Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Platefire on June 12, 2023, 09:54:09 pm
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I thought I had gone over all of my amps and had them working right but I just discovered it again hiding. I fired it up and it does have some noise problems. Does anybody remember GroundhogKen, Brad S Bryant and EL34(Doug) helped me with this in 2001?? They all pitched in and helped me get r dun. I really did a lot of playing over this amp but it's been sitting up for several years now. What little I've tried it, it has a humm problem. I changed most of the coupling caps way back then but didn't mess with the electrolytics.
So I tore it down today and checking the resistors and caps. Going to change all the filter caps. Making a list and checking it twice;>)I pretty much left the power amp as is and modded the preamp for guitar. This was a thrift shop find I got for $20. Its got 6SC7, 12AX7 pres, 12AX7 PI, PP 6L6's and a 5Y3 Rec.
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Why not the complete schematic ?
Clean tubes's sockets contact and tubes contacts.
Clean pots
In both cases, good contact cleaner is a must ; I only use Deoxit
In an infrequently used tube amp, these contacts oxidize and can cause problems.
The tone stack as you show
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Picture inside the amp may help.
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This was way before ExpressSCH was available or wasn't aware of it. I spent $32 and got the full set of documentations from SAMS Technical Publications on the amp. I may draw a proper schematic while working on it. Since it still has the 1959 filter caps in it, I'll change those first and see what that gets me. The amp was sounding pretty good as is but would start growling out of no where, stop and clear up. I've found some resistors that are pretty far off also. Yelp I use Deoxit.
I remember being really excited when I got this amp because it was in really good shape. So I had a really good platform and tube compliment to work with. There were several different Integrated hi-fi amps brands in this style that came out in the late 50's.This GE one is really built like a tank:>)
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Radio Museum has clean copies of that schematic. I have one of those amps waiting for a soldering iron.
https://www.radiomuseum.org/r/general_el_pa_20.html (https://www.radiomuseum.org/r/general_el_pa_20.html)
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I'm Thinking the best way to replace the filter caps is remove the old Filter Can, plug the hole with a cover plate and install axle filter caps on terminal strips. Here is a gut shot of the situation at the Can Caps. The old can is a Mallory 40/450, 40/450, 5/350, 100/50 and 1 3/8 Dia x 3" high. Not sure if any direct replacements are available but pretty sure it would be cheaper to do F & T Axle caps on terminal strips.
Also it has a umbilical cord from the power amp to preamp section where there is a 30-30/350V axle cap. I found an F & T 33-33/450V axle cap for that. Two + leads on one end and ground lead on the other. It's also 1" Dia same as the old paper cap, so the new F & T will fit the existing holder bracket
I know this is a dangerous question but, After having a look, whatayathink?
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I suggest using a F&T 50/50 can to replace the two 40s. The can will fit the hole from the removed original can. Need the clamp too. Then use an 8uf@450V and 100uf@100V axials for the other two caps that were in the can. Plenty of room to install these small caps.
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I like that idea!!! Also Thanks for the schematic. I had marked up my old original so much that it's good to have a clean slate to look at.
I got a couple of questions I'm thinking on:
Discovered the NFB resistor is 3K and a metal film resistor. The schematic shows 27K. I don't remember changing the resistor but nobody else could have done it because the amp was untouched when I got it. Well it sounded pretty grand with the 3K but I realize that's a lot of NFB going back to the circuit. So I'm thinking you would say experiment with the value or put a 50K pot it or put the 27K in there with a on/off switch on it?
I installed a standby switch on it originally but I had to extend the wires all the way out of the chassis to a switch on a bracket because of the chassis odd configuration. I did get bit once when my finger got on the wrong side of the switch. Is there a safe way I can make this work or should I just do away with it for safety reasons? I does have a 5Y3, so the B+ would come up slowly
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Put a 50K pot in series with your 3K resistor.
If you must have a standby switch put it inside a small enclosure. I suggest this Bud Box (https://www.amazon.com/BUD-Industries-CU-3003-Aluminum-Electronics/dp/B005T7T0EG/) from Amazon, but to save $$$, you could even use a small PVC electrical box (https://www.lowes.com/pd/CANTEX-1-Gang-Plastic-Old-Work-Electrical-Box/5001717179) from Lowes.
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What about coating the inside connections of switch with Liquid Electrical Tape? As per Link:
https://www.amazon.com/Star-brite-Liquid-Electrical-Tape/dp/B0000AXNOD/ref=sr_1_3?keywords=Liquid+Electrical+Tape&qid=1686685364&sr=8-3
On the NFB, the problem with that is finding a place to put a pot. I'll have to study that one.
Maybe use a pot in series to find the best operation setting and then replace it with a fixed resistor.
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If you must have a standby switch
That Sluckey comment is telling. Unless you intend to be gigging the amp and taking multiple liquid or dry refreshment breaks, I would see no need for the standby switch.
BTW - this is great. I often find that if I let some thrift shop treasure sit on a shelf long enough, someone else here will work on one and I get to lurk and learn.
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I can't really truly justify a standby switch other than I like um and it's important to me :dontknow:
The hardest part of these type integrated hi-fi amps like this one is the configuration of the chassis makes it harder to access everything. Once you get by that, it makes a nice head for what ever cabs you have. I use to have a old Peavey Musician 2-12 cab that I run this head through and it just sounded great and was a great match. My youngest Daughter run off with that to go with the Bogen CHB-50 Bassman conversion I made for her. BTW, she was in the Army at the time, so I did it like this:
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Where does a fellow get a 7.5K 5watt power resistor these days. Guess that's an odd value? The existing one is measuring 6.3K
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Where does a fellow get a 7.5K 5watt power resistor these days. Guess that's an odd value? The existing one is measuring 6.3K
https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=7500+ohm+5+watt+resistor&_sacat=0
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I love that US Army Bogen. I bet it sounds great. I hope she still plays it. I have found Bogen, Stromberg Carlson, and Grommes to be really solid amps. As you say, the PA amps are easier projects than the integrated hi-fi amps.
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I have had the Bogen CHB-10, CHB-20(still have) and CHB -50. She was coming from Ft. Bragg to visit, pick it up and I was doing some last minute mods on it adding a master Vol. Moving some of the pots around and left the holes in the face plate. My intention was to install plastic plugs in the unused holes and put some text to identify the pre vol and master vol but she drug it off before I had time to finish. I made that amp when she was really into guitar playing. She had such great potential, fast learner but all of a sudden she dropped it and that was it. She bought an acoustic several years ago and bangs on that sometimes. She still lays claim(and has possession) to the Bogen anyway;>/
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sluckey Thanks, I bought that NOS Wirewound Sprague Koolohm 7.5K 5watt
Wow! I found a mistake in the power supply going to the pre-amp section. R30 in the power amp section feeding B+through umbilical cord to pre-amp is suppose to be 110K but with three orange bands is 33K??
I'm going to leave it as is for now. When I get it up and running, I will check voltages in
preamp and see if I need to change it out
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Wow! I found a mistake in the power supply going to the pre-amp section. R30 in the power amp section feeding B+through umbilical cord to pre-amp is suppose to be 110K but with three orange bands is 33K??
You probably did that on the advise of someone in order to increase the B+ for the preamp to more common guitar amp voltages. No need to change it if you are happy with the sound.
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yelp! Will wait and see the final results and go from there. Some of the voltage readings recorded way back then appear to be rather low for guitar amps, especially in the pre-amp section. Main thing right now is just identify and replace all bad components with new. I can tweak resistor or cap values after I get it up and running and see where I'm at.
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Outch! The more I look at this amp, the more I'm amazed that it worked as good as it has? Looking at the preamp today starting with the input jack. It appears that how you would normally hook up a 1 Meg to ground on the grid, I had a 15 Ohm:>))
Color bands Brown/Green/Black where the 1 Meg Brown/Black/Green
I must of got the color band arrangement confused and didn't bother to do a follow up check on the Ohms with a MM. Common greenhorn mistake. When I fix this thing right it will probably never sound the same again---but I hope it's for the better :l2:
Here is a link to some handy resistor color band charts:
http://www.bpesolutions.com/atechnical/resistorqv.pdf
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Outch! The more I look at this amp, the more I'm amazed that it worked as good as it has? Looking at the preamp today starting with the input jack. It appears that how you would normally hook up a 1 Meg to ground on the grid, I had a 15 Ohm:>))
In addition to the resistor mixup, there's another flaw in that mod. See attached pic...
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So both of those 3.3 Meg grid to ground resistors should remain?
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R13 is not causing any problems but it is not needed either. I'd clip it out.
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Well I got the input wired up as per your drawing for pretty near happy Grid leak with R13 gone to happy resistor grounds
I think I've looked it over enough to make a fairly intelligent parts order. There's always going to be something you missed :think1: no way out!
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Question? and I think I know the answer, but not sure? This amp on the 6SC7 Socket has the socket isolated from the chassis supported by two screws with nut on bottom side. The screws are surround by thick & tall rubber washers top and bottom, so the socket kind of floats on its own separate from the chassis.
I think it may be because the 6SC7 is known having micro-phonic tendencies and this cushioning helps to minimize that tendency but may be other reasons also. May have something to do with grounding, but not sure.
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I think it may be because the 6SC7 is known having micro-phonic tendencies and this cushioning helps to minimize that tendency but may be other reasons also.
Good guess. That was common practice for high gain preamps for mag cartridges that are very low output.
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> 6SC7 is known having micro-phonic tendencies
ALL tubes are microphonic. (This was a claimed advantage for that newfangled solid-state junk.) Mounting on rubber was standard for the first audio tube, if the maker cared.
Guitar amps are *just* enuff less sensitive than mag-phono amps that you can usually omit the rubber. Or when hi-gain amps evolved, they had so many 12AX7 in them that you could swap-around and get a good tube at the input.
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After doing the changes to the pre amp section I couldn't resist hooking it to the power amp even though the main cover panels are still disassembled. While I was installing all the tubes and umbilical cord I deoxeted the tube sockets and tube pins. The pre amp worked good and I didn't notice any of that hummm breaking in and out as before, a lot quieter.
I also checked a few voltages. The voltages coming off the rectifier in the power amp under full load is running 17 to 23 volts over schematic values. With schematic values first then actual readings next, 1st node 425/448, 2nd node 325/354, 3rd node 315/332. After 332 runs through the 33K to umbilical cord, pre amp gets 252 with sch showing 255----so that's pretty close. Whoever resized the 110K down to 33K for the pre amp voltage knew what they were doing, I sure don't remember doing it! The plate voltage to the 6SC7 is 66:>)
So the pre amp is ready to go except for the two 30/30 uF 450V cap I'll be ordering with the other filter caps to the power amp.
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What about coating the inside connections of switch with Liquid Electrical Tape? As per Link:
Don't put a band aid on a potentially dangerous shock hazard! Put that switch in a proper enclosure or just eliminate it. You don't really need a STBY switch. If you need a mute switch there are several ways to safely do that with no worries about shock hazard.
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Got a grounding question on installing my new can cap. I don't install these every day, so I'm not too sure of myself. Attached is a picture of the new can:
1-Don't I need to run a chassis ground wire to the negative terminal?
On the old can, (see picture) the PT center tap, 250 R41 Power tubes cathode Res, 25K R45 Grid 2 power supply ground and ground to umbilical cord are all grounded on old can ground term!
2-Can I just transfer all these grounds to the new can negative term?
Also I got a 100uf/100V for power tubes cathode bypass and a 10uf/500V individual capacitors to take the place of C1d and C1c formally provided by old can that will be removed.
3-I call your attention to the 4 term, 1 ground terminal strip at bottom of old can cap pix. My plan is to remove the old standby switch green and black wiring and install the replacement caps for C1c and C1d to that terminal strip and attach the old leads from the old can cap to that.
This is what I'm thinking, please tell me if it's Kosher. I'll also attach the schematic for your ref
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yes
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Thanks! :thumbsup:
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Got a basic Tube Amp 101 Question? I know how I've done it in the past, but I want to know what would be considered the proper way to do it. That is-----Extending component leads when they are too short and you need to add a section on for it to reach.
Attached is a picture of my Pre Amp chassis showing the old 30/30uF /Cap in a metal clamp with long red leads and laying on top of that the blue F & T 33/33uF replacement.
So my intent is to clip the old red leads off the old Paper Mallory cap full length and use those to attach to the F & T. Of course I will then remove the old Mallory cap and insert the new F & T in the clamp and attach the new leads to the old Red leads. Question is, what would be the best way to attach the leads, solder it and finally insulate it?
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I have replaced a lot of filter caps in old Gibson amps. Many of them have that exact setup - dual firecracker caps. Those F&T dual axial caps are perfect for that. I don't know if there is one "proper" way, but what I do is clip the leads on the F&T down to about an inch or so and make an eyelet. I slide a short length of heat shrink over the old cap leads, make a solid solder connection and then insulate with the heat shrink. I've tried twisting the leads and soldering them, but the F&T leads are stout, solid wire and the firecracker leads are stranded and flexible - I have a hard time making a good twisted connection with the two different wire types.
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Thanks very much for your reply. In the past, I have twisted the leads together in a overlap of about an inch. It never worked well and was hard to do. I did a little research and found a pretty common practice was to bend a small hook in each lead, mechanically tighten the hook with pliers and then solder--kinda like the attached picture.
It hadn't occurred to me that the leads on my old firecracker might be stranded. I checked and thank goodness it isn't. So unless someone enlightens me about a better way, I think I'll try the hook method.
We do these things in our builds/repairs but never talk about it. Rather than do what I normally do, I just wanted to maybe find a better way I could feel good about after its all said and done
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The hook is a good method. However, I would cut the new cap lead about 1/2" long and put the hook as close to the cap body as possible. Then slip a short piece of heatshrink tube over the old wire before making the solder connection at the cap body. Finally, slip the heatshrink over the solder joint and apply some heat.
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Got Ya! Thanks sluckey
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Well I've been busy on this last couple of days. Got all the new filter caps in. It's sounding much better and running nice and quiet.I'm now checking a lot of the old carbon resistors to see if they are holding or drifted. On V2 the driver and recovery for the Baxandall tone stack has resistors that I've measured in place that's is reading pretty off:
V2A R25 680 Blue/gray/Brown Pin #3 Cathode Resistor to ground reads 853 measured in place
V2B R26 470K Yellow/Violet/Yellow Pin 7 Grid to Ground reads 90K measured in place
V2B R28 1800 Brown/Gray/Red Pin 8 Cathode Resistor to Ground reads 2100 measured in place
Don't know if these are off enough to sweat blood over? Problem is these resistors are what I would say, almost inaccessible because of the "V" shaped pre amp chassis. To get your hands plus a soldering iron in that space??? See picture below of the chassis.
Also here is another top view of the chassis and right below the pilot light is V2 socket with subject resistors. Maybe you know some tricks to pull such a task off. Won't really know how far off they are unless I get one leg disconnected and measure it that way
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Hmm - those pics are curbing my enthusiasm for working on mine. Any chance of forcing that bend open enough to get in there? Anyways, R25 and R28 I suggest leaving as is for now. As bypass resistors, those values might work out fine. R26 - I wonder if a parallel path is affecting that reading? Does not seem likely that it went from 470K to 90K.
Edit - although the ground side of that 470K does not look to too tough to reach - I think I would make notes and continue on to the point where you can sound check and then reassess.
BTW I have a Hako desoldering tool with a built in vacuum - they are pricey but are great for such work.
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Here's a tip when measuring resistors in circuit...
If the resistance is higher than expected, the resistor has likely drifted up in value. If the resistor reads lower than expected there is usually something in parallel to the resistors. Resistors rarely if ever drift down in value. If the resistor shows signs of being overheated the value could measure either higher or lower.
Always have a schematic handy when checking values. Cathode and plate resistors can usually be measured accurately in circuit. Consult the schematic for most others to see if there is something in parallel with the resistor. It's usually apparent.
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Attached is the schematic for the input and tone stack stages I made 22 years ago. It's pretty much correct except for the Grid Leak changes recently done on the input. Also R13 has been removed. I don't see anything I would consider in parallel with those 3 resistors, do you?
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According to the schematic you should be able to measure R26 accurately in circuit. It's highly unlikely that a 470K has drifted down to 90K. I would recheck R26 and verify the amp is actually wired according to the schematic.
Does the amp really have that .1µF cap just prior to V2 pin 7? If not, that could explain the 90K reading.
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Hmm, after comparing your schematic to your pic, I'm gonna say your schematic is WRONG! I see a dark green wire that appears to go from pin 7 directly to the junction of a 100K and a small ceramic picoFarad cap. No .1µF cap in sight.
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Thanks sluckey
The .1 uf just prior to V2 pin 7 does not exist. The dark green wire from pin 7 goes directly to center output term of the treble pot. The 100K coming from the bass pot is correct.
The small 330pf ceramic cap is being fed from V2A pin 1 through .1 coupling cap through the ceramic to input term of treble pot, just like the schematic. So even though the 330pf appears to be connected to the 100k your seeing, it's not. However the 330pfis connected to a 100K underneath the terminal strip that is connected to the input term of the bass pot.
I hope this clears thing up a bit. I can't believe the 470K on pin 7 could have drifted down to 90K either. Something is effecting that reading!
I did start a new ExpressSCH schematic on this amp but haven't been able to make much progress lately. I know a good schematic would clear a lot of things up and help identify all my original mistakes :icon_biggrin: but we will git r dun
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The .1 uf just prior to V2 pin 7 does not exist. The dark green wire from pin 7 goes directly to center output term of the treble pot. The 100K coming from the bass pot is correct.
There you go. That's the parallel path. The blue path is parallel to the red path. With the Bass pot turned down you have a 100K + 10K = 110K parallel to the 470K. That calculates to 89K. With the Bass pot turned up you have 100K +1M + 10K = 1.11M parallel to the 470K. That calculates to 330K.
Measure the 470K again but turn the Bass pot. You should see the meter reading change.
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bmccowan
I've got an old Weller 8200 N that's literally held together with electrical tape. I use that for everything these days. I've tried the simple 30 watters and have a bunch of them I never use. They just didn't work for me. That Hako sounds great with a vacuum to suck that solder down. I wouldn't know how to act with one of these modern total solder stations.
On the old amp, knowing what I know today, about the difficult configuration, I would have probably sold the amp or parted it out and used the parts to build something else. It was in like brand new condition when I got it and I was so excited because I though it was so Cool. So I dove right into it not giving it a second thought. The crazy thing about it, is spite me being a total greenhorn and had no idea what I was doing, the Hoffman forum walked me right through it and it ended up sounding great to my ears. So I've used it quite a bit over the years but lately it was starting to moan and groan. So I'm a little cautious about what I change on it because that tone and feel I've enjoyed may be part of the mistakes I made---I don't know. However on the other hand, it could be improved because it does have some quirks that I've never flushed out. So it hard to know how far to go with it. I've been running some of my different guitars through it and it seems to be friendly to every guitar. Just amazed at the tone and feel. Platefire
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With the bass pot full up I'm reading 372K across the 470K resistor. That's about 42K more than you calculated. Does that mean my 470K is salvageable? Those kind of calculations are really valuable, don't see how you retain so much. Mine seems to leak out :dontknow:
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With the bass pot full up I'm reading 372K across the 470K resistor. That's about 42K more than you calculated. Does that mean my 470K is salvageable? Those kind of calculations are really valuable, don't see how you retain so much. Mine seems to leak out :dontknow:
What that 372K reading means is your bass pot is really more than 1M. The 90K reading you had with the bass pot turned down (and out of the circuit) means your 470K is perfect. I calculated 89K and you measured 90K. I bet my calculator is more accurate than your meter. :l2:
The parallel resistor equation is easy... It's the reciprocal of the sum of the reciprocals. Ain't that easy? :wink:
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Great and Thanks! If I put a new battery in my MM, it would probably read up to snuff :thumbsup:
What about R25(680/853) and 28(1800/2100). I think I could access R28 fairly well with extreme caution but R25 is a pure bugger :BangHead:
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bmccowan
I've got an old Weller 8200 N that's literally held together with electrical tape. I use that for everything these days. I've tried the simple 30 watters and have a bunch of them I never use. They just didn't work for me. That Hako sounds great with a vacuum to suck that solder down. I wouldn't know how to act with one of these modern total solder stations.
On the old amp, knowing what I know today, about the difficult configuration, I would have probably sold the amp or parted it out and used the parts to build something else. It was in like brand new condition when I got it and I was so excited because I though it was so Cool. So I dove right into it not giving it a second thought. The crazy thing about it, is spite me being a total greenhorn and had no idea what I was doing, the Hoffman forum walked me right through it and it ended up sounding great to my ears. So I've used it quite a bit over the years but lately it was starting to moan and groan. So I'm a little cautious about what I change on it because that tone and feel I've enjoyed may be part of the mistakes I made---I don't know. However on the other hand, it could be improved because it does have some quirks that I've never flushed out. So it hard to know how far to go with it. I've been running some of my different guitars through it and it seems to be friendly to every guitar. Just amazed at the tone and feel. Platefire
Weller 8200N - Is that one of those big pistol grip things? Oh man!
The Hakko desoldering iron - F-301 is also pistol grip, but its really only good for desoldering. Tip is too bulky for soldering. I have another Hakko station for that. I have a modern Weller station too, but the damn thing has a mind of its own. Sometimes powering down to 350 from 750 - other times staying right on the setting. I should have returned it long ago.
The amp - I forgot that you previously had it operating to your satisfaction. Mine I have yet to touch, and may resort to putting the trannies and a few other parts in a new chassis. That folded chassis looks like a nightmare.
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What about R25(680/853) and 28(1800/2100). I think I could access R28 fairly well with extreme caution but R25 is a pure bugger :BangHead:
Those are cathode bias resistors. Slightly larger value means the tubes are biased slightly cooler. No big deal in a preamp stage. I'd let it be.
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Oh Good! That should wrap up the pre amp section. Got some resistors in the power amp section I need to take a secondlook at. I need to get it back together while I can still remember where all the panels and screws go at. I want to take
some finish pictures gut shots and assembled shots.
Oh yeah, this amp has a hum balance pot hooked to the heater wiring. I just happened to check the heater voltage theother day and one side was 3.8 VAC and the other was 2.8 VAC. So I adjusted it where it was balanced at 3.3/3.3
Thanks :happy1:
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Oh yeah, this amp has a hum balance pot hooked to the heater wiring. I just happened to check the heater voltage theother day and one side was 3.8 VAC and the other was 2.8 VAC. So I adjusted it where it was balanced at 3.3/3.3
Thanks :happy1:
A better way to adjust the hum balance is just turn the pot for minimum audible hum. If your ears can't decide then connect your AC voltmeter across the speaker and adjust for minimum voltage.
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uh oh, got a problem! On the V3B Phase inverter grid pin 7, I got about 40VDC. Schematic says -2VDC there.
I know I'm suppose to have 65V on the cathode which I reading 78VDC up through the 3K cathode resistor where it is then 76DCV up to the 470K. Then on the other side of the 470K it turns to the 40VDC that is on the grid.
So I'm thinking C12 .047uf is leaking. Don't know where else it could be coming from?
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The -2V on the schematic is wrong! Nothing is wrong with the amp.
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Well Thanks, that's good to know. I'll ride on that :grin:
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Weller 8200N - Is that one of those .....
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Yeah, PRR that's just like mine but it appears yours is in better shape than mine. Are you having any trouble finding solder tipsfor yours. I just installed my last spare tip a while ago and the last one I bought was from Radio Shack and they are closed now.So I hope there are other places to pick those up at
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The -2V on the schematic is wrong! Nothing is wrong with the amp.
I should clarify... There's nothing wrong with the voltage reading on the PI. Of course there may be other problems in the amp.
Yeah, PRR that's just like mine but it appears yours is in better shape than mine. Are you having any trouble finding solder tipsfor yours. I just installed my last spare tip a while ago and the last one I bought was from Radio Shack and they are closed now.So I hope there are other places to pick those up at
Amazon... (https://www.amazon.com/s?k=weller+8200+soldering+gun+tips&crid=28GNDE8CREKZU&sprefix=weller+8200+soldering+gun+tips%2Caps%2C101&ref=nb_sb_noss_1)
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Weller 8200N - Is that one of those .....
Yup - I've got one tucked away somewhere. Was my Dad's. I've used it a few times remelting chassis solder puddles. Hey if it works for you, Platefire, go for it. I think I'd be burning wire insulation and film caps even worse than I do now. :icon_biggrin:
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That 8200 is also useful for cutting synthetic braided rope using the proper tip.
https://www.amazon.com/Weller-RCT-Rope-Cutting-Black/dp/B0002BSQR6/
These tips are very easy to make with a bit of 12AWG solid wire, a hammer, and a vise. Most marine supply stores have an 8200 in the rope section.
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I understand sluckey on the PI voltage. One crises averted, looking for the next one :laugh:
Thanks also for the source on the 8200 solder tips.
bmccowdan--I have burn up a few things with that 8200. You have to be easy on trigger and keep that tip clean
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...yours is in better shape than mine.
Sorry-- that's grabbed from eBay, just to remind me what a 8200 is. (I have two Guns, neither an 8200; I know Weller made many variations.)
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That 8200 is also useful for cutting synthetic braided rope using the proper tip.
https://www.amazon.com/Weller-RCT-Rope-Cutting-Black/dp/B0002BSQR6/
These tips are very easy to make with a bit of 12AWG solid wire, a hammer, and a vise. Most marine supply stores have an 8200 in the rope section.
I rebuilt a 20' wooden sloop many years ago (doing this will make you think the amp hobby is super cheap) and the local marine store had just that - and they let the customer use it themselves! I wonder if that still happens?
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...marine store had just that - and they let the customer use it themselves! I wonder if that still happens?
The Hamilton Marine in Southwest Harbor might. It's way off the highway, not many jerk landlubbers find it.
The Hamilton in Searsport is right ON Rt1 next to the flea markets, so customers can't be running loose in there.
Gotta respect a company which blends their own Lobster Buoy Paint and then posts a Stress Test with wire brush, scraper, files, and parking-lot.
YouTube "Lobster Buoy Paint Stress Test"
For away folks: the modern Lobster Buoy is Styrofoam, so it is amazing any non-toxic paint even sticks, much less takes abuse.
Catch the Mount Desert Water plug at 2:36.
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Love that video, especially the parking lot test. I have a pair of oars that need painting - that paint might be just the ticket.
“Believe me, my young friend, there is nothing - absolutely nothing - half so much worth doing as simply messing about in boats.”
― Kenneth Grahame, The Wind in the Willows
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Trying to calculate bias for this cathode bias amp. So here goes:
I=v/r, I=Voltage across cathode R/Measured cathode R Ohms, I=26.16VDC/241Ohms, I=.10818, .10818/2 tubes=.05409
54.09 DC Milliamps Bias
am I even in the ballpark?
It's got Sovtek 6L6WXt's
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You're in the ball park. More specifically, you're on second base. Gotta do a little more math to get to home plate.
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Trying to calculate bias for this cathode bias amp.
Why?? Do you think GE got it wrong? Make Rk equal 250 ohms and quit fretting. (Or start fretting, your choice.)
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Well PRR, no doubt the cathode resistor is correct, I'm just curious. What little I been studying on cathode biasing, it appears that the mA are normally set higher/hotter than fixed. I really slow on this math, so it don't hurt to give it a run every once and a while
I think what sluckey was wanting me to do was calculate the wattage and also the dissipation to reach home plate
My plate voltages are 440/436. so averaging those out is 438VDC. So to calculate power P=IV, P=438 X .05409=23.69 Watts
I think my little ol brain is fried, hafto come back to this latter :think1:
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I think what sluckey was wanting me to do was calculate the wattage and also the dissipation to reach home plate
My plate voltages are 440/436. so averaging those out is 438VDC. So to calculate power P=IV, P=438 X .05409=23.69 Watts
Well, now you're on third. That 23.69 watts you calculated was for the tube and the cathode resistor. But you need only the tube dissipation in order to reach home plate. So, subtract the cathode voltage from the measured plate voltage to get the voltage across the tube. Then multiply the voltage across the tube times the calculated cathode current.
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OK, I think go get cathode current I need to divide the measured voltage at cathode by the measured value of the cathode resistor
So 26.16/241.8=.108
so to get % of dissipation multiply voltage across tube 411.84 X calculated cathode current .108 =44.48 % dissipation
Well, am I home yet?
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44.48 is static dissipation in watts. I don't know what "% of dissipation" is. And 44.48W is for two tubes.
Well, am I home yet?
It's a close call. What about it judges? Shall we give it to him? :wink:
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:dontknow: 44.478 :icon_biggrin:
Looks good to me
Thanks for the lesson...again :worthy1:
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Sooooooo-44.48 Static dissipation in watts. Trying to figure what kind of information I've got with that? Does this relate to the totalMaximum Plate Dissipation in the Tube specs?
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This is purely a guess, and Steve (or other really smart guys here on this forum) will correct, but I think it relates to the efficiency somehoe of the tube/how it is performing. Again, a guess. Heck, I'm on my 2nd bourbon now-and listening to a Pink Floyd (David Gilmore) concert-and he just pulled out the BLACK STRAT!!...
; )
Vr
J
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Well I know on fix bias amps we set the bias at 60 to 70 % dissipation to get the most efficient tone/sound but at the same time not burn up the tube because if it was set at 100% dissipation, it wouldn't last too long. So I'm thinking this is related to the same thing---but don't recall put it in these terms "static dissipation in watts".
I'm thinking it's the way of measuring the amount output the amp is producing at idle with no guitar signal??
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Don't let the word "static" confuse you. Static simply means at rest. No signal applied. Maybe I should have used the word "idle". Means the same thing.
Yes, it relates to the max plate dissipation seen in the tube charts. For fixed bias many of us like to set the static/idle dissipation at about 70% of max dissipation. For cathode biased amps the static/idle dissipation typically runs about 90% to 110% of max dissipation.
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OK thanks for the clarification. So we have some calculations and figures. So let me get at the question I wanted to know all along. On this cathode biased amp, what is the % of dissipation this amp is set at? or what calculation we have already done will tellme that?
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The % of dissipation usually is the ratio of your measured idle dissipation to the tube chart max plate dissipation.
You measured/calculated 44.48W. But that's for two tubes. One tube would be half that (if the tubes are perfectly matched). So let's use 22.24W for one tube. And JJ 6L6 says PaMax is 30W. So...
22.24\30 = 74.1%
However, most tube charts say the PaMax is only 19W. So, in that case...
22.24\19 = 117%
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Thank you so much!! :bravo1: This brings me to the home base I was wanting to reach for myself on this amp. Learning more than I ever have on cathode bias. BTW-does PaMax mean "Peak Maximum"? Using the 19watt reading seems pretty hot but on the other hand, I have had those Sovtek 6L6WXT 22 years in this amp and they are still kicking pretty strong. I paid $20.00 for the set back in 2001. So I'm glad we got this all documented, on hard copy and also on this forum, so I can go back and grab it when my forgetter gets to working real well.
Guess I will push on to doing final checks on the power amp.
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PaMax means Power Anode Max. Anode is same as plate.
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I've got 5 resistors in the power amp that are reading out of spec or more than 10% off and no problem because I've got replacements for 4 except for the 3K cathode resistor R37 PI V3B that is reading 3448 in place but it appears to me it does have a 470K in parallel to it plus a 110K in series to ground with it. So that may be whats throwing it off. Physically it don't look burnt and colors is good.
I will lift a leg and measure it that way before I make a special order for that one
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There's nothing in parallel with R37. You can accurately check it in place. I'd let it be unless I was super bored.
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I'm not super board :icon_biggrin: I got burgers to grill and got to run to Wally world to get some ice cream salt and ice. Hope they are open?
OK to be officially in parallel, the resistor would have to be connected to the resistor in question at both ends. Right? One end
don't count
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I'm not super board :icon_biggrin: I got burgers to grill and got to run to Wally world to get some ice cream salt and ice. Hope they are open?
Don't forget the watermelon!
OK to be officially in parallel, the resistor would have to be connected to the resistor in question at both ends. Right? One end don't count
That's right. And the parallel connection may not be obvious. Usually (but not always) cathode or plate resistors can be checked in circuit because one end terminates at the tube pin with no other resistors connected, as is the case with R37. Always helpful to have a schematic handy when checking resistors in circuit. For example, look at V3 pin 3. You'll see a 3K to ground. But if you follow that wire up and to the right you'll see it connects to a 27K that connects to the OT secondary. The OT secondary has very low resistance so ignore that. That means the 27K is practically connected to ground, which puts it in parallel to the 3K at V3-3. Your meter will be measuring 3K||27K so the reading will be about 2.7K.
So, the take home on this is... When measuring resistors in circuit and the reading is lower than the resistor value there is probably another resistor in parallel even if it's not obvious. Looking at the schematic will usually give you an idea what your meter should measure. If the circuit is more complicated though, you'll have to disconnect one lead of the resistor.
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On V3B PI R35 the 47OK R reads 579K, R37 3K cathode resistor reads 3448 Ohms and R32 110K reads 123K.
I didn't realize it but was just checking but you can't buy a 3K or a 110K resistor anymore from most sources. I was thinking for R37 the 3K, I could install a metal oxide 3.3K(that measures 3.25K) and for the R32 to ground, install a 100K metal oxide. That would be installing common values now available and would be close to the values its already drifted to. That way it won't be drifting further out hopefully anytime soon. Good or bad idea?
Also got a metal oxide 470K for R35.
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> you can't buy a 3K or a 110K resistor anymore
So make it 100k and 2700 (drop 10% all around).
This isn't government work.
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> you can't buy a 3K or a 110K resistor anymore
So make it 100k and 2700 (drop 10% all around).
If you decide to do this then also change R36 to 100K just to keep the cathodyne outputs balanced.
I probably have 3K and 110K resistors. I'll look tomorrow. Looks like you need 1/2 watt?
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I checked and found some metal oxide 2.7K that measures in the 2650 range. Got plenty of 100K's
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I probably have 3K and 110K resistors. I'll look tomorrow. Looks like you need 1/2 watt?
I checked my stock. Plenty of 3K and 110K but they are all way over spec. :sad2:
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Time to shake and bake them, dip them in secret sauce, scrape them, paint them - whatever it takes.
This ebay seller has those values in 1 watt metal film. https://www.ebay.com/itm/324683051749?hash=item4b989e32e5:g:DdUAAOSwJ2hgzXRu&amdata=enc%3AAQAIAAAA4K99NfT63TdFIG75dikOW14Uwl%2FVnDdTq9mDeBFLiMYOhebMtrCWG%2BGooM8CS8EJAAFllyV0PUJNtggeBxJbjT7Fv5PX39Zkldd1Zby6UTLM0Je6orYaoBZTsLYeumfryBuViE4M4YMqpvOY%2Bfr9uucj2Xr%2Fw6TlVEiv1GAlzzG82usv8I9QCRKsY5z30Pp2ftvkV7OVWkCv7tOftfwSv7zWbkcthB1eMYRGmIr%2F2jy3cNvCiuJRPAlcA%2BAxbPgypAIw2GCO1MXQI5rNAcDzdV%2F1TKJTC3HkGtAqI7kRhNPo%7Ctkp%3ABFBM2KKa96Ri
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Well thanks for looking sluckey. I got a bunch of CC resistors I picked up years ago but no 3K or 110K in mine either. Well I guess I forgot about e-bay and free shipping too. Thanks bmccowan
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Well I ordered them. Won't ever have to worry about running out of those values for this amp because for $2.99 they send10 Ea:>)
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sluckey----Got a question for you. It's regarding the "Loudness 500K, Tap 120K & 350K" or we would call "Master Volume".
Only thing this 4 term 500K pot has a component with multiple leads attached to it as shown on the schematic(attached for your convenience). This is something I left in place from the original circuit that I really don't understand its function. So if you could explain to me what this is doing, I truly would appreciate it.
What sort of Monster do we have here?
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It's a tone compensating network. Probably improves the bass sound at lower volume levels.
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Thanks sluckey for having look and letting me know. As I remember I started to rip all that out and just install a 1Meg Audio pot in its place but all the pots have long split end shafts to reach the face plate and for the existing knobs to fit, so I left it as it is.
On the parts sheet is says K1 is "Tone Compensation" that was a special single component with multiple leads in the old tone stack that was removed.
On the parts sheet is says K2 is "Loudness Compensation" that is also a single special component with multiple leads that remains
I'm thinking that possibly you could disconnect the special Loudness Compensation Component and just use the three top terms input/output/ground like a regular pot. Don't know what that would do to the sound that I'm use to? The loudness pot has 5 terms on it, 3 at the top, two at the bottom.
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Just remove K2 and you will have a standard 500K volume control.
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Been thinking about it and believe that's exactly what I'm going to do, cut the cord to K2. That's more of the design I had in mind way back then will finally be fulfilled. K2 is mostly Hi-Fi design, not guitar amp.
Also been thinking about that existing 3K NFB resistor in the place of the 27K shown on the schematic. I'm thinking of taking the standby switch and wiring and installing it as a 27K NFB on/off switch. I noticed that some vintage Fender 6L6 PP amps use the 27K NFB resistor. Off course I can test the 27K NFB operation before I put it all back together again.
Thanks for your help. Trying to do a good job on it :icon_biggrin:
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Correction!! The NFB resistor I thought was 3K is actually 27K like it should be. It's just in parallel with the 3000 CC cathode resistor and measures 3.14K. A 3K and a 27K in parallel is 2700 Ohms. So considering drift, 3.14K seems about right. After all,the 27K color code red/blue/orange can be clearly seen + it's a metal oxide 1 watt, so I doubt its drifted much. It's the CC 3K that's done most of the drifting
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Good to see you using your noodle to trace the parallel path and calculate that resistor puzzle. Now you need to spend some time on the resistor color code. The color code for 27K ain't red/blue/orange. :icon_biggrin:
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Well OK, Red/Violet/Orange
It looked blue on my Radio Shack, 3 wheel, Resistor/Capacitor Color Code Tool
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> It looked blue on my Radio Shack, 3 wheel, Resistor/Capacitor Color Code Tool
That's been out in the sun too long.
I suppose your Shack shut-down??
You can find them, but shipping is silly.
https://www.tequipment.net/ElencoCC-100.html
https://vetco.net/products/resistor-color-code-calculator-card
Table on back of their business card, free for the asking:
https://www.allspectrum.com/store/resistor-color-code-chart-all-spectrum-electronics.php
Stickers:
https://www.redbubble.com/shop/resistor+stickers
T-shirt:
https://www.redbubble.com/i/t-shirt/Resistor-Color-Chart-by-Cheesy-Puffs/25352665.WFLAH
On-line cheat:
https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-resistor-color-code
Print yourself:
https://www.electronics-tutorials.ws/resistor/resistor-colour-code-wheel.html
https://www.electronics-tutorials.ws/wp-content/uploads/2018/05/pdf-colourwheel.pdf
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me> shipping is silly.
https://www.tequipment.net/ElencoCC-100.html
Product Total $1.90
Shipping Estimate
- Ground - $11.92
- 3 Day - $32.67
- 2nd Day Air - $40.10
- Next Day Air - $89.56
- USPS - $12.95
Total $13.82
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Maybe one of the Navy boys will chime in and tell you how real techs memorized the resistor color code. My trade school instructor was retired navy so I got it a long time ago.
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Wikipedia has an AMAZING collection. Most of them gibberish, and apt to confuse black and brown.(*) (There are three "b" colors.)
https://en.wikipedia.org/wiki/List_of_electronic_color_code_mnemonics
(*)My trick, beyond offensive sayings, is to KNOW that 100 Ohms, a super-common value, IS Brown-Black-Brown.
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The one I learned made the top of the list in the Offensive/outdated section.
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Wow! That's a lot of Resistor color code help there, Thanks! PRR. I actually started trying to memorize number/colors combinations tonight. Got a good start on them. Seems to me if you get the color/number combinations memorized plus the multipliers, you've got a good start on getting correct values. If you can't calculate it in your head, using your calculator on your phone, you can crunch the number pretty quick. I'm talking about 4 band color codes, not 5 or 6---not there yet :think1:
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There are many apps for resistor color codes for android or apple.
I've been using the 4 band color code all my life. It's ingrained in my brain just like the alphabet. I recognize resistor values without having to think about it. I'm a bit slower with the 5 band color code and do have to think about it. Since I changed over to metal film I've gotten quicker. As long as the third band is always black (0) I'm pretty quick. But as soon as the third band changes color (such as 472K) I have to make a deliberate calculation in my head. Thankfully, I don't own any of those weird values. :icon_biggrin:
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The one I learned made the top of the list in the Offensive/outdated section.
When you said Navy, I knew that was the one. :laugh: Not safe anywhere anymore.
I like that one from Canada: Black Bears Roam Our Yukon Grounds But Vanish in Gray Winter
Although I find it strange the way the mind works. I tend to just recognized the value with some kind of photographic memory - but I have the hardest time with the multiplier.
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The multiplier is easy if you don't think of it as a multiplier. I think of it as the number of zeros to add after the first two digits. For example if the third color is orange I just add three zeros. For example, yel-violet-orange is 47 with three zeros, or 47000. Or, red-red-black is 22 with no zeros.
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That should help me. I have a Nephew who is both color blind and dyslexic. He would love this hobby.
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The multiplier is easy if you sort resistors by multiplier color. I had a black bin, a brown bin, red, orange.....
If I needed "about 3k", look in the red bin, find a red-vio-red (2.7k) or org-org-red (3.3k), whichever came to light first.
When I needed 2.2Meg I learned to look in the green bin. So only 8 bins, 7 if you admit that the blue-stripe parts can sit in the green bin (IF you have good blue/green light and vision).
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Got the metal film resistors for this project in today. Weird, the ticket said for $2.99 you get 10 pieces for each R value. They were actually 17 each. Not complaining:>) Also I got a little concerned because one package was marked 2K, but I didn't order 2K but 3K. No worry when I checked it with my MM, it was 3K.
Installed the needed resistors today, about 11. Fired it up with a light bulb limiter, no problem! When put it on household current that voltages are running 10 to 45V higher than schematic. I jammed on it this evening and it's sounding really good. No red plating. No unwanted noises
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Attached is my draft ExpressSCH. that was never done back in 2001. So I welcome you to check me out! I plan to add some more info as I get time, but this is the basic draft.
It calls for a 5AMP fuse. That seems a little stiff to me. Seems like a 2 AMP would be more in line to provide some real protection?
The voltages shown are recent readings.
Like to note a difference in what the original sch shows and what's actually been in
place. R30 shows a 120K but there is actually a 22k in that place. Also R47 snows a
22K and there is actually a 120K in that place. When I installed new resistors, I replaced
to match what's in place rather than the schematic.
FYI-The main changes has been to the Preamp section. The power amp is stock like the original schematic. I have attached the original schematic for reference. Platefire
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Congrats on bringing her back. Tough chassis to work on! And thanks for posting a draft completed schematic.
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Thanks! I think it's a lot of sentimental value with this one. Of all the hunting I use to do for old tube stuff, coming up empty handed most of the time, this old amp I picked up in like new condition for $20 really boosted my desire to continue the hunt.There is a lot of those old PA-20's still floating around, maybe someone will use my plan to re-build one. But you really have to be patient with this one, because it's an extra level or two more of difficulty.
On ExpressSCH, I haven't used it in a while and was working on this amp sch. Spent the better part of 3 hours working on it and all of a sudden the program shut down and disappeared. So about that time I remembered that I always used to be hitting the "save" disk symbol because this program, as far as I know, has no automatic save. Sure enough I opened the file back up and lost everything. So when I'm working on it, I'm now hitting save every minute of so!
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Looking back at this amp after latest restoration, I don't understand the power nodes routing. Usually the second power node(B)goes to the power tube screens but on this amp, it goes to the PI. Then the 3rd power node goes to the screens. This seems to be going against the norm--any engineering rational behind this?
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Looking back at this amp after latest restoration, I don't understand the power nodes routing. Usually the second power node(B)goes to the power tube screens but on this amp, it goes to the PI. Then the 3rd power node goes to the screens. This seems to be going against the norm--any engineering rational behind this?
You mean B and C are split rail (from the same B+ node) but with C feeding the screen (and C is lower than B). Probably to keep the screens sitting well below the plates, likely to do with having cathode-biased 6L6s idling at near Pmax and I suspect the load line would otherwise cross the Vg0 grid curve below the knee if the screen voltage was the same as the B+ (so having a lower screen voltage lowers the grid curves to keep the load line through the knee - helping to keep the screens happy when driving the amp hard*). No doubt these were over-designed to ensure longevity.
*screen current tends to shoot up when the plate voltage swing bottoms out ('below' the point where the grid curves all drop suddenly to the origin). If the load line crosses below the Vg0 knee, then the probability of over-dissipation of screens is higher when plate voltage swing bottoms out. Lowering Vg2 is one way to avoid this. Biasing the output tubes colder is another way. Running a lower reflected load resistance is another way.
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Old 6L6 had low Vg2 ratings; also we though we had enough power at 300V on G2. But it had to be non-saggy, hence the divider with heavy bleeder.
But also: you are looking at a 2023 interpretation of a 1959 design. Might want to look at how it used to be. Lower voltages, not very different; but one feed much cleaner.
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OK, thanks for taking a look and the explanations. I'm use to the screen being just a few volts below plate, not use to that much difference. I was getting ready to put the amp back together and wanted to make sure everything is OK before hand.
The original tubes were originally 6L6GB's and I've been running Sovtek 6L6WXT's last 22 years, so I don't guess I've overloaded my wagon, it's still kicking and no burn outs so far.
That lower screen grid voltage may have been my saving Grace
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sluckey---I did remove original K2 component off the master volume pot. Made quite a difference. Before I was having to cut bass fully counter clockwise and boost treble fully clockwise to get the tone I wanted.
Now---The amp is brighter and the tone controls works more like they are suppose to with adjustments landing closer to the 12:00 position. I guess I was kind of afraid to remove that because I wondered if it would remove some of the original charm, but it only improved the amp. So that old myth is abolished. Thanks!
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I'm use to the screen being just a few volts below plate, ....originally 6L6GB's
6L6GB specs: https://frank.pocnet.net/sheets/127/6/6L6GB.pdf
400V Vp, 300V G2
The GE runs 415V-25V= 390V on plate-cathode, 315V-25V= 290V G2 to cathode. Pretty much all they dared.
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Both transformers have a 1957 date, so it may be a year so older than I had it dated(1959). I put it all back together and been testing it out with different guitars.
Inside the top cover panel right above the power tubes, it had a 1/8" layer of asbestos about 4" x 4". I guess that was to protect the panel from the heat. The power tube are only about 1/2" below the cover. I carefully removed it. I'm pretty familiar with that, we did a lot of asbestos abatement contracts at Fort Polk. Never seen it in a tube amp before. I've been reaching over to feel that spot on the panel when I using the amp but it only gets warm, not hot. I might ought to replace that with some modern/safe heat resistant material. If anybody has one of these amps, you need to be aware of that.
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If anybody has one of these amps, you need to be aware of that.
Thanks Platefire - As you know I have one of those GEs sitting on my "maybe one day" shelf. But my intent is to use the iron and a few other parts and build into a non-nightmare chassis. But when taking it apart I will watch for that patch and be careful. My understanding is that you want to avoid the asbestos becoming friable. It's inhaling the fibers that is not so good for you, eh?
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Yes, I remember you saying you have one. Believe me, your saving yourself a lot of problems by putting the trannys in a more conventional chassis. When you tear it down for parts just be careful removing that top cover not to disturb it and put the whole top panel in a sealed HD bag and dispose of it in the proper place.
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Thinking about something? The asbestos heat shield over the power tubes is removed, but I noticed last night the cover plate was getting pretty hot, especially on the under side. Guess they had it there for a reason:>) I've been looking a self adhesive heat shield material that protects up to 1000 degrees to put on there but I'm kind of concerned about the self adhesive letting go after so long. The attached picture shows the situation, the small space above the tubes between the top panel, about 1/2". Playing outside on a hot day it could really really get hot.
Any recommendations for this or am I making a mountain out of a mole hill.
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Drill some ventilation holes in the case?
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It's a metal case and the back is completely open. I'd let it be. Really, how often will you be "Playing outside on a hot day"?
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... how often will you be "Playing outside on a hot day"?
And I love to live so pleasantly
Live this life of luxury
Lazin' on a sunny afternoon
In the summertime
In the summertime
In the summertime (https://genius.com/The-kinks-sunny-afternoon-lyrics)
As a 1957 Hi-Fi you might build it into pine cabinetry and leave it on all the time. Or your child might touch the top and get a blister.
Live dangerously!! (And smarter than the average bear.)
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Lazin' on a sunny afternoon
In the summertime
These days it's more like...
Hot town, summer in the city
Back of my neck getting dirty and gritty
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Where you live - yes.
In Maine its more like:
If the rain comes
They run and hide their heads
They might as well be dead
If the rain comes
Either way, I like your solutions better. With Sluckey's you need do nothing. With PRR's you just need to mix up a gin and tonic and Beware the Demon Bowler.
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Wow! I've had one crazy day :dontknow: The good part is I get home and find ya'll have my problem solved and as it turned out, I didn't have a problem at all.
My Amp, My Amp
has got me thinking bad thoughts
My Amp, My Amp
Could of been cooler and got mellowed out
but keeps picking at me, oh what shall I do?
Nothing said I, Nothing is what I'll do :icon_biggrin:
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Made A Cover for it :icon_biggrin: