Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Platefire on October 03, 2012, 10:49:27 pm
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I build an 5F1 Champ for my Son about 10 years ago when I first started building amps. He is wanting to use it more but wants to get some of the normal champ hum out to use it also for recording.
He sent me a link to Marsh Amplification "Reducing Noise in Tweed 5F1" at the link as follows:
http://www.marshamps.com/5f1noise (http://www.marshamps.com/5f1noise)
It's main focus is twisted heater wires and installing an virtual center tap if needed, using sheilded input wires from input up to the second stage preamp and also increasing the filter cap values on the first and second stage of filtering(OT and Screens)
OK-It already has the twisted heater wires and has a heater center tap-check. I can change the hookup wire out to shielded as they recomend-check.
On the filter caps they recomend for diode rectifier first main filter 100uf/500v(improve low end tightness), 40uf/450(no real change in tone) for screen supply and no change on the preamp filtering 8uf/450V. So my question is does this sound right on the filtering? My son does have a plug in SS rectifier he uses sometimes but mostly uses a 5Y3. IMHO 40uf/40uf or 40uf/20uf would be less a drastic change? Has anybody had any experiance with this? Platefire
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Increasing the existing caps is brute-force with poor leverage.
Humongious first-caps still have a lot of ripple.
+Add+ an R-C section in front. First-cap reasonably-large (not gigantic). Then the added 250 and now-second 40uFd will knock-down first-cap ripple another 14:1 before it goes to the OT/6V6. Net result is 35 times (31dB) less buzz at OT/6V6... from "some" to "none". The screen is reasonably well filtered already (due to large 10K resistor) but with today's cap-prices you may as well round-up from 8 to 20uFd. Voltage ratings similar to the voltages you measure now plus a margin for high wall-voltages in studios or other dives.
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Hello PRR
One thing I noticed about you and I---we seem to do our posting late-late :dontknow:
Thanks for the nice schematic and explination. My problem with that plan is finding the space for the extra cap and resistor. Not saying it's imposible, just will have to get real inventive. The noise reduction you indicated looks sizeable and desirable as long as it don't hinder the normal champ tone response much. I see you did leave the last 8uf stage intact. I'll talk this over with my Son. Thanks, Bob
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Before put more filtering , how did you build this amp ?
Good wiring and component installation it a must. Resistor, capacitor and wire going to tube's grid must be very short. if not they act like antenna.,
Did you use shorting jack for guitar input ?
Can you send us some photo of your wiring , inside the amp
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Well I admit this amp was done early in my amp building experiance. I learned a lot since then. So re-checking the lead dress and grounding will be one thing I do. I did do it under the direction of a very knowlegable and experianced Casey4s. In fact he hand built the kit himself for me and I just put it together and he was advising me through the whole process. So I think its pretty much assembled OK.
I don't have my Son's amp presently in my possesion. We have been talking about it by phone. He's also talking about replaceing the coupeling caps with sozo--so we are trying to get all the needed mods figured out so I can do it all at one time when he brings the amp down. Attached is a Flicker link to that project photos that does have some not so good pictures of the circuit. Platefire
http://www.flickr.com/photos/12623503@N04/sets/72157601870694030/with/1328143458/ (http://www.flickr.com/photos/12623503@N04/sets/72157601870694030/with/1328143458/)
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Sorry photos can't help , Poor resolution and no close-up.
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Har! He must of read this post because he sent some new pixs. Here they are:
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Yeah! In 2002 I also learned board building from Caseys4 tutorial available at the time.
is he still here? thanks.
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+Add+ an R-C section in front. First-cap reasonably-large (not gigantic). Then the added 250 and now-second 40uFd will knock-down first-cap ripple another 14:1 before it goes to the OT/6V6.
Thanks for the drawing, I'm going to try it with mine. I'm calculating that the 220-330 ohm only needs to be a 2 watt. Am I miscalculating? (I'm short on 5W space).
Thanks.
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I think if I mounted a terminal strip between the brass ground plate and the end of the board, I could mount the extra 40uf cap on the terminal strip and run the 250 Ohm/5 watt resistor from terminal strip cap + to existing 16uf location. Of course the 16uf and next 8uf would be changed out to 40uf and 20uf. I believe I can make that work! Attached is a rough sketch of the plan. Platefire
Woolly: I exchanged e-mails with Casey4s about a year ago. He was doing alright then best I could tell. He was testing effects pedals for some company and doing reviews for them. Great
Guy! Platefire
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> only needs to be a 2 watt
At start-up, the first cap comes to (say) 400V very quickly due to high current available from PT, the second cap starts from zero and charges through (say) 250 ohms.
So at that first instant the power dissipation is 640 Watts !! (400V in 250 ohms.)
The second cap will be largely charged-up within 0.01 seconds. That's actually fast enough to shave the apparent 640 Watts, but the math is tedious. There's also PT resistance.
Duncan PSU shows:
Peak of 293V 1.33A or 390 Watts near 0.05 seconds after turn-on.
RMS-averaged over first 0.1 seconds, 86V 0.34A RMS or 29 Watts. Over first second, 4.6 Watts. Over first 10 seconds, 2.15 Watts.
Yes, resistors will take short-term overloads. But the spec is usually 10X, not 390/2 or 200X.
If you ever had these filter resistors blow "for no reason?", that's why. Smack a 2W with 390 Watts for an instant, it doesn't smoke, but it gets micro-cracks. Do it 2 or 20 or 200 times, the crack goes right through and it quits.
_39W_ is the for-sure safe size. However we know from experience that the 10X spec is conservative. Basically we don't turn-on as often as some industrial punch which might apply thousands of overloads an hour 9-5 M-F.
Even long-term you have 1.9 Watts. A 2W part is going to fail.
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Sorry but there is a poor lead dress.
Easy to understand you have some noise.
Wires going to the small preamp tube are like an antenna.
And many other wire are un-properly dress. Some shielded wire will help.
Don't spend money on other cap until you fix that first.
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Can't we use a small value R, like 10 or 20 ohms at 5w as the first thing in the B+ PSU to knock/slow down the inrush current a little and then be able to use a lower wattage for the 250R safely?
Would this give a little extra margin of safety/longer life to the 250R at 5w listed value in PRR's reply #1?
Brad :think1:
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Well my Son(Greg) wants to proceed with PRR plan. I will also be installing sheilded wiring in the input/preamp section, addressing lead dress issues and installing a couple of sozo .022 coupeling caps.
On another note----This amp presently has a Weber 10F150T speaker that has a ribbed cone for extra treble. This is a little to bright for him. He's looking for a 10" that will compliment the champ tone with a little more thickness and a little less harsh highs. Weber Silver Bell had been a consideration but not tried yet. Any suggestions on what might fill the bill? Platefire
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> 10 or 20 ohms at 5w as the first thing
How does that help?
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How does that help?
to knock/slow down the inrush current a little and then be able to use a lower wattage for the 250R safely?
Brad :dontknow:
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What PRR's really getting at is you already have a comparable amount of resistance in the PT secondary winding (even more if you add the PT primary resistance, referred to the secondary).
So you're not doing much by adding the 10-20Ω resistor except adding something else throwing heat in the chassis.
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OK, thanks. Just a thought.
Brad :icon_biggrin:
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Regarding these mods on installing sheilded wire. I've had some experiance installing shielded but not a whole lot! So I understand that you only ground one end. So far when I have installed sheilded wire on an input its has always been in a case where the wire went straight from input jack to the preamp tube attached directly to the grid stopper resistor that was connected directly to the tube socket pin #2.
In this case we go from jack to board turret--68K grid stopper resistor--board turret to tube pin #2. It almost seems like your defeating your purpose with the exposed resistor on the board right in the middle of your run to the 1st stage preamp? Is that Ok, or should I bypass the board and go staight to the tube with my sheilded and 68 K R to tube socket?
So the rest of the sheilded run from #1 Plate to board turret-coupeling cap-board turret to volume control/tone stack. Same situation here only exposed turrets and cap on the board?
Also should the sheilding continue from volume/tone stack to second stage preamp?
Just trying to get my ducks in a row before I do this. Thanks, Platefire
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Regarding these mods on installing sheilded wire. I've had some experiance installing shielded but not a whole lot! So I understand that you only ground one end.
ANSWER ; YES
.. or should I bypass the board and go staight to the tube with my sheilded and 68 K R to tube socket?
ANSWER ; DO LIKE THAT
So the rest of the sheilded run from #1 Plate to board turret-coupeling cap-board turret to volume control/tone stack. Same situation here only exposed turrets and cap on the board?
Also should the sheilding continue from volume/tone stack to second stage preamp? ANSWER ; YES
Just trying to get my ducks in a row before I do this. Thanks, Platefire
Keep the "center " wire of your shield wire as short as possible 1/4 inches.
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Getting Preped to work on this Saturday. I was wondering on installing sheilded cable on the input to the first preamp stage, I'm thinking of doing away with the second input jack and that way it would be direct with no other connections from the jack to 68K R(maybe 33K better?) connected directly to V1. Run the sheilded right accross the top of the board by the existing abandoned in place 68K Resistors to its destination. Please take a look at the previous pixs and see if you agree. Thanks, Platefire
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On your photos I see that you don't use shorting jacks with 1 meg resistor to ground , you must use it to kill noise.
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Save time, just add the extra C-R filter.
I'll bet $100 the hum goes down. Others on this board have made the same exact change and reduced hum.
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Well I guess your going to have to spell it out for me--not reading C-R Filter? Capacitor-Resistor?
I am planning on adding an additional Filter Caps and resistor more or less like PRR proposed.
So your saying the champ humm is coming from the old under-filtered situation accociated to the tweed champ?
The plan is still in accordance with this pitiful sketch:
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Platefire, I think that yes, he means the extra cap and resistor that PRR inserted in that schem from his post. That extra cap should reduce your hum quite a bit.
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Don't forget to move the red wire from the standby switch. Move from the board to the junction of the 250 ohm resistor and the new cap. Leave the OT connected to the board. You probably knew this. Your pic was not obvious to me.
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Yeah, Thanks sluckey! One of the hardest parts of doing this will be un-doing my inexperiance in those days with multiple wraps of component leads around turrets, globs of solder, poor lead dress and about everything else you can think of a green horn does. I've got my solder sucker, solder wix, iron ready and will need a big dose of patience. Platefire
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Just to jump in....i used a four section cap can when I rebuilt my kay 703. I used a similar cap and resistor setup as prr mentioned and connected the ot to the second node.....amp is really hum free. I always add extra filtering to se amps fwiw
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So your saying the champ humm is coming from the old under-filtered situation accociated to the tweed champ?
Well, yeah. That, and this was a student-level amp to play with your new Fender Champ lap steel...
First, it's a single-ended amp; there's no cancellation of power supply hum in the output transformer, as there is with push-pull. And the more expensive push-pull amps were targeted at gigging musicians and professionals, and had such expensive frills as a choke to reduce the ripple at the screen node (a BIG ripple here can introduce hum into the output section).
But hum was also mitigated in the original Champ with a 6" or 8" speaker and a tiny cabinet, which didn't really have good bass response down to 120Hz. So overall, it didn't seem to bad, especially in comparison with other similar entry-grade amps.
But we now want to crank up Champs and give them nice 10" or 12" speakers (some people even play a Champ through a 4x12" with surprising volume). The better speakers and larger cabinets start to reveal the extra hum, especially if you're accustomed to higher-quality designs with less hum.
So while you're paying more for all the other upgraded bits, you really have to add another capacitor and resistor ahead of the plate node to knock the hum down to acceptable levels.
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So while you're paying more for all the other upgraded bits, you really have to add another capacitor and resistor ahead of the plate node to knock the hum down to acceptable levels.
This is THE standard "power supply design 101 for SE amps" as we know it. Fender (& other makers of the day) saved money by their design method, the slight hum/noise trade-off wasn't nearly as big a deal as were the $$ savings. Most know how frugal Leo was. Filter caps weren't cheap back in Fender's day - and still aren't today! Modern SE amps' power section should not be designed how Leo did it and should be as PPR's schem shows. BUT, that's not all as far as Leo's methods that were less than ideal. His heater wiring, component layouts, lead dress, grounding, etc. all had flaws but he worked though these challenges to accomplish his goals and yet he still made great amps that performed well. It also proves that there's not just one way to build a nice amp.
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Yeah, I'm beginning to think that abandoning the two 68K's input grid stoppers on the board and installing a sheilded cable from the main input jack to the maybe a 33k soldered direct to the #2 grid pin may be a waist of time(Like HBP said). I probably should just install the new caps and 250/5 watt resistor first and see if that does what is needed. Platefire
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Plate - I rarely ever plug into the 68k input if an amp has one and I always design/build everything running the shielded cable to 33k resistor soldered directly to socket - standard way of doing things. Sometimes this is lowered to 22k or 15k depending on amp/tube in V1.
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I just changed my SE Supro Super Six to include PRR's C and R mod. It worked great and the amp can now even handle noisy fender single coil pups that like to buzz and hum. Here is the Supro (Valco) Super Six schem. if anyone wants to try this one? The Triodestore.com has a reproduction OT for this exact amp made by Mag Components. I like it and it doesn't break the bank.
Have a blessed Sunday. Jack D
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We also did the mod this weekend, busy now will talk and post pictures later. Plate
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We did the mod this last weekend. My son Greg brought the amp down Friday night and we demo-ed old filter caps and installed the new caps/resistor that night. He was looking over my shoulder and lending a hand when needed. The funny thing that happened was after we got the filter caps and 250 Ohm resistor installed we decided to do a test. We hooked it up to the speaker cab with nothing plugged in and flipped it off of standby---nothing! My heart kind of sank becasue I was looking for smoke or what I may have hooked up wrong. Greg got a cord, plugged it in and touched the opposite end with his thum--and buzzz--that was a relief! So he grabbed a guitar and plugged it in and it was working fine--only without noise. Even full volume with only a hint of noise. Needless to say Gregg was very happy.
We installed the Sozo caps next morning tested again got a short scare when we couldn't get it to work but a speaker lead had fell off. Everthing was find after attaching the speaker lead. Couldn't tell a lot of difference between the orange drops and the Sozo. I was then ready to install the sheilded input wire but he chose not to do that. Can always install it latter if needed. Thanks very much PRR for supplying the plans. Platefire
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So I'm guessing you're a believer in the extra filter cap ahead of the plate now? :icon_biggrin:
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Well I haven't done it to my DIY Champ yet. I played on Greg's quit a bit before he took it home. He liked the SS rectifier in it. He was going for getting the biggest loudest sound out of his champ he could and was shooting for little cleaner headroom. We tried several tubes--JJ 6V6, EH 6L6 and Tung Sol 5881. On preamps a EH 12AX7 and GE 5751. He left the GE 5751, Tung Sol 5881 and Weber SS rectifier in there.
I hate to make judgements on a bedroom test. To me a band situation forces you into another situation where its got to compete with a drummer, bass amoung other things which and I didn't do. In my test if felt a little stiff at about 1/3 volume and loosened up a little better at 1/2 vol. I like a nice response at pretty low volume. The champ hum is not that big of issue with me but I do enjoy a quiet amp. It's something to consider! I'll be interested in hearing Greg's opinion when he works it with his band. Platefire
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Sorry for the intrusion
a guy that I know has the same noise problem with his 5F1 clone
I would like to use the PRR mod adding the extra capacitor and resistor
What about the max value of the first capacitor in reference with the used rectifier tube ?
Which is the tube used before of the SS rectify ?
as from datasheet 5Y3 max first cap value is in order of 20uF, a change of rectifier tube is a must as to use the PRR mod ?
Thanks
K
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I'd stay with the 5Y3 if using a 6V6. Unless you just want to run a higher B+. If you do want higher B+ use a 5AR4.
Adding another 20uF and a 250Ω will still give much hum reduction. BTW, I've used a 33uF with 5Y3 in many of my builds with no problems.
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Hi Steve
Many thanks for sharing your experience
I'll go with a 33uF :thumbsup:
K
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I got my DIY Champ out last night I built in 2001 and gave it a good listen. Its noise floor is just not bad enough to me to warrent the cap change. Very slight hum but not bad and it don't get louder as I turn the amp up. So for now I will leave it as is.
One thing I did notice is the JJ 6V6S I had in there was extremly bright. I don't know if it just me but it seems every JJ power tube I've tried lately seems way to bright?? Why am I just now noticing that? I tried a old GE 6V6 that sounded great but it started red plating. Also an EH 6L6 that was pretty good. The best that I left in there was a vintage RCA 6L6GT---real nice. I really would like to find a new 6V6
that isn't way bright like the jj. Any suggestions? Platefire
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> it don't get louder as I turn the amp up
Then the hum in the output stage is greater than the maximum hum in the preamp. You could do better. Of course if it is acceptable, then don't.
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PRR---Your statement "You could do better" reminds me of a term we used in Government Contracts for inspection using the "Performance Requirements Summary"(what the contractor could get away with before corrective measures were taken) which specifies a "Acceptable Level of Deviation". I guess for my purposes the champ noise floor is my "Acceptable Level of Deviation". :icon_biggrin:
One of my all time favorite Government terms is "Overcome by Events" :l2:
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I'm wondering if you are seeing any flash over in the rectifier tube when you first turn on the amp and the PS caps are discharged. You see if as a brighter flash when you turn on the amp. If yes, it can harm the tube. This can happen when larger capacitors are in place than what the tube rectifier can cope with. This can usually be fixed by adding two diodes (1N4007 is fine) in line with the plates of the rectifier tube. You can see an example of how they did this in the reissue '57 Fender Deluxe (http://support.fender.com/schematics/guitar_amplifiers/57_Deluxe_Fender_57_schematic.pdf (http://support.fender.com/schematics/guitar_amplifiers/57_Deluxe_Fender_57_schematic.pdf)). The diodes serve to protect the rectifier tube. Not a bad idea to install them.
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Well we are discussing two DIY champs here practically identical originally. My son's champ that we moded was using a Weber SS rectifier in his. So if there any feedback of electrical current to the rectifier there wouldn't be no obvious flash and of course in my champ is all original cap set up like tweed era amp complete will classic champ hum;>/ which is "My Acceptable Level of Deviation" Thank you Leo. :icon_biggrin:
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Talked to my Son Greg today. He said he tried his modified Champ out with his band. Looks like it didn't quiet cut the mustard in getting enough clean headroom to compete with his drummer. He said the amp sounds great but the clean stuff he plays is a little too distorted when he turns it up loud enough to get over the drummer. This is un-miked and would probably work that way with the support of a good PA man--which he don't always have. Platefire
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Well my Son(Greg) wants to proceed with PRR plan. I will also be installing sheilded wiring in the input/preamp section, addressing lead dress issues and installing a couple of sozo .022 coupeling caps.
On another note----This amp presently has a Weber 10F150T speaker that has a ribbed cone for extra treble. This is a little to bright for him. He's looking for a 10" that will compliment the champ tone with a little more thickness and a little less harsh highs. Weber Silver Bell had been a consideration but not tried yet. Any suggestions on what might fill the bill? Platefire
The Weber Silver Bell by itself is very midrange limited. I have a champ with a celestion blue and it sounds very close to the 12 and it is not harsh at all. The silver bell 10 is sort of flabby.
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Looks like it didn't quiet cut the mustard in getting enough clean headroom to compete with his drummer. He said the amp sounds great but the clean stuff he plays is a little too distorted when he turns it up loud enough to get over the drummer.
It's a Champ, not built to have clean headroom over a drummer. It was a student model amp.
Drives me crazy, big, loud amp needs to be quiter and small quite amp needs to be louder.
Need a bigger pay load then he needs a bigger truck.
Brad :laugh:
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That's why I hang on the my Pignose G40V that I moded from a 10" speaker to a 12". It's smaller than my DIY Champ and has 40 watts of good fender(5f6A/MV) sounding get up and go. If I need a little power in a small package, that's what I grab.
On Greg's amp, I told him we have stretched the champ circuit about as far as you can go without changing it to something else. Any more milage would have to be accomplished maybe by more efficent speaker as Ed suggested. Platefire
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I'm wondering if you are seeing any flash over in the rectifier tube when you first turn on the amp and the PS caps are discharged. You see if as a brighter flash when you turn on the amp. If yes, it can harm the tube. This can happen when larger capacitors are in place than what the tube rectifier can cope with. This can usually be fixed by adding two diodes (1N4007 is fine) in line with the plates of the rectifier tube. You can see an example of how they did this in the reissue '57 Fender Deluxe (http://support.fender.com/schematics/guitar_amplifiers/57_Deluxe_Fender_57_schematic.pdf (http://support.fender.com/schematics/guitar_amplifiers/57_Deluxe_Fender_57_schematic.pdf)). The diodes serve to protect the rectifier tube. Not a bad idea to install them.
I may be wrong, but aren't the diodes connected differently on the first page schematic (which seems correct to me), than on the other pages (SERVICE MANUAL DOCUMENT 5/31/07), which seems backwards?
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I may be wrong, but aren't the diodes connected differently on the first page schematic (which seems correct to me), than on the other pages (SERVICE MANUAL DOCUMENT 5/31/07), which seems backwards?
They're connected the same on all pages.
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now I see it too :icon_biggrin: :BangHead:
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Hi Ya'll! I'm tagging on to this because of the valuable info and conversation contained.
My Champ quit! On inspection and a little voltage testing discovered my high voltage DC was stopping at the OT/6V6 plates. No voltage at screens and beyond, the 2 watt/10K was gone. Also after more checking found out the 8uF/450V cap for the screen had failed and caused a direct path to ground. I suppose the cap failed and took the resistor out.
So since I got to do a repair I decided I may as well upgrade a bit to maybe defeat some of the normal Champ hummm. So referring to the original plan I did to my Son's amp back in 2014, would like to do something similar. I will be using the 5Y3 rectifier---so where the upgrade plan used a 40uF before the 250 Ohm R and another 40uF after the 250R to plates. My thought was to use a 30uF in those two locations and maybe my 5Y3 could still survive? So the plan would be the Identical as attached sketch with the exception, the two 40uF's on sketch will be 30uF's/500V.
Thought I would run it by ya'll to see if there are any holes in it? Thanks, Platefire
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Since nobody responded to my previous post question, am I to assume that it's correct, workable and and i should proceed with vigor? I feel pretty good about the plan, just concerned about the 5Y3's health. I think I remember sluckey said he had gone as high as 33uF with a 5Y3 without any problems.
Come on sluckey, PRR, tubenit, Willabe----Please bless it or kick it in the head--I need to order parts!
:dontknow: Platefire
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I think I always use 33 uF right after the 5Y3, and have never suffered a failure.
Go for it.
Jon
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Bless you. My only concern would be the 250Ω/5W. May be too light. Measure the voltage drop ***ACROSS*** that 250Ω and do some math. Actual power will equal V2÷250. For example, you measure 25V across the resistor, so...
252 ÷ 250 = 2.5W actual power dissipated by that 250Ω resistor. Double that value for safety so use a 5W resistor.
So, if you actually have less than 25v across the resistor that's even better, but if you have more than 25v across the resistor better to use a 10 watt resistor.
If you don't want to do the math then just get a 250Ω/10W resistor to start with. :icon_biggrin:
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Doing the math isn't the problem, I don't have a 250 resistor to hook up and measure. I haven't ordered any of the parts yet. Guess I could order a 5 watt & 10 watt and which ever I need, use.
I know the voltage on plates is about 378VDC. Tried calculating voltage drop across the resistor but the answer doesn't make sense:
Current I=v/r=378/250=1.512 Amperes
Voltage across Resistor V=I/R=(1.512)(250)=378V
Somethings hay wire? That formula shows no drop in voltage. Must of done something wrong. Platefire
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You're not dropping 378 across that resistor, or at least you shouldn't be. Sluckey said to measure *across* the resistor. The way you have it figured your amp is pulling 1.5 amps. No way! I haven't read the whole thread but it's a Champ, so should be well under 100 mA, or .1 amp (I think... 1 power tube, 1 preamp tube ) So anyway....
250r * .1 = 25 volts, so you should be dropping about 25 volts *across* that resistor at the most.
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Guess I could order a 5 watt & 10 watt and which ever I need, use.
Since you must buy the resistor just get a 10 watter and be done with it. And when you mount the resistor give it plenty of air space around it. It will get hot. It's supposed to get hot. Keep at least 1/2" clearance around the resistor. Don't put your extra filter cap up close to that resistor.
When you get this mod done if you feel that your B+ is too low just put a 5AR4 tube in the rectifier socket.
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Thanks sluckey, to be honest I'm still having a debate in my mind rather
To go with the upgraded or go back with original set up. The hum never bothered
Me a lot and I sure love the tone and response. So if I go back with the same,
I know I won't loose any of that, but with the upgrade---I'm not sure.
Other than a quieter amp, what are the pluses for the upgrade? Platefire
BTW---the reason I'm deliberating so hard on this is I've been using the champ a lot
Lately and it's sounding so good, don't want to loose anything, if anything I want to gain some!
From my research the trade off might be loose a little responsiveness for a tighter bottom end?
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If you like the way it sounds leave it alone.
If you want to know how the sound is affected with the extra filter stage then experiment with it. Leave the original caps just as they are and add a 33µF cap and 250Ω/10W resistor between the rectifier and the board. If you like it both ways consider using a DPDT switch so you can switch the extra filter stage in and out. One way is original, flip the switch for the additional filtering. That's the only way you'll ever know how what you hear changes.
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If you like it both ways consider using a DPDT switch so you can switch the extra filter stage in and out.
VOX do something like that on the AC30CC2 and call it Smoothing Switch VINTAGE / MODERN
(http://i.imgur.com/4GfAZ0r.jpg)
Note the way they do it, using a resistor to keep cap charged
Franco
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> VOX do something like that
Yes; but that is a fix-bias push-pull BIG amp and we are talking about a Champ (self-bias SE). The big amp sucks more when pushed. The Champ doesn't. "Sag" won't happen.
It isn't 370V across 250r. The Champ's bias makes it act somewhat like a resistor. If it is happy bias/load, it acts about like the audio load. Say 7K. So for a first easy-number crack, it pulls 350V/7K= 50mA. (Probably less.) 50mA or 0.050A in 250r is 0.625 Watts. Need >=2W part.
> just get a 10 watter and be done with it
Don't disagree. I just put a full 6.0"x6.2" post where I only needed a 3.5x3.5. However the 6V6 is a 12W-14W tube. We only want a little drop, not to drop as much as the 6V6. It is likely we don't need anything like 12-14W, even 10W. With math we find <1W, and do need a 2W part, so it is indeed a Power resistor and the 10W may be the same price? (I used the 6-by because it was already in the house and far cheaper than driving to the Depot for a lesser stick.)
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Ciao PRR
> VOX do something like that
Yes; but that is a fix-bias push-pull BIG amp and we are talking about a Champ (self-bias SE). The big amp sucks more when pushed. The Champ doesn't. "Sag" won't happen.
I agree with you, an SE amp didn't Sag, only one thing, the AC30 is a Cathode Bias amp, not a Fixed Bias
and ... thinking to the option suggested by Steve, I remembered that VOX used on that amp a capacitor switch with a resistor between
the added capacitor and ground as to keep it in charge (no pops ?) with a switch that shorts the resistor to insert the function of the cap on the
circuit, my intention was to point the attention to the use of the resistor (if to add the switch + cap was the choice)
Franco
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Even though I'll probably not attempt the switching option between vintage and modern, I have been playing
With the switching arrangement how a DPDT might be hooked up to accomplish this. It's a good brain teaser! Platefire
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........ how a DPDT might be hooked up to accomplish this
(http://i.imgur.com/FtW9ro5.jpg)
Something like this ?
Franco
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Thanks Franco! Looks interesting. My problem in understanding what
Your doing with the connections to the DPDT? DPDT has 6 terminals.
I only see 4 terminals on yours and they are not in the normal
Locations----so I can't really see the power flow in switched positions.
Platefire
Edit: ok I studied it a Little more and see. In one switch position the
Path around the 250 is provided and the path to ground on the 40uF is
Blocked by the 10k R---vintage. The other switch position the path around
The 250 is cut off and voltage must then pass through the 250 plus the
Ground connection is then connected to the neg lead of the 40uf.
That's pretty cool!
But! If I'm not mistaken, I think I found a routing of voltage through
The DPDT without having to mess with the ground. Platefire
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Connections are six but only four are user
Ciao
Franco
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OK! The minuet sluckey suggested using a DPDT, I started working on a plan. Certainly not because I was sure I want to try to install it but, i though that was a pretty good puzzle to work out.
So this is just a rough sketch but it's doing all the voltages transfers through the switch. I was thinking, boy! if you did it this way, you would need a good heavy duty switch because if it shorted out you would have major smoke.
OK, so here it is, tell me if it's workable? Platefire
BTW---The hardest part of actually performing this from my end is trying to find a spot
to install the DPDT without stringing out the leads.
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I think your scheme will work but is possible you have a pop when you switch, also there will be an inrush current when the 40uF cap is connected to B+
Not that I'm sure about my version, but with it I'm trying to avoid those problems
Here my version with the DPDT draw in one other way
(http://i.imgur.com/Mr2pOpL.jpg)
Franco
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K, I don't like having B+ and ground on the same switch. Just makes me nervous. I prefer something like the attached drawing.
Plate, here's an idea. Just get a choke and convert to 5E1 schematic. Forget about any switch.
EDIT... Fixed pic IAW shooter
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I don't like having B+ and ground on the same switch
+1
Sluckey, I did a small edit on the snipit, I think that's what you were going for
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Thanks! I never even notices that. :icon_biggrin:
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(http://el34world.com/Forum/index.php?action=dlattach;topic=14464.0;attach=66541;image)
I like that :thumbsup:
Franco
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Guys! I really appreciate your combining your tube amp skills together to come up with this plan, Great! I don't think such a thing as this has been done to a champ?? So I guess it's decision time for me--either put up or shut up:>)
I have looked into the physical aspect of this and I'm really thinking it is possible. If you will refer to my picture and my notes, I'll run my thoughts by on what I think can be done.
1-See the small board next to the main board? That was originally a Pentode/Triode switching setup that I never liked the operation so I used the space for switchable cathode bypass cap for V1b. The resistor on the left is not connected to anything can be removed and the 22uf/25V V1b switchable cathode bypass cap can be relocated and mounted on the switch. OK that clears the small board for this mod. So I think the 100K/1 watt could go on the left turrets and the 40uF on the right turrets of the little board.
2-The 250/10 watt. I think I could mount a terminal strip in between the main board and the small board for that.
3-The choke. I think the choke can be mounted on the front bottom of chassis between PT and OT with the wire access hole for the leads right below in line with the bottom of the small board.
4-The DPDT. There is 1 3/4 Inches between the 5Y3 and the 6V6. I think the switch could go centered between those two tube sockets.
So I think that's it, other than doing the hookup/lead dress as well as possible. I think the space can be eaked out to do it. Platefire
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I never even notices that. :icon_biggrin:
I only noticed cuz the sun got eclipsed :icon_biggrin:
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Look closely at my drawing. Both resistors will mount directly to the switch. Look at how I mounted a 10W resistor on my standby plus mod on page 1 of this pdf...
http://sluckeyamps.com/VAC15/Vox_AC15_mods.pdf (http://sluckeyamps.com/VAC15/Vox_AC15_mods.pdf)
If you want the bestus of all, replace the 250Ω/10W with a choke.
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_I_ don't understand why you would want to switch-out this mod. You don't want hum/buzz! (the axe gets enuff already.) The few-percent drop of raw power output is not going to spoil any show where you brought a Champ.
_I_ would clip-lead it in (carefully!!) and try both ways before committing to a switch and a hole.
Anyway, what is wrong with the attached SPDT solution? The 40uFd is not totally out of circuit but with 250r in line it isn't doing much. It does charge to full voltage for less POP when you flip switch with power on.
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which way is standby? :icon_biggrin:
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:icon_biggrin: I'm letting all this soak in a bit
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Well I made my decision. I decided not to go with the vintage/modern switchable arrangement.
I decided to up the vintage arrangement a bit to 22uF OT/Plates, 10uF screen and vintage original 8uF for preamp.
Hope you guys aren't to put out with me after all the hard work you did on the other plans. I appreciate your help! Just getting my Champ working again will be a thrill because I had been using it almost every day and kind off missing it. Thanks, Platefire
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You might look in to putting a 6L6 in his amp. It has been discussed on this forum in the past. As I recall the conclusion was that a 6L6 can be swapped for the 6V6: it will provide more clean head room and wider tonal response; It will not increase the output by much if any and it will not have the same tone as the 6V6 (if that is good or bad will be in the ears of the beholder).
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Yes, I have ran a 6L6 in it. When I built the amp it was designed with a PT with enough heater amps to handle a 6L6, so the first few years that's all I ran in it. Later on I tried a JJ 6V6 and I liked the tone and response a lot better. I also got a plug in SS rectifier I tried and ran with it. To me the thing just sounds awesome with the 5Y3, 6V6 and 12AX7. The original Champ hum never bothered me that much but for my Son it was intolerable, so we upgraded his as on the beginning of this thread.
I guess I'm kind of funny about my DIY Champ being my first amp build 17 years ago and I have been very happy with its performance, so it's not a whole lot of incentive to do much different. If it was just another project amp that I was throwing stuff at just to see what happens, it would be different.
I do think this thread may be very beneficial to someone down the line who is looking to do something like this to their original champ circuit, who do a search, then find these power supply circuits and try them. I hope when they so, they share their results. Platefire
BTW--These first Champ projects was brought about by an amp man known on the net as "Casey4s".
Some of you may of heard of him. He encouraged me to build it, that I hesitantly decided to take it on, he then hand built the kit, provided detailed instructions, provided e-mail communications through the whole process that included not only building the amp but the cab to. So he also provided the same thing for my Son's Champ. So these amps are good as they are because I got started on the right foot by Casey4s.
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I agree:
there is a lot of outstanding information on this thread. It would make a nice article for publication if the information were organized;
the sound of a 5Y3, 12AX7 (in fact I prefer a 12AY7) and 6V6 in the 5F1 format is about the best there is.
I have built five of these amps. I have found the ones using turret boards typically have more hum than the ones using circuit boards. I think the wires between the turrets act like little antennas picking up stray stuff that finds its way into the signal.
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I've been running my Champ in parallel with my converted
Bogen CHB 20A in 1-10 cab. It's not stereo but just a big &
wide sound. I set the Champ fat/thick and dial up the highs
On the Bogen. So I set the Champ volume like I want and
Then bring the Bogen up to balance with it. That is the most
Satifying sound to my ears. I'm anxious to get the champ going
Again. Just the right power level for my home playing. Platefire
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Need some help, re-thinking my amp failure. I thought I completely understood what happened based on this:
1-First I determined the first 10K power resistor between plate/OT and screen failed because is was open.
2-I attempted to clip in a new resistor around the open resistor to see if the amp would start working. When I did this the clipped in resistor started smoking, so I quickly shut it off.
3-I got to checking with my MM I finally found that the filter cap on the screen was completely closed to ground because I could get continuity with my MM from the positive pole to the negative/ground.
4-So this was a "Ah Ha" moment. The cap failed and took out the original resistor and was smoking my new test resistor. That was my analysis!
So tonight with parts ordered, I decided I would take the old parts out in prep for installing the new parts. So when I pulled out the failed 8uF filter cap, I checked it with my MM and it was not closed or no continuity from positive to negative like before??? The amp had 378VDC on the 6V6 plates in it's failed condition, all heaters were working but no voltage downstream to 6V6 screen or to 12AX7.
So I wondering, could the cap test failed one time and seem OK the next? Is there something else that could cause the resistor failure? Don't think there was a short anywhere else. This is kind of blowing my mind, because I thought I had it figured out. Platefire
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You fell for the old obvious culprit trick. :icon_biggrin:
With cap still out... Check resistance from 6V6 pin 4 to ground. Does it read low resistance? If so, pull the 6V6. Does the socket pin 4 still show low resistance?
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No. Shows .4 Ohms with tube in and 0 Ohms with it out. Platefire
Just testing the tube pulled out of the socket, I have continuity between Cathode pin #8 and Grid #2/screen pin #4.
Soooo--That's how I got continuity on both ends of the 8uf filter cap, it was shorting internally in the tube through Pin #4 tube screen to cathode pin #8 to cathode resistor/bypass cap to ground.
Just de-soldering and lifting one leg of the 25/25 Power tube Cathode bypass cap---the 470 Ohm cathode resistor then measured 466 Ohms(OK) but the cap had continuity from positive to negative leads. So apparently the 25/25 cap gave up the ghost and provided a path to ground.
Boy! this one sure made a Monkey out of me :dontknow: not the first time and probably won't be the last!
Thanks for your help sluckey.
In actuality, if I had my original analysis was correct(instead of incorrect), I could have replaced the faulty 25/25 cathode bypass cap, the 22K 2watt resistor and the 6V6, and it would been good to go. Guess it's not a bad thing I'm installing new filter cap and power resistors, it has been 17 years. Looks like I ended up going around Robin Hood's barn on this one. Platefire
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Good news, my parts came in this morning and are installed. I'm running
my EH 6L6 that I was running before the now dead JJ 6V6S. Plate voltage is
376, screen 334, to pre-amps 292, 12ax7 plates 198 each. Sound good
and a little quieter!
Thanks sluckey for pointing the right direction on my trouble shooting.
Glad to have my Champ back up! Platefire
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> Shows .4 Ohms with tube in and 0 Ohms with it out.
Zero? Or Infinity? They are different.
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nice! glad to read you got it sorted out.
--pete
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PRR----I was just trying to provide the answers/readings he ask for. In the
Process I just discovered my tube had a short. Yeah, my terminology
Is probably way off.
Thanks, Pete! I been jamming with the EH 6L6 in it but I'm missing my
JJ 6V6S---hafta get one ordered. Platefire
BTW-Here is a updated schematic of the Champ--may have mistakes!
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I was complaining about missing the old JJ6V6S that shorted out but----------Man! This amp has got a great clean sound like never before since making the changes and using my old EH 6L6. Platefire
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A working amp usually sounds better than a non-working amp. :icon_biggrin:
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Har! Har! :laugh: you let me off lite! Thought for sure you was going to bring up something about running a 6L6 on a 470 cathode resistor, but since you didn't, we'll just let that slide------ :happy1:
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Just thinking out loud, maybe no action here :dontknow: but----since my 6v6 failed with an internal short from screen to Cathode, I keep thinking of adding a screen resistor. I've been reading up on it some.
I've read the higher you go, the more it sucks tone. Some say install it 5watts or more so it won't fail. Others say make it 1watt because you want it to fail and act like a fuse.
I've also read that fender originally installed them because screen voltage was running higher than plate voltage and installed them to prevent Parasitic oscillation.
So my main interest is protecting my next 6V6 and maintaining tone. I'm hesitant to change anything but need to put the screen thing to rest--any Wisdom here? Platefire
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Since my tube failure and damaged parts, I'm a little concerned with bias of the EH 6L6 I'm now running. It does feel very hot to the touch. So I calculated tube dissipation on this one to see where I am at. Let me show you what I did, and then I have a question?
I've got an old EH 6L6 from Doug in 2001 but the data sheet I found on it says maximum dissipation is 30 watts. That data sheet is attached.
Voltage Readings:
Plate voltage=382
Plate to Cathode=325.4
Across Cathode=34.64
Cathode 470 Resistor=466
V/R=34.64/466=.0743A=74.3mA Plate Current
(325.4)(.0743)=24.18 Watts Dissipation (80.6 %)
Normally it is recomended 70% Dissipation. It's my understanding the Class A can be biased a little hotter that Class B or AB.
So my question is(if my math is correct) am I safe in leaving it a 80.6%??? Platefire
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Plate voltage=382
Plate to Cathode=325.4
Across Cathode=34.64
These voltages don't add up. At least one of these readings is wrong. If your plate voltage is 382v and your cathode voltage is 34.64v, then plate to cathode must be 382 - 34.64 = 347.36. Yet you say it is 325.4. So, somethin ain't right.
Normally it is recomended 70% Dissipation.
That 70% is recommended for fixed bias amps. It's OK and quite common to run cathode biased amps at 100%.
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I got my data incorrect. It is 382 at the A node to OT but 360 on the
Plate. So 34.64 + 325.36=360. So that should straighten that out. So
I'm good to go at 80%------good! Platefire
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good to go at 80%
fwiw, I've found on many SE builds 80% is where the PA comes alive, with big bottles, something north of 90%
it's pretty easy to do some math and || R's to the cathode R for "fine tuning"
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So using the old water pipe analogy, wouldn't you reduce
the resistance to open up the flow? I have read that 6L6's
Should be around 250 Ohms. Platefire
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I have read that 6L6's
Should be around 250 Ohms.
Yes. If you are talking about a pair of push/pull 6L6s. But for a single 6L6 you need twice that amount, 500Ω. 470Ω is close enough.
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OK Gotya, Thanks!