Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jeff_free69 on July 09, 2024, 09:22:36 am
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Greetings Gang,
This 1966 Blackface Vibro Champ followed me home a while back. Finally found a little time to start the rescue operation, so I opened her up.
The good news is that it appears to be 100% original and unmolested.
The bad news is that the interior has significant rust on the chassis (my guess is that before getting kicked to the curb it was relegated to a garage with a leaky roof; could be that water splashed on top and infiltrated through the chassis bolts and also down the back etc.)
It seems these modest amps are fairly highly valued, so I think its worth it to invest some time and resources to restore. I'm loving how relatively simple the circuit is (Several years ago I restored a 72 SF Twin Reverb and lived to tell the tale, with a lot of handholding from this forum)
For starters I'm planning to do the basics:
- 3 prong cord , death cap etc
- Replace the Filter cap can
- Replace the 4 onboard Electrolytics
It also seems that these amps were biased very hot, at least for more modern line voltages (mine is a constant 121VAC). So I'd like to get your opinions on how to handle that.
Otherwise I'll keep it as vintage as possible unless there's some compelling improvements to be recommended. (For example, are 20/20/20 uF filters caps still considered the best way to go? I've seen variations that increase some of the values)
More photos with all the gory details to follow...
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If it was mine, I would want to know it the PT and OT lives. Everything else can be cleaned up and fixed
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Once you complete the things on your list, measure voltages and adjust bias (or B+) to suit.
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1- Check if the fuse is the right one.
2- Caps in a can. You can go for JJ brand 20 X 3 at 500 volts . Check for diameter of amp hole to clear the connectors
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Can Capacitors: CP Manufacturing brand are a good choice too . More looking like the original one
https://www.tubesandmore.com/products/capacitors?filters=3109a3124c112a3109
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Thanks for the tips.
I've started the initial cleanup. What a mess! Here's some of the before shots.
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Thank' for the pictures.
Chassis need a good cleaning
I'll try with electronic cleaner and a brush, more harder than soft
I is a good amp.
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more, before...
tha capcan is General Electric 4x20uF
Tubes are RCA; 12AX7's are Mullard
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After first round of cleaning
WD40 on the chassis rust; Isopropyl on the circuit board
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more, before...
tha capcan is General Electric 4x20uF
Amp use only 3
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Can Capacitors: CP Manufacturing brand are a good choice too . More looking like the original one
https://www.tubesandmore.com/products/capacitors?filters=3109a3124c112a3109 (https://www.tubesandmore.com/products/capacitors?filters=3109a3124c112a3109)
That's interesting I don't think I've seen a GE brand cap can in any of my original Champ/VCs. Seen a bunch of Mallorys though.
The CE Manufacturing don't just look like the originals, they are made on the same equipment as the original mallory's. If the amp were mine a CE Man. cap can is all I would use.
https://www.tubesandmore.com/products/capacitors?filters=3109a3124c112a3109c2341a2356 (https://www.tubesandmore.com/products/capacitors?filters=3109a3124c112a3109c2341a2356)
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This amp looks great compared to many. I don't see mouse droppings . . .
Don't overdo the cleaning. You want this amp to work well, but have the patina. Don't replace stuff that doesn't need replacing. The more original it is, the more it is worth.
Platefire has got the priorities right (as always).
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As far as biasing, these amps ran hot from the factory, upping the bias 470 ohm resistor will lower the bias but might not be the best option. Hot Blues Plate and many other recommend leaving the 470 ohm and swapping the first two dropping resistors, putting the 10K first to the screen and the 1K to the pre-amp. This change will lower the screen voltage and cool the bias a bit. They say these amps run best at a dissipation of 14/15 watts, hot, if you raise the bias R, the word is it kills the tone. There's a video on this change that was posted here a short while back, do a search, good info on the change.
You could use a 5K first then 5K to pre-amp, but I would go no lower than 5K first dropping R, a 15K to pre-amp would give the amp more of a brown tone, experiment. These are fantastic amps. IMO, the OT is too small, over-saturates too much. I used an old paper wound SE 10W OT that fit the holes and loved the change, and a 10" alnico helped the tone too. Of course you don't want to butcher the the amp, try an extension speaker. Nice amp.
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Thanks for all the great info, this is a great starting point. Touching on some comments..
FUSE: confirmed the fuse is is 1A sloblo
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TRANSFORMERS
The overall condition may not be too bad cosmetically , but the evidence is that there was a significant amount of water dripping in. eg the screws holding down the circuit boards are rusted. That has me wondering about the transformers, as suggested by Platefire
1) What's the recommended way to investigate this, before investing in a full order parts?
Should I just pull the tubes and take basic measurements? I have a light bulb current limiter.
2) If they aren't good, are there replacements readily available? (in a quick search I haven't seen anything close )
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BIAS
Not sure if its the same video referenced by dude , but I have seen one that did the 1k - 10K swap instead of just bumping up the 470R.
Said it had more stable operating voltages and was similar to how it is done on Tweeds.
I will definitely have a look when I get to that point
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FILTER CAP CAN
CE looks like the way to go ( gotta love good ol Pete for cranking them out).
Yes it only needs 3, but I like the versatility of 4x20 being able to double up to reduce hum
UncleDoug put it on the first node (Rectifier), citing the fact that later versions did this
Lyle@Psionic put it on the second (6V6 screen node), to avoid stress on rectifier.
I suppose there are many opinions; its easy enough to experiment once the basics are taken care of
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SPEAKER
If I do have to replace it, I would not hack in a 10". I have a few extension cabs I can use to beef up the sound with a bigger spkr. BTW are the original 8" alnico?
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" FILTER CAP CAN
CE looks like the way to go ( gotta love good ol Pete for cranking them out).
Yes it only needs 3, but I like the versatility of 4x20 being able to double up to reduce hum. "
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I will not reduce hum . Hum is because a bad heater wiring in this amp. See below
It may help to keep sound tight at hi volume, you won't loose bass.
For no more hum ; remove one heater side from chassis ground , add a new wire twisted to the one it had.
Make a an artificial heater center tap with 2 X 100 ohms 1/2 watts each side of the transformer heater wire. Other side of each resistor must go to chassis ground.
https://www.valvewizard.co.uk/heater.html
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Cool project - a couple of comments:
I suggest getting the amp working in stock configuration before deciding to modify. Modding first makes it hard to find problems.
If you do not have a variac, I think using the light bulb limiter is the way to go. There is a start-up procedure here on the Forum - good to follow. Transformers are pretty robust, so you are likely to luck out.
In my experience, that speaker will not last if playing at loud volumes. You may want to ensure it works and replace it and save the original. I like the Webers, but WGS speakers have been suggested by many.
You mentioned Lyle@Psionic, his videos on bringing back crudded up Fender amps are great - they make me want to go back and redo some of my projects. I could never do videos as I repaired amps - too much swearing. :laugh:
Heater wiring - although I think Latole is correct about best practice for these, I did exactly that on my Vibro-Champ and found it made no discernable difference.
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Cool project - a couple of comments:
I suggest getting the amp working in stock configuration before deciding to modify. Modding first makes it hard to find problems.
Very wise advice.
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120hz hum can be reduced substantially in these little amps by installing another power supply filter node off the rectifier. Add a 1k 3w or so off the rectfier and use that 4th cap in the cap can for the first filter, then onward as normal.
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I'll start to fix the bad heater's wiring with no ground at PT heater secondary.
Sure you will not ear any hum
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Latole do you actually know the difference between 60 hz and 120 hz hum?
And I have several vintage blackface Champs and Vibrochamps that have original unmolested filament wiring and they don't exhibit 60hz hum. One reason is the little 8" speaker. When you connect these little amps up to a 2x12 cab you will hear things (sometimes) that are not really audible through the 8 inch speaker.
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Latole do you actually know the difference between 60 hz and 120 hz hum?
And I have several vintage blackface Champs and Vibrochamps that have original unmolested filament wiring and they don't exhibit 60hz hum. One reason is the little 8" speaker. When you connect these little amps up to a 2x12 cab you will hear things (sometimes) that are not really audible through the 8 inch speaker.
To be honest, not really.
But I do know how to get rid of all that noise.
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I will add a grid stopper resistor to 6V6 pin 4.
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I don't think it runs as hot as it may look.
Granted, there's 21V over the cathode resistor, .0446A.
Screens will drop that to around .04A
(342-21)*0.04= 12.84W
Even at modern voltages you're likely still safe.
If anything, I'd personally add a dropping resistor off the rectifier just to get it to the specs on the schematic, and then the rest of the amp falls where it should. If you have an extra cap in your can, you could use that to make another filter there if you want.
A screen resistor may be a good idea.
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Thanks - so much great info to digest.
Of course, i will be starting with the baby steps and see where things go.
Re: Heater wire twisted pair / artificial center tap...
After Uncle Doug doubled the filter on the rectifier node, he then did the twisted pair conversion. He discovered that the transformer had an unused centertap tucked away and used that. His was an early 66, mine is a later 66 , and i think I have the same unused wire. ( In each case he demonstrated a reduction in hum with a close-miked external 12" spkr .)
But this is above my paygrade at the moment.
I think I'm just about set do the basic 3 prong conversion and to start the power up.
Couple of more shots :
The pilot light, showing more corrosion (a little blurry , but there's some rust on the base)
My fear is that the pots have the same damage internally. BTW there's something odd about the Volume and Bass pots. Any ideas about what's going on here?
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I’d pull the tubes and test the transformers and speaker first. If those work, then maybe replace the cap can and see what voltages you get. Avoid changing anything else if it works (except to ensure it has good tubes). Don’t bother cleaning away the patina. The more stock it is, the more $ it will fetch
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Don’t bother cleaning away the patina. The more stock it is, the more $ it will fetch
I agree on the stock components. But I do not think cleaning the amp hurts the value of the amp at all, unless you take abrasives to it. If you want to play it - and who does not want to play a Blackface Fender, cleaning the circuit board may be necessary to get it sounding good. Stray voltage tends to leak across a cruddy eyelet board.
Your best approach depends on your intentions - fix and sell or fix, play and maybe sell later. If the later, I'd avoid destructive mods and save any parts that were changed. I'm always in the play-it camp - that's what it was made for. Of course I have a vintage Alfa Romeo that I drive. Am I hurting the value - probably. Do I care? Not a bit - guitars, amps, and cars are to be enjoyed. And now a little voice is telling me to step off the soapbox.
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Tubes socket 's contacts could be rusty too.
Follow last Tubeswell advice ; check PT
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. BTW there's something odd about the Volume and Bass pots. Any ideas about what's going on here?
Are you talking about the plastic center shafts protruding from the backs? Any markings on the backs of those pots? I haven't seem many like that before.
Or is it something else?
Pots should be cleaned and lubed with some de-oxit spray and worked back and forth a bit. The brass plate looks really clean.
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. BTW there's something odd about the Volume and Bass pots. Any ideas about what's going on here?
Are you talking about the plastic center shafts protruding from the backs? Any markings on the backs of those pots? I haven't seem many like that before.
Or is it something else?
Pots should be cleaned and lubed with some de-oxit spray and worked back and forth a bit. The brass plate looks really clean.
Yes, those. I'll see if can get a close up next time. Haven't Deoxited yet, but they are super stiff. And the power switch is completely shot. So i suspect that, like the pilot light, there may be internal rust on these components
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" FILTER CAP CAN
CE looks like the way to go ( gotta love good ol Pete for cranking them out).
Yes it only needs 3, but I like the versatility of 4x20 being able to double up to reduce hum. "
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I will not reduce hum . Hum is because a bad heater wiring in this amp.
Fender made a step-backward in engineering in the late-50s 5F1 Champ (https://el34world.com/charts/Schematics/files/Fender/Fender_champ_5f1_schem.pdf), and all Champ/Vibro Champ amps afterward. They connected the single-ended output transformer to the main reservoir cap at the rectifier output. There is significant ripple at this cap, that ripple gets impressed across the OT primary, and results in hum at the speaker.
The 5E1 Champ (https://el34world.com/charts/Schematics/files/Fender/Fender_champ_5e1.pdf) (and every earlier Champ amp) fed the 6V6 screen and output transformer from the same cap: the 2nd cap of a C-R-C or C-L-C filter. The result is substantially reduced ripple at the 2nd cap, less ripple impressed across the OT primary, and less hum in the speaker.
"Going backward" to the good 5E1 power supply (even if using a 470Ω resistor instead of a choke) has been standard practice on this forum for a good 15 years or so.
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Schematic of added for Vibro champ with 4 E cap can. But I have a question with about the screen supply, why is the screen voltage X, taken from the pre-amp supply (X & A) and not at point A..? It's the same voltage at both points, why isn't the (X & A) near the power supply? Is there a reason for getting voltage down the board.
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Sweet! will definitely put this on the top of the list once the basics are taken care of. (plus your copy of the layout is so much clearer than the one i have - I can actually read the values. Mine must be a 100th generation copy from a mimeograph machine)
I think I get the idea - very clever. looks like the tricky part will be fishing out the wire from the rectifier to the gopher hole on the board, so it can be moved to the extra cap element.
got a little vacation time coming (gotta make sure i don't miss all the sweltering heat). So in a few days I should be able get moving ...
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Schematic of added for Vibro champ with 4 E cap can. ...
I drew that schematic & posted it on another forum. It is merely Fender's original Vibro Champ schematic with an additional filter-section and an added resistor. These "added components" are placed before the existing caps/connections.
... But I have a question with about the screen supply, why is the screen voltage X, taken from ...
Look again: "X" is at Pin 5 which is the control grid (G1), not the screen grid (G2) which is at Pin 4.
- "X" connects to a 0.02µF cap and 220kΩ resistor. There is no power supply connection here.
- "A" connects from an eyelet in the power supply, then passes through a hole (that "X" also passes through), and connects to the eyelet that the 100kΩ plate resistors share. This IS a power supply connection, from the last filter cap to the 12AX7 plate loads.
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I drew that schematic & posted it on another forum. It is merely Fender's original Vibro Champ schematic with an additional filter-section and an added resistor. These "added components" are placed before the existing caps/connections.
Great! I believe it was mentioned that the resistor value between the 20uF elements should be 470 ohms, correct?
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470 ohm or 1k ohm are acceptable HBP did mention 470 ohm above.
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I did the powerup and basic measurements, and I'm quite relieved to report there was no smoke or flames.
The power switch is shot, so I had to modify my light current limiter rig to add one there, and build a little test jig.
Turns out the 1A SB fuse was no good either (guess what I didn't have and had to buy.)
Here's what i did / measured. All are done with the current limiter and 4 ohm dummy load
Some measured voltages were way higher than spec and some way lower (I assume the limiter has an effect )
1) Removed all tubes
PT secondary: 366 VAC
Lamp/heaters: 7.2 VAC
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2) Installed Rectifier
PT secondary: 347 VAC
Rectifier p8: 458 VDC
Filter caps: 460/468/470 VDC
Lamp/heaters: 6.6 VAC
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3) Install ALL tubes
Rectifier p8: 210 VDC
Filter caps: 183/194/205 VDC
Lamp/heaters: 4.9 VAC
6V6
p3 188V
p4 195V
p8 0V
12AX7 Vibrato (on)
p6 195V
p1,2,7,8 varying
12AX7 preamp
p1 131V
p3 1.0V
p6 129V
p8 0.9
See anything alarming, before I go ahead and retest without the current limiter? (I'm hoping to see voltages closer to the spec).
Or should i just go ahead and order the electrolytics etc now ?
Please let me know your thoughts and any other suggestions for next steps...
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Assuming the limiter behaved as expected, yes I would test without it and record voltages.
I also suggest that you go ahead and order and replace the electrolytics.
And if you don't already have them - two varieties of Deoxit will help those pots - D5 for heavy duty cleaning and F5 to lubricate them after cleaning. A lot of folks just use the D5, but I am a sucker for emptying my wallet. Actually I think the F5 helps the pots stay smooth longer.
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Post #35 shows pin8 cathode voltage at 0v with the 6v6 plugged in with the current limiter.
Did the current limiter show any shorts or did it pass initial start up?
I would check the pin8 connections.
Could be bad 6v6 tube.
Even with current limiter I would expect some pin8 voltage unless the bulb wattage is really high.
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Post #35 shows pin8 cathode voltage at 0v with the 6v6 plugged in with the current limiter.
Did the current limiter show any shorts or did it pass initial start up?
Could be bad 6v6 tube Even with current limiter I would expect some pin8 voltage unless the bulb wattage is really high.
OK - back to work
To get restarted I double checked my measurements. The bulb was glowing dimly; (I'm not experienced enough to know for sure how much is acceptable. The bulb is 100W. I measured the voltage at the fuse at 86 VAC.
I don't have a big stock pile of spare tubes, but i did happen to have an old Fender 6V6 GTA, of unknown provenance. It also showed 0V on p8, and no significant change on any of the other pins.
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I then retested without the Current Limiter. While the voltage coming out of the transformer looks reasonable, everything else still looks way too low ( based on the AA764 schematic)
Expected / Measured (no current limiter)
5Y3 p4,6 red 315 AC / 338 AC
p2,8 yel 355 DC / 305 DC
FILTER CAPS
320 / 253
340 / 282
355 / 305
6V6
p3 342 / 266
p4 340 / 281
p8 21 / 0
12AX7 vibrato (on)
p1,7 170 varies
p2 1.6 varies
p6 340 / 281
p8 175 varies
12AX7 preamp
p1 205 / 170
p3 1.6 / 1.6
p6 200 / 79
p8 1.5 / .9
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FWIW I powered down, discharged and took some measurements on the components - all in circuit
The can filters were all about 22 uF
The cathode bypass electrolytic caps were all in the general neighborhood except the 25 uF Bias cap on the 6V6, where there's 0V on p8.
It measured off the scale on the highest range (400uF).
It also appeared to be a dead short (assuming the 470 ohm resistor was not).
I'm not sure how measuring like this affects the results, but wondering if this is might go away when i replace the caps?
What might be the cause of the generally low voltages
- bad Rectifier? I don't have any spares to test)
- Bad filter can caps?
- Do you think the PT is healthy?
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What might be the cause of the generally low voltages
- bad Rectifier? I don't have any spares to test)
- Bad filter can caps?
- Do you think the PT is healthy?
Low voltages=something is drawing too much current.
Bad filter can caps=definitely possible
Healthy PT=yes....so far
Have you tried testing resistances on your caps? They could read perfect on your meter and still be basically a resistor to ground.
It almost seems like the bypass cap on your 6v6 is a direct short, which would definitely explain low voltages. This isn't an amp you are just going to throw away or give up on, you want to repair it, so a good plan of action may be to start repairing before you break something that isn't already broken.
Replace the electrolytics and check values of all resistors, make sure your transformer windings aren't open or shorted, clean the pots and switches, and then start checking voltages. Seems like you're putting the cart before the horse, at least to me.
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I would replace the 6V6 cathode capacitor. I'd use 47Uf 100v electrolytic there. Save the old one.
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Definitely replace the 6v6 cathode capacitor. Measure the 470ohm resistor and measure the 6v6 pin8 to ground which should be ~470ohm.
Do this/these first and test as opposed to doing a bunch of stuff at one time and then testing.
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I don't really understand the mojo of a 60 year old part that is waiting to fail. Sure, save the caps in a bag, cut them open and put new caps inside so it looks vintage. Make it so you don't have to wince every time you turn it on.
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I've still got a couple of bags of old Fender mojo hanging around - don't really know why. Most of the blue ones are the dreaded death caps.
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I've still got a couple of bags of old Fender mojo hanging around - don't really know why. Most of the blue ones are the dreaded death caps.
:laugh: I have same. I went to the Crossroads in Clarksdale, Mississippi and sprinkled them around in a circle. I sat in the center for an entire night. Nothing.
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:l2:
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..Healthy PT=yes....so far
Have you tried testing resistances on your caps? check values of all resistors, make sure your transformer windings aren't open or shorted,
1) Transformer: removed tubes and took a bunch of measurements for shorts etc around the transformer:
Primary:
Black1- Black2 5 ohms
Shorts? (X = no)
Black – Gnd X
Black - Red p6 X
Black - Red p4 X
Black - Yel p8 X
Black - Yel p2 X
Black - Green X
Red p4 - Red p6 380 ohm
Red p4 - Gnd 195 ohm
Red p6 - Gnd 185 ohm
Yel p2 – Yel p8 .3 ohm
Yel p2 – Gnd X
Yel p8 – Gnd X
Green - Gnd .5 ohm
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2) Lifted leg of bias resistor and cap
470R = 650 ohm
Capacitor : capacitance - no reading ; 1ohm
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It doesn't look to me that there are any shorts on these measurements, but i am perplexed by the HT secondary reading (red wires) being so high = 380 ohm
Is that normal?? With the Rectifier tube out of the circuit, shouldn't the impedance through the windings be pretty low?
The "470" bias resistor = 650 ohm seems reasonable, at least for now ( I've seen many people increase it to this or higher to cool the bias)
The cap is shot, for sure...
Anything else i should look at ? Do i need to unsolder the PT connections to make these measurements?
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That cathode resistor is way out of tolerance. Id replace the 470R with a new 1 watt metal film flameproof resistor probably around 600-750R. It depends on what the power dissipation of the 6V6 is when you check it. Might still be ok with 470R. Some people replace the KR with up to 5 watt resistors, but I should think 1-2 watt would be ok. Isn't that a 1 watt in there stock?
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Your HV winding on the PT is fine 195+185=380 IMO
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Thanks for confirming.
I noticed the math was fine, but the high value had me concerned.
Pure ignorance on my part and not terribly important, but if the measurement is just through the windings, i wondered why not something lower, like the Primary side (Black1-Black2 = 5 ohms)
Anyway as long as nothing looks horrendous here I will order some parts...
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HBP could inform on the why.. the primary 117VAC side is larger wire and it is a step-up tx where the secondary side winding wire is much smaller I think.
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Here are photos of my 1965 Vibro Champ that may or may not assist you in your endeavor. I bought this in an estate batch with more than one C and VC. I was told the old man that owned them had recently died.
Some parts were replaced. It also came with a brown frame 10" Jensen 4 ohm speaker. I have never looked to see how hacked up the baffle might be - seems ok to me.
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This amp is free of hum and as you can see the filaments are still wired stock.
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SWEET! - hard to believe how something almost 60 years old looks so new, especially compared to my rust bucket.
Looks like you have a resistor hanging on your 6V6 that i dont have. something new/old?
Do you recall what value you used for the bypass cap on the preamp second stage ? The layout posted on this thread shows 2 uF, but my unit has a 10 uF, which matches the AA764 schematic. My understanding is that this affects the bass cutoff frequency ( eg 2uF = 100Hz; 10uF = 20hz; 4.7uF = 40 Hz)
General question
- any preferred brands for these caps (I haven't done any shopping for several years and dont know the current state of affairs)
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... i am perplexed by the HT secondary reading (red wires) being so high = 380 ohm
Is that normal?? With the Rectifier tube out of the circuit, shouldn't the impedance through the windings be pretty low?
...
It's fine, valve rectifiers require >100ohms of 'protecting resistance'.
The HT secondary will have more than 650/120 = 5.5 times the number of turns used for the primary.
The use of thin wire allows the protecting resistance to be built into the transformer.
That avoids the amp builder needing to use external resistors.
And allows the transformer builder to save on copper etc.
>180ohms per rectifier anode is probably more than the minimum necessary, but that's the designer's choice, it won't cause a problem per se.
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Looks like you have a resistor hanging on your 6V6 that i dont have. something new/old?
Do you recall what value you used for the bypass cap on the preamp second stage ? The layout posted on this thread shows 2 uF, but my unit has a 10 uF, which matches the AA764 schematic. My understanding is that this affects the bass cutoff frequency ( eg 2uF = 100Hz; 10uF = 20hz; 4.7uF = 40 Hz)
General question
- any preferred brands for these caps (I haven't done any shopping for several years and dont know the current state of affairs)
That is a 470R screen resistor. I haven't been in this amp for a few years, and should probably pull tha amp out and implement HBPs recommendations.
The cap for the second stage is 10uf as seen in this photo blown up some.
I think axial caps would be best but you could use Orange Drops, Mallory poly caps, but since there are so few maybe get some Sozo or Dougs Jupiter caps which a lot of people like.
For electrolytics you could use Sprague or the new Mod caps.
Doug has all you need here in his store.
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Parts are on the way!
I lifted the legs on a bunch of components and took measurements. The other Mallorys are out of spec by 40-50% and there's a one suspect resistor.
Of course no 1 vendor had everything, but hopefully everything will be in place next week
Have a great weekend!
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Latest update -
Got my parts and installed them. Of course some complications to keep things interesting
Quickly discovered i really need the bigass iron for chassis grounds.
Found a Switchcraft slider switch rated for 3A that fit perfectly (original Carlson was 4A). But of course the mounting holes on the wings are not threaded like the original, making a major pain to get a nut&washer onto the screw. but at least i was able to keep the original screw.
The screws that secure the boards to the chassis were completely rusted, and one completely rounded out, and it was really bugging me.. Plus i had the nagging feeling that a washer may have disappeared underneath (see above). I ordered a screw extractor kit and got them out - BTW anybody know exactly what a replacement would be?
I was then able to lift the board a little and take a partial peak underneath. I discovered that I was actually able to completely slide out the insulator board, since the filter cap can was still not wired. I was then able to give it a fiberoptic colonoscopy, underneath the main board, without unwiring anything. Wasn't pretty , but relieved that there was no sign of any stray metal parts.
The insulator board needed some real cleaning, especially the rust on the bottom side. I've tried cleaning it a couple times, most recently resorting to 60 grit sandpaper. I figure its the bottom of the insulator board so no need to worry about patina. But I still can't get all the rust stains out, and i know how conductivity can be such a sneaky problem.
Anyway here's the current state. I'm debating if its ready to give it shot
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It's your amp, but there's no way I'd re-use that eyelet board. Too much work to re-wire/solder that all up and then run into trouble. And if not right away, it's coming. :w2:
And I would not use a new eyelet board made of that same material. I'd go with a fiberglass board. Our host has them in his store as do other on line stores.
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1) Transformer: removed tubes and took a bunch of measurements for shorts etc around the transformer:
...
Red p4 - Red p6 380 ohm
...
... i am perplexed by the HT secondary reading (red wires) being so high = 380 ohm
Is that normal?? ...
Hammond Champ Replacement PT (https://www.hammfg.com/files/parts/pdf/290AX.pdf) ---> 277Ω
Hammond has lower winding resistance than vintage power transformers (as you see with yours).
... the high value had me concerned.
Pure ignorance on my part and not terribly important, but if the measurement is just through the windings, i wondered why not something lower ...
High Voltage winding is many-many turns of Skinny Wire ---> Big Volts step-up (120v up to 650v), but small current (only 0.1A for Hammond's part)
Low Voltage winding is a few turns of Very Fat Wire ---> Big Volts step-down (120v down to 5v or 6.3v), big current (2-3A for Hammond's part)
Primary has to have some goodly number of turns for Inductance (40x the resistance of the low-volt windings in Hammond's part), and carries a moderate current (about 0.775A to 0.8A for Hammond's part).
- Gotta have fatter wire for higher current.
- Gotta have many-turns for big step-up.
- Skinnier wire is higher Resistance per unit-of-length.
- Add It Up: High Voltage (B+) winding is typically high-resistance.
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It's your amp, but there's no way I'd re-use that eyelet board. Too much work to re-wire/solder that all up and then run into trouble. And if not right away, it's coming. :w2:
And I would not use a new eyelet board made of that same material. I'd go with a fiberglass board. Our host has them in his store as do other on line stores.
Thanks, I hear ya - I have to acknowledge that the board may be more trouble than its worth.
But with that in mind, i dont see any reason not to reinstall the insulator board (after i take another crack at cleaning and removing mositure) to see what's what. The new caps are already installed (i just have to wire up the filter can).
Hopefully will see some improvement and then can start chasing down what other gremlins there are.
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Maybe try Evaporust on the board, then alcohol and finally acetone. Did that on my Champ board and got all the DC off. It is true these boards will absorb grit it time and then might go conductive again. Oh, and stick it in the sun to get it real dry, with a weight on it.
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Thinking about the condition of the insulator board and what i saw peeking under the main board made me realize there's a lot more to be done here. A deeper cleaning is called for.
To get underneath the main board I thought I'd have to unsolder at least a few things , like the but the 4 grounds on the brass plate. But I figured out a way to get complete access without unsoldering anything else at all. By unbolting the front panel pots and jacks I was able to flip out the whole harness along with the brass plate.
Gently folding it all back revealed the loveliness lurking beneath.
Using dozens of Qtips I did a couple of passes with copious amounts of isopropyl. On the last one I hit with a heat gun (at a low setting)
Also did another drenching of the insulator board and hit that with the heat gun. Put it under a weight while it felt a little more pliable.
I'll give it another pass and focus on the rust on the chassis bottom...
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Yikes, that's a bunch of rust....not good. You can get that all out in time.
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Yikes, that's a bunch of rust....not good. You can get that all out in time.
After its down to bare metal again it has to be treated properly .. or it will rust again when exposed to humidity. I would suggest Por-15 or something similar.
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Hey I been watching this one. I thought I had a lot of cleanup on my highly neglected and abused 1974 Pro Reverb head but your really going the extra mile to bring this back. The guy that sold it to me told me it would never work again unless you replace the board, but I guess I was determined to prove him wrong. So I hope this one works well for you after you finish the cleanup process :thumbsup:
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Hey the solder joints for the cap can and other joints to the chassis requires a lot of heat, you'll need an 80W to 120W iron to get those done properly. I use and great big old 80w Wall pencil iron. Also take care not to melt or damage components and wire around the joint when applying heat. I usually use some heavy duty tin foil doubled over and then create a heat barrier around those areas to solder.
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Hey the solder joints for the cap can and other joints to the chassis requires a lot of heat, you'll need an 80W to 120W iron to get those done properly. I use and great big old 80w Wall pencil iron. Also take care not to melt or damage components and wire around the joint when applying heat. I usually use some heavy duty tin foil doubled over and then create a heat barrier around those areas to solder.
Yes, thanks. just realized i wasn't completely clear in that i had already picked up the Weller 80W (WLIRPK8012A). Even together, my old 100W Radio Shack gun + modern 80W digital workstation dialed up 900 degrees weren't hacking it.
The Weller has the big fat tip for stained glass work, plus a screwdriver tip which is perfect for prying up the tabs once you get the solder flowing. Worked like a charm. I just haven't gotten around to soldering in the new can yet.
I learned to keep some teflon oven sheets handy ( because i tend to focus on the tip and loose track of everything else )
I'll be making a few more passes of WD40 soakings and various scraping tools sporadically over the weekend.
I think i have some silver Rustoleum on the shelf...
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Consider applying some rust converter prior to topcoating. Here's some on the cheaper end of the spectrum
https://www.amazon.com/Rust-Oleum-Automotive-248658-10-25-Ounce-Reformer/dp/B003HG48AC/ref=sr_1_5
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scraping/cleaning is coming along very well, just about done
The Rust Reformer looks like an excellent option. But just to be clear, is this going to be appropriate only in the completely stripped areas? Seems like its not suitable for galvanized steel (needs alkyd paint?) and isn't that what the chassis is, except where the zinc coating completely rusted off?
I was originally thinking of taping off and spraying the whole area under the boards, but maybe i need to be more precise.
Now that i think about it, i recently redid my metal fence using Rustoleum "rusty metal" primer ( thick redish brown stuff) its oil based and also not suitable for galvanized metal, but it would be easy to paint in the affected areas, then use the silver paint.
Thanks for the metallurgy lesson
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Here's how she's looking down to bare metal. Scratched the heck out of it , but patina under the board took a back seat to getting it clean.
I expected to see more damage, but it looks like there's just some pitting.
not sure how this affects my previous comments above ...
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Moving along. What's your next step? This amp is for you yes? On rusted machinery I have sandblasted to remove all rust then use a primer and then paint. Clear primer may be the ticket to seal it up as is and get back to making music. A labor of love.
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You should be able to polish the nasty scratches out... The patina is definitely gone, but there's no reason you can't get it shiny again.
Some sort of a preserver may be a good idea? But it looks like you'd have to paint the chassis, which... on the outside is fine, but the inside? I dunno.
Normally amps aren't subject to that much moisture.
Maybe just wiping it down with oil or something would suffice.
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Agree, maybe just a bit of polish and light wipe of anti-rust (WD or light oil). Less is more. It's not gonna get abused now so it should stay clean for ages. Looking forward to a sound take.
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thanks for confirming that any rustoleum paint is pure overkill at this point.
The more i look at the chassis here the less worried I am about it. i now realize all that rust which accumulated underneath likely dripped down from above over time. And the pitting on the bottom is relatively minor. Here's one of the original photos of the cabinet top with the rusty screen stapled to it. of course these conditions will never happen again. Check out the rust stalactites!
I have a product called Everbrite that is a clear coating for metal. I used it a few months ago on a brass mailbox and some door hardware, and its held up very nicely. I thought about just coating it with wd40, but as long as i have this think it is the way to go.
I did a couple more passes trying to polish with some higher grits of sandpaper. IF the project is a success, no human being is likely to ever look underneath the insulator board again. If not then a new board is probably needed, and a few scuffs wont matter.
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Crank that baby up.... but first. bulb limiter :icon_biggrin:
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Got her all back together, all cleaned up (or at least as clean this poor old girl is going to get for now)
After all the jostling i thought it'd be a good idea to reflow all the solder joints on the tube sockets and pots.
The tube sockets were tightened up and cleaned.
The boards are temporarily secured with some #4 bolts & nut (because they slide through without stripping the threads) . The 2 original machine screws are rusty trash, so I'd appreciate it if somebody knows exactly what they are (and where to get them if they're unique/hard to find). They seem to have a tapered tip and I'm guessing #6 , but with a low thread/inch.
Here's how the chassis bottom looks with the clearcoat .
Also took some closeups of the pots while they were dangling. those 2 weird ones ( Vol, Bass) are hard to make out). They've all been DEOXIT-5ed several times over the past couple of weeks.
Tomorrow I'll do the powerup and test...
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https://darrenriley.com/homepage/fender-screw-php-zi-6-32-x-14-0038900000/
https://www.mojotone.com/Self-Tapping-Stainless-Steel-Screws-BF-Chassis-Fiberboards
Either is a source for the 6/32 screws.
--Pete
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Good sources - or just stock up
https://www.amazon.com/1000Pcs-Nickel-Plated-Phillips-Self-Tapping-Electronic/dp/B07HF46Q4R/ref=asc_df_B07HF46Q4R/?tag=hyprod-20&linkCode=df0&hvadid=693675560295&hvpos=&hvnetw=g&hvrand=9113095291155701964&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9061105&hvtargid=pla-571118526895&psc=1&mcid=7c908a653d9339e5af567711be3c135c&gad_source=1 (https://www.amazon.com/1000Pcs-Nickel-Plated-Phillips-Self-Tapping-Electronic/dp/B07HF46Q4R/ref=asc_df_B07HF46Q4R/?tag=hyprod-20&linkCode=df0&hvadid=693675560295&hvpos=&hvnetw=g&hvrand=9113095291155701964&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9061105&hvtargid=pla-571118526895&psc=1&mcid=7c908a653d9339e5af567711be3c135c&gad_source=1)
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I usually use a tap to chase the treads then any screw will work, no need for self-tapping.
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I think Doug has those screws right here in the parts catalog.
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Looks like I have a good update for you - Its actually making music ! (though my playing is rustier than the chassis was).
I rigged up my 2x12, 4 ohm TwinReverb cabinet and was happily surprised when the Strat rang out , loud and clear.
But there is that nagging hum, even with all the controls set to 0. Maybe its just what's inherent to the design, and with 2 12's right in your face you're going to hear it
Miraculously the Volume and tone pots are quiet.
The only issue is the tremolo intensity seems to go full on by the time you get to 2 . Sounds really good , and the speed control is good. But its practically on / off .
Also kinda feels like there's a lot of gain. I'm not familiar with the VC, but it seems to break up a little early. not complaining, just assumed it would be on the polite / clean side
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The Bias is coming in at 12W / 85% dissipation.
I still have the original cathode resistor in there, and it measured 650 ohms (instead of 470)
Voltage across it is 22.5
Plate to cathode measured 368V
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Most of the other voltages are looking much more robust. Before the cap update they were all pretty low. Now they are higher than the design calls for. But considering the higher AC coming out of the transformer (line in is 120VAC), i guess this could still be acceptable (?)
The exception would be the Tremolo 12ax7, which are actually lower than expected. so this is something that needs to be looked into. I can try swapping some tubes - any other ideas?
For reference here are the latest
Expected / MEASURED
5Y3
p4,6 red 315 AC / 356 AC
p2,8 yel 355 DC / 405 DC
FILTER CAPS
320 / 375
340 / 395
355 / 405
6V6
p3 342 / 390
p4 340 / 394
p8 21 / 22
12AX7 preamp
p1 205 / 224
p3 1.6 / 2.0
p6 200 / 240
p8 1.5 / 2.1
12AX7 vibrato (OFF) ***
p1 210 / 180
p2 ? / 1.6
p3 2 / ?
p6 410 / 390
p7 210 / 180
p8 215 / 186
***EDIT : The expected values on the vibrato tube shown here are wrong; see below
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... But there is that nagging hum, even with all the controls set to 0. Maybe its just what's inherent to the design, and with 2 12's right in your face you're going to hear it
The only issue is the tremolo intensity seems to go full on by the time you get to 2 . Sounds really good , and the speed control is good. But its practically on / off .
Also kinda feels like there's a lot of gain. I'm not familiar with the VC, but it seems to break up a little early. not complaining, just assumed it would be on the polite / clean side
As far as the hum goes, how's your lead dress? Audio wires routed away from any AC voltages and HT DC. Don't route near the heaters. Do wires cross at right angles if they must cross? Try a chopstick pushing on B+ wires. wires here and there to see if the hum level changes.
Tremolo intensity sounds like your intensity pot is a log taper. If so, you might get a better range of control if you swap in a linear taper pot of the same value.
I wouldn't have expected a too-much-gain problem with that circuit. Interesting. One easy thing to try is to swap in a 12A_7 cousin for the 12AX7 in the gain stage. This trick worked to tame a Marshall 1975 clone I built. I currently have a 12AU7 in the first gain stage, and the range of overdrive vs. clean is much more manageable than it was with a 12AX7.
12 AT7 µ = 70
12AY7 µ = 40
12AU7 µ = 17
Try a 12AT7 in the first position. Play some. You may find you like a different tube so you can adjust µ to your taste.
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That filament circuit in the VC is prone to hum, unless you convert it to layer BF circuit filament wiring, getting rid of using the chassis as the ground, the hum probably isn’t going away.
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... But there is that nagging hum, even with all the controls set to 0. Maybe its just what's inherent to the design ...
The hum is normal; it's not heaters or lead dress.
Look up the 5E1 Champ, and compares its power supply to your Vibro Champ. You will see the reason for the hum in the Vibro Champ.
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If your 470R cathode resistor reads 650R, try changing it out for a new part within spec.
That will bring down your voltages.
I've had good luck with higher wall voltages by using a small value dropping resistor after the rectifier. I've found if I can get voltages at the front end to line up with the schematic, the rest of the amp falls into place.
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The hum is normal; it's not heaters or lead dress.
Look up the 5E1 Champ, and compares its power supply to your Vibro Champ. You will see the reason for the hum in the Vibro Champ.
I have a 67 VC, with a different SE OT (vintage 10 watt paper wound) and 10" alnico spk, only differences but I have rewired the filaments not using the chassis ground and have "no hum". OK, I did reverse the two dropping R's. Is the choke what your talking about in the 5E1, or the 8uf filters..? You have 100 times more experience than I have, maybe it's the different OT I installed..? Or maybe the the 4th filter off the 5Y3, I installed..?
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Is the choke what your talking about in the 5E1, or the 8uf filters..?
It's the choke BEFORE that feeds the OT CT.
Push/Pull power stages have natural noise cancellation that SE amps don't have.
So for a long time now, here and at other forums, guys add at least another filter cap and a low value B+ dropping R before what's already there. It works great. On new SE builds a choke works even better.
I built a Princeton 5F2a, Champ with single tone control. I used a Fender choke for a PP 6L6GC Fender amp for a pi filter before what's already there; filter cap/choke/filter. Amp is very quite.
The Fender PP 6L6GC choke can supply enough current for a SE 6V6 amp and it's preamp tubes. IIRC it has enough to use a 6L6GC SE and it's preamp tubes.
A Fender Deluxe PP 6V6 amp's choke is fine if you want to also add a 2nd choke for the screen supply node on a SE amp. It could be smaller but where do you find it easily?
And a choke fro the screen supply.
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Thanks All!
I'm going to try to live with the hum for a little while and get used to how she sounds. When playing it wasn't noticeable. I'm going to hook up the original speaker ( haven't tested it at all)
The hum reducing suggestions are still cooking in my mind, as I have the extra 20uF element in the can... And I will also review the bias.
But I was thinking about the Tremolo situation.
1) the voltages seemed lower than expected: I realized I had wrong expected values , as i must've been looking at the AB version for that. With the trem OFF, the values are now slightly above expected.
I also wanted to try measurements with the Trem ON (which of course will vary on certain pins) . When ON they go way higher . So I guess this is a bit more consistent
12AX7 TREMOLO
Exp / OFF / ON
p1 170 / 180 / 10-260
p2 ? / 1.6
p3 2 / ?
p6 340 / 390 / 390
p7 170 / 180 / 10-260v
p8 175 / 186 / 10-260
2) Intensity. I haven't swapped in another tube yet, so will do that next session. Any other thoughts on what to check ? The obvious culprit looks to be the pot
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When playing it wasn't noticeable.
I've built a handful of xSE amps, my "hum test";
IF it does away when I move the guitar volume from 0 to 1, it's not a problem
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But I was thinking about the Tremolo situation.
1) the voltages seemed lower than expected
If you hear trem when you turn up the Intensity knob, you're good. The trem works fine.
The amp won't sound better when you exactly-match the numbers on the schematic (which weren't what I measured in a 100% stock 1965 Vibro Champ, either).
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be looking forward to the finish pixs :icon_biggrin:
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Concerning the cabinet.. that shield screen is hideous.. time for some staple pulling.. also have to remove the carry hanlde IIRC.
Its also obvious that Leos bean counters had their eyes on staple usage - they sure didn't waste any securing the grill cloth.
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Concerning the cabinet.. that shield screen is hideous.. time for some staple pulling.. also have to remove the carry hanlde IIRC.
Its also obvious that Leos bean counters had their eyes on staple usage - they sure didn't waste any securing the grill cloth.
OH yeah, that rust collector is long gone! Along with hundreds of staples
Washed it down good, and only getting around to thinking about what to do about replacing it.
Foil tape?
Would it even make a difference , considreing the internal hum thats already in there?
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I've used and some manufacturers use the 3M foil tape and it seems to work well. Not very expensive and easy to try.
It's really up to you.
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Here's my Princeton 5F2a schematic drawing showing how I did the power supply and dc standoff voltage.