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5580.jpg)[/img]
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5581.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5581.jpg)[/img]
A little crate amp I can rob stuff from, maybe the OT:
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5585.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5585.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5583.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5583.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5584.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5584.jpg)
This is my first build ever, its the preamp section on an Ampeg B15n. I can rob it of parts it need be because it doesnt sound very good:
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5590.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5590.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5593.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5593.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5592.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5592.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5591.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5591.jpg)
And HERE Is the coolest part of the whole deal
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5589.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5589.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5586.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5586.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5588.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5588.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/IMG_5587.jpg (http://i268.photobucket.com/albums/jj26/thebattens/IMG_5587.jpg)
Looks like I can squeeze a 12" speaker in with a little work. That will make a HUGE difference.
OK boys, start throwing your ideas at me. I really want to do this! I bet with what I have and what I want to do, I can do the whole thing for $100 out of pocket.
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It really should be repaired.
It would be a sweet guitar amp.
Recall that Tweed Champ and Princeton _ARE_ radio parts and circuit minus tuner. The parts Leo got in late 1940s are cheap generics of what Philco used in 1935.
'75 is a half-12AX7.
'42 is a 6F6 on a 6-pin base. 6F6 is a lower-gain precursor to the 6V6.
'80 is 5Y3 on 4-pin base.
A '75 is only $13. '42 can be had for $23 or less. '80s go for $70!
If you drill the socket rivets you can probably drop octal sockets right in, use 6F6 directly or 6V6 with mods.
The only non-Tweed bit is the SE power stage and interstage transformer, and the final stage working in AB2 Triode mode. Might not be raucous enough for a tavern, might be fine for the parlor.
Challenges: the grid bias is back-bias, needs ALL the radio current including tuner plus the field-coil current. The wiring will probably make you insane.
There's something wired to the top of the volume control. Cut it. Feed an iPod here, it will work fine (after repair). Tone self-adjusts at lower volumes, you also have four switch settings. For guitar you would need one more stage.
The PT, choke, and OT can be used directly in any modest two-6V6 plan.
3-pin cord of course.
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The Phico OT is surely 3-4 ohms. Your 16 ohm monster will be a very poor match.
I like the look, but a parlor radio should be 6" or 8", maybe 10" if the power stage is weak. A 12" is really too big IMO.
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PRR- Thanks for taking the time to type all that. There is really some good info there.
I can get a 4 ohm 12" if I need to. Or I have the 4 ohm 8" in the crate amp. What would a modest two-6V6 plan be? And should I lean towards that instead of a champ? I wish I knew more about electronics and building amps, but at this point about all I can do is follow a schematic. Its funny, I used to be an electrician. I can wire a mansion by myself, but cant wire an amp without a guide.
Also, I like the idea of using a volume now withe the on/off built in. Now I have room for a 3 knob setup!
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And what would happen if I used the Philco output transformer in a champ build? Is it a problem to use a transformer that is too big?
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OK, well after spending half the night up looking up info and staring at schematics I think I have figured out where I am going with this.
It will basically be a Champ AA764 with a switched volume. That way I have volume, bass and trebel control. I will use the 4ohm 8" speaker from my little crate amp. And I will use the transformers from the original radio. This is assuming that I can use the output transformer with a single 6V6GT without any problems, and assuming the original transformers work. I think the only thing I will have to buy are a couple tubes, and the switched volume pot.
If anyone has input, I would greatly appreciate it!
Other option is a Champ 5F1 with an added tone stack.
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Output Transformer- All you need is the proper impedance ratio and the wattage capacity. It sounds like the OT is big enough to handle an SE 6V6. You get impedance for a given load, your 4 Ohm speaker, by multiplying the square of the turns ratio by theactual load impedance. To determine turns ratio, put a convenient small AC voltage on the primary, like 10 VAC from your Variac. Measure the secondary voltage right around .27 VAC- This would give you a turns ratio of 37:1. Square that for 1369. 1369 x 4 =5476 Ohms. Pretty good for a 6V6 class A nominal load of 5500. If you don't have a variac, see if you have a wall wart transformer that has an AC output, like 9VAC. Use the same formulas substituting 9VAC. Allbest, Larry
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Output Transformer- All you need is the proper impedance ratio and the wattage capacity. It sounds like the OT is big enough to handle an SE 6V6. You get impedance for a given load, your 4 Ohm speaker, by multiplying the square of the turns ratio by theactual load impedance. To determine turns ratio, put a convenient small AC voltage on the primary, like 10 VAC from your Variac. Measure the secondary voltage right around .27 VAC- This would give you a turns ratio of 37:1. Square that for 1369. 1369 x 4 =5476 Ohms. Pretty good for a 6V6 class A nominal load of 5500. If you don't have a variac, see if you have a wall wart transformer that has an AC output, like 9VAC. Use the same formulas substituting 9VAC. Allbest, Larry
Great info. Thanks.
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From the "For what it is worth" section..I built a Fender 5B3 earlier this year using the volume pot with the switch built in to switch the power on and off. I got quiet a bit of hum from it. Had to go to a regular switch. Those old Fenders did not use a stand-by switch so that saves you one position.
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Cool thanks. I can always do a rear toggle switch for power. That will keep the 3 year old from turning it on too!
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Push-pull OT is a terrible SE OT.
I think it should be restored nearly as it was designed. However this would be an incredibly tedious job. Really not sure what you should do.
Short-term, get a little chip-amp and tuck it inside the Philco case.
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Restoring it really isn't an option. I simply don't have the cash or knowledge to do it. However, it seems that I have most of what I need to make it an amp. I thought the PT, choke, and OT can be used directly in any modest two-6V6 plan. Is there a difference with a champ? And if so, whats a good 2 6v6 plan?
Thanks again for the input. I am hoping to get out to check the transformer tonight. I guess worst case, I can buy a Champ OT, they are only like $30.
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I pulled the chassis out to see what I was starting with. Its pretty ugly inside. The cloth covered wire is coming apart, and there have been repairs done in the past. I figured I had nothing to loose, so I plugged it in to see what would happen and it tripped a breaker. No biggie.
I took some more pics:
Just to make sure I know what I am talking about, are these labeled correctly?
http://i268.photobucket.com/albums/jj26/thebattens/Philco/Trans.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/Trans.jpg)
The Guts:
http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5607.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5607.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5604.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5604.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5605.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5605.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5609.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5609.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5606.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5606.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5608.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5608.jpg)
The Enclosure:
http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5598.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5598.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5597.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5597.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5599.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5599.jpg)
Rubber mounted Chassis:
http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5600.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5600.jpg)
Speaker:
http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5594.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5594.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5595.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5595.jpg)
Transformer on a Speaker???
http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5596.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5596.jpg)
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Since it needs a total rewire, and tubes just to start, restoring it is out of the question. I may just completely gut it and use the chassis and enclosure to build a champ.
I am trying to make a list of parts that can be reused.
1. Chassis
2. Enclosure
3. Power Transformer ??
4. Choke ??
5. Output Transformer ?????
6. Recifier tube socket ??
Is it worth reusing any of the old caps and resistors to get a vintage sound? Or will that just cause more problems?
The speaker is definatly an 8", so I will use the speaker out of my little Crate amp.
I have a Hammond 262F6 transformer in another build. Will that do me any good here?
Any input is appreciated!
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> are these labeled correctly?
No. The OT is on the speaker. What you marked is the driver transformer. Innerestingly the DT cost more than the OT. (In part this is SE/PP difference.)
PM me an email address where you can take a 2MB PDF file, I'll give you full schematic and excellent parts layout diagram. Philco documented this stuff well.
I spotted one odd thing. The schematic shows "1.11 ohm" next to the speaker voice coil. This may actually include the OT winding. Can you break a connection from OT (which is on the speaker!) to voice coil, and read the voice coil alone? If it is 3 or 6 ohms, then it is wound for 4 or 8 ohm close-enuf. If the VC is really 1.11 ohm DC, then it is a "1.3 ohm nominal" speaker and that's very awkward.
The main filter-caps are the round cans inside "TV-shape" shields, a very odd thing. The service data confirms they are electrolytic (thus now scrap) and you can get new ones for $1.10-$1.80 (if this were the 1930s all over again!).
Otherwise....
Have parts been removed other than tubes? Has the cloth rotted off the wires real bad? Have you seen seriously burnt/melted parts?
If not, then it really is less work, the way I see it, to replace the rotten and obsolete audio bits than to rip and ravage. (I'm NOT talking about fixing the tuner; it likely just needs tubes, and is the best AM you will ever hear, but the AM band is crap and better SW radios are cheap.)
replace these caps:
C55 0.02u
C63 0.05u
C59 0.25u
C77 10u 75V + 8u 450V (100u 50V + 22u 450V)
C78 8u 450V (22u)
C71 3u (22u 400V)
three '42 sockets change to octal H H K G1 G2 P
verify 9-11 ohms at "10 ohm" tap on R66
short live end of R66 to center-tap, adjust for 200 ohms to ground
replace field-coil with 1K 10W resistor
two 1N4007 and two 250r 10W resistors on '80 socket (genuine high-loss 5Y3 hard to get)
three 6V6
smoke-test, quickly verify -1.0V at 10 ohm tap
squirt volume pot, tone switch
snip top of volume pot, feed iPod
build 1-stage guitar preamp
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none of the wires are totally bare. But if you move them too much they come apart. In my pics there is a little square box that looks like the bottom has melted, and there is a cap that may have leaked.
I have the bassman preamp that I posted pics of earlier. I can use that as a preamp I guess?
I need to see if the OT comes off of the speaker. I will let you know tommorrow.
When you say snip top of volume pot, feed iPod. Do you mean to make it an iPod player, or is that where I would feed a preamp into?
Thanks!
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The OT is riveted to the speaker cage, so it can come off.
It has 4 wires in and 2 out to the speaker.
http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5616.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5616.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5615.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5615.jpg)
http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5614.jpg (http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5614.jpg)
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That speaker is repairable with rubber cement and tissue.
me> build 1-stage guitar preamp
Got it.
RF stage is type '78, sells for $8. Pentode is fine preamp. Most wires may be left as-is.
Replace R32 with 0.68u (standard Marshall cathode cap).
Break plate lead. Add 33K from plate to a B+ source. Add 0.01u from plate to top of volume control.
For added fun: snip some wires on AM/SW switch, use that to select guitar preamp or line (iPod) input.
> The OT ...has 4 wires in and 2 out to the speaker.
The field-coil is also in there. One end is high B+ and OT CT. The other end comes back to power the low B+ loads.
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ok Cool.
I understand MOST of what you have told me. I will get started on it ASAP and will have many questions I am sure!
"Pentode is fine preamp." Is that what my Bassman preamp is? I just followed a schematic when I built it.
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I met a local guy today that has vintage tubes and radio parts stacked floor to ceiling in his house. I took the radio over to him and he ran a few tests on it. Its pretty bad. He gave me a lot of information that was way over my head but basically the inside of this radio has alot of bad caps and burnt resistors that I didnt notice.
He is a radio restoration guy, and he suggested that I gut it and make it into an amp. He did have some of the original tubes I would need, but I dont want to spend extra $$$ on it until i figure it all out.
So now I am really confused, and am leaning towards gutting it and building a champ.
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I spoke some more to the local guy this afternoon, and now I am looking for a simple push-pull 6V6 schematic with 1 or 2 12AX7 preamp tubes. This should allow me a decent little amp and still allow me to use my transformers.
Does anyone have a good idea for a simple push-pull 6V6 schematic with 1 or 2 12AX7 preamp tubes?
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How about this one?
A 5E3 Deluxe- you can modify the volume controls to one concentric stacked pot maybe.
http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/fenderamps/deluxe_5e3_schem.pdf (http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/fenderamps/deluxe_5e3_schem.pdf)
Have fun, Larry
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Ya that may be the way to go. THANKS
I just found some interesting info:
1. The Power Transformer is 110v 25hz, which if I am not mistaken will be fine to use, but may give me a higher B+
2. The output transformer is a part number 32-7078. Its a push pull. The Secondary impedance = 1.25 ohms and the Primary impedance = 5,580 ohms. From what I understand I would need a 1.25 ohm speaker. Thats not really going to do the trick!
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You can use a 2ohm speaker and the reflected impedance will rise a bit
or you can use 2 paralleled 2ohm speaker (like if it is a 1ohm speaker) and the reflected impedance will drop a bit
1.25ohm and 2ohm or 1ohm are not so different
the impedance of an OT a Speaker is not constant referring to frequency
Kagliostro
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Thanks again for the speaker info.
I may be wrong, but it seems that it would be cheaper to get another OT then to get a 2ohm speaker. I will need to look into that though...
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I took my radio to a local amp builder that I know. He is a super cool guy and he helped me get together a game plan. I also got my tubes, sockets, and a nice used 20watt output transformer that has 4, 8, and 16 ohm output. He also gave me some grille cloth.
So, Heres the plan.
Use the stock power trans, and choke.
5y3 Rectifier
1 12ax7 preamp for now, but possible add 2 more preamp tubes later.
2 6v6 power stage
20 watt, selectable impedance out put trans.
When I got home, I stripped the chassis, got the enture thing to bare metal, and painted the outside metallic gold.
Here are some cell phone pics:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-09-19_200320.jpg)
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-09-19_200332.jpg)
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When I got home, I stripped the chassis, got the enture thing to bare metal, and painted the outside metallic gold.
OK, come on now, how did you strip it/get it to look like that??? :undecided:
Looks great!
Brad :icon_biggrin:
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I used a flap disc in a 4" grinder. I am also building a 51 Chevy and have been doing a lot of custom body work. So for me the metal work is the easy part.
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I got the layout for the chassis figured out today. Here is a a couple pics of where I am at so far, If you see any problems with this layout, please let me know:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-09-24210140.jpg)
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-09-24210227.jpg)
Here is the inside, nice and roomy:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-09-24210717.jpg)
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-09-24210204.jpg)
I had a few extra tube sockets, so I used them to fill in some of the holes. There are 2 extra octal and 2 extra small 9 pin sockets. I would like to use them and was wondering if I should plan a second channel or something while I am at this stage. What do you think?
Here is how my power tranny read out:
There are 9 wires total. I marked the primary before I took it out of the original setup, so that was easy.
There are 2 wires that are black, they are 120v each or 6v together. This must be for the Filaments
Thera are 2 brown wires that are 74v and 75v each or 5v together. These must be for the rectifier
And then there are 3 tan colored wires: Here lies the question.
The separate voltages to ground are 33v, 252v, and 109v
The 33 and 109 together = 410v
The 252 and 109 together = 410v
And the 33 and 252 together = more than my meter can read.
This leads me to think that the 109 wire is the center tap, but I may be wrong. Also I THINK that originally the 109 wire went off by itself somewhere...
Does this make sense?
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-09-24215607.jpg)
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When ALL winding ends/taps are lifted from ground, the "to ground" voltage readings are practically bogus.
I wish you had noted goes-to-what before you (expertly) chopped. It hasn't changed much. Two HV ends go to rectifier plates. The CT often goes to ground, though Philco took it a few hundred ohms "below ground" to get bias.
Resistance readings should suggest which is CT.
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I checked the resistance readings and from what I found the CT is what I thought it was. Yesterday I got all the heater wires and the rectifier done. I am getting 368v on the B+ coming off the rectifier (using a 5Y3GT).
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> I am getting 368v on the B+ coming off the rectifier
With or without a filter-cap?
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Without a filter cap. That is straight off the tube.
WHY? is that bad?
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The DC on the cap will be up-to 1.4/0.9 times higher, 578V.
With a couple 6V6 sucking 80mA-100mA through a true 5Y3 it will be 430V-410V DC.
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Is that good?
Sorry....
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Plan a place for a HOT dropping resistor to get down in the 300s.
There's a 1K slide-tap back-bias resistor on the original which might be the cat's meow.
Unless you are gigging-out, a couple 6V6 shouldn't need more than 300V 350V or so. In cathode-bias with reasonable OT impedance, you really don't want more than this, tubes melt or clean power drops. (400V works for fixed-bias max-scream stage amps.)
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I am planning on running the choke that was in the original radio. Hopefully it will drop the voltage some, if not then your suggestion sounds great. Thanks again for all your help and info!
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According to PRR's schematic, there was a 1140 ohm field coil in series with a 7750 ohm resistor placed directly across the B+ line. That's a pretty hefty load and will probably knock the B+ down to a managable level. You could use the original 7750 resistor plus a 1K/10W to simulate the field coil, or you could use a single high wattage resistor.
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I mis-remembered. There is a 10+110+130= 250 ohm high-Watt resistor, near a dead spider, known to be good for dropping quite a few Volts at full radio current (more than your amp will pull). It may not drop a lot, but it is worth saving, if you didn't grind it out.
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I am pretty sure I still have it. I know the mounts for it are still there. I would more it though, since my preamp board sits there now. Thanks for the info!
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Well I dont have that big resistor any more. I thought I saved it, but I guess not. While I was at it, I made a list of everything I need, and what I have. I have alot less than I thought I would. Oh well, time to make an order unless I can substitute...
Here is my schematic:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/ampschematiccopy.jpg)
Here is the list of what I need:
Resistors Quantity Capacitors Quantity
820 2 0.01 1
1500 2 0.1 2
2700 1 0.25 2
4700 1 .0001 1
6800 1 .00025 1
100K 3 .0005 1
100K 5% 1 .002 1
10K 1 .1-200 1
1K 5W 5% 1 20-600P 3
1M 5 25-25 2
220K 3 25-50 +- 1
27K 1W 5% 1 40-450 +- 1
470 1W 2
4700 1W 1
470K 1
47K 1
56K 1
56K 1W 1
68K 1
82K 5% 1
And Here is what I have on hand:
Resistors Quantity Capacitors Quantity
2.1 2 .0005 8
680 2 .001 3
.33 5w 1 .0047 3
1.5K 2 .01uf 4
1.5M 2 .022 3
100K 6 .047 1000V 1
100K 2W 2 .047 630v 1
120K 4 .1uf 5
150K 5 .56uf 2
1K 2W 4 0.33uf 3
1M 2 100uf 2
220K 3 32uf-500v 2
22K 2 470pf 5
250 10W 2 47pf 2
270K 40 50uf 500v 2
3.3K 9
3.9M 10
39K 10
470K 2
47K 8
5.6K 3
5.6M 3
56K 10
Sorry its hard to read, The columns were straight before I hit post.
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That bias supply will NOT work. It makes NEGative 400V, you want -50V.
There's two typical supplies. One comes off a 50VAC PT lead maybe through a small resistor. The other comes off a 350VAC PT lead through a BIG resistor.
Since you don't have a 50VAC PT lead, you better find values for the other kind.
The 470K before the .0005 is leftover from a mix network. Since you are not mixing, you probably do not want this resistor.
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I thought the -54 on the schematic was correct for the bias? How do I figure out what I need?
Thanks for the tip on the 470k reistor
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About bias from HV tap see page 5 of Sluckey Amp Scrap Book
(and also the other pages that are interesting)
http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf (http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf)
Kagliostro
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> I thought the -54 on the schematic was correct for the bias?
Yes, that is what you want.
But the 1K from a 300VAC winding will give -400V bias, NOT what you want.
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OH Ok. I see what you are saying. Thanks again!
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Since I have so much room in this chassis, I decided to do separate boards for the preamp and power stage. I got the preamp board done this morning. I know it isn't the cleanest board ever, but its my first one and I am pretty happy with it.
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-10-02080023.jpg)
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Late last night I finished wiring this amp up. I got up early to fire it up and check voltages.
The good new is that it passed the smoke test, but my 6V6 tubes started glowing red hot. But I think my voltages are all screwed up.
Here is what I got:
Filter cap 1: 314v
Filter cap 2: 307v
Filter cap 3: 307v
Filter cap 4: 169v
The B+ at the rectifier tube is 315v
The bias is -314. This is from the tube side of the bias to filter cap 1. I was not sure exactly where to check it at.
I need to get the bias to around -50. Should I wire in a pot between the PT and the bias?
Thanks!
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The bias is -314. This is from the tube side of the bias to filter cap 1. I was not sure exactly where to check it at.
Which side of filter cap 1??? Check the bias voltage between pin 5 of the 6V6s and chassis ground. If it really is -314v you'll need to replace the bias cap because it has been severely stressed.
I need to get the bias to around -50. Should I wire in a pot between the PT and the bias?
Did you forget to heed PRR's post? Your schematic shows a 1K resistor between the PT and the bias diode. That resistor needs to be MUCH BIGGER. Try 100K to begin with. Then change the value up/down as needed to get the desired bias voltage.
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ok cool. I have a 100k pot as well. But I will start with the resistor to see where I end up at.
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For some reason I am burning out the 1k 1watt resistor in between the PT and the Diode? I never even got to try a different value. They burn out as soon as I turn the power on. Any ideas?
The Bias cap is shot too, it literally popped.
And should I be doing this with the tubes in or removed? So far the tubes have all been in.
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I have a 100k pot as well.
Pull the 6V6s and don't put them back in until the bias voltage is right!
Forget the pot. Replace that 1K resistor with a 100K resistor. Replace the diode with a KNOWN good diode. Replace that bias cap. Now what is the voltage reading between the 6V6s pin 5 and chassis?
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ok, I replaced the 1k resister with 100k, put in a brand new diode, and replaces the blown 25/50 cap with a 50/50 (it was all i could get). Now I am getting -11 volts at pin 5 of the 6V6.
Also, with all the tubes out except the rectifier. I am getting 568v and filter cap 1,2 and 3. And 528 volts at Filter cap 4. Should there be that big of difference with the tubes in or out?
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Now I am getting -11 volts at pin 5 of the 6V6.
That's better. But -11v is not enough. You want about -35 or -40 on pin 5 of the 6V6s. Replace the 100K with a 47K and see what you have. Then change the value up/down as needed to get the desired bias voltage.
Also, with all the tubes out except the rectifier. I am getting 568v and filter cap 1,2 and 3. And 528 volts at Filter cap 4. Should there be that big of difference with the tubes in or out?
There can be a BIGGG difference with tubes out, but 568V is probably exceeding the voltage rating on your filter caps. If so you risk blowing them just like you blew the bias cap.
Reread reply #54 for my solution to bring the voltage down to managable levels.
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I am having a really hard time getting the bias correct.
I have tried several different ways to get -35 to -40 volts on pin 5 of the 6V6
Here is what I have so far:
1.1k resistor by itself burns up instantly
100k resistor gives me -11 volts
I hooked 8 resistors in parellel and got up to -22. Here are the resistors I used: 100k, 100k, 100k, 100k, 27k, 27k, 27k, 4.7k
This should have gotten me a total of 2.74k
Then I tried 4 resistors in series: 250ohm, 250ohm, 470ohm, 470ohm = 1.4k. This got me to -28 but the 250ohm resistors were literally smoking.
All the resistors I used are at least 1 watt. Part of me thinks the problem is that my voltage without the tubes is so high, but if I put in a large resistor like was suggested, wont that drop my voltage with the tubes in as well. Voltage with the tubes in is almost too low. Will the bias voltage change when I put the 6V6 tubes back in and my B+ drops?
What am I doing wrong here?
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need pictures.
--DL
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Here is the built before I tested it:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-10-10010743.jpg)
Here is the bias circuit currently. The resistor isnt in it because I need to know what value it needs to be. These is a resistor under the cap:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-10-11171757.jpg)
The outside:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-10-10085338.jpg)
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Will the bias voltage change when I put the 6V6 tubes back in and my B+ drops?
No. Not if it's wired IAW your schematic.
What am I doing wrong here?
I don't know yet, but I suspect something is not wired correctly. Look at the attached simplified schematic and make the two voltage measurements indicated. One meter lead needs to be on chassis ground. Be sure to use a 100K resistor. The Princeton Reverb uses this exact circuit and feeds 340VAC to the 100K and puts out -34V on the bias cap. The circuit is a proven one. I didn't just make up that 100K value. The 100K should not get hot.
While you're checking the AC voltage, also check the AC voltage on the 5Y3... pin 4 to chassis and then pin 6 to chassis. Report back with the readings. There are only 4 components involved. There's gotta be something simple amiss. If you can post some high rez pics we may be able to spot something.
PS... For anyone that has trouble looking at the bright green schematic, look at this one instead (http://www.el34world.com/charts/Schematics/files/fender/BASSMAN_6G6B.pdf). The green schematic is a 6G6-B Bassman without the normal channel and the rectifier is replaced with a 5Y3.
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Your diode is currently installed backwards. The banded end connects to the "100K" resistor and the black end connects to the negative end of the bias cap.
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(http://i268.photobucket.com/albums/jj26/thebattens/Philco/6G6Bvoltage.gif)
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_8993.jpg)
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_8994.jpg)
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_8995.jpg)
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Your diode is currently installed backwards. The banded end connects to the "100K" resistor and the black end connects to the negative end of the bias cap.
WOW, now we are getting somewhere!!! I will reverse it and report back with the new voltages.
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OK Diode is correct now.
Here are the voltages:
Pin 4 & 6 on 5Y3 = 412vac
100k to diode = 222vac
Diode to cap = -43vdc
Pin 5 on 6V6 = -35 !!!!!!!! SWEET!!!
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BTW, what's a "blown 240"? volvo with a roots blower?
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Diode to cap = -43vdc
Pin 5 on 6V6 = -35 !!!!!!!! SWEET!!!
Those two voltage readings should be the same!
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BTW, what's a "blown 240"? volvo with a roots blower?
EXACTLY!! It was the first car I built. 13psi with 13:1 compression. Had to run it on 116 octane or it would ping. Watch this vid:
http://youtu.be/C3UzMk2ViY0 (http://youtu.be/C3UzMk2ViY0)
Those two voltage readings should be the same!
Is that a problem? There is a resistor in parallel with the cap too, wouldn't that change the voltage some?
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> Is that a problem?
Yes.
What are you using for a volt-meter, something you bought for the Volvo? 1K/V needle-meter? Or a proper digital meter?
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I am using a Fluke T5-600 voltage, continuity, and current tester.
I am using a 50-50 cap instead of the 25-50 that is in the schematic. Could that have anything to do with it?
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I am using a 50-50 cap instead of the 25-50 that is in the schematic. Could that have anything to do with it?
No. But mentioning that cap reminded me of something I forgot to mention... All the experimenting with all those resistors has severely stressed thatr bias cap. E-caps don't like reverse polarity voltages. Even if the cap seems OK now, I'd still replace it just because I know it's been stressed. This bias circuit is such a simple circuit that it's easy to think that it's not very important. But if that cap fails you risk losing your output tubes and maybe even some transformers. Replacing that cap is real cheap insurance.
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ok, will do. I will get one tomorrow. But what about the difference in voltage?
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But what about the difference in voltage?
The only thing between the diode and the tube grid is a 220K resistor. No current flows thru the 220K, so no voltage can be dropped across it, therefore the voltage would be the same.
Your tester is the cause of the difference. Your tester has a 1meg input resistance. When you connect the tester to pin 5 now you create a voltage divider and current can now flow thru the 220K causing a voltage to be dropped. You basically have a 220K in series with a 1meg (your tester) across a 43 volt source. Doing a little math yields 35v dropped across your meter.
So, don't be concerned about the difference. It's legit with your tester. You may want to get a meter that is more appropriate for this type work.
Your next big challenge should be to get the B+ lowered to a safe value. You are risking blowing those big blue filter caps.
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But what about the difference in voltage?
So, don't be concerned about the difference. It's legit with your tester. You may want to get a meter that is more appropriate for this type work.
Your next big challenge should be to get the B+ lowered to a safe value. You are risking blowing those big blue filter caps.
Very cool, thanks for the info.
If I drop the B+ down with a large resistor, will it effect the voltage with the tubes in? I think the voltage with the tubes in is already a bit low. If it wont effect it, what value should I begin with to try to drop it. And does it just go between the 5Y3 and the first filter cap?
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Before doing anything to drop the B+, re-evaluate now that you have a reasonable bias voltage. Put all the tubes in and recheck voltages. I'm mainly interested in the DC voltage readings on the 5Y3 pin 8, 6v6s pin 3, and 6V6s pin 4. Measure quickly and be prepared to turn the amp off if tubes glow red or any other odd thing turns up.
What are the new numbers?
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5Y3 pin 8 = 485vdc
6V6 pin 3 = 474vdc
6V6 pin 4 = 483vdc
Bias is -35
The tubes are not glowing, except for the filaments, and nothing smoked!
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Those voltages are way too high for 6V6s. Find that big power resistor from the original radio and I'll give you some ideas using it to drag the B+ down. If we can get the voltage down to 400-425 range (Deluxe Reverb territory) then some good 6V6s will handle that OK. Find the resistor in this pic...
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ok I found that resistor and measured the resistance. It still works. Now what?
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ok I found that resistor and measured the resistance. It still works. Now what?
put in series with the PT high-volt secondary center tap. IOW, lift the center tap lead of the high volt secondary off ground and connect it to one end of that resistor; the other end of the resistor connects to ground.
make sure that that resistor is bolted in well to the chassis - it's gonna get really hot - so hot you'll probably get a good scent of that new paint baking...
peace,
--DL
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Sweet. Hopefully I can get that done tonight after work. I will post the new voltages...
Thanks!
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Give DL's suggestion a try first. Then try this...
Temporarily connect that 7750Ω between the 5Y3 pin 8 and ground and measure the voltage from 5Y3 pin 8 to ground. (Turn the amp off as soon as you get the voltage reading.) What is the B+ reading with the 7750Ω in place?
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sim shows about 40-45V drop, so neighborhood of 440V?
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I will get answers to both of you guys tomorrow. I just got home from an 18 hour work day and a 450 mile drive.
Thanks for the replies though!
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> sim shows about 40-45V drop
Simming what? Do you have good data for winding resistance of a Zenith PT?
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> sim shows about 40-45V drop
Simming what? Do you have good data for winding resistance of a Zenith PT?
e-s-t-i-m-a-t-e. SWAG. i'm new at it, but i i have figured out it's not an exact method.
--DL
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OK, here we go.
Starting voltage this morning was 485 at pin 8 of 5Y3
Sluckey's idea got me to 450v
Dummyload's idea got me to 394v
So what is better? Voltage a little high, or voltage a little low?
And how hot is this resistor going to get? The best place to mount it will put it about 1/8th inch off of the wooden box of the radio and about 1/4 inch from a filter cap can.
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My idea was an experiment to see what kind of sag was involved with your PT and that 5Y3. It may not be a real world solution for you.
It's gonna get hot. Can you mount it in the original location?
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The original posisition was the rear of the chassis. Now the rear of the chassis is full of other parts. I have a place to mount by the PT and filter caps. I moved the caps so they are about 1/2 from where the resistor will be. If need be I can make some kind of heat shield.
Are we talking 200 degrees or 800? It makes a big difference when its sits close to 80 year old wood. Or maybe I can find a place on top of the cab. Maybe by the power tubes.
I like the amount of voltage your idea gives me. What would be a more real world solution?
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I was looking at the original location on the chassis and it has MANY cooling holes drilled on either side of the resistor. So I ended up mounting it in the outside of the chassis.
Here is a pic:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-10-15122103.jpg)
I checked the voltages.
Pin 8 on 5Y3 and the first 3 filter caps are all around 395v. The last filter cap is around 210v.
So I plugged in a speaker and my guitar. NOTHING. You can hear a bit of a hum thru the speaker, but no signal. I think I may have smoked the 6V6 tubes. Will bad output tubes keep ALL the sound from coming thru? Is there anything else I should check?
Thanks!
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If it were me, I would first trace every wire and component by the schematic, and do that at least twice. Because, I checked and double checked the schematic on my first time build, and it wasn't till the 5th or so time thru that I caught my huge mistake, and it should have been glaringly obvious (and would have to a normal person :laugh: )
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Ya I have already done that a couple times. When my voltages were still very high; I was able to get a faint signal thru it. But now that the voltage is lower, I get nothing thru it.
Could it be that the higher voltage was forcing some signal thru the dead 6V6 tubes, and now that its lower nothing gets thru? OR how can I test the 6V6s?
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UPDATE:
I just got back from taking my amp to the local amp guy. We tested all the tubes and they are all good. Then we plugged it into a known good cab, and there is a signal getting thru. It is a clean signal but VERY weak.
Then we plugged in a Bias tester. Its was a tester that plugged into the tube sockets, and then the tubes plug into it. There was NO reading on the bias tester. He suggested that I have the wrong screen resisters. Then it dawned on my that the schematic that I am copying has 5881 tubes and I am using 6V6gt tubes. Maybe I need to put a different value resistor to the power tubes?
Thoughts?
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I can't believe your amp guy didn't take the extra 5 minutes to "know" why he had no reading on the bias tester.
Fender used 470Ω screen resistors on almost all his amps. 6V6s, 6L6s, 5881s, all the same. If you used 470Ω screen resistors , you're fine. Something is still not right. Check voltages on ALL pins on both 6V6s and report your findings.
RE: the experiment with the 7750Ω power resistor.... I never meant to use that resistor as a 'fix' for lowering the B+ in your amp. I just wanted to see how much the B+ would drop when you added the additional load. The voltage dropped to 450V. That's an additional load of 58ma. IMO, 450V is still too hot for 6v6s, but it is very common to see 450V on a pair of 6L6s.
I don't know why you chose to put 6V6s in this amp, but here's my recommendation based on the power resistor experiment. Use a pair of 6L6s. And change the rectifier to a GZ34/5AR4. And get a meter that's capable of accurately reading voltage and resistance. $20 should get a cheap DVM that will get the job done. That Fluke T5-600 (http://assets.fluke.com/manuals/T5______iceng0100.pdf) is a fine instrument but it's more suited for an auto mechanic or electrician. It just isn't intended for poking around inside an amp.
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Would swapping out to 6L6 and a 5AR4 take care of the voltage issues, or would I still need to figure out a way to drop my B+? Maybe the different rectifier tube would do that itself. I initially used the 6V6 because I already had them. And I was/am trying to keep the cost down on this project.
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Here are the 6V6 voltages, These are all to ground with the amp on and tubes in:
Pin 1- 0
Pin 2- 29vac
Pin 3- 401vdc
Pin 4- 401vdc
Pin 5- -43 vdc
Pin 6- 400vdc
Pin 7- 33vac
Pin 8- 0
And just for reference
5Y3 Pin 8- 402vdc
Bias Connections:
rec pin4 to 100k - 416vac
100k to diode - 193vac
diode to cap - -51
cap to gnd - 0
And I learned that it does suck to get shocked off a filter cap. Glad it was unplugged and I wasn't grounded. Still not as bad as when i got shocked off of the ignition coil on my old 66 Suburban, or when i got hit by a 277 stinger leg in a commercial building!
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Would swapping out to 6L6 and a 5AR4 take care of the voltage issues, or would I still need to figure out a way to drop my B+?
6L6s have a higher max plate voltage than 6v6s: 500 vs 350. 5u4 or 5ar4 handles higher PT voltage as well. you can remove that 7K resistor and let the 480v stand as is...
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[Duplicating my post from the other thread, since we're talking about this amp.]
Here are the 6V6 voltages, These are all to ground with the amp on and tubes in:
...
Pin 2- 29vac
...
Pin 7- 33vac
...
I hope you really mean 2.9vac and 3.3vac for these pins. The nominal filament voltage is 6.3vac, but when measuring to ground, you'll see half of that at each pin, or 3.15vac. Each would be within tolerance is the decimal is in the wrong place. If not, your filament voltage is way wrong.
Pin 5- -43 vdc
That may be too much bias voltage for 6V6's. That's roughly 1/10th of screen voltage, which should bias a 6V6 near cutoff. You'll want to experiment with reducing the resistance to ground across the bias cap to reduce the negative voltage and increase tube current.
The "easy" way to do that is to use a 100-250k pot, wired as a variable resistor, across the resistor to ground. Start at the position of maximum resistance, and slowly turn the shaft while monitoring tube current. You'll eventually land on your desired current. At that point, shut off the amp. Measure resistance across the parallel combination of stock resistor and pot, get the nearest standard-value resistor, and replace the stock resistor and pot with this new resistor to ground. Bias done.
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On a side-note, I think you're learning for yourself why a number of us advocated leaving the radio in it's stock form, and just ripping out the RF portion of the circuit. It's easier and faster to let it be what it is, and just nudge it towards working with a guitar signal.
Kinda like trying to take a Volvo 240 and turn it into a Ferrari... :icon_biggrin:
But these are valuable lessons to experience. I think you're also learning a LOT about all facets of amps messing with this project.
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On a side-note, I think you're learning for yourself why a number of us advocated leaving the radio in it's stock form, and just ripping out the RF portion of the circuit. It's easier and faster to let it be what it is, and just nudge it towards working with a guitar signal.
Kinda like trying to take a Volvo 240 and turn it into a Ferrari... :icon_biggrin:
But these are valuable lessons to experience. I think you're also learning a LOT about all facets of amps messing with this project.
Oh, I absolutely agree with you. But also you need to know that I am having a great time figuring it out and learning. Just like when everybody told you that you could NOT make a Volvo fast with a stock motor. Well that was a fun project too. And when I am done I am going to have something I can be proud of. I already have 2 more projects/builds I want to once I get a little $$$.
I was just going over the schematic and layout for the 10th time and I found something I missed. There is a jumper from pin 3 to pin 8 on the 3rd 12ax7. Well I missed that jumper! I just put it in and once my kids wake up I will see if that did anything.
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Strange things are happening! :w2:
I put the jumper in that I missed and now there is HUGE signal coming thru. But its a terrible sound. You can hear the guitar signal, but its very distorted (in a bad way) and there is lots of scratching, popping, and ringing. And is seems that the controls dont do what they should.
I double checked my voltages and Pin 3, 4, & 6 are down in the 300s. But my heaters really are at 29 and 33 volts! Even with the speaker and guitar cables unplugged you can hear the amp ringing. I am assuming this is the heater voltage being too high.
I am going to take out the big voltage dropping resistor to see if that brings my heater voltage back down. I really hope that I dont have a bad PT.
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Get the tubes out until you figure out that heater issue. 10x too much voltage on the heaters will kill tubes.
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Here are the 6V6 voltages, These are all to ground with the amp on and tubes in:
...
Pin 2- 29vac
...
Pin 7- 33vac
...
Those filament readings are bogus! Your PT has no center tap for the filament winding and your schematic shows no artificial centertap. Therefore, your readings to chassis for the filament AC voltage are meaningless. The correct way to measure your filament voltage is between tube pins. For the 6V6s put one meter lead on pin 2 and put the other meter lead on pin 7. Now what voltage does your tester say you have?
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Good Point! I will have to double check that.
Can anyone point me in the right direction to start figuring out why its making all these crazy sounds? I already checked all my solder joints and grounds. It all looks pretty good. Next I am going to take a small wooden dowel and tap all the components to see if I can find any that are bad. I figure if I tap something and it makes the sound worse, or better, than thats something to investigate further.
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Can anyone point me in the right direction to start figuring out why its making all these crazy sounds?
That's what we're trying to do... :laugh:
Hoffman's rule applies here: "If it was wired right, it would be working right now."
There's got to be wiring errors, etc going on. You've found one already, which resulted in a big change/improvement. There are almost certainly others. The best way to proceed is find and resolve all the issues.
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Can anyone point me in the right direction to start figuring out why its making all these crazy sounds?
1. Remove the 7750 ohm resistor.
2. Disconnect the NFB loop.
3. Measure tube voltages and put the results in Doug's tube chart so we can see them.
What about the filament voltage???
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OK.
1. I removed the 7550 resistor.
2. I unhooked the feedback loop from the speaker jack side.
3. Heater voltage is 6.
4. Here are my voltages: http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11927 (http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11927)
Looks like there may be a problem with Valve 2. But I dont really know...
I am going to see about getting some 6L6 tubes and a 5AR4
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V2 and V4 are messed up. Divide and conquer. Fix V4 first.
Compare to the voltages on your green schematic. Your Fluke tester will not be able to read the grid voltages on V4 but it should read the cathode voltage OK. Looks like V4 cathodes may not be connected to ground properly. Do a resistance check with amp off--- V4 pins 3/8 to chassis. Should be approx 10K to 12K.
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Holy Crap You Just Found Something!
I deleted the presence control from the original Bassman schematic and never put in the ground! Looks like I need a 4700 resistor to ground.
Also I got a 5U4 and two 6L6 tubes.
EDIT:
I installed the 4.7k resistor to ground from what would have been the presence knob, also the new tubes. Now I am able to get an improved signal thru it, even though its still terrible. At this point we are making some good progress.
Also, I may have wires the pots backwards. Does this matter other than them working backwards? Looking at a layout of an amp, Does it assume you are looking at the back of the pot, or the side with the stem?
EDIT 2:
I hooked up the feedback loop again to see if there are any changes. There arent any differences. Also I looked at how V2 is wired and I dont see anything jump out at me as being wrong.
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Here are my new voltages:
http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11930 (http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11930)
filter caps:
1=401
2=394
3=393
4=210
Also I notice that when I turn the volume knob, I get odd vibrations and squeels from the amp. Even with the speaker and input unplugged.
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Your new voltage readings look OK except for V3-7. No way can V3-7 be -37v! Check that again.
Also, I may have wires the pots backwards. Does this matter other than them working backwards? Looking at a layout of an amp, Does it assume you are looking at the back of the pot, or the side with the stem?
Doesn't matter. The typical Fender layout is viewed from the back side of the pots.
Keep an eye on the PT. That 5U4 needs 3A filament current and the original 80 tube only required 2A. I'd be more comfortable with a 5AR4.
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I will grab a 5AR4 when I order more parts. Its not worth the $15 of a new tube to risk the PT.
I will double check the voltage on V3-P7. But that does lead to the new ground and 4.7k resistor that I put in. I wonder of there is a problem...
Also, why would my volume know be doing such odd things? I think I did end up using 500k pots for volume and treble, and a 50k for bass. I may have the original volume pot from the radio still...
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OK so V3 Pin 7 is actually -37 volts. It changed when I put in the 4.7k resistor to ground for V4 cathodes. What would cause that?
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I suspect that V3-7 does not have a path to ground. It should measure approx 1 meg to ground (with treble pot set to max CCW). But I don't think your tester can measure that much resistance. Use some clips to temporarily connect a 1meg (heck, use anything between 100K and 2meg) directly between V3-7 and chassis. What's the voltage now?
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With the 1m resistor in place it reads from -32 to 0v depending on where the treble pot is set.
The treble and volume pots are both tied into this tube. I wonder if maybe the tube is bad?
EDIT:
I swapped my tubes around and nothing changed. The weird sound I am getting is coming from my power tubes based on where the volume is. The volume has to be all the way up, or the tubes scream at me.
Also, I double checked everything on the schematic and a layout of the original amp I am copying and other that the bass pot being wired backwards (I fixed that) everything seems to be wired correctly. But I am still at -35 on t3-p7.
EDIT 2:
Are these black caps polarized (I dont think they are)?
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-10-18152840.jpg)
If they are polarized, and are installed backwards, could that be causing the problem? The lightning show in my power tubes must be tied into the negative voltage on valve 3 pin 7.
This is the preamp board, ground for those caps is at the bottom of the picture:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-10-18153319.jpg)
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The volume has to be all the way up, or the tubes scream at me.
do this: ground the grid on the last 12ax7 stage of each preamp. on the "bass" channel, that would be the pin that the center lug on the treble knob is connected to, and on the "normal" channel, its the pin the volume pot is connected to. ground the grids with a jumper wire of something. It the amp gets quiet, you have a problem in the preamps (disconnecting one of the two jumper wires at that time, will tell you which preamp).
If the amp stays noisy, its either the PI, the output section, or the OT.
at that point, I might connect the two grids of the 6L6s together with a .1 cap, and with another .1 cap, connect to one of the 6L6's grid to ground. For the purpose of troubleshooting, this will isolate the problem to either the PI or the power section. In a healthy amp, this would be like having the volume knobs at zero.
Here's one other thing: are your input jacks both properly wired switching jacks? when you don't have an instrument plugged in, the signal should be grounded.
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With the 1m resistor in place it reads from -32 to 0v depending on where the treble pot is set.
Plain and simple. It ain't wired like your green schematic.
Or your voltage tester is lying.
Get a decent/cheap DVM.
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This amp only has the bass channel. I grounded the center pin on the treble pot and still got noise. It just didnt change when I turned the volume or treble.
I only have 1 extra .1 cap, I have three .01 caps and a 100 uf cap. Is there any other way to test this?
There is only 1 input jack and it is not switching. I grounded the 2 terminals together for testing purposes.
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I will try to get a DVM tonight or tomorrow. Its gonna be a harbor freight one though, thats all I got the $$$ for.
EDIT:
I went to HF and got a DVM. I tested several resistors and checked its voltage against my Fluke and it seems accurate. My other meter wasnt lying. I am at -32 on pin7 of tube 2. Time to trace my schematic AGAIN!
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A couple more strange things!
I was testing the pots in my amp and for some reason the volume pot was acting strange. It would read from 0-250-0 as you turned it. I replaced it with a known good 500k.
Then when I was testing it, and turned it on standby from the ON position, I got a big spark from the choke. It was big enough that it killed my standby switch for a bit. I had to cycle it on and off several times before it would latch off. I also took the choke out of the circuit.
And finally just now as I turned it to standby from ON, i got a spark from the resistor between filter cap 2 and 3. Now its smoked.
This is getting VERY frusterating. :cussing: :BangHead: :help:
At this point I have several issues:
1. Why is there -34 volts on tube 2 pin 7?
2. Why are my tubes screaming at me?
3. What is causing a huge spark when I go to standby from ON?
4. Is my choke dead from sparking when I went to standby?
5. Is my standby switch bad?
6. I need a new 4700 1w resistor
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If it was me, with my VERY limited knowledge (just so you know I'm no expert) I would unhook the PT from the rect. tube, so you're only turning on the PT. Plug into your light bulb limiter - if you don't have one already, make one, very cheap, pays for itself the first time something's wrong. Make sure there's no issue with the PT, nothing shorted to ground. If not, plug into wall outlet, Check your heater voltages.
Then, re-hook to your rect. tube. If possible, only have your B+ "power rail" connected. Check again with limiter. Again, check your heater voltages.
If it were me, somewhere in there I'd find at least one mis-wire. (at least, I did on mine :icon_biggrin: ) Hope this helps... remember, divide and conquer.
If all that's okay, hook up the next little "section".
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If it were me, somewhere in there I'd find at least one mis-wire.
+1 +1 +1
I suspect the white wire is not connected correctly. :icon_biggrin:
This project seems to have regressed quite a bit in the last 24 hours. Hang in there.
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Things seem to have gone down hill since I found that the volume pot was acting up. I think that there may be a ground issue now since my preamp grounds all went thru a wire bus on the back of the pots.
I am going to be out of town for a couple days, so it may be nice to have a couple days off from the amp.
When I get back the first thing I am going to do is reground the whole thing, see if it still sparks. Then go from there.
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so it may be nice to have a couple days off from the amp.
Yep. I found my big mistake because I didn't have time to fret over it for a few days, went back, re-traced (yet again) and thought "how did I miss THAT???". Good luck!
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Thanks guys!
I really do appreciate all the help. I know that I will get this thing straightened out, its just a matter of patience (from my wife). :icon_biggrin:
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I got a tiny bit done today. Mostly just re-grounding.
I used a 12gauge speaker wire between all the pots with both ends bolted to ground, one with a ring terminal, and the other to a ground block that is bolted to the chassis. I soldered all the preamp grounds to it, along with the grounds for the shielded wires (preamp board to power board & input jack to preamp board). The ground that would have gone to the presence knob is also soldered to the speaker wire buss. The other side of the ground block also has the ground for the 3rd and 4th filter caps.
Here are some pics:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-10-19143204.jpg)
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-10-19143221.jpg)
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-10-19143237.jpg)
The PT, Power tubes, Bias, and filter caps 1 & 2 are grounded to the PT bolt along with the power cord ground.
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-10-19143312.jpg)
I need to get more solder and a new soldering iron before I do more.
On another topic, the 12" speaker fit NICE:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-10-19144406.jpg)
Leaving town for work in the morning, so its on hold till Saturday at least.
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I was comparing my schematic with the original fender schematic and I may have discovered a mistake.
Here is the original schematic:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/BASSMAN_6G6B-2.jpg)
Here is the schematic that I built:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/myamp1.jpg)
And here is one showing a change that I may need to make. I think that by deleting the normal channel, I may have removed a b+ feed into the first power stage cap. The red shows the modifications from the one I built.
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/ampschematicidea.jpg)
Would this be better? Maybe even correct? And does the red line need a resistor in it? Maybe a 220k.
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be sure to connect "B" ("B" inside square,, not B+) to "B" (the left choke lead), you originally had that connected to the wrong side of the the 4700 R.
you don't need the red line. the last 12ax7 stage of preamp gets its B+ from the "B" rail (follow the 100k plate resistor), not the "C" rail. if you follow the original schematic, from "C" rail to the plate of that 12ax7 section, there is about 1.1M of resistance in between.
academically speaking:
if you used that red line, you absolutely do need a resistor in there, otherwise the 20/600 filter cap at the bottom end of that red line will look like a short to ground for the AC signal voltage. you'd more or less be giving that 12ax7 section two plate resistors,, I don't know what you'd end up with really....
also, 12gauge wire is overkill, especially for anything that doesn't carry any DC current (like volume pot grounds). Don't feel that if you have a ground problem, 12g is going to fix it. even the 12ax7 cathode grounds have current measured in milliamps. you can use 22awg, or 26awg..
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Perfect thanks. Hopefully this is why there is a negative voltage on the 2nd preamp tube pin 7.
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At this point I have several issues:
1. Why is there -34 volts on tube 2 pin 7?
2. Why are my tubes screaming at me?
3. What is causing a huge spark when I go to standby from ON?
4. Is my choke dead from sparking when I went to standby?
5. Is my standby switch bad?
6. I need a new 4700 1w resistor
OK, so I fixed the filter cap wiring as suggested. Also I sorted out some of the issues.
1. Voltage is now -24 on pin 7 of tube 2.
2. The tubes are still screaming at me.
3. The spark is gone, must have been a ground issue.
4. Choke seems fine.
5. Standby switch seems fine, but I got a replacement if need be.
6. I replaced the resistor with a new 2watt.
Bias is -44 now.
Here is a video clip of my tubes screaming at me. The change in the sound of it is from me turning the pots. The sound stops when all the pots are turned all the way up. At the end of the video where the tube noise starts and stops several times I am turning the volume from 10-9 and back and forth. The other noise in the background is my 1 year old.
http://youtu.be/3_tf3uhBGl4 (http://youtu.be/3_tf3uhBGl4)
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I could be totally wrong,and often am. But, it looks to me as if you have arcing going on in your tubes. If so, that is Not Good.
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Ya maybe I do, all I know is that it did the same thing with the last set of tubes as well. So I am assuming its in the amp, not the tubes.
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1. Why is there -34 volts on tube 2 pin 7?
something is wired incorrectly. look at the schematic, where would -34 volt come from? it should be 0. there is no negative voltage supply for the grid there. something is wired incorrectly there.. pull tubes, turn on and figure out why its not 0v.
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I traced the schematic again and I dont see anything jumping out at me. The only thing is that I dont have a switched output jack that automatically grounds itself when its unplugged. I used a jumper wire to ground the 2 terminals together and the amp got a little quieter, but the negative voltage didnt change. It goed from 0 volts when the pots are at full; to -27 when the treble out is at 0.
Also there is no ground switch before the PT, but with a 3 prong cord you dont need it right?
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No ground switch with 3 prong plug, right.
If it were me, I'd look really hard at your treble pot wiring. It seems like it's acting like a bias pot. Something you might find useful is to get one of those 5-pack of colored Sharpies and color up some of the wires to make it easier to trace.
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Is it possible that a bad or mis-wired OT can cause this? I have retraced the treble knob 10 times, and its correct as far as I can tell.
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Is it possible that a bad or mis-wired OT can cause this?
It's highly unlikely. I think it's much more likely that you'll find wiring errors, or your wiring has poor solder connections, or that ground bus ain't connected to chassis very well, or something associated with those white wires. I really suspect multiple problems. Since you are sure that V2 and the treble pot are wired correctly, I suspect a poor connection somewhere between pin 7 and chassis.
I want you to make a few resistance checks with amp turned off. Clip one meter lead on chassis. Clip the other meter lead directly on the following tube pins. V2-8 and then V2-7.
I want one number for V2-8 and I want two numbers for V2-7 (one with treble pot max CW and another with treble pot max CCW.
Also, how is that ground buss connected to chassis? Is that chassis painted on the inside? If so, be sure to scrape off the paint at any point where you make a connection to the chassis. Don't try to solder to your chassis. Use screws, nuts, and star washers (or kep nuts) to make chassis connections.
And one final thing. Connect a jumper wire to chassis and connect the other end directly to V2-7. Be careful to get the right pin and only the right pin. Turn the amp on. Does V2-7 read zero volts now?
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I want you to make a few resistance checks with amp turned off. Clip one meter lead on chassis. Clip the other meter lead directly on the following tube pins. V2-8 and then V2-7.
I want one number for V2-8 and I want two numbers for V2-7 (one with treble pot max CW and another with treble pot max CCW.
Also, how is that ground buss connected to chassis? Is that chassis painted on the inside? If so, be sure to scrape off the paint at any point where you make a connection to the chassis. Don't try to solder to your chassis. Use screws, nuts, and star washers (or kep nuts) to make chassis connections.
And one final thing. Connect a jumper wire to chassis and connect the other end directly to V2-7. Be careful to get the right pin and only the right pin. Turn the amp on. Does V2-7 read zero volts now?
v7-p2 treble down = 995; treble up = 1501
v2-p8 is 002 either way
Ground wire it bolted to the chassis at both ends, one end is a ring terminal, the other side is thru the ground block that is bolted to the chassis, no star washers.
Chassis is bare clean metal at both places.
Jumper wire from p2-7 to chassis makes between 0 and .1 volts. It makes the tubes arc though, and none of the knobs do anything.
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v7-p2 treble down = 995; treble up = 1501
v2-p8 is 002 either way
I'll assume all three of those numbers need to be multiplied by 1000.
Jumper wire from p2-7 to chassis makes between 0 and .1 volts. It makes the tubes arc though, and none of the knobs do anything.
Which tubes arc? All of them?
You have at least one more wiring error. If you can't find it, then consider ripping out all the wiring and start fresh. Or take it to someone who can find it. Your odd layout and schematic combined with all white wiring will make it a huge challenge for any of us to spot the problem via pictures. Someone needs to have the amp in hand to unravel this mystery.
Think about this... The first time you posted voltages, V2-7 was fine. Now it's not. What changed? Also, your bias supply is the only source of negative voltage in that amp. And V2 ain't supposed to be connected to the bias supply.
I've held off saying anything about your layout and wiring job because I didn't want to sound condescending or discourage you. But it's time to say it. Your wiring sucks and it's probably the root of most of your problems. This criticism is meant to help, not to hurt your feelings.
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You didnt/wont hurt my feelings. If I didnt want constructive criticism/help I wouldnt be here. I have already considered starting fresh with my wiring.
The thing that changed was when I found there was a jumper missing from pin3 to pin8 of the 12ax7 in the power stage. Before that I had good voltages, but no output.
only the 2 power tubes arc. I have considered pulling the 3rd 12ax7 and power tubes to see what my voltages are, then put them back in 1 by 1 to see when the problem surfaces.
Could a bad tube socket do this? they are all used...
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The thing that changed was when I found there was a jumper missing from pin3 to pin8 of the 12ax7 in the power stage.
When you connected 3 and 8 of the phase inverter (the 3rd 12ax7), perhaps that's where something went wrong? If you have a little strand of solder touching something it's not supposed to that can really mess you up. Someone else on here -I forget who- mentioned that a long time ago. And it can be only a hair width, really hard to see.
If you do re-do your wiring, try using at least 3 colors. You'd be surprised (at least, I was :BangHead:) at how easy it is to be dead sure you're tracing a certain wire and you're not.
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The thing that changed was when I found there was a jumper missing from pin3 to pin8 of the 12ax7 in the power stage. Before that I had good voltages, but no output.
Actually, shortly after finding the missing jumper, you also found a missing 4.7K resistor in the cathode circuit of the PI. The PI voltages got happy after you put that resistor in, but about that same time V2-7 went to negative 37V.
A bad socket will not put a negative voltage on V2-7. Bad wiring will.
There cannot be negative 37 volts on V2-7 unless it's wired wrong. I can't see that wiring error without having my hands on the amp, but it's there. You just have to keep looking.
In one of your pics I see a three lug grounding block (like you'd see in a circuit breaker distribution panel). Looks like it is soldered to the chassis? If so, take it out and put those wires on a ring lug and bolt to the chassis. A small Weller iron is not capable of reliably soldering to the chassis.
One more experiment... dealing with the arcing output tubes. Set your meter to read voltage and connect your leads between pin 5 of one of the output tubes and chassis. You should read approx -40v. Leave your meter connected and monitoring the negative bias on pin 5. Now use that jumper clip lead to connect V2-7 to chassis. What happens to the meter reading?
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Pn 5 on the output tube get me -34. When I turn the volume and treble to get the tubes really screaming, pin 5 gets to about -37 to -38. Doing the jumper from v2-p7 to ground keeps the pin5 at -34.
The3 lug grounding block wasnt soldered to the chassis. It was bolted, but I took it out and put a ring terminal in its place.
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> There cannot be negative 37 volts on V2-7 unless it's wired wrong
If the previous section is oscillating violently (and we have evidence of that), grid rectification will drive a grid negative.
I have not grokked the layout. High-gain amps (all guitar amps) are layout critical. Even a 2-toob Champ can oscillate; slight change of treble pot wiring can eliminate that.
Find the scream.
(http://www.google.com/images?q=tbn:ANd9GcSOenGoLoKZYZtntFA2n0BmObVy6yNyXcIyMkXMrp7Ldu8ubRo3bgMZIQ)
Or remove/short preamp path to kill the scream while diagnosing power-stage voltages and currents (so the tubes don't die), then find the scream.
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I may have just found it. There was a broken pin inside the socket on v2-p6. I am swapping in a new socket now and will report what happens...
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If the previous section is oscillating violently (and we have evidence of that), grid rectification will drive a grid negative.
What evidence? I'm not seeing that. The previous stage voltages look typical. In fact, the plate and cathode of this tube look normal even with -37Vdc on the grid. I would have thought the tube would be cut off with full B+ on the plate and zero on the cathode. I'd love to stand over this amp for a few minutes.
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WE HAVE SOME PROGRESS!!!!!
The internally broken pin at V2-P6 was what was causing the -37 on pin 7. I swapped in a new socket, and the voltage at pin 7 is now 0.
Tubes are still screaming at me though...
And just for the sake of testing, i plugged in a guitar and speaker. The amp just screams feedback no matter where any of the pots are.
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>> If the previous section is oscillating
> What evidence?
"Tubes are still screaming at me though... The amp just screams feedback no matter where any of the pots are."
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Guess the screaming had nothing to do with -37v on the grid. That negative voltage was the gremlin I was chasing. I expect there are still others about.
Pull the 2 preamps tubes leaving only the PI, power tubes, and rectifier. Does it still scream?
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Pull the 2 preamps tubes leaving only the PI, power tubes, and rectifier. Does it still scream?
Ya it still does. When I pull the PI tube the screaming all but goes away. You have to really listen to hear it and that may be normal.
Now I did notice that one of the power tubes has alot more blue coloration in it. This also may be normal, but I dont know, so I took some pics.
The tube closest to the camera has more blue in it, and was the one I really noticed was screaming. Would it hurt anything to pull one of the power tubes to try to isolate some problem, or is that a bad idea? Or maybe I can swap the 2 power tubes to see if its a tube or amp problem.
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-10-23204740.jpg)
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-10-23204712.jpg)
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Do you still have the NFB loop disconnected? If not, disconnect it now. Does it still scream?
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I do have it hooked up. I will unhook it and report back in a couple days when I get home. I had to go out of town for work.
Thanks again.
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Disconnecting the NFB loop is a method to check if the feedback phase is correct.
If the screaming stops when you disconnect the NFB loop the feedback phase is incorrect. The fix is to swap the OT plate leads going to pin 3 of each output tube. Then reconnect the NFB loop.
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Ok cool.
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Any progress?
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Not yet. As soon as I got back from work. I left on vacation. Won't be home till next Saturday. But I will report progress a.s.a.p.
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OK, Back from Vacation and time for an update:
Disconnecting the NFB helped alot with the squeeling. So I swapped Pin 3 on the 2 power tubes.
Now with the speaker and guitar plugged in, and all the controls at full, you can get a halfway decent, fuzzy, tone. Lots of hum though and no where near a good sound. BUT if you turn the volume down a little it screams and makes all sorts of terrible sounds. With the volume down, it sort of has a robotic sound, if that makes sense. And if you hook up the NFB it screams again.
what do you guys think?
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I did a little experiment this morning after tracing all the connections in the PI and power stage. Everything was correct.
First I pulled the preamp tubes. Then I took my phone and clipped it in where the preamp section and the power stage come together. This is with the NFB disconnected.
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_3847.jpg)
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_3848.jpg)
Here is the result:
http://youtu.be/38xrKoFLW00 (http://youtu.be/38xrKoFLW00)
So other than what sounds like a heater hum, I would say the power stage is fine. Does anyone disagree?
Since I have a test method that seems to work, I came up with some other test points that I will try:
1. Already tried. Seems to point to the Preamp being the issue
2. With the first preamp tube out, tie in right before the tone stack.
3. With the second preamp tube out, jump from the first tube to the powerstage. This will bypass the tone circuit and the 2nd tube.
4. With the first preamp tube out, tie in right before or after the volume.
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/Testpoints.jpg)
Thoughts?
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Well thanks to all your help, this thing is FINALLY done!
I wanted to share a few pics of how it turned out.
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_0059.jpg)
Here is a shot of the wood with a flash, it catches the color pretty good:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_0060.jpg)
The upper knob is on a piece of 1/4 inch dowel. It comes out and the input jack is behind it. Also I cut a piece of the original dial to put behind the original dial opening:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_0061.jpg)
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_0063.jpg)
Here is a pic of the back:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_0064.jpg)
The Speaker:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_0065.jpg)
The Chassis:
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_0066.jpg)
Again, thanks to all your help and advice. For my first build it sounds pretty good!
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That is cool stuff, glad you stuck with it!
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Thanks. I am pretty stubborn with stuff like this when input my mind to it. I do think I has a bad power tube. I was playing it today and if I hit the strings hard I get a bad sound. It almost sounds me its clipping. I will try to post a sound clip tomorrow.
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Here is a sound clip. You can hear it start to sound bad when I turn the volume up and hit the strings hard, that at about 15 seconds into the video. The camera didnt really pick it up too good, but it get REAL loud when it starts to sound bad.
(pardon the crappy playing, I usually play bass)
http://youtu.be/ctrIhRG8HTg (http://youtu.be/ctrIhRG8HTg)
I am thinking that maybe I need new power tubes, or that it is clipping, but before I go spend the $$$ for tubes I wanted to get your opinions.
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Saw your clip.. it just sounds like you have a bad solder joint ,( maybe where the heater wires are connected to the sockets). When you turn up the volume, the speaker vibration reveals the problem.
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I looked at my solder joints and I dont see any bad ones right off.
I was thinking it is bad power tubes because it get ALOT worse if I swap the 12AU7 PI tube for a 12AX7. I was thinking the higher gain of the 12AX7 is pushing the tubes too hard, and the AU is just pushing them a little hard.
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Here is a better sound clip. Any ideas whats wrong here?
http://youtu.be/oEvYsksBF-o (http://youtu.be/oEvYsksBF-o)
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Two questions:
1. Have you tried to reproduce the noise with a guitar?
2. Does the amp makes the same noise when a different speaker cabinet is plugged in (not using the speaker in the Philco cab)?
It seems from the pics that the speaker is a much bigger diameter than the cutout in the baffleboard. With a bass, you might get the cone pumping with loud, low peaks. I'm thinking maybe the speaker surround is buzzing against the baffleboard during those loud peaks.
I had some concerns about a similar potential problem on my current amp project, but experience from others says it doesn't happen much (if at all) with guitar signals.
If you can confirm the problem as being the items above (speaker buzz against the baffle due to large cone excursions on bass peaks), then you might want to install a shim between the speaker and baffle to get extra clearance. I'm thinking you could cut a wood ring a bit wider than the speaker gasket, drill holes for the mounting screws, and install between the baffle and the speaker. That would probably keep the speaker surround from buzzing against the baffle.
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It does the exact same thing with guitar. In fact it does it worse with the higher strings.
Its a good thought, but there is already a 1/4 inch ring spacing the speaker from the grille cloth.
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I noticed today that my treble knob is a little scratchy. could there be DC on this pot that is causing all my problem? or would that not give me sounds like this?